★ The Modern Sailplane



(1) Fiberglass
– Whilst being adequately strong, are quite flexible, and often bend to what appears to be an alarming amount.
(2) Grass Reinforced Plastic (GRP)
– As plastic resins reduce in strength with heat, all fiberglass sailplanes are white on the upper surfaces in order to reflect heat.
– Colored markings to increase visibility may only be carried on the extremities of the structure.
○ Construction
– The skin carries all loads and becomes the only structure, is capable of being molded to accurate tolerances and superb finish.
– An increase in performance that may only be achieved in wood or metal structure at much greater expense.
★ Three Forces

– It steady flight, the total reaction force balances the weight.
– The point about which the sailplane would balance is called the center of gravity designed to be close to same point at which the total lift force acts ; the center of pressure.
– Naturally different pilot weights, and different airspeed, will cause small changes in the position of the two centers.
– The sailplane is designed so that the center of gravity is always ahead of the center of pressure.
– The tail-plane is designed to make a small down load which can be varied to achieve a balance at different weights and speeds.
★ Airfoil

– The wing cross section.
– Its shape is designed to provide only a little resistance as it moves through the air, but at the same time, provide a pressure difference in the airflow over the upper and lower surface.
– The air pressure difference results in an upward force being exerted on the wing.
– Almost all gliders use an asymmetrical airfoil in which the upper camber is greater than the lower camber.
– This characteristics of an airfoil section produces good lift at slow airspeed.
(1) Leading Edge
– This part of the airfoil meets the airflow first.
(2) Trailing Edge
– This is the portion of the airfoil where the airflow over the upper surface rejoins the lower surface airflow.
(3) Camber
– The curve of an airfoil section from the leading edge to the trailing edge.
(4) Chord Line
– A straight line connecting the leading edge and trailing edge of an airfoil.
– An imaginally straight line between the leading and trailing edges of an airfoil section.
★ Wing Lift

– The force which supports the weight of the aircraft enabling it to fly.
– The amount of lift a wing produces depends on the airspeed and the angle of attack.
– The faster the airspeed, the greater the lift.
– The greater the angle of attack, the greater the lift – but only up to a critical angle.
– The net force developed perpendicular to the relative wind.
(1) Angle of attack

– The angle at which the wing meets the air.
– The acute angle between the chord line of an airfoil and the resultant relative wind.
(2) Relative wind

– The airflow relative to an airfoil.
-Parallel to and opposite the flight path of the airfoil.
○ Bernoulli’s Principle (Swiss) (80% of effect.)

– As the velocity of fluid (or air) increases, it’s internal pressure decreases.
– A positive–pressure lifting action from the air mass below the wing, and a negative pressure lifting action from lowered pressure above the wing.
○ Newton’s Third Law (20% of effect.)
– The force of equal and apposite reaction pushes against the action pushes.
– The air stream strikes the relatively flat lower surface of the wing when inclined at a small angle to its direction of motion, the air is forced to rebound downward and therefore causes an upward reaction in positive lift.
– While at the same time air-stream striking the upper curved section of the “leading edge” of the wing is deflected upward.
○ Center of Pressure = “CP”

– Conversely, the center of pressure moves ahead of the center of gravity at high angle of attack.
★ Stalling

– Beyond a critical angle, the smooth flow of air over the surface breaakc down and becomes turbulent.
– It always occurs when the wing is at its critical or stalling angle, regardless of weight or airspeed.
– The smooth air flow over the wing can be upset by trying to fly at too great an angle of attack, usually achieved by flying too slowly, but can be brought on by a very abrupt pull up, or very steeply banked turn.
(1) Stalling Speed

– For the same weight, the sailplane in straight flight will always stall at the same speed.
– A stalling speed will be increasing by
– Increasing the weight,
– Increasing the load factor,
– CG moving to the forward, and
– when wet.
(2) Aileron Stall

– Coarse aileron use at, or near, the stall can induce the side where the aileron goes down to stall and cause the wing drop sharply to the side.
(3) Lateral Damping
– The tendency of a glider to resist movement in the rolling plane, because of the increased angle of attack (and hence increased lift) of the down going wing.
– Loss of lateral damping is the primary cause of the one set of autorotation.
○ Recognition

– A turbulent feel from the controls.
– Buffeting
– Controls become excessively slow in response.
– It becomes quiet.
– Increased back pressure required.
– Wing – drop.
– The nose drops.
– A rapid loss of height.
○ Recovery

1.Stick Move forward.
2.Accelerate Until normal control be restored.
3.Wings level.
4.Pitch up To normal attitude.
– Contrary to what we would at first imagine, holding the stick back to raise the nose does not effect a recovery as this continues to keep the wing at the critical angle, and therefore it remains stalled.
– The aircraft goes into a steep descent, as long as the wing is held at a high angle of attack (by keeping the stick back) it has high drag, hence no great speed is achieved.
– To effect a recovery the angle must be reduced, the stick must be moved forward.
– Immediately on doing recovery the sailplane will accelerate and normal control will be restored very rapidly.
○ Prevention
– To recognize the situation and be able to recover with minimum loss of height.
Most sail plane have warning signs of an impending stall.
The alert pilot should acquaint himself with each types warning features, and be able to take the correct action if necessary.
– It is normal practice to always use a speed with a good margin above the stall when near the ground, known as a “safe speed near the ground.”
Safe speed is calculated as 1.5 times the stalling speed.
– When thermalling, the efficient speeds to use are often within five knots of the stall speed.
– Gusts can cause a temporally variation well in excess of the stall speed.
Thus it is not unusual to inadvertently stall.
– The recommended circuit speed for each type is usually at safe speed or slightly faster.
★ Spinning

– Where one wing has stalled before the other and the aircraft tries to yaw and roll towards that wing.
○ Spin stage
1. Slow pre-stall.
2. Stall.
3. Stall with wing drop.
4. Stall with commencement to spin. (Incipient spin)
5. Full spin. (More than one complete turn)
– Modern aircraft have very docile stall and spin characteristics and can be recovered from any stage.
○ Autorotation
– The loss of lateral damping leading to one wing stalling and the commencement of rotation in the direction of falling wing.
– Due to the large increase in angle of attack as this “inner” wing drops, with no lateral damping to stop it, the angle of attack increases even further, the drag increase is very large and a continuous rotation is encouraged.
– The “over” wing remains virtually un-stalled.
○ Recovery
1. Rudder. Apply full opposite to the direction of turn.
2. Stick Forward until the spinning stops.
3. Ailerons Central.
4. Spinning stop Check.
5. Rudder Center-rise.
6. Wings Level.
7. Pitch up Carefully from the resultant dive.
– The spinning will not stop on the application of rudder.
– It is the forward movement of the stick that un-stalls the wing, which stops the spinning.
★ Skidding & Spin

1. To compensate for over banking
(R)Aileron – UP
(L)Aileron – DOWN
2. While left turn
(R)Wing speed – Fast
(L)Wing speed – Slow
3. The angle of attack
(R)Wing – Small
(L)Wing – Great
4. When skidding airflow comes from out side
(R)Great angle of attack and lift wing is going up.
(L)Small angle of attack and lift wing is going down.
5. To compensate for increasing over banking.
(R)Aileron more up and still producing some lift.
(L)Aileron more down and exceed critical angle
– A spin toward the low wing, if you were to stall a sailplane in a skidding turn.
★ The total reaction of two forces

– Increased rearwards with respect to the airflow due to the airs resistance to the wing’s movement.
– For analysis we break the total force into components, one at right angle to the direction of travel, called “Lift”, and one in the opposite direction to the travel, called “Drag”.
★ Drag
– The parts of the glider have drag, so while lift comes only from the wing, drag is produced by the whole aircraft.
– A force opposing the motion of a body through the air.
○ Induced Drag

– The part of total drag which is created by the production of lift.
– In separable from the process of producing lift from the wing and it is proportional to the angle of attack of the wing.
– The drag is induced by the lift-producing process.
– Induced drag gets less as the aircraft speed increases, the opposite effect to that accurring with parasite drag.
○ Wing Tip Vortex

– One of the major drag producing areas are wing tips.
– There, the air tries to flow around the tips in order to equalize the top and bottom, surface pressure differences.
– The continuation of this flow and the forward speed creates a vortex at each wing tip.
– To minimize
– The wing are tapered to make the tips as small as in practical.
– Various shapes are used to reduce it as well.
– Some aircraft have vertical winglets as well to reduce vortices.
○ Parasite Drag
– That part of total drag created by the form or shape of aircraft parts.
– The aircraft speed increases approximately as the square of the speed.
(1) Form Drag
– The drag caused by the shape.
– To minimize
– Making the non-lifting parts.
(2) Skin Friction Drag
– Any roughness of the aircraft skin.
– To minimize
– Keeping the surfaces as smooth.
(3) Profile Drag
– Form + Skin friction drag.
(4) Interference Drag
– Combines the effects of form and skin friction drag. (Turbulence)
– To minimize
– Selecting the shape of components and positioning.
○ Total Drag

– The minimum drag and the highest L/D ratio (L/D Max) of a glider occur at the angle of attack and airspeed where the parasite drag and the induced drag are same.
– A glider will travel the maximum distance through the air when it is operating at an airspeed that produces the L/D max.
★ L/D Ratio

– The L/D ratio of a glider is an aerodynamic function that is determined by the angle of attack and is not affected by the weight of the glider.
(The L/D max always occurs at the same angle of attack.)
○ Best Glide Speed

– The maximum horizontal distance through the air can be obtained during the glide.
– With an equilibrium in the forces, the path of the glider in terms of horizontal distance covered / height lost, is equal to the lift / drag ratio.
– Thus the gliding angle is the flattest when lift / drag is at maximum corresponds to a definite angle of attack, which we must fly at achieve this performance.
– As we do not have an angle of attack gauge, we find that for the normal weight range, airspeed corresponds closely to angle of attack and this is used as pur glide for performance.
★ Wing Plan-form
(1) Elliptical wing
– A minimum of induced drag.
– Stall characteristics are inferior to the rectangular wing.
– Difficult to construct.
(2) Rectangular wing
– Stall first at the wing root and provides stall warning, aileron effectiveness, and quite stable.
– Low cost. (Easier to build)
– More drag.
(3) Taper wing
– Desirable weight and stiffness.
– Not as efficient aerodynamically as the elliptical wing.
– Very good stall characteristics when designed with a wash out.
– Decrease in drag.
– Increase a lift. (Aspect ratio)
(4) Sweep forward
– Moving the lifting area forward.
– Allow for a small change in CG whether flying solo or with the rear seat occupied.
★ Washout

– Most sailplane have a small amount of twist, or washout, built in to wing.
– This places the tip at a lesser angle than the wing root.
– The critical angle is reached first by the inner portion of the wing so that only a portion of the wing stalls.
– This allows the tip portion of the wing to remain un-stalled and also keeps the ailerons functioning effectively.
★ Aspect Ratio

– The ratio of wing span to wing chord.
(1) High Aspect Ratio
– Decrease the drag at high angle of attack.
– Less wingtip vortexes.
– Improving the performance when in a climbing.
– High lift coefficients.
– Increase in the length of span and the weight of the wing structure.
(2) Low Aspect Ratio
– Very high speeds demand greater aerodynamic cleanness.
– Greater strength.
– Increase in drag.
– High wing loading.
– High stall speeds.
★ Flaps

– When deflected, it increases the camber of the aerofoil and produces an increase in lift with an accompanying increase drag.
– Reduce stalling speeds and usually alter the stalling angle as well.
– The flap most commonly found on high performance sailplanes is the plain flap.
○ Negative Flap
– Upward or negative flap deflection reduces drag at high speed by allowing the wing to remain at its mast efficient angle of attack.
★ Airbrake

– Spoil lift and increase drag.
– An increase stalling speed of 2 – 5 knot but have no effect on the angle.
– Effective in controlling descent rate, is speed–limiting in a 30 degree dive and some older airbrake designs be speed-limiting in a vertical dive.
– When extended, they usually cause a nose-down change in the glider’s trim.
★ Spoiler

– The primary purpose of spoilers is to break up the smooth flow of air over the portion of the wing “spoiling” the lift. (Decrease lift)
– Limit ability to control descent rate and is not speed-limiting.
★ The Three Axes
– The sailplane is controlled around three axes of movement.
(1) Pitching

– About the lateral axes.
– The elevator controls the pitch of the glider and thereby controls it speed.
(2) Rolling

– Around the longitudinal axis.
– The ailerons control the bank or roll of the glider.
– The primary turning controls.
(3) Yawing

– About the vertical axis.
– The rudder controls the yaw of the glider.
★ Stability
– Sailplanes are designed to be either neutrally or positively stable.
○ Static stability
(1) Negative Stability

– If disturbed the ball will never return.
– Not stable enough
– The Pilot will be unable to achieve any precision in control.
(2) Neutral stability

– If disturbed the ball will move and continue unless a force is applied to stop it.
– Conveniently ignoring friction, of course.
(3) Positive stability

– If disturbed the ball will return to its original point.
– It may make a number of oscillations before stopping.
– Too stable
– The pilot will have great difficulty in maneuvering.
○ Dynamic Stability
(1) Negative stability

– The oscillations increase in amplitude.
(2) Neutral Stability

– Where oscillations remain at the same amplitude.
(3) Positive Stability

– The decreasing oscillations in its flight path.
★ Flatter
– An oscillation of a control surface or surface which can cause an excitation of the main surface (wing, tail-plan, etc..) of the aircraft.
– aileron movements at speeds in excess of the placard can start wing flexing that rapidly develops into a series of self-reinforcing movements.
These will increase in amplitude until the structure breaks.
– Once initiated it can only be stopped by reducing speed.
– Mass balances on control surface help to limit flutter to speeds that are too fast to be of any useful value.
○ Factors
– Excessive free play in the control surface.
– Incorrect to loose mass-balance weights.
– Loss of control circuit stiffness due to other factors such as broken control-rod supports etc.
– Flying outside placarded speed limits, usually too fast at high altitudes.
★ Longitudinal Stability (Pitching)

– The wings center of lift (CL) is to the rear of the CG.
– Nose heavy, keep the nose from continually pitching downward.
○ Trim Tab
– Gives the pilot an improved level off “FEEL” in the pitching plane, especially at high speeds.
(1) Balance Tab

– To provide a force movement of a control surface.
(2) Anti-Balance Tab

★ Lateral Stability (Rolling)
– Common procedure for producing lateral stability are dihedral, keel effect, sweep back, and weight distribution.
○ Dihedral

– The upward angle of the sailplane’s wings with respect to the horizontal.
– Low-wing sailplanes commonly have more dihedral than high-wing sailplane.
○○ In A Side Slip
– Low Wing
– Increased angle of attack
– Increased lift
– High Wing
– Reduced angle of attack
– Decreased lift
○ Sweepback

– The wings taper backward from the root of the wing to the wingtip.
– Improve lateral stability and aid slightly in directional stability.
○○ In A Side Slip
– Low wing
– An effective decrease in sweepback
– Meets the relative wind more perpendicular
– Increased lift
– Increased drag
– High wing
– An effective increase in sweepback.
– Decrease lift
– Decrease drag
★ Directional Stability (Yawing)

– The area of the vertical fin and the sides of the fuselage aft of the CG are the prime contributors which make weathervane.
★ Turning

– The sailplane is turned by banking
– This allows a portion of the lift force to provide the turning force.
– In order for the sailplane to remain in steady flight, the lift must increase when it is banked to provide a component equal to weight.
– The Steeper the bank, the more lift is required.
– A vertical bank is no possible as there is no lift available to balance the weight.
– This increased lift is obtained by increasing the angle of attack, increases the drag also.
– Thus, in a turn, the sailplane sinks more rapidly than at the same speed in straight flight.
– Centrifugal acting out ward is opposite and equal to the inward turning force.
○ Load Factor

– The ratio of the amount of load imposed on an aircraft structure to the weight of the structure itself.
– A 1-G load factor is one in which the load on the structure is equal to the weight of structure.
– As the lift required in turning is greater than the weight.
– The inertia forces increase its apparent weight, and the structure must support a weight greater than that of the sailplane at rest.
– The greater the bank angle the greater the load factor.
– The centrifugal force acting on the sailplane as it is being pulled around in the turn.
○ Stall Speed

– Requires a higher angle of attack to provide the needed lift and as a result, there is a correlation between the angle of attack, the load factor, and the increase in stall speed.
○ Radius of Turn

– The rate at which the sailplane changes direction is governed by the angle of bank.
– The steeper the bank angle the more rapid the change of direction.
– The size of the circle made is similarly affected.
– For efficient flight we need to bear in mind that steeper turns increase the sinking speed as well.
– It should be noted that there is very little deterioration of sinking performance occurs.
– Thus in practical use turns of 20 to 40 degrees are generally used.
– Steeper banked turns are used where a rapid change of direction is necessary.
– A part from turning to position the sailplane, the turning performance is most important.
– Thermals are of limited size, and only by turning can the sailplane remain in them continuously, and thus gain height at the fastest rate.
★ Adverse Yaw

– The down-going aileron produce an increased angle of attack on that part of the wing, the up-going aileron produces a decreased angle of attack on the other side.
– increased angle of attack means increased lift (which causes the desired roll), but also means increased drag, which causes a yaw in the adverse direction.
○ Minimize The Effect
– Ailerons which are ridged in such a way that their upward movement is greater than their downward movement.
★ Ground Effect


– A condition of improved aircraft performance when operating near a surface.
– A usually beneficial influence an aircraft performance which occures when it is less than height of the aircraft’s wing span above the surface.
– A lower than the normal angle of attack produces the same amount of lift.
(1) Wing Tip Vortex
– Out board portion of the wing more efficient.
– Reduces wing tip vortex.
(2) Down Wash
– Reduces induced angle of attack and induced drag.
– Reduces down wash velocity.
★ Left Turning Tendency
(1) Torque

– The reaction to the turning of the propeller system.
– If the propeller system turns counter clockwise, the fuselage reacts by turning clockwise. (Newton’s 3rd Law)
(2) P-Factor

– The difference in lift exists between the downward half of the blade and the upward half.
(3) Corkscrew Effect

– Spiraling slipstream strikes the vertical fin on the left, it cause a left turning moment at high propeller speeds and low forward speed.
– Corkscrew flow of slipstream causes a rolling moment to the right.
– Counteracting torque reaction to the left.
★ Gyroscopic Precession

Force Response
Nose Up Turn Right
Nose Down Turn Left
Turn Right Nose Up
Turn Left Nose Down

– A phenomenon in rotating systems that makes all forces react with a movement 90 degrees from the point of force in the direction of rotation.
– The reaction to a force applied to a gyro acts in the direction of rotation and approximately 90 degrees of the point where force is applied.




★ Spin

1. Throttle Partial Power
2. Elevator Full Off
3. Prior to the stall “Break” C’K
4. Rudder Deflect
5. Elevator & Rudder Hold Full
6. Aileron Neutral

– Recovery
1. Aileron Neutral
2. Throttle Idle
3. Rudder Full Opposite
4. Control Briskly Forward
5. Rotation Stops C’K
6. Rudder Neutral
7. Pitch Smooth Up