Wing geometry and twist - RC Groups
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Mar 13, 2017, 09:40 PM
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Jon Himself's Avatar
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Wing geometry and twist


Hi guys, wonder if I can pick some brains.
I'm a newbie plane builder but just finished my first balsa design (and first flying wing) and to be fair it is not as good as I had hoped. Lessons learned and I am now heading back to the CAD board.

However I wanted to try and clarify some of the geometry aspects and design intent.

1) Where do you usually pick up the dihedral from ? The way I have done it is to build the wing in two halves and incorporate the sweep, dihedral and twist into each half....trying to be clever I have ended up with a quite weird warped wing that was a pig to skin as the wing is bowed.
My ribs are not perpendicular to the spars which I think has caused build issues.
Should I have built dihedral into a centre section and gone from there instead with a pair of straight wings coming out at an angle ?

2) Where should the twist originate from ? I took mine from the trailing edge, again with the concept I could keep the aileron shape as easy as possible to construct. Now however I think I should have twisted about the hinge line of the control surfaces (ie rear spar location) as I am having binding issues - enough to abort the build.

Shame because the wing was strong and the twist held up great, aerodynamically I know it would have flown well if I could have finished it.
Any help much appreciated - cheers
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Mar 14, 2017, 10:16 AM
Registered User
What kind of twist did you use?

The simplest way is to block up the wingtips ("linear" twist).

But you could also use the (recently discussed) "nonlinear Horten" twist.

Might also be informative if you had some pics to show how you hinged your elevons.

Finally, it might also be good for you to share some of the specifics of your planform ... sweep, amount of twist, etc. You shouldn't need all that much dihedral with a swept wing, due to the dihedral affect of sweep.

-Dave
Mar 14, 2017, 03:15 PM
B for Bruce
BMatthews's Avatar
Yeah, pictures would help a lot.

What you did isn't bad in itself. Lots of folks have used the same techniques. It may be that you just missed the target slightly on a few aspects. A slightly different approach or different amount might be all that is needed.
Mar 14, 2017, 04:11 PM
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Jon Himself's Avatar
Hi chaps and thanks for the replies,
TBH I have learned a lot since I designed the wing - as the 1st wooden plane I ever tried it is a bit embarrassing, but I am more of a trial and error person. The 1st foam version of this I built flew incredibly well apart from the ridiculous low wing loading !

I designed the wing to give a bell shaped lift curve from the sweep and taper ratio confered by nothing other than aesthetics.
Using the local coefficient of lift I plotted the shape and twist using an autostable foil at the root and a naca 0010 at the tips.

It's about +1.5 at the root, -2.0 at half way and -10.0 at the tips if I remember right.

From there I gave it a 3 degree dihedral, again nothing other than guess work. All the trailing edges sat on that theoretical dihedral line and the nose sections are pointed down as if they were hinged at the trailing edge.

From there I cut my plan with little feet - it actually worked really well but the design could have benefited from more torsional strength as the wing has tried to straighten out (not helped by the leading edge design).

Hopefully the pics make sense - I am aware I made a fair few errors with the construction, esp the leading edge.
The aileron spar is the main issue as it is not linear and I neglected to brace the ailerons enough, the result is warping.
Last edited by Jon Himself; Mar 14, 2017 at 04:18 PM.
Mar 14, 2017, 04:57 PM
Registered User
10 d seems excessive at tips. Especially if your Foil transitions.
Tip twist has relationship to the anticipated/planned Flying speed envelope. (fast requires much less twist)
It's Important to have/keep the Elevon hinge line straight, as you now understand
3d Dihedral total or per side ? 5 would be okay. Dunno about more though.. although 6 d isn't excessive
IF emulating a Horten effort ? Note that the Leading edge is Not a straight line.
Mar 14, 2017, 07:01 PM
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Jon Himself's Avatar
10d was from a cl calc, it was designed as a slow flier (bungee glider only)

On the twist, instead of pivoting from the trailing edge, my guess is that I should pivot from the rear spar (elevon hinge) ?

3degrees both sides yes, so 6 all in all.
Mar 15, 2017, 10:08 AM
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Knoll53's Avatar
Quote:
Originally Posted by Jon Himself
2) Where should the twist originate from ? I took mine from the trailing edge, again with the concept I could keep the aileron shape as easy as possible to construct. Now however I think I should have twisted about the hinge line of the control surfaces (ie rear spar location) as I am having binding issues - enough to abort the build.
Yes, this is correct. With all the wild variables in a Horten wing, every rib is different. In order to avoid binding at the control surfaces, you must lay out the airfoils about the hinge line. Top or bottom hinge line makes a difference. This would cause the leading edge to droop with zero dihedral at the hinge line, so the designer must layout the hinge lines with dihedral is such a way to sculpt the leading edge. This task is quite doable with 3D modeling.

BTW. there is a work around for the binding issue. Cut off your continuous hinges and simply use just 2 pin hinges per control surface.

Kent
Mar 15, 2017, 11:20 AM
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EdSoars's Avatar
I recently set up the hingeline for a central elevator with one servo on a 2m plank. I wanted a little dihedral, so I calculated how much dihedral could be accommodated by running the hingeline from the top surface in the center joint to the outboard ends of the elevator. It isn't difficult to carve, as all the surfaces are planar. Hinges will be pin hinges or a continuous line of silicone cement.

And now that I've offered that wisdom, I realize that it won't help your Horten twist distribution at all.
Maybe someone else can use the method.
ed


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