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Jan 11, 2021, 10:38 AM
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Thread OP
Discussion

F3-RES Trailing Edge


I realize this may open a can of worms but I happen to be a fan of a razor sharp trailing edge.

With the limits on the use of materials, is obsessing over this worth it?
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Jan 11, 2021, 11:04 AM
Registered User
whacker's Avatar
why not use a piece of sitka as your trailing edge, 3/32 x 1/4, cut off 1/4 from the existing material of the TE, and add your sitka in place


Buta Bing Buta Boom.

sharpen away.

whacker
in the sunny Waughs River valley
Jan 11, 2021, 01:03 PM
Screwing up is an art
mabrungard's Avatar
I’ve used a strip of 1/64” plywood laminated to 1/8” or thicker balsa to create a trailing edge that’s pretty sharp and durable. The strip width needs to be something like 1 inch in order for the overall TE to be reasonably rigid.
Jan 11, 2021, 02:28 PM
B for Bruce
BMatthews's Avatar
I've used 1/64 ply as well. And it makes for perhaps not a razor edge but pretty darn trim for sure. Especially when you sand it down to half the 1/64 thickness.

If you do this then also consider cutting the strips across the grain and use multiple strips. That way the stronger and stiffer long grain of the outer ply layers is running chord wise so it resists splintering away.

Using a piece of firm'ish medium density C grain for the balsa part will also go a long way to allowing the balsa and ply lower lamination to be sized down pretty small and still be nice and stable when shaped. I'd have no issue for using this down to something like 3/32x1/2 then laminated to the 1/64 and shaped to final size on a control surface for example.
Jan 11, 2021, 03:49 PM
Duane, LSF IV
Wazmo's Avatar
Note that, for mass balance/flutter purposes, the trailing edge is the worst place to add weight, especially with the lighter structure of F3RES planes. I believe that some argument has been made that, for lower Re, a slightly blunt trailing edge is lower drag than a sharp one.
Jan 11, 2021, 04:03 PM
Registered User
Weird idea about grain...imagine you've decided to laminate the last bit of the edge from 2 sheets of balsa, but one of them was cross-grain. Lots of work to do, but I bet that cross-grain would be quite strong and not have that pie-crust breakoff of parallel grain (usually from clumsy handling). (Confirmed with a test sample...it is very stiff, but you'd have to watch it for dinging anyway)

Also, is carbon rod for the edge against the rules?

BTW, I think the original question was more about if these techniques are worth it, aero-wise.
Jan 11, 2021, 04:35 PM
Screwing up is an art
mabrungard's Avatar
Quote:
Originally Posted by Wazmo
Note that, for mass balance/flutter purposes, the trailing edge is the worst place to add weight, especially with the lighter structure of F3RES planes. I believe that some argument has been made that, for lower Re, a slightly blunt trailing edge is lower drag than a sharp one.
Good point about weight on trailing edge, but is a plane in this class going to go that fast?? I donít believe so.

I donít buy the argument that blunt has lower drag. Do incorporate a reasonably thin and strong TE.
Jan 11, 2021, 05:10 PM
Duane, LSF IV
Wazmo's Avatar
The lighter and less stiff the structure, the lower the flutter speed. Certainly launch or a dive could potentially be in reach of flutter speeds.
Jan 11, 2021, 06:19 PM
Registered User
Gratter's Avatar
Now let’s add some more fuel to the fire.
You have that razor shark edge and you are now covering it with your favorite iron on covering.
Do you wrap it from the top to the bottom or the bottom to the top?
And what does that edge do to the drag?
My opinion has always been there are only a few guys flying who might be able to actually be able to notice drag produced by a lot of the building techniques we debate.
Jan 11, 2021, 07:37 PM
Registered User
whacker's Avatar
But, personally I wouldn't want to be responsible for denying the OP his eureka moment and doing the TE of his 16 oz plane exactly the way he wants to.
Make it as sharp as two thickness's of Ultrakote

Airhead may be onto something here and surely his next build will be his best build ever.

So I say go for it have a blast , go where ya want , do what ya want.
and send us some pictures on the Trail

whacker

in the dark Waughs River Valley
Jan 11, 2021, 07:58 PM
Registered User
Thread OP
Thanks for the input guys, this is helping

I'm thinking 1/64 ply sanded down top and bottom, and end grain balsa. Yes a lot of work but im not trying to sell a kit, just optimize the airfoil inside the construction parameters.

Wondering how much the trailing edge detail effects performance and if there would be any noticeable benefits.

CGordon -Carbon trailing edge is against the rules in my read thus the dilemma?

Wazmo - Can you point to any data confirming this? I thought I read that also but I also read more about having it sharp.

Whacker- I'm trying for something closer to 10 ounces than 16 for a bit more of a challenge, but its going to be higher aspect ratio with less area and profile drag. Something along the lines of a composite type airfoil but following the guidelines for these models. Yes getting the covering not to stick to itself at the back end of the wing is going to be a challenge but I have a stash of some contest grade that wont weigh much, I may do a solid wing and route some lightening holes. I have a few ideas im bouncing around but was primarily wondering how much the thickness would change performance. I think overall a think airfoil will be the biggest difference from what seems to be the common denominator out there.
Last edited by Air Head; Jan 11, 2021 at 08:07 PM.
Jan 11, 2021, 08:45 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by Wazmo
The lighter and less stiff the structure, the lower the flutter speed. Certainly launch or a dive could potentially be in reach of flutter speeds.
That's true. But we're talking about very little weight being added. And with a slight change of the structure to enhance the stiffness the flutter tendency can be counteracted anyway. And there's no doubt that while minimal adding the 1/64 ply is going to add some stiffness. And at the same time being able to run the shape down thinner means less balsa. And that'll make up a little for the added plywood and glue.

As for thick vs thin TE edges thin is always going to be more efficient. But if we're going to live with a blunt rear edge then sharp corners will have less drag than the same thickness and rounded. But thin will win every time if the structure can support thin without being fragile or allowing the edge to deform.
Jan 11, 2021, 09:22 PM
Duane, LSF IV
Wazmo's Avatar
Quote:
Originally Posted by BMatthews
That's true. But we're talking about very little weight being added. And with a slight change of the structure to enhance the stiffness the flutter tendency can be counteracted anyway. And there's no doubt that while minimal adding the 1/64 ply is going to add some stiffness. And at the same time being able to run the shape down thinner means less balsa. And that'll make up a little for the added plywood and glue.

As for thick vs thin TE edges thin is always going to be more efficient. But if we're going to live with a blunt rear edge then sharp corners will have less drag than the same thickness and rounded. But thin will win every time if the structure can support thin without being fragile or allowing the edge to deform.
Plywood is heavy, at least compared to balsa. Yes, the trailing edge will be stiffer, but mostly in bending fore/aft and some in bending up/down. The most common flutter mode for wings is in torsion. The trailing edge will do very little to change torsional stiffness.

"Thinner is better" used to be the conventional wisdom, but I believe that's been questioned more recently, at least in some Reynolds number ranges. I don't have a specific paper handy, but I recall reading something about unstable vortex shedding causing periodic changes in chord-wise flow. If I find something, I'll post it.
Jan 11, 2021, 09:49 PM
Registered User
I seem to recall that one of the Soartech publications has a test with a thicker trailing edge. Also, Xfoil has a trailing edge thickness setting which might provide sone insight. I used a little strip of spruce at the t.e. to allow a thinner trailing edge. It worked well and was easy.

Whether the trailing edge is thinner or not, it may be better not to wrap the covering around the t.e., as that adds a radius.
Jan 11, 2021, 10:13 PM
Registered User
Norm Furutani's Avatar
I have used this technique but instead of spruce, I used basswood. Basswood has a less prominent grain (not ring porous). As for the covering, if the planform has straight tapers, you could cover in one piece and put the overlap at the leading edge. This is standard practice for carbon T.E.

Or you could go with a carbon trailing edge, but then you will probably be dealing with carbon cap-strips.

Quote:
Originally Posted by whacker
why not use a piece of sitka as your trailing edge, 3/32 x 1/4, cut off 1/4 from the existing material of the TE, and add your sitka in place


Buta Bing Buta Boom.

sharpen away.

whacker
in the sunny Waughs River valley


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