Deep stall

Deep Stall is a aerodynamic stall condition that is difficult or impossible to correct and unstall the wing. In some flying tail aircraft it is the blanketing of the rear control surfaces by a stalled main wing. In a canard it is typically a stalled main wing, which the canard has no aerodynamic ability to nose down and correct.

The revised designs ensure that deep stalls occur only if the canard aircraft is built or operated beyond its approved limits. The most likely cause of a deep stall today is flying with the CG beyond the aft limit, either knowingly or unknowingly. Building errors that result in incorrect incidence angles on the canard or main wing can also be the cause. These issues should be identified and resolved during the aircraft’s test phase. Despite the rarity of deep stalls, some pilots express concern that unusual attitudes could alter the normally benign canard stall behavior. This is particularly worrisome when attempting a loop where airspeed decays rapidly in an unusual attitude where the weight might tend to make the aircraft slide backwards. Since there is no prop-wash on the rudders of a canard, it is not possible to use the rudder to recover from a hammerhead maneuver.

After deep stall testing was completed on the Velocity and Cozy IV, there was evidence that the strakes contributed to unrecoverable deep stalls. The delta wing planform of the strake delays its stall and the strake’s center of lift is forward of the aircraft’s CG. This was confirmed during low speed testing of the Velocity (Sport Aviation, July 1991 - Sport Aviation, September 1991 - Kitplanes November 1991) which showed the wing and canard stalled at 18 and 20 degrees pitch angle while the strakes did not stall until 26 degrees. At aft CGs, lift from the strakes were pulling the nose higher even after the canard had stalled; whereas canard stall normally results in a nose-down pitch. Strakes are known to be destabilizing and this demonstrates one reason why.

All the reports state that the application of power had almost no effect or resulted in a slight nose-up pitch. Some pilots reported that the propeller was “blocked” or “cavitating”. What would cause the nose to pitch up when adding power? Velocity test pilot Carl Pascarell reported (in that November 1991 kitplanes article) riding the full stalled Velocity to the ocean in N81VA without injury.