Technical Report Boost Gliders

8/8/2019 Technical Report Boost Gliders

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R E P O RTREPORT T ECH N I CA LTECHNICAL

by Thomas E. Beach

I. IntroductionFew sights in model rocketry can match the

beauty and grace of a boost glider rocketingswiftly into the air, then returning in a slow,gentle glide to the surface of the earth.Building and flying a successful boost glider isone of the most challenging projects in modelaeronautics, and it can be one of the mostrewarding.

In this report we will discuss the principles of flight by which gliders operate, as well as thepractical aspects of designing, building, and fly-ing boost gliders. Do not be discouraged if some of the concepts in this report are difficultto understand it is not necessary to fullycomprehend the theory of flight in order tobuild a glider that will awe the spectators as ittraces lazy circles in the sky overhead. Evennovice rocketeers can successfully fly boostgliders built from Estes kits if they follow theinstructions and carefully build and trim the

glider. But the more you know about how aboost glider works, the better you will under-stand the reasoning behind each step, and thegreater are your chances of a perfect flight.

II. Basic DefinitionsA boost glider is a model rocket that ascends

vertically into the air by rocket power, then part orall of the model returns as a glider supported byaerodynamic lifting surfaces. During boost phasethe boost glider is a stable, ballistic vehicle justlike any model rocket; it must not ascend in a shal-low climb using lift from the wings. Near peak altitude the boost glider transitions from boostconfiguration into glide configuration. The transi-tion from rocket into glider is the most basic prob-lem of boost glider design, and modelers havedeveloped many ingenious methods of making themetamorphosis, which are covered in section VI.

Historically, the first boost gliders were essen-tially model rockets with overly-large fins thatcould transition into a glide configuration. Theserear-engine boost gliders were not very efficient asgliders, but at the time it was amazing to get a suc-cessful glide recovery to work at all. The originalEstes Technical Report TR-4 documented some of these early efforts.

The state of the art of boost glider design took agiant leap forward with the development of the front-engine boost glider. Rather than trying tomake a rocket that glides back, Larry Renger, amodel rocketeer with experience building modelairplanes, adapted a conventional glider to boostlike a rocket. The early Estes Technical ReportTR-7 discussed front-engine gliders. Variations of this front-engine design are still the standard todayfor high performance free-flight boost gliders.

To make the transition from boost to glide con-figuration, many boost gliders separate into multi-ple parts. A special sub-class of boost gliders,

called rocket gliders , makes the transition withoutseparating or ejecting parts. Rocket gliders aremore complex to design and fly.

T R - 4

B o o

s t

G l i d e r s

1

boostphase

glidephase

transition

Rear engine boost glider


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III. How Gliders FlyIn order to understand how boost gliders work, we

must first discuss the theory of flight by which all glidingaircraft operate.Terminology

The basic parts of a glider are shown in figure 1. Thisconfiguration is typical of standard, high performanceboost gliders. We will see that other arrangements of themain aerodynamic surfaces are possible, as with the SpaceShuttle which combines the wing and horizontal stabilizerinto one delta wing.

Any object in flight, be it a rocket or glider, will reactto an offset disturbing force by rotating around the Center of Gravity (CG) of the object. The CG, also called theCenter of Mass, is essentially the balance point of theobject. Any force that acts through the CG of a rocket orglider will cause the model to accelerate in the directionthe force is acting. A force that does not act directly

through the CG will also produce a torque, or turningaction, that will cause the model to rotate around the CG.A torque, also called the moment of the applied force, canbe caused by aerodynamic forces on fins or wings, an off-axis engine thrust, or any other force acting on the model.The strength of a torque is the product of the strength of the force and the distance of the force line from the CG.

Figure 2 defines the main axes and rotational motionsof a glider. A motion around the lateral axisthat causes the nose of the glider to rise or fall is called a

pitch . A motion of the glider turning to either side iscalled a yaw . A rotation around the longitudinal axis of the glider is called a roll .

Aerodynamic Forces

Four important forces act on an aircraft during flight, asshown in figure 3. Thrust is the force provided by the air-planes engine, usually by a spinning propeller or thereaction from jet or rocket engine exhaust. A glider, afterit has been launched to altitude, does not have a thrustforce acting on it. Weight , the force of gravity acting onthe mass of the aircraft, acts through the CG of the planein a downward direction. The aircraft must counteract theweight force in order to stay in the sky. The motion of theaircraft through the air produces an aerodynamic forcewhich we break into two components: Lift , which actsperpendicular (at right angles) to the aircrafts direction of motion; and drag , which acts directly opposite the direc-tion of motion through the air.

For a powered airplane flying at constant velocity at aconstant altitude, the upward pointing lift force is used tobalance the weight force. The thrust of the engine coun-terbalances the drag force (which tends to slow down theairplane) so that the aircraft can continue moving forward.This forward motion is important because it creates thelift force.

A glider has no thrust force to counteract the drag forcethat tends to slow down the glider. As a result, a glidercannot maintain a level flight at constant speed and alti-tude. An aircraft in steady gliding flight is alwaysdescending relative to the air around it. The glider isessentially trading off altitude to maintain its velocity.Figure 4 shows the balance of forces acting on a gliderdescending in a steady glide at constant velocity.

The glider follows a path which slopes at an anglebelow horizontal called the glide path angle , which wedenote by the Greek letter . The direction of the glidersmotion along the glide path is shown by the velocityarrow on the diagram. (Notice that the gliders longitudi-nal axis is not pointing along the direction of motion, but

2

wing

fuselage

nose stab

tail

fin

Figure 1 - Conventional Glider Configuration

figure 2 - Rotation Axes and Motions

Figure 3 - Aerodynamic Forces on an Aircraft

yawaxis

CG

pitchaxis

rollaxis

rightyaw

pitchup

leftyaw pitch

down

rightroll

leftroll

lift

drag

weight

thrust

velocity

Front engine boost glider

CG


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slightly above the glide path at angle , called the angleof attack ). The lift force, which always acts at a rightangle to the velocity through the air, is tilted forward fromvertical at an angle equal to . The lift and drag forces on

the glider add together as shown to form the resultantforce F, which balances the weight force. Because allforces on the glider are balanced there is no net force tocause an acceleration, so the glider will continue at con-stant velocity, moving along the glide slope at constantspeed (this is Newtons first law of motion in operation,by the way).

The angle of the glide slope depends on the ratio of thelift to the drag (L/D). The larger the lift-to-drag ratio is,the shallower the glide slope and the farther the glider willtravel horizontally before reaching the ground. Some full-sized sailplane gliders have L/D ratios as high as 40. Atypical model rocket boost glider may have a lift-to-drag

ratio around 3. Contrary to what you might expect, trim-ming your glider to fly at its maximum L/D will allow itto fly farthest horizontally, but it will not result in thegreatest duration (the length of time in the air). Durationdepends on the sink rate, or downward velocity, whichdepends on the glide angle and the glide speed.Maximum duration is achieved by trimming the glider tofly at a higher angle of attack, which results in a slowerglide speed that more than compensates for the slightlysteeper glide slope.

Lift and DragAs an object moves through the air, it experiences an

aerodynamic force. Lift is the component of that aerody-namic force which acts perpendicular (at a right angle) tothe direction of motion through the air. Air flowing pastthe wing of an airplane produces the lift that keeps theplane in the air. Lift is also the force that acts on the finsof a regular model rocket to keep it stable in flight.

Drag is the component of the aerodynamic force that actsin the same direction as the relative airflow. Any time lift isproduced, you also get drag its unavoidable. But an effi-cient glider wing will produce as much lift as possible withas little drag as possible. This is accomplished by usingwings with specially shaped cross-sections called airfoils .

Figure 5 shows a selection of different airfoil shapes.

Figure 6 shows an airfoil shape typically used on smallboost gliders. This airfoil is certainly not the most effi-cient shape possible (no single airfoil shape is best for allsituations) but it is fairly easy for modelers to produce thisshape. The distance between the leading and trailingedges is called the chord (or chord length) of the airfoil.The reference line drawn between the leading and trailingedges is also sometimes called the chord (or chord line).The maximum thickness of the airfoil is typically 5% to15% of the chord length. The high point of the airfoil islocated 25% to 35% of the chord length back from theleading edge. The leading edge may be rounded (asshown) or may be nearly sharp. The trailing edge has asharp taper.

How does an airfoil produce lift? Figure 7 shows anairfoil held at angle of attack ( ) to the relative wind coming at the air foil with speed v (this is equivalent tothe airfoil moving forward at speed v through still air).You may think that the air flow hitting the underside of

the tilted wing produces the lift force, but that is onlypartly correct. The angle of attack and shape of the airfoilcauses the air moving over the top of the wing to movefaster than the air moving over the bottom of the wing.

3

Figure 5 - Airfoil Shapes

flat plate (poor airfoil)

symmetrical (streamlined)

flat bottom

under cambered

convex - concave

high point

chord

25-35%of chord

roundedleading edge

maximum thickness 5-15%flat bottom

sharptrailingedge

Figure 6 - Typical boost glider wing airfoil

faster airflow,low pressure

slower airflow,high pressure

relative windvelocity

v

angle ofattack

Figure 7 - Airflow past airfoil

velocity

lift

F

weight

glide angle

horizontal

d r a g

g l i d e p a t h

Figure 4 - Balanced Forces on a Glider


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This results in a high pressure area under the wing and alow pressure area above the wing, resulting in the liftforce. In fact, most of the lift (about two-thirds) resultsfrom the suction of the low pressure area above the wing.

The amount of lift and drag produced by a wing isdescribed by the equations:

L = 1 / 2 CL A v2 (Eq. 1)

D = 1 / 2 CD A v2 (Eq. 2)

where is the density of air, A is the wing area, v 2 is thesquare of the air speed (v times v), C L is the lift coeffi-cient , and C D is the drag coefficient . The lift coefficientdepends on several factors including the wings airfoilshape, aspect ratio (defined later), and the angle of attack.Figure 8 shows how the lift coefficient (C L) changes withangle of attack for a typical boost glider airfoil*. Theangle of attack is measured between the chord line of theairfoil and the relative wind direction (the velocity vector)as shown in figure 9. As the angle of attack increases, C Lincreases and the wing produces more lift. Notice that theairfoil shown produces lift at a zero angle of attack, and

even produces some lift at small negative angles of attack.This effect is typical of cambered airfoils which have anupward bulging shape. At a certain (negative) angle of attack the airfoil produces no lift; this occurs when the

zero lift line (see figure 9) is parallel to the relative wind.The zero lift line can be approximated by drawing a lineconnecting the trailing edge to a point halfway betweenthe upper and lower wing surfaces at 40% of the chordlength back from the leading edge (see figure 9).

A symmetrically shaped airfoil (even a flat plate air-foil) will also produce lift when held at an angle of attack to a relative airflow; but these profiles producemore drag for a given amount of lift than do properlyshaped cambered airfoils. A symmetrical airfoil produces

zero lift at a zero angle of attack (its zero lift line runsalong the line of symmetry).

The lift produced by a wing increases as the angle of attack increases but only up to a point! Beyond a cer-tain angle of attack, called the stall angle , the lift pro-duced decreases (notice how the C L plot drops off forlarger ). The wing is said to stall (this has nothing to dowith the engine of an airplane stalling; this is an aerody-namic effect). At the same time the lift of the stalledwing is decreasing, the drag on the wing continues toincrease rapidly (see figure 8). This is a bad situation foran aircraft; if not immediately corrected, the drag willslow the aircraft, the lift will decrease further, and the air-craft will plummet.

What causes the wing to stall? Remember, it is thepattern of airflow past the wings airfoil that produces lift.When the angle of attack of the wing increases beyond thestall point, the layer of air flowing over the top wing sur-face separates, creating a large turbulent wake behind thewing. The low pressure area above the wing which pro-vides most of the lift is destroyed and the lift drops, asshown in figure 10. A smoothly rounded leading edgecan help keep the boundary layer attached, which is onereason good airfoils have this feature. A flat plate airfoil

has very poor stall characteristics.

Sources of DragWhere as lift is a desirable aerodynamic force, drag is

usually undesirable, and aircraft builders (including modelrocket hobbyists!) take great pains to reduce drag wherev-er possible. Lets look at some of the causes of drag andpossible methods of reducing it.

Equation 2 shows that drag depends on a dimension-less drag coefficient (C D). For most subsonic aircraft thedrag coefficient is described by the equation:

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

0.20

0.16

0.12

0.08

0.04

0.00

Figure 8 - Lift and Drag Coefficient vs. Angle of Attack

(*Note: The C L and C D plots in this report are presented only to illustrate airfoil behavior.Very little experimental data exists for thin airfoils in the Reynolds number rangeapplicable to boost gliders. Take these plots with a grain of salt)

Figure 9 - Zero-lift line of an airfoil

stall

CL

CD

D r a g

C o e

f f i c i e n

t ( C D

)

L i f t C o e

f f i c i e n

t ( C L

)

Angle of Attack ( )

o zero lift angle

4 0 % o f c h o r d

angle of attack of zero-lift line

angle

ofattack

relativewind

chordline

zero-liftline1 / 2

1 / 2

Boundary layer separates

Largeturbulentwake

Figure 10 - Airflow past stalled wing-4 0 4 6 8 10


5/16


6/166

wing, and e is less than 1 for other planforms. Althoughthe tapered wing planform is slightly less efficient thanthe elliptical planform, it is much easier to make, so manyboost gliders use tapered wings. The sharply swept wingsused on high-performance fighter planes are not very effi-cient for use on model rocket boost gliders; swept wingsare great at supersonic speeds, but not at slow gliderspeeds.

The shape of the outer edge of the wing tip alsoaffects the induced drag. A rounded edge makes it easierfor air to flow around from bottom to top and increasesthe drag. Sanding the edge of the wing tip either sharp orflat will inhibit airflow that creates the tip vortex andreduces the induced drag.

You may wonder why we cant simply glue small

plates to the wing tips to inhibit the vortex airflow. In theo-ry, this can work; in fact, some newer passenger jets havesmall vertical surfaces on the wing tips for this purpose.But, in practice, your tip plates would probably increase theparasite drag much more than they reduce the induced drag.

Glide StabilityA glider must fly at the proper attitude , or orientation

in space. Stability on the pitch axis keeps the wing at thecorrect angle of attack to produce lift. Yaw stability isrequired to keep the glider pointed forward into the rela-tive wind. Roll axis stability is required to keep the winghorizontal so the lift acts upward. A free-flight boostglider must have this stability built into the design so thatit will automatically enter a stable glide after transitionand recover from disturbances like wind gusts.

Pitch Trim and StabilityThe wing of a boost glider is not stable in pitch all by

itself. The lift of the wing is assumed to act through apoint called the aerodynamic center which is located nearthe quarter chord point (1/4 of the chord length back from

the leading edge). The center of gravity (CG) of the wingis typically behind the aero-center, resulting in a pitchingmoment that would cause the wing to pitch up and tumbleend-over-end. The most common way to achieve pitchstability is by adding a second, smaller aerodynamic sur-face called the horizontal stabilizer (or stab ). The stabproduces an opposite pitching moment to counter thepitching moment of the wing (see figure 15). The glideris said to be trimmed (or balanced) in pitch when theupward pitching moment due to wing lift (given by x wtimes L w) is equal in strength to the downward pitchingmoment of the stab (given by x s times L s).

When trimmed and in a steady glide, the wing of theglider will be flying at a certain angle of attack. Whatthat angle of attack will be depends on the relative inci-dence angles of the wing and stab. The incidence angle issaid to be positive if the leading edge is higher than thetrailing edge, relative to some reference line.

To understand the situation better, lets start with theequation describing the trim condition

xwLw = x sLs (Eq. 4)

then substitute in the definition of lift from equation 1 and

Figure 15 - Pitch trim of a glider

LW

wing

CG

xW xS

LS

stab

stabincidence(negative)

Pitch trimmed when x WLW = x SLS

Figure 14 - Wing planforms

elliptical

tapered

rectangular

swept

delta

A = area

A = areaS = span

C =

c h o r d

S = span

in general:

A.R. = S2

A

for rectangular wing:

A.R. = S2

= S2

= S

A SC C

Figure 13 - Aspect ratios of wings

A.R. = 12

A.R. = 2


7/16 7

cancel terms giving

xwAwCLw = x sAsCLs (Eq. 5)

where A w and C Lw are the area and lift coefficient of thewing, A s and C Ls are the area and lift coefficient of thestab, and x w and x s are the distances that the wing lift andstab lift are acting from to the CG. Equation 5 shows usthat there are two ways we can trim a glider. The firstmethod involves changing the position of the center of gravity. Moving the CG changes the values of x

wand x

s(see figure 15). In practice, we change the position of theCG by adding or removing weight from the nose or tail of the glider.

The second method of trimming the glider involvesadjusting the relative incidence angle between the wingand stab. If the wing and stab are at different incidenceangles, they will meet the relative wind at different anglesof attack. Remember that the lift coefficient depends onthe angle of attack (see figure 8). So, we can adjust thelift coefficients (the C L values) to balance equation 5 bychanging the relative incidence (and therefore, the anglesof attack) of the wing and stab. In practice, we usuallychange the incidence of the stab by warping or shimmingthe trailing edge of the stab up or down as needed. Somemodels have adjustable control surfaces, called elevators ,on the trailing edge of the horizontal stabilizer to adjustthe trim.

So far we have discussed the pitch trim of the glider,but how can we insure that the glider will be stable ? Tobe stable in pitch, the glider must return to its trim orien-tation after a disturbance, such as a gust of wind, changesits attitude. If the angle of attack of the glider increases,the forces on the wing and stab must change in such away that the glider will pitch back down to its trim atti-

tude. If the angle of attack decreases, the changing forcesmust cause the glider to pitch up (see figure 16).

To insure that our gliders meet the stability conditionsshown figure 16 we must have the stab flying at a lowerlift coefficient than the wing (C Lw > C Ls) . Because thelift coefficient is related to the absolute angle of attack,this means the zero-lift line of the stab must be at a lowerangle of attack than the zero-lift line of the wing.

Why does having the zero-lift line of the stab at alower angle than the zero-lift line of the wing produce astable glider? To understand this, you must know that theslope of the C L vs. curve (see figure 8) is roughly the

same for all airfoils (it is actually slightly lower for thestab because it operates in the airflow wake of the wing,but close enough for this explanation). This means thatwhen the glider pitches up, the C L of the wing and stabincrease by about the same amount. But wont themoments produced by the wing and stab then increase bythe same amount? No, they wont, as we can see fromequation 5: When the glider is flying in trim attitude, thetwo sides of the equation are equal to each other. But,remember that we have built our glider so that C Lw > C Ls.This means that a greater fraction of the stabs pitching

moment comes from the x times A part of the term, sothe C L of the stab has a greater effective lever arm thandoes the C L of the wing. Therefore, when C Lw and C Lsincrease by the same amount, the pitching moment of thestab (with its greater effective lever arm) will increasefaster than the pitching moment of the wing, and the glid-er will pitch back down toward its trim attitude. An oppo-site situation occurs if a disturbance causes the glider topitch below its glide trim angle: The lift coefficients of both wing and stab decrease by about the same amount,but this causes the stabs pitching moment to decreasefaster than the wings, so the resulting moment causes theglider to pitch back up toward its trim attitude.

For convenience of construction, most boost gliders

have the wing mounted flat on the fuselage (at nearly 0incidence). The horizontal stabilizer is then typicallyattached to the glider with a negative incidence of 1 to 3degrees, depending on the design. This puts the stab at alower angle of attack than the wing, making the glider sta-ble in pitch. The negative incidence of the stab is some-times provided by gluing a shim under the trailing edge of the stab, giving some up elevator (see figure 17).

On a typical boost glider, the surface area of the stabis 30% of the wing area. The front of the stab is locatedabout 2.4 times the wing chord length behind the back of the wing (assuming wing and stab aspect ratios around 5).

The glide CG of a typical boost glider usually liesbetween 30% and 100% of the chord length behind theleading edge of the wing. Although it is possible to trima glider to fly with the stab at 0 incidence to the wing(because the zero-lift line of the airfoiled wing is stillhigher than the zero-lift line of the symmetrically airfoiledstab), these zero-zero gliders have difficulty pulling outof a dive, so are best avoided.

Some glider designs locate the horizontal stabilizingsurface ahead of the wing (see figure 18) where it iscalled a canard . To be stable, the zero-lift line of thecanard must have a positive incidence relative to the wing

Figure 16 - Pitch moments on a stable glider

LW

trimangle

relativewind

Ls

Glider at pitch trimangle:

xWAWCLW = x SAS CLS(no net rotation)

Pitch angle above trimangle:

xWAWCLW < x S ASCLS(rotates nose down)

Pitch angle below trimangle:xWAWCLW > x S ASCLS

(rotates nose up)


8/168

(canard gliders are essentially gliders with small wings infront and large stabs in back).

Other glider designs, like the Space Shuttle orbiter, do

not have a separate horizontal stabilizing surface. Instead,they use the rear section of the wing as the stabilizer (seefigure 19). These gliders usually have movable control sur-faces, called elevons , on the trailing edge of the wing thatcan be adjusted to provide the negative incidence necessary

to keep the glider stable.

Yaw StabilityThe glider must be stable in yaw to recover from any

disturbances that would turn the glider to the left or right.Keeping the glider pointed straight into the relative windinsures that the wings will produce the same lift, and itminimizes the parasite drag on the model by keeping thefrontal area presented to the airflow at a minimum. Yawstability is provided simply by adding a vertical stabilizer (or fin ) at the rear of the model. The fin provides stability

in the same way the fins of a regular model rocket providestability: If the model yaws to a non-zero angle relative tothe airflow, the lift force (which points sideways in thecase of the fin) creates a moment which rotates the modelback into the relative wind. This works if the lateral cen-ter of pressure of the glider (as seen in the side view) islocated behind the CG of the model.

The fin must be large enough to provide yaw stability,but a bottom-mounted fin that is too large can interferewith the roll stability of the model (discussed next). Thefin should typically have an area that is 5% to 10% of themodels wing area. Most front-engine boost gliders havethe fin attached to the underside of the fuselage to keep itout of the exhaust blast of the motor. While this may look funny compared to the arrangement on regular airplanes,it works just fine.

Roll StabilityMost free-flight gliders are stabilized in roll by using

dihedral . The wings are attached to the fuselage at anangle (called the dihedral angle ) with the wing tips higherthan the center of the wing (see figure 20).

The wing may use a simple V dihedral or may use amultiple-panel polyhedral.

The greater the dihedral angle, the greater the roll sta-bility of the model, but too much dihedral is undesirable.Dihedral creates roll stability by sacrificing some of thewing lift; only the vertical components are available tosupport the glider against gravity. A dihedral anglearound 15 degrees is typically used on boost gliders.

Figure 21 shows how dihedral produces roll stability.If a disturbance causes the model to roll, part of the liftforce is directed to the side, which causes the model toaccelerate toward that side. This sideways motion iscalled a sideslip . As seen from above, the models actualdirection of motion is now a combination of its forwardand sideslip velocity. The relative wind is now approach-ing the glider at an angle. Figure 21c shows the glider as

Figure 20 - Wing dihedral

winglift

dihedralangle

simple Vdihedral

three panelpolyhedral

four panelpolyhedral

LV

LH

FRONTVIEW

Figure 17 - Shimming trailing edge of stab for negative incidence

stab

fuselage

fin

shim

nose

canardwing

d i r e c t i o n o f f l i ght

Figure 18 - Canard Glider

Figure 19 - Delta wing glider with elevons (no separate horizontal stabilizer)

Un ited S ta

te s

elevon

elevon

wing


9/16

seen from the direction of the relative wind. Notice thatthe wing on the side toward the sideslip is facing the windat a higher angle of attack than the other wing. This wing

(which was on the side of the glider that had rolled low)thus produces greater lift which causes the glider to rollback into position with the wings horizontal. Meanwhile,the yaw stability of the glider points it into the new rela-tive wind, and the model is back in a stable glide. Thislast step is why the gliders fin must not be too large: Alarge fin may correct the yaw disturbance caused by thesideslip before the dihedral can correct the roll.

A swept wing planform also produces roll stability, asseen in figure 22. When the glider rolls and develops asideslip, the air flows over the gliders wings at differentangles. The airflow over the lower wing is closer to per-pendicular to the wings leading edge, resulting in a faster

relative airspeed over the wing chord. This producesgreater lift on this wing, so the model rolls back to hori-zontal. Ten degrees of wing sweep produces about thesame roll stability as one degree of dihedral.

Turns and Spiral DivesBoost gliders are generally trimmed to fly in a gentle

turn which keeps the glider over the launch area for alonger time and makes it easier to recover. The turn isusually produced by adding a small amount of weight to

one wing tip of the glider. This causes a shallow bank , orroll angle, which directs the lift vector slightly to one side,producing the turn. A turn can also be produced if thestab is attached at a small tilt angle (no more than 2degrees from horizontal) so that a small part of its lift isdirected to the side. You must be careful not to overdothe turn: If the turn is too tight, a spiral dive can result.Figure 23 shows how this problem can develop.

When the glider is following a curved flight path, thestab meets the air at a higher angle of attack, relative tothe wing, than it does in level flight. This increases the

stabs pitching moment and tends to make the glider pitchdown. It also reduces the pitch stability of the glider(which depends of the relative incidence of the wing andstab). A glider requires more up trim in a turn than itdoes in level flight.

Even if the glider is properly trimmed for a gentleturn, it may run into trouble if a disturbance causes it toroll too far. If the model is too slow to correct the bank,the resulting tighter turn can cause the glider to nosedown and lack sufficient pitch stability to pitch up, result-ing in a spiral dive and crash. This can occur if the pitchstability margin is too small, if the dihedral angle is toolow, or if an oversized fin prevents the dihedral from cor-recting the roll.

IV. Boost StabilityDuring boost, a boost glider must be a stable ballisticvehicle just like our regular rockets. The center of pres-sure (CP) must be located far enough behind the center of gravity (CG) to produce the correcting moments needed tokeep the model pointed into the relative wind (pointed up,in this case). The profile of a boost glider is usually dif-ferent when seen from the top or from the side, so the CPpositions for pitch and yaw are usually in a differentplace; the boost glider must be stable on both axes.

Rear-engine boost gliders usually present few prob-lems on boost: The thrust line of the engine normallypasses through the CG and the rear mounted wings com-mon on these models make them act like large-finnedrockets. The pitch control surfaces (such as elevons) arenormally held flat during boost, so they produce no liftforces to affect the trajectory.

Front-engine boost gliders present several boost prob-lems, especially in pitch stability. The engine must bemounted off-center on a pylon so that the exhaust blastwill not damage the model, but this offset thrust line caus-es a strong pitch-down moment during thrusting. Keepingthe pylon low (about 1/2" for mini-engines and 3/4" forstandard engines) can minimize this effect. The wing and

9

Figure 21 - Roll stability from dihedral

a)

b)

c)

d)

FRONTVIEW L i

f t

horizontal component oflift causes sideslip

Glider as seen from direction ofoncoming air (exaggerated)

Sideslipvelocity

Forwardvelocity

netvelocitythrough

airgreater lift force on low wing rotates

glider back to horizontal

TOPVIEW

U S A

more lift

morelift

airflow direction due to sideslip

less lift

lesslift

Figure 23 - Effect of turn on glider trim

Figure 22 - Roll stability of swept wing

airflow (exaggerated)

stab at higher angle to airflow

GLIDER IN TURN


10/1610

stab can produce large amounts of lift at the high boostvelocities, so these models tend to loop or arc over in apitch-up direction. This looping effect can be minimizedby using a far-forward CG position (the front mounting of the engine helps in this regard) because this keeps thewing at a lower angle of attack.

V. Boost Glider PerformanceNot all boost gliders are designed with ultimate dura-

tion performance as the primary goal, just as most of ourmodel rockets are not designed to achieve ultimate alti-tude performance. Many model rocket designs are dictat-ed by aesthetic concerns; we want our models to look impressive! But we can improve the duration perfor-mance of any boost glider, even one built primarily for itscool looks, if we keep a few principles in mind.

The glide performance equation tells us how long ittakes a glider to descend through each foot of altitude:

where C L and C D are the coefficients of lift and drag, isthe air density, and W and A w are the weight and the wingarea of the glider. We want the value of t to be a large aspossible to give the greatest glide duration time.

The first term, C L3/2 /CD, shows us that we canincrease duration by increasing the lift coefficient, even if it means increasing the drag coefficient by a similaramount (because C L in the numerator has a higher powerthan C D in the denominator). This is why the maximumduration does not occur at the glide angle with the greatestlift-to-drag ratio (C L /CD), but occurs at a higher angle of attack. The drag increases at this higher angle (so the

glider flies slower) but value of C L3/2 increases even more,so the sink rate is less.

The factor W/A w, the weight divided by wing area, iscalled the wing loading . Anyone constructing a glider canimprove its duration performance by building it as light aspossible. Choose lightweight balsa for construction, donot use excessive amounts of glue, and do not finish themodel with a heavy layer of paint. There are limits, of course: If the model is structurally too weak, it will shred apart on boost.

A boost glider designer must also keep in mind that thetotal duration performance of the model depends on boostperformance (how high the glider boosts) as well as glideperformance. A feature that increases glide performancemay decrease boost performance (a large wing area is anexample). Another feature that improves boost performance(such as a slick finish) may decrease glide performance(due to the increased weight). Having to make trade-offs(between boost and glide performance, between low weightand sufficient structural strength, between roll stability andmaximum lift, etc.) is the essence of engineering.

VI. Boost Glider DesignsDesigning a boost glider that can streak skyward atover 100 miles per hour as a rocket, and then descend atless than 10 miles per hour as a stable glider, is not aneasy task. Modelers have developed several methods toaccomplish the transition from rocket into glider, and wewill briefly describe the most common techniques. Thesemethods fall into two general categories: Shift the centerof gravity; or shift the aerodynamic surfaces. Many boostglider designs use a combination of these methods.

Parasite GlidersThe simplest form of boost glider is the parasite glid-

er. One or more gliders are attached to a carrier rocketand released in flight. An example is the Estes SpaceShuttle kit. The glider must not interfere with the stabilityof the carrier rocket; this can be accomplished in threeways:

1) Use a small glider on a large, very stable rocket.The forces on the glider are small and easily overcome bythe fins of the large rocket.

2) Mount the parasite glider in a way that minimizesthe disturbance caused by the glider. The glider can bemounted at a slight down pitch so the wing will not pro-duce lift during boost. The glider must also be positionedin such a way that the forces which do occur will not

interfere with the stable CG/CP relation of the model (theglider should be located at or behind the CG of the rocketin boost configuration).

3) Arrange multiple gliders so that any disturbing liftforces cancel each other. The Estes A.R.V. Condor kit isan excellent example.

A parasite glider is typically attached to the carrierrocket with a simple hook near the nose of the glider (seefigure). When the ejection charge blows the nose cone off the carrier rocket, the reaction force causes the carrier tosuddenly slow down, allowing the inertia of the parasiteglider to slide it free. Supports (or two more hooks) are

Figure 24 - Boost stability

rear engine front engine

CG

CGCP

thrust line

stab lift

wing lift

CG

winglift

stab lift

thrustline

Thrusting - wing and stab liftbalance thrust pitch

Coasting - stab liftbalances wing lift

(Eq. 6)t =

CLCD

3/2 12

WAW


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usually needed at the rear of the glider to keep it from tilt-ing into a roll when mounted on the carrier.

Shifting the Center of GravityEjecting Engine

Many front-engine boost glider designs use the CG

shift that results from jettisoning the engine to make thetransition from boost to glide configuration. With theengine in place at the front of the model, the CG is farforward of the center of pressure, so the model is a stablerocket. When the engine is jettisoned, the CG shifts back-ward to the position of stable glide trim.

Early boost gliders simply ejected the engine casing andallowed it to free fall, but this is now considered bad form.For increased safety, a streamer should be attached to the

ejected engine. This can be accomplished by using an enginemount tube that is large enough to allow a streamer to bewrapped around the engine; or, the engine could pull anattached streamer out of an additional side tube.Pop Pod

The Pop Pod design is an improvement of the simpleengine ejection method for front engine gliders. At ejec-tion, the entire engine pod separates and deploys arecovery system. This removes as much excess

weight and drag as possible from the glider toreduce the wing loading and increase glide per-formance. This also eliminates the safetyhazard of the free-falling engine casing.The hook system that attaches the pod tothe glider must be secure enough to keepwind from jiggling the glider off whilethe model is on thelaunch pad, but itmust be loose enoughto pop free with agentle tap on the front of the pod. The reaction force

from ejecting the nosecone and recovery sys-tem is what separatesthe pod at ejec-tion.

The shock cord and stream-er (or parachutelines) of the pop pod should be kept short or they may snagon the glider after ejection. This results in the tangled podand glider fluttering out of the sky, and is known as a Red Baron .

Ejecting Internal Power PodRear-engine boost gliders often shift the CG from

boost position backward to glide position by ejecting aninternal power pod out the back of the glider. The podcontains the engine, additional weight up front for booststability, and a recovery system. The recovery system isoften stowed alongside the motor tube of the power pod

11

Pop Pod

G l i d e C G

B o o s t C G

Pop Pod Boost Glider

Typical Pop Pod Hook System

Glider

Glider

Fuselage

hookand sidesupports

PopPod

A T M O S P H E R I C R E S E A R

C H V E H I C L E

USA

LOCKDOWN

HOT GA S

A T M O S P H E R I C R E S E A R C H V E H I C L E

1 2

Estes Space Shuttle Estes A.R.V. Condor

Parasite Glider Attachment

launch lug rear wing supports

1/8 dowelon balsastand-off

Ejecting internal power pod

recoverysystem

engine

power pod

glider

nose weightfor boost

U n i t e d S t a t e s

Ejecting Engine to shift CG


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when the pod is inserted into the main body tube of theglider.

Ejection of the power pod often releases aero-surfacesof the glider (such as the elevator) to move into glide set-ting. The pod may also have fins attached at the rearwhich provide additional boost stability by moving the CPbackwards.

Slide PodRecall that a rocket glider is a boost glider which does

not eject or drop parts. One type of front-engine rocketglider that uses CG shift is the slide pod design. Theengine pod is held in a forward position during boost, butat ejection the pod is released and elastic bands pull itbackward, moving the CG to glide position.

Control Surface ShiftMany boost gliders employ a movable control surface,

usually the elevator, that is set in one position for boost andwhich flips to a new position for glide. This method is com-monly used on rear-engine designs: A string or hook on thepower pod holds the elevator flat during boost. When the ele-vator is released, an elastic line or spring system moves theelevator up. The stop that halts the elevator in glide positionmay be adjustable (a nylon screw or bendable wire) to allowtrimming the glider.

Radio-controlled boost gliders essentially use this samesystem: The pilot holds a down-elevator setting to counter thegliders tendency to loop up during the high speed boost, thentransitions to glide settings as the glider slows near apogee.

Variable GeometrySome boost gliders and most rocket gliders use vari-

able geometry to transition from boost to glide by moving

the aerodynamic surfaces (usually the wing). These glid-ers may use a latching mechanism to hold the surfaces inplace until ejection, but often use burn string actuation (athin string holds the surfaces in boost position until theengines ejectioncharge burns throughthe string). Elasticbands or a springmechanism then shiftthe surface to glideconfiguration.Swing Wing

On a swing wingglider the wings pivotback for launch. Thismoves the center of pressure of the modelrearward for a stableboost. It also reducesthe aerodynamicforces on the wingsduring the high

speed boost, allow-ing more efficienthigh aspect wingsto be used. TheEstes Tomcat is anexample of a swingwing glider.Slide Wing

The wings of a slide wing gliderare mounted on arail or box that canslide along the fuse-lage. When heldback in boost position,the wings move the CPof the model well back of the CG. Elasticbands pull the wingspulled forward at ejec-tion for glide. The glid-er can be trimmed byadjusting where thewings stop.Flop Wing

The wing of a flopwing glider has ahinge at mid-span thatallows the outer half of the wing to be folded under the root half during boost.When folded in boost position, the wing has a symmetri-cal airfoil shape, so it produces less lift to disturb theboost. The wing area is effectively reduced (it is now athick wing with smaller surface area) so the CP of themodel is moved back for boost. Reduced flight stresseson boost also allow higher A.R. wings (when unflopped)to be used.

Slide pod rocket glider

Swing-wing rocket glider

Slide-wing rocket glider

glideposition

boostposition

glideposition

boostposition

glideposition

wing tipfoldsunder forboost

glideposition

boostposition

Elevator held flat by powerpod during boost

elasticthread pullselevator upafter ejectionof power pod

Control surface shift

Flop wing rocket glider


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Scissors WingThe wing of a scissors wing glider has a central pivot.

The wing is aligned along the body tube for boost andpivots out perpendicular to the body for glide.

Flex-Wing GlidersFlex-wing

gliders have aero-dynamic surfacesmade from thinplastic filmattached to arigid framework.Some glidershave used a flexi-ble-wing on aconventionalglider configura-tion, but mostflex-wings usethe Rogallodesign with threespars. Theseflex-wings can befolded up andinserted inside a carrier rocket. After ejection, springcoils in the nose spread the spars and plastic film intoflight position. Flex-wings have very low wing loading.

VII. Radio Controlled Boost GlidersRemotely controlling the flight of your boost glider is

a truly exciting experience, but it is not an activity that anovice can jump into and expect instant success. Beforeyou attempt to build and fly a radio-controlled (RC) boostglider you should have some experience building and fly-ing RC aircraft. A beginners RC sailplane is a goodplace to start learning the construction methods used andthe basics of RC piloting. If possible, get assistance froman experienced RC flyer before your first flight. Check with the local model airplane club; many model aviatorslove to help rookies learn the ropes. Some modelers alsorecommend learning to fly fast gas-powered model air-planes to learn how to handle the high speeds you will

encounter during boost. Unlike most boost gliders, whichare ballistic rockets during boost phase, most RC boostgliders must be guided during boost, which requires quick reactions. Taking care to build your model with all sur-faces properly aligned and true, and carefully followingthe directions, goes a long way to ensuring a safe boost.

Most beginner RC aircraft have elevator and ruddercontrol. If you want to fly an RC boost glider with eleva-tor and aileron control (such as the Estes Astro-Blasterand Strato Blaster) you may want experience flying an RCmodel with ailerons. (Note: A rudder is a movable con-trol surface on the trailing edge of the fin used to turn themodel in yaw; ailerons are control surfaces on the trailingedges of the wings used to control the roll motion of theaircraft.)

VIII. Construction and FinishingYou need to pay special attention to a few simple

rules when building a boost glider:1) Build the glider straight and true. The wing, stab,

and fin must all be properly aligned and positioned andfree of warps. On a regular model rocket, a fin misalign-

ment results in a wobble during boost and some lost alti-tude; on a boost glider, a similar misalignment can resultin a crashed model. Use your best craftsmanship.

2) Build light, especially when good duration perfor-mance is desired. Avoid excessive amounts of glue; large,heavy fillets are not required. Avoid heavy paint jobs.Sand all balsa parts smooth with 400 grit sandpaper andapply a light coat of spray paint. For optimum duration,some modelers prefer a finish of model airplane dope, oreven just bare balsa colored with marker pens. Othermodel airplane finishing methods are useful, such asmodel aircraft tissue applied with dope, and iron-on filmfor large gliders.

Boost gliders are built primarily from balsa, but balsavaries considerably in density. Use lightweight, warp-freebalsa. When sanding the wing airfoil, start with coarsegrit sandpaper (100 grit) to save effort, then progress tofiner 220 grit paper for final shaping. Placing the trailingedge of the wing at the edge of your work surface andusing a sanding block is helpful when shaping the longrear taper of the airfoil. Finish by sanding smooth with400 grit paper. For the thick wings of larger gliders, arazor plane can save a lot of time in the initial shaping.Very large boost gliders often use sheeted built-up wing orfoam core wings. The fuselage of small gliders may bemedium or hard balsa, but spruce is useful when extrastrength is desired.

IX. Flying Boost GlidersGlide Trimming

Be sure to trim your boost glider for proper glide beforeyou launch it! To trim , or balance, the glider you will needto glide test it. This is best done in an open area, preferablywith tall grass to cushion any hard landings, in calm condi-tions. If the model is a rocket glider, perform the glide testswith an expended engine casing in place.

Give the glider a gentle toss into the wind (if any),

Scissors wing glider

Rogallo flex-wing glider (folds inside carrier rocket)

glideposition

boostposition

spar

nosesprings

plasticfilm


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aiming just below the horizon. Throw in a straight line,with the wings level. You should release the model at itsglide speed; if you throw too hard, the model will pitchway up, stall, and land hard.

Make several test glides and watch how the modelbehaves (see figure). If the glider stalls or dives, youmust adjust the trim, either by adding/removing trimweight, or by warping the trailing edge of the stab.Weight can be added easily by sticking small pieces of

modeling clay onto the glider.If the glider pitches up repeatedly (stalls), the CG is

too far back. Add a small amount of weight to the noseof the glider (or reduce the incidence of the stab; set it forless up-elevator). If the glider dives, remove noseweight or add weight to the tail (or increase the incidenceof the stab; set it for more up-elevator). If the gliderturns sharply to the right or left, add weight to the wingtip on the outside of the turn.

Repeat the glide tests and adjust the trim as neededuntil the glider follows a smooth glide path. If the gliderdoes not turn, add a small amount of weight to one wingtip until the model glides in a very gentle turn (not tootight!), then recheck the glide trim. Balsa surfaces canwarp over time so you should check the gliders trimbefore each flying session.

FlyingFly your glider with recommended engines. In gener-al, low thrust engines are best because they avoid putting

too much stress on the wings. Most glider designs havehigh drag (compared to normal rockets) so they requireshort-delay engines.

When launching front-engine models, the micro-clipscan snag the tail of the glider. To avoid this, set up anumbilical tower (a stake or other support) and tape thelaunch control system wire to the tower so that the clipswill swing away from the model after ignition (see fig-ure).

If the boost glider separates into multiple pieces, havesomeone assigned to watch and recover each part if possi-ble. Watch the glide of the model in flight; it is easier tospot small trim errors during an actual flight than it is on ahand toss. If the model does not transition into glide (andit survives anyway), increase the incidence of the stab(more up-elevator) and re-trim the glider using trimweight. If it turns too sharply and dives to the ground (aspiral dive) add weight to the outside wing tip to decreasethe turn.

During glide, your model may encounter rising bub-bles of warm air, called thermals , that can greatly increasethe duration. Remember, a glider in a stable glide alwaysdescends relative to the air around it. But if that air is ris-ing faster than the glider is descending, its possible forthe glider to rise relative to the ground.

X. Other Types of Model AircraftModelers interested in boost gliders can learn more

about how gliders work by building and flying other typesof model aircraft, such as the Estes Light Gliders . Unlikeboost gliders, which have to be built strong enough to takethe stresses of a rocket launch, the Light Gliders are con-structed of lighter materials, so their glide performance issuperior and they are less likely to be damaged in a hardlanding. They are perfect for making flight after flight topractice glide trimming and test out your understanding of glider aerodynamics.Hand Launch Gliders

Hand launch, or chuck gliders , are thrown high intothe air where they settle into a gentle glide. The design

Umbilical tower prevent clips from snagging tail Hand launch of a glider

microclips launch

wires

umbilicaltower

frontengineboostglider

rightspiralclimb

throw hardat steepangle withright bank

rolls out into glide

glidertrimmedfor leftturn

Glide testing trim of a glider

stall

divegood glide


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used in high-performance front-engine boost gliders isbased on hand launch glider technology.

The stab of a chuck glider has more incidence (moreup-elevator) than a typical boost glider to enhance theirglide stability (a boost glider with this much stab inci-dence would tend to loop violently on boost). To success-fully fly a hand launch glider, trim it for a steady glidewith a left turn. The model is thrown hard, with an over-hand or side-arm motion, upward at a steep angle with thewings banked to the right. You are trying to achieve aright spiral climb trajectory. As the model slows down,the dihedral and left-turn tendency will level out themodel and transition it into a steady glide. This takespractice, so dont be discouraged if it doesnt work rightaway. (If you throw left-handed, reverse the directionsabove.)

Tow Line LaunchA glider can be towed into the air using a string, just

like you would tow a kite into the air. A loop on the endof the string slips over a backward facing hook on theunderside of the glider. When the glider reaches peak altitude and the tow line is allowed to go slack, the dragon the line (a parachute or streamer is attached to the lineto increase this drag) slides the loop off of the hook andthe glider flies free. Radio-control sailplane gliders arelaunched in a similar manner with elastic tubing or awinch motor providing the pull on the line to tow theselarger gliders into the air.

Rubber-band Powered Gliders

A twisted rubber band supplies the energy to rotatethe propeller that provides the thrust force that causes rub-ber powered gliders to climb to altitude. Then theyreturn to the ground as gliders.

XI. ReferencesTR-11 Aerodynamic Drag of Model Rockets , Dr. Gerald

M. Gregorek. (Estes Industries)

TR-7 Front Engine Boost Gliders , Original Edition,Gordon Mandell.

TR-4 Rear-Engine Boost -Gliders , Original Edition,Gordon Mandell.

Available from the National Association of Rocketry TechnicalServices (NARTS), P.O. Box 1482, Saugus, MA 01906:

Basic Glider Stability , Robert Parks.

Basic Design Rules for Boost and Rocket Gliders ,Dr. Gerald Gregorek.

Available at your local library: Textbooks on basic andadvanced aerodynamics. The ones I happened to use are:

Theory of Flight , Richard Von Mises, Dover Publications,New York, 1959.

Foundations of Aerodynamics , Arnold Kuethe and Chuen-Yen Chow, John Wiley & Sons, New York, 1976.

Tow Line glider

Rubber-band powered glider


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Technical Report Boost Gliders

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