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Stall speed in an established slip

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Stall speed in an established slip

Old 16th Apr 2014, 10:21
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Stall speed in an established slip

I have seen articles claiming that in a sideslip stall speed can actually be lower than in balanced flight, because of lift produced by the fuselage. There are also articles that claim that it's higher. Conceivably it could be both, depending on the aircraft type.

Does anybody know what the actual difference is, for any reasonably common types?

In practical terms, I suppose the utility of knowing it is that if you were to find yourself low and slow in a slip, you may be well advised to push the nose down prior to uncrossing the controls. However, really I'm asking more out of curiosity.
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Old 16th Apr 2014, 11:09
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Bearing in mind that the pitot pointing sideways in a slip will be misreading airspeed. How would you know?I was taught to set the speed, then slip, the airspeed would appear to reduce, as you "un slip" correct airspeed would return.
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Old 16th Apr 2014, 11:37
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I'm no airframe designer, but wouldn't the drag induced from a fuselage moving sideways counteract any lift its form might produce?
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Old 16th Apr 2014, 12:04
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Not a designer or engineer either, but I should think that IF the fuselage produces lift in the slip THEN it produces even more in straight flight. As I was taught side-slipping, it is a way of making the airframe less efficient aeronautically - which includes the wing(s) as well as the rest.
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Old 16th Apr 2014, 12:52
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Sorry Glum, but if your supposition was true then logically, knife-edge flight by the aerobatic chaps wouldn't be possible, and it clearly is.

Absolutely, different designs will produce differing amounts of lift in a side slip, but it is difficult to imagine a design which would produce more total lift in a side slip. Don't forget that lift is the upward force supporting the aeroplane and if you are canted over in a bank, only the vertical component of your wings' lift counts. Factoring in the amount of wing blanked by the fuselage as well and its not looking great for a higher overall vertical lift component in a slip. Imagine a 30 foot wing at a 30 degree bank angle. Its only producing 86% of it's normal lift before fuselage blanking is considered. If you add 2ft of blanked wing at the root then you've got a total of 5.5 feet of wing producing no lift at all. There probably are not many GA aircraft designs that can produce more lift with a slipping fuselage than it can with five feet of wing.

Sure, your fuselage will be producing some lift but at the price of very high form drag (from being side -on to the airstream) and induced drag (from the large hole left in the air behind it that is a region of lower pressure 'sucking' the aeroplane backwards). Its this drag that we really want as we are side slipping in order to dump height without gaining too much speed.

All this drag however, does not act against the lift force because they act in different planes (axes). Drag is the horizontally retarding force that restricts our forward progress, whilst lift is the upward force that keeps us aloft, hopefully counteracting our weight.

As for knife-edge flight, you simply need enough power and at least some side-area to your fuselage. The more power you have, the less side are you need to produce a vertical lift component.

Simples.

In terms of real life though, I try my slips at altitude in a new type first. When coming out of the slip, I aim to come out in a nose down attitude which I then bring up once the controls are uncrossed and everything is stable.

Last edited by FleetFlyer; 16th Apr 2014 at 13:12.
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Old 16th Apr 2014, 13:58
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The wing behind the fuselage in a slip will get less air moving over it, therefor generating less lift. I suspect (gut feeling) that the possible lift generated by the fuselage does not compensate that, so I suspect you end up with less lift in a slip.

Anyway, the stall speed is dependent on the angle of attack, not on the amount of lift produced. If you keep your wings level, and perhaps compensate the extra drag and loss of lift with extra engine power, then you should have about the same stall speeds. But to keep the wings level, you have to compensate for the roll effect caused by the yaw, increasing the angle of attack of the wing behind the fuselage, causing that wing to stall sooner.

You have to take into account the shape of the wing: swept or not, dihedral or not, that all have an influence because of how the air will flow over the wing in a slip.

My best guess is that your stall speed goes up in a slip.
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Old 16th Apr 2014, 14:59
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Anyway, the stall speed is dependent on the angle of attack
Eeerm. There's no such thing as 'stall speed', only stall angle-of-attack (I wish I had the proverbial £1 for every time I've had to post that on PPRuNe).

I've stalled the Yak at 140 kts and flown it at zero indicated air speed unstalled.

The only thing that stalls a wing is flying it beyond the stalling angle. Which for any given wing configuration (slats, flaps etc.) does not change. NOTHING else can cause it to stall.
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Old 16th Apr 2014, 15:05
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Just to complicate it a bit further, most slips done in anger are to lose height very quickly without any increase in air speed, as in a real forced landing into a small field. So they'll be done with full flap, and in some types, the Cessna 150 family for one, the rate of descent with the engine out, full flap and maximum controllable slip is dramatic.

As some one has said, pitot errors prevent any correct indication of air speed; all I know is that if you keep the airspeed indicating the stall speed for the weight and configuration plus about 10%, using the elevator to control it, you won't stall. Or at least I don't.

But I'm pretty sure that the stall speed must increase, for all the reasons cited above and the diagonal airflow; but that is compensated by the pitot under-reading in a slip.
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Old 16th Apr 2014, 16:45
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If the stall speed increases in a steep bank because of the reduced effective span then it follows that with the reduced span with the wings yawed will do the same, dunnit?Please ignore the use of the term "stall speed". I KNOW it's ANGLE of ATTACK. Some one told me once or twice!!
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Old 16th Apr 2014, 16:58
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Some one told me once or twice!!
But you (and not just you) weren't listening?

Stick & Rudder... The bible!
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Old 16th Apr 2014, 16:59
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If the stall speed increases in a steep bank because of the reduced effective span
Stall speed doesn't increase in a steep bank due to reduced effective span. It increases because of increased G-loading, thus increased angle of attack.

Slips do more towards reducing airspeed by putting the fuselage into the airflow than anything about span-wise flow. The increased rate of decent is a result of putting the nose down to maintain airspeed given the increased drag.
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Old 16th Apr 2014, 18:42
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Quote:
Some one told me once or twice!!
But you (and not just you) weren't listening?

Stick & Rudder... The bible! Unquote :

Jesus H Christ. What part of, I KNOW !! Don't you understand?
I am extremely pissed off by the number of smart assed instructors, pilots and anyone else who wants to express their superior knowledge telling anyone who mentions the S word just how wrong they are.
Just how thick do you think we all are? Yes it is all to do with angle of attack, I too have stalled a K13 at 80knots, that was thirty years ago, I know how I did it, I did it on purpose, I still know why it stalled! I don't need told again thank you very much! Is that clear enough?
Having ranted all that, without an angle of attack gauge some form of reference is required. Therefore in STRAIGHT & LEVEL flight a SPEED is used instead of an ANGLE. This speed can vary depending on weight balance air temp pressure what the pilot had for breakfast who he slept with etc, who cares? My a/c stalls straight & level, solo, half tanks, clean, standard atmosphere, at 2000ft, at 40 knots. at this point the wing has reached an angle of attack of 17.5 degrees leading edge up. (Chord line) just in case.


Stick & Rudder...the Bible my ass. Yes I've read it!
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Old 16th Apr 2014, 19:25
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Crash one, you sound like a gibbering wreck!

If you know the correct terminology, why not use it?

The misnomer 'stall speed' is used in basic flight instruction because neophyte students can't be expected to grasp AoA and need a simple reference to stay safe while they struggle to learn to drive an aeroplane. 'Stall speed' is a nursery tool in aviation and serves that purpose. Far too many pilots who should have put aside childish things by now, haven't.

There's nothing smart arsed about using correct terminology. To continue to believe in stall speed as you become more experienced and should know better is nothing to be proud of.

Wings make aeroplanes fly. Wings don't know anything about speed as far as stalling is concerned. They only understand AoA. And so should a pilot once past the primary learning stage.

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Old 16th Apr 2014, 19:51
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'Stall speed' is a nursery tool in aviation and serves that purpose.
Wings don't know anything about speed. They only understand AoA.
I wish I could agree with you Shaggy, but I can't do so. It's too simplistic a view.

There's a reason why aircraft are certified based on stall-speed rather than AOA. Stall speed tells you how slow you can get before you reach critical AOA.

Let's put it simply: For a given wing (to simplify it, taking out the constants, making huge assumptions), Lift = speed^2 * AOA. You can keep a critical AOA but you need speed to generate lift or you'll be descending.

Wings DO know speed. They know speed and AOA. They need both. This continued new-fangled over-concentration on AOA is gonna get some kid killed as they keep hearing "Speed doesn't matter, AOA does."

Edit: Shaggy, after your edit, NOW I agree with you. ;-) I still think it's a valuable reference number though.

Last edited by slam525i; 16th Apr 2014 at 20:02. Reason: because Shaggy edited too
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Old 16th Apr 2014, 19:58
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I was beginning to doubt my own wisdom... Any wing at zero airspeed creates zero lift at any AoA. All other things equal, lift follows airspeed by the second power. I.e. twice the airspeed, four times the lift. All other things equal, mind you.
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Old 16th Apr 2014, 20:02
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Stall speed tells you how slow you can get before you reach critical AOA.
Unfortunately, Slam, it doesn't. Wing loading (so weight an 'G') are factors as well, so the 'stall speed' isn't always the 'stall speed'. Which is why one can stall at 140 kts and not stall at zero kts in the same aeroplane. Which is why it shouldn't be used beyond basic training - it's potentially misleading.

Stall AoA is always stall AoA, however, for any given wing configuration (flap / slat setting).

I was beginning to doubt my own wisdom... Any wing at zero airspeed creates zero lift at any AoA. All other things equal, lift follows airspeed by the second power. I.e. twice the airspeed, four times the lift. All other things equal, mind you.
Quite so (as long as the wing is not stalled and disregarding high lift devices such as flaps and slats), which is why I edited my post on 'what wings know about' to include "as far as stalling is concerned". I made that edit straight away on reading my post and realising the error, before any answering posts had appeared here.

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Old 16th Apr 2014, 20:35
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I know Shaggy. I should clarify to say that Stall speed (by the book) is only for 1 G, max gross weight, wing config etc. etc.

I really don't understand why this is so hard to grasp. Stall = critical AOA. lift = speed^2 x AOA. That's all there is to it.

You get too slow in level flight. Your speed drops. Your lift drops. You keep pulling back to maintain constant lift as you slow. You exceed critical AOA, you stall.

You steep turn while flying slow(ish). The G increases. You try to maintain altitude by pulling, increasing AOA so your lift = weight * G. You exceed critical AOA, you stall.

It's really not that difficult. Why do these discussions keep coming up again, and again, and again?
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Old 16th Apr 2014, 21:03
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Well, I agree about the static port problem, but you could also try stalls at the same altitude and heading (therefore same wind) using the GPS speed.

I'm sure we are all clear about the difference between stalling speeds in steep turns, stalling speeds in a straight line, and stalling angle of attack. For clarity, I was referring to stall speeds in a straight line. I can't imagine it would be fun or necessary to validate an angle-of-attack meter for slips.
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Old 16th Apr 2014, 21:05
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Because, Slam, folk still keep using the term 'stall speed' as if that's a 'carved in stone' speed below which the wing will stall and above which it won't. That would be merely semantics if it weren't for the fact that people die in low level stall spin accidents while religiously observing 'the stall speed' without realising there isn't one. Or that there is an infinite number of them - the same thing really.

The term is meaningless, a hang-over from basic training where studes need something simple to observe which, in the limited envelope of initial flight training, will keep them safe.

It's a term that should be left behind for the correct one, stall angle, in post PPL flying. Continued use of such an inaccurate term indicates that pilots might not really understand how a wing works, and why it sometimes doesn't. That can be fatal as one extends the envelope beyond straight and level and rate one.
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Old 16th Apr 2014, 21:27
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Which is why one can stall at 140 kts and not stall at zero kts in the same aeroplane.
I'm really intrigued by the second part...is this a "trick" answer...IE it's not stalled atzero airspeed, simply because there's no airflow to start any lift?

Ihave, ingrained in my memory, a report of a Spitfire at an airshow ,which pancaked into the ground at the bottom of a loop. Plane was going "full bore" but the pilot had started too low andthe nose-high pancake ensued......a classic HIGH SPEED STALL
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