PERFORMANCE

★ Weight And Balance
○ Too Heavy
(1) Taxi
– Cause structural damage as a result of applying brakes.
(2) Take off
– Increase take off roll.
– Reduce its climb performance.
(3) Cruise

図

– Increase induced drag.
– Decrease cruise speed.
– Increase stall speed.
– Cause structural damage as result of turbulence, maneuvering.
(4) Landing
– Increase touch down speed.
– Cause structural damage as a result of hard landing.
★ Center Of Gravity : CG

図

– The theoretical point where the entire weight of the aircraft is considered to be concentrated.
– Must be positioned in a very small range.
– The maximum pilot weight will position it at the forward limit, and the minimum at the rear limit.
– The rear limit is more critical than the forward.
– Moving the balance behind the rear limit may result in the aircraft becoming uncontrollable one it has been upset, with dire result.
○ Ballast
– Most sailplanes are designed to take pilots from 150 lbs to 240 lbs.
– Heavier pilots have a difficult problem, but lighter ones will find it competitively easy to obtain and secure suitable ballast to bring them up to the minimum cockpit weight.
○ Extreme Forward CG
(1) Taxi
– Cause structural damage as a result of applying brakes.
(2) Take Off
– Nose too heavy.
– Increase take off roll.
– Reduce its climb performance.
(3) Cruse
– Impossible trim
– Increase tail down weight.
– Too stable in pitch.
– Impossible to up the nose from dive.
– Impossible to recover from stall.
– Increase induced drag.
– Decrease cruise speed.
– Increase stall speed.
– Cause structural damage as a result of turbulence manuvering.
(4) Landing
– Difficult to flare.
– Increase touch down speed.
– Cause structural damage as a result of nose gear landing.
○ Extream Aft CG
(1) Taxi
– Less effect nose gear
(2) Take Off
– Pitching up before reach lift off speed.
– Stall after lift off.
(3) Cruise
– Uncontrollable
– Impossible to trim to a safe speed.
– Decrease tail down weight.
– Unstable in pitch at all speed.
– Stability will be degraded.
– Impossible to lower the nose at stall.
– Impossible to recover from a spin.
– Flat spin.
– Decrease induced drag.
– Increase cruise speed.
– Decrease stall speed.
(4) Landing
– Impossible to lower the nose.
– Cause structural damage as a result of tail contact.
★ Definition
(1) Empty Weight
– The empty weight of the aircraft.
– Without pilot, fuel baggage.
(2) Gross Weight
– The maximum permitted flying weight.
(3) CG Range
– The range of movement of the center of gravity at various load combinations.
★ Sailplane Performance
– Calculated at the design stage.
– It is then tested to verify the calculations usually done by measuring the sinking speed at a series of different airspeeds.
– For accurate measurement, additional instrumentation.
– Observational checks can be made by flying with a type which has already been throughly.
○ Performance speed
Polar diagram at msl.

図

– The points are plotted to give a graph.
– From this performance curve, gliding angle and many other related factors can be calculated.
★ V-G Diagram

図

– The curved lines extending upward and down ward from a load factor of zero are the positive and negative lift capability lines.
– The horizontal line at 3.8G’s indicates the positive load factor limit.
○ V Speeds
(1) Vso
– The stalling speed in the landing configuration.
– Aircraft stalls on less of this speed with max. gross weight.
(2) Vs1
– The stalling speed in a specified configuration.
(3) Va
– The maneuvering speed.
– The speed above which only one-third deflection of the aileron and rudder controls are permitted and the elevator must be used in such a way as to keep applied “G” force within permitted limits.
– The speed at which full abrupt control travel at maximum gross weight may be used without exceeding the load limits.
– Control become more sensitive with speed, so when flying fast care should be taken to make only small smooth control movements.
– Maximum speeds are not always a matter of strength.
(4) Vno
– The NEVER exceed speed in rough air.
– The speed above which strong gusts in the atmosphere may cause damage to or failure of structure.
– Most fiberglass sailplanes have the same smooth and rough air placard.
– Glass is very strong but flexible, while it has more than enough bending strength, its twisting strengh is poor, and this sets the limit.
(5) Vne
– The NEVER exceeded speed in smooth air. = Maximum permitted speed.
– Flying in excess of this may cause damage.
– The greatest danger is from gusts or maneuvering causing sudden additional loading when at high speed.
– At less than the placard speed the aircraft will withstand the loads, above it damage may occur.
– Vne must be reduced with an increase in height, the reduced IAS with height as a result of decreasing air density.
– In the absence of detailed placard information, reduce Vne by 1.5% per 1,000ft from the absolute value on the placard.
(5) Vdf
– Velocity, demonstration flight.
– The maximum speed at which the aircraft is tested for certification purposes.
– 5% higher than Vne but lower than the theoretical structural limit (non-tested) of Vd.
– A test figure and must not under any circumstances be used as a flight limit in routine service.
★ Water Ballast
○ Use of Water Ballast
– Water ballast is used in some high-performance sailplane to increase their cruising speed. (In the wings or fuselage)
– The L/D ratio is a function of aerodynamic consideration and is independent of the sailplane weight.
– A glider carrying water ballast can increase its lift by flying faster, rather than by increasing its angle of attack.
– As the weight of the glider increases the airspeed must be increase to maintain the same glide ratio.
○ Additional Restriction
– Sailplane empty weights and maximum weights are listed so that where water ballast is fitted, pilots can determine how much they can carry.
– Most sailplanes are such that the ballast tanks may only be filled to capacity with pilot well below the maximum pilot weight.
– Ensure you check before filling the tank right up.
– The water is carried near the CG for minimum effect on CG and handling characteristics.
○ before Landing
– Water ballast should be jettisoned as the inertia loads are high and damage can result from anything but a perfect landing.