Glider Design

[Richard Miller]

Dave,

- or whomsoever...

I've got the size of the vertical surface [of a hlg] nailed. It's so simple, just keep cutting till the tail begins to wander and there you are.

I have no such assurance insofar as tail moment arm and stab size are concerned, however. I just go by what I inherited from The Tradition, and what looks right. I do see what appear to me to be longish moment arms and kind of small horizontals on some gliders, however, and wonder.

Anything to say about that?

-Richard

[Dave Robelen]

Hello Richard,
Well, yes, I can speak to the issue of tail moments a bit. If we allow that the basic formula for static stability in pitch takes the percentage size of the stab and multiplies that by the momemt arm (1/4 chord-1/4 chord)/average chord. This value shows that a small stab on a long arm is equally as effective as a larger stab on a short arm. That covers the STATIC case.
When you get to the dynamic case a small stab on a long momemt arm behaves like the moment of inertia term, not linear but distance squared. A glider with a long boom and small stab will damp much quicker, both after launch, and in turbulence. Meanwhile the vertical can become smaller also for a modest weight gain. Additionaly, the weight of the added boom is generally less than the shorter boom + larger tail.
Happy tossing, Dave

[Richard Miller]

Dave,

That was something I needed to know; another of those things that other people seemed to know, somehow, but had passed me by. From now on it's Go Long!

By the way, I'm a dowswer, and use the pendulum. I used it yesterday to rectify my current hlg design - there seems to be a new one every week, and ain't it grand! - and it said longer on the moment arm, lesser on the stab area. So be it.

I feel that one has to do a lot of thinking and building and flying, then some more thinking, and have a lot stored away in Intuition before the pendulum will provide answers like that.

So, thanks,

-Richard