C172 : "Avoid slips with flaps extended". Why?

[alexka]

C172P Flight Operation Manual briefly mentions :

"Avoid slips with flaps extended"

No explanation is given about why is this not or might not be recommended? Can please somebody clarify the reason behind?

E.g. what might happen in case flaps are 10 (20) and sideslip maneuver is executed?

Under term "sideslip" I understand a maneuver, where ailerons and rudder are deflected to practically full angle in order to descent rapidly due to increased drag.

Thanks,
Alex

[Rod1]

Because the flaps allegedly blanket the rudder which could ruin your day.

Rod1

[S-Works]

Blanking of the rudder I believe. My 172 is not allowed to side slip with full flap 40' but fine on the others.

[FullyFlapped]

I no longer have my C172 manuals, but I must admit I thought it was because of the risk of blanking the elevator, not the rudder, especially with 40 degs of flap on (i.e. barn-door mode).

Surely in a slip the rudder is fully deflected in any case, otherwise how can you be slipping ?

FF :ok:

[PAPI-74]

The C172, as you say, has barn doors. These give a huge ROD. Approaching the ground below 1000' with a 2000 FPM + ROD in low hours hands is not a good idea. This is one reason.
The other is as you say....blanking.
A sideslip needs all 3 controls: Ailerons for the bank angle, Rudder to control the descent path and Elevator for speed control.
Some types are ok to slip with full flap, but not the 172, as the downwash from the lowered flap, reduces the effectiveness of the Rudder and Elevator.

You will also notice a speed change due to the pitot position (try opening the door....)

If you have too much angle of bank, you will need loads of rudder to keep on your flight path. This may cause the rudder to exceed it crit AoA. Don't forget the RAF/Flight Path is now in the direction of the lowered wing and 90 deg to the rudder keel.
Again the elevator has also a RAF/FP change. This blanks the raised section of the Elevator, as the aircraft slips towards the lower wing or surface.
Not good if the slipstream off of the wing surface is deflected down by flap, producing more downforce on the elevator and an increased load factor.
With it being blanked and at a higher load factor, you might not get a lot of warning when she departs controlled flight.

[FlyingForFun]

Almost all the replies here have mentioned the 40 degrees "barn door" flaps.

But I think, from memory, the same advice is given in the manual of all variants of C172, including those that only have 30 degrees of flap?

FFF
--------------

[alexka]

Surely in a slip the rudder is fully deflected in any case, otherwise how can you be slipping ?

Apparently some people call wing-down approach for crosswind landing as "sideslip". On the other forum I have also read about term "forward slip" which is claimed to be an American version of what "we" call descend sideslip ... Not sure, but there seems to be no common official term for the sideslip maneuver, please correct me if I am wrong. That's why I explicitly explained what I mean.

[PAPI-74]

A side slip is a manoeuvre in which the aircraft is placed in a banked attitude, and the natural tendency for the a/c to yaw is countered with opposite rudder. This just looses height rapidly.
Entering and recovering from a forward slip is similar, but more skill is required as there is an intended descent path to follow. This is done by using sufficient rudder input to track towards a feature such as an initail aiming point in a field.

[FullyFlapped]

Alexka,

I am not a "technical flyer", so we'll have to wait for others to come along and give us the correct definition of a "slip". However, I believe it simply refers to flying with "crossed controls", i.e. rudder and ailerons deflected in different directions. I think whether it's a "forward" or "side" slip just depends on the direction the nose is pointing in : if the nose is aligned with the runway centreline, I'd call it a forward slip, if you're "crabbing" while maintaining track, then it's a side slip. However, I'm probably wrong (not an unusual situation! ;) )

What I can tell you, however, is that you most certainly can slip a 172 (and every other Cessna I've flown) with flaps. I personally wouldn't take 40degrees of flap when attempting a wing-down landing, because the book says not to, and it knows more than I do. 10, 20 or even 30 though, and as long as you watch you airspeed carefully (bearing in mind the ASI will read incorrectly) there is no problem - there can't be, or you couldn't land in a crosswind : even a "crabbed" approach obviously has to be kicked straight sooner or later).

I am not recommending this, but I can also tell you I've been in a 172 when it was landed in a crosswind of 32kts, straight across, and the guy flying (12,000 hour+) took 40 degrees of flap. Scared the crap out of me, and if I'd still had the power of speech when I saw what he was doing, I'd probably have said something !!

FF :ok:

[PAPI-74]

Slip is not enough rudder for the selected angle of bank. If you were in a 30deg turn to the right and the ball on your turn co-ordinator was to the right, it is telling you there isn't enough rudder (yaw) to balance the turn.
This is slipping.
A Side Slip is as you said, cross controlled.
e.g. right roll, left rudder giving not enough rudder or yaw to balance the turn. But you are not intending to turn, that is why you use oppostite rudder and maintain a heading (forward slip due to a forward reference).
You can side slip in a turn to really get down, thus no longer a forward slip, just a side slipping turn.

A Skid is the opposite.....too much yaw or rudder input. Don't get slow and do this.

Crab is an angle the nose is offset with wings level. If you were approaching a runway with a cross wind, you would crab the nose into wind to allow you to track the extended centreline. Keep this crab on during the flare and as you get your first sink, hold off with the usual back pressure and start to straighten the nose. As you do this, the secondary effect being roll, apply a small amount of opposite aileron to prevent the roll. Keep this on and start applying more and more. This will keep the centre line as you touch down and during braking.

Landing like that is a bit risky due to the blanking and gust/mechanical turb. that may happen. In short, you can stall the tailplane and stress the flaps. He may be able to feel a problem before it happens, but cessna's can bite.

[RatherBeFlying]

From the C-172L manual:If flap settings greater than 20 [degrees] are used in sideslips with full rudder deflection, some elevator oscillation may be noted at normal approach speeds. However this does not affect control of the aircraftNow if the crosswind is strong enough to call for full rudder, there's likely enough mechanical turbulence that you won't notice. I suspect the certification pilots picked up on this behavior at altitude.

After full flaps are applied you may still want to steepen the approach. Sideslip is the tool to use.

[Sunfish]

If you have full flaps in a C172 (Older 40 deg, newer 30 deg) and are on speed and still wish to steepen the approach, it must be an "interesting" approach or you have miscalculated.

My understanding is that Ratherbe flying is correct. The flap airflow disturbes the elevator airflow, leading to "oscillation", which disturbs the pilot, which disturbs Cessna's lawyers. Be aware also that some manuals caution against sideslips on low fuel because you can uncover the inlets.

POH's for American aircraft seem to be full of legally approved lame generalisations, like Piper's "it is good practice to contact the runway at the slowest speed possible."

The switch from 40 degrees to thirty degrees was made to increase payload - its related to the aircrafts ability to maintain height at gross weight. I found this out when a flap switch stuck at 40 on a go around in a C150. We never exceeded fifty feet and only just made it back to the end of the strip.

[waldopepper42]

Just to add further fuel to the debate. :E

I thought the danger was due to blanking of horizontal tail surfaces. Now, on most aircraft, these surfaces are set to produce a down force rather than lift. Therefore the effect of blanking is to remove this downforce, leading to an abrupt pitch nose down. Close to the runway, this can spoil your day!

I think the leading exponent of this manouvre was the Miles Magister - which had a few unexplained fatalities until some brave and enterprising soul decided to test the theory by side slipping inverted - and got the sudden and expected pitch up!!! :eek:

The RAF cured the problem by telling pilots - "Don't do it"! Can't imagine this enlightened philosophy replacing modern ADs though!

Over to the experts - fact or urban myth?

[turniphead]

Waldo is right.
Subject 1.Old cessna with full 40 deg flap
2.Full slip with full rudder
3.Low airspeed. But not a lot lower than normal speed for a 40deg final approach speed.
can't say as what speed though as position error on IAS is huge (and dependent on slip to left or right)
There is a slight turbulence felt through the control column which is a sure sign you are close enough to disaster. Ignore this and maintain the controls(or knock another knot or two off the speed) and there is a violent instantaneous pitch down.
Truly alarming.