The effect of sweepback on adverse yaw on flying wings

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John Newton

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I have been thinking about adverse yaw as this seems to be the number one problem in the adequate control of swept flying wings. The key to a successful flying wing design (without fins) seems to me to design a simple means of negating the adverse yaw for all normal flight regimes.

Does anyone have any thoughts on how wing sweepback effects adverse yaw? My inital thoughts are that it will get worse with increasing sweep for two reasons:
1. The increased dihedral effect will make greater control deflections necessary to achieve the same roll and therefore greater adverse yaw will be generated. (I suspect this is what is occuring on my current model which has 30 degrees sweepback).
2. The assymetric effect of the sweep as the wing yaws may cause increased drag on the wing halve on the outside of the roll/turn opposing the yaw/roll.

If anyone has any insight or data on this it would be much appreciated.
 

WonderousMountain

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There was an earlier thread on this site, the distance from CG, amount of deflection, speed of wing, incidence, slip angle/breeze, and pendulum effect all come into play. Even without an engine these are a lot of things to look at all at once "through" a turn.

Could you publish the views and weights of your sailplane? This is one of my favorite questions...
 

John Newton

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One solution is to increase the angle of the elevon hinge line. As the elevon moves up, it begins to act like a small rudder.
I'm not sure I quite follow, do you have a sketch, photo to illustrate?

I notice that the full size tiger moth (and models of) is frequently said to suffer from adverse yaw, I wonder if the sweepback of the wings (whilst only small) contributes to this effect?
 

John Newton

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There was an earlier thread on this site, the distance from CG, amount of deflection, speed of wing, incidence, slip angle/breeze, and pendulum effect all come into play. Even without an engine these are a lot of things to look at all at once "through" a turn.

Could you publish the views and weights of your sailplane? This is one of my favorite questions...
I am simply tring to ascertain the general trend, before I commit to a given sweep, i,e. does sweepback, in general, increase adverse yaw, I realise that other factors my come into play.

I find knowing the centre of gravity location and wing loading of similar designs is useful, for comparrison, especially with flying wings where the location of the precise neutral point and centre of gravity are often crucial. Researching the subject is made harder by the fact that these figures are often not given.


John
 

PaulS

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Divorced ailerons such as the Junker style will diminish or completely do away with the adverse yaw associated with induced drag. They are also more sensitive to angular deflection verses rate of roll - especially at low speeds and high angles of attack.
 

Norman

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Divorced ailerons such as the Junker style will diminish or completely do away with the adverse yaw associated with induced drag.

No they don't. The constructive interference reduces friction on the leading surface quite a bit but pressure drag is increased. Induced drag is entirely a planform effect and external airfoil flaps don't have any advantage in that area over any other hinging method. They are more powerful than plain flaps so a smaller surface, or a smaller deflection of an equal sized surface, could be used. One thing I noticed in the NACA reports on their work with external airfoil flaps was that there is a very short lag between the increase of lift and the drag buildup but that lag is only a fraction of a second and wouldn't have much effect on adverse yaw.

Adverse yaw is caused ENTIRELY by the induced drag of the up-going wing. It's not necessarily caused by the greater lift of that wing but rather by the lumpy shape of the lift distribution. Say you have a perfectly elliptical lift distribution before you deflect the elevons. When you deflect both elevons the same amount you get a similar ugly distortion to the lift distribution on both wings but the one on the down-going wing produces a smaller yawing moment because, even though the lift distribution is a bad shape, the lift near the tip of that wing has decreased thus moving the center of lift {and the induced drag that that lift makes} inboard. So not only is there less induced drag but the vector is smaller and on a shorter lever arm from the CG.

Meanwhile the up-going wing has this big ugly bump in its lift distribution out near the tip. Not only does it produce more drag than an elliptical lift distribution but the center of lift is farther outboard and therefore works on a longer lever.

The solutions to this problem are to either use some effective way of producing a lot of form (parasite) drag near the wing tips or resort to some trick to make the lift distribution on the up-going wing improve with elevon deflection. The only way that I know to do that, in the absence of vertical fins, is to have the basic lift distribution be slightly less than optimum for the span. Then when you deflect an elevon down the lift distribution becomes more elliptical thus allowing you to increase lift without as much increase in induced drag. At the same time the lift distribution on the down-going wing has gotten just horrible thus producing lots of drag on the inside of the turn.

They are also more sensitive to angular deflection verses rate of roll - especially at low speeds and high angles of attack.
Yep, constructive interference AKA "slot effect" improves the pressure gradient on the forward surface thus delaying boundary layer transition and delaying separation to a higher AoA but not nearly as much as a leading edge slot. They're also less sensitive to main wing stall. All in all a very interesting type of control surface but they're not a cure for adverse yaw any more than any other profile tweak. Induced drag just isn't an airfoil section characteristic
 
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John Newton

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Thanks Norman and Paul S, had been looking into Mitchel wing Junkers style flaps versus Kasper flaps, drag rudders etc. as a solution to adverse yaw on my wing, did not realise that the Junkers/Mitchel type did not reduce adverse yaw, can rule that one out then. I am currently leaning toward a strengthened version of the Kasper flap, seems a nice and simple solution (if it works!).

I am currently avoiding going the bell shaped lift distribution route due to the complexity of designing/building it accurately.

I Would still be interested to know if adverse yaw is effected by wing sweep angle, took a look at other threads on this site and elsewhere and could not see anything.
 

jrhirsch

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Elevon angle.GIFPicture an elevon straight up. It becomes more and more like a rudder as the hinge angle is increased as in the lower wing. When the hinge angle increase is combined with a tip plate or fixed rudder and a few degrees of dihedral it turns more like you're using elevator and rudder. It has a very controllable mush as well. The drawback is that it washes out the inboard section of the wing more than the outer and likely causes greater drag when the elevon is out of it's normal position. I gave up on flying wings. They look beautiful, but they're never as efficient as a conventional planform.
 

Aircar

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You are right that aft sweep does inhibit yaw rate (as the forward moving wing exposes more projected span --the directional stability contribution of swept wings is opposing the yaw/turn input --anhedral can cancel this out to a large degree . The OTHER contribution to adverse yaw not mentioned by Norman is the THRUST felt by the down going wing (just as a flapping bird feels ) --this also opposes the turn and requires a zero lift condition on the downgoing wing to avoid --a wing down force or negative local alpha is sure to create the forward force you don;'t want just then. It is possible to get no roll damping by judicious control of local wing angles of attack across the span (as birds seem to do ) or a net lift increase during the turn ... Curiously I had a sit on the Russian Prakt ultra light at the localm airshow (Avalon) and noted that it has REVERSE differential with the flaperons in positive settings --NO UP going aileron at all with full flaps but only downgoing on one side , The 1960s Victa vAirtourer had 'automatic' wing twist buiolt in from the two piece flaperons which drove the outer aileron segment cia a two to one gearbox on their outer end (regardless of whether flap or aileron the outer end moves half as much as the inner -seemed to work quite well ) This sort of mixing set up (with more variations) is common on flying wings --the YB 49 was an example.

Some model testing can teach a great deal about dutch roll and other behaviour of swept or otherwise non standard layouts --some maybe unprecedented.

BTW has anyone heard of how the Synergy is going ? -is it flown yet ? (an example of both forward and aft swept surfaces and claims proverse yaw )
 

John Newton

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View attachment 22754Picture an elevon straight up. It becomes more and more like a rudder as the hinge angle is increased as in the lower wing. When the hinge angle increase is combined with a tip plate or fixed rudder and a few degrees of dihedral it turns more like you're using elevator and rudder. It has a very controllable mush as well. The drawback is that it washes out the inboard section of the wing more than the outer and likely causes greater drag when the elevon is out of it's normal position. I gave up on flying wings. They look beautiful, but they're never as efficient as a conventional planform.
Thanks for the diagram, I see what you mean now, I may well have a play with some small models to see how this works in practice, looks like a nice and simple solution to adverse yaw if not the most efficient way of negating it. I wonder if it would work without tip plates/winglets?
 

John Newton

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While it's not exactly adverse yaw, all swept wing aircraft are susceptible to a related phenomenon known as Dutch Roll.

Dutch roll - Wikipedia, the free encyclopedia
I have experienced this on one of my tailless models (a Lightning Dart), I assume it is down to lack of sufficient yaw damping, Having spoken with other members it seems to me this can be cured by using a fin ahead of the c of g combined with enlarged winglets aft of the c of g to give required yaw damping without inreasing tail volume?
 

John Newton

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Thinking about Kasper flaps, does anyone know if they work in practice to negate adverse yaw? I have heard conflicting reports, don't want to commit to using them in my model if they won't work.
 

henryk

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captarmour

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What about roll spoilers? If they can work they give yaw in the correct direction and can couple in a bit of pitch up to reduce up elevator in the turn. probably too much of a good thing on a swept wing though?
 

John Newton

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What about roll spoilers? If they can work they give yaw in the correct direction and can couple in a bit of pitch up to reduce up elevator in the turn. probably too much of a good thing on a swept wing though?
I had though about using these and mixing in down (?) elevator to compensate for the pitch change they would produce when deployed, not sure where best to locate them so that they would not interfere with elevator control surfaces though, ideally both the elevator and spoilers want to be located near the tips on a swept back wing.
 

cavelamb

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The Northrup YB35/YB49 had yaw problems that had to wait for powerful microcomputers that could correct it.
A simple mechanical mixing probably would not work very well over a large speed range.
This is one of the niche areas where FBW might be used to advantage in a home built - as a yaw damper rather
than primary flight controls.
 
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