Chapter 6: stability and control

So that the F-16A’s static margin is:

Figure 6.16 plots the neutral point locations calculated for the F-16C vs Mach number and compares them with actual values. Note that, despite the F-16’s relatively complex aerodynamics, the method produced reasonably good estimates.

S-6.1 The YF-22 and X-31 have demonstrated the ability to maneuver at angles of attack above 60 degrees. At these extreme angles, well beyond stall, conventional control surfaces sometimes lose their control authority, or even work in reverse. Brainstorm 5 concepts for control mechanisms which might be used to control an aircraft in pitch, roll, and yaw at very high angles of attack, up to 90 degrees.

S-6.2 Flying-wing airplanes (including delta-wing jet fighters) have no canard or horizontal tail to serve as a trimming surface. They are trimmed entirely by changing the pitching moment coefficient of the wing. This limits their ability to use highly cambered, high-lift airfoils, since one of the inevitable consequences of high camber is a strong nose-down pitching moment. Brainstorm at least five ways to allow a flying wing to use a highly-cambered airfoil, at least on the inner 40% of its span, but still be trimmable.

S-6.3 The area of the F-16’s stabilator was increased in order to increase its pitch control authority. One of the consequences of this change was an increase in the aircraft’s static margin. Brainstorm at least five ways to increase an aircraft’s pitch control authority without increasing its stability.

A-6.1 Fill in the table below.

A-6.2 How many degrees of freedom does an aircraft have?

A-6.3 Define static and dynamic stability.

A-6.4 Explain why a weathervane is stable (points into the wind).

A-6.5 Explain the tradeoff between stability and maneuverability.

A-6.6 A conventional aircraft (tail to the rear), is in trimmed, level, unaccelerated flight. The wing is generating 40,000 lbs of lift and has a moment around the aerodynamic center of -20,000 ft-lb. The aerodynamic center of the wing is located at 0.25c, the center of gravity is located at 0.45c, the aircraft has a chord of 5 ft, and the symmetric tail aerodynamic center is located 10 ft behind the center of gravity. What is the lift generated by the tail and what is the weight of the aircraft? {Hint: Draw a sketch and assume thrust and all drag forces act through the center of gravity.}

A-6.7 An aircraft with a canard is in trimmed, level, unaccelerated flight. The wing is generating 40,000 lbs of lift and has a moment around the aerodynamic center of -20,000 ft-lbs. The aircraft has a chord of 5 ft, the aerodynamic center is located at 0.25c, the center of gravity is located at 0.10c, and the canard a.c. is located 5 ft ahead of the center of gravity. What is the lift generated by the canard, and what is the weight of the aircraft

A-6.8 a. How would increasing the tail volume ratio change the longitudinal static stability of a conventional aircraft?

A-6.9 An aircraft has the following data: The center of gravity is located 0.45c behind the leading edge of the wing, the aerodynamic center of the wing-body is at 0.25c, the tail volume ratio is 0.4, the wing lift curve slope is 0.08/deg, the tail lift curve slope is 0.07/deg, / = 0.3, the tail setting angle is 3^o,

b. Calculate the static margin.

c. Is this aircraft stable?

d. Calculate C_{M } , C_{M o} , and _e and plot the aircraft’s trim diagram

e. What is this aircraft’s trimmed lift coefficient?

f. What is this aircraft’s trim speed?