Aerodynamics of Flight

aviation training aircraft. The development of the ability to factor. Abrupt pull ups at high diving speeds may impose judge load factors from the feel of their effect on the body is critical loads on aircraft structures and may produce recurrent important. A knowledge of these principles is essential to the or secondary stalls by increasing the AOA to that of stalling. development of the ability to estimate load factors. As a generalization, a recovery from a stall made by diving A thorough knowledge of load factors induced by varying only to cruising or design maneuvering airspeed, with a degrees of bank and the V aids in the prevention of two of gradual pull up as soon as the airspeed is safely above stalling, A the most serious types of accidents: can be effected with a load factor not to exceed 2 or 2.5 Gs. A higher load factor should never be necessary unless recovery 1. Stalls from steep turns or excessive maneuvering near has been effected with the aircraft’s nose near or beyond the the ground vertical attitude or at extremely low altitudes to avoid diving 2. Structural failures during acrobatics or other violent into the ground. maneuvers resulting from loss of control Spins Load Factors and Flight Maneuvers A stabilized spin is not different from a stall in any element Critical load factors apply to all flight maneuvers except other than rotation and the same load factor considerations unaccelerated straight flight where a load factor of 1 G is apply to spin recovery as apply to stall recovery. Since spin always present. Certain maneuvers considered in this section recoveries are usually effected with the nose much lower than is are known to involve relatively high load factors. Full common in stall recoveries, higher airspeeds and consequently application of pitch, roll, or yaw controls should be confined higher load factors are to be expected. The load factor in a to speeds below the maneuvering speed. Avoid rapid and proper spin recovery usually is found to be about 2.5 Gs. large alternating control inputs, especially in combination with large changes in pitch, roll, or yaw (e.g., large sideslip The load factor during a spin varies with the spin characteristics angles) as they may result in structural failures at any speed, of each aircraft, but is usually found to be slightly above the including below V . A 1 G of level flight. There are two reasons for this: 1. Airspeed in a spin is very low, usually within 2 knots Turns of the unaccelerated stalling speeds. Increased load factors are a characteristic of all banked turns. As noted in the section on load factors in steep turns, 2. An aircraft pivots, rather than turns, while it is in a spin. load factors become significant to both flight performance and load on wing structure as the bank increases beyond High Speed Stalls approximately 45°. The average light plane is not built to withstand the repeated application of load factors common to high speed stalls. The yield factor of the average light plane is reached The load factor necessary for these maneuvers produces a at a bank of approximately 70° to 75°, and the stalling stress on the wings and tail structure, which does not leave speed is increased by approximately one-half at a bank of a reasonable margin of safety in most light aircraft. approximately 63°. The only way this stall can be induced at an airspeed above Stalls normal stalling involves the imposition of an added load The normal stall entered from straight-and-level flight, or an factor, which may be accomplished by a severe pull on the unaccelerated straight climb, does not produce added load elevator control. A speed of 1.7 times stalling speed (about factors beyond the 1 G of straight-and-level flight. As the 102 knots in a light aircraft with a stalling speed of 60 knots) stall occurs, however, this load factor may be reduced toward produces a load factor of 3 Gs. Only a very narrow margin zero, the factor at which nothing seems to have weight. The for error can be allowed for acrobatics in light aircraft. To pilot experiences a sensation of “floating free in space.” If illustrate how rapidly the load factor increases with airspeed, recovery is effected by snapping the elevator control forward, a high-speed stall at 112 knots in the same aircraft would negative load factors (or those that impose a down load on produce a load factor of 4 Gs. the wings and raise the pilot from the seat) may be produced. Chandelles and Lazy Eights During the pull up following stall recovery, significant A chandelle is a maximum performance climbing turn load factors are sometimes induced. These may be further beginning from approximately straight-and-level flight, increased inadvertently during excessive diving (and and ending at the completion of a precise 180° turn in a consequently high airspeed) and abrupt pull ups to level wings-level, nose-high attitude at the minimum controllable flight. One usually leads to the other, thus increasing the load 5-36