Stall speed in an established slip

[abgd]

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.

[Crash one]

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.

[glum]

I'm no airframe designer, but wouldn't the drag induced from a fuselage moving sideways counteract any lift its form might produce?

[Jan Olieslagers]

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.

[FleetFlyer]

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.

[Pirke]

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.

[Shaggy Sheep Driver]

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.

[old,not bold]

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.

[Crash one]

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!!

[Shaggy Sheep Driver]

Some one told me once or twice!!

But you (and not just you) weren't listening?

Stick & Rudder... The bible!

[slam525i]

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.

[Crash one]

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!

[Shaggy Sheep Driver]

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.

[slam525i]

'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.

[Jan Olieslagers]

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.

[Shaggy Sheep Driver]

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.

[slam525i]

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?

[abgd]

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.

[Shaggy Sheep Driver]

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.

[cockney steve]

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

[Pirke]

Most GA planes don't have an AOA indicator, so speed is the most indicative. That works for flying straight and level, anything else is a best guess. Most GA planes don't do aerobatics either.

So to get back on topic: in a straight and level slip with constant speed, do you think the speed at which the plane reaches critical AOA on a wing is higher, lower or the same? My guess is higher, see my reasoning above. Due to the air flow coming from a side angle, the indicated speed could be lower, higher or the same, depending on the position of the static port and pitot tube.

[Mach Jump]

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?

Ok. Lets not get carried away here.

When an aircraft stalls, there will be something showing on the ASI. This is the stall IAS in that aircraft, under those conditions. If the aircraft stalls under identical conditions, the stall IAS will be the same, so to suggest that there is no such thing as stall speed is unhelpful.

In a steady heading sideslip, in unaccelerated, level flight, there will be some wing blanking, some turbulence, some spanwise airflow. and some angling of the lift vector away from the vertical. For a given speed therfore, assuming for our purpose that the ASI has no position errror due to the sideslip, in order to maintain sufficient vertical lift component to support the weight, the AOA will have to be increased. This means that the aircaft will reach it's critical AOA at a higher speed.

In a sideslip, there will however, always be some element of lift generated by the fuselage, and there will be some vertical component of this lift. The question is, will this fuselage vertical lift component be sufficient to compensate for the vertical lift component lost. If it is, then there will be no increase in wing AOA to maintain the total vertical component, and the stall speed will remain the same. If it isn't, then the stall speed will be higher. If it is more than required just to compensate for the lost vertical component, then the stall speed will be lower.

In practice it will be difficult to evaluate the effect, without a special flight test pitot/static system, as the position errors in a sideslip can be huge, so I suggest that few pilots, if any, will be able to say which common types of aircraft fall into each category.


MJ:ok:

[slam525i]

If you slip, your wing is operating at less efficiency, so a lower coefficent of lift, so you need more speed for the same lift, or more AOA at the same speed, so my guess would be a higher stall-speed.

A better question would be what are you doing slipping with a high AOA/low-speed? It seems like a great way of getting yourself killed.

When I do a slip to lose altitude on final, and I need to lose as much as I can in as short a distance as I can, I go full flap, full rudder, and keep the speed up, since drag is speed^2. I think J. Mac McClellan (or was it Richard Collins?) wrote this in Flying Magazine a long time ago as more effective than going slow and waiting. You can go above or below best glide speed to get rid of energy faster, but going faster is safer since you're not as close to stall. Just don't get too close to the flap limits.

[Miserlou]

Abgd,
No need to lower the nose PRIOR to uncrossing the controls It works like this.
As you enter the slip the nose will have a shallower angle. You maintain this pitch angle. The ASI will show a lower speed due to position error. As you straighten up the nose 'falls' back to the original pitch angle and the indicated speed on the ASI is restored. It never actually changed.

Slam,
Choosing a higher than normal approach speed just leaves you in ground effect with excess speed which makes the whole thing pointless. Don't see what's wrong with doing it at the proper speed.

The whole point of the exercise being to increase the drag to increase the descent angle.

[Crash one]

Shaggy sheep. Gibbering wreck indeed. Well thanks for that, excuse me for my existence. You are coming across as one of the head up his ass instructors who never listen to any body.As for stall speed/angle of attack. I do know the difference but you obviously can't grasp that. So get on with it. Good night.

[slam525i]

Choosing a higher than normal approach speed just leaves you in ground effect with excess speed which makes the whole thing pointless. Don't see what's wrong with doing it at the proper speed.

Jesus Christ on rollerskates... no one LANDS like that. :ugh: You straighten out on short final, slow down, and land normally at normal speeds. The whole point is to have the right energy as you come over the fence in a normal landing config. Isn't that obvious? One would have to be a complete, 100%, undiluted moron to think anyone advocates actually touching down while still in a full slip (other than a forward slip for cross-winds) or while carrying excess speed.

(We've gone from a nuanced discussion about the effects of a slip to stall-speed into stating the bloody obvious apparently.)

By the way, I'd much rather float a little carrying 5-10 knots extra over the numbers on a long runway than miss the fecking runway completely. If you think coming out of a slip a few knots fast "makes the whole thing pointless" compared with not slipping... I don't even know what to say to that.

I think I'm done with this discussion. Too many people stating the bloody blatantly obvious. (Not you, Shaggy, or the OP.)

[john_tullamarine]

pitot errors prevent any correct indication of air speed

Probably one need not worry too much about the pitot. However, the static PEC can be expected to be shot to pieces unless it was addressed in the FT program for some strange reason.

[abgd]

An example of an article that claims that stall speed is reduced in a slip.

Aerodynamics of a Final Approach Slip Explained | Aviation International News (http://www.ainonline.com/aviation-news/ainsafety/2013-12-09/aerodynamics-final-approach-slip-explained)

Incidentally, for a thought experiment imagine a cylindrical fuselage and a normal wing, rotated from normal flight to 45 degrees to the direction of the airflow.

The angle of attack of the wing will be reduced, but the angle of attack of the fuselage will stay the same - zero (assuming initial airflow was axial).

Now increase the angle of attack of the wings until it is the same as the original. Obviously there will be some blanking by the fuselage, and the wingspan will be effectively reduced (and chord increased). However, the angle of attack of the fuselage will now no longer be zero.

Ergo we may expect the amount of lift caused by the fuselage to increase from zero when the angle of attack was zero, to something. Assuming the amount of lift created by the wing remains constant, and given that the amount of lift contributed by the fuselage has risen from nothing to something, we can now reduce the angle of attack of the wing by a small amount until the total amount of lift (fuselage plus wings) is equal to the weight of the aircraft - i.e. the same as in the initial unslipped condition.

And putting some numbers into it, let's take a more reasonable slip of 30 degrees (still probably rather a lot).

The height of the leading edge above the trailing edge will be identical for an aircraft with a horizontal wing.

The chord will be 1.15 x greater.

Now, if your initial angle of attack was 10 degrees just before the stall, it will now be about 1.2 degrees less, so you will perhaps have to increase the angle of attack by another 1.2 degrees to compensate. You will have to increase the angle of attack of the fuselage by about the same amount to compensate for lost lift, and you will generate a little lift by doing this.

Looking at a top view of a Cessna 172 the fuselage seems to have about half the surface area of its wings, so assuming that it is equally efficaceous in producing lift per unit area (which it probably won't be - and I don't mean efficient because I'm expecting lots of drag) you might expect that it would now be producing about 6% extra lift relative to the unslipped condition.

In practice, I'm betting that the wings are likely to produce at least 6% less lift in a slip due to shadowing, turbulence caused by the fuselage so on reflection it seems unlikely to me that stall speed is lower in a slip.

[Crash one]

I was under the impression that angle of incidence was the angle of the wing chord line to the fuselage mean line, and would be difficult to alter without major surgery.

[abgd]

OK - made 2 small edits for clarification.

But obviously the relative angles of attack of the fuselage and wings can change, whilst the angle of incidence remains the same.

[shortstripper]

Slam525i

You've taken Miserlou's perfectly reasonable reply and jumped on him. I don't for a minute think he meant you were landing in such a configuration. I think he meant that if flown the way you describe, you straighten out just before land then have that extra speed to lose in ground effect and float down the runway. That's fine on a long runway, but if like often the case with me, I need to slip coming into a very short strip over woods to lose that last bit of height. Any float would make the exercise pointless!

You'll probably think I'm having a dig too, but it's really just constructive criticism .... Which is the whole point of these threads surely?

SS

[slam525i]

You're right, shortstripper. I was already worked up over the huge fuss everyone is making of the AOA/stall-speed thing. (It's such a simple concept.)

My apologies, Miserlou.

I will say though, that I don't see anything wrong with pushing the airplane down with a full slip at higher than normal approach speeds. It bleeds off more energy in a shorter distance, and will give you the same landing roll if you're doing it right (not carrying extra speed as you get to the runway). In fact, you're less likely to end up carrying extra speed because you'll have bled off that energy earlier in the approach.

The alternative of doing it at normal approach speeds (basically best-glide speed), will give you a shallower angle of descent. At slower than normal approach speed, you'll again get better angle of descent, but you're flirting with a stall.

[shortstripper]

Hi Slam,

Yes I see your point as a pure height losing exercise and without thinking about it too hard I think you're right and reasonable on that score. I suppose the only concern might be a little extra strain on the rudder post, but that is only if taking it to the extreme and I know that's not what you meant.

SS

[Pirke]

Let's look at it differently. If you're flying straight and level at critical AOA, what would the effect of entering a side slip be? To stall or not to stall?

Another one. Let's say you're just passed the critical AOA in a straight and level flight and are in a stall. Will entering a side slip recover you?

In both cases I wouldn't bet on the side slip being a help. Maybe that's my inexperience as a fresh PPL holder, but I find it hard to believe that in the same circumstances a side slip will reduce the AOA. If only for the roll input required to compensate for the yaw induced roll. Roll input at critical AOA will make the wing with down aeleron stall. That's why you only use rudder during recovery. People die from mistakes such as this, especially on final.

[hegemon88]

Another one. Let's say you're just passed the critical AOA in a straight and level flight and are in a stall. Will entering a side slip recover you?


I guess it will help you... spin. Unless you're very precise when crossing controls.

But I am a 130-hour PPL myself so let's wait and see what the big boys have to say.



/h88

[shortstripper]

Hi Pirke,

It's actually not easy to spin from a slip if that's what you were implying?

The upper wing will generally stall before the lower but will induce corrective yaw by doing so and pull you out of the slip and full stall unless you are really ham fisted. I'm rubbish at explaining this but try it sometime. A skid is a very different beast and will spin you at the drop of a hat!

Once learned and ingrained, slipping becomes instinctive and very intuitive as you get to just know by feel how much back pressure is required and the natural buffeting created by the airflow disruption gives you a good idea of speed / AoA

SS

Edited to add

The first bit of what you write is interesting. When on the point of stalling, almost any control imput could tip you over the edge and possibly into a spin and so you are correct. However, when already established in a slip it changes the dynamic completely and with a few caveats is a very safe manoeuvre.

[Shaggy Sheep Driver]

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?


Steve, no trick. It simply requires that the wing loading be zero as well as the speed - at the top of a stall turn for instance (though there's no stall - the US term 'Hammerhead' is better!), or any other 'ballistic' manouvre. This is why if you roll on 90 degrees of bank but don't pull, the aeroplane will not stall. That is quite a good way to 'throw away' an aerobatic manouvre where one has pulled up for it and then decides not to proceed with it.

If there's no load on the wing it won't stall as the stalling angle of attack will not be reached. Conversely, pulling out of the 4th quarter of a loop the speed may be very high but pulling too hard can cause the critical angle to be exceeded (usually on one wing because the ball may not be exactly in the middle), and the result is that wing stalls and the aeroplane flicks, quite violently.

[Lightning Mate]

As an ex-RAF QFI I find this thread intriguing.

[Miserlou]

Slam,
Apology accepted but……
What you say about flying higher speed, although theoretically correct, requires a greater horizontal distance where you are already 'too close' ('too high' equals 'too close') and has a much less significant effect than slipping.

I still don't see what's wrong with doing the whole process at the correct speed.

Happiness is……

ending the final turn in a side slip and straightening up in the flare to a 3 pointer!

[Keef]

An interesting thread!

The little Jodel I fly these days doesn't have an AOA indicator - indeed, it has very few instruments. So I use the ASI to tell me how things are going. It also has no flaps, and glides quite well for an old lady. It will stall, but complains through the controls first.

The standard method of losing height quickly is to sideslip it. It behaves well doing that, and fortunately (or not, depending on your point of view) the ASI indications when slipping are useable. I find that keeping the same IAS while slipping works reasonably well. Roll out of the slip, and the IAS remains unchanged. It actually needs a slight amount of up-elevator to stop it speeding up in the slip.

I'm sure there's less lift in a slip, because it's going down like a lift. As to what the "stalling speed" is, I have no idea. 80kph indicated seems to work well.

[slam525i]

Miserlou:
What you say about flying higher speed, although theoretically correct, requires a greater horizontal distance where you are already 'too close' ('too high' equals 'too close') and has a much less significant effect than slipping.


I said:
I go full flap, full rudder, and keep the speed up, since drag is speed^2
Which means I'm slipping and increasing the airspeed, not just diving, which would do nothing except make the runway go by faster.

[abgd]

I'm sure there's less lift in a slip, because it's going down like a lift.

Except that if you weigh yourself in a lift descending at a steady rate, the answer will come out the same as on solid ground. It's only when the lift starts going down that you will temporarily weigh less, or when you slow down your descent at the bottom where you will transiently weigh heavier.

I wonder whether this could be a real reason to expect stalling speed to increase when converting from a slip to level flight - your rate of descent is liable to decrease in level flight, so whilst transitioning you will require more lift and therefore a greater angle of attack than when in an established glide. You're effectively pulling out of a dive.

[Miserlou]

Slam,

You just carry on carrying your 5-10 knots extra.
I'll carry on doing it at the correct speed, thanks.

[Shaggy Sheep Driver]

It actually needs a slight amount of up-elevator to stop it speeding up in the slip.

I don't know whether that's universal, but it's true of every aeroplane I've slipped. You do need a bit of back stick to prevent the speed building up in the slip. Not much point arriving accurately at the threshold by slipping, only to float half the length of the strip through excess speed!

As regards the lack of AoA indicators in aeroplanes, it's always struck me as a major omission. Fast jets, Concorde, and gliders (the ubiquitous 'wool tuft') have them. Not many other aeroplanes I'm aware of do.

There are several instruments in the aerobatic aeroplanes I've flown I'd gladly trade for one.

[Miserlou]

Less lift in a slip?

Ok, I'll bite on this one. Yes, there is.
As the effective lift acts perpendicular to the leading edge (check your swept wing aerodynamics) then there will be slightly less lift.

However, I believe it's the significant increase in drag which is the major factor.

Abgd,
As one is releasing the slightly increased back pressure on the controls, the risk of stalling is not an issue when re-establishing straight flight.

[abgd]

I'm not quite sure what you're getting at regarding lift perpendicular to the leading edge?

If an aircraft is descending at a steady rate, ascending at a steady rate or flat on the tarmac, the force counteracting gravity must be the same - whether it's due to the wings, terra firma, or helium bladders in the fuselage. For our purposes it's likely to come from airflow over the aircraft, so the question isn't how much lift there should be - we know the answer to that - but how it's produced.

Perhaps we're talking about slightly different things - lift defined as force perpendicular to the airflow will change slightly because the angle of descent changes in a slip. In fact, it must increase as we need the vertical components to remain equal.

[Lone_Ranger]

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.

Rubbish, Glum is correct, the 'lift' produced by a fuselage in knife edge is 90 degrees from lift produced by a wing.
In a side slip, this 'lift' simply becomes a drag component to the flight direction

[The500man]

Fuselage lift is so inefficient you need a lot of pull from the donkey to stay in knife edge flight for any appreciable amount of time. Couldn't tell you what the stalling AoA is on the fuselage but it does plummet the same when you stall on it!

For anyone that thinks you can't fly on the fuselage have a read up on slow rolls.

[Lightning Mate]

?????????????????

[The500man]

For the sake of seeing how a transition to fuselage lift and back works. Not because it has anything to do with slips! :)

[lasseb]

I wonder how you would actually deduct the airspeed when you are in a slip. It's hard enough when flying clean with all the temp/pressure errors this thing has.

When in a slip the airflow is at an angle on the pitot tube yielding errors. And depending on the design of the static port(s), airflow could be forced in here giving errors as well. The C172 is notorious for this as there is only one static port on the left side.
In total the ASI reading is very unpredictable when slipping. Without a correct reading ASI it can be a bit tricky to determine if you are actually gaining or loosing lift, as airflow speed is part of the equation...

[mikehallam]

Bearing in mind all the above caveats & that no two a/c are exactly the same, one does in my sort of light a/c rely on the ASI with usually 1.3 X stall for a normal approach.
That's quite a big fudge factor to help deal with all sorts of errors & local wind effects in the airs over the hedge.

Consequently when folk start discussing the finer points of changed direction of airflow - sort of angled across the pitot - one assumes they mean it will read lower than the actual forward speed of the 'plane. They also should simultaneously note that the wing generally sees the same angled airflow.

In both cases the pitot and wing experience much the same net airflow at approx. right angles to the leading edge.

In view of the relative crudity of the pressure sensing system used as a speed measuring device I am of the opinion that when usefully side slipping, if one only keeps to the usual margin over the IAS it a) won't be closer to the stall than normal
and b) one's actual forwards speed is higher, thus requiring a certain amount of care to reduce momentum, restore a normal attitude & a slow enough speed shortly before touch down.

mike hallam.

[Piper.Classique]

Re AoA indicators, the wool tuft on a glider is there to indicate yaw. It won't give you any other useful indication, but is more sensitive than a slip ball. (and cheaper)

[shortstripper]

Cover your ASI and practice without it every now and then!

Does nobody fly by attitude and feel? On finals I very rarely look at the ASI unless in a new type I'm unfamiliar with. I say again that a slip is very intuitive and the aeroplane will tell you if it's happy or not. The buffeting it creates gives a good indication of speed and though it's a different picture attitude is still relevant. What do you need other than your sight line to landing point to tell if it's having the desired effect on descent rate? In most of the types I fly you do need less nose down attitude in the slip to maintain the same speed and the fact that you require a bit more back pressure on the stick would indicate less lift is being generated by the wings maybe? As you straighten up the back pressure on the stick needs to be relaxed or if you are slipping at a minimum speed to get into a short strip for instance ... you will be close to the stall for sure.

SS