REFLECTOR:Stall Characteristics

[Ronnie Brown]

Jim, Nicely stated!!!

Here's a link to the Cozy articles

http://www.cozybuilders.org/newsletters/news_35.html#deep_stall

http://www.cozybuilders.org/mail_list/topics99/Ang_of_Att.txt

Ronnie


----- Original Message -----
From: "Jim Sower" <canarder@frontiernet.net>
To: <reflector@tvbf.org>
Sent: Wednesday, November 12, 2003 2:07 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 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.
| >
|
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