david@infopro.UUCP (David Fiedler) (09/11/85)
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). It seemed almost obvious later, and explained why you can
cross-control in a proper slip with nose down, but you would be in trouble
in more normal attitudes. My question is: since there don't seem to be any
absolutes in flying, is this true all the time, or are there some odd
combinations of weight, balance, and winds that might cause problems when
slipping? And why is it that 172s (and other planes) are placarded against
slipping with flaps down? Does it have to do with the flaps blocking
airflow to the tail section, or what?
--
Dave Fiedler
{harpo,astrovax,whuxcc,clyde}!infopro!dave People Phone: (201) 989-0570
USMail: InfoPro Systems, 3108 Route 10, Denville, NJ 07834
Flight Instructor: "What would you do if I jumped out right now?"
Student: "Close the door and retrim?"cfiaime@ihnp3.UUCP (Jeff Williams) (09/12/85)
In article <763@infopro.UUCP> david@infopro.UUCP (David Fiedler) writes: >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). It seemed almost obvious later, and explained why you can >cross-control in a proper slip with nose down, but you would be in trouble >in more normal attitudes. The first law of flying (even before "pay your CFI") is: An airplane can stall in any attitude and an any airspeed. That is to say that nose position makes no difference as to the ability to stall an airplane, whether in a slip, skid, or straight and level. However, most airplanes (most, not all) don't have the elevator authority at low airspeeds and in a slip to stall. This is not to be considered a guarantee. Where a stall in a slip happens, the airplane will break into a spin "over the top" where any competent pilot can recover from the incipient spin before the airplane actually departs into the classic nose down attitude of a spin. By the way, a stall from a skid will break "under the bottom" where most pilots take up to a half turn before the airplane is recovered. In the traffic pattern, this is the one that kills. As a suggestion, Sammy Mason has written a very good book, "Stalls, Spins, and Safety." Mason is an ex-Lockheed test pilot with very good credentials. Jeff Williams ihnp3!cfiaime
ark@alice.UucP (Andrew Koenig) (09/13/85)
> 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.
daver@hp-pcd.UUCP (daver) (09/18/85)
When you take your written test, one of the questions the FAA seems to always ask is under what conditions an airplane can stall. The correct answer is at any attitude and any airspeed, and they mean it. I have accidentally stalled an airplane twice, both times on the same flight in a Cessna 150, and in both cases the nose was pointing straight towards the ground and the plane was flying at over 100 mph. The plane was an aerobat and was close to gross for aerobatics, I was used to flying acro in Citabrias, with much lighter wing loading, and I tried to pull up out of a loop too hard. It really does happen. Dave Rabinowitz hplabs!hp-pcd!daver
dsmith@hplabsc.UUCP (David Smith) (09/26/85)
> When you take your written test, one of the questions the FAA seems to always > ask is under what conditions an airplane can stall. The correct answer is at > any attitude and any airspeed, and they mean it. This answer may be correct, but is pretty nebulous. Isn't it better to say "when the angle of attack is too great"? David Smith hplabs!dsmith