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Building Tips and technical articles. => Building techniques => Topic started by: Motorman on May 29, 2021, 09:37:38 PM
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Intentionally left blank
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Vertical grain for sheer webs on main spars and vertical boxing of trailing edge.
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This is a case of reality vs purity of a theoretical concept.
If the TE is made of thin sheet, for example an upper and lower layer of 1/16", and overlays the tails of the ribs, then the beam is interrupted by each rib which is in cross-grain compression. So the additional strength of a shear web that has vertically oriented grain between the ribs does not achieve what you might think. Instead you have a discontinuity at each rib. One of the rules for designing structure is to avoid--or be very careful you understand--discontinuities.
Unless you are using superlight balsa, or unusually thin balsa for TE shear webs, there likely little reduction in strength, even in a theoretical TE with no rib discontinuities.
That said, it is just as easy to cut either type, so have at it. Either is stiffer than none, which would appear to be the goal here, not to decide which one has the higher ultimate strength.
Dave
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I fully believe in vertical grain TE shear webs to avoid the TE pieces from sagging between the ribs. Never had a problem with them, but did have some problems before I started installing them...
Years ago there was a feature in Mechanics Illustrated (or was it Popular Mechanics...) called "Wordless Workshop." In that feature each month a cartoon character would build something for his house, but the actual instructions were the photos/illustrations only. Attached here are a few photos that make up a "Wordless Workshop" piece on making and installing TE shear webs. I guess in this case I'm the cartoon character...
Enjoy - Bob Hunt
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As always, pics are worth a 1000 words, thanks Bob. y1
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I don't understand what I'm seeing. On picture 10 there is a piece glued to looks like the top of the trailing edge that looks to be rough balsa. What is that part for? Looks to me like it runs the span of the trailing edge.
The fit and finish of your work is outstanding!!
Quite frankly, I'm jealous of your ability....
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Can I piggy-back a related question?
How does one generate the inside angles when doing sheeted trailing edges? I have three kits using this instead of a solid TE.
And if I manage to cut the inside angles, which one would be best?
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Hi gang,
I have to make a small comment there. I agree with everything presented so far, with the exception
of one thing. In an effort to maintain perfect alignment, I don't like to remove the wing from its
jigging until as much of the fabrication is completed as possible.
I was reminded of why this is my policy, when I started to remove the wing from it's jigging, (Imitation kit
from Mike Griffin) and one tip lifted up about 1/8" - 3/16". I had neglected to install the trailing edge
sheer webbing, so the wing stayed in the jig, and the sheer webbing will get installed before removal.
A little tip here....be sure and install your TE sheer webbing BEFORE installing cap strips, because they
make installation a lot harder. As you can see from the attached photo, it's going to be cumbersome to
install the sheer webbing at this point, but I'll get 'er done ! In the future, I might even consider adding
the TE sheer webbing before adding the TE top sheeting...we'll see how that works out.
As always, thanks Bob for taking the time to do a "Wordless Workshop". They are always invaluable.
Cheers.
Warren Wagner
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I don't understand what I'm seeing. On picture 10 there is a piece glued to looks like the top of the trailing edge that looks to be rough balsa. What is that part for? Looks to me like it runs the span of the trailing edge.
The fit and finish of your work is outstanding!!
Quite frankly, I'm jealous of your ability....
Hi Jerry:
What you are seeing is the 1/4-inch wide TE cap. I install that cap while the wing is still in the Lost-Foam fixture and then carve and sand it to shape after the shear webs are in
Later - Bob
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So is that rough strip to hold the trailing edge perfectly flat while you are adding the shear webs? Then you plane and sand it away?.
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So is that rough strip to hold the trailing edge perfectly flat while you are adding the shear webs? Then you plane and sand it away?.
That is the actual trailing edge. It is planed and shaped to match the airfoil, and it is what the hinges for the flaps are mounted to, or if no flaps, as solid sheet trailing edge addition is glued to to help shape the wing outline. Without this glued on you would have open space between the ribs. Between this trailing edge piece and the shear webs as described, that is where the strength of the trailing edge comes from.
Type at you later,
Dan McEntee
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I see now...if I would have looked at the first picture closely, I would have seen what was done.
Thank you for clarifying....sometimes I think I'm loosing it...
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Hi Jerry:
Just a bit more for clarification: Attached are some photos that depict how I prepare the TE of the wing to accept the trailing edge cap. Also shown is how I hold the trailing edge caps in position on the back of the wing using foam covered bricks. I coat the TE cap with Titebond II glue and then place it against the back of the TE strips that are on the wing. Note that I use the Lost-Foam method of building that insures the wing will be straight and have no stresses built into it.
I just realized that I'll have to post the photos in two separate responses due to file size.
Hope this helps. If you would like to have a copy of the Lost-Foam PDF manual, please PM me your email address. The manuals are free.
Later - Bob
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Last two photos...
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Hi Guy’s,
Just a point, sheer webbing is very important to the wing structure for stiffness. Consider this, think about how strong a cardboard box is, it’s always stronger when you close all the sides of a box. As in leading edges, the sheer webbing has to go vertically, it provides strength in compression, and tension to the wing spars. For trailing edges, I don’t think it has to be vertical (I’ve done both ways) but length wise does about the same thing. Some of the designs I’ve built over the years, the sheer webbing was run span wise and it was still very stiff, because it closes the box (that is the important thing).
Later,
Mikey
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Bob, I now see why the trailing edge will always come out straight with your method. The top and bottom of the trailing edge HAS to be straight, because the two parts have to follow the straight pieces of foam on the top and bottom. Makes perfect sense now that I see it.
Thank You for explaining and the photos.
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Finding a straight good piece of 1/2x1/2 balsa for a I/Beam trailing was always difficult. I use to build them up with a web in the front. No shaping needed and they were always straight and stronger than the old way.
Ed
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EddyR,
What made them stronger?
Dave