Slips

[boofhead]

Long time since I was taught this stuff...
I cannot see why the US makes a big deal out of what they call "forward" slips as distinct from "side" slips. The airplane does not know the diffference, and the result is the same. In one case you point the fuselage away from the intended direction of flight, in the other you point it down the runway, but in both you have crossed controls.
Has anything been written about this matter that I can read and quote? I want to understand the difference, if there is any, or to refute the point if there is none, and would need someone with some clout who has written an article or maybe a chapter in a book? I doubt if my word will be taken, and after all, I could be wrong! (if a tree falls in the forest and no-one is around to hear it, why is it still my fault?).

[FlyingForFun]

From an aerodynamic point of view, I agree that the two are identical.

However, the reason for doing them, and therefore the method of teaching them, is quite different. I recently had a student who had become very confused about the whole idea of cross-wind landings when he confused the two different types of slip. Once I broke the slip down into its two different types, and explained the whys and hows of each type, it made a lot more sense to him (and his cross-wind landings improved overnight).

I don't know of any references off the top of my head, but I'd be surprised if Langeweische didn't have something to say on the subject?

FFF
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[boofhead]

FFF, that is exactly my point; we make the subject complicated when it is not. It matters not whether the slip is "forward" or "side". All we need to teach is how to slip, and what to look out for. For example, when I set up a slip I note the IAS at the point the slip commences and maintain that, it might be lower or higher than the original speed prior to the slip, depending on which side the pitot is located, but by maintaining the speed, you know you are safe and don't need to work out what it should be.
Put in bank then use rudder to maintain the desired direction of flight, whether that is toward the runway or in a turn. See the sink, what a thrill. Easy to get into, easy to fly, and easy to recover, provided you don't get all lost in theory. If you want the theory, do it on the ground.
On the runway, who cares what the maneouvre is called, you don't need to know it is a slip. Just a mechanical process of keeping the fuselage aligned with the runway heading and enough bank to stay on the centreline. Watch out for too much bank, don't want to scrape a wing or pod, and if the crosswind is too great (can't maintain centreline), go around. It is an unnecessary complication to describe it as a slip, it is a wing down crosswind landing is what it is. Sure you are slipping but who cares. That is not the point of the exercise.
So there is only one slip, and even a skid is really a slip, since by definition the controls are "crossed" which is all you need to identify it. If you are not balanced, you are slipping.
Getting lost here, feel free to step in!
What was my point? There is no need to identify slips by when they are being used. A slip is a slip end of story. We confuse students by making them learn stuff they don't need to know, and frighten them to boot. Better to keep it as a practical exercise in how to fly, and keep the crosswind landing as a landing exercise, not a slip exercise.
Now I need someone who is an expert (or at least has been published) to agree or disagree with me, hello out there?

[Genghis the Engineer]

Well for what it's worth I've been published on a fair number of aspects of flight mechanics, and studied it to TPS and PhD level.

For my money there is sideslip (also known as "Beta"). "Skid", "forward slip", and any other bits of terminology are just complicating an issue that doesn't require it.


Incidentally, you really shouldn't teach constant IAS in a sideslip, you should teach constant attitude. Constant IAS + blocked static on one side = either too fast + rudder (risk of structural damage) or too slow + rudder (risk of spin).

G

[boofhead]

I flew for 30 years not knowing there was a difference in terminology, and not even realising that a wing down crosswind landing was a slip. If the technique was used when there was no crosswind (self induced) then I could see it as a slip, but balancing the forces to maintain runway heading did not seem to me to be a slip at all. Then I started flying in the US and got my US CFI and had to learn about "forward" and "side" slips to pass the exams. It was hard for me, with a brain starting to gell, to get around it, and I have seen students have a great deal of difficulty with the concepts, to the point that I believe we are wasting time and effort teaching something that is at best a theoretical classroom point and has no practical application (not the slip; the name). When a student is confused, it is harder for her to fly the manoeuvre, and more chance she will screw it up, as a glider pilot in Picton, Ontario might have done earlier this year. (http://www.airforce.forces.gc.ca/dfs/pdf/REPORTS/FSIR/CGDZF/CGDZF_e.pdf)
And on that point, when we are approaching to land and need more drag, we swing the nose away from the runway and use rudder to maintain track, so that we are flying sideways. Yet we call it a Forward Slip. On the runway in crosswind we drop a wing and use rudder to maintain runway heading, so as to fly forward down the runway, yet we call that one a Side Slip.
Maybe the guy who wrote it in the US pilot training manuals was dyslexic.
Kermode uses the term Side Slip, but does not cover the crosswind case. I was hoping someone would have already challenged this one, so I could quote him/her, and if anyone knows of such, either way, please let me know. Meanwhile I will of course quote you, Ghengis.
I figured that if I had, say, 60 at the point when I started the slip, as I yawed the nose of the airplane the airspeed would change depending on the pitotstatic configuration and the IAS would either drop or rise. If I flew that new IAS I would in fact still be flying 60 in the forward direction, ie my real speed would still be 60. I would adjust attitude to maintain that new speed. If I flew constant attitude, would that not lead to a "real" speed decay due to the increased drag?

[P.Pilcher]

About 37 years ago I went to Canada for a holiday with my brand new PPL in my pocket hoping to sample light A/C/ flying out there. During a check out my Canadian instructor assessed my cross wind landing technique and stated "Guess you were taught by an ex-RAF instructor huh? O.K. here we drop the wing into wind and keep the A/C in line with the runway with rudder. With this technique you can put a Cessna 150 down with 30 knots across"

I've never forgotten that, and it works with Aztecs, Partenavias, Twiin Otters, Jetstreams, Shorts 330/360's and, I am relaibly informed, Boeing 707's - as well as your Cessnas, Cherrytrees, Robins and so on.

P.P.

[1McLay]

Its a matter of opinion isnt it...

and I'm inclined to keep the ball in the centre and teach that too...:p

[Genghis the Engineer]

This, if I may say, is diverging into a different topic.

There are three ways of flying a crosswind landing in a light aeroplane:-

(1) Wing-down

(2) Initially crabbed, transitioning with rudder into wing-down in the roundout.

(3) Land crabbed (generally reserved for aeroplanes with 2-axis controls, such as for example a flying flea derivative).


(1) and (2) involve at the end what is essentially a steady heading sideslip in landing configuration. Many GA aeroplanes are happy to accept either. But not all - for example a wing-down in a G109b will almost certainly result in a wingtip striking the runway, whilst the roll due to sideslip response is slow enough that when straightening for landing, it's possible to land it BEFORE the bank angle comes on enough to become a problem.

Conversely I know one aeroplane (the X'air - a microlight, but otherwise entirely conventional) which wouldn't be amenable to Percy's preferred technique, the reason being that the lateral stability is TOO HIGH, resulting in the pilot and passenger being effectively squashed into one side of the cockpit and pushing up pilot workload too much.

Incidentally, although crabbed is the RAF's preferred method, it's not universal - the RAF's L1011 pilots for example fly wing-down.

G

[FlyingForFun]

Back onto the subject of written references, and back onto my own suggestion of Langewiesche. (I've spelt his name correctly this time, since I've got his book in front of me!) What he has to say on the subject is:The rudder is also used in cross-wind landings. In order to have the airplane going straight over the ground at the moment when it makes contact with the ground, it is made to sideslip through the air......He doesn't appear to use the term "forward slip" at all, but he does use the time "sideslip" to refer to the cross-wind landing technique as well as the technique for loosing height.

Incidentally, though, one thing I had forgotten (since it's been a while since I read his book) is that he is not a big fan of the rudder at all. His vision of the aircraft of the future doesn't have a rudder. He sees ailerons designed well enough that adverse yaw becomes negligable, the use of side-slip to loose height replaced by flaps, and no need to straighten up the nose prior to landing because the "new" tricycle undercarriages which were being used could land crabbed. So he may not be the best reference to quote on this particular subject! ;)

FFF
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[boofhead]

Thanks all, for the replies.
I agree with the comments re crosswind landings and such. There is a real effort in the US to teach that the case of slipping while on approach is a specific exercise, different from slipping in other areas and deserving of a special name, "Forward Slip". Students are expected to treat this case as special, not because of its hazards but because it has that special title. This makes it a big deal and causes many students grief and confusion lest they get it wrong in the test.
My campaign is to de-identify the slip on approach from all other slips, and simply identify the slip as any case when the ball is not centered. Of course the cross controls used on approach might be considered rather extreme, depending on the airplane and its characteristics. I believe de-identifying the technique would make learning the procedure, and understanding the effects of controls, easier for the average student.
Confusion as to the best way to slip the airplane when flying into small strips has been identified as a contributing factor in some accidents and I think the terminology has something to do with that.

As an aside, I was always amused when giving a landing in the B747 to First Officers of an Asian airline that shall remain un-named, when they would set up the airplane at around 300 feet with cross controls, so as to offset the drift of a crosswind, and fight to maintain that all the way down to landing. Setting the bank and rudder in close proximity to the ground was way too frightening, and as for crab, forget it! Never mind that the wind at 300 is not the same as on the ground (I have seen it change from right to left crosswind in the last couple of hundred feet, with no change in the FO setup of controls; imagine the result) and that if the rudder and ailerons are offset enough, there will be a degradation of lift, just when you don't want it. I don't know how, but they all managed to avoid a pod strike, did not need my hand under the controls to stop excessive aileron, as it turned out. So maybe I was the one confused.