Man, not sure if I spelled that correctly, but; A friend is kind of rebuilding a fuse for a 1.20 sized Cap. Not sure of the model of Cap, or maker of the kit. Anyway, he told me that he wants to round off the leading edges of the fin and stab. I remember reading somewhere, and maybe it was in the profile/hotrod/funfly types of planes, that flat would be a better choice. Don't have time to search right now so I though I'd ask the Gurus
Thanks_bob

its the trailing edges you have to leave flat, the leading edge is best rounded.
flat leading edges on funflys are just lazyness

bob_nj,

Ideally, the leading edge should be nicely rounded, the surface should have an airfoil section ( as opposed to a flat surface with constant thickness ), and the trailing edge should be as sharp as possible. Many planes are built with flat ( as opposed to airfoil section ) tail surfaces, which is probably OK as a time saver. There is no reason in the world not to round the leading edge and taper the trailing edge to a fairly sharp edge. I believe all the claims about blunt trailing edges preventing flutter to be nonsense, but I could be wrong.

banktoturn

for stick type built surfaces you couldnt sand it sharp and keep the strength unless the TE was much deeper than the 1/4x1/4 sticks normally used, more like 1/4x1
apart from the weight being undesirable, it also makes flutter more likely.
to date i've never had flutter on a funfly surface even at speed

phillybaby,

Maybe I'm not sure what you mean, but I don't think tapering the trailing edges would have a big effect on strength. The tapering would be on the elevator and rudder surfaces, which are usually solid pieces. The trailing edge member of the built-up horizontal and vertical stabilizers would not be affected by tapering the elevator and rudder.

banktoturn

usually solid pieces yes, that works, and i do it on those types
but FunFlys have surfaces made from 1/4x1/4 sticks, you cant taper the TE coz it'll end up like a bit of triangle stock, which is pointless.
if you do it like a soild surface, the TE will break off, or the covering snap it!

phillybaby,

Now I see your point. I've never had a fun-fly plane, so I was picturing built-up stabilizers and solid control surfaces. I suppose that even built-up control surfaces, could be tapered, but the effort may not be worth it.

banktoturn

Also fun fly planes are not looking for speed, so any drag or in-efficiency from squared-off trailing edges won't make that big a difference. Not a scientific fact, just a WAG based on my observations. But the same WAG tells me you wouldn't want a squared-off LE, either.

I've been told [turn on the hearsay alarm here] that a tapered control surface on a flat stabilizer (verticle or horizontal) may be blanketed by the flow off hte stab and have a noticeable neutral/dead zone about center as a result. I'd expect you not to want that on an aerobat unless you were looking for very true straight-line performance.

On flutter, I have seen an elevator decide to take flight on its own... in which case its surfaces were not quite flat. They were a little convex, which should certainly lead you to expect them to be doing some lifting. If you do taper them, get the surfaces as flat as you can. BTW, lots of full sized planes have untapered, flat plate empennages built up out of metal tubing, so a flat surface is not a cardinal sin. (I'm not too sure whether the CAP(s) is (are) among those).

Al Stein,

If a control surface is blanketed, that's the same as saying that separation occurs at the leading edge of the control surface. This is certainly possible, and my be aggravated by the gap. For a well designed lifting surface, we would like the flow to remain attached without having to resort to untapered control surfaces. This is not impossible, and I don't think that tapering control surfaces usually causes separation at the gap. One solution mentioned in a post here at RCU is to make the leading edge of the control surface just a little thicker than the trailing edge of the surface it is mounted on. This seems like a reasonable plan.

Ideally, the curvature, on both top and bottom surfaces, of the control surface should be a continuation of the curvature of the lifting surface to which it is attached. There is nothing magic about zero curvature ( flat ) as far as preventing flutter. It is, however, much easier to make the surfaces flat.

You're right, a flat plate as a lifting surface is not a cardinal sin, particularly for the empennage, which should not generate huge amounts of lift anyway. It's a matter of how much mediocrity you want to live with. The original question was about what would be a "better choice". An airfoil section with a nice sharp trailing edge is best. A flat plate with flat leading and trailing edges is worst.

banktoturn