Grassstrippilot wrote:A while back I came across some information regarding the aerodynamics involved with performing forward slips in aircraft. I thought I had posted it but when this subject came up on another thread, I looked and couldn't find it. So, here it is. We've all heard about the warnings about slipping with full flaps (I can tell you with certainty that it can be done at least in 172s and 182s) and I think most people reference the "blanketing of the elevator". So when I read this, I thought it was very interesting. This was the first time I've ever read an explanation of the aerodynamics of what is happening.



Here’s what Bill Thompson, former Manager of Flight Test & Aerodynamics at Cessna, had to say about the issue of slipping with full flaps in the 172 (Cessna — Wings for The World, by William D. Thompson, Maverick Press, 1991, p. 41):

With the advent of the large slotted flaps in the C-170, C-180, and C-172 we encountered a nose down pitch in forward slips with the wing flaps deflected. In some cases it was severe enough to lift the pilot against his seat belt if he was slow in checking the motion. For this reason a caution note was placed in most of the owner’s manuals under “Landings” reading “Slips should be avoided with flap settings greater than 30 deg. due to a downward pitch encountered under certain combinations of airspeed, side-slip angle, and center of gravity loadings”. Since wing-low drift correction in crosswind landings is normally performed with a minimum flap setting (for better rudder control) this limitation did not apply to that maneuver. The cause of the pitching motion is the transition of a strong wing downwash over the tail in straight flight to a lessened downwash angle over part of the horizontal tail caused by the influence of a relative “upwash increment” from the upturned aileron in slipping flight. Although not stated in the owner’s manuals, we privately encouraged flight instructors to explore these effects at high altitude, and to pass on the information to their students. This phenomenon was elusive and sometimes hard to duplicate, but it was thought that a pilot should be aware of its existence and know how to counteract it if it occurs close to the ground.

You'll notice that beginning with the 1972 model year ('73 for European-built models) 172s have a larger dorsal fin. This apparently eliminated the "pitch-down" problem.

However, there is also an unrelated, more benign phenomenon that Thompson described in newer models in full-flap slips: “a mild pitch ‘pumping’ motion resulting from flap outboard-end vortex impingement on the horizontal tail at some combinations of side-slip angle, power, and airspeed.” This really isn't a problem and it's not limited to Cessnas -- my Sport Cub does it, as well, and it doesn't keep me from slipping with full flaps in either airplane.

So although the 172L’s larger dorsal apparently solved the pitch-down issue, they kept the cautionary note in the POH because of the latter phenomenon.

Unfortunately Cessna contributed to the “end of the world” fear of slips with flaps, by not explaining the pitch-down phenomenon in the manuals; and in fact, many earlier C-172 manuals expressly said that slips with full flap were prohibited. I rummaged through my collection of old Cessna owners manuals:

1958 C-172: “prohibited”
1959 C-175: “prohibited”
1966 C-172F: “prohibited”
1972 C-172L (first year of the big dorsal): “should be avoided”

The manuals and TCDS for these older models have been revised since then, and there is now no legal prohibition against slips with flaps -- but that’s what a lot of us old-timers read back then and remember.


And some further clarification:


The pitch-down that might occur with flap-down slips in Cessnas is not from "elevator blanking." Cessna's aerodynamicist and test pilot said, "The cause of the pitching motion is the transition of a strong wing downwash over the tail in straight flight to a lessened downwash angle over part of the horizontal tail caused by the influence of a relative 'upwash increment' from the upturned aileron in slipping flight."

In other words, in extreme slip conditions some of the upwash from the upturned aileron on the lowered wing might hit the horizontal stabilizer. Flaps cause downwash on the horizontal stabilizer -- that's why retrimming is necessary when flaps are extended. When some of that flap downwash is replaced by upwash from the aileron, there is very suddenly too much nose-down trim, and the nose pitches down.

"Elevator blanking" makes it sound as if the elevators lose effectiveness, and that's not what is happening.

The pitch-down is not a consistent phenomenon in 172s; Cessna test pilots found it "elusive and sometimes hard to duplicate." And apparently the larger dorsal fin from 1972 onward prevents the 172 from yawing far enough in a slip to allow aileron upwash to hit the tail, so the issue is moot.

It's interesting that the old (pre-1972) C-172 owners handbooks (which do not carry the force of law) said slips with full flaps were "prohibited" -- while the current version of the TCDS (which is the "law") only calls for placards saying "Avoid slips with flaps down" (172 thru 172E) or "Avoid slips with flaps extended" (172F thru 172N). The old "prohibited" language is nowhere to be found.

There is no caution against slips with flaps in the 172P and newer, in which maximum flap extension is reduced to thirty degrees.

contactflying wrote:Staying in low ground effect over that long runway would have been a lot safer. Once they had gotten well out of ground effect and onto a wooded hill they could not out climb, they should have flown all the way to the crash like Les did with his Stinson and lived.

This is something that makes complete sense to me but no one teaches. In fact, the FAA teaches the opposite by instructing all climb outs be Vy or Vx.

contactflying wrote:cstolaircraft,

I wouldn't try to fix the nose pumping that happens in Cessnas with full flaps and strong slip. If we just leave it alone it works out dynamically. I think the problems Bagarre and Robw56 brought up were attempts to fix it with elevator input.

Again, where altitude and time are available we shouldn't need a forward slip with Cessna's good fowler flaps.

If you lose an engine on takeoff or while conducting a maneuvering flight operation, I suggest you go ahead and do every thing you can to get into that spot right there right now. Three times I had that situation and found full flaps and full slip to be appropriate and useful to get into the beginning of very small landing zones. Low level engine failures are six second deals. We can see only the one or even two very close survivable landing zones. The one we want is almost always obvious because of the few choices. We are almost always high and fast. That sounds crazy at 100' but at 100' the LZ is very close. Getting down in the beginning of the LZ can mean saving tin, but most importantly it means saving skin.

Thanks for you comments.

Contact

cstolaircraft wrote:....From what I understand the problems with cessnas falling out in a full flap slip are from the flaps blanking out the elevator. adding the power was a way for me to increase the effectiveness of it. From my experience with full flaps slips cause the nose to pitch down a good bit causing an increase in airspeed, adding the extra power helps prevent the loss of airspeed while having little effect on the rate of decent.