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Author Topic: Will it be strong enough?  (Read 3160 times)

Offline KenP51

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Will it be strong enough?
« on: August 10, 2013, 03:51:30 PM »
Please weigh in on a structural question I have.

In the Open Forum there is a thread "Solid Sheet Wings"    http://stunthanger.com/smf/index.php?topic=32251.0   that got me to ponder building a plane with sheet wings. Purposed power to be an unused ST .34 I have.

I am thinking about a profile fuse bi plane with (2) 7 x 36 inch wings. But instead of sheet balsa wings I want to do composite wings. Build the wings with 2 sheets of $tree foam board (paper removed) glued together and then sheet both sides of the wings with either 1/32 or 1/16 light sheet balsa. Scotch spray 77 adhesive to laminate all the sheets together. And add some carbon spars as well as some carbon caps. I have a bunch of Dave Brown .05 Oz./ft Carbon Fiber Tape to use as caps. It is an inch wide but splits to narrower widths super easy. Maybe under the balsa skins to be smoother and less unsightly (even though I know the strongest place would be at the extreme sides I.E outside of the skins).

The foam core would be about 3/8" thick. So the over all thickness would be about 1/2" with 1/16" sheeting or 7/16" with 1/32" sheeting.

The struts would lock the structure together the 2 wings would form.

I know flat plates do not make the best airfoil, but that is not what I am questioning. Rather would the wings be structurally adequate.

The wings would be flapped.

Ken

But as for me and my house, we will serve the Lord

Offline phil c

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Re: Will it be strong enough?
« Reply #1 on: August 14, 2013, 07:19:15 PM »
A half inch thick wing is pretty thin to be stiff, even with sheeting.  Assuming the wings are supported in the center and tied with struts near the tip, an 18 in. panel length they should be OK.  I'd suggest using a couple strips of 1/16 in. spruce, lr VERY hard balsa as the spars at 25% chord, tied together with a vertical web across the center.  Put another web between the spars where the struts attach too.

The easiest way to stiffen things up would be to make the wings with a real airfoil, but 1 in. thick.  Top and bottom 3/16 in. square balsa spars.

Phil C
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Offline Tim Wescott

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Re: Will it be strong enough?
« Reply #2 on: August 14, 2013, 08:28:52 PM »
1/32 inch just seems too thin.  1/16, maybe.

Another way to stiffen things up would be struts (since it's a biplane, and all).  Flying wires would work, too, but would be a pain in the patootie to rig.

I think I'd want the wings airfoiled just for performance; getting strength and stiffness would be a nice plus.
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Offline KenP51

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Re: Will it be strong enough?
« Reply #3 on: August 14, 2013, 08:56:43 PM »
I agree that 1/16 would be the safer bet. Add some strips of Carbon. Carbon spars should help a bunch.

Would some Kevlar thread (1/2A flying lines) on diagonals ( X pattern ) help in torsion? Would the correct term be drag lines?

I have been thinking hard about flying wires. They would really help. A real pain, but worth it.

Ken
But as for me and my house, we will serve the Lord

Offline phil c

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Re: Will it be strong enough?
« Reply #4 on: August 17, 2013, 04:20:52 PM »
The "problem" with using carbon fiber strips is that they immediately become THE structural member.  The only thing that keeps them straight is the foam in between which has almost no strength.  Once it bends it collapses.

Much simpler just to make it one inch thick=three X as stiff with an airfolil.  And limit the unsupported length.  Half as long is 8x stiffer.
For something like this you don't really need a foam cutter.  A couple of templates and a long sanding block would trim the foam down in a hurry.

If you really want to use 1/4 in. foam board, I'd leave the paper on(it is a good stiffener).   Use interplane struts, maybe even full chord at the tips.  Then stiffen the whole box up using X arranged interplane struts running from the upper strut joint to the lower fuselage and vice versa.
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Offline Tim Wescott

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Re: Will it be strong enough?
« Reply #5 on: August 17, 2013, 04:24:50 PM »
I agree that 1/16 would be the safer bet. Add some strips of Carbon. Carbon spars should help a bunch.

Would some Kevlar thread (1/2A flying lines) on diagonals ( X pattern ) help in torsion? Would the correct term be drag lines?

I have been thinking hard about flying wires. They would really help. A real pain, but worth it.

Aren't you approaching the amount of work to make an airfoiled foam wing?
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Offline Brett Buck

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Re: Will it be strong enough?
« Reply #6 on: August 17, 2013, 05:13:41 PM »
The "problem" with using carbon fiber strips is that they immediately become THE structural member.  The only thing that keeps them straight is the foam in between which has almost no strength.  Once it bends it collapses.

Much simpler just to make it one inch thick=three X as stiff with an airfolil.  And limit the unsupported length.  Half as long is 8x stiffer.
For something like this you don't really need a foam cutter.  A couple of templates and a long sanding block would trim the foam down in a hurry.

If you really want to use 1/4 in. foam board, I'd leave the paper on(it is a good stiffener).   Use interplane struts, maybe even full chord at the tips.  Then stiffen the whole box up using X arranged interplane struts running from the upper strut joint to the lower fuselage and vice versa.

    Phil has encapsulated the issue nicely, it is going to be very difficult to get this thing to stay together. Many of the design choices in the larger airplanes are made based on structural considerations more than aerodynamics.

     The larger the airplane the more you have to consider it, because the forces go up with the cube of the linear dimensions and the stiffness only goes up with the square of the linear dimensions. The bigger it gets, the more disproportionately the strength to weight ratio goes down. That's why you can get away with real thin slab wings on 1/2Aa.

 

    Brett

Offline KenP51

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Re: Will it be strong enough?
« Reply #7 on: August 17, 2013, 05:40:56 PM »
    Phil has encapsulated the issue nicely, it is going to be very difficult to get this thing to stay together. Many of the design choices in the larger airplanes are made based on structural considerations more than aerodynamics.

     The larger the airplane the more you have to consider it, because the forces go up with the cube of the linear dimensions and the stiffness only goes up with the square of the linear dimensions. The bigger it gets, the more disproportionately the strength to weight ratio goes down. That's why you can get away with real thin slab wings on 1/2Aa.

 

    Brett


I have gave this thing a lot of thought, and it is approaching excess work for what was conceived as very little work.

Some of the things I was going to try would work much better and for not much more work on an 1" thick true foil and still cost about the same.

True foiled wings would fly and turn better, that I am pretty certain of.

But it was a fun thought experiment, at least for me.

Maybe with a 1" thick wing I can get away with 1/32 sheet, with the use of a full depth spar, and some carbon caps.

Thanks for the input guys. I had fun with it.

Ken
But as for me and my house, we will serve the Lord


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