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Old Oct 06, 2012, 02:36 AM
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rayonnair's Avatar
Singapore, Singapore
Joined Jun 2012
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Idea
EPP flying wing (covering film for washout)

So I have been playing around with a scratch build - an EPP flying wing with a KFM airfoil.

Even after inserting a spar through the centre section, I had a fair amount of torsional flex. To make the wing more rigid under torsion, I decided I'd put covering film all over it. I figured, lose the KFM airfoil, but gain some torsional rigidity.

An interesting thing happened when I covered the wing... I got washout! It's only a few degrees, but on eyeballing it seems even across both wings. The covering film has made it fairly rigid torsionally, too.

Please note, I know nothing about aerodynamics. I just thought it was interesting to see and maybe someone who actually knows something about something can figure out a use for it.

Happy building!

Update 30 March 2014: Plans and pics are in post #5.
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Last edited by rayonnair; Mar 30, 2014 at 01:25 AM. Reason: Add instructions and pics
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Old Oct 06, 2012, 04:51 AM
Damn this gets addictive.
Thechittyfather's Avatar
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Just out of interest. What covering did you use? Do you think you could replicate what happened? And did you bond the film to the EPP?
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Old Oct 06, 2012, 07:41 AM
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Covering film used: this one from hobbyking.

For the EPP, I am using a 15mm thick sheet for the main wing and a 12mm thick sheet for the step. The thicker sheet has a bevel cut in the leading edge at 45 degrees to the bottom surface. The thinner sheet has a bevel cut in the leading edge at 70-ish degrees to the top surface.

Replicable: I suspect it is easily replicable, because I did my wing sides one by one. When it happened the first time I thought "crap, now I have to iron the other side and get it even". I left it for a week because I had no time to work on it, then came back to do the other side. Same result, and I was surprised at how even the washout was on both sides.

I don't have a piece of scrap EPP large enough to attempt another replication. I'd actually like to replicate these effects on a bigger 1.6m wing, because that's the goal I'm working to (this one is only 1m in span and was actually supposed to be a test for a KFM wing).

If I get around to building the 1.6m wing I'll update this thread with results.

Examining the wing closely, I think I've figured out why the wing twisted like this. From the end of the KFM step to the trailing edge of the main wing, the covering film shrinks and tightens. The tension from the trailing tip of the wing to the beginning of the step near the centre of the wing flexes the foam up. The front of the wing is too thick to flex easily. So what you get is washout.

Adhesive: I didn't put any on at all. The covering film seems to have no issues sticking to the EPP, which is unusual because there's only one tape I have that sticks to EPP. In fact the covering film seems to stick better to the EPP than it does to itself .
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Old Oct 06, 2012, 02:41 PM
Damn this gets addictive.
Thechittyfather's Avatar
United Kingdom
Joined Oct 2011
530 Posts
Looking at that photo I think the way you have the step run back to the TE at the root is also stopping the wing twisting at that point. Might be useful to take a photo of the whole underside of the plane so anyone wanting to try this can see how it's layed out.
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Old Mar 30, 2014, 01:21 AM
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Update with plans and flight video

Welp. Haven't been here in a while! I have been distracted with 3D planes and recently, multicopters. Been getting into the whole FPV thing and now we've come full circle - the flying wing is a perfect platform for a basher FPV ship.

I've therefore taken the time to set her up and fly her. Everything is very much TLAR, but it turned out well.

On a 3S 2200mah battery, she flies for 17m 30s until the beeper goes off (beeper is set to 3.5v/cell).

I've yet to install FPV gear and will continue to experiment. Meanwhile if anyone wants to give making her a go, I'm attaching build instructions along with some close up pictures of the setup.

Tips:
  • She is very much a TLAR wing, and I don't think you need to follow the setup exactly. My EPP dimensions are so varied because that's what I had on hand. You could probably make one out of 15mm thick EPP all the way. That might give you the option to fly finless, too.
  • The covering material gets eaten up by grass / dirt landings. Get some packing tape on the LE.
  • Wingtip fins are whatever size you want them to be. I cut scrap pieces of EPP and stuck 'em on with double sided tape. They'll fall off eventually.
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Old Mar 30, 2014, 03:35 AM
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Singapore, Singapore
Joined Jun 2012
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Flight video

Here's some flight video of the current configuration:

TLAR Flying Wing Flight Demo (2 min 19 sec)


Buddy got some nice chaser shots on the goggles too.
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