"Ask ATC" Blog

Stall Speed

What's the stall speed of your aircraft?

The idea that an aircraft will stall at a particular speed can sometimes be misleading. It implies that as long as the airspeed is "in the green" the aircraft will not stall. Sometimes it's good to be reminded that this just isn't true. An aircraft can stall at any airspeed and at any attitude.

Stalls happen when the relative airflow intercepts the wing at such an angle that it cannot adjust to the change in direction required to maintain a smooth, laminar airflow over the wing. Air has inertia and will only make gradual direction changes on a wing before it begins behaving badly.

When an aircraft does stall, it's important to know that most wings incorporate some design so that the entire wing does not stall the same time. Some aircraft incorporate "washout" whereas the wing is slightly twisted (roughly 3 degrees) so that the outboard edges have a lower angle of attack than the inboard half. Typically, the inboard half of the wing will stall first and in reality, it is very rare for the entire wing to ever stall completely.

One of the tell-tale signs of an impending stall is the "buffeting." It is interesting to note that it is not actually the wing that causes the buffet, rather the turbulent air from the inboard half of the wing striking the horizontal stabilizer. Downwash from the laminar airflow is typically what keeps the tail down, however during a stall, when the downwash becomes insufficient, we get the sudden nose drop. One of the reasons power-on stalls typically happen at lower airspeeds than power-off stalls, is the fact that the propeller preserves lift on the inboard part of the wings, thereby maintaining downwash on the tail. For this reason, when the stall does happen, more of the wing is typically beyond the critical angle of attack, and the nose or wing drop associated may be more intense.

Though most pilots don't often think of the aerodynamics involved with stalls, a basic understanding is required to keep them from happening. While airspeed is a good indicator, it is only that... an indicator. In reality, it all comes down to relative airflow and angle of attack.