Cross Control, Flaps and Wings

[SoundBarrier]

Hello Peeps!

I have a question on cross controlling (side-slip specifically) in a high wing single. While this sounds easy enough, and it is, I have heard several schools of thought on performing said operation with flaps down in a high wing aircraft specifically. This is what I have heard:-

Theory 1
Do not side-slip with them flap thingy's down as it blankets the airflow over the tailplane reducing effectiveness of elevator and rudder.

Theory 2
Theory 1 is not true you can do it just don't get caught.

Now, in the event of an engine fire, I will no doubt be using said manoeuvre to get my athma on terra firma with the shortest of time!

Does anyone know of anymore information I can get on this?

[barit1]

I know that some aircraft behave somewhat nonlinearly if slipped with flaps (do I recall some Cessnas placarded thus?)

But I've never thought that's a general rule for all aircraft - while I'm a few years removed from active flying, I wouldn't hesitate to slip a ship if it's not placarded. :hmm:

[Tinstaafl]

Unless the aircraft's flight manual prohibits sideslips with flap extended then feel free. After all, if you use any wing down for a x-wind landing you're in a sideslip with full-, part- or no-flap.

[bwicker]

I read somewhere, maybe the POH of the 152 and 172, that the reason they mention precaution when slipping with full flaps is because of buffeting on the tail from the disturbed airflow from the flaps. Sometimes this could cause a minor oscillation in the vertical plane. However, I don't think it's limiting. And Also, I believe it's just full flaps that they mention, nothing about 10 or 20 degrees. I reguraly slip a 172 when needed with full flaps, but i have that in the back of my mind while i'm feeling for any oscillation.

Brian

[SB4200]

Answer here is simply to consult your flight manual. The 172 models I am familiar with allow sideslipping with any flap setting however sideslipping with more than 20 degrees is to be avoided (but not prohibited) if possible due to a slight tendency for the tailplane to oscillate.

[Happydays]

The Cherokee P28 is allso certified to sideslip at any flap setting. 15, 30, 45

[On-MarkBob]

In any aircraft it is important to use your airspeed indicator while side-slipping. The pitot tube, for the best part, and being in the direction of normal airflow, will in fact influence the airpeed indicator so that it tells you the component of airflow over the wings in the normal sense. During the side-slip you would, thus, expect to see the airspeed drop if the deck angle remains the same. Nose the aircraft down and recover the airpeed to your approach speed. What the airspeed indicator tells you is what the wings are getting in the normal sense, the tail should not be affected or 'balnked' if you have the correct airspeed reading.

[sander82]

One of the arguments I once heard about not trying to make a forward slip with flaps extended, is that during a sideslip there is no direct indication of the actual airspeed. Since the pitot tube is not aligned with the airflow. For this reasons it would be easy, if one is not carefull, to exceed Vfe.

[ahramin]

In any aircraft it is important to use your airspeed indicator while side-slipping. The pitot tube, for the best part, and being in the direction of normal airflow, will in fact influence the airpeed indicator so that it tells you the component of airflow over the wings in the normal sense. During the side-slip you would, thus, expect to see the airspeed drop if the deck angle remains the same. Nose the aircraft down and recover the airpeed to your approach speed. What the airspeed indicator tells you is what the wings are getting in the normal sense, the tail should not be affected or 'balnked' if you have the correct airspeed reading.
I have not heard an intructor-pseudo-explanation-babble of this calibre in quite a while. In a side-slip, like in high AOA maneovers, the airspeed indicator is not reliable. In no case is the airspeed indicator capable of telling you the "component of airflow over the wings in the normal sense". A pitot tube becomes inacurrate once the airflow is more than a few degrees off the angle of the pitot tube (Check the CAS vs IAS chart in your POH for an example). On the other hand there is very little effect on the lift produced by a wing with a much bigger yaw. In no way will a "correct" airspeed reading have anything to do with whether or not the tail gets "balnked". In fact it will get you dangerously close to Vfe.

On a serious note: Yes, in many high wing airplanes full flap and an aggressive sideslip will produce a small bit of buffeting on the tail. While there is a small oscillation in pitch, there are no control problems. This is not a dangerous manuever, and i have never seen a flight manual prohibiting it, though as already stated many of the cessna manuals have a precautionary statement about this. I have never seen any remarks whatsoever in the limitations section with regards to flaps and sideslips.

[On-MarkBob]

Well Ahramin, There is no need to be derogatory! As the old saying goes. "it is sometimes better to keep silent and be thought a fool, than to open your moth and remove all doubt.

For the record I have been a test pilot for many years, I have been involved with prototype air testing and wind tunnel testing. Particularly, I have been involved with STOL aircraft and modifications to aircraft. I have also written flight manuals and supplements that cover just this kind of thing. I did not say that the airflow was going in that direction but that the pitot head is measuring a 'component' in that direction.

Some of you are correct with regard to Vfe and you have to bear that in mind when carrying out such manoeuvres. Also, as I am a qualified and licenced Aircraft engineer as well, I can assure you that some aircraft are prohibited from large slipping manoeuvres due to the side loads placed on the flap 'hangers'. I have never known a problem with a Cessna 152, 172, 182. The PA28 does not have that kind of flap 'hanger'. The Ralley MS880, 890 has a problem, big Flaps on wing 'hangers'. Furthermore, most Cessnas are well endowed when it comes to flap, slowing the aircraft down applying full flap then nosing the aircraft down can be just as effective as a side-slip in the amount of hight loss and you retain full control.

[barit1]

When the 170B first appeared (about 1951?) it was the first Cessna with the BIG flaps. The pilot community called them "Barn Door" flaps - and I think that once extended, you'd have to work hard to exceed Vfe.

Full flap on earlier Cessnas (140/140A, 170/170A) was about equivalent to 1/4 flap on later models.

[old,not bold]

The C152 would, if side-slipped agressively with full flap. achieve remarkable rates and angles of descent. Some precision was needed to straighten up without losing some control and immediately land gently at an aiming point close to a notional hedge, in a practice forced landing. Fishtailing was an acquired art.

The whole aircraft would buffet a little if pressed hard enough, but there was no loss of rudder/elevator control during the slip.

As far as I can remember, it was advised to use the elevator to keep the IAS 5-10KTS above the full flap, no power, level flight stall speed.

The Auster was similar, but with slightly less powerful flaps.

I guess that the thinking was that the actual net velocity was immaterial, and that if the pitot head, meauring the forwards component, showed that this component was well above the likely stall speed the wing would probably keep flying. I now wonder if the pitot location (ie which side of the fuselage) made a difference.

No doubt an expert aerodynamicist can explain better, and indeed one already has, to an extent.

[john_tullamarine]

Couple of thoughts ..

(a) one significant risk is loss of horizontal tail surface forces resulting in a nose down pitch .. might not be a good idea coming over the fence if one is slipping aggressively. ie probably a good idea to heed the OEM's warnings and/or know one's aircraft's characteristics in some detail

(b) re IAS errors, I would be more concerned with the effect on static errors (PEC), especially for aircraft with static sources on the keel surfaces


Having said that .. it is a wonderful sensation to fly an approach to a spot landing in an aircraft which can do these things .. SuperCubs forever ...

[old,not bold]

Couple of thoughts ..

(a) one significant risk is loss of horizontal tail surface forces resulting in a nose down pitch ........ SuperCubs forever ...

Did one maintain a little power (a) in a futile attempt to ward off carb icing (b) to keep some airflow over the tail?

I never flew a Supercub...one of life's many regrets...

Ahhhhhh, the smell of hot oil on grass.........

....back to the fireside.

[ahramin]

In regards to components of airspeed, we are not talking about simple vector addition here. Air is a compressible fluid, not a vector, and a pitot tube is not a magic measuring instrument, even for test pilots.

Anyone here fly glass gliders? Seen speeds of 5-20 kts on the airspeed indicator while in a sideslip. I don't think the wings were only seeing 5-20 kts of useful airflow.

Airplanes certainly don't stall when the airspeed indicator hits a certain IAS, they stall when the wing does.

[barit1]

Couple of thoughts ..
(a) one significant risk is loss of horizontal tail surface forces resulting in a nose down pitch .. might not be a good idea coming over the fence if one is slipping aggressively. ie probably a good idea to heed the OEM's warnings and/or know one's aircraft's characteristics in some detail...

I found (thanks to friend Ozzie Ozborne) an early 172 manual which cautions against this - but I'm sure it never bothered me much. There are light planes which I've slipped down to about 20' before recovery, but probably not a 172 with full flaps.

Also Ozzy found a caution against prolonged slips/skips in the 182 when fuel selector is on "both" because of unporting and drawing air into the fuel line. This might apply to many aircraft I guess. :ouch: