C172 flap question

[shumway76]

In the C172, it is mentioned that sideslipping is prohibited with flaps extended. Why?

[Aphrican]

I wondered that too.

Could it be because the lift differential on the two wings increases massively with flaps extended during a sideslip given the position of the flaps in relation to the fuselage and relative wind? This could mean that you might run out of aileron quite quickly and too close to the ground.

I also get further confused as the generally accepted cross wind landing technique essentially calls for a very moderate sideslip anyway.

[Unusual Attitude]

I owned a share in a C172 with 40 flap for about 10 years and never really got to the bottom of this.

Only issue I ever noticed was that you got a fair bit of buffet over the tailplane when sideslipping with 40 flap. I only ever practiced this at altitude as with 40 flap it dropped like a brick anyway, if you needed to sideslip also you'd have to have made such a pigs ear of the appraoch you should be throwing it away for another try.

Only reason I ever tried it out was to keep it up my sleeve in the event of a forced landing as a 'last resort to get you in'.

Heard all sorts of conflicting rumours from the tailplane being blanked giving you no pitch authority (not the case when I've done it) to the buffet potentially being confused with stall buffet etc

Interested to hear if there is a definitive answer....?

[mad_jock]

Its because the barn dooor flaps will blank/ throw dirty air onto the rudder which will distrupt the airflow round it and there is a possibility of a fin stall or getting control issues.

[Mark1234]

To the best of my memory, it is not *prohibited*, I believe the wording is 'not advised'. Further it varies from model to model. The reason is that under some circumstances it can affect the elevator. For the full story, I'll quote PilotDAR:

Irish Stu, Though I will suggest caution when slipping fully flapped 172's, to my knowledge (the older ones for sure), doing this is something to "Avoid", it is not prohibited. This stems simply from non ideal handling in pitch at certain speeds. Similarly, a door off on a 172 can induce this at about 65kts, and it just feels a bit spongey in the elevators. It certainly won't hurt you.

During flight test, all of this era of planes had to meet this requirement:


3.118 Directional and lateral stability
(a) Three-control airplanes.
(1) The static directional stability, as shown by the tendency to recover from a skid with rudder free, shall be positive for all flap positions and symmetrical power conditions for all speeds from 1.2 Vs1 up to the maximum permissible speed.
(2) The static lateral stability, as shown by the tendency to raise the low wing in a side-slip, for all flap positions and symmetrical power conditions, shall:
(i) Be positive at the maximum permissible speed.
(ii) Not be negative at a speed equal to 1.2 Vs1.
(3) In straight steady sideslips (unaccelerated forward slips) the aileron and rudder control movements and forces shall increase steadily, but not necessarily in constant proportion, as the angle of sideslip is increased; the rate of increase of the movements and forces shall lie between satisfactory limits up to sideslip angles considered appropriate to the operation of the type. At greater angles, up to that at which the full rudder control is employed or a rudder pedal force of 150 pounds is obtained, the rudder pedal forces shall not reverse and an increased rudder deflection shall produce increased angles of sideslip. Sufficient bank shall accompany sideslipping to indicate adequately any departure from a steady unyawed flight.
(4) Any short period oscillation occurring between stalling speed and maximum permissible speed shall be heavily damped with the primary controls (i) free and (ii) in a fixed position.


The 172 does meet the requirement, but is marginal.

I shall quote William Thompson, former Cessna test pilot, who writes in "Cessna, Wings for the World"...

...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 degrees due to a downward pitch encountered under certain combinations of airspeed, sideslip angle, and center of gravity loadings".

I very highly recommend that any Cessna pilot read that book, it is facinating!

Use caution, discuss with your instructor, and learn. Do not do "prohibited" things in planes. "Avoid" or "Use caution" are just that. Don't worry about flap tracks, just fly well maintained planes, and do walk around inspections as if you're about to hang in the air from the plane.

Be careful, but go and learn, that's what you're up there for.

Permalink
http://www.pprune.org/private-flying/360563-c172-rate-descent-sideslipping.html#post4691361

[rapidshot]

good point Mad Jock,
one from me, due to increased downwash angle downward force on elevator should also increase but i think this increase in downward force reduces the nose down pitching moment created by flaps,

[mad_jock]

Read Pilot DAR stuff he knows way more about these things than I do.

[cessnapete]

I owned a C170 ( forerunner of the 172 with a tailwheel) for several years. Early on in my ownership I wondered why the pilots handbook cautioned against sideslips with full flap.
One day I tried it at a safe height. On the third attempt with full flap at 70mph the usual approch speed, I applied full rudder and slipped, after a few seconds the nose dropped through almost 90 deg at a rate that threw me against the seat straps. The elevator lost its feel as though the cable had snapped.

I recovered in 300-500 feet. At certain angles of slip the tailplane is obviously blanked. I didnt do it again-the handbook was correct!!

[RTN11]

Only issue I ever noticed was that you got a fair bit of buffet over the tailplane when sideslipping with 40 flap.

Control Buffett, as apposed to airframe buffett, would indicate a tail stall which would decrease elevator authority, to nil in the event of a full tail stall, so I would be careful about experimenting too far with this.

Was it control buffett or airframe buffett you encountered?

[C-dog]

Quote from my 1962 Cessna 172C Owner's Manual, page 3-6, Landing:

"Normal landings are made power off with any flap setting. Slips are prohibited (my emphasis) in full flap approaches because of a downward pitch encountered under certain combinations of airspeed and side-slip angle"

It's a wonderful document - the same section on landing says "landings are usually made on the main wheels............."

[Mark1234]

I'm inclined to suggest it's pretty futile trying to guess what is actually happening to the flow; what comes off those barn doors will be all over the place, and the way it interacts highly variable.

That said, I'm inclined to think if we had stalling tail feathers and 90 degree pitch excursions, there would have been a few incidents, and something would have been done. There are enough 172's around, and they date back to when the majority probably regarded sideslips as normal, we're hardly talking about a hot new design.

For comparison, with C-dog, my 172N, manual says, (on page 4-19) "Steep slips should be avoided with flap settings greater than 20deg due to a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle and centre of gravity loadings" Emphasis mine.

So yep, it varies by model. Hopefully we've also answered why!

[Pilot DAR]

To offer the more complete story, here is the entire passage pertaining to this subject, from Cessna, Wings for the World, by Thompson, page 41:

With the advent of 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 degrees due to a downward pitch encountered under certain combinations of airspeed, sideslip 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 manoeuvre. The cause of the pitching motion is the translation 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 up turned 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.

When the larger dorsal fin was adopted in the 1972 C-172L, this sideslip pitch phenomenon was eliminated, but the cautionary placard was retained. In the higher powered C-172P and C-R172 the placard was applicable to a mild pitch pumping motion resulting from flap outboard-end vortex impingement on the horizontal tail at some combinations of sideslip angle , power and airspeed.

[Big Pistons Forever]

The advice "You can't slip with flaps in the C 172" is another flight school urban legend that should be stomped out. There are no restrictions on slipping in the "limitations" section in any of the POH's I have seen (C 172 L,M,N,or P). The note to avoid slips is contained in the "Normal Operations" section.

Frankly the C 172 has plenty of flaps. If full flap won't get you on the desired final profile then I would suggest your approach is so bent out of shape, you should go around.

But if you are high on a forced approach you want to use everything at your disposal including slips. I am dismayed at the number of students who were told they could never slip the aircraft with the flaps deployed even in the forced approach scenario.

[Genghis the Engineer]

The advice "You can't slip with flaps in the C 172" is another flight school urban legend that should be stomped out. There are no restrictions on slipping in the "limitations" section in any of the POH's I have seen (C 172 L,M,N,or P). The note to avoid slips is contained in the "Normal Operations" section.

Frankly the C 172 has plenty of flaps. If full flap won't get you on the desired final profile then I would suggest your approach is so bent out of shape, you should go around.

But if you are high on a forced approach you want to use everything at your disposal including slips. I am dismayed at the number of students who were told they could never slip the aircraft with the flaps deployed even in the forced approach scenario.

Oh it gets worse than that BPA - I was told the same my my instructors on my CPL course, which I flew initially in a PA38 then in a Piper Arrow.

I did what I was told to keep them happy, but so far as I've ever been able to establish there is absolutely no good reason not to sideslip a high approach, and in particular a PFL, in any Piper single. I can only assume that this scuttlebut had found it's way from somebody somewhere who did their instructors course on a C172 and was given duff information there, which they took as gospel for all flying.

More people need to read the (**(&&*&* POH and use it properly.

G

[lasseb]

I'm guessing here but:

When you lower flaps in a C172 you get a heavy downwash on the elevator causing the plane to pitch up. (just try flying straight and level and apply flaps)...

When you sideslip, some of this downforce might miss the elevators because you are flying the plane sideways into the wind. Also the tail fin will block some of the downfoce - again because you are flying sideways. All this will happen quite turbulent, and will cause the downforce on the elevator to vary quite a bit, causing all sorts of pitch issues.
Again - just a guess..

[GeeWhizz]

At best glide in a C172 (~65kts) with the full 40 degrees of flap you have a stonking descent profile! Approaches that are still high in this configuration should be abandoned and flown again properly. There's never much need to side slip a C172 IMHO.

Considering side slipping however, if the 40 flap with idle power wont bring the bloody thing down (which I fail to believe it wouldn't) and going around for another attempt is impossible it's a forced landing situation. In this case a flapped side slip would be the only option regardless of what the POH recommends. It's probably the only time going against the POH is a good thing if the aeroplane occupants survive.

[lasseb]

At best glide in a C172 (~65kts) with the full 40 degrees of flap you have a stonking descent profile! Approaches that are still high in this configuration should be abandoned and flown again properly. There's never much need to side slip a C172 IMHO.

I have flown a C172 with full flaps and 45 bank with a ROD of less than 300 fpm... This was due to heavy thermals in the area.

And in an emergency engine failure situation where you are about to overshoot it is good to know what will happen if you try sideslipping. The alternative might be hitting something really hard.
So practising sideslip with full flaps (at a safe altitude) is not a bad idea at all.
(geewhizz->the latter not aimed at you, as this was you point exactly ;-) )

[cessnapete]

A 1954 170B. 172 type fowler flaps.
I must not practice manouvers that are prohibited in the POH !!

[Red Top Comanche]

I tried this once (at high altitude)

Its OK until you let the speed bleed off, then without any warning it goes into a cross between a flick roll and a bunt. Quite scarey but the lesson was simple, watch the speed but it does give a great rate of decent. Only way to get it better ROD is get in a stable phugoid at about 35kts.

Remember aircraft bite fools quickly.