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Design => Stunt design => Topic started by: Motorman on March 09, 2021, 04:01:35 PM
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Are there guide lines for how thick you can make a stab/elev? I'm building a 550 sq in plane and not happy with the flex I'm seeing with 1/4"stab. I laminated 1/32 top and bottom, inlayed spruce two places and even glassed it with 1.6 and it's still too bendy. I want to make a built up stab that's thick enough to be stiff.
Motorman 8)
Assuming a PA not a profile I make mine 1/2" framed and airfoiled with a 1/4" diameter 14" arrorw shaft imbeded in the Stab TE then planked with 1/32. Stiff and light. On small profiles like a Twister I cut a slot in the stab, imbed the arrow shaft then plank it again with 1/32.
The same result can be had with a carbon fiber strip on the stab TE.
Ken
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Thickness is more aerodynamics .
If you get a good sheet of 12 Lb/Ft quater grain from the jungle , and bend ? twist it over your knee , youll find its pretty stiff . None of this namby pamby girly stuff for Stab.s , or flaps .
(https://external-preview.redd.it/Pdk6gFTeTyxJBsEGYTtovV_1jGlKbRiTyTvhuGM_3ZI.jpg?auto=webp&s=9667b1cd9e28c35971b767ca395c7f967f93846e)
The plantation needs the right kudos, for good wood ,
(https://assets.catawiki.nl/assets/2018/8/21/a/8/c/a8c7fc0b-0760-4eda-ad29-764509ec848d.jpg)
It makes these guys so strong, they can carry a whole tree . You need it from a hidden shaded one , so it took a long time to grow so the grains really close , with 18 month min. seasoning .
(https://lh3.googleusercontent.com/proxy/Y50pKNOu_jKwz8ZJwDqlBHWWpZbNky4540XOwqhHmsP966RCOHxG1hRV8u2RtKv1eoa563XZ7akkphncL6I3Nvv0rSTTeYAIXvxTLRYTg0M38F_jHvGnrJ22mWFHLj7CeA)
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(https://d3i71xaburhd42.cloudfront.net/6dee5d011c78397cf2f30049be281911f2d3738a/38-Figure10-1.png)
(https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcRRhUuPmm00xhBS0Xn8tbLbtRlWC0IOIJHGFw&usqp=CAU)
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Here's a stiff horizontal tail section for my 503 square inch plane. The stab consists of a 3/16" trussed balsa core and 1/16" balsa sheeting for a thickness of about 5/16". Carbon veil is used between these laminations, which are epoxied together under the weights shown. The elevator is solid medium-weight (probably 7-8 lb) balsa. The entire surface is covered with .56-oz glass and just enough epoxy to attach and fill the glass. The cross section is shown in one of the photos below. The blue tape was used to keep epoxy out of the cloth hinges during construction - extremely important. The assembly flexes very little under hand loads and would be even stiffer, if I had applied the glass with a 45-degree bias. Much of the stiffness comes from the diagonals themselves. This horizontal tail assembly weighs a bit less than a like one I built earlier with carbon veil on the elevator and silkspan on the stab, but no veil under the 1/16" sheeting.
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Some factoids:
- I've heard of folks making stabs on "big" (i.e. 700 square inches) up to 1" thick -- and then needing to sharpen up the leading edge so the plane wouldn't "hunt" relentlessly in level flight.
- The stab on my Atlantis, a hand-me-down airplane from Paul Walker, is a tad more than 1/2" thick, built up but flat, with a fairly pointy leading edge -- and it was originally 1" thick, but the airplane relentlessly hunted in level flight... And we don't argue with Paul over here on the Left Coast
- I've heard tales of thick-stabbed planes hunting relentlessly in level flight until music wire turbulators were taped onto the stabs' leading edges
- Flat stabs are easier to align -- and that Atlantis sure flies nice
So -- make the stab as thick as you want, just expect that you want a small radius on the LE (I've seen anywhere from 1/16" to 1/4" recommended. I haven't done experiments myself, though). Or make a nice-looking blunt leading edge, and bring some 1/16" music wire and scotch tape to the field the first time you fly it.
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I used thread rubber cemented or doped along the rounded leading edge of the thin SkyRay stab I was gifted. 'seems to work OK, but I made too many rebuilding changes to draw conclusions. This is inexpensive and easily reversible.
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Serge,
How much "is a bit less".......?
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Serge,
How much "is a bit less".......?
Christ -
I don't have the old weight tallies at hand, but I think they were within 1/8 ounce of each other, when my old scale worked. They had the same thicknesses and structures, except for the carbon veil laminate on the lighter one. I just weighed the heavier one at 2 oz +/- 1/16 oz, but the other is part of a plane I've flown over the past four years and cannot be weighed.
Areas:
Lighter one (has carbon veil between laminations): 100.8 in2
Heavier one (no veil between): 104.5 in2 (about 3.7% larger in area, W = 2 oz to nearest 1/8 oz)
So,...if they were built of identical materials, there would be about .07 oz difference in weights (close to but greater than 1/16 oz). This would be just enough to (perhaps) show up on the only scale I have available.
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Christ -
I don't have the old weight tallies at hand, but I think they were within 1/8 ounce of each other, when my old scale worked. They had the same thicknesses and structures, except for the carbon veil laminate on the lighter one. I just weighed the heavier one at 2 oz +/- 1/16 oz, but the other is part of a plane I've flown over the past four years and cannot be weighed.
Areas:
Lighter one (has carbon veil between laminations): 100.8 in2
Heavier one (no veil between): 104.5 in2 (about 3.7% larger in area, W = 2 oz to nearest 1/8 oz)
So,...if they were built of identical materials, there would be about .07 oz difference in weights (close to but greater than 1/16 oz). This would be just enough to (perhaps) show up on the only scale I have available.
Thanks Serge. See you guys this summer!