Thread: Discussion Plank 101
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Old Sep 29, 2012, 10:28 AM
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Originally Posted by Knoll53 View Post
Thanks Herk. I've always considered the rolling moment from the fin above the CG to be minor, but as you point out, it has more of an influence due to the small wing span. The attached sketch shows the first fin with a low portion that is to be used at a bungee launch handle and skid. Although it adds a little weight, I'll go with it.

Those are some good reasons for winglets. Any thoughts on why most planks sport a central fin? Forward sweep would be one reason.

I'm sure, at some point, I will try to Discuss Launch this glider. Thanks for the reminder...this plane needs a throwing peg. Kent
Hi Kent, I've always used a center fin when I wanted an active rudder control. I never wanted to deal with the complexity of running rudder controls to the wing tips. Since I'm mainly a thermal flyer, I've found that these planes need aileron-rudder coordination to fly at slow thermal-circling speeds.

Most of the fixed fin planes seem to be primarily slopers where most of the flying is at lower AOA - don't feel the need for an active rudder control. So why do they put the fixed fins in the center - convenience? imitation?, - added tail moment????

Regarding discus launch. I don't know if you've done much of that, but if it's structurally feasible, you probably want to move the peg aft a bit. At the start of the spin the plane lags pretty far behind your arm and the tip structure will dig into your hand if the peg is too far forward.

Here is where I put the peg on that DLG experiment -- worked well because the trailing edge of the tip slipped between my fingers instead of digging into them.
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