Deep Stall Recovery? A deep stall occurs when the main (rear) wing of a canard aircraft stalls before the forward canard.  Unless recovery is initiated quickly, the aircraft’s angle of attack continues to increase until the canard airfoil also stalls.  With both wings stalled, the aircraft may lack enough control authority to recover.  The pitch attitude becomes “locked-in” and the aircraft descends at low speed and high angle of attack.  Unrecoverable stalls are rare, but several types of canard aircraft have experienced deep stalls during their development.  This resulted in various design modifications (shorter canards, vortilons, revised CG limits, etc.) to provide adequate safety margins. 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. The Apollo’s low wing and strake-less design are different enough from other canards (Long-EZ, Cozy, Velocity) that we wondered how it might behave in a deep stall scenario.  After studying the subject, we are cautiously optimistic that the Apollo’s deep stall behavior will be more benign than comparable aircraft.  To explain our optimism, we must review how airflow behaves during unrecoverable deep stalls.  These incidents are well documented in pilot reports and flight tests of other canard aircraft. The picture at right shows a “steady state” deep stall with the canard and main wing stalled.  The angle of attack is around 45 degrees and the canard elevator has lost effectiveness.  Forward speed is 35 to 50 mph and the descent rate is between 3000 to 4500 feet-per-minute.  The application of power has no effect or causes the nose to pitch slightly upwards.  With the wing and canard stalled, what are the forces that prevent this aircraft from recovering?  There are several explanations: 1. 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) 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. 2. The fuselage is also destabilizing because it has a large area and the center of lift is located forward of the CG.  The shape is not as effective as the strake in producing lift, but the fuselage doesn’t stall like an airfoil does.  The strake and fuselage lift eventually reach a state of equilibrium with other aerodynamic forces to hold the nose steady, but they may resist attempts to push the nose down.  When the strake is stalled, pushing the nose down unstalls the strake first, which pulls the nose back up. 3. 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?  Was this the result of the strake unstalling itself or is there another reason?  If we review the previous picture (above), we can see that the upper half of the propeller is operating in turbulent flow from the stalled wing/strake, whereas the lower half of the prop is exposed to relatively clean incoming air.  The application of power may cause the lower half of the prop to produce more thrust than the upper half, which pushes the nose UP!  Makes perfect sense once you think about it. Now let’s examine why the Apollo might be better off in a deep stall scenario.  We can see the following differences in the picture at right: 1. The strake has been removed and its destabilizing effect is eliminated.  Lift from the strake can no longer pull the nose up after the canard has stalled.  We still have fuselage lift pulling the nose up, but the force is less strong without the strake’s contribution.  This may give the pilot more time to recognize the deep stall and push the nose down before the elevator becomes ineffective. 2. The Apollo’s low wing obstructs approximately equal areas of the propeller’s upper and lower halves.  Without strakes, the turbulence that affected the upper third of the prop arc during a deep stall is gone.  It appears the upper and lower parts of the propeller can produce fairly equal amounts of thrust. 3. Lowering the wing and fuel mass moves the aircraft’s CG about 0.8” lower.  The Apollo’s thrust centerline is also 1.25” higher than the reference aircraft, so the thrust centerline is now 2.0” higher relative to the CG.  This change ensures a stronger nose down moment when the Apollo’s power is applied.  Since the thrust asymmetry was also reduced, I am hopeful that application of power may now effect recovery from deep stalls.  Please Note: This is a theoretical discussion about deep stall behavior.  I am not claiming that deeps stalls will be more benign or recoverable in the Apollo and this capability is not a design goal.  I am merely speculating that it is a possibility based on the Apollo’s configuration.  Flight testing of the actual aircraft will determine whether there is any real benefit.  And even if there is, main wing stalls on a canard aircraft will remain dangerous and must be avoided. Site Map Email the Designer Copyright © 2012 Apollo Canard