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