Aircraft Engine Sizing

8/13/2019 Aircraft Engine Sizing

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CONTENTS

ABSTRACT ................................................................................... 1

ACKNOWLEDGEMENT ................................................................ 2

LIST OF FIGURES ......................................................................... 3

LIST OF TABLES ........................................................................... 4

NOTATIONS ................................................................................ 5

1.0 INTRODUCTION .................................................................... 6

1.2


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1.0 INTRODUCTION

1.1 Project Background

Ab initio flying training aircraft are a class of aircraft specifically designed

to facilitate transition of pilots-in-training (student pilots) to pilots. Ab initio

flying training aircrafts are characterised by additional safety features which

include; forgiving flight characteristics, stiff structure, simplified cockpit

arrangement, excess power and good visibility. These characteristics serve

to accommodate the mistakes by the inexperienced and enable instructors

to allow students more time to correct their own errors which increase

learning speed. The above mentioned qualities however are consideredtogether with the operators requirement of cost effectiveness.

A cost effective aircraft must have comparatively lower sum of initial

investment and operating costs. Additionally, availability of parts and ease

of maintenance are major considerations to the operator.

In search of a good trainer, the NAF incorporated the Scottish Aviation

Bulldog into service in the early 80s for use as trainer. However, the high

operating cost, which comprises of cost of fuel and spares made if

impossible for the NAF to sustain the use of Bulldog as trainer.

Consequently the ABT-18 aircraft was introduced in 1995 to replace the

Bulldog. Since then, the air beetle has been the ab initio trainer of the NAF.

The air beetle is known to have many inadequacies, the major of which are

the unforgiving nature of its nose wheel strut, and frequent high cylinder

head temperature. These maintenance problems made it impossible for the

NAF to sustain local training. Replacing the air beetle therefore, with an

easier to maintain, more robust aircraft would assist the NAF to actualize its

dream of self reliance in pilot training.


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1.2 AFIT Light Trainer Concept

The department of Air Engineering, Air Force Institute of Technology is

exploring the possibility of designing a light trainer aircraft for the Nigerian

Air Force. The project when successful is going to replace the ABT-18

Aircraft currently in service as the ab initio trainer of the NAF. The AFIT

light trainer aircraft project considers a two seater low wing light aircraft. It

is intended to be an engineering data gathering research to lay the grounds

for future development work in subsequent years. The project consists of a

number of individual research topics that stand in their own right

Figure 1: AFIT LIGHT TRAINER AIRCRAFT

1.3 Project Specifications

The conceptual design of the aircraft has been carried out by a team of

instructors. The conceptual design was able to establish all the features of


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the aircraft. The following are the established parameters and payload

configuration of the light aircraft:

SPECIFICATIONS

Interior Layout

Accommodation and capacity

Maximum specified passenger capacity: 2

Configuration: side by side

Doors: 1 overhead canopy

Powerplant

Type: Piston Engine

Capacity: To be determined

Configuration: Tractor

Performance:

Max take-off weight: 1,066 kgEmpty weight: 669 kg

Range: 540 nm

Service ceiling: 10,000 ft

Cruise speed: 46.96 m/s

AERODYNAMIC INFORMATION

Lift CharacteristicsMaximum lift coefficient: 1.448

Basic wing Max CLangle 15o

Stall angle 9o


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Drag Polar:

Cruise condition M = 0.14

Sea level takeoff, undercarriage and flaps extended

Landing gear increment

1.4 Authors Responsibility

The author was primarily responsible for engine sizing (determining power

required), engine selection, installation and performance calculation for theAFIT light trainer aircraft. Engine selection is an important part of the

aircraft design process. Because the engine size was not determined

during the conceptual design, this requires the determination of power

required by the aircraft, selecting an engine capable of producing that

amount of power, determining how it will be installed and calculating the

key performance indicators. Evidently, certain interface issues are present

with components such as fuselage, nose landing gear and systems such as

the fuel system and the mass and C.G of the aircraft. Thus, it is paramount

to work in collaboration with the various designers with which interface

issues exist for synchronization of efforts.

1.5 Design requirements

Definition of necessary air worthiness requirements and goals are key

aspects of an aircraft design. This provides a means of monitoring the

project goals and ensuring that the design outcome is not conflicting with

the pre-stated requirements.


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1.5.1 Certification Requirements

The marketability and the integrity of a design are ensured by it strict

adherence to standard design regulations as documented by the

appropriate governing bodies. In this case, AFIT light trainer aircraft is

designed in conformity with design requirements of EASA certification

Specification for normal, utility, aerobatic and commuter category airplanes

(CS - 23). The air worthiness code of CS23 is applicable to this class of

aircraft as it is an aerobatic aircraft with maximum certified take off weight

of greater than 750kg (which is the limit for CSVLA), and less than 56kg.

The sections of the regulatory document that apply to the power plant areas listed in Appendix A.

1.5.2 Functional Requirement:

It can be argued that the correct choice of power plant is half the success

of an aircraft design. It does not matter if you build a really efficient

structure or have the best aerodynamics if the choice of power plant is

poor, then the design will not be successful.

The primary role assigned to the aircraft engine is to provide the required

thrust to move the aircraft. In addition, there is the need for the engine

weight and fuel consumption to be within acceptable limits of todays

technology.


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2.0 LITERATURE REVIEW

2.1 Aero Engines:

Aircraft need some form of power to keep them flying. Since the first

powered flight, numerous types and models of engines have been

developed. Aviation propulsion system varies according to construction,

configuration and purpose and has gone through several changes over the

years. The most popular propulsion systems are Gas Turbine and

Reciprocating Engines.

2.1.1 Gas Turbine Engines

Most modern passenger and military aircraft are powered by gas turbine

engines, which are also called jet engines. The gas turbine engine is

essentially a heat engine using air as a working fluid to provide thrust. To

achieve this, the air passing through the engine has to be accelerated; this

means that the velocity or kinetic energy of the air is increased. To obtain

this increase, the pressure energy is first of all increased, followed by the

addition of heat energy, before final conversion back to kinetic Energy in

the form of a high velocity jet efflux.There are several types of jet engines.

They include:

2.1.1.1 Turbojet Engines

The first and simplest type of gas turbine is the turbojet. Turbojet engine

derives its thrust by highly accelerating a mass of air, all of which goes

through the engine. Since a high "jet" velocity is required to obtain an

acceptable thrust, the turbine of turbo jet is designed to extract only enough


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power from the hot gas stream to drive the compressor and accessories .

All of the propulsive force (100% of thrust) produced by a jet engine is

derived from the exhaust gas.

Figure 2: A TURBOJET ENGINE

2.1.1.2 Turboprop Engines.

Many low speed transport aircraft and small commuter aircraft

use turboprop propulsion. There are two main parts to a turboprop

propulsion system, the core engine and the propeller. The core is very

similar to a basic turbojet except that instead of expanding all the hot

gasses through the nozzle to produce thrust, most of the energy of the

exhaust is used to turn the turbine. The shaft on which the turbine is

mounted drives the propeller through the propeller reduction gear system.

The propeller produces most of the thrust in a turboprop and the exhaust


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contributes little thrust. Approximately 90% of thrust comes from propeller

and about only 10% comes from exhaust gas.

Figure 3: A TURBOFAN ENGINE

2.1.1.3 Turbofan Engines.

A turbofan engine is the most modern variation of the basic gas turbine

engine. As with other gas turbines, there is a core engine. In the turbofan

engine, the core engine is surrounded by a fan in the front and an

additional turbine at the rear dedicated for driving the fan. The incoming air

is captured by the engineinlet.Some of the incoming air passes through

the fan and continues on into the core compressor and then theburner,

where it is mixed with fuel and combustion occurs. The hot exhaust passes

through the core and fan turbines and then out thenozzle,as in a basic

turbojet. The rest of the incoming air passes through the fan and bypasses,
http://www.pilotfriend.com/training/flight_training/tech/turbo/inlets.htmhttp://www.pilotfriend.com/training/flight_training/tech/turbo/burner.htmhttp://www.pilotfriend.com/training/flight_training/tech/turbo/nozzles.htmhttp://www.pilotfriend.com/training/flight_training/tech/turbo/nozzles.htmhttp://www.pilotfriend.com/training/flight_training/tech/turbo/burner.htmhttp://www.pilotfriend.com/training/flight_training/tech/turbo/inlets.htm


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or goes around the engine, just like the air through a propeller. The air that

goes through the fan has a velocity that is slightly increased from free

stream. So a turbofan gets some of its thrust from the core and some of its

thrust from the fan. The ratio of the air that goes around the engine to the

air that goes through the core is called the bypass ratio. Because the fuel

flow rate for the core is changed only a small amount by the addition of the

fan, a turbofan generates more thrust for nearly the same amount of fuel

used by the core. This means that a turbofan is very fuel efficient. Most

modern airliners use turbofan engines because of their high thrust and

good fuel efficiency

Figure 4: A TURBOFAN ENGINE

2.1.2 Reciprocating Engine

A reciprocating aero engine is a fuel-burning internal combustion pistonengine specially designed and built for minimum fuel consumption and light

weight in proportion to developed shaft power. Reciprocating aircraft

engines operate on a four-stroke cycle, where each piston travels from one

end of its stroke to the other four times in two crankshaft revolutions to


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complete one cycle. The cycle is composed of four distinguishable events

called intake, compression, expansion (or power), and exhaust, with

ignition taking place late in the compression stroke and combustion of the

fuel-air charge occurring early in the expansion stroke. These reciprocating

engines burn specially formulated aviation gasoline and produce shaft

power by the force of combustion gas pressure on pistons acting on

connecting rods turning a crankshaft. Major parts are the crankcase,

crankshaft, connecting rods, pistons, cylinders with intake and exhaust

valves, camshafts, and auxiliary operating systems such as ignition, fuel

injection or carburetion, and fuel and oil pumps.

Figure 5: PISTON ENGINE PRINCIPLES

2.2 Light trainer Aircraft Engines

2.2.1 Selection of Engine Type

The following factors play a role in selecting the type of propulsion system

to be used:


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1. Required cruise speed and/or maximum speed

2. Required maximum operating altitude

3. Required range and range economy

4 Noise regulations

5. Installed weight

6. Reliability and maintainability

7. Fuel amount needed

8. Fuel cost

9. Fuel availability

10. Specific customer or market demands

Overall fuel efficiency, cost and installed weight often dominate the

arguments about the pros and cons of a certain type of propulsion system.

Most modern aircraft using engines with up to 450hp output are powered

by air-cooled, horizontally opposed, reciprocating engines. The biggest

reason for this is cost: other than avionics, the system that contributes most

to a vehicles price is its propulsion system, and a turbine engine cancost

up to five times more than a comparable piston engine. Additionally, recent

years have seen large technological advances in piston engine

manufacturing, making them lighter, more powerful, and more efficient.

Finally, piston engines are known to have greater flexibility with respect to

transient power requirements than turbine engines, which not only

increases safety, but also performance and efficiency. Because of these

factors, many general aviation manufacturers are using piston engines in

an attempt to reduce vehicle price and increase the potential market.


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The smaller turbine engines that fall below 450hp have failed to meet the

low cost and fuel economy necessary to compete with existing

reciprocating engines. Therefore reciprocating engines are the cost

effective choice for light trainer aircraft like the AFIT Light Trainer Aircraft.

2.2.2 Determination of Engine Size:

Before choosing an engine for an aircraft, the total thrust (power) required

must be known. The thrust-to-weight ratio (power loading for propeller-

powered aircraft) and wing loading (w/s) are the two most important

parameters affecting aircraft performance. Wing loading and thrust-to

weight ratio are interconnected for a number of performance calculations,

such as takeoff distance, climb rate and maximum speed. These

performances are critical design drivers used to size the engine.

2.2.3 Getting the Required Engine:

An aircraft can be designed using some existing engine or a new to-be-

designed engine. However, rarely is a new general aviation airplane design

enough incentive for engine manufacturers to go for the time and expense

of designing a new engine. This only happens in the case of major military

fighter or bomber program. Designer of a general aviation or aerobatic

aircraft mostly rely on selecting the best of the existing engines. To do this,

the designer determines the performance characteristics he needs and

search for an engine that is capable of delivering those parameters.


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2.3 Engine Installation

Haven decided on the type and the number of engines to be used, the next

decision is: how should these engines be installed? There are a number of

possible options. The following factors play a role in deciding on the engine

installation:

1. Management of exhaust gases

2. Management of cooling air

3. Stability and control considerations

4. Safety Considerations

5. Noise considerations, and6. Structural Considerations

2.3.1 Management of Exhaust Gasses

When the exhaust stack is mounted perpendicular to the free stream, it is

very bad from a drag viewpoint. Figure 7 shows how this can be improved

somewhat. Reference 6 shows that poorly designed exhaust configurations

can increase the zero lift drag coefficient of an airplane by 16 percent. By

directing the exhaust rearward, drag can be reduced and some thrust can

be recovered as well.


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Figure 6: A BAD WAY OF MOUNTING EXHAUST PIPE

Figure 7: AN IMPROVED WAY OF MOUNTING EXHAUST


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2.3.2 Management of Cooling Air

Most piston engines are designed with the assumption that the cooling air

will flow from top to bottom: downdraft, as shown in figure 10. Updraft

cooling arrangement, as shown in figure 11, may look good, but can result

in the cooling air being heated by the exhaust stack thereby reducing

cooling effectiveness. Mismanagement of cooling air can cause drag

Increase of up to 9 percent of zero lift drag according to Ref6

Figure 8: A DOWNDRAFT

COOLING

Figure 9: AN UPDRAFT

COOLING


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2.3.3 Stability and Control Considerations

The stability and control effect that should be considered in a tractor engine

is the effect of engine/propeller thrust line location and inclination. In

preliminary design, a good rule of thumb is: if the engine disposition differs

significantly from that of existing certified airplanes, considerable power

effects on stability and control characteristics can be expected. In such

cases the safest thing to do is to perform the necessary stability and control

calculations before freezing the design.

2.3.4 Safety Considerations

AII engines and other heat generating equipment must be isolated from the

rest of the airplane by means of firewalls and/or other suitable shrouds.

This requirement is of great importance in isolating engines from fuel tanks.

Fire walls should be made out of stainless steel and/or titanium.

2.3.5 Noise Considerations

Airplanes and their engines create a substantial amount of both interior and

exterior noise. The interior noise levels should not be as high as to cause

discomfort to the passengers or to make safe operation by the crew

impossible. The exterior noise levels should meet the requirements of CS -

36. These requirements impose severe restrictions on the type of engine

and/or propeller technology which can be utilized.


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2.3.6 Structural Considerations

The structural consideration that plays an important role in the integration

of the propulsion system into the airframe is the transmission of thrust into

the airframe. To transmit thrust forces into the airframe it is necessary to

have a number of 'hard points' where the engine is physically attached to

the airframe. The number of these hard points depends on the type of

engine used. Figure 12 shows an example of the principal method used to

mount piston engines in airframes. This is accomplished with a truss

(usually made of welded steel tubes) or with a support cradle. Note that

where the truss or cradle is attached to the airframe, thrust and engine

weight determine the attachment point loads.

It is important to note that the attachment (mounting) points on the engine

itself cannot be changed easily. Their location depends on the internal

design of the engine which is determined by the engine manufacturer.

Changing these attachment points is very expensive. Also, since most

piston engines transmit significant vibrations into the airframe it is essential

to use some type of shock mount(s) to reduce these vibrations.

2.4 Engine Mount

The engine mount is primarily used to connect the engine to the airframe or

fuselage. Some of its secondary features are to distribute the weight of theengine and spread the torque and vibration generated by the engine. Most

engine mounts are made from tubular steel chrome-molybdenum (4130)

and welded together. This is a lightweight and strong construction. After


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points are parallel with the firewall, so there is no awkward angle when

installing the engine bolts and shock mounts. The disadvantage of this type

of mount is that it is not effective in cushioning vibration and engine torque.

Figure 11: A CONICAL MOUNT

2.4.1.2 Dynafocal Mounts

These are the best types of mounts, and they do a perfect job of

cushioning the vibrations and movements from the engine. Dynafocal

mounts also produce lower cockpit noise. However, they are more

expensive to build and construct. The engine is held in four attach points

(located in a ring), under a certain angle and point to the center of gravity of


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the engine. During welding this angle must be held in perfect alignment or

else the four bolts will not fit when installing the engine in the mount.

There are two types of dynafocal engine mounts: Type 1 and Type 2. Type

1 is used in Lycoming engines up to 180 hp and the type 2 is used in the

IO-320 and IO-360 model engines from Lycoming.

Figure 12: ADYNAFOCAL MOUNT

2.4.1.3 Bed mount

In a Bed mount, the engine is mounted using four points underneath the

crankcase and then hang to the firewall, as shown below:


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Figure 13: A BED MOUNT

2.5 Aircraft Performance

2.5.1 Introduction

Performance is a term used to describe the ability of an airplane to

accomplish certain things that make it useful for certain purposes. For

example, the ability of the airplane to land and take off in a very short

distance is an important factor to the pilot or operator who operates in andout of short, airfields. The ability to climb fast, carry heavy loads, fly at high

altitudes at fast speeds, or travel long distances are essential performance

for operators of airline and executive type airplanes.

The main elements of performance are the takeoff and landing distance,

rate of climb, ceiling, payload, range, speed, maneuverability, stability, and

fuel economy. Some of these factors are often directly opposed: for

example, high speed versus shortness of landing distance; long range

versus great payload. It is the compromise between two or more of these

factors which dictates differences between airplanes and explains the

degree of specialization of modern airplanes.


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Therefore, whenever the question of aircraft performance is asked, what

the questioner wants to find out are:

How fast can the airplane go?

How high can it go?

How fast can it climb?

How far can it travel without refueling?

What length of airfield is required to operate it?

2.5.2 Engine and Aircraft Performance

The various items of aircraft performance result from the combination of

airplane and engine characteristics.

The aerodynamic characteristics of the airplane generally define the power

and thrust requirements at various conditions of flight while engine

characteristics generally define the power and thrust available at variousconditions of flight. The matching of the aerodynamic configuration with the

engine is accomplished by the designer to provide maximum performance

at the specific design condition; e.g. range, endurance, and climb.

Below are examples of engine role in key performance parameters:

2.5.2.1 Maximum Speed

The maximum level flight speed for the airplane will be obtained when the

power or thrust required equals the maximum power or thrust available

from the powerplant. So maximum speed is directly related to the engine

capacity. On the other hand, the minimum level flight airspeed is not


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usually defined by thrust or power requirements. Instead, it is determined

by conditions of stall or stability and control problems.

2.5.2.2 Climb Rate

Climb depends upon the reserve power or thrust. Reserve power is the

available power over that required to maintain horizontal flight at a given

speed. Thus, if an airplane is equipped with an engine that produces 200

total available horsepower and the airplane requires only 130 horsepower

at a certain level flight speed, the power available for climb is 70

horsepower.

2.5.2.3 Range Performance

The ability of an airplane to convert fuel energy into flying distance is one of

the most important items of aircraft performance. Range is another

performance parameter that is determined partly by the engine; the specific

fuel consumption (SFC) of the engine.

2.5.2.4 Takeoff performance

The minimum takeoff distance is of primary interest in the operation of any

airplane because it defines the runway requirements. The minimum takeoff

distance is obtained by taking off at some minimum safe speed that allows

sufficient margin above stall and provides satisfactory control and initial

rate of climb.

To obtain minimum takeoff distance at the specific lift-off speed, the forces

that act on the airplane must provide the maximum acceleration during the

takeoff roll. The powerplant thrust is the principal force in providing the


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acceleration and, for minimum takeoff distance, the output maximum thrust

should be high.


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REFRENCES

1. ANDERSON, D. Aircraft performance and design, McGraw

Hill,1999.

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thesis, Cranfield University, 2007.

3. DINGWAL, S. Conceptual design of an aerobatic byplane ,Msc

Thesis, Cranfield University, 2007

4. Dr Roskam J., Airplane design, Parts I - VIII, Roskam aviation

and engineering corporation, 1990

5. European aviation safety agency, Certification specifications For

Normal, utility, aerobatic, and Commuter category Aeroplanes(

CS-23) Amendment 2 (corrigendum), 2010

6. HOLLMANN, M., Modern aircraft design, 3rd edition, 1986.

7. HOWE, D. Aircraft conceptual design synthesis, professional

engineering publications, Cranfield, 2000.

8. HIGHLEY, J.L.,A thermodynamics based model for predicting

piston engine Performance for use in aviation vehicle design

Georgia Institute of Technology, April 2004

9. KARUZIC, G., Conceptual design of green and silent airliner,

Cranfield University, 2006.


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10. KROES, MJ.and WILD, TW. Aircraft powerplants, 7th edition,

2002.

11. RAYMER, D., Aircraft design: a conceptual approach, AIAA

education series, 1992.

12. ROLLSROYCE PLC, The Jet Engine 5thedition, 1996

13. SADRAEY, M. Aircraft conceptual design Daniel Wester

College.

14. STINTON, P., The design of the aeroplane, Oxford Blackwell

Science, Oxford, 2001.

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