Why would full length flaperon cause wing tip stall?

[cloud-9]

I understand that outboard ailerons used as flaperons can induce wing tip stall because the deployed aileron increases effective angle of attack, thus reducing stall speed, causing the wing tips to lose lift sooner (relative to the wing root) than they would without the flaps down.

But this falls apart if the flaperon extends the whole length of the wing. Relative angle of attack should remain constant, thus the wingtips should not decrease stall speed any more than the wing root, thus keeping the ratio of stall speed for root and tip just as it is without flaperon deployed.

??

Jim

[vintage1]

Well its BETTER, but any sort of undercamber stalls a lot more sharply at low airspeed/high AofA..so thats why we use inboard flaps..basically in a tight turn, if the inner tip which is travelling slowest stalls, that wing will drop, and if there isn't instant yaw correction and decent airspeed increase, you have a classic spin.

[cloud-9]

Well its BETTER, but any sort of undercamber stalls a lot more sharply at low airspeed/high AofA..so thats why we use inboard flaps..basically in a tight turn, if the inner tip which is travelling slowest stalls, that wing will drop, and if there isn't instant yaw correction and decent airspeed increase, you have a classic spin.


Ah, right! The inner tip moves slower. THAT'S what makes the root and tip different. Thank you sir, that's what I needed to know.

[JetPlaneFlyer]

if the inner tip which is travelling slowest stalls, that wing will drop,

Vintage,
This is a common misapprehension..
Stall speed is not directly related to flying speed (other than in a minor way due to Re number).. Stall speed is mainly a function of angle of attack (AoA) and in a turn, assuming no control surface deflection AoA this is the same for both wings. The fact that the inner wing is moving through the air slower than the outer wing has little direct bearing on which wing will stall first.
What could stall the inner wing is application of opposite aileron (or rudder) to 'pick-up' the inside wing, equally the outer wing could stall first if ‘into the turn’ control deflection was added.

Getting back on the original poster’s question: ‘Strip’ flapperons should not make the wing more tip stall prone providing the flaps are an equal percentage of wing chord over the whole span… But they may, as Vintage says, make the stall sharper once it comes.
Give it a try (at safe altitude) and see how it works.
Steve

[Brandano]

I think Steve is right, the inner wing will fly at the same AOA of the outer, and so the inner wing stall isn't DIRECTLY related to airspeed. However, travelling through slower air it will produce less lift, and tend to tighten the turn. This causes the pilot to correct the turn with more aileron, and promotes the inner wing stall. So we can safely say that usually the inner wing stalls first, as a consequence of flying through slower air, but through a slightly more complex mechanism than just reaching the "stall speed", a term I always dislike! I think that the persistent quoting of a stall speed in the performance figures of a plane causes people to have a wrong idea of what actually is causing a stall.

[biber]

Dynamic pressure gets smaller towards the circle's centre, so if you don't have an increase in CL there, you will get an unballanced lift and thus rolling moment and it will increase its bank.
You needn't even speak in AoAs, as CL is what really counts.

biber

[cloud-9]

Dynamic pressure gets smaller towards the circle's centre, so if you don't have an increase in CL there, you will get an unballanced lift and thus rolling moment and it will increase its bank.
You needn't even speak in AoAs, as CL is what really counts.

biber


CL = lift?

Jim

[JetPlaneFlyer]

CL = lift?

Jim

Close... CL = Coefficient of lift

[biber]

Quite right, sort of.
CL = coefficient of lift.

To be exact,
L = A * rho/2 * v^2 * CL
with
L = Lift (Force)
A = surface area
rho = air density
v = velocity
CL = coefficient of lift

CL is pretty much linear to AoA for most airfoils, as long as flow is fully attached.
And the term rho/2 * v^2 is called the dynamic pressure.
So Lift is calculated as the 'coefficient of lift' times the dynamic pressure times the wing area it#s working on.

biber

[vintage1]

Vintage,
This is a common misapprehension..
Stall speed is not directly related to flying speed (other than in a minor way due to Re number).. Stall speed is mainly a function of angle of attack (AoA) and in a turn, assuming no control surface deflection AoA this is the same for both wings. The fact that the inner wing is moving through the air slower than the outer wing has little direct bearing on which wing will stall first.
What could stall the inner wing is application of opposite aileron (or rudder) to 'pick-up' the inside wing, equally the outer wing could stall first if ‘into the turn’ control deflection was added.



I don't think you are right, or an aircraft would never spin.

Yep. Basically let's say we have a given angle of attack, which leads to a given stall speed.

The outer wing is above stall speed, the inner wing is below it.

Ergo spin.

So although you are correct in that its the AofA which determines stall speed, the fact that the two halves of the wing are travelling at different airspeeds, is what causes a tipstall when no aileron is pplied.

I fly a lot of models on RET - no ailerons - and I can assure you that if you slow them and turn them, especially with no washout, they can and do drop a wing like a :censored:

Particularly long slender wings.

Ergo anything which increase the propensity to stall at the tips is to be avoided. You can do that with washout, or inboard flaps.

Strip flaps shouldn't make the model worse than it is with no flaps, except I have found that my undercambered vintage wings 'break' under stall more rapidly than e.g. a Clark Y type profile..and a stock wing with down flaperons looks pretty undercambered to me..

[Brandano]

Hmm, vintage1 is (as usual?) right. Actually, in a spin the two wings are at different AOA's, since the radius of the "turn" is really tight. The stall on the outer wing is less severe, and this forces the plane to keep spinning towards the wing that isn't producing lift. The same happens in a turn too, though the effect is less massive. If anything the pilot's input on ailerons will worsen it. Couple this with a pilot that is sticking to its "stall speed" without accounting for bank angle and consequent G loads, and hey presto: planes stall more easily when turning. As a matter of fact, how often does a "straight on" stall occur outside of training?

[mnowell129]

Stall is related to speed by virtue of reynolds number (rho * V * L / mu) so as speed slows down the reynolds number falls and the stall angle may change, although the changes in stall angle may be small due to reynolds number change in a relatively narrow range of airspeeds (models?).
Clearly as the airspeed approaches 0 the reynolds number does also so the stall angle drops as well.
Dropping flaps causes the lift curve slope to shift up and to the left, so if you are already at the hairy edge, dropping a flap/aileron can suddenly create a situation where the angle of attack is greater than the stall angle. It's double jeopardy too, since the flap dropping raises the effective angle of attack as well. Add this to the fact that the dropped flap/aileron has more drag, causing a slowdown and the picture is complete for a tip stall. Can happen both ways however, I've had models that would spin into the turn, some would spin to the outside.
The snaps to the outside are always a surprise :eek:

[JetPlaneFlyer]

The outer wing is above stall speed, the inner wing is below it.


This is illustrating the point I was making... In reality there is no such thing as 'stall speed'. Speed is not the dominant factor in determining when a wing will stall, it's AoA that causes stall (Re number factors accepted as I stated in my original post). I've stalled models when flying 'flat out' plenty of times by pulling too tight a turn.

Having said that, what was said by Brandano is also correct in that due to slower flying speed the inner wing tends to make less lift than the outer, so to fly in a steady state turn it has to have greater CL to compensate for lower airspeed, therefore is likely to stall first. However it's perfectly possible that the outer wing will go first if the pilot is holding 'into the turn' aileron.
I've had models tip stall 'outer wing first' on several occasions :(

Rudder application, at least on aircraft with dihedral, also alters AoA of the wing, that's how it creates a turn.

To be honest i think we really agree on the issue and perhaps we are splitting hairs. I'm certainly not denying the fact that aircraft can tip stall in a turn and drop into a spin. Ultimately the differential in airspeed between the inner and outer wing is a big factor in this... but it's not likely to be the direct cause of tip stall.

[vintage1]

For a given angle of attack there most definitely IS a stall speed.. and for given lift and wing area there is..also

High speed stall happen because the plane needs to generate more lift in turning: so more AofA so higher stall speed.

You can compute the stall sped as a function of turn radius and speed..for a given plane.

[BMatthews]

A little confusion here over terms or concepts. For a given airfoil there's a very short range of stall angle and that stall will occur at any speed if that angle of attack is reached. It can happen when in a slow approach or in a pylon racing turn. When the lift coefficient reaches the critical angle the flow separates and we have a classic stall. The "range" is caused by the operating reynolds number of the moment. A pylon racer in a high G turn has a far higher Re than the same plane trying to hang it high in a landing approach. That higher Re delays the final catastrophic flow separation to a small degree and the wing stalls at a high angle of attack which translates to a higher Cl. But all in all an airfoil will stall at a given angle of attack. That angle depends on the camber and shape and to a small degree (at our sizes at least) to the operating speed and size of the wing chord.

[mnowell129]

For a given airfoil there's a very short range of stall angle and that stall will occur at any speed if that angle of attack is reached. .... The "range" is caused by the operating reynolds number of the moment.
Agreed, however the range can be 3, 4 , 5° over a range of reynolds numbers, a 4° variance out of 15° is enough that if you are close to stall and slow down at the same angle you will stall.
I had a helicopter once that would autorotate just fine right until touchdown then things would go to H#$%. I finally figured out that right at touchdown the blades were slowing and hitting a critical Re where they suddenly got very awful and stalled and the heli plopped, usually causing a boom strike. A painful lesson about reynolds number.

[JetPlaneFlyer]

I had a helicopter once that would autorotate just fine right until touchdown then things would go to H#$%. I finally figured out that right at touchdown the blades were slowing and hitting a critical Re where they suddenly got very awful and stalled

Surely the problem there was that the blades were slowing down therefore they produced less lift (lift being proportional to velocity squared)... To compensate for the lower velocity you would add more pitch and 'hey presto'.. stall. Nothing to do with Re number, or at least Re would be a minor factor.

Back to theory that the turning model tip stalls because of lower Re of the slower inner wing...
In a turn I'd estimate that the speed differential between the inner and outer wings would never be more that 2:3 (probaly MUCH less in reality) I run a few simulations in Profilli/Xfoil using a Cark Y. Given a 2:3 airspeed speed differential the difference in stall AoA was virtually undetectable, certainly less than 0.5 Deg.

[biber]

Exactly.
With the Lift being proportional to the product of dynamic pressure and lift coefficient,
and speed going squared into the dynamic pressure you get the following.
A glider with a turn radius of 3.5 times its wingspan (that's not too far off the scale) will have a wingtip speed ratio of 3:4 (if bank is not too steep at close to 90 dgrees bank angle the speed difference will approach zero again).
That gives a ratio of dynamic pressure of (3/4)^2 = 9/16.
I.e. to maintain the current low bank angle the CL at the tip pointing into the circle will have to be 16/9 (178%) of the value the other tip is working at.
Don't pick on the exact numbers too much, they just ought to show about what we are dealing with.
However, Quite meaningless by what means you achieve that higher CL, be it by aileron deflection or by dihedral combined with slip angle.
It's likely to get the inner tip stalled first (though there are designs that might repeatably stall the outer tip first under certain circumstances).

biber

[JetPlaneFlyer]

For a given angle of attack there most definitely IS a stall speed..

Vintage,
I should leave it but I can’t help myself challenging you on this statement (the use of bold font and capitals is what does it).

If you think this statement is literally true then please tell me; what's the stall speed for a Clark Y airfoil, 8" chord, operating at a given angle of attack of 3 Deg :confused:


I'm not denying that Re number has an influence on stalling AoA but over the airspeed differential that you would experience between one wing and the other in a turning aircraft this effect is insignificant... Biber explains the real reason for the inner wing tending to stall first in his post above.

[relyon]

Vintage,
I should leave it but I can’t help myself challenging you on this statement (the use of bold font and capitals is what does it).

If you think this statement is literally true then please tell me; what's the stall speed for a Clark Y airfoil, 8" chord, operating at a given angle of attack of 3 Deg :confused:


I'm not denying that Re number has an influence on stalling AoA but over the airspeed differential that you would experience between one wing and the other in a turning aircraft this effect is insignificant... Biber explains the real reason for the inner wing tending to stall first in his post above.I won't speak for vintage1 but suspect this may be a confusion between stall and loss of lift. A stall always causes a loss of lift; a loss of lift is not always caused by a stall. This is probably the single biggest area of confusion I see when quizzing helicopter private pilot candidates about airflow differences between the advancing and retreating blades of a helicopter rotor system in translation.

Bob

[BMatthews]

There's often little lift lost in a stall. You just need to look at all the 3D models "harrier'ing" all over the place with the wings and controls working just fine but they are in a fully stalled state. The real beef with the stall is the sudden and huge build up in drag. That drag typically makes the airspeed drop drastically in a very short time and that leads to a sudden loss of lift and the typical nose drop.

[vintage1]

Vintage,
I should leave it but I can’t help myself challenging you on this statement (the use of bold font and capitals is what does it).

If you think this statement is literally true then please tell me; what's the stall speed for a Clark Y airfoil, 8" chord, operating at a given angle of attack of 3 Deg :confused:


I'm not denying that Re number has an influence on stalling AoA but over the airspeed differential that you would experience between one wing and the other in a turning aircraft this effect is insignificant..

It most certainly is NOT...a 6 ft wingspan model with a turn radius of under 10ft is what you may experience with a glider.. that is 13:7 ratio of tip airspeed if the model is 'flat'.

For a low wing loading model, the centripetal acceleration is quite low..V squared over R..if its say 15mph (22fps) its about 1.5g..so a bank angle of a little over 56 degrees. So even at that bank angle there is a significant difference in airspeed between inner and outer tips.

The faster you go of course, the more G you need to pull to maintain a radius, until the bank angle approaches 90 degrees..which is why at high speed stalls, the aircraft may not break into the turn at all..it may break out.



. Biber explains the real reason for the inner wing tending to stall first in his post above.

[vintage1]

There's often little lift lost in a stall. You just need to look at all the 3D models "harrier'ing" all over the place with the wings and controls working just fine but they are in a fully stalled state. The real beef with the stall is the sudden and huge build up in drag. That drag typically makes the airspeed drop drastically in a very short time and that leads to a sudden loss of lift and the typical nose drop.

Indeed. a stall on the inner wing will slow the wing even more. Lowering the lift.

[JetPlaneFlyer]

It most certainly is NOT...a 6 ft wingspan model with a turn radius of under 10ft is what you may experience with a glider.. that is 13:7 ratio of tip airspeed if the model is 'flat'..
You can turn a 6ft span model in a 10ft radius.. without banking it :eek: Maybe you can but certainly my experience is a little different, maybe I'm just not a good enough pilot.

If you take the 56 Deg bank into consideration (which would have to be a VERY lightly loaded model) then the ratio is about 11/8 (or 8/11 depending on which way you look at it)... i.e. slightly LESS airspeed differential than the 3/2 (or 2/3) figure I speculated on in my previous post ;) .. So even by your own calculations my guesstimate was pretty accurate and if anything slightly exaggerates the speed differential.

For a more average R/C model the flying speed would be much higher in such a tight turn (if it could turn that tight at all), the bank angle steeper and the ratio of tip airspeeds even closer.

PS... You did not answer my question on the 'stall speed' of an airfoil at 3 Deg AoA :confused: ... this was the main point I was getting at in my post.

Anyway, enough of this, I think we both agree that the inner wing is the one most likely to stall in a turn even if we cant quite agree on the main reason why this is so... Possibly time to 'agree to differ' on this minor detail?

[Cory]

The coordination of the turn has been neglected in this conversation. In a plane with any di or polyhedral the two wings will be flyng at different AoA's, unless the turn is perfectly coordinated. Lacking a turn coordinator, RC pilots don't really have a good way to know how coodinated their turn is. I beleive that you will find that the inner or outer wing will stall first depending on whether you are in a slip or a skid. I seem to remember that making the difference in which way a Cessna 150 would roll when doing accellerated stalls years ago.

[biber]

For me at least in full size it's quite clear what a reasonable scenario does look like.

Take a 15 m ship.
V_min is about 20 m/s, thermalling airspeed is typically from about (case 1) 25 m/s to (case 2) 30 m/s,
allowing for about 1.5 g to 2g turns.
That gives maximum bank angles from roughly 45° to 60°.

Turn radii corresponding to that are:
(case 1) 64 m
(case 2) 62 m

The radii the wingtips are moving on:
case 1 - inner tip/outer tip: 54.5 m / 74.5 m = 73%
case 2 - inner tip/outer tip: 54.5 m / 69.5 m = 78%

That squared gives the ratios of dynamic pressure from the inner tip to the outer one:
case 1: 61%
case 2: 54%

And that's what the Joe gliderpilot faces over and over on every normal flight.
It's just normal to have something upto 75% or so average outward aileron applied in a coordinated (!) slow thermalling turn, depending on the type of glider, your mileage may vary.
Problem is, that too much of downward deflection (aileron) may increase Cl_max a bit, but also tends to worsen stalling characteristics of that section.

biber

[Ralph Walton]

My take on the arguement of full length flapperons causing tip stall, ( which seems to be the case I've observed) is that a flapperon at the tip creates "wash in".

Most planes are designed with washout at the tips to help prevent tipstalls, If you go putting flaperons at the tips, you create the opposite. Thats why crow braking uses the flaps down, and the ailerons up. Very stable.

Go for spoilerons if you can. They are much more stable.They can be trickey to get the geometry of the servo and linkages right, but if you start with the servo arms leaning back ( if the servos on top of the wing) as far as you can while having good flight control ( gives favourable aileron differential too) and connection hole in the horn on the aileron back a bout 5 mm from the hingeline, you're on the right track.

[Brandano]

biber, how pronounced is aileron differential on this sort of ship? Could it be that most of the counteracting roll moment is given by loss of lift on the outboard wing rather than increased lift on the inboard wing? I expect that especially on high AR wings the effect of adverse yaw will be massive.

[biber]

There is significant differential on most full size gliders.
On many of them it's more or less close to an up deflection twice as big as downward.
As it's done mechanically the diff is about zero for small deflections.
Anyway, it always cancels out just some fraction of the adverse yaw and for speeds about below 30 m/s you still need pretty much full rudder added to get a coordinated rolling motion at full aileron.

It wouldn't pay to exceed a certain limited downward deflection, anyway, since extra lift wouldn't further increase, while airfoil drag will literally explode.
A reasonably effective upward deflection can be much bigger.

The diff is certainly not as big a factor in minimising the adverse yaw of full size gliders than most people would think.
Otherwise it would be a disaster to fly inverted and in fact it is not.
(You can roll it into bank and enter turns maintaining centered yawstring during inverted.)

In fact there are lots of gliders where the minimum speed limit in thermaling is because you are running out of outward aileron to maintain bank and going even more slowly would let you roll steeper into the turn.
That may happen with partly stalled aileron or wingtip but need not involve any stalled parts to get your aileron control maxed out.

biber

[Andrew McGregor]

Even in a perfectly coordinated turn, the inboard wing tip is running at a lower airspeed AND higher angle of attack than the outer tip; remember, coordination is at the fuselage, and in a turn the flow field is curved relative to the plane. So, you have lower airspeed inboard, meaning lower Re and therefore lower stall AoA, combined with a geometrically higher AoA. Of course the inboard wingtip will stall first.

Full length flaperons do increase the AoA; they've moved the trailing edge down. Now, that amounts to a change in decalage angle, and therefore a change in trim speed as well. That means that you're trimmed to fly slower after deploying flaperons, you have more drag, requiring more power to maintain flight at that speed, AND you have less margin between trimmed AoA and the stall. The higher power requires more torque from the motor, which means you're likely to be uncoordinated. And then you fly a typical RC landing approach, which involves some pretty tight turns, and snap out of one because your inboard wingtip stalled...

If you set up a throttle to rudder mix to keep the plane coordinated in the face of torque reaction, flaperons become much more civilised... but you still have to fly pretty carefully with them on.

Full length spoilerons, on the other hand, make lots of drag, reduce the AoA making it hard to truly stall the model (although the drag level might well make it fall like a rock anyway), and are generally a better idea. Only downside is that at extreme deflections, you tend to lose aileron authority altogether.

[talkthetorque]

True that airspeed has little to do with stalls and that AoA is what counts but I dont agree that in a turn the wing tip speeds dont play a role. Remember that AoA is measured from the direction of airflow coming into contact with it. The slower wing will have a high AoA because the airflow angle is now different. Imagine a plane in a conventional spin. Picture how the air is hitting the outboard wing and how it is hitting the inboard wing :) The angles are very different

Regards