The Tip Stall

When an airplane is flying at an airspeed just slightly faster than its stall speed and is placed into a tight radius turn, it may flip over on its back and rapidly lose altitude. This startling and undesired maneuver is caused by a tip-stall.

This type of stall is caused by the slower airspeed of the wing half at the inside of the turn, which is pointed toward the center of the turn (especially the wing's tip), together with the faster airspeed of the wing half on the outside of the turn. Since your plane was flying near its stall speed before the turn was negotiated, turning further decreased the airspeed of the inner wing half, causing it to stall. Even though the outer wing half has increased airspeed, your plane is, in reality trying to fly with only half a wing, which, of course, is impossible.

WARNING: The altitude loss from a tip-stall is unbelievably rapid! If you do not have enough altitude to recover from a tip-stall your airplane will most certainly crash. This could cause property damage or personal injury.

Can a tip-stall tendency be minimized? YES, by one of three methods.

  1. Do not make a tight-radius turn a low airspeed.

  2. Use a wing with "washout" built in. Washout simply means that the positive incidence of the wing decreases progressively outward to the wing's tips. The trailing edge of the wing is higher at the tip than at the root of the wing. This decreases the angle of attack of the wing tips and minimizes their tendency to tip- stall.

  3. Use a wing with a varying airfoil. A wing having such an airfoil may have a root rib that is fully symmetrical, but its tip rib may be a flat-bottom airfoil (or close to it). In other words, progressing outward from the symmetrical root rib the wing's tip ribs, the wing's airfoil will gradually become semi-symmetrical then continues to change on outward to become a flat- bottom airfoil at the wing's tips. Since a flat-bottom airfoil creates the greatest amount of lift of any of the airfoil types used by R/C aircraft, this decreases the tendency of the wing tips to stall.

NOTE: Methods No. 2 and 3 only decrease the tendency of a wing to tip-stall, and will allow the aircraft to negotiate tight-radius turns at a slower airspeed. However, if an airplane makes a slow enough tight-radius turn, regardless of its wing-type, a tip-stall can still occur.

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© Copyright 1997 All rights reserved Tom Denham