Gibbs Guides, Gibbs Guides offers an expanding range of ...

Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 3 – Power System Solutions

© Copyright Andrew Gibbs

1 2013

Electric Power Systems Part 3

Power Power Power System System System

SolutionsSolutionsSolutionsby

Andrew Gibbs

gibbsguides.com


The Gibbs Guide to Electric Power Systems

Part 3

Power System Solutions

A Gibbs Guides e-book

By Andrew Gibbs

gibbsguides.com

2 © Copyright Andrew Gibbs 2013


Messages from Andrew Gibbs

Thank you for purchasing this e-book to Electric Power Systems. I sincerely hope it will be an enjoyable read, and that it will be a great deal of help to you. If you like the guide, please tell your modelling friends. If you have suggestions for improvements or alterations, please feel free to contact me.

Copyright matters

Several months of full-time work plus a great deal of effort went into creating this guide. As the author of this e-book, I hold the copyright to it. I make my living from helping my fellow modellers by writing about modelling matters, so I do have to charge for some products such as this e-book. There’s lots of high quality free-to-access information on my website at www.gibbsguides.com which you are welcome to access and share. There’s also a completely free, high quality newsletter which you are most welcome to sign up for at this site. I make sure my products offer excellent value by providing high quality information at a reasonable price. In return, I ask that you respect my right to copyright and do not share this publication with others without my permission. If you did this, you would deprive me of the chance of a sale. And if that happened I might have to stop writing Gibbs Guides and get a proper job!

Andrew Gibbs

Cover photograph

The cover photograph shows John Ranson’s superb Corsair warming up for an atmospheric scale sortie.


http://www.gibbsguides.com/


DISCLAIMER

Electric power system components such as motors, batteries, electronic speed controllers, propellers, chargers and so on are supplied with instructions. Additional information may also be available on the supplier’s and/or manufacturer’s website. You are strongly encouraged to read and implement any such instructions, especially those instructions relating to safety. Such instructions must be accurately followed without deviation. None of the information within this guide is intended to overrule such instructions, and where any disagreement exists, always follow the instructions of the manufacturer and contact them or the supplier of your equipment for advice about the particular circumstances of your application. All possible care has been taken with this guide and the information within it is offered in good faith; nevertheless, in using this guide you do so at your own risk and absolve the author of any liability whatsoever in respect of death, personal injury, damage to property or any other kind of accident. The Gibbs Guides terms and conditions state that by purchasing this guide, you have already indicated that you accept these terms.

Copyright notice This guide is copyright Andrew Gibbs. All rights strictly reserved. Storage on a retrieval system, reproduction or translation of any part of this work by any means electronic or mechanical, including photocopying, beyond that permitted by Copyright Law, without the written permission of Andrew Gibbs is unlawful.

Printing your guide By buying this e-book, you have bought a license to print a single copy single copy for your own use. If you choose to print your copy, I suggest using good quality 90 gsm paper or heavier for the pages, and thin card for the front and rear covers.



Contents Chapter 1: Introduction to Power System Solutions 6 Chapter 2: Comparing I.C. and Electric Power Systems 7 Chapter 3: Introduction to the Quick Reference System 11 Chapter 4: How to use the Quick Reference System Tables 17 Chapter 5: 3-cell QRS Tables: 100 Watts – 500 Watts 23 Chapter 6: 4-cell QRS Tables: 400 Watts – 650 Watts 39 Chapter 7: 5-cell QRS Tables: 600 Watts – 850 Watts 46 Chapter 8: 6-cell QRS Tables: 750 Watts – 1,000 Watts 53 Chapter 9: Motor Kv and Propeller Variations 60 Chapter 10: Sizing Considerations for Power Systems 61 Chapter 11: Purchasing Motors, Batteries and ESCs 63 Chapter 12: Installing Power System Components 66 Chapter 13: Testing & Setting Up Power Systems 73 Chapter 14: Propeller Power Absorption Tables 75 Index 77



Chapter 1 Introduction to Power System Solutions

Welcome to the third part of the Electric Power Systems series. The heart of this guide is an easy to use system of tables allowing the modeler to simply and quickly select an effective power system for a wide range of model aircraft. A great deal of work went into creating this simple but useful system to provide an easy way to choose a power system. The Quick Reference System (QRS) of tables presented here is the result. Andrew Gibbs Southampton, UK, Mar 2013

The Quick Reference System allows you to easily and quickly determine a suitable power system for a wide range of sport and scale models. This superb Hawker Fury was designed and built by the prolific electric modeller Chris Golds. The 72 inch (1,828 mm) span model uses a 6-cell 4,200 mAh LiPo battery supplying an AXI 4130/16 brushless motor. This motor has a Kv of 485, and turns a 16 x 10 APC E prop providing the model with a delightfully aerobatic and powerful performance. Chris, a retired RAF squadron leader and talented aviation artist says the model is capable of all aerobatics including erect and inverted spins.

Acknowledgements Grateful thanks are extended to all those who helped me to bring this guide to completion. In particular I would like to acknowledge the valuable contributions made by Toni Reynaud for comments and sage advice. Grateful thanks are also due to Toni, Chris Golds and my father John Gibbs, who each kindly reviewed the rough drafts and offered their suggestions.



Chapter 2 Comparing I.C. and Electric Power Systems

Many modellers new to electric flight already have considerable experience working with glow or diesel internal combustion (i.c., or nitro) engines. Such experience leads of course to a feel for what size of engine would be most likely to suit a given model. Consequently, it’s natural to try to use this same mindset to think in terms of which electric motor would be equivalent to a particular size of i.c. engine. Electric motor manufacturers recognise that there is a demand for this type of equivalence, and some label their motors accordingly, E-Flite’s successful Power series of motors is one example. However, there are so many variables in electric power systems that in practice, direct comparisons of i.c engines and electric motors are impossible - one obvious reason why is that the power output of an electric motor can vary widely since it depends on both the battery voltage and the propeller selected. For this reason, instead of thinking in terms of i.c. engine equivalents, I encourage the modeller new to electric flight to think about electric power systems in terms of system power, which is measured in Watts. Of course, i.c. engines and electric power systems are fundamentally different – i.c. engines burn liquid fuel to drive a piston up and down, while electric systems use the principles of electricity and magnetism to create rotary movement. Another difference is that i.c engines have to be designed to turn relatively quickly, whereas electric motors can be designed to operate at a very wide range of speeds. About the only thing the two systems have in common is that they are both used to turn a propeller!

This beautiful Spitfire is the work of long time electric aeromodeller George Worley. The model uses a large, relatively efficient propeller and flies very well. Power of an i.c. engine A typical manufacturer’s claimed power output for a modern 0.40 two stroke (two cycle) sport engine is 1.2 hp. One horsepower is 746 Watts, so 1.2 hp is equivalent to 895 Watts (1.2 x 746 = 895). At first sight, it may appear that we will need the same 895



Watts of electrical power to get the same model performance as a 0.40 engine can provide. In fact, an electric model can achieve the same performance with less power than this, for several reasons: Actual power output of an i.c. engine To produce its full 1.2 hp, the i.c. engine needs to be able to rev quickly, perhaps to about 15,000 - 16,000 rpm. This engine speed can only be achieved by using a very small propeller such as a 9 x 6. However it is found in practice that the best flight performance of sports models is gained by using a considerably bigger prop such as a 11 x 6, even though the achieved engine speed is now lower, perhaps 11,000 rpm. At this lower engine speed, the engine might well only develop somewhere around 0.8 - 0.9 hp (600 - 670 Watts).

Claimed i.c. engine power output may only be achieved using what is for most sport models, an impractically small propeller. This vintage Webra 40 and OS35 accompany (centre) a Speed 600 & gearbox, Jeti 45/3 with 2:1 ratio gearbox and a Mega 16/15/3. Propeller efficiency The reason that the i.c engine gives a model a better performance using a larger prop at lower revs is because the efficiency of the larger propeller is higher than the smaller alternative. This increased efficiency more than makes up for the engine operating at a lower speed where it makes less power. A typical electric system using an outrunner motor will turn at a lower rpm than a typical i.c. engine operates at. This allows the electric motor to use a larger and more efficient propeller. Also, the propellers designed for electric use are typically more efficient than those intended for i.c. models, since they do not have to be designed to withstand the starting and vibratory loads involved with i.c. engines. This allows the designer of the electric propeller to use a thinner, lighter and more efficient airfoil.



Left: the drag of an exposed cylinder head and muffler is significant, and can be much more than may be assumed. In contrast, electric motor installations are almost always aerodynamically far cleaner. Right: Electric motors generally fit into the nose of a model, meaning that the installation is very clean, aerodynamically speaking. This Plettenberg electric motor and RCV 52 engine have similar power outputs. Aerodynamic Drag Most i.c engine installations involve the engine’s cylinder head and muffler (silencer) protruding from the natural lines of the model. As well as being ugly, these projections create a very significant amount of drag, sapping model performance. The amount of drag is considerable, and is usually a great deal more than it might appear. In contrast, the motor of an electric power system is invariably enclosed within the cowl. For this reason, electric motor installations are almost always aerodynamically very clean, leading to better performance. The following table summarises these differences between i.c. and electric power:

Some differences between i.c. and electric power i.c. Electric Propeller size Relatively small Larger and thus more efficient Operating rpm Small useful maximum

rpm range Wide range of motor speeds available to suit application

Aerodynamic factors

Significant drag from cylinder head and exhaust muffler

Significantly cleaner, lower drag installation

Rpm range of electric systems For modellers only used to i.c. engines, the range of speeds that electric motors operate at can be surprising. For example, the motor for a large model might operate at only 5 - 6,000 rpm, and the motor for an average club sports model at perhaps 8,000 rpm. These speeds are a little slower than for comparably sized i.c. powered models. In contrast, small high performance electric models employ motors turning at 20,000 rpm or more, and many small electric ducted fan (EDF) systems turn considerably faster than this. For



this type of model, small, high pitch, high rpm props (or fans) are the only way to provide the required performance, even though the propeller efficiency is relatively low.

Electric motors are available in a wide variety of designs, allowing propeller rpm from just a few thousand per minute up to more than 30,000 rpm. This charming Blackburn Monoplane uses an outrunner motor turning at a fairly low speed. i.c. and electric differences – propeller load and engine/motor performance It is worth remembering that a fundamental difference between i.c. and electric power systems is the way the engine or motor responds to different propeller loads. An i.c engine’s power output will fall if the propeller diameter and/or pitch is increased, while in contrast, the power output of an electric motor will increase in response to a larger load. The tables below summarise these different characteristics:

i.c. engine Small prop Large prop Load Low High Rpm High Low Power output High Low

Electric motor

Small prop Large prop Load Low High Rpm High Lower Power consumption Low High

An approximate comparison An approximate i.c. to electric comparison is that for each cubic centimetre of displacement, a bit less than 100 Watts of electrical power is required. Alternatively, we can say a little less than 150 Watts per 0.1 cu in. For example, a 0.40 cu in size model (6.5 cc) will need up to about 600 Watts.



Chapter 3 Introduction to the Quick Reference System

The Quick Reference System (QRS) offered here gives a convenient and very quick method of choosing a power system for a wide variety of sport or scale models. The heart of the system is a series of simple, carefully designed tables. These detail suitable combinations of brushless motor, LiPo battery, brushless ESC and propeller. Altogether, the tables detail almost 400 different power systems to suit a wide variety of models. The system ensures that the suggested components are well matched to each other and to the requirements of the model. The system can help to save you money by avoiding poor or inappropriate choices of equipment. Also, it allows you to see if an existing motor could be used for a project.

Which motor, battery, propeller and ESC combination will work for a model? The vast array of motors, propellers, batteries and ESCs offer a huge choice to modellers, but this means it can be confusing to try and make a good choice. The Quick Reference System will help to make sense of it all.

Limitations of the Quick Reference System The suggested power system will usually draw a power close to that indicated. However there are a number of variables which cannot be predicted in power systems which will affect the current drawn. Such variables include any difference between a manufacturer’s stated motor Kv and the actual motor Kv, any difference between stated and actual propeller pitch, the propeller blade profile and prop blade shape, the characteristics of the battery and its state of charge. For these reasons, please understand and appreciate that the power systems suggested by the QRS can only be relied on to produce a power system which will draw approximately the predicted power. Unless you are exactly copying an existing power system for installation in an extremely similar model, some experimentation with propellers may well be necessary to get your system to draw the required power and to become optimally matched to your model.



The Quick Reference System allows you to quickly and painlessly determine a suitable power system for a wide range of sport and scale models. This small ‘Elfi’ owned by Belgian electric enthusiast Ronald Steenacker is powered by a 3S 1,500 mAh LiPo and a small outrunner. The propeller is an 8 x 6.

About the tables The tables are arranged in order of increasing power starting at 100 Watts, and going up to 1,000 watts. For each power level there are three tables, each one covering a different propeller diameter. For example, there are three tables for 100 Watt systems; one for smaller props, one for medium props and one for larger propellers. Power level increments From 100 to 300 Watts the tables go up in increments of 25 Watts. After 300 Watts, the tables go up in increments of 50 Watts all the way up to 1,000 Watts. Each of the tables contains within it four different power systems. In total therefore, for each power level there are 12 suggested power systems to choose from. Cell count increments The power systems suggested in the tables use cell counts between 3S and 6S LiPo batteries. The table below details the power level ranges covered by each cell count.

Cell count increments & power ranges 3S LiPo systems 100 – 500 Watts 4S LiPo systems 400 – 650 Watts 5S LiPo systems 600 – 850 Watts 6S LiPo systems 750 – 1,000 Watts

In order to provide plenty of choice and flexibility, there is some overlap between the cell count categories. For example, there are both 3S and 4S systems suggested for a power system of 400, 450 and 500 Watts.



Examining the tables Each of the tables actually details four complete power systems. An example of one of the tables is shown below:

Model speed category SLOW LOW MED HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 926 926 926 926On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 9.3 9.3 9.3 9.3System power in Watts (approx.) 100 100 100 100Motor kv 1,350 1,250 1,200 1,500Preferred continuous motor current capability 12 12 12 12Propeller rpm (approx.) 12,101 11,205 10,757 13,446Propeller diameter in inches 7 7 7 6Propeller pitch in inches 4 5 6 5.5Propeller pitch/diameter ratio 0.57 0.71 0.86 0.92Propeller pitch speed in mph (approx.) 46 53 61 70Preferred ESC current rating 14 14 14 14Estimated current consumption check in Watts 103 102 108 104

100 Watt system (Smaller props) 3S LiPo

Figures emphasized in bold relate to battery, motor, prop and ESC details and provide a simple reference for equipment specifications when choosing a system. The remaining data allows you to see the context of these suggestions: Heading Banner For easy reference, each table is headed by an orange coloured banner announcing the power level and the propeller size category. In the example above, the heading banner shows the table is for 100 Watt power systems, and covers the smallest of the three propeller size options. The propeller terms ‘small’ ‘medium’ and ‘large’ are of course relative to the power level; what is a large propeller for a 100 Watt system will be a small propeller for a 250 Watt system. Model speed The tables provide for models in four speed categories, labelled slow, low medium, high medium and fast. An example of a model in the ‘slow’ category is a particularly slow or ‘draggy’ model such as a vintage style model or a SE5a. A model in the ‘low medium’ speed category would be a Piper Cub. A model in the high medium category would be a moderately powered WW2 fighter, while one in the ‘fast’ category would be a more highly powered WW2 fighter or sport model. Number of LiPo cells The tables detail the number of LiPo cells for a given system. These vary from 3S systems (3 x LiPo cells) up to 6S systems. Suggested battery capacity in mAh The battery capacity suggested will give a discharge rate of approximately 10C at full throttle. This should provide approximately 5 minutes at full power for 80 % of the battery



capacity. In practice, much longer flights can be had by throttling back. The actual battery capacity installed can of course be different in capacity to suit your preferences. A smaller battery will cost less and will reduce the model’s weight a little at the expense of flight time, while a larger battery will cost and weigh more, but will give longer flights. Battery voltage The battery voltage is assumed to be 3.6 Volts per cell. This is a reasonable on-load figure for a LiPo cell. The actual voltage will vary depending on a number of factors including the battery’s C rating, its state of charge and the current demanded. System current in Amps The system current is the current which will give the power level shown with the chosen battery voltage. In practice, the actual current may be higher or lower depending on the many variables involved. This is another reason to allow a little headroom for ESC and motor current ratings. System power in Watts The power of the system is repeated as a check on the combination of cells and current. It should always be the same as the power level given in the heading. Motor Kv The suggested motor Kv is chosen to give a power level close to that indicated with the suggested propeller. Minimum continuous motor current capability The tables recommend a minimum continuous motor current capability. The current capability suggested is 1/3 (33 %) higher than the current predicted to be drawn by the power system. There are three reasons why a more powerful motor is suggested than may seem necessary:

(a) This excess capacity ensures that there is some ‘headroom’ for increasing the power if this proves necessary.

(b) A larger motor operated well within its limits will be more efficient than a small motor pushed hard. A 33 % margin means that the motor will be operating at around 75 % of maximum current at full throttle, yielding a useful efficiency gain.

(c) The actual power which a system draws can only be found by testing. Since the exact power consumption cannot be precisely predicted, it is worthwhile to build in a margin of safety in terms of motor power.

Whether an electric model is an advanced type or a vintage design such as this Junior 60, choosing a motor with some excess current capability has a number of advantages.



There is nothing magic about the figure of 33 %. If you prefer, you can of course use a motor with a smaller margin of capability in terms of current, bearing in mind the above points. However, I would definitely not recommend dropping below a margin of 25 %. When choosing a motor, make sure you choose it based on its continuous current capability, and not its burst rating. Propeller rpm The propeller rpm figure given in the tables is derived from the Kv of the motor multiplied by the battery voltage. A suitable allowance has been made for the difference between the motor Kv and the reduced speed of the motor when turning against the load of a propeller. The table assumes an on-load voltage of 3.6 Volts per cell (Vpc) and that the motor will turn at 83 % of the motor’s Kv figure with a typical prop attached. For example, a motor with a Kv of 1,000 is assumed to rotate at 8,300 rpm on 10 Volts. The actual rpm figure will vary somewhat depending on factors such as the design of the motor, its actual Kv, the make and size of the propeller and the characteristics of the battery. Smaller props which present a lower load to the motor will tend to rotate a little faster than indicated, while larger props will tend to rotate a little more slowly.

The system current detailed in the tables is the approximate current which will equate to the power level shown. Because there are many variables involved in power systems, it may be necessary to change the propeller to achieve the indicated system current. This superb model is a 50 inch (1,270 mm) span Curtis Shrike, designed and built by Chris Golds. The model is powered by a Mega 16/25/6 and has been flown with both 10 x 7 and 11 x 5 props. It consumes approximately 16 Amps / 180 Watts from its 3S LiPo. Propeller diameter and pitch in inches All of the suggested propeller sizes are to be found in the APC electric propellers range, and the power absorption figures tables are most likely to be close to the indicated figures if these propellers, or those with a similar load factor are used. The propeller diameter and pitch are shown which should result in approximately the indicated current being drawn. Remember you have three options for propeller size for



each power level. The table for medium size props makes a good starting point. If you are limited on prop diameter then the small prop option may offer a solution. Propeller pitch/diameter ratio The propeller suggested in the tables is chosen with an appropriate pitch to diameter ratio in mind for the type of model. For example, a suitable prop for a slow model will have a p/d ratio at or close to 0.5, while faster models need a higher ratio. Depending on the available propellers, some flexibility in this regard may be necessary. The pitch/diameter (p/d) ratio is discussed in detail elsewhere in this series. Propeller pitch speed, in mph This line shows the pitch speed of the system. Comparing this speed with the expected flying speed of the model provides a useful guide as to the suitability of the system for a particular airframe. The ideal pitch speed of the chosen propeller should be approximately 25 % higher than the flying speed of the model in level flight. For example, if the model is expected to fly at 30 mph, the ideal pitch speed would be around 36 mph. In practice it is not always possible to arrange for an ideal pitch speed and a faster pitch speed may be necessary. A greater pitch speed margin may be beneficial for models for which high speed diving performance is required. Suggested ESC minimum current rating A suggestion is made for the minimum continuous ESC current capability. This is 50% higher than the current predicted to be drawn by the motor. For example, for a system drawing 20 Amps, a 30 Amp ESC is suggested. The excess current capability ensures that there is some headroom for increasing the power if this proves necessary. Also, a larger ESC operated well within its limits will probably be more efficient than a smaller unit worked harder. Except for the small cost and weight penalties, there is no harm in using an even higher ESC rating if you prefer. If desired, you can of course use a lower rated ESC, but in any event make sure that there is at least some excess capacity. The voltage rating of the chosen ESC must of course always be suitable for the number of cells used. Attempting to use an inadequately rated ESC is dangerous, and will probably result in destruction of the ESC and may result in the loss of the model. Estimated current consumption check in Watts The estimated current consumption check is an estimated figure, calculated using reasonable values for all of the important parameters. It is included as a check that the suggested propeller dimensions are broadly correct. However, it is important to appreciate that the actual current consumption of a new power system cannot be predicted with precision, so it will be necessary to test any new system to determine the actual current and power. It may be that the current and power are close to the expected values, and if so then flight testing can commence with the existing propeller. If the current is lower than expected, a larger propeller may be tried, and vice versa. In any event, the QRS tables suggest components which are capable of providing the required power, so there should be no problem in obtaining the expected performance, subject only perhaps to a little propeller swapping.



Chapter 4 How to use the Quick Reference System Tables

To use the Quick Reference System Tables, a very easy 3-stage process is all that is required:

1. Estimate the weight of your model 2. Decide on a suitable power loading using the supplied table. 3. Refer to the appropriate quick reference system table.

These stages are now discussed in some detail: 1 – Estimate the weight of the model. For this first stage, we need to make an estimate of the total weight of the finished model in a ready to fly condition. You should be able to find a figure for the likely flying weight somewhere in the documentation for an ARF model, or on the plan for plan-built models. If converting an i.c. model to electric power, we can expect the electric version to weigh very close to the same as the i.c. version using a brushless/LiPo set up. If you can find no weight information at all, and the model is an ARF or ready-built type, simply weigh all the components and then add 40% to allow for the weight of the power system and RC system components. For example, if the weight of all the airframe components is 1,000 g, a reasonable estimate for the weight of the model in flying condition would be around 1,400 g (1.4 times the bare airframe) If the model is to be built from plans, then you may have to find a similar model for which the weight is known in order to make an estimate for your own model.

When selecting a power system, the first stage involves estimating the weight of the finished model. It is better to over-estimate the model’s likely finished weight, as this will help to avoid a model ending up under powered. This 68 inch (1,727 mm) span Seagull Decathlon was built by Andrew Weight and weighs a little over 7 lb (3,200 g)



2 – Decide on the power loading. The power loading of a model (i.e. its power to weight ratio) is the power drawn by the motor divided by the weight of the model. For example, a model weighing 5 lbs which draws 500 Watts would have a power loading of 100 Watts per pound (100 W/ Lb). An appropriate power loading will depend on the type of aircraft and the sort of performance required from it. Full size aircraft illustrate the diversity of power loadings very well. For example, a Piper Cub flies on 40 W / Lb, while a Spitfire has a power loading more than three times greater. Some examples of the power loadings of various full size single engine aircraft are compared in the table below:

Aircraft Engine Max weight Power loading Fournier RF4 motor glider 80 hp = 60 Kw 1,653 lbs 36 Watts / Lb Piper Cub 65 hp = 48 Kw 1,220 lbs 40 Watts / Lb Cessna 172 R 160 hp = 119 Kw 2,450 lbs 49 Watts / Lb Piper Super Cub 150 hp = 112 Kw 1,750 lbs 64 Watts / Lb Sopwith Camel 130 hp = 97 Kw 1,482 lbs 65 Watts / Lb F6F Hellcat 2,000 hp = 1,492 Kw 15,415 lbs 97 Watts / Lb Hurricane 1,185 hp = 884 Kw 8,710 lbs 101 Watts / Lb P51 D Mustang 1,720 hp = 1,111 Kw 12,100 lbs 106 Watts / Lb FW 190 A8 1,675 hp = 1,250 Kw 10,800 lbs 115 Watts / Lb Pitts Special 260 hp = 194 Kw 1,625 lbs 119 Watts / Lb

Models will generally be capable of flying with an approximately scale level of performance using a similar power loading to full size machines. However, there are several reasons why it is a good idea to use a significantly higher power loading for all types of RC models:

1. The availability of high power brushless motors and high capacity LiPo batteries has led to substantially increased performance expectations compared to some years ago. 2. Scale power loadings may not provide sufficient performance entertainment. For example, a Piper Cub (or a similar sport model) with a scale level of performance will be a extremely sedate machine. This may suit the scale purist, but most pilots will enjoy having more power. Since the purpose of our fine hobby is entertainment, why not? In contrast, a scale level of performance will more often be considered acceptable for a model such as a WW2 fighter. 3. In some cases, a higher than scale power loading is necessary to emulate convincing scale flight. For example, a WW2 warbird such as a P51 Mustang, F6F Hellcat or FW 190 may not be able to execute a really large, scale-like loop with a scale power loading. 4. In some respects low powered models are harder to fly than those with more power. For example, a modestly powered Piper Cub can only climb at a shallow gradient. An inexperienced pilot may not appreciate this, and may attempt to get the model to climb too steeply, resulting in a stall.



For the above reasons, a higher power loading is almost always chosen for models. The suggested power loading for various model types is shown in the table below.

Power loadings for various types of single engine model in W / lb Performance level Model type Reasonable Good Very good Powered glider, flying wings e.g. Fournier RF4, Sophisticated Lady

40 - 50 50 – 60 60 +

Vintage cabin model e.g. Junior 60, Black Magic

40 - 55 55 – 70 70 +

Civil light aircraft e.g. Cessna 172, J3 Cub

60 - 70 70 – 80 80 +

Trainer e.g. Standard trainer

60 - 70 70 – 80 80 +

WW1 fighter e.g. Sopwith Camel, Fokker DVII

70 - 85 85 – 100 100 +

WW2 fighter e.g. P51 Mustang, Spitfire etc

80 - 100 100 – 125 125 +

Sport aerobatic model e.g. Four Star 40, Wot 4, Acromaster

80 - 100 100 – 125 125 +

High performance biplane e.g. Pitts Special, Hawker Fury

80 - 100 100 – 125 125 +

Seaplane/floatplane e.g. North Star, Cub on floats, Icon A5

n/a n/a 125 +

Performance Level Categories Wing loading considerations and performance expectations both mean that a 3 lb Piper Cub will require more power than, say, a Spitfire of the same weight. It will be helpful to discuss what is meant by each of the performance category descriptions.

Reasonable The performance of a model with a ‘reasonable’ power loading approximately corresponds to what was considered satisfactory some years ago. Take off from grass should be possible provided wheels are not too small and the grass is not too long. There should be sufficient power for a perfectly flyable model, however, by modern standards, a power loading this low may well now be considered unsatisfactory. Good A ‘Good’ power loading should provide a model with more rapid take offs, a better rate of climb and a higher top speed than a lower powered equivalent. By modern standards, this level of performance might still be considered a little sedate for some model types. Very good A ‘Very good’ power loading corresponds to modern performance expectations. This level of power should provide a model with brisk take-off performance from either grass of a hard surface. The model should also have a strong rate of climb and the ability to fly relatively fast in level flight.



It is only fair to point out that expert opinions vary as to an appropriate power loading for various model types. My personal opinion is that a power loading in the ‘Good’ category is ample for most models. However, when discussing this with experienced modelling friends, some agreed with me, some felt it was too much power and others too little! Models with insufficient power can be disappointing, so I suggest that you opt for a power loading in the ‘Good’ or preferably ‘Very good’ category unless you particularly want a sedate, relatively low performance model. It is not expensive to install a bit more power, and if you find you have too much, you can always throttle back and/or change the propeller so that less power is available. Seaplanes, floatplanes and flying boats Seaplanes are a special case since, depending on the quality of the design of the floats or hull they may require a surprisingly high amount of power to unstick from water. For this reason, for reliable water take-off performance, I suggest at a power loading of at least 125 W / lb for seaplanes, although depending on the model, they may be able to rise off water (ROW) with less power than this.

Models that are required to operate from water such as this Multiplex MiniMag need more power than if they were configured as land planes. Aim for least 125 W / lb. 3 – Consult the QRS (Quick Reference System) tables for a suitable system At this stage, you will have decided how much power you require. It is then a simple matter of finding the corresponding power table, and using it as the basis for choosing a power system. Note that it is not necessary to select components that exactly match the suggested specification. A deviation of up to about 10 % or so from the suggested Kv figure should not make too significant a difference to the power system. For example, if a motor Kv figure of say, 1,200 is given, it is reasonable to consider motors with a Kv figures between approximately 1,080 and 1,320. Increasing the motor Kv means that it becomes more likely that you will need to use a smaller prop than suggested, and vice versa. In any case, always remember that some experimentation with propellers will usually be required to find the best match for the model and to tweak the power level to exactly that required.



Worked example: Piper Cub As an example, let us suppose we have a 1/5 scale 82 inch ARF Piper Super Cub, with an estimated flying weight of 8.5 lbs. For this model, the relevant row is clearly Civil Light Aircraft. For our model, let’s say we decide on a power loading of 90 W / lb. The model weighs 8.5 lbs, so the amount of power required = 8.5 x 90 W / lb = 765 Watts. The closest available table for 765 Watts is 800 Watts. The tables offer a choice of 5S or 6S systems. We already have a pair of 3,300 mAh 3S batteries, so in this case, we decide to investigate using a 6S system, as a 6S battery can be made by joining the pair of 3S batteries in series. The full size machine has a prop diameter of 74 inches, equivalent to 14.8 inches for this 1/5 model, so the maximum practical propeller diameter is 14 or 15 inches. Checking the model’s propeller clearance confirms this. Looking at the 6S tables for larger props, the second column, for medium speed models appears to fit this subject best:

The estimated system current is 34.7 Amps, and of course this current will flow through all parts of the power system including the battery, ESC and motor. Note that the figures given for items such as battery capacity (3,472 mAh) and ESC rating (52 A) are approximate figures and were derived by calculation. These figures should be used for guidance only, and are targets rather than a figure which must be strictly adhered to. Let’s look at each of the suggested components in turn. Battery The suggested battery is a 6-cell LiPo of around 3,472 mAh. We know that the capacity suggested in the tables is relatively generous, so there is no problem in using smaller batteries. We can use two 3S, 3,300 mAh in series to make a 6S 3,300 mAh battery, and the slight reduction in capacity will be insignificant. Motor Kv and current capability The suggested motor has a Kv of approximately 405 and is able to operate at a current of 46 Amps or more continuously. In this case, the chosen motor should not have a Kv of



less than 405 - there is limited propeller clearance, and the lower the Kv, the larger the prop must be for a given current. Propeller The table suggests a 14 x 8.5 prop. If we find that the motor does not absorb enough power turning this prop, we still have the option to increase the diameter to 15 inches. Note that it is always worth obtaining some alternative propellers for experimentation. ESC The table suggests an ESC rating of 52 Amps. Manufacturers generally rate their ESCs with ‘round’ numbers such as 50 or 50 Amps. In this case, an ESC rating of 50 Amps would be fine, since this still provides a healthy margin between the ESC’s capability and the actual current flowing.

Choosing a motor with some spare current capability is always a good idea if motor efficiency is important. This Purple Power motor is installed using a rack mount.



Chapter 5 Quick Reference System Tables

For 100 to 500 Watt systems & 3-cell LiPo batteries

This chapter contains the Quick Reference System tables for 3-cell LiPo power systems. Note that these 3-cell (3S) tables are for power systems between 100 and 500 Watts. The tables for 4S systems start at 450 Watts, so if you need a 450 or 500 Watt system, you will need to decide first as to which of the two Voltage options best suits your requirements. To use the tables, simply follow this process:

1. Decide on the power required. 2. Find the page with the three tables for your chosen power level. 3. Decide on your preferred propeller size – this will lead you to the correct table for your needs. 4. Now locate the column appropriate to the flying speed of your model. The associated propeller pitch speeds shown may help you to choose this. 5. The components of the suggested power system are shown highlighted in bold.

Important reminder! The power system specifications detailed in the tables are offered as a starting point. It is not guaranteed that the systems will consume exactly the target current with the suggested prop. To achieve this, some experimentation with different propellers may well be necessary.

For any new power system, some experimentation with different propellers will very likely be necessary and beneficial. Trying different propellers will allow you to find the propeller which best suits your particular airframe and flying style. Trying alternative props may also be necessary to ensure the actual current consumption of your power system matches the desired current consumption. This 45 inch (1,143 mm) span Edge 540 uses a 900 Kv motor turning a 12 x 6 propeller at 7,500 rpm. The system consumes 31 Amps from its 3S 2,500 mAh LiPo, and draws 315 Watts at full throttle. The specification and performance of this particular system is closely comparable to the one seen in the 325 Watt tables.



Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 926 926 926 926On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 9.3 9.3 9.3 9.3System power in Watts (approx.) 100 100 100 100Motor kv 1,350 1,250 1,200 1,500Preferred continuous motor current capability 12 12 12 12Propeller rpm (approx.) 12,101 11,205 10,757 13,446Propeller diameter in inches 7 7 7 6Propeller pitch in inches 4 5 6 5.5Propeller pitch/diameter ratio 0.57 0.71 0.86 0.92Propeller pitch speed in mph (approx.) 46 53 61 70Preferred ESC current rating 14 14 14 14Estimated current consumption check in Watts 103 102 108 104


Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 926 926 926 926On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 9.3 9.3 9.3 9.3System power in Watts (approx.) 100 100 100 100Motor kv 1,125 980 1,250 1,170Preferred continuous motor current capability 12 12 12 12Propeller rpm (approx.) 10,085 8,785 11,205 10,488Propeller diameter in inches 8 8 7 7Propeller pitch in inches 4 6 5 6Propeller pitch/diameter ratio 0.50 0.75 0.71 0.86Propeller pitch speed in mph (approx.) 38 50 53 60Preferred ESC current rating 14 14 14 14Estimated current consumption check in Watts 101 100 102 100

100 Watt system (Medium props) 3S LiPo

Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 926 926 926 926On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 9.3 9.3 9.3 9.3System power in Watts (approx.) 100 100 100 100Motor kv 930 840 980 900Preferred continuous motor current capability 12 12 12 12Propeller rpm (approx.) 8,337 7,530 8,785 8,068Propeller diameter in inches 9 9 8 8Propeller pitch in inches 4.5 6 6 8Propeller pitch/diameter ratio 0.50 0.67 0.75 1.00Propeller pitch speed in mph (approx.) 36 43 50 61Preferred ESC current rating 14 14 14 14Estimated current consumption check in Watts 103 101 100 104

100 Watt system (Larger props) 3S LiPo





Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 1157 1157 1157 1157On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 11.6 11.6 11.6 11.6System power in Watts (approx.) 125 125 125 125Motor kv 1,200 1,050 1,340 1,260Preferred continuous motor current capability 15 15 15 15Propeller rpm (approx.) 10,757 9,412 12,012 11,295Propeller diameter in inches 8 8 7 7Propeller pitch in inches 4 6 5 6Propeller pitch/diameter ratio 0.50 0.75 0.71 0.86Propeller pitch speed in mph (approx.) 41 53 57 64Preferred ESC current rating 17 17 17 17Estimated current consumption check in Watts 123 124 125 125


Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 1157 1157 1157 1157On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 11.6 11.6 11.6 11.6System power in Watts (approx.) 125 125 125 125Motor kv 1,000 900 1,060 960Preferred continuous motor current capability 15 15 15 15Propeller rpm (approx.) 8,964 8,068 9,502 8,605Propeller diameter in inches 9 9 8 8Propeller pitch in inches 4.5 6 6 8Propeller pitch/diameter ratio 0.50 0.67 0.75 1.00Propeller pitch speed in mph (approx.) 38 46 54 65Preferred ESC current rating 17 17 17 17Estimated current consumption check in Watts 128 125 127 126








Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 1389 1389 1389 1389On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 13.9 13.9 13.9 13.9System power in Watts (approx.) 150 150 150 150Motor kv 1,060 960 1,125 1,020Preferred continuous motor current capability 18 18 18 18Propeller rpm (approx.) 9,502 8,605 10,085 9,143Propeller diameter in inches 9 9 8 8Propeller pitch in inches 4.5 6 6 8Propeller pitch/diameter ratio 0.50 0.67 0.75 1.00Propeller pitch speed in mph (approx.) 40 49 57 69Preferred ESC current rating 21 21 21 21Estimated current consumption check in Watts 153 151 152 151






Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 1620 1620 1620 1620On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 16.2 16.2 16.2 16.2System power in Watts (approx.) 175 175 175 175Motor kv 1,110 1,010 1,180 1,080Preferred continuous motor current capability 22 22 22 22Propeller rpm (approx.) 9,950 9,054 10,578 9,681Propeller diameter in inches 9 9 8 8Propeller pitch in inches 4.5 6 6 8Propeller pitch/diameter ratio 0.50 0.67 0.75 1.00Propeller pitch speed in mph (approx.) 42 51 60 73Preferred ESC current rating 24 24 24 24Estimated current consumption check in Watts 175 176 175 179


Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 1620 1620 1620 1620On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 16.2 16.2 16.2 16.2System power in Watts (approx.) 175 175 175 175Motor kv 930 880 940 880Preferred continuous motor current capability 22 22 22 22Propeller rpm (approx.) 8,337 7,888 8,426 7,888Propeller diameter in inches 10 10 9 9Propeller pitch in inches 5 6 7.5 9Propeller pitch/diameter ratio 0.50 0.60 0.83 1.00Propeller pitch speed in mph (approx.) 39 45 60 67Preferred ESC current rating 24 24 24 24Estimated current consumption check in Watts 175 178 177 175






Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 2030 2030 2030 2030On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 20.3 20.3 20.3 20.3System power in Watts (approx.) 225 225 225 225Motor kv 1,020 950 1,025 960Preferred continuous motor current capability 27 27 27 27Propeller rpm (approx.) 9,143 8,516 9,188 8,605Propeller diameter in inches 10 10 9 9Propeller pitch in inches 5 6 7.5 9Propeller pitch/diameter ratio 0.50 0.60 0.83 1.00Propeller pitch speed in mph (approx.) 43 48 65 73Preferred ESC current rating 30 30 30 30Estimated current consumption check in Watts 230 223 230 227








Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 2315 2315 2315 2315On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 23.1 23.1 23.1 23.1System power in Watts (approx.) 250 250 250 250Motor kv 1,050 990 1,060 1,000Preferred continuous motor current capability 31 31 31 31Propeller rpm (approx.) 9,412 8,874 9,502 8,964Propeller diameter in inches 10 10 9 9Propeller pitch in inches 5 6 7.5 9Propeller pitch/diameter ratio 0.50 0.60 0.83 1.00Propeller pitch speed in mph (approx.) 45 50 67 76Preferred ESC current rating 35 35 35 35Estimated current consumption check in Watts 251 253 254 256








Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 2546 2546 2546 2546On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 25.5 25.5 25.5 25.5System power in Watts (approx.) 275 275 275 275Motor kv 1,090 1,020 1,090 1,030Preferred continuous motor current capability 34 34 34 34Propeller rpm (approx.) 9,771 9,143 9,771 9,233Propeller diameter in inches 10 10 9 9Propeller pitch in inches 5 6 7.5 9Propeller pitch/diameter ratio 0.50 0.60 0.83 1.00Propeller pitch speed in mph (approx.) 46 52 69 79Preferred ESC current rating 38 38 38 38Estimated current consumption check in Watts 281 276 277 280








Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 2778 2778 2778 2778On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 27.8 27.8 27.8 27.8System power in Watts (approx.) 300 300 300 300Motor kv 950 880 1,000 890Preferred continuous motor current capability 37 37 37 37Propeller rpm (approx.) 8,516 7,888 8,964 7,978Propeller diameter in inches 11 11 10 10Propeller pitch in inches 5.5 7 7 10Propeller pitch/diameter ratio 0.50 0.64 0.70 1.00Propeller pitch speed in mph (approx.) 44 52 59 76Preferred ESC current rating 42 42 42 42Estimated current consumption check in Watts 300 303 304 306


Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 2778 2778 2778 2778On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 27.8 27.8 27.8 27.8System power in Watts (approx.) 300 300 300 300Motor kv 825 750 840 825Preferred continuous motor current capability 37 37 37 37Propeller rpm (approx.) 7,395 6,723 7,530 7,395Propeller diameter in inches 12 12 11 11Propeller pitch in inches 6 8 8 8.5Propeller pitch/diameter ratio 0.50 0.67 0.73 0.77Propeller pitch speed in mph (approx.) 42 51 57 60Preferred ESC current rating 42 42 42 42Estimated current consumption check in Watts 303 304 301 303




Model speed SLOW MEDIUM HI MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 3009 3009 3009 3009On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 30.1 30.1 30.1 30.1System power in Watts (approx.) 325 325 325 325Motor kv 1,150 1,075 1,150 1,090Preferred continuous motor current capability 40 40 40 40Propeller rpm (approx.) 10,309 9,636 10,309 9,771Propeller diameter in inches 10 10 9 9Propeller pitch in inches 5 6 7.5 9Propeller pitch/diameter ratio 0.50 0.60 0.83 1.00Propeller pitch speed in mph (approx.) 49 55 73 83Preferred ESC current rating 45 45 45 45Estimated current consumption check in Watts 330 324 325 332


Model speed SLOW MEDIUM HI MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 3009 3009 3009 3009On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 30.1 30.1 30.1 30.1System power in Watts (approx.) 325 325 325 325Motor kv 975 900 1,025 910Preferred continuous motor current capability 40 40 40 40Propeller rpm (approx.) 8,740 8,068 9,188 8,157Propeller diameter in inches 11 11 10 10Propeller pitch in inches 5.5 7 7 10Propeller pitch/diameter ratio 0.50 0.64 0.70 1.00Propeller pitch speed in mph (approx.) 46 53 61 77Preferred ESC current rating 45 45 45 45Estimated current consumption check in Watts 324 324 327 327


Model speed SLOW MEDIUM HI MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 3009 3009 3009 3009On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 30.1 30.1 30.1 30.1System power in Watts (approx.) 325 325 325 325Motor kv 845 770 865 845Preferred continuous motor current capability 40 40 40 40Propeller rpm (approx.) 7,575 6,902 7,754 7,575Propeller diameter in inches 12 12 11 11Propeller pitch in inches 6 8 8 8.5Propeller pitch/diameter ratio 0.50 0.67 0.73 0.77Propeller pitch speed in mph (approx.) 43 52 59 61Preferred ESC current rating 45 45 45 45Estimated current consumption check in Watts 326 329 329 326






Model speed SLOW MEDIUM HI MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 3241 3241 3241 3241On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 32.4 32.4 32.4 32.4System power in Watts (approx.) 350 350 350 350Motor kv 1,000 925 1,050 930Preferred continuous motor current capability 43 43 43 43Propeller rpm (approx.) 8,964 8,292 9,412 8,337Propeller diameter in inches 11 11 10 10Propeller pitch in inches 5.5 7 7 10Propeller pitch/diameter ratio 0.50 0.64 0.70 1.00Propeller pitch speed in mph (approx.) 47 55 62 79Preferred ESC current rating 49 49 49 49Estimated current consumption check in Watts 350 352 352 349








Model speed SLOW MEDIUM HI MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 3472 3472 3472 3472On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 34.7 34.7 34.7 34.7System power in Watts (approx.) 375 375 375 375Motor kv 1,025 950 1,075 955Preferred continuous motor current capability 46 46 46 46Propeller rpm (approx.) 9,188 8,516 9,636 8,561Propeller diameter in inches 11 11 10 10Propeller pitch in inches 5.5 7 7 10Propeller pitch/diameter ratio 0.50 0.64 0.70 1.00Propeller pitch speed in mph (approx.) 48 56 64 81Preferred ESC current rating 52 52 52 52Estimated current consumption check in Watts 377 382 378 378








Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 3704 3704 3704 3704On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 37.0 37.0 37.0 37.0System power in Watts (approx.) 400 400 400 400Motor kv 1,050 975 1,100 980Preferred continuous motor current capability 49 49 49 49Propeller rpm (approx.) 9,412 8,740 9,860 8,785Propeller diameter in inches 11 11 10 10Propeller pitch in inches 5.5 7 7 10Propeller pitch/diameter ratio 0.50 0.64 0.70 1.00Propeller pitch speed in mph (approx.) 49 58 65 83Preferred ESC current rating 56 56 56 56Estimated current consumption check in Watts 405 412 405 409










Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 3 3 3 3Suggested approx. battery capacity in mAh 4630 4630 4630 4630On load battery Voltage (approx.) 10.8 10.8 10.8 10.8System current in Amps (approx.) 46.3 46.3 46.3 46.3System power in Watts (approx.) 500 500 500 500Motor kv 980 890 1,000 980Preferred continuous motor current capability 62 62 62 62Propeller rpm (approx.) 8,785 7,978 8,964 8,785Propeller diameter in inches 12 12 11 11Propeller pitch in inches 6 8 8 8.5Propeller pitch/diameter ratio 0.50 0.67 0.73 0.77Propeller pitch speed in mph (approx.) 50 60 68 71Preferred ESC current rating 69 69 69 69Estimated current consumption check in Watts 508 508 509 509





For 400 to 650 Watt systems & 4-cell LiPo batteries

This chapter contains the Quick Reference System tables for 4-cell LiPo power systems. Note that these 4-cell (4S) tables are for power systems between 400 and 650 Watts. The tables for 5S systems start at 600 Watts, so if you need a 600 or 650 Watt system, you will need to decide first as to which of the two Voltage options best suits your requirements. To use the tables, simply follow this process:



This 54 inch (1,370 mm) span Banchee uses a 770 Kv motor turning a 15 x 8 propeller at 8,500 rpm. The system consumes 40 Amps from its 4S 4,000 mAh LiPo, and draws 450 Watts at full throttle. Comparing the specification and performance of this particular system to those seen in the 450 Watt tables, it is clear that an unusually large propeller had to be fitted to get this system to consume sufficient power. This is probably due the battery having an unusually high internal resistance, limiting current flow. It could also be that the actual motor Kv was lower than the stated Kv. This model highlights that some prop experimentation will often be required to get a power system to perform as desired.




For 600 Watt to 850 Watt systems & 5-cell LiPo batteries

This chapter contains the Quick Reference System tables for 5-cell LiPo power systems. Note that these 5-cell (5S) tables are for power systems between 600 and 850 Watts. The tables for 6S systems start at 750 Watts, so if you need a 750, 800 or 850 Watt system, you will need to decide first as to which of the two Voltage options best suits your requirements. To use the tables, simply follow this process:



For any new power system, some experimentation with different propellers will very likely be both necessary and beneficial. Trying out different propellers will allow you to find the propeller match which best suits your particular airframe and your flying style. Trying alternative props may also be necessary to ensure the actual current consumption of your power system matches the desired current consumption. This beautiful Aichi Val is the work of Chris Golds.







Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 5 5 5 5Suggested approx. battery capacity in mAh 3333 3333 3333 3333On load battery Voltage (approx.) 18.0 18.0 18.0 18.0System current in Amps (approx.) 33.3 33.3 33.3 33.3System power in Watts (approx.) 600 600 600 600Motor kv 480 450 510 475Preferred continuous motor current capability 44 44 44 44Propeller rpm (approx.) 7,171 6,723 7,619 7,097Propeller diameter in inches 14 14 13 13Propeller pitch in inches 7 8.5 8 10Propeller pitch/diameter ratio 0.50 0.61 0.62 0.77Propeller pitch speed in mph (approx.) 48 54 58 67Preferred ESC current rating 50 50 50 50Estimated current consumption check in Watts 598 598 609 615





For 750 to 1,000 Watt systems & 6-cell LiPo batteries

This chapter contains the Quick Reference System tables for 5-cell LiPo power systems. Note that these 5-cell (5S) tables are for power systems between 600 and 850 Watts. The tables for 6S systems start at 750 Watts, so if you need a 750, 800 or 850 Watt system, you will need to decide first as to which of the two Voltage options best suits your requirements. To use the tables, simply follow this process:



This colourful Hangar 9 P51 is enjoying a cloudless blue sky! A 400 Kv motor is installed which turns a 15 x 8 propeller at 7,500 rpm. The system consumes a current of 48 Amps from its 6S 5,000 mAh LiPo, and draws 1,050 Watts at full throttle. The specification and performance of this system is comparable to one seen in the 1,000 Watt tables. However, it is not necessarily the case that this will occur. For any new power system, it is important to test the power system before flying the model, and establish the current and power being consumed. It is nearly inevitable that a model can be improved by trying out different propellers. This allows you to find the prop which best suits your particular airframe and your preferred flying style.



Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 6 6 6 6Suggested approx. battery capacity in mAh 4630 4630 4630 4630On load battery Voltage (approx.) 21.6 21.6 21.6 21.6System current in Amps (approx.) 46.3 46.3 46.3 46.3System power in Watts (approx.) 1000 1000 1000 1000Motor kv 475 445 500 465Preferred continuous motor current capability 62 62 62 62Propeller rpm (approx.) 8,516 7,978 8,964 8,337Propeller diameter in inches 14 14 13 13Propeller pitch in inches 7 8.5 8 10Propeller pitch/diameter ratio 0.50 0.61 0.62 0.77Propeller pitch speed in mph (approx.) 56 64 68 79Preferred ESC current rating 69 69 69 69Estimated current consumption check in Watts 1,001 1,000 992 997

1,000 Watt system (Smaller props) 6S LiPo

Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 6 6 6 6Suggested approx. battery capacity in mAh 4630 4630 4630 4630On load battery Voltage (approx.) 21.6 21.6 21.6 21.6System current in Amps (approx.) 46.3 46.3 46.3 46.3System power in Watts (approx.) 1000 1000 1000 1000Motor kv 435 415 425 400Preferred continuous motor current capability 62 62 62 62Propeller rpm (approx.) 7,799 7,440 7,619 7,171Propeller diameter in inches 15 15 14 14Propeller pitch in inches 7 8 10 12Propeller pitch/diameter ratio 0.47 0.53 0.71 0.86Propeller pitch speed in mph (approx.) 52 56 72 81Preferred ESC current rating 69 69 69 69Estimated current consumption check in Watts 1,013 1,005 1,024 1,025


Model speed SLOW MEDIUM HIGH MED FASTNumber of LiPo cells 6 6 6 6Suggested approx. battery capacity in mAh 4630 4630 4630 4630On load battery Voltage (approx.) 21.6 21.6 21.6 21.6System current in Amps (approx.) 46.3 46.3 46.3 46.3System power in Watts (approx.) 1000 1000 1000 1000Motor kv 380 355 385 360Preferred continuous motor current capability 62 62 62 62Propeller rpm (approx.) 6,813 6,364 6,902 6,454Propeller diameter in inches 16 16 15 15Propeller pitch in inches 8 10 10 12Propeller pitch/diameter ratio 0.50 0.63 0.67 0.80Propeller pitch speed in mph (approx.) 52 60 65 73Preferred ESC current rating 69 69 69 69Estimated current consumption check in Watts 999 1,018 1,004 985

1,000 Watt system (Larger props) 6S LiPo



Chapter 9 Motor Kv and Propeller Variations

For each power system detailed in the tables, a particular motor Kv is indicated. The Kv figure given is purely a target, and it is perfectly possible to deviate from this figure, as long as a suitable adjustment is made to the propeller specification as well. If using a motor with a higher Kv than suggested, this will result in a higher propeller rpm, which will increase the current drawn by the motor. Similarly, if the chosen motor has a lower Kv than the table shows, the propeller rpm will be lower, and the current the motor draws will also be less. It is always necessary to check on the current consumption of any new power system, and to have some alternative propellers available in case the current consumed is too low or too high. This check is even more important if using a motor with a different Kv to that indicated in the table. For motors with a higher Kv, expect to use a smaller prop than indicated in the table, and vice versa. Changes in Motor Kv Because of the power absorption characteristics of propellers, relatively small increases in motor Kv will cause relatively large increases in the current drawn by the motor. Propeller changes When considering adjusting the prop, it is useful to have some idea of the consequences of changing from one propeller to another are. The propeller power absorption tables in a later chapter will allow you to compare the power absorption of various propellers at different speeds.

This Multiplex HiMax HC 3516-1130 outrunner has a Kv of 1,130. If this motor was used instead of one with a Kv of 1,000 for example, we would expect the current consumed to significantly increase unless a smaller than suggested propeller was used. It is worth reminding ourselves that the actual current drawn by a motor will vary according to a number of factors which are impossible to predict. Such factors include the accuracy of manufacturer’s Kv values, the state of charge and the internal resistance of the battery, the accuracy of the stated propeller pitch value, the airfoil, blade shape and area distribution of the propeller blades and many more factors besides.



Chapter 10 Sizing Considerations for Power Systems

It is important to appreciate that the motor, like all of the power system components must be considered as a part of the whole power system and not in isolation. Generally, it is far better to choose a motor with some spare capacity and run it a relatively modest power level, rather than installing a small motor which is operated on the edge of its capabilities. This will help to ensure excessive heating is not experienced, and will also result in a more efficient power system. Sizing of motors, batteries and ESCs Choosing an appropriate size of brushless motor is of importance when it comes to model performance. A balance needs to be struck between the issues of weight and electrical efficiency.

Aerobatic models like this one benefit from a reasonably light wing loading, and a relatively high power loading, i.e. a relatively high power to weight ratio. Whilst the model will probably fly quite adequately with 80 Watts per pound, much more enjoyment will be gained with 120 Watts per pound, or even higher. This type of model is not intended for 3D maneuvers such as prop hanging, so it is unnecessary to select an extremely light weight power system.

Sizing a power system for 3D flight If light weight is the most important consideration, such as may well be the case for a 3D model that is required to prop hang or climb vertically, then it makes sense to choose a relatively small, light weight motor and run it relatively hard. The same philosophy would also apply to the battery; in this case a relatively small battery would be appropriate. Short flight times would be the consequence of using a relatively small battery, but provided the battery had an appropriately high ‘C’ rating (i.e. low internal resistance), this would be quite okay. The use of smaller, lighter cells would keep the weight down, and would allow the model to be more agile in 3D flight. However it would mean there will be limited scope for increasing the power of the chosen set up if this should be required.



Sizing a power system for aerobatic sport and scale models Conversely, if factors like high efficiency and long motor and battery life are more important than ultimate performance, as is the case for the great majority of sport and scale models, then choosing a power system with some spare capacity makes a lot of sense.

The average club sport or scale model will fly very well using a motor sized to operate somewhere around 75 % of its maximum current capability. This gives scope to increase power at a later date, and also helps to ensure that the motor will be operating at a reasonably high level of efficiency. This Bristol Bulldog was built by Colin Low. Photograph by Colin Low and used with his kind permission

By choosing a larger motor, and operating it significantly below its maximum current capability, you can be sure the motor will be running reasonably nearer than otherwise to its best efficiency current. By choosing a motor than can be operated efficiently, flight times are longer, especially when combined with a generous battery capacity. This philosophy also means that the possibility exists to increase the system power at a later date, simply by increasing the size of the propeller. Of course, it is important to make sure that motor and battery are not too large, or the model weight will be unnecessarily high, and the flight performance will suffer. For average sport and scale models, it should be clear by now that if a model calls for say, 500 Watts of power, one should look for a motor rated significantly in excess of this. You should not buy a motor rated at 500 Watts, especially if this is a ‘burst’ and not a continuous rating. Instead, it is much better to buy a more highly rated motor with a suitable KV and cell count. A margin of between 33 % or more would not be not too much. For example, supposing a 4-cell (14.4 V, assuming 3.6 Vpc on-load) 500 Watt system was required for a model, the continuous current would be 34.7 Amps. In this case, I would suggest a motor capable of 46 Amps or more, 46 Amps being 1/3 higher than the target current of 34.7 A. The power system tables reflect this philosophy.



Chapter 11 Purchasing Motors, Batteries and ESCs

When making a purchasing decision, it can be difficult to obtain the necessary information to make a properly informed decision. It is relatively straightforward to choose a suitable battery or ESC, but the matter of motors is altogether different. Let’s look at each of these: Choosing a battery The three most relevant factors to consider when choosing a battery are quality, ‘C’ rating and capacity. Battery capacity is easily determined, but the quality and the actual ‘C’ rating are less easy to assess. The ‘C’ rating is the ability of the battery to deliver a sustained current without becoming damaged. The rating indicates the ability of a battery to deliver current, expressed as a multiple of its capacity. For example, a battery of 3,300 mAh capacity can theoretically deliver a current of 3,300 mA, or 3.3 Amps, for one hour before it is exhausted. So, the 10 C current for this battery would be 10 x 3.3 = 33 Amps. This is 10 times the capacity, hence the term ’10 C’. Similarly, a battery with a capacity of 2,200 mAh rated at 20 C should be able to sustain a continuous current of 44 Amps. All manufacturers wish to portray their products in the best light possible, so in some cases there may be a tendency to inflate ‘C’ rating claims. Higher quality batteries tend to have higher ‘C’ ratings, so price is usually a reasonable indicator of quality. Choosing an ESC When choosing an ESC, the most relevant factors to consider are current rating, quality, and factors such as BEC output and any other facilities which you may need. Once again, price is usually a reasonable indicator of quality.

The choice of battery, motor and ESC for a model is important. To some extent, the quality of components can be tailored to the model in question; for simple park flyers such as this one, a budget system might be appropriate, while for a more demanding application such as this high performance aerobatic model (right) like this RBC Tigercat, a higher quality system would be much more appropriate.



Choosing a motor There are a number of factors to decide on when choosing a motor. Assuming a motor is specified to do the job we need in terms of Kv, cell-count (Voltage) and current, the most relevant factors to consider are probably price, quality and efficiency. As usual, price is a good indictor of quality; higher priced motors tend to be of higher quality and tend to be more efficient than cheaper alternatives. Nevertheless, there are plenty of cheaper motors delivering good service to their budget-conscious owners. Deciphering motor names When looking for a motor, it quickly becomes apparent that manufacturers name their motors in a variety of ways. Names along the lines of MaxPower 23/45/500 are common (although I did make that particular one up), yet these mean little unless we know what the manufacturer intends by their naming system. Very often, the naming system is to do with the physical dimensions of the motor such as the diameter of the length of the rotor. This may seem relevant to the engineers who design the motors, however to the average modeler, this information is of little use – we need to know what a motor can actually do for us, and of course also what its limitations are. Even for someone who knows a bit about power systems, selecting a motor is not always straightforward since retailers do not always provide sufficient information to allow potential customers to make an informed choice. Often, some detective work is required to try and work out what a motor’s properties are. As a minimum, I like to know the following:

● What voltage range is the motor designed to operate at? ● What is the Kv of the motor? ● What is the maximum safe continuous current? ● What is the maximum efficiency current (or at least a sensible current range)? ● What are its exterior dimensions – the motor must fit in the available space. ● Examples of typical power systems using this motor.

What do all those numbers mean!? A number of manufacturers use the first two numbers to mean rotor diameter and length – these are not a lot of help when trying to find out what a motor will actually do for us! This is an MP Jet AC28/20-10



Missing information Some retailers do not supply information such as the maximum safe continuous current for a motor, or its maximum efficiency current. If information you need is not clear from a website or catalogue, don’t be afraid to ask the retailer. Continuous and ‘burst’ ratings All motors have a maximum current they can be safely operated at on a continuous basis. The maximum continuous current is only safe for the motor provided cooling arrangements are satisfactory. Some manufacturers and retailers specify a ‘burst’ current for motors, and a time period this applies for. For example, a motor might be rated at 40 Amps continuously, with burst of up to 50 Amps for 60 seconds. Unless it is important to have the lightest possible system, it is better to choose a larger motor which will not need to be operated at or close to its maximum burst current. So, for normal sport modeling use, I recommend motors are chosen only on the basis of their maximum continuous current capability, and that any burst rating is ignored. The cost versus quality decision As with any market place, the quality and price of the available electric power system components varies from lower quality, budget-level items right up to high quality, expensive components. The cost/quality decision is clearly a matter of personal choice, but my advice is to invest in reasonable or higher quality components that can be expected to give a long life. Some suppliers offer two levels of quality. Generally speaking, better quality components will run cooler and be more reliable than cheaper alternatives. If you do prefer to purchase lower quality components, then I suggest you buy items with plenty of spare capacity so that in relative terms, they do not have to be pushed so hard. Poor quality components pushed hard will not last well and may well turn out to be more expensive in the longer run. The following two quotes from John Ruskin (1819 - 1900), a prominent English social thinker, may be of interest at this point:

“It's unwise to pay too much, but it's worse to pay too little. When you pay too much, you lose a little money - that's all. When you pay too little, you sometimes lose everything, because the thing you bought was incapable of doing the thing it was bought to do. The common law of business balance prohibits paying a little and getting a lot - it can't be done. If you deal with the lowest bidder, it is well to add something for the risk you run, and if you do that you will have enough to pay for something better”. “The bitter taste of poor quality lingers long after the sweetness of low price is forgotten”.

If Mr Ruskin had been born a hundred years later, he might have been talking about electric motors, batteries and ESCs!!



Chapter 12 Installing Power System Components

To get the best from the components of a brushless power system, it is necessary that they are all installed carefully. Every power system installation is different, so each must be considered separately. The most important aspects of a power system installation are to find a suitable location for each component, and then to mount each component in a secure manner and to ensure that each is adequately cooled. Planning an installation A little time spent in planning an installation will be repaid when it comes to carrying out the work. The location of components, the method of motor mounting and cooling issues all need to be decided. An inlet as well as an outlet must be provided for cooling air. It is also helpful to arrange matters so that the battery can easily be removed for recharging. As far as possible, the wiring for the power system should be kept well away from all of the components of the RC system. This particularly applies to the receiver, but includes all components such as servos and also any RC system wiring. This issue applies to all types of installation, particularly to those using non - 2.4 GHz RC systems. Installing motors Electric motors can be surprisingly powerful, so it is important to ensure they are firmly secured to the model. A variety of motor mounting techniques available; the most common methods are to screw the motor directly to a fuselage former, or to attach it to the fuselage using a rack mount. It is important to ensure that the motor is mounted with appropriate down and side thrust angles.

Outrunner motors can generally be mounted in two ways. Left: Where mounting to a nose former is not possible, the motor can be attached to a plywood spacer box using an ‘x’-shaped motor mount. The depth of the box has been made such that the propeller driver of this E-Flite Power 60 motor is correctly positioned. Spacers can be added to move the motor forward slightly if required. Right: In this example, the design of the model allows the motor to be attached directly to the front former of the model. Care was taken to use screws of a suitable length.



Left: This Purple Power brushless outrunner has been installed in a Wot 4 using a rack mount. Right: The cooling air intake area on a model should if possible be at least twice the area of the cooling holes of the motor. An outlet for the warmed air must be provided, and this should be slightly larger than the intake area. Length of mounting screws When mounting to a nose former, the screws used to attach the motor will penetrate the case which means that some portion of the screw will project inside the motor. If the screws are too long, they may contact the motor windings, causing damage. For this reason, take care that the screws are the correct length for the job. Cooling of brushless motors Make sure the cooling arrangements for your motor are carefully considered. All motors, including brushless types, will operate more efficiently and more powerfully when they are kept cool. It is impossible to overcool a motor. If motors are allowed to overheat, their magnets may become degraded and the motor will be permanently damaged.

The gyroscopic forces developed by the propeller can be very considerable, especially for aerobatic models. It’s important to ensure that the motor is fixed firmly to the model, and also that the structure of the model is adequate to take the in-flight loads. The gyroscopic forces exerted by this prop caused the motor mount of this ARF model to rip itself away from the fuselage in flight, during aerobatics!



Cooling of outrunners Brushless outrunners are generally the easiest type of motor to keep cool since it is usually easy to arrange for a copious flow of cooling air to reach the windings where the majority of heat is generated. The rotating case of the motor may act as a fan, helping the motor to stay cool.

Left: A typical outrunner motor with mounting accessories and a pair or propeller drivers. The x-shaped component is a motor mount, which allows the motor to be fitted to an internal former as an alternative to attaching it to a former at the very front of a model. Right: This transparent curved plastic air scoop is an excellent way to ensure that plenty of cooling air will reach the casing of a brushless motor. Care has also been taken to perforate the motor mounting plate to allow cooling air to be admitted to the front of the motor as well. This would be the best place to direct cooing air for an outrunner, but in this model the perforated former is a largely token gesture since the presence of a spinner will prevent air from flowing into the front of the motor by ram effect. Cooling of inrunners The cases of inrunner motors are usually perforated with holes to allow cooling air to be admitted. The windings of an inrunner are positioned in contact with the motor case, so as well as ensuring some air flows through the motor (where possible), it may also be beneficial to provide a flow of cooling air over the case of the motor. Heat sinks are available for some inrunner motors, and these will help the case to be cooled. Heat sinks are especially worth considering for helicopters, which spend a lot of time operating at high power and zero or very low airspeed. The magnets of an inrunner are located at the centre of the motor, and may well become hotter than the outside case of the motor, so if the case becomes more than moderately warm, the magnets may be in danger of becoming damaged. ESC position The ESC emits electronic noise in the radio frequency spectrum. Some of this noise can be picked up by the RC system components and fed back to the receiver through the RC system wiring, resulting in a reduced ability for the receiver to ‘hear’ the transmitter (i.e. a reduced ability to detect signals from the transmitter). It is therefore a very good idea to locate the ESC in a position that is reasonably distant from the receiver and servos, and



any associated RC system wiring. It is often convenient to position the ESC adjacent to the motor. This generally automatically keeps the ESC away from the RC components, and places it in an area where cooling can easily be arranged. ESC cooling ESCs should be attached to the model with airflow in mind. If possible, allow airflow to reach both sides of the ESC for maximum cooling. Plastic cable ties are a suitable way of attaching the ESC to the model. The unit should not be mounted in protective foam, as this will prevent it from being cooled.

It is important to provide both an inlet for cooling air, and an outlet for the warmed air. Left: This inlet is in the chin of a P51 model. This is a high pressure area, so air naturally tries to enter the opening in flight by ram effect. The central portion of the inlet is directed upwards straight to the motor. Right: The air outlet on the same model is in an area of low pressure, so the movement of the model helps to ‘suck’ air through from the nose. ESC mounting It is important to ensure that the leads of the ESC are not under strain; we do not want them to become disconnected in flight. For smaller models, Velcro may be used to attach the ESC to the model. However the use of Velcro will tend to insulate one side of the ESC, reducing the ability of the ESC to be cooled, so this method should only be used for ESCs which have plenty of spare current capability. Also, if you do use this method, make sure the Velcro is not attached to the side of the ESC which has a large smooth metal plate just under the heat shrink tube, as this is where the FETs are, which are the main source of heat. Extending ESC leads Occasionally the leads of the power system components are not sufficient to allow the components to be placed where we might want them to be. In this case, consideration may be given to lengthening the leads to or from the ESC. Extending motor lead length The leads connecting the motor to the ESC may be lengthened without concern. The motor or the ESC leads may be lengthened to accomplish this.



Left: The three intakes here provide a copious quantity of air for the motor, battery ESC. The left hand intake is a NACA duct, which provides an effective, low drag way to draw air into a fuselage without using a protruding air scoop. Right: If space is really tight, the ESC may be mounted outside the model. Although this is not the most elegant solution, it does at least ensure a copious supply of cooling air.

The ESC of this Hangar 9 P51 was installed in a short balsa tunnel. Air is admitted into the cowling (not shown) and a portion passes down the tunnel. Two rails were glued to the floor of the tunnel to allow air to pass beneath the ESC for maximum cooling. Extending battery lead length You will probably be able to see that the ESC includes one or more relatively large capacitors (of the electrolytic type) at the battery end. These perform a smoothing function, and are necessary to protect the ESC from voltage ‘spikes’ which are created by ESCs rapid, continuous switching of current on and off. The ESC is constructed with capacitors which are sized for an average battery lead length. A short addition to the length of the leads connecting the battery to the ESC (perhaps 50 - 75 mm or 2-3 inches) will probably cause no problems, but if any greater increase in length is needed, additional capacitors should be installed between the battery wires, as close as possible to the ESC itself. The greater the increase in length, the larger these capacitors will need to be.



Left: The three wires to the motor may be extended at will. However, the same freedom does not exist for extending the ESC to battery wires. Right: Usually it is possible to see capacitors located at the battery end of the ESC. These examples protect this ESC from voltage spikes. They are rated at 470 μF, 35 Volts. Retaining batteries in the model It is essential to make sure that battery cannot become detached from the model in flight – should it fall out, will be a serious hazard to any bystanders, and the model will probably crash. The G-forces on components in flight and the forces due to vibration during take off and landing can be surprisingly high. Any model may accidentally become inverted in flight, so even if your model is not an aerobatic type, the battery retention method must work well, whether the model is upright or inverted. You should allow for both positive and negative G-forces. Don’t rely on a hatch to retain the battery - secure the battery with its own fixing method. Cooling of LiPo batteries Some cooling should be provided for batteries, but this need not be excessive. LiPo batteries work best when they are slightly warm, so it is counterproductive to supply them with too great a quantity of cooling air, especially in cold conditions. Arranging the supply of cooling air is a matter of judgment; clearly a small battery used in a low power model will have a much lower need for cooling than a large battery in a high powered model. Power system wiring As well as the need to physically separate the RC system wiring and components from the power system components, it is also important to make sure that connectors cannot become detached in flight – the G-forces on components in flight and the forces due to vibration during take off and landing can be surprisingly high. The radio frequency radiated by wiring can be reduced somewhat by twisting the power system wires together. About 1 turn per inch (25 mm) is sufficient. RC system wiring Particularly for non - 2.4 GHz systems, the receiver’s aerial (antenna) should be routed well away from any power system components and also as far as possible away from servos. If possible, a spacing of at least 2 inches (50 mm) between RC system and the power system components is a good idea. If using equipment with crystals, make sure



that these cannot fall or vibrate out of the receiver. Care must also be taken to ensure that servo plugs and any RC system extension leads cannot become detached in flight, again remembering that there can be some surprisingly high forces on model components in flight. It is worth considering tying extension leads together using a commercial product or alternatively using cotton thread. Finally, in a similar way to the power system wiring, twisting of the servo wires can reduce the amount of interference picked up by them.

Arrangements need to be made for easy access to the battery, so it can be removed for recharging. Left: The battery of this moulded MiG 15 is firmly attached to the model with a Velcro strap. The hatch is not relied on to help retain the battery. Right: This small all foam warbird has a removable top deck, making battery changes very straightforward.

Power system wiring and RC system wiring should be kept physically separate as much as possible. Little attention has been given to this issue with this model.



Chapter 13 Testing & Setting Up Brushless Motor Power Systems

It is always wise to test a model on the ground before committing it to a flight. As well as a range check, a thorough check of the power system is also wise. This involves running the system up and checking for any problems such as unusual noises, vibration and so on. Brushless motors do not require breaking in. Testing models on the ground A stationary model will have a reduced flow of cooling air to the motor, battery and ESC. Care should therefore be taken to avoid prolonged static running at high power.

This is John Ranson’s large Hawker Tempest taxiing out for another thrillingly realistic flight. When testing the power system of a model on the ground, take care not to allow components to become overheated. The model should face into any prevailing wind, and power runs should be kept short.

Avoid overloading your motor It is worth reminding yourself that the current draw is determined by the propeller load. Before testing any new installation, it is important to measure the motor current using a ‘wattmeter’ or similar. Without this information, you have no way of knowing what your power system is doing, and therefore no way of knowing if your power system components are overloaded. Model safety An electric motor is potentially more dangerous than an i.c. engine, so do not be fooled into thinking that electric systems lack the power to cause injury. A rotating prop is not much different from a sharp, whirling scythe, and it may do great injury if care is not taken to avoid this. I recommend that motors are not run up indoors; instead, take the model outside and secure it firmly in some kind of restraint. It may be wise to remove the prop before testing a system for the first time. When running the motor, stand behind the model and keep any spectators behind the model as well.



Before starting to use a brushless motor, it is important to check the installation is safe. Check the security of the motor in the model, check the prop is balanced and firmly fixed to the motor shaft etc. If you are in any doubt, remedy the situation before attempting to start the motor. Direction of rotation Before starting a motor for the first time, be aware that it may run backwards. If this is the case, simply change over any two wires between the ESC and the motor. Brushless motors will rotate just as well one way as the other. The ESC will handle all commutation issues, so there is no question of adjusting motor timing as a consequence of changing the direction of rotation, as may be required for brushed motors. One advantage of the ability of brushless motors to operate in either direction is that for pusher installations, a special opposite rotation pusher prop is not necessary, since an ordinary tractor prop can be used simply by changing the direction of motor rotation. First flight precautions It is wise to limit the flight time of a new model to begin with, assessing the motor temperature after flight. Air may not behave as we think it will/should, so the power system components may not receive the expected degree of cooling. Charge quantity It is a good idea to note the charge quantity required to replace the energy used for a flight, and compare that with the flight time. This will allow you to estimate a safe flight time, avoiding the risk of a flat battery in flight.

Flying at reduced throttle can greatly extend flying times compared to flying at full throttle. Throttling back to about 70 % throttle (judged by throttle stick position) equates to approximately half of the full throttle current, doubling flight time. Experiment with props Surprising gains can be realized from a little experimentation with different propellers, so it may be well worth trying several before deciding on the one which best suits your needs. Be sure to check the current consumption any time a propeller is changed for one with more pitch, and especially one with more diameter.



Chapter 14

Propeller Power Absorption Tables

Propeller power absorption characteristics The amount of power required to turn a propeller depends on its pitch and diameter, and the rpm it is turned at. Expressed mathematically, we can say that the power required is proportional to rpm³ x diameter4 x pitch. This can also be summarised this in a non-mathematical way:

Relationship between pitch and required power Small increases in propeller pitch will require a proportionate increase in power, and vice versa. For example, if the propeller pitch is increased by 10 % (e.g. an increase in pitch from 10 inches to 11 inches), the power required to maintain the same rpm also rises by 10 %. Relationship between rpm and required power Small increases in rpm will result in a disproportionately large increase in demand for power. This means that if for example, the propeller rpm is increased by 10 % (e.g. an increase in rpm from 7,000 rpm to 7,700 rpm), the power required to maintain the same propeller rpm will rise by 33 %. Relationship between diameter and required power Small increases in diameter result in a disproportionately very large increase in power demanded. For example, if the diameter is increased by 10 % (e.g. an increase in diameter from 10 inches to 11 inches), the power required to maintain the same rpm will rise by 46 %.

It is worth Note that when changing from one propeller, to another that presents a higher load to the motor, the rpm will fall, meaning that the increase in current and power will not be as large as if the same propeller rpm was maintained. Left: Propeller load will depend on a number of factors such as diameter, pitch, rpm, blade shape and many more. Right: Power and current consumption should always be checked. Many different tools are available to do this, including simple to use ‘wattmeters’. A more sophisticated variation on this theme is this power analyser.



The following tables show the approximate input power required to turn a variety of propellers at a range of different speeds, using a motor operating at 80 % efficiency. It is very instructive to study the tables, and see how changes in rpm, pitch and diameter affect the power absorbed by the propeller.

Diameter Pitch P/D ratio 6 7 8 9 10 11 12 13 14 157 4 0.57

0.710.860.500.630.751.000.500.670.831.000.500.600.701.000.500.640.730.770.91

13 20 30 42 58 77 100 127 159 1957 5 16 25 37 53 72 96 125 159 199 2447 6 19 30 44 63 87 116 150 191 238 2938 4 21 34 51 72 99 131 171 217 271 3338 5 27 42 63 90 123 164 213 271 339 4178 6 32 51 76 108 148 197 256 325 407 5008 8 43 68 101 144 198 263 341 434 542 6679 4.5 38 61 91 130 178 237 308 391 488 6019 6 51 81 122 173 237 316 410 521 651 8019 7.5 64 102 152 216 297 395 513 652 814 10019 9 77 122 182 259 356 474 615 782 977 1201

10 5 65 103 154 220 301 401 521 662 827 101710 6 78 124 185 264 362 481 625 795 992 122110 7 91 145 216 308 422 562 729 927 1158 142410 10 130 207 309 439 603 802 1042 1324 1654 203511 5.5 105 166 249 354 485 646 839 1066 1332 163811 7 133 212 316 450 618 822 1068 1357 1695 208511 8 153 242 362 515 706 940 1220 1551 1937 238311 8.5 162 257 384 547 750 999 1296 1648 2059 253211 10 191 303 452 643 883 1175 1525 1939 2422 2979

Propeller Power Absorbtion in WattsRPM in Thousands

Diameter Pitch P/D ratio 4 5 6 7 8 9 10 11 12 1312 6 0.50

0.670.831.000.310.500.620.770.500.610.710.860.270.400.470.530.670.800.500.630.750.59

48 94 162 257 384 547 750 998 1296 164812 8 64 125 216 343 512 729 1000 1331 1728 219712 10 80 156 270 429 640 911 1250 1664 2160 274612 12 96 188 324 515 768 1094 1500 1997 2592 329613 4 44 86 149 236 353 502 689 917 1190 151313 6.5 72 140 242 384 573 816 1119 1490 1934 245913 8 88 172 298 472 705 1004 1377 1833 2380 302613 10 110 215 372 591 882 1255 1722 2292 2975 378314 7 104 203 350 556 830 1182 1621 2158 2801 356114 8.5 126 246 425 675 1008 1435 1968 2620 3401 432514 10 148 289 500 794 1186 1688 2316 3082 4002 508814 12 178 347 600 953 1423 2026 2779 3699 4802 610515 4 78 153 264 419 625 890 1221 1625 2109 268215 6 117 229 396 628 938 1335 1831 2437 3164 402315 7 137 267 461 733 1094 1557 2136 2843 3691 469315 8 156 305 527 837 1250 1780 2441 3250 4219 536415 10 195 381 659 1047 1563 2225 3052 4062 5273 670515 12 234 458 791 1256 1875 2670 3662 4874 6328 804616 8 202 395 683 1084 1618 2304 3160 4207 5461 694416 10 253 494 853 1355 2023 2880 3951 5258 6827 868016 12 303 593 1024 1626 2427 3456 4741 6310 8192 1041517 10 322 629 1088 1727 2578 3670 5035 6701 8700 11061

Propeller Power Absorbtion in WattsRPM in Thousands




Index 3D flight 61 3-cell tables 23 - 38 4-cell tables 39 – 45 5-cell tables 46 – 52 6-cell tables 53 – 59 Aerodynamic drag 9 Air scoop 68 Axi 4130/16 6 Battery 21 - capacity 13 - choosing 63 - cooling 71 - extending lead length 70 - purchasing 63 - retention 71 - sizing 61, 62 Blackburn monoplane 10 Bristol Bulldog 62 Burst rating 65 Capacitor 71 Charge quantity 74 Continuous rating of motors 65 Cooling 67 - 70 Cost vs quality 65 Current 14, 16 Curtis Shrike 15 Decathlon 17 Drag 8, 9 Edge 540 23 Electric motor - size 61 - choosing 64 - continuous rating 65 - cooling 67 - direction of rotation 74 - extending leads 69 - installing 66 Electric Motors - Axi 4130/16 6 -- E-Flite Power 60 66 HiMax HC 3516-1130 60 - Jeti 45/3 8 - Mega 16/15/3 8 - Mega 16/25/6 15 - MP Jet AC28/20-10 64 - Speed 600 8 Elfi 11 ESC 16, 22, 71

- capacitor 71 - choosing 63 - cooling 69 - mounting 69, 70 - position 68 - sizing 61 - extending leads 69 Floatplane 20 Flying boat 20 Gearbox 8 Gibbs, John 6 Golds, Chris 6, 15 Gyroscopic forces 67 Hawker Fury 6 Hawker Tempest 73 Heading banner 13 i.c. engine 7 - comparison with electric 7 - 10 - drag of exposed cylinder etc 9 - power output 8 Installing components 66 Kv 6, 14, 15, 20, 21, 23, 60 - changes 60 LiPo - number of cells 12, 13 Low, Colin 62 MiniMag 20 Model speed 13 Motor - size 61 - choosing 64 - continuous rating 65 - cooling 67 - installing 66 Mounting screws 67 Multiplex MiniMag 20 NACA duct 70 Number of LiPo cells 12, 13 Outrunner 8, 10, 66 Piper Cub 18, 21 Planning an installation 66 Power 14 Power absorption tables 75 Power system wiring 71


Propeller 8, 22 - changes 60 - diameter 16, 75 - load, loading 18, 19, 75 - efficiency 8 - experimenting 74 - pitch 75 - pitch/diameter ratio 16 - pitch speed 16 - power absorption characteristics 75 - rpm 75 Purple Power 22, 67 Quick Reference System 11 - limitations 11 Quality 65 Rack mount 22 Radio Modeller Magazine 6 Ranson, John 73 RC system wiring 71 RCV 52 9 Reynaud, Toni 6 RM trainer 7 Ruskin, John 65 Safety 73 Seaplane 20 Spitfire 7, 18 Steenacker, Ronald 11 Tempest 73 Tigercat 63 Voltage 14 Watts 8 Webra 40 8 Weight estimating 17 Weight, Andrew 17


Gibbs Guides, Gibbs Guides offers an expanding range of ...

Documents

Transcript of Gibbs Guides, Gibbs Guides offers an expanding range of ...