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 distanc