Airlerons etc - what's the best place for that gap!

[quincross]

I've got a specific issue that has made me think more generally about gaps between wing and control surface - any help appreciated!!

A while ago I built a tailless 'V' swept wing with an odd elevon configuration (i.e. like paoli wing). The elevon's bisected the tips of the wings at 90 degrees to the airflow, meaning they're really wide&thick at the ends and thin on the inboard. The wing flew great! The trouble was I'd built to hinge at the top, which due to the weird elevon configuration was of course now a curved surface...Doh!...hence, to make it work there was a huge gap between wing and elevon! Anyhoo, now that I know the wing flies, is stable and controllable, it's time to invest some time putting this right.

Rather than a more extensive rebuild to fix the 'gap', I could just rebuild the elevon to hinge at the bottom surface.

Now I came to thinking does a bottom hinge for airlerons, flaps or elevons cause any adverse aerodynamic issues? Surely, it should actually be better than hinging at the top aerodynamically speaking as not only is airflow unobstructed over the bottom of the wing, but there may be some turbulator effect (especially in my 'weird configuration' case).

ok,ok, I know I'd be best off aerodynamically with a round hinge (difficult to build) or a standard central ' >< ' hinged surface (can't do it due to internal pushrods) - but if you have to have a gap, where should it go?

So please let me know what you think - am I going to have issues? or do we hinge control surfaces at the top surface purely for convention and it has no aerodynamic function?

Cheers guys
QX

[BMatthews]

If the gap is a wide one it will affect some aspects.

I've got this electric powered fun fly model that I built the wing all in one piece and then "cleverly" sliced the ailerons out from the structure. Various strips and insets being placed to allow this to work easily.

So... there's no gap on the bottom and a big open V on top. The gap is sealed thanks to using monokote hinges so there's no leakage other than at the very thin gaps at the ends of the surfaces.

It definetly loops outside better than inside. The big open V's force an earlier stall when loaded positive compared to negative and the model will "hinge" or fall off a wing when looping tightly to the inside (up elevator) and not do this when looping at the same size outside (down elevator) when the smoother side is now the "upper" or "inside" surface.

[quincross]

That's exactly what I have done, in that the elevons are cut from the wing ribs (see attached photos).

So basically what your experience has been is that with an open large V gap flying characteristics have been 'better' (outside loop) than when closed (inside loop)? Or if not better, less likely to bite!

I may not be following here as it's difficult to follow and link one to the other - probably my '8mnth old baby' addled brain, sorry!

[Brandano]

hinging the ailerons on the top of the wing automatically gives you a little aileron differential, a sort of poor man's frisé ailerons. By contrast, hinging them on the bottom will give you an inverse differential, and disrupt more the airflow on the top of the wing. I am not really sure this is more desirable than the gap you have in your current design

[eflightray]

Perhaps a bit late as you already have a chamfered L.E. to the aileron?, but how about the tape 'Z' hinge? They give a double hinge.

Here's a link on how - 'Z' hinge post (http://www.rcgroups.com/forums/showthread.php?t=884171#post10027579)

[BMatthews]

Quincross, I think you got it right.

Basically to take advantage of my own experience I'd suggest that if you're doing a hinged at one surface V that you make it like |\ with the hinge at the upper surface rather than |/ with the hinge at the lower side. This will produce a cleaner and less stall prone upper surface which is where it'll count for the most on a glider that doesn't see a lot of inverted time. Now if you've already chamfered the leading edge of the control surfaces so that the hinge line looks like |< then you're sort of in the middle stall wise.

[quincross]

Thanks to all - some great advice!

Basically I'm probably going to be stiffed into hinging at the bottom surface as I'd need to rebuild the wing to hinge at the top properly (that's how it's hinged now). I reckon this is probably the lesser of the two evils - currently with hinging at the top there has to be a permanent 'gap' between control surface and wing to get round the 'curved' top edge hinge line!!! That's got to be doing weird things for the airflow and to be honest I'm really suprised the prototype has had such good flying characteristics!

I could rebuild with a Z-hinge (it'd be easier than rehinging at the top which would require a complete rebuild). I can actually really vouch for Z-hinges as I've used them on many a vintage model - although these are stitched Z's rather than tape, tape kind of brings this great solution into the 21st century which is excellent!!!

Either way, I'll let you know what the results are!

[JetPlaneFlyer]

Why not put a sealing strip over the gap. The strip could be made of mylar or similar thin flexible plastic. The strip would be bonded to the wing rear edge and just rest on the upper surface of the aileron alowing it to move freely. The wing covering could be extended over the strip to give a nice seamless look.

Worth a try?

Steve

[Brandano]

I'd rearrange the shape of the ribs at the wingtip so that the top surface will allow for a straight hinge. I think this would also automatically build in a little washout, which fo a swept flying wing is usually beneficial

[quincross]

Why not put a sealing strip over the gap. The strip could be made of mylar or similar thin flexible plastic. The strip would be bonded to the wing rear edge and just rest on the upper surface of the aileron alowing it to move freely. The wing covering could be extended over the strip to give a nice seamless look.

Worth a try?

Steve

I did think about a solution like this (although not exactly). The issue is that the strip would only seal the gap in one direction - i.e. if the gap was on the top edge it would seal on 'up' airleron and open up on 'down'. It could be made to work if the strip went inside the control surface and slid 'in and out' but would then need to have cutouts in the ribs etc. etc.

It could work, but I don't know how reliable it would be - at least that was my conclusion

[quincross]

I'd rearrange the shape of the ribs at the wingtip so that the top surface will allow for a straight hinge. I think this would also automatically build in a little washout, which fo a swept flying wing is usually beneficial

I totally agree with the sentiment, but there's a risk/reward element here!

What you suggest would require an extensive rebuild at the tips because of the D-box construction (very stiff intentionally!). So would the suspected degredation/improvement in flying characturistics be worth this effort? In other word would the simple solve of having a gap at the top really be such a pain that it's worth a rebuild to prevent it?

After all washout is already built in and can easily be increased by warping the wing along it's length rather than just at the tip.

That's really my problem!

[Texas Buzzard]

hinging the ailerons on the top of the wing automatically gives you a little aileron differential, a sort of poor man's frisé ailerons. By contrast, hinging them on the bottom will give you an inverse differential, and disrupt more the airflow on the top of the wing. I am not really sure this is more desirable than the gap you have in your current design
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I have hinged ailerons & elvons by placing monocote ON TOP for years.
That does give more up and less effective down. This lessens Adverse Yaw.

Never hinge on the bottom. That gives adverse yaw.

[quincross]

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I have hinged ailerons & elvons by placing monocote ON TOP for years.
That does give more up and less effective down. This lessens Adverse Yaw.

Never hinge on the bottom. That gives adverse yaw.

interesting!

are you saying that you get aadverse yaw from hinging at the bottom because of areodynamical effects or just because mechanically the airleron movement is restricted in the up movement? In other words is it only adventageous to hinge at the top because mechanically you can program into your tx as much up as you like (gap opens) but are restricted in how much down movement you can have because the airleron gap closes?

Adverse yaw is one of the few things that would cause me to walk down the more difficult rebuild route and hinge at the top. If this is an aerodynamic effect I'd not want to increase the amount of adverse yaw already inherent in the design which I am currently removing to some extent via airleron differetial.

[BMatthews]

He's referring to differential travel. But Texas, that doesn't come from using a top or bottom hinge line. You can only get that by placing your control horns or other arms at an angle to the pushrods. I'm assuming you don't run the surfaces until they bind.

Quincross, with a flying wing you can't use differential travel. Trying to do so would be like adding up elevator every time you use the ailerons. The normal mode of travel to avoid adverse yaw is to use more up travel than down.

[MarkusN]

Yeah, but Brandano's poor mans Frise aileron effect comes on top. And that can be used on wings as well (The Hortens made ample use of it.)

I definitely think that such a gap has less damaging effects on the pressure surface of a wing. But on a reflex profile it is debatable which side is the pressure side at the trailing edge, especially with added control surface deflection.

[quincross]

Quincross, with a flying wing you can't use differential travel. Trying to do so would be like adding up elevator every time you use the ailerons.

You're right, but yes you can - I'm not using it in the 'traditional' airleron differential sense, see below:

The wing originally had some adverse yaw in certain situations, primarily at low speed, directly into the wing. Permanent programmed airleron differential would not be appropriate for precisely the reason you give - you'd get effective up elevon. However, by programming in a mix between the tx rudder stick and airleron so that application of rudder would give a single up elevon/airleron response, you get an effective virtual yaw axis you can use to control adverse yaw in these situations.

However, like you say you have to be careful with it as it causes the nose to rise.

I hope I explained that clearly!

[BMatthews]

Well, remember that with PURE aileron input the tendency for any model be it flying wing or conventional is to drop the nose as it rolls away from level. So mixing in a little "differential" will be like adding just a little up elevator on a flying wing. At the same time it will also reduce the adverse yaw. Mix in a small enough amount and you'll see a reduction in the adverse yaw but still have a little of the nose dropping. It sounds like you've tuned in on this happy middle ground. Your rudder moving only the one surface is actually an example of 100% differential in action but at lower speeds the pitching isn't as noticable as the drag and roll effects and I think that's why you're able to make it work that way. But likely you're using it in momentary amounts just to stab some drag in but neutralize it before the nose rotates up further? If you use the "rudder" at higher speeds it likely does some funky flying antics?

At low speed with the wing operating up near the higher life coefficient range that is where adding some "down" surface angle will have the worst effect since you're adding camber AND increasing the local angle of attack which pushes up the Cl in a range on the lift drag curve where there's a much faster increasing Cd for Cl increase. That's where the adverse yaw comes from. And on something like your Paoli flying wing where there's no rudder or even any fins to try to counter or damp this effect you need to be careful when flying at that slower flying, high Cl point. Using differential reduces the down travel in favour of more up travel on the other side in the hopes of producing a better matched deflection drag on the up travel side to counter the lift related drag of the down travel side.

In the end though the real answer to flying nose up and slow is to use the absolute minimal control input and just be patient while it slowly rolls in the desired direction. Even models with differential will tip stall and fall off the opposite way if you jam the stick while "hang" flying.

Does all this sound like your situation?

[quincross]

Does all this sound like your situation?

Pretty much in that the flying speed is very slow, largely as a result of the wing loading which is now down to around 4 oz/sq ft. The wing was designed for thermalling from slope on dead calm days. I've got to say one of my favorite times for flying is at the end of summer, on a hot afternoon with a 1-3mph breeze driving thermals off the fields onto the hill - perfect! Unfortunately very rare for those of us in the UK, but I live on the side of a hill so when I get one of those days I'm there! That's what SkySurfer was built for and so far has proved just the ticket.

Due to the low wing loading (I think) there really isn't much of a stall, and I've not been able to get it to tip stall or spin. That said I tend to fly the wing in big slow thermal turns, so I guess I'm not exactly stressing it regularly and I may just have got 'lucky' on the test flights.

I tend to use virtual 'rudder' in two ways - as you describe (short stabs) to correct in a fast bank (e.g. turn to thermal) and then more gently but for a prolonged period if really trying to wring the best out of a patch of lift during a thermal turn. The former causes the nose to rise quickly and you can get porposing if you're not careful, the latter seems to have little effect but then the amount is much smaller.

If you take it that I'm probably flying faster from bank to thermal than in a thermal turn, I guess that would add up.

However, if this is the case, I doubt adding very small permanent differential would work because of the need to fly across a speed range rather than at one set speed i.e. rather more but shorter stabs of differential are needed to correct at speed whereas at the designed flight speed a more 'traditional' prolonged but less agressive differential is used.

[Texas Buzzard]

He's referring to differential travel. But Texas, that doesn't come from using a top or bottom hinge line. You can only get that by placing your control horns or other arms at an angle to the pushrods. I'm assuming you don't run the surfaces until they bind.

Quincross, with a flying wing you can't use differential travel. Trying to do so would be like adding up elevator every time you use the ailerons. The normal mode of travel to avoid adverse yaw is to use more up travel than down.
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Assuming a flat-bottomed airfoil ( ala Clark Y ). Yes, BMatthews, by hinging on the top surface and nothing else you are correct in that both ailerons will move the same number of degrees up and down. BUT, since the ailerons are hinged on the top side here is the effect: The up aileron will be forcing more air upward causing that wing to be forced down AND the other wing's aileron will be in the down position BUT not acting as effectively as the other Up Aileron. Why is this?
If you were to look from the aft end you would see that the Up aileron has moved more up into the airstream than the down aileron moved into it's airstream. This is true even tho' they both moved the same # of degrees.
What we have here is the Effect of differential ailerons without action of horns or servo. Yes both move the same # of degrees But the Up alileron is more effective because it sticks Up in to the airstream MORE than the Down aileron does.
I learned this way back when I added barn door ailerons to a standard trainer - a Lou Andrews Explorer ( sorta' like Sig Kaydet). If I tried to just turn with only elevator and ailerons to the left, the drag of the right aileron which was Down would force the nose over to the Right. I had to coodinate the turns with the rudder. A touch of left rudder when applying left aileron gave a coordinated , nice , bank and a tad of elevator brought it around.
This was long but I tried to be complete.

[BMatthews]

drag of the right aileron which was Down would force the nose over to the Right[/B]. I had to coodinate the turns with the rudder. A touch of left rudder when applying left aileron gave a coordinated , nice , bank and a tad of elevator brought it around.
This was long but I tried to be complete.

Texas, what you had there in bold with your Explorer is a classic case of adverse yaw and there's not really any differential effect at work. Or at least nothing you could put your finger onto from that description. The down traveling aileron is both increasing the wing section's angle of attack and camber all at the same time. And when this happens there's an increase in drag. At the same time the wing with the upward traveling aileron is being asked to provide LESS lift and with that comes a reduction in drag. And it's these changes in drag that was skewing the nose to the right.

Sailplanes with ailerons get this a lot. And moreso when flying slower. But if they keep the speed up a little so the model isn't anywhere near the stall they act with more of a pure rolling action. That sound at all like the Explorer?

[Texas Buzzard]

A very respected poster, BMatthews, and I are saying the same thing - - -I think.

Here is what I had saidL "I learned this way back when I added barn door ailerons to a standard trainer - a Lou Andrews Explorer ( sorta' like Sig Kaydet). If I tried to just turn with only elevator and ailerons to the left, the drag of the right aileron which was Down would force the nose over to the Right. I had to coodinate the turns with the rudder".
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Here is what BMatthews said: "Texas, what you had there in bold with your Explorer is a classic case of adverse yaw and there's not really any differential effect at work. Or at least nothing you could put your finger onto from that description. The down traveling aileron is both increasing the wing section's angle of attack and camber all at the same time. And when this happens there's an increase in drag."
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All of this came from the question of the effect of hinging ailerons on the Top surface rather than on the bottom or the middle. ( that is what I understand.)
Look at what I tried to say. "When trying to make a left turn with only ailerons initiating the bank (turn) the nose was forced over to the right. The reason was the DRAG of the right (down) aileron. This was noticed when std. hinges were used and placed in the middle. The right aleron was way down in the effective air causing Drag while the left aileron didn't cause as much drag.
When hinged on the top that right aileron's T.E. did not go down as far as it did when hinged in the middle. Thus less drag w/ top-side hinge.
Finally, BMatthews analysis is a good one but maybe he didn't see one word in my post - "EFFECT".
From that post of mine:"If you were to look from the aft end you would see that the Up aileron has moved more up into the airstream than the down aileron moved into it's airstream. This is true even tho' they both moved the same # of degrees.
What we have here is the Effect of differential ailerons without action of horns or servo. Yes both move the same # of degrees But the Up alileron is more effective because it sticks Up in to the airstream MORE than the Down aileron does."

It is good to know poster read carefully, thanks BMatthews.

[Texas Buzzard]

interesting!

are you saying that you get aadverse yaw from hinging at the bottom because of areodynamical effects or just because mechanically the airleron movement is restricted in the up movement? In other words is it only adventageous to hinge at the top because mechanically you can program into your tx as much up as you like (gap opens) but are restricted in how much down movement you can have because the airleron gap closes?

Adverse yaw is one of the few things that would cause me to walk down the more difficult rebuild route and hinge at the top. If this is an aerodynamic effect I'd not want to increase the amount of adverse yaw already inherent in the design which I am currently removing to some extent via airleron differetial.
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Yes I am saying that on a high-winged trainer type that I experienced "adverse yaw" when the plane was fitted with barn door ailerons that were hinged right in the middle of the aileron ( not on the top or the bottom). No differential modifications were used at the servo.( one servo used). Why?
Trying to keep it simple I would say that this setup made the "down" aileron more effective in producing DRAG. It was sticking down into the airflow more than the "up" aileron.
Of course I will agree that if one actually leans the ailerons back a bit from the hinge line or attaches the pushrods on the servo wheel behind the center line mechanical differential would be achieved. But a "poor boy" fix is to hing the ailerons on the top with monocote. This will produce an EFFECTIVE aerodynamic differiential. Why? The up aileron will be sticking up more than the down aileron is sticking down relative to the airflow across the wing.
A DIAGRAM MAKE THIS EASIER TO UNDERSTAND.
Yes it is a quick fix to hinge at the top, but is easier than other methods. It is for an airfoiled wing - not a flat plate.

[BMatthews]

Texas, I should have added is that I'd be willing to bet 12 inches of balsa on the fact that you would have gotten the same effect with the ailerons on your Explorer if the hinge was on top, in the middle or on the bottom. In this case I'd say it was just the model itself and nothing to to with where the hinge lines were located.

Now this is not to say that there may not have been SOME differences. As I noted way back early in this thread a model of mine with rather prominently open V hinges showed a big issue when the open side of the V is on the inside of any looping maneuvers. So there definetly is some effect on where the open side of the V is located. But your results with the Explorer are consistent with issues having nothing to do with hinge lines.

THat help out any?

[kcaldwel]

Just a note that almost all Discus Launch Glider wings have bottom hinged full span flaperons, with a substantial open V on the top surface, and a Kevlar flex hinge on the bottom surface. Top surface gap seals have been tried, but there doesn't seem to be any difference.

The European DLG pilots, especially the Swedish pilots, are using "reverse" differential (more down than up travel) on the flaperons with no rudder control (one piece fixed fin). It sounds like a recipe for huge amounts of adverse yaw, but the main effect they seem to get is some pitch up on turn initiation, which reduces the elevator input required. With all the low speed thermal turning required with a DLG, it has me questioning a lot of the conventional aileron adverse yaw ideas.

A Swedish pilot, with no rudder control, did win the German Open recently, which is the present equivalent of the World Championship for DLGs. This would indicate there isn't a big performance penalty. There is a slight weight advantage - maybe 10 grams - but they typically build heavier models anyway and almost everyone was using ballast in windy conditions.

Kevin

[Brandano]

it might just be that the adverse yaw isn't bad enough to overcome the vertical stab effect. How big is the stab and how large is its moment arm on those discus launch gliders??

[kcaldwel]

DLG's have big fins and long tail moments, so they are more yaw stable than most gliders at least.

Dr. Drela's analysis of a DLG using no differential, and no rudder, rolling into a 30 degree bank shows 13 degrees of sideslip, which I would have thought would be enough to cause a performance degradation. The European reverse differential would presumably be worse, yet it seems to work:

http://polecataero.com/handlaunchu/using-the-rudder-in-thermal-flying

K.