Mark Steitle wrote:

         Reply to:   RE: Stall proof
I have read that a deep stall can also be caused by modifications to the canard which increases its authority, thereby defeating the "canard stalls first" requirement.  Such mods might include changing the angle of incidnece on the canard, installing canard/elevator gap tape, etc. as well as too far aft cg.  (I'm no expert, just surf the web and read a lot!)  Here is an explanation of deep stalls from the Cozy web site...
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DEEP STALLS

 The July '91 issue of Sport Aviation (and also Nov.'91 Kitplanes) contains a very fascinating article on the investigation of the deep stall characteristics of the Velocity 4-place canard aircraft. In 1988 and 1989, there were 3 reported instances where 3 different Velocities 'locked in" a deep stall condition from which the pilots were unable to recover. The aircraft descended almost vertically, but in a flat attitude, at an unusual ly slow rate of descent, variously reported as 1200 fpm or 15 to 20 mph. Per Burt Rutan, this is several times slower than theory would predict. Two of the aircraft splashed down in water, and one on land. One pilot was uninjured, and the other two received "minor" injuries.

 The article describes the ingenious test program Danny Maher conducted on the ground to determine what it took to stall a Velocity, what happened when it stalled, and what airfoil modifications were most effective in preventing stalls. He was then able to confirm his findings in flight test, with a very clever way of avoiding a recurrence.

 Following publication of the July article we were asked to comment on how this related to the 3 and 4 place Cozy. We think this is a very important subject which we should discuss:

1.First of all. even though the Velocity and the Cozy, particularly the Mark IV, appear to be similar, there are some important differences, most notably in the canard, which is the key to stall resistance. The 3- place Cozy uses the GU airfoil, and the Mark IV the R1145MS airfoil, both of which are used on the Long EZ. The Velocity uses a different airfoil with considerably more area (about 50 % more) than either of the Cozys.

2. It has been demonstrated that the original Velocity could be made to enter a deep stall while being operated within its approved c.g. range. We have not been able to demonstrate this with either the 3-place or Mark IV. The 3-place Cozy was tested 1/2 in. aft of its approved c.g. range and the plans model 1-1/2 in. aft of its approved c.g. range without entering a stall. Admittedly, we haven't tried to force a stall at these extreme c.g.'s by pumping the stick, however.

3. We think the Cozy (and other canard aircraft as well) could be forced into a deep stall if the c.g. were moved far enough aft of the aft c.g. limit and the attitude aggravated by pumping the stick. We see no justificafion for trying to demonstrate this, however, if the approved c.g. range appears to be safe from stalls, because deep stalls do not appear to be recoverable (except by very skillful test pilots) unless a mechanism is in stalled to change the c.g. in flight. What happens is that the excessive drag of the stalled wing causes the loss of all forward movement, rendering the controls
ineffective, so the aircraft simply mushes.
Apparently, even full power is not enough to overcome the drag of the stalled wing and reestablish enough forward motion to make the controls effective.

4. It should be emphzised that deviation from the plans and/or builder modifications could reduce the stall resistance of theCozy. It is mandatory that vortilons be installed on the main wing to prevent airflow separation at high angles of attack.  Extending and/or changing the shape of the nose can affect pitch stability. Canard airfoil shape, incidence and span affect pitch stability, and should not be changed. Even increasing the size of the fuel tanks can be dangerous by allowing too great a shift in c.g. with pitch. You should not fly your airplane without having made an accurate weight and balance evaluation, and without knowing exactly where the c.g. is. You cannot assume that your acceptable c.g. range is the same as our prototype due to individual variances in homebuilt aircraft. You must confirm the acceptable c.g. range of your aircraft by stall tests at least 10,000 ft. above ground while wearing a parachute. If you see any hint of an unusual or uncommanded pitch!

 up, or

any hesitance in nose down contro l power when at full aft stick, go to full power and full forward stick immediately and recover! If your aircraft hangs in a high sink condition, rock it out with ailerons and rudder using maximum available engine power. Ballast your aircraft to a more forward c.g. and retest. If you do not want to take the risk of doing this stall test program, do, at least, limiit your flying to mid or forward c. g.
5. Notwithstanding the fact that we have not been able to stall a Cozy at the aft c.g. limit and beyond, we still advocate using ballast during solo operation to place the c.g. near mid range. The main reason is to avoid a large difference in trim and flying qualities between solo and duo operation, although it also greatly increases the margin of safety as regards stall
resistance. It goes without saying that at any c.g. we don't recommend flying approaches at minimum speed and full aft stick, and aggravating attitude by pumping the stick.

6. We would be remiss if we did not point out the advantage that canard aircraft have over conventional aircraft, as regards stall behavior:

            1.It is said that 25% of general aviation fatalities are due to stall/spin accidents. To our knowledge none of these can be attributed to canard type aircraft.

            2.Most (all?) conventional aircraft will stall at full aft stick while being operated within their approved c.g. range.  With rudder and/or aileron input, they will spin. Canard aircraft are designed to be stall proof (or at least stall resistant) at full aft stick within the approved c. g. range.

            3.When a conventional aircraft stalls, the nose drops and speed builds up rapidly, and the aircraft often spins. This is most likely to occur during approach to landing (or during take off) when there isn't sufficient altitude to recover, and is usually fatal to the occupants. If a canard aircraft could be made to stall, it would descend in a flat attitude at a sink rate not likely to be fatal.

On balance, we believe that canard aircraft are much safer than conventional aircraft for the reasons detailed above.

Danny Maher should be commended for the very innovative test program he conducted and the information he published which greatly increases the knowlege and understanding of the stall behavior of canard aircraft. If you have any further questions on this subject, please contact us.

 
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