High-performance aircraft also use laminar-flow airfoils. In this design, drag is reduced by maximizing smooth air streams called laminar flow. This smooth airflow tends to separate more abruptly from the wing at high angles of attack, resulting in a full stall that occurs immediately after exceeding the critical angle of attack. Like the swept wing, this design improves high-speed efficiency at the expense of low-speed performance.
Pilots know that an airfoil can be stalled at any attitude and at any air speed. The part of this statement referring to attitude is easy to understand. Because the relative wind is opposite to flight path, the relative wind comes from beneath the airfoil when an aircraft is in descending flight. Even with a pitch attitude level with the horizon, the stalling angle of attack can be exceeded with a steep angle of descent. The air speed part of the statement is a bit more complicated. If the stalling angle of attack is always the same, why isn't the stalling air speed always the same? It's because four factors affect the stalling air speed: gross weight, load factor, altitude, and location of the center of gravity.
First let's examine the lift equation L = Cl S Greek letter sigma Vktas2/295, where L = lift in pounds, Cl = coefficient of lift, S = wing area in square feet, Greek letter sigma = air density ratio to that of standard sea level, and Vktas = true air speed in knots.
If the aircraft is in straight and level flight, lift is equal to weight and the equation becomes L = W = Cl S Greek letter sigma Vktas2/295.
Solving for velocity, the equation is arranged as Vktas2295 W / Greek letter sigma S Cl and Vktas = the square root of 295W / Greek letter sigma S Cl.
As the air speed is decreased, the Cl must be increased by increasing the angle of attack to keep the lift equal to weight. As the critical angle of attack is reached, the Cl has reached the maximum Clmax and the air speed is at the minimum speed, or stall speed. This speed is abbreviated as Vs.
When calculating an aircraft's stall speed the equation is Vs (knots true air speed) = the square root of 295W / Greek letter sigma S Clmax. From this equation, each factor affecting the stall speed can be evaluated.
As the weight of an aircraft increases, so do the stall speed and the required angle of attack. If a heavy aircraft and a light one are both flying at the same speed, the heavy aircraft is at a higher angle of attack. If both aircraft decrease their air speed at the same rate, the heavy aircraft will reach the critical or stalling angle of attack first, at a higher air speed than the lighter aircraft. As the center of gravity is shifted forward, the greater nose-down moment must be offset by a greater tail-down force. This tail-down force increases the effective gross weight of the aircraft, increasing the stall speed as described previously.
The extension of flaps has a pronounced effect on stall speed. Some flaps, such as Fowler flaps, will increase the wing area (S) and thus decrease the stall speed. The extension of flaps also increases the maximum coefficient of lift by increasing the wing camber (curvature), and, with some flaps, the boundary layer energy. The lowering of the trailing edge with the extension of the flaps will increase the angle of attack for a given pitch attitude by increasing the angle of the chord line to the relative wind. (For a further explanation of flaps see "High Lift Devices" in the May/June 1998 issue of Woman Pilot.)
As altitude increases, the density ratio (Greek letter sigma) decreases. The higher the altitude, the higher the true air speed of the stall. The indicated stalling air speed remains the same (as does the calibrated and equivalent air speed) with increasing altitude, but the actual speed through the air mass (TAS) increases.
An aircraft's bank angle in a turn has an important effect on stall speed. As an aircraft banks, the lift vector is displaced away from the vertical. The horizontal component of this deflected lift vector acts to horizontally accelerate (turn) the aircraft. As the lift vector is deflected, less of it remains in the vertical direction. To maintain level flight, the vertical component of lift must remain equal to the aircraft's weight. To accomplish this, the entire lift vector must be increased by increasing the angle of attack. Pilots learn that banking an aircraft requires an increased back pressure on the control yoke. As the bank angle is increased in level flight, the angle of attack is increased, and will eventually reach a stall, no matter how high the air speed. Hence, the aircraft can be stalled at any air speed.
Load factor is the ratio of the lift the aircraft is producing to its weight. In level coordinated flight, the load factor is equal to the inverse of the cosine of the bank angle (1/cos Greek letter theta). In wings-level flight, the load factor is 1 (cos 0 = 1). As the bank angle increases, the load factor increases. At a bank angle of 60 (cos 60 = 0.5) the load factor is 2. The stall speed increases with the square root of the load factor as it does with weight. At low speeds, an increasing load factor will result in a stall. At high speeds, structural damage may occur before the stall. For example, normal-category aircraft are designed to withstand a load factor of 3.8 g's. If attempting to turn with a 75 bank, the stall speed is approximately doubled. If flying at an air speed that is less than twice the normal stall speed (Vs x 2), the aircraft will stall. If flying above this speed, structural damage may occur because a load factor in excess of 3.8 would be experienced. This is why the speed of an aircraft must be reduced to a value known as maneuvering speed (Va) before attempting maneuvers that might possibly exceed the maximum load factor.
A spin is an aggravated stall in which one wing is more fully stalled than the other and thus experiences less lift and more drag. In the resulting autorotation, the aircraft yaws and rolls while descending. A spin requires a significant amount of altitude to recover, and some aircraft can become unrecoverable in a fully developed spin. Pilots should practice spins only in an aerobatic aircraft with a certified flight instructor.
To summarize, an airfoil stalls when it exceeds its critical angle of attack. To recover from a stall, the angle of attack must be decreased, usually by lowering the pitch attitude. As the pitch attitude is being adjusted, full power is added to minimize altitude loss. The proper stall recovery technique can make the difference between life and death. That's why competent pilots routinely practice stall recoveries in various configurations. It also helps if pilots understand the aerodynamics of the stall. In this case, an inch of prevention is definitely worth hundreds of feet of cure.