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Old Aug 16, 2008, 01:47 PM
Ride, Surf, Fly, Sleep
Joined Dec 2007
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Question
Airlerons etc - what's the best place for that gap!

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
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Old Aug 16, 2008, 09:54 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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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.
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Old Aug 18, 2008, 12:12 PM
Ride, Surf, Fly, Sleep
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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!
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Old Aug 18, 2008, 01:07 PM
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hinging the ailerons on the top of the wing automatically gives you a little aileron differential, a sort of poor man's frisť 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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Old Aug 18, 2008, 01:27 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,257 Posts
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
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Old Aug 19, 2008, 11:19 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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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.
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Old Aug 20, 2008, 02:38 PM
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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!
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Old Aug 20, 2008, 02:52 PM
Grumpy old git.. Who me?
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Aberdeen
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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
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Old Aug 20, 2008, 05:06 PM
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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
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Old Aug 21, 2008, 01:34 PM
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Quote:
Originally Posted by 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
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
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Old Aug 21, 2008, 01:38 PM
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Quote:
Originally Posted by 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
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!
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Old Aug 22, 2008, 11:43 AM
Texas Buzzard
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Amen.

Quote:
Originally Posted by Brandano
hinging the ailerons on the top of the wing automatically gives you a little aileron differential, a sort of poor man's frisť 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
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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.
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Old Aug 22, 2008, 05:24 PM
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Quote:
Originally Posted by Texas Buzzard
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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.
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Old Aug 22, 2008, 08:32 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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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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Old Aug 22, 2008, 11:11 PM
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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.
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