Aerodynamics of Flight

is descending at the same power as used in straight-and- drop during a stall, reducing the AOA and “unstalling” the level flight. wing. The nose-down tendency is due to the CL being aft of the CG. The CG range is very important when it comes to As forward pressure is applied to the control yoke to initiate stall recovery characteristics. If an aircraft is allowed to be the descent, the AOA is decreased momentarily. Initially, operated outside of the CG range, the pilot may have difficulty the momentum of the aircraft causes the aircraft to briefly recovering from a stall. The most critical CG violation would continue along the same flight path. For this instant, the AOA occur when operating with a CG that exceeds the rear limit. decreases causing the total lift to decrease. With weight now In this situation, a pilot may not be able to generate sufficient being greater than lift, the aircraft begins to descend. At the force with the elevator to counteract the excess weight aft of same time, the flight path goes from level to a descending the CG. Without the ability to decrease the AOA, the aircraft flight path. Do not confuse a reduction in lift with the inability continues in a stalled condition until it contacts the ground. to generate sufficient lift to maintain level flight. The flight path is being manipulated with available thrust in reserve The stalling speed of a particular aircraft is not a fixed value and with the elevator. for all flight situations, but a given aircraft always stalls at the same AOA regardless of airspeed, weight, load factor, or To descend at the same airspeed as used in straight-and- density altitude. Each aircraft has a particular AOA where the level flight, the power must be reduced as the descent is airflow separates from the upper surface of the wing and the entered. Entering the descent, the component of weight stall occurs. This critical AOA varies from approximately 16° acting forward along the flight path increases as the angle to 20° depending on the aircraft’s design. But each aircraft of descent increases and, conversely, when leveling off, the has only one specific AOA where the stall occurs. component of weight acting along the flight path decreases as the angle of descent decreases. There are three flight situations in which the critical AOA is most frequently exceeded: low speed, high speed, and turning. Stalls One way the aircraft can be stalled in straight-and-level flight An aircraft stall results from a rapid decrease in lift caused by by flying too slowly. As the airspeed decreases, the AOA the separation of airflow from the wing’s surface brought on must be increased to retain the lift required for maintaining by exceeding the critical AOA. A stall can occur at any pitch altitude. The lower the airspeed becomes, the more the AOA attitude or airspeed. Stalls are one of the most misunderstood must be increased. Eventually, an AOA is reached that results areas of aerodynamics because pilots often believe an airfoil in the wing not producing enough lift to support the aircraft, stops producing lift when it stalls. In a stall, the wing does which then starts settling. If the airspeed is reduced further, not totally stop producing lift. Rather, it cannot generate the aircraft stalls because the AOA has exceeded the critical adequate lift to sustain level flight. angle and the airflow over the wing is disrupted. Since the C increases with an increase in AOA, at some L Low speed is not necessary to produce a stall. The wing point the C peaks and then begins to drop off. This peak is L can be brought into an excessive AOA at any speed. For called the C . The amount of lift the wing produces drops L-MAX example, an aircraft is in a dive with an airspeed of 100 dramatically after exceeding the C or critical AOA, but L-MAX knots when the pilot pulls back sharply on the elevator as stated above, it does not completely stop producing lift. control. [Figure 5-38] Gravity and centrifugal force prevent an immediate alteration of the flight path, but the aircraft’s In most straight-wing aircraft, the wing is designed to stall AOA changes abruptly from quite low to very high. Since the wing root first. The wing root reaches its critical AOA the flight path of the aircraft in relation to the oncoming air first making the stall progress outward toward the wingtip. determines the direction of the relative wind, the AOA is By having the wing root stall first, aileron effectiveness is suddenly increased, and the aircraft would reach the stalling maintained at the wingtips, maintaining controllability of angle at a speed much greater than the normal stall speed. the aircraft. Various design methods are used to achieve the stalling of the wing root first. In one design, the wing is The stalling speed of an aircraft is also higher in a level turn “twisted” to a higher AOA at the wing root. Installing stall than in straight-and-level flight. [Figure 5-39] Centrifugal strips on the first 20–25 percent of the wing’s leading edge force is added to the aircraft’s weight and the wing must is another method to introduce a stall prematurely. produce sufficient additional lift to counterbalance the load imposed by the combination of centrifugal force and weight. The wing never completely stops producing lift in a stalled In a turn, the necessary additional lift is acquired by applying condition. If it did, the aircraft would fall to the Earth. Most back pressure to the elevator control. This increases the wing’s training aircraft are designed for the nose of the aircraft to 5-25