REFLECTOR:Stall Characteristics

[Jim Sower]

You may recall that Danny and Nat puffer both experimented extensively with deep stall
situations.  At forward CG, neither could produce a "steady state" deep stall - the
airplane, if left alone, recovered itself.  IIRC a hammerhead was the preferred method
of entry.  They used a weight that they could move fore and aft in flight to alter the
CG so as to produce the steady state (unrecoverable) deep stall condition we all dread
so much.  That is how they documented where their CG limits should be to avoid the
situation.  When they did encounter steady state deep stall, they noted the position
of the adjustable ballast so they could compute the CG at which it occurred, and then
recovered by the simple expedient of sliding the ballast forward and thereby shifting
the CG forward to where "steady state" deep stall could no longer occur.

All of this is pretty well documented.  Both Danny and Nat have been in and out of
deep stall extensively.  They determined the CG at which steady state deep stall
occurs, and in so doing, proved that at more forward CG conditions, it CANNOT occur.

A possible explanation for the phenomenon that occurred to me:
When flying, the airfoil produces lift in such a pattern that the sum of the lift
produced, or the lift vector, occurs at about 25% of the MAC (Mean Aerodynamic
Chord).  When in a deep stall, the whole airplane is behaving like a flat plate, and
the "lift" (actually flat plate drag) vector of the wing acts through the geometric
center, or about 50% MAC.  Since the CG is forward of the Aerodynamic Center in flight
(Aerodynamic center being the sum of the moments of wing lift and canard lift), it
will be somewhat farther forward of the AC if the wing lift is at 50% MAC rather than
25% MAC.  This would tend to increase positive pitch stability (make the airplane
pitch down out of a flat vertical descent), and render the flat attitude unrecoverable
deep stall impossible.  However, the fuselage, which produces no lift if flight will
produce "lift" in a deep stall condition effective at the geometric centroid of the
fuselage, which is forward of the aerodynamic center of the wing-canard combination in
flight.  This would reduce pitch stability (the tendency of the airplane to pitch down
(and out of the deep stall condition)).  So what happens in a deep stall is that
although the AC of the wing migrates aft, the total AC of the airplane migrates
forward - to the point that, in an aft CG condition, we encounter neutral pitch
stability and the airplane attitude will remain where it is, which is "flat".  At
forward CG, although there is much less positive pitch stability, there is still
enough to allow the nose to fall through and end up flying in a steep dive (which is
easily recoverable).  Nat's and Danny's experiments tend to bear this out.

This would explain why so many canards that have done hammerhead type maneuvers in a
safe CG range have had no deep stall problems, but with sufficient aft CG they will
all encounter unrecoverable deep stall.

Just a theory (except the part about Nat's and Danny's test programs) ... Jim S.

Scott wrote:

> Scott,
>
> I did not realize you were a CFI and also was unaware of how many hours in
> a Velocity you have. I am revising "my opinion" of "your opinion"! :-)
>
> This conversation has gotten away from us.
>
> It started with a statement that you could deep stall the airplane if you
> flew it outside of "Normal Flight".  The problem being how to define normal
> flight.
>