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 Post subject: Preventing Wing Twist
PostPosted: Wed May 09, 2007 6:43 pm 
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Paper Model Overlord
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Joined: Fri Oct 06, 2006 1:15 pm
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Location: Alameda CA
Designing tapered wings that don't twist at the tip I'm finding is a real bear. Has anyone worked out what variable to control? I thought I had it figured out by calculating the true trailing edge to tip chord intercept angle for both the upper and lower surfaces, but even then the wing sometimes glues up with twist.

I don't crease the leading edge, which makes this problem more difficult.

If I can get this issue properly identified as to what factors control it, I can exploit it for propellors, which NEED twist.

Thanks for whatever assistance anyone can render!

Mark Baird
Alameda CA


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 Post subject:
PostPosted: Thu May 10, 2007 6:05 am 
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Paper Model CINC
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Location: Pensacola
Designing a model is sometimes a trial and error thing. IMHO, you can go overboard with calculating, and sometimes you just have to rely on zen. What I do is look at the overal profile of the wing. On a FG type model, the wingtip will usually be nearly flat, while the upper curved surface will increase by a percentage, based on the profile. I take the flat wing profile, and using the perspective option on the MOVE tool in Photoshop, I expand the wing root approximately the amount I estimate I'll need. I haven't had much problem with wing twist using this method, on swept, delta or straight wings. Soemtimes, it'll take a try or two to get the right amount of increase, but paper and ink are cheap, and building sub-assemblies to check fit and alignment is a standard design function.

One tip when doing this--make sure that the area the wing that will have the greatest thickness is perfectly vertical.

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 Post subject:
PostPosted: Thu Jun 14, 2007 5:31 pm 
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Paper Model Overlord
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Location: Alameda CA
Rob wrote:
... sometimes you just have to rely on zen.

Zen works if you know where to give something the english, so to speak. With wing twist I couldn't figure out what variable to tweak.

But I think I licked it.

Aft of the mainspar on most wings the curvature of the airfoil toward the trailing edge is so small as to be virtually flat. The plan view of such a section would be three sides of a rectangle with the fourth side being the angled trailing edge forming a trapezoid. With curvatures close to nil, simple trigonometry can be used to calculate what the true shapes would have to be to achieve a desired amount of twist out toward the tip (wash-in or wash-out).

The key to the whole thing was recognizing that the trapezoid can't actually be a trapezoid, but has to be two joined triangles, with the joint being a "fold" that allows the tip chord to be set to its degree of twist WITHOUT lifting the trailing edge off a base plane. That "fold" runs from the top root corner to the trailing edge tip corner.

Assuming no twist, the front triangle has the tip chord flat on the base plane, and tilts up off the base plane until its apex is as high off the base plane as the root thickness at the mainspar. The trailing edge triangle joins that triangle, with its root chord edge tilting back down to meet the base plane, and the trailing edge itself flat on the base plane.

Use trigonometry to calculate the true shapes of the joined triangles and voila - you've got the true shape of the wing pattern aft of the mainspar (at least close enough for the tolerances involved in paper).

Once the formulas are derived, you can start to add twist and/or dihedral to desired amounts and work out the true pattern shape every time.

I've applied this method to three different wings already, and it's been working perfectly.

Without pictures it's a bit difficult expressing the concept clearly, but I hope there's enough here that it helps anyone who's been struggling to prevent their wings from twisting.


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 Post subject:
PostPosted: Fri Jun 15, 2007 9:51 am 
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Paper Model CINC
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Location: Pensacola
We do a lot of complex math, usually withot even realizing it, each and every day of our lives. Every time you wad up a piece of paper, and toss it across the room into a trash can, you're actually calculating the arc required based on the distance and mass of the paper wad, for instance. If a fan is blowing, additional calc's fg\have to be take for windage. Pretty complex calculus, yet we manage quite well....well some of us....
There's math behnd everything--so I congratulate you on deciphering the wing twist problem and putting it into terms we can readily use. Math rules!

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Currently working on: see avatar


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 Post subject:
PostPosted: Fri Jun 15, 2007 1:32 pm 
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FG Origami Master
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Joined: Fri Sep 17, 2004 6:33 am
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Location: San Antonio TX
I didn't take physics in high school.

That MUST be why I suck at 8-ball!

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Edward Merica
AKA "The Sarge"


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 Post subject:
PostPosted: Fri Jun 15, 2007 2:06 pm 
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Paper Model CINC
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Don't need it--just some geonetry, trig and calculus. Physics s science--not math. I only took enogh physics to get my degree, although it's an interestin subject, too.

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 Post subject:
PostPosted: Tue Jun 19, 2007 12:01 pm 
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FG Origami Master

Joined: Mon Nov 20, 2006 3:48 pm
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Location: liberty lake, wa
no offense, Rob but it looks like you need to take some spellin'
(nice brewster; nice of you to share pics of the process, although
you make it look easy)


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 Post subject:
PostPosted: Tue Jun 19, 2007 3:14 pm 
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Paper Model CINC
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Joined: Thu Oct 21, 2004 9:34 am
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Location: Pensacola
I've got a bit of arthritis in my hands, and if I type too fast--I get lots of mistakes....mostly of the 'fat finger' variety. Guess that what happens when you learn to type on an old Underwood manual.

the Brewster looks simple--but doing the cowling and landing gear is a bit fussy. Best bet is to let every sub-assembly dry solid before continuing on. (have another model to work on between those steps)

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-Rob-

Currently working on: see avatar


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