Aerodynamics of Flight

Associated with “drag rise” are buffet (known as Mach buffet), trim, and stability changes and a decrease in control force effectiveness. The loss of lift due to airflow separation results in a loss of downwash and a change in the position of the center pressure on the wing. Airflow separation produces a turbulent wake behind the wing, which causes the tail surfaces to buffet (vibrate). The nose-up and nose-down pitch control provided by the horizontal tail is dependent on the downwash behind the wing. Thus, an increase in downwash decreases the horizontal tail’s pitch control effectiveness since it effectively increases the AOA that the tail surface is seeing. Movement of the wing CP affects the wing pitching moment. If the CP moves aft, a diving moment referred to Spanwise flow as “Mach tuck” or “tuck under” is produced, and if it moves forward, a nose-up moment is produced. This is the primary reason for the development of the T-tail configuration on many turbine-powered aircraft, which places the horizontal stabilizer as far as practical from the turbulence of the wings. Sweepback Most of the difficulties of transonic flight are associated with shock wave induced flow separation. Therefore, any means of delaying or alleviating the shock induced separation improves aerodynamic performance. One method is wing sweepback. Airspeed sensed Sweepback theory is based upon the concept that it is only the by wing Mach 0.70 component of the airflow perpendicular to the leading edge True airspeed of the wing that affects pressure distribution and formation Mach 0.85 of shock waves. [Figure 5-68] On a straight wing aircraft, the airflow strikes the wing leading edge at 90°, and its full impact produces pressure and lift. A wing with sweepback is struck by the same airflow at Figure 5-68. Sweepback effect. an angle smaller than 90°. This airflow on the swept wing has the effect of persuading the wing into believing that it is flying because the boundary layer tends to flow spanwise toward slower than it really is; thus the formation of shock waves is the tips and to separate near the leading edges. Because the delayed. Advantages of wing sweep include an increase in tips of a swept wing are on the aft part of the wing (behind critical Mach number, force divergence Mach number, and the CL), a wingtip stall causes the CL to move forward on the Mach number at which drag rise peaks. In other words, the wing, forcing the nose to rise further. The tendency for sweep delays the onset of compressibility effects. tip stall is greatest when wing sweep and taper are combined. The Mach number that produces a sharp change in coefficient The stall situation can be aggravated by a T-tail configuration, of drag is termed the “force divergence” Mach number and, which affords little or no pre-stall warning in the form of tail for most airfoils, usually exceeds the critical Mach number by control surface buffet. [Figure 5-70] The T-tail, being above 5 to 10 percent. At this speed, the airflow separation induced the wing wake remains effective even after the wing has begun by shock wave formation can create significant variations in to stall, allowing the pilot to inadvertently drive the wing the drag, lift, or pitching moment coefficients. In addition to into a deeper stall at a much greater AOA. If the horizontal the delay of the onset of compressibility effects, sweepback tail surfaces then become buried in the wing’s wake, the reduces the magnitude in the changes of drag, lift, or moment elevator may lose all effectiveness, making it impossible to coefficients. In other words, the use of sweepback “softens” reduce pitch attitude and break the stall. In the pre-stall and the force divergence. immediate post-stall regimes, the lift/drag qualities of a swept wing aircraft (specifically the enormous increase in drag A disadvantage of swept wings is that they tend to stall at the at low speeds) can cause an increasingly descending flight wingtips rather than at the wing roots. [Figure 5-69] This is path with no change in pitch attitude, further increasing the 5-48