Aerodynamics of Flight

loading of any aircraft at which a “flat” spin develops. A • A forward CG location increases the need for greater flat spin is one in which centrifugal force, acting through a back elevator pressure. The elevator may no longer CG located well to the rear, pulls the tail of the aircraft out be able to oppose any increase in nose-down pitching. away from the axis of the spin, making it impossible to get Adequate elevator control is needed to control the the nose down and recover. aircraft throughout the airspeed range down to the stall. An aircraft loaded to the rear limit of its permissible CG A detailed discussion and additional information relating range handles differently in turns and stall maneuvers and to weight and balance can be found in Chapter 10, Weight has different landing characteristics than when it is loaded and Balance. near the forward limit. High Speed Flight The forward CG limit is determined by a number of Subsonic Versus Supersonic Flow considerations. As a safety measure, it is required that the In subsonic aerodynamics, the theory of lift is based upon the trimming device, whether tab or adjustable stabilizer, be forces generated on a body and a moving gas (air) in which capable of holding the aircraft in a normal glide with the power it is immersed. At speeds of approximately 260 knots or off. A conventional aircraft must be capable of a full stall, less, air can be considered incompressible in that, at a fixed power-off landing in order to ensure minimum landing speed altitude, its density remains nearly constant while its pressure in emergencies. A tailwheel-type aircraft loaded excessively varies. Under this assumption, air acts the same as water and nose-heavy is difficult to taxi, particularly in high winds. It is classified as a fluid. Subsonic aerodynamic theory also can be nosed over easily by use of the brakes, and it is difficult assumes the effects of viscosity (the property of a fluid that to land without bouncing since it tends to pitch down on the tends to prevent motion of one part of the fluid with respect wheels as it is slowed down and flared for landing. Steering to another) are negligible and classifies air as an ideal fluid difficulties on the ground may occur in nosewheel-type conforming to the principles of ideal-fluid aerodynamics such aircraft, particularly during the landing roll and takeoff. The as continuity, Bernoulli’s principle, and circulation. effects of load distribution are summarized as follows: • The CG position influences the lift and AOA of the In reality, air is compressible and viscous. While the effects of wing, the amount and direction of force on the tail, these properties are negligible at low speeds, compressibility and the degree of deflection of the stabilizer needed effects in particular become increasingly important as speed to supply the proper tail force for equilibrium. The increases. Compressibility (and to a lesser extent viscosity) is latter is very important because of its relationship to of paramount importance at speeds approaching the speed of elevator control force. sound. In these speed ranges, compressibility causes a change in the density of the air around an aircraft. • The aircraft stalls at a higher speed with a forward CG location. This is because the stalling AOA is reached During flight, a wing produces lift by accelerating the airflow at a higher speed due to increased wing loading. over the upper surface. This accelerated air can, and does, • Higher elevator control forces normally exist with a reach sonic speeds even though the aircraft itself may be flying forward CG location due to the increased stabilizer subsonic. At some extreme AOAs, in some aircraft, the speed deflection required to balance the aircraft. of the air over the top surface of the wing may be double the • The aircraft cruises faster with an aft CG location aircraft’s speed. It is therefore entirely possible to have both because of reduced drag. The drag is reduced because supersonic and subsonic airflow on an aircraft at the same time. a smaller AOA and less downward deflection of the When flow velocities reach sonic speeds at some location on stabilizer are required to support the aircraft and an aircraft (such as the area of maximum camber on the wing), overcome the nose-down pitching tendency. further acceleration results in the onset of compressibility effects, such as shock wave formation, drag increase, buffeting, • The aircraft becomes less stable as the CG is moved stability, and control difficulties. Subsonic flow principles are rearward. This is because when the CG is moved invalid at all speeds above this point. [Figure 5-64] rearward, it causes a decrease in the AOA. Therefore, the wing contribution to the aircraft’s stability is Speed Ranges now decreased, while the tail contribution is still The speed of sound varies with temperature. Under standard stabilizing. When the point is reached that the wing temperature conditions of 15 °C, the speed of sound at sea and tail contributions balance, then neutral stability level is 661 knots. At 40,000 feet, where the temperature is exists. Any CG movement further aft results in an –55 °C, the speed of sound decreases to 574 knots. In high- unstable aircraft. 5-44