Stalls and Spins
Stalls and resulting spins have caused aircraft accidents since the beginning of flight. Even though airplanes have evolved to have better stall characteristics, stalls and spins continue to be a leading cause of accidents (Landsberg). A stall occurs when airflow separates from all or part of the upper surface of a wing, resulting in sudden loss of lift. This is caused by exceeding the critical angle of attack (angle of attack is the angle between the relative wind and the chord line of the airfoil). Below the critical angle, airflow over the wing surface is relatively smooth. Above the critical angle, the thin layer of air above the wing, or "boundary layer," becomes turbulent and separates from the airfoil (Dole and Lewis, 53). Lift is destroyed and drag increases, causing the aircraft to rapidly lose altitude. Pilots are trained to recover from this condition. However, if the stall occurs too low to the ground, there may not be enough altitude to recover. A study of aircraft accidents from 1992 to 2002 reveals that approximately 80 percent of fatal stall accidents occurred within 1,000 feet of the ground (Landsberg). Stalls are usually associated with slow flight in a nose-up attitude, but they can occur at any airspeed or attitude. Spins are of even greater concern because recovery requires more altitude and more actions on the part of the pilot. Simply stated, a spin is an autorotation resulting from one side of the wing stalling more than the other. Spins cause rapid loss of altitude. If the pilot does not recover, the aircraft will spin into the ground. Aircraft design affects the ease of entering and recovering from spins. Adding devices to improve stall characteristics will generally reduce spin-related accidents; preventing a stall or making it gentler can reduce accidental spins. Straight-wing aircraft must stall before they will spin (swept-wing aircraft do not necessarily have to stall first) (Dole and Lewis, 186).
Favorable Stall Characteristics
Aircraft are designed to have the most favorable stall characteristics possible given the compromises involved. A good aircraft should give the pilot adequate warning of the stall, stall gradually, and tend not to spin after the stall (USCFC). This means the aircraft wing should stall at the roots first, rather than at the tips where the ailerons are located. (Stalling at the tips first renders the ailerons ineffective for roll control.) Usually a twist, or "washout," is built into the wings so that the tips are always at a lower angle of attack. However, this is not always enough to create good stall characteristics. Some aircraft require further modifications to the wing. In the interest of safety, several types of fixed devices can modify a basic wing in order to improve stall characteristics. These include winglets, leading edge cuffs, stall strips, stall fences, slots, and vortex generators.
Winglets, which are vertical extensions of the wingtips, improve stall characteristics by reducing induced drag. This induced drag comes from high pressure air under the wings flowing around the tips to the lower pressure area above, creating vortices. Winglets redistribute the intensity of wingtip vortices over a larger area. They increase the maximum coefficient of lift, resulting in a lower stall speed (BLR, Inc.). NASA studied the effect of winglets on the performance and handling qualities of aircraft. Using windtunnels, NASA tested two versions of a model airplane: one with winglets and one without. At stall, the airplane without winglets tended to "roll off" and "drop a wing" (Van Dam et al.). The airplane with winglets demonstrated improved stall characteristics. According to a NASA report, "The winglets appeared to prevent the wing tip from stalling firstÉ reducing the tendency to roll off" (Van Dam et al.). In another study during the fuel crisis of the 1970s, NASA aerodynamicist Richard Whitcomb found that winglets reduced drag...
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