All About Stalls

Air, like every other mass, has inertia and resists making sharp turns. Air flow about a wing behaves similarly: it can make only gradual changes in direction. This raises the question: If a stall develops progressively and the wing is always developing some lift, what causes the sudden “break” or “nose drop” associated with a stall? The answer is only incidental to the loss of lift. In normal flight, downwash from the wing strikes the upper surface of the tail plane. This action helps the elevator-stabilizer combination to produce a downward force that keeps the nose up in straight and level flight. As a stall is approached, turbulent air from above the stalled portion of the wing strikes the tail. This is usually the cause of the familiar stall buffet. In other words, the wing doesn’t buffet, the tail does. When enough of the wing stalls, insufficient downwash remains to keep the tail down. In a sense, the horizontal stabilizer stalls too and the nose drops.

The rectangular wing has the most ideal stall pattern which is an aft root stall. Such a stall provides a tail buffet to warn of an impending stall and allows the wingtips to remain flying as long as possible. Since ailerons are positioned toward the wing tips the rectangular wing preserves maximum aileron control.

To the contrary, a tip stall is bad news. The tail planes are not behind the stalled portion of the wings and therefore may not provide the warning buffet. The ailerons become ineffective early in the stall and cannot be counted upon to provide roll control during flight at minimum airspeed.

Aircraft designers go to great lengths to make certain that their aircraft exhibit optimum stall patterns that begin at or near the wing root. Four methods commonly used to achieve this are: Wing twist, a stall strip, variable airfoil wings, and wingtip slots.

Without experience in a particular aircraft, it is difficult to predict which wing will drop during a full-power stall. This is because the factors causing one wing to stall before the other might consist of minute flaws on a leading edge such as a dent, a flat spot, or even a landing light. Engine and propeller forces often cause the left wing to drop during a power-on stall, but only if both wings are identical, a condition rarely found on production airplanes.

During a climbing turn, the outside wing is at a slightly larger angle of attack than the inside wing. If the aircraft is stalled under these conditions, the outside (or high) wing usually stalls first, resulting in an abrupt reversal in the direction of bank, otherwise known as an “over-the-top” stall. Failure to execute a timely recovery can lead to a full roll followed by a conventional spin.

During a descending turn, the converse occurs. The inside wing has the larger angle of attack and tends to stall first, resulting in an increased bank angle. An attempt to recover using ailerons can aggravate the “under-the-bottom” stall and result in an increased bank angle and possible spin.

Airspeed, or lack of it, is not the primary cause of a stall. A stall occurs for only one reason: the pilot has tried to fly the wing at too large an angle of attack. Recovery is just as simple. Reduce the angle of attack.

(The above information has been extrapolated from Barry Schiff’s, The Proficient Pilot vol. 1.)