Slipping with Flaps

[bagarre]

It was mentioned in another thread that slipping with full flaps will blanket the horizontal stabilizer and thus cause a stall.

I'm confused as a stall occurs when the angle of attack becomes too great and the wing stops flying due to airflow separation.

In order to INCREASE the angle of attack, your horizontal stabilizer needs to apply DOWNWARD pressure on the tail.
So, if slipping with flaps blankets the tail...the tail is applying LESS downward pressure. This (in an aircraft within CG limits) will cause the nose to lower and DECREASE the angle of attack....the exact opposite of a stall.

I've done full flap slips at altitude to see what happens and the plane never tried to enter a stall. In fact, the nose would pitch down now and then and decrease the AOA as mentioned above.

I don't understand how this can cause a stall unless you let the nose come up and you bleed off all your airspeed...which can happen in any airplane in a slip.

Here is an excerpt from the horse's mouth on the subject. It mentions nothing about stalling the airplane.
http://www.boundvortex.com/ReadArticle. ... ticleID=34

[lowNslow]

It doesn't cause a stall so much as blanket airflow to the elevator and reduces elevator control effectiveness.

[tweiss]

It was mentioned in another thread that slipping with full flaps will blanket the horizontal stabilizer and thus cause a stall.

I'm confused as a stall occurs when the angle of attack becomes too great and the wing stops flying due to airflow separation.

In order to INCREASE the angle of attack, your horizontal stabilizer needs to apply DOWNWARD pressure on the tail.
So, if slipping with flaps blankets the tail...the tail is applying LESS downward pressure. This (in an aircraft within CG limits) will cause the nose to lower and DECREASE the angle of attack....the exact opposite of a stall.

I've done full flap slips at altitude to see what happens and the plane never tried to enter a stall. In fact, the nose would pitch down now and then and decrease the AOA as mentioned above.

I don't understand how this can cause a stall unless you let the nose come up and you bleed off all your airspeed...which can happen in any airplane in a slip.

The stall that is spoken of is not the wing stalling. When the flap blanks the relative wind over the horizontal stabilizer due to the alignment of the two in a slip, it can stall the tail.
Many people have done the the full flap slip with and without the knowledge of what may transpire. Sometimes you may get away with it and experience no effect. Other times you may get a little burble of a nose down nod.
But make no mistake, if the airplane is loaded just right (fore, aft, one gas tank filled more than the other.......) and the right airspeed is achieved with the right angle of attack, you will get a very violent windshield view of earth faster than you can blink.
The test pilots did not tell Cessna to placard against full flap slips on these airplanes just because they didn't think it was cool. That HS works hard to keep the tail where we want it as far a pitch goes. Without that HS back there, the tail will be in front of you faster than ground looping.
My 'unprofessional" advise is DON'T DO IT. The innocent people you may kill on the ground may be someone we know.
Don't mean to rant, but as pilots of these airplanes we've been warned. And at least as far as the B-models go, I don't see the need to dive any faster than what the barn door flaps will do all by themselves already, and my flaps are already ruined by gap seals.
I don't feel the need to tell everybody on my post to fly safe. I think we do a pretty good job as a group. But just be mindful of the parameters in our POH's. These airplanes are antiques for cripe sakes.
Tom

[Bruce Fenstermacher]

Tom has it about right. The elevator quits flying (stalls) and the nose can point straight down. Most of the time however the elevator will become completely limp. Removing the slip, even a little, regains elevator control. If you are not looking for the elevator to go limp and you have the aircraft trimmed pretty well, one might not notice.
I had a VG kit which places VGs under the elevator. I thought it improved the issue but I would not say it eliminated it.

As Tom has sayed and I think most would agree. With the B model flaps deployed fully there is no reason to also have to slip.

The '48 and A model does not have the problem, at least I have not been able to duplicate it in mine, as they do not have the same flaps.

[bagarre]

But make no mistake, if the airplane is loaded just right (fore, aft, one gas tank filled more than the other.......) and the right airspeed is achieved with the right angle of attack, you will get a very violent windshield view of earth faster than you can blink.

The test pilots did not tell Cessna to placard against full flap slips on these airplanes just because they didn't think it was cool. That HS works hard to keep the tail where we want it as far a pitch goes. Without that HS back there, the tail will be in front of you faster than ground looping.
My 'unprofessional" advise is DON'T DO IT. The innocent people you may kill on the ground may be someone we know.
Don't mean to rant, but as pilots of these airplanes we've been warned. And at least as far as the B-models go, I don't see the need to dive any faster than what the barn door flaps will do all by themselves already, and my flaps are already ruined by gap seals.
I don't feel the need to tell everybody on my post to fly safe. I think we do a pretty good job as a group. But just be mindful of the parameters in our POH's. These airplanes are antiques for cripe sakes.
Tom

So, Cessna took the time to PROHIBIT performing stalls with someone in the back seat...but this violent earth view faster than ground looping from performing a slip with flaps is just "Not Recommended" ?

Seems to me if it was actually that bad, they would have said "Dont Ever Do This".

Perhaps there has been a little bit of embellishment on this behavior of the airplane in the last 60 years.

[Bruce Fenstermacher]

David,

I believe this behavior had Cessna continued production, of the 170B as it is, would have lead to the restriction.

As it was they redesigned the rudder and elevator for the 172 which increased the area of the elevator. Wonder why? They also restricted full flap slips in the 172. Wonder why? Later they restricted flap deflection to 30 degrees though this was probably due to other changes like higher gross weight.

Limitations put on aircraft are often because that is what it took for them to pass certification tests at the time. Perhaps there was no requirement to perform a full flap full slip maneuver throughout the CG envelope and the behavior was not voluntarily restricted.

I've personally blanked out the elevator on purpose at altitude and experienced total loss of elevator authority. I did not wait until the nose pointed down but do not believe the stories of it doing so are embellished.

[pdb]

I had a chance to speak with one of Cessna's test pilots and he said substantially the same thing as Bill Thompson, the Engineering Test Pilot and later Manager of Flight Test and Aerodynamics at the Cessna Aircraft Co. as quoted in the reference above: http://www.boundvortex.com/ReadArticle. ... ticleID=34

"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 counter-act it if it occurs close to the ground."

However, he further said that since the phenomenon was so rare and hard to duplicate, they figured try would deal with the problem with a $.25 placard rather than an expensive and uncertain test regimen to try to design this problem out. I think they made the right choice. If you have to slip a 170 with full flaps to get it down, you have really screwed up and need to try again.

[voorheesh]

Another part of this equation is the center of gravity vs the center of lift. The cg is in front of the center of lift exerting a nose down force which is countered by the down load on the horizontal stabilizer. If the download goes away, the nose will go down. If it happens suddenly, it can be very abrupt. There are several turboprops that have restrictions on flap deployment in icing conditions for this same reason. If I remember, it involves downwash from the flaps interrupting airflow over the horizontal stabilizer causing an immediate loss of download (negative lift). Even a small accumulation of ice on the leading edge of the stabilizer would allow this to happen. Twin Otters and Jetstreams have had accidents associated with this. A fwd cg might aggravate the situation.

Related subject. When I was learning to fly, my instructor insisted on all students being proficient at spins. He demonstrated a spin in a Cessna 150 with full flaps (a maneuver that is probably prohibited). He demonstrated a power off entry in a simulated skidding base to final turn and that little plane went upside down so fast, my head spun. No buffet or warning what so ever. It went straight down and lost about 800' before recovery even after he selected flaps up, opposite rudder, and release of back pressure on yoke, all immediately. I am pretty sure that a 170B will react in a similar fashion which behooves us to be careful in the pattern.

[T. C. Downey]

I've tried to get N2801C to do that and was never successful.

[Blue4]

I will respectfully disagree with a few of the posts here, but attempt to explain each point.

1) I recommend trying slips with your airplane at altitude for practice. I've flown a lot of Cessna singles, while never having a problem with a combination of slips and flaps, so I always wondered why the "Slips with Flaps not recommended" note was in all the books. Well, its readily apparent in a B-model! Try it AT ALTITUDE. When the nose comes noticeably down quickly, it certainly will get your attention! The good news is the most natural thing in the world is to remove the slip, and you'll have your friendly old airplane again. In short, it is a good idea to know your ride inside and out.

2) There are times when slips ARE necessary during landing, notes or not. When flying from approved airport to approved airport, you may not have many occasions where the technique is needed--enough said. To provide stronger evidence, the wing-low method of landing in a crosswind necessitates slipping the airplane. The counter argument might be that a side slip is different than a forward slip. Aerodynamically, however, there's no difference. I'm quite certain that you still use some flaps while landing in a crosswind, don't you? You're violating that note when you do.

3) Therefore, to know how your airplane acts, re-read statement number 1. There airplane will talk, you just have to listen.

-Scott

[GAHorn]

Good words.
(and on a related matter, in strong crosswinds I avoid the use of full flaps in my B-model in order to increase flight control effectiveness, touching down somewhat faster than usual, then applying brakes, retracting flaps, etc. to get below flying speed quickly....but that's another discussion.)

baggare, sometimes unfortunate word-choices occur in everyday discussions and misunderstandings occur.

It may be that anecdotally, folks refer to the sudden pitch-down associated with this manuever as similar to that which may occur with a common stall..... but that is not what technically/aerodynamically happens.

Technically, the "stall" which occurs with B-models slipping with full flaps refers to the tailfeathers.
Here's what happens: The tail, as already noted, provides downwards-lift... counterbalancing the CG (aircraft loaded center-of-gravity) which is forward of the CL (wing center-of-lift.)

When the B-models large flaps are deployed, a strong down-wash is created just forward of the horizontal stabilizer. Now, remember the horizontal stab is an upside-down wing...and that downwash mimics a large increase in AOA (angle-of-attack)....therefore the horizontal stabilizer is working at or near it's maximum....at it's job of hold the nose up.
Now...slip the airplane, and the fuselage will blank-out the opposite-side horizontal stabilizer....suddenly reducing by HALF the authority of the tail....and the nose DROPS, suddently, sometimes a violent pitch-down, and with very little altitude for recovery if in the landing pattern on short final.

I hope that helps understand the admonition by the manufacturer to "Avoid Slips with Full Flaps" in the B-model. I read that as, not advice,....not a recommendation...but an operating limitation! In order to comply with the Type Certificate operating limitations one WILL.....avoid slips with full flaps in a B-model.

[blueldr]

As the old Packard Motor Car slogan used to say, "Ask the man who own one."

When I first got my '52 C-170B airplane, I did not feel that I really needed to spend much time on the little book on how to fly it. After all, I was an Air Force Command Pilot, an airline captain with an ATP and type ratings, and I had considerable experience flying other small airplanes.

When I was quite new to the C-170B, I was making a full flap approach to Cameron Park Airport here in California. Clever pilot that I am, I discovered that I was somewhat high to make the 1st third of the field. I advised myself, "Self, you need to slip off some of this excess altitude, particularly since someone may be watching you."

Use lots of left stick and plenty of left rudder.

OMG! Why is the windshield full of mother earth instead of runway? How did all that stuff from the rear get up here under the panel and windshield? S.O.B.! This is a p--s poor place to be!

Had I been twenty feet lower at the start if this maneuver, I would not be telling about it now.

Shortly threrafter, when I had stopped shaking enough to turn the pages, I found where the Cessna test pilots were a lot smarter than I had given them credit for.

[daedaluscan]

I have tried slipping with full flaps at altitude. What I felt was a softening of elevator authority, and a lack of centering - when I pulled up the elevator italmost wanted to pull me in that direction. Definitely a loss of authority but I never got the full nose down attitude, probably due to my lack of comfort at continuing.

My question is what about a slip with 20° of flap? I often want to lose a bit of altitude without engaging full flaps as I want to land a little faster due to crosswind/gusty conditions. I feel pretty comfortable doing this, and have never experienced a loss of elevator authority?

[bagarre]

This is making sense to me now.

If the horizontal stab was to be completely blanked out it makes sense that the nose would 'flop down'.
It would depend greatly on the AOA and configuration when entering the slip to get it 'just right' to nullify the entier tail at once.
Else, you'd just get the soft feeling described (or the slight bobbing I mentioned) as the tail would only by partially obscured.

But if you did null the tail, it makes complete sense for the plane to pivot around the center of lift as the center of gravity is forward of that. Arguably, the more forward the CG is from the CL, the more dramatic the effect would be.

The pivot point would be at the center of lift which is very different from a traditional stall.
In a wing stall, the wing falls thru the air as the nose drops to re-establishes the right AOA.
In a tail stall, the wing is still flying. There wouldn't be a 'fall' but rather a see-saw pivot.

Would be interesting to do some wind tunnel tests or model it in software.

wow.

[Bruce Fenstermacher]

Charlie, 20 degrees and a slip should be OK. The warning is against a full flap slip.