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From: ark@alice.UucP (Andrew Koenig)
Newsgroups: net.aviation
Subject: Re: Slipping and Cross-Control Stalls
Message-ID: <4308@alice.UUCP>
Date: Thu, 12-Sep-85 17:03:37 EDT
Article-I.D.: alice.4308
Posted: Thu Sep 12 17:03:37 1985
Date-Received: Fri, 13-Sep-85 04:43:27 EDT
References: <763@infopro.UUCP>
Organization: Bell Labs, Murray Hill
Lines: 34

> I am just beginning to get into slips. My instructor surprised me a bit
> during a "pretend" emergency landing, by doing steep (45-degree) S-turns 
> while only 75 to 150 feet off the ground. I
> expressed some mild worry about stall/spins (having read a lot about same
> especially at low altitudes), whereupon he noted that since our nose was
> pointed down quite far, we couldn't stall (I'm not sure of our exact
> airspeed at the time but it WAS at least 60 KIAS; clean stall speed on a
> 172 is 50).

You might want to go back to your instructor and ask him if he really
meant that the airplane couldn't stall because its nose was down.
THAT STATEMENT IS INCORRECT!  An airplane can stall in ANY attitude
at ANY airspeed!!!

What makes an airplane stall is not airspeed or attitude but angle of
attack.  Now it is true that in unaccelerated flight the angle of attack
is closely related to indicated airspeed, but it is definitely NOT
true in unaccelerated flight.  You can stall out of a tight descending
turn very easily with the nose well below the horizon, for instance.

Cessna 172s have a placard cautioning the pilot to "avoid slips with
flaps extended" because under certain combinations of loading, power,
trim, etc.  the turbulence from the flaps can blanket the tail,
causing pitch instability during a slip.  While I have not had it
happen to me, I am told that the instability is not dangerous and
disappears immediately as soon as you get out of the slip.  It is
for that reason that the placard does not say something like "slips
prohibited with flaps extended" -- they aren't prohibited, just
inadvisable.

Incidentally, I have never found anything in the flight handbook
for the Cessna 150 that says anything about slips with flaps in
that airplane, so I expect they're OK.  The manual for the Cardinal RG
explicitly says that slips are permitted in any configuration.