1.8 Airplane Stability
Loading an airplane to the most aft CG will cause the airplane to be a) less stable at all speeds b) less stable at slow speed, but more stable at high speeds c) less stable at high speeds, but more stable at low speeds
A : Airplane becomes less stable at all speeds as the center of gravity is moved backward. The rearward center of gravity limit is determined largely by considerations of stability.
What determines the longitudinal stability of an airplane? a) The location of the CG with respect to the center of lift. b) The effectiveness of the horizontal stabilizer, rudder, and rudder trim tab. c) The relationship of thrust and lift to weight and drag.
A : The location of the center of gravity with respect to the center of lift determines, to a great extent, the longitudinal stability of the airplane. Positive stability is attained by having the center of lift behind the center of gravity. Then the tail provides negative lift, creating a downward tail force, which counteracts the nose's tendency to pitch down.
What is the effect of advancing the throttle in flight? a) Both aircraft groundspeed and angle of attack will increase. b) Airspeed will remain relatively constant but the aircraft will climb. c) The aircraft will accelerate, which will cause a turn to the right.
A : When advancing the throttle, initially the groundspeed increases due to the corresponding increase in airspeed. This causes the aircraft to pitch up, increasing the angle of attack. Airspeed and lift continue to increase until the opposing forces equalize. Then, the aircraft will climb at a relatively constant airspeed due to the increase in lift caused by excess thrust and additional airflow over the wing.
An airplane said to be inherently stable will a) be difficult to stall b) requires less effort to control c) not spin
B : An inherently stable airplane will usually return to the original condition of flight (except when in a bank) if disturbed by a force such as air turbulence. Thus, an inherently stable airplane will require less effort to control than an inherently unstable one.
An airplane has been loaded in such a manner that the CG is located aft the aft CG limit. One undesirable flight characteristic a pilot might experience with this airplane would be a) a longer takeoff run b) difficulty in recovering from a stalled condition c) stalling at higher-than-normal airspeed
B : The recovery from a stall in any airplane becomes progressively more difficult as its center of gravity moves backward. Generally, airplanes become less controllable, especially at slow flight speeds, as the center of gravity is moved backward.
What causes an airplane (except a T-tail) to pitch nosedown when power is reduced and controls are not adjusted? a) The CG shifts forward when thrust and drag are reduced. b) The downwash on the elevators from the propeller slipstream is reduced and elevator effectiveness is reduced. c) When thrust is reduced to less than weight, lift is also reduced and the wings can no longer support the weight.
B : The relative wind on the tail is the result of the airplane's movement through the air and the propeller slipstream. When that slipstream is reduced, the horizontal stabilizer (except a T-tail) will produce less negative lift and the nose will pitch down.
Changes in the center of pressure of a wing affect the aircraft's a) lift/drag ratio b) lifting capacity c) aerodynamic balance and controllability
C : Center of pressure (CP) is the imaginary but determinable point at which all of the upward lift forces on the wing are concentrated. In general, at high angles of attack the CP moves forward, while at low angles of attack the CP moves aft. The relationship of the CP to center of gravity (CG) affects both aerodynamic balance and controllability.