REFLECTOR:Stall Characteristics

Chuck Jensen
Wed, 12 Nov 2003 15:47:58 -0500


I read your explanation and started thinking about it...then my head started
to hurt and was about to explode, so I stopped thinking.  All the CGs and
ACs and chords and dischords are interesting, and may even be right, but for
a simple ole country boy, the arrow with feathers on both ends analogy still
seems to capture the issue.  

Drop an arrow with feathers on both ends, and nothing much happens.  Put a
heavy arrowhead on one end and we all know what happens; the same thing as
happens with a foreward CG.  Where the CG is, and whether its affected by
flight attitudes and positions is an interesting discussion, but the key
seems to be "aft CG; good-bye!"  Recovery, if any, is likely to be based on
luck as skill, even though the pilot would be highly motivated.  Yes?

Chuck Jensen

-----Original Message-----
From: []On
Behalf Of Jim Sower
Sent: Wednesday, November 12, 2003 2:08 PM
Subject: Re: REFLECTOR:Stall Characteristics

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
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
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
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
produced, or the lift vector, occurs at about 25% of the MAC (Mean
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
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
pitch down out of a flat vertical descent), and render the flat attitude
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
although the AC of the wing migrates aft, the total AC of the airplane
forward - to the point that, in an aft CG condition, we encounter neutral
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
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
> flight.

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