LouW

The term “tip stall” as it appears from time to time in this forum appears to be a catchall phrase covering any tendency for an airplane to drop a wing when operating slowly. In many cases the wing drop has little to do with a stall at the wing tip.

Consider a wing of rectangular shape built without twist. Downwash and spanwise flow influence the angle of attack at each section across the wing such that the effective angle of attack near the tips is less than that near the center. When it stalls, the flow separation begins in the center and proceeds outward toward the tips. If the wing is moving straight forward when the stall occurs, it merely pitches down. However if there is a yawing motion at the moment of stall, the wing, in the direction of the yaw, will stall first and will drop suddenly even though the tip area is not stalled.

Now consider a sharply tapered wing also built without twist. Downwash and spanwise flow also effect the angle of attack at each section, but in this case the effective angle of attack near the tips is greater than that near the center. It stalls first at the tips and then progresses toward the center. Again, if the wing is stalled while moving straight ahead, the wing will simply pitch down. However even a little yawing motion at the moment of stall will cause the wing in the direction of the yaw to drop sharply. This is a true tip stall.

Notice that in both cases, ailerons were not used, and the cause of the sudden drop of the wing was the yawing motion at the moment of stall. Now consider the rectangular wing with ailerons just on the outboard part of the wing. When ailerons are deflected, the upgoing aileron reduces the angle of attack on that side increasing the stall margin. The downgoing aileron increases the angle of attack, but the downgoing aileron also acts like a flap, changing the camber, which allows the section to go to a higher angle of attack before stalling. Since the rectangular wing stalls inboard first, the tips are still flying and aileron deflection alone isn’t likely to cause a wing to drop.

A similar thing occurs for the sharply tapered wing. The downgoing aileron still functions as a flap allowing a higher angle of attack before stall, but since the tips are operating with a much smaller stall margin, it may under some conditions result in tip stall.

Strip ailerons, where the entire trailing edge is hinged, generally have less effect at the tips because they have a narrower chord and deflect less for the same amount of roll moment. For this reason they are less likely to induce tip stall even on highly tapered wings.

An elliptical wing, and a moderately tapered wing with rounded tips that approximates an ellipse, has a distribution of lift such that the entire wing operates close to the same angle of attack at each section. Such a wing tends to stall all at once rather than progressively. When a yawing motion exists at the moment of stall it tends to drop a wing suddenly but it may be due to tip stall or not depending on the circumstances.

Another effect of aileron deflection, that is most significant near the stall, is “adverse yaw”. The upgoing aileron causes a reduction of drag at that tip while the downgoing aileron increases drag at that tip. If this is not resisted by rudder input, the wing will yaw toward the downgoing aileron. As seen above, a significant yawing motion at the instant of stall will cause a wing to drop whether the tip is stalled or not. The effect is of course greater if the tip is already operating close to the stall angle. In many cases of sudden wing drop, the cause is likely due to adverse yaw rather than tip stall. For lightly loaded rectangular wings such as found on trainers and many sport aircraft, I suspect this is always the case. If the habit were formed of always using rudder when ailerons are used in order to compensate for the adverse yaw, instances of “tip stall” would be greatly reduced.

There are some design features that can minimize the tendency to tip stall on highly loaded tapered wings. Washout, where the wing is twisted a little such that the tips have less incidence than the root, is one such time honored method. It works quite well for normal flight attitudes, however for inverted flight the washout becomes washin and aggravates the tendency to tip stall when performing inverted maneuvers. Another consideration when using washout on an aerobatic design is that it makes snap maneuvers more difficult. Washout is most useful on scale warbirds and other high wing loading aircraft. It is generally not required on trainers and sport aircraft with rectangular wings and light wing loading.