Sideslip
 

I find it interesting that instructors, from different countries, use different names/definition on basic aviation concepts.

USA: Slip/skid - slip=yawing nose outside the turn, skid= yawing the nose inside the turn. The slip can be a tool, the skid not so (stall/spin). Sideslip= used for crosswind("side"wind) landings, using only enough rudder to parallel centerline, while bank correct drift. Forward slip= used to lose excess hight, usually with full rudder, bank contol track, normally in to wind to make transition to sideslip safe and easy. Sliping turn=forward slip in turn.

In some other countries the word skid is not used...
Every slip is called sideslip....:rolleyes:
How do you then explain the transition from forward slip to sideslip. What do you call a skid?

Question:

Which countries use the term skid? Forward slip?

I've read somewhere that the Brits used the term sideslip for landing in sidewind. Do you know the history of these terms and why there is so much confusion out there?

Not quite- there's there is indeed the sideslip or "wing low" technique...some Brits also use "crab" for the wings level "kicking off drift" method.

Be interesting to see the answers you get, might be a case of " you say tomatoes and we say tomatoes "...,,.:E :}

I can't speak for all Australians and not saying this is a 'textbook' answer but I generally think of slip/skid as out of balance flight that occurs when the aircraft is turning and sideslip as out of balance flight when tracking straight.

Slip when the tail of the aircraft is slipping inwards on a turn, skid when it is skidding outwards in the turn. Usually (but not always) slipping or skidding is unintended.

I think of sideslip as out of balance flight when the aircraft is not yawing (ie when travelling in a straight lateral path), and is something often done deliberately.

eg sideslip is used (particularly on aircraft with no flap) to increase the rate of descent on approach or to get better visibility on approach (eg in an aircraft like a Pitts where over the nose visibility is not great) or to allow the aircraft to stop drifting when there is a cross wind on a landing etc.

I have heard people use terms like 'forward slip' and recall someone arguing there are multiple types of sideslip (and who had different names/uses for all of them - high nose/low nose/forward etc) but for me I just think in terms of skid/slip occurring when the aircraft is out of balance in a turn (due to over or under ruddering) and sideslip when the aircraft is out of balance when travelling straight.

Like I said, that may not be technically correct but is how I personally use (and think about) those terms in practical flying and how I used the those terms when I was instructing.

Probably I'm just a slow coach sort of chap.

Sideslip, to me, just means that the wind is coming other than from dead ahead. Can apply in straight or turning flight. One can be really fancy and talk about slip and skid, if that's useful.

Push your luck too much with slip angles and it can get interesting ...

Beyond that, I get a bit weary.

I am with JT.
The only term I ever use, irrespective of aircraft attitude, flight phase or configuration, is sideslip, either left or right, sometimes knowledge of the magnitude is useful.
Never use the term "balanced" as I can never be sure what people mean by this term. That is, do they mean zero sideslip, or ball in the centre?

That is, do they mean zero sideslip, or ball in the centre?

Now, that just has to cause some head scratching and bring out a question or two, I would warrant .....

for practical purposes I think that elegantly sums it up.


I used to teach it as when your bum was not being pushed left or right in the seat (and sometimes got students to close their eyes while I flew out of balance and got them to tell me me which way). Not saying it was the best way but thought it might help develop an instinctive feel for it rather than only know by staring at the ball.

Jongster, as I said is "out of balance" flying with sideslip, ball not centred, flying with zero sideslip ball not centred, or flying with sideslip with the ball centred?
That is why I never use the term.

Feeling humbled, I just learnt something :O

I'd never really thought about situations where the aircraft could have the ball (or your bum) centred but be sideslipping before but yes... wings level, aircraft tracking straight but the longitudinal axis not dead on to the airflow

eg with propellor P factor trying to yaw nose one way, compensating for that with rudder and keeping wings level so tracking straight - straight and wings level would mean ball in the middle.

In that situation - is it in balance? Buggered if I know!

Thanks a lot zzuf :mad: I can no longer say "keep it balanced". Bugger. :uhoh:

Thinking something like a glider's yaw string would indicate difference from a balance ball in that situation...

Hi Jonkster, have a look at the theory of asymmetric flight as well - small bank angle changes can change sideslip angle markedly, even change the direction of sideslip.
Yaw strings were good enough for Air NZ DC8's......

As I suspected, there is a confusion out there on very basic concepts.

The US Private Pilot PTS/ACS states clearly what is required knowledge and skill for the checkride. The same for the glider pilot certificate. A correct forward slip shall be demonstrated, and if no wind, the sideslip shall be explained. If unable, you may fail the checkride.

In a glider, a skid in the circuit to land could fail the checkride. To demonstrate the difference between skid and slip I have this done in a turn at altitude. A skid + stall (as in base to final turn) will start a spin, a slip, on the other hand, will first level the wings.

This is how I teach the forward slip:

https://youtu.be/yxy2MnUnfUM

Thanks Wingman - pretty clear I would fail those tests as I have no idea of what is required! The terms are meaningless to me. I would suggest that sideslip in either direction, at the stall, upright, inverted, turning or straight, is a recommended spin entry technique.
The video was a full rudder sideslip to the left in the landing configuration. I couldn't see the forward part. Perhaps a video of a backwards sideslip would be enlightening.

zzuf, yes I'm aware of the resistance to these concepts out there, as I fly internationally. That's why I started this thread.

As long as you can explain these ideas, the words you use don't matter much to me, except in preparation for a FAA checkride. I do favor these definitions, though, as they make explanations better. I suppose you don't use the word "skid" either, eh..?

Zuff - Well this was the point with this thread, to illustrate the different opinions about about a basic concept.

As long as you can make a meaningful explanation for your student, then I don't care what words you use, except in the preparation for a FAA checkride.

I favor these definitions. They makes it easier for students to understand and differentiate them, i.e. sideslip for crosswind (sidewind) landing, forward slip to lose hight.

I suppose you don't use the word "skid" either?

Zuff - Well this was the point with this thread, to illustrate the different opinions about about a basic concept.

As long as you can make a meaningful explanation for your student, then I don't care what words you use, except in the preparation for a FAA checkride.

I favor these definitions. They makes it easier for students to understand and differentiate them, i.e. sideslip for crosswind (sidewind) landing, forward slip to lose hight.

I suppose you don't use the word "skid" either?

Zzuf, I would have to disagree a little bit there - I know of at least one reasonably common, spinnable aircraft, that (from practical experience) you can stall while sideslipping and so long as you are not yawing (ie doing the kind of sideslip you might do on an approach to lose height), it will stall in a reasonably stable and docile way without entering a spin.

The first time I tried this I was very green and had been arguing with another much more experienced instructor, that doing this would cause it to spin. He said OK let's do it and see. We went up to altitude and did it - I was quite surprised (and firmly put in my place) when it stubbornly refused to enter a spin that way.

However the very same aircraft enters a spin sharply if you stall it while yawing in a 'skid' (by that I mean applying enough rudder to cause the aircraft to yaw but while holding off bank) eg what might happen low to the ground if a pilot is scared of banking but wants to change the aircraft direction to line up with a strip and so subconciously boots it around with their feet.

At that point I agree - pretty generally I think that form of sideslip in a stall will spin most spinnable aircraft (and similar to what you say, for some aircraft 'out spin aileron' is the recommended spin entry technique).

For me I think yaw and its direction is the issue that results in different behaviour when sideslipping in the stall. ie - when excessive rudder is causing yaw, when stalling, it gets interesting.

NB for the earlier case where it won't spin - I most certainly cannot say it applies to all aircraft and am curious to hear about ones that behave differently.

JT, responding to earlier comments that you edited, I would say yep, fair enough.

I think you are right - my terminology and description (and my recollection of what inputs I am actually using) is fuzzy. It would not be helpful for a student at all if I was to explain this like I have here.

What is concerning is I know I have explained it much like that in the past :eek:

This has opened a can of worms for me!

I am now sitting here in a swivel chair pushing myself left and right visualising exactly what control inputs I put in and how the aircraft responds and am now thinking that maybe what I am thinking I do is not what actually I do! :)


I am looking at my hand and feet position in the swivel chair for the two imagined configurations and... they seem to be pretty similar - yet one I know from experience will result in a spin and the other a floppy stall :hmm:

The difference seems to be one involves continuing yaw, the other is tracking straight (but with the nose yawed to the side).

I am wondering what causes the the sharp spin entry I described when I imagine I am 'over ruddering a turn, holding off bank' sideslip compared to the more docile stall behaviour when I imagine myself in a 'cross controlled' tracking straight, one wing down nose sideways approach type sideslip.

Could it relate to perhaps the stall in the former is an accelerated stall due to the aircraft turning? Almost a subtle flick manouvre?

I need to go and try it and see what actually I am doing with the controls and what happens rather than relying on what I think I have been doing.

JT, responding to earlier comments that you edited, I would say yep, fair enough.

I deleted those comments because, on re-reading your post, it was clear that I had misread it first time around .. mea culpa.

You would do well to heed zzuf's comments .. his background is more than eminently suited to this sort of discussion. Hopefully, he will continue in the discussion.

Likewise, I shall endeavour to get djpil interested in joining the discussion.

Between the two of them, you have a wealth of relevant knowledge and experience.

As to your comments, above, it probably involves not so much the mere fact of slip .. but what happens to rolling moments, drag asymmetry, etc when the aircraft lets go for whatever reason ...

Jonkster, I have had a little difficulty visualising your spinning scenerio. To me yaw is rotation around the vertical axis, this may be as a result of rudder application, wings level, a normal turn, or a combination.
For your spin resistant aircraft I would add that some certification requirements specify stalling at up to 5 degrees of sideslip with no marked deterioration of stall handling qualities. They also require a similar result stalling in rain.
I would be loath to comment on your experience without viewing a time history of all parameters relevant to spinning. The problem is that this is a highly dynamic situation and even an experience TP may have difficulty assessing cause and effect without instumentation and data recording.
Wingman, no, I don't use the term skid - a sideslip is a sideslip!
However you are correct in that the terminology seems to be national in origin - my TP training was at Boscome Down, so I guess that there is a good bit of British influence in the terminology I use. Also, when looking at a flight regime such as sideslip I will be invariably assessing the aircraft handling qualities and the compliance with various stability and control requirements.
I note that FAR 23 directional static stability test require that the aircraft recover from a "skid", rudder free. There is no requirement to recover from a "slip" - very strange. Luckily both "slip" and "skid" are just sideslip, so directional static stability demonstrated in one will guarantee compliance in the other.

I was going to stay out of this...
 

No. They are not. They are opposites.


An airplane has a stick. And rudder pedals. They move independently of each other. If the stick is to the left and the rudder to the right, that's a slip. The ball will be well to the inside of the turn.


If the stick is to the left and you have too much left rudder, that's a skid. The ball will be to the outside of the turn.


In both cases "step on the ball" will correct the situation.


Regardless of what prefix you put in front of slip, sideslip, forward slip, etc. it's the same "crossed control" combination. Only the purpose has changed. I can barely remember which is which, but I can slip to lose altitude, and I can slip to correct for a cross wind (prefer to crab, but that's a different topic).


"The dreaded stall spin on the base to final turn" is a skid. Skids are inherently more dangerous as the inside wing stalls first.


Terry

LTCTerry, Thanks for the egg sucking lesson.
Perhaps you could explain what is the opposite of sideslip, bearing i mind that sideslip is simply a velocity along the aircraft lateral axis, what is the opposite?
The two examples you give are indeed opposites, the first the sideslip is to the left, the second the sideslip is to the right.
It seems a strange aircraft in the second example, too much left rudder implies sideslip to the right, in a normal aircraft which has conventional lateral static stability, sideslip to the right will require right aileron to balance the rolling moment due to side slip.
In your case of left stick and (excessive) left rudder, a normal aircraft will be rolling left - one component of the rolling manoeuvre will be the rolling moment due to aileron deflection (left), the other component will be the rolling moment due to sideslip (dihederal effect). That is a left rolling moment due to right sideslip.
I don't use prefixes, so please don't verbal me.
If you seriously believe that the ball is always a proxy for sideslip angle all I can say is that your education is lacking somewhat.

Please forgive the Tread drift - but I thought for a minute that this discussion was going to be about 'sideslipping' ... obviously not ... :confused:

I thought for a minute that this discussion was going to be about 'sideslipping'

You got it in one .. even if it might challenge some of your fondest thoughts on the subject ...

Jonkster,
You win the chokky frog for independently working out that the ball position, in many cases, has no relationship to sideslip angle.

This should explain the general tendency for an aggressive spin entry from a skidding turn:
http://apstraining.com/traffic-pattern-stalls/
Especially refer Diagram C which is from Rich Stowell.

If you redraw Diagram C in a steady, straight slip you should see why the general tendency for it to be spin resistant.

I've been training instructors in these exercises over the last 15 years or so, mostly in a Decathlon and some in a Cessna Aerobat. Managed to catch a lot of trainee instructors unaware of the approaching stall/spin as the usual symptoms of an approaching stall are not present (or at least nowhere near as obvious as the common straight stall) in a skidded turn stall.

I regard Rich Stowell's book as the best on this subject:
http://www.richstowell.com/shop/book...pin-awareness/
Thoroughly recommended.

Strange indeed: FAR 23.177 talks about lateral stability with tendency to raise the lower wing in a sideslip and directional stability with tendency to recover from a wings level sideslip. AC23-8c has this clarifying note: "... the determination of "appropriate" wings level sideslip (previously referred to as skid) angles will depend on sound judgment ....".

So, playing instructor, I'll use the term skid per Rich Stowell. I try to keep my engineering life separate.

just wanted to say I have learnt a lot in this thread - embarrassingly about things I thought I knew.:O

:ok:

djpil
How remiss of me quoting an out of date FAR! Of all people I should know better! Somewhere along the line "skid" has been amended to "sideslip" in both the FAR and AC. How perceptive of the authors!

Forces In A Turn - Free CFI Tool | Boldmethod

click on this, its an interactive gif , click on the slip, skid and coord buttons and see what happens to the forces.