A Closer Look at Stall Speeds
Stall speed is the slowest speed a plane can fly and maintain level flight. When a plane slows down, it produces less lift. If the plane tilts its wings up, thereby increasing its angle of attack, it can compensate for any lift lost. However, if it tilts its wings too far, the air pulls away from the top of the wing and the plane loses lift. This is known as a stall. Stall speed can be reached by increasing the angle of attack as close to stall as possible and slowing down until weight and lift balance out. Stall speed increases as weight increases, as wings need to fly at a higher angle of attack to generate enough lift for a given airspeed.
The increase in load factor in a turn also increases stall speed. For example, in a level, 60-degree-bank turn, the weight on the wings doubles and stall speed increases by about 40 percent. Furthermore, contamination such as frost or ice on the wing can reduce the amount of lift produced by the wing, also raising the stall speed. Changes to the airfoil geometry from high-lift devices such as flaps or leading-edge slats increase the maximum coefficient of lift and thus lower stall speeds. In this blog, we will look at two lesser-known factors that affect stall speed: center of gravity and thrust.
In a 2,300 pound aircraft, the Cessna C172N Skyhawk, the stalling airspeed is 53 KCAS (knots calibrated airspeed) for its most forward center of gravity and 50 KCAS at its most aft center of gravity. However, why does stall speed depend on the center of gravity at all? To understand this, it is important to reconsider the misleading conceptions surrounding the forces that affect flight which all act through the aircraft’s center of gravity. These forces are lift, thrust, drag, and weight.
Lift acts through the center of pressure, which is usually just behind the center of gravity. The center of pressure moves forward as the angle of attack increases rearwars while the angle of attack decreases. The horizontal tail’s angle of incidence (the angle between a ray incident on a surface and the line perpendicular to the surface at the point of incidence) is usually negative, and the tail commonly produces tail-down force to offset the pitch-down moment from the main wing lift in level flight. Additionally, the thrust and drag almost never act through the airplane’s center of gravity. Instead, they create their own torque-couples, depending on multiple factors.
The tail-down force (tail lift) opposes wing lift and increases effective weight. As the center of gravity moves forward, the wing is now forced to produce more lift, meaning the stalling airspeed rises. Adversely, as the center of gravity moves rearward, less tail downforce is needed and the stalling speed lowers. Dynamic wing-loading reduction of 10 percent will cause an approximate 5 percent reduction of the stalling speed. This can go as far as totally unloading a wing in which case the stalling speed becomes zero (nothing to lift). Conventional airplanes are designed so that the center of gravity and center of pressure are close to each other on the longitudinal axis in the normal operating range. Many conventional airplanes often pitch down in a stall because the horizontal tail stalls before the main wing.
Lower tail-down force with an aft center of gravity also results in less tail trim drag, leading to higher cruising airspeeds and better fuel efficiency. Modern long-range jets are able to transfer fuel automatically in flight to dedicated tail tanks, resulting in significant increases in cruise-speed. Despite this, moving the center of gravity too far rearward negatively affects pitch stability and makes it easier to structurally overwork an airplane while maneuvering. Moving the center of gravity too far forward generates excessive pitch stability, less maneuverability, and causes serious landing flare control problems such as running out of up elevator, which is also degraded by reduced main-wing downwash in ground effect, causing an additional pitch-down moment.
Stalling speed is also affected by thrust. If all forces are projected on vertical and horizontal axes, the horizontal thrust component must offset total drag (including tail trim drag) in unaccelerated straight and level flight. Because of this, the total thrust intensity must be larger than drag. On the other hand, the vertical thrust component opposes weight and less lift is required, decreasing the stalling airspeeds. The vertical thrust component is normally larger than the tail’s downforce at high pitch angles. If enough thrust is created, the effects can go as far as completely removing the need for lifting surfaces. Many short-takeoff-and-landing (STOL) and propeller-driven airplanes have powerful engines. These engines, when they work in tandem with efficient wing boundary layer control, significantly decrease the stalling speeds at high pitch angles, allowing for short takeoff and landing distances.
Despite this, the propeller thrust in single-engine aircraft makes things more complex. This added thrust, whether induced flow or propwash, is often destabilizing. The propeller’s induced flow increases with thrust, consequently energizing the boundary layers on the wing root and horizontal tail. This is not negative in and of itself, but it often causes sudden and rapid nose drop as the normally thin airfoil tail surfaces experience sharp leading-edge stalls at high angles of attack. Therefore, power-on stall entries are typically much smoother than power-off stalls (depending on the location of the center of gravity).
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