Roy Bourgeois wrote:
That's right Eric - Think of the fuselage/rudder as a lever moving the
glider about the center of the yaw axis. The farther away the rudder is
from the center of that axis the farther it must move to have the correct
effect.

Being further back puts it in a slightly different airstream, giving it
a slightly lower angle of attack (less than 2 degrees, even in a tight
circle) compared to one half as far from the wing. Since it has twice
the lever arm, it only needs half the lift of the closer one to counter
the adverse yaw, so losing two degree in AOA isn't a problem; in fact,
it's probably a help because so much less force is needed.

Only by increasing the size can we make it move laterally far and
fast (but that creates a weight problem). A longer fuselage reduces the
"force" needed - but the tail it needs to travel laterally farther faster
to have the appropriate effect. Thus, the 1-34 is very stable in yaw - but
it really doesn't have adequate yaw response when you step on the
rudder. Consider for a minute the size of the rudder on the Genisis
2: It's downright tiny (maybe 25% the size of the 1-34's) - but it is so
close to the center of yaw axis that it is more than adequate - and much,
much, more responsive than the 1-34.

I think the Genesis is a poor comparison, because it's reflexed airfoil
creates less adverse yaw in the first place. My flapped glider acts the
same way at any given speed: positive flap requires more rudder in a
turn than when negative flap is used. Another reason: the rudder is
actually further from the wing that it appears, because of the sweep
forward. You can't measure to the wing root to get a meaningful
distance, and you need to account for the sweep backward in the rudder.
These factors put the rudder farther from the wing than you might guess
from looking a side view drawing.

The 1-34 poor rudder response may due things I can only guess at:
perhaps the fin/rudder airfoil is poor (note how old gliders tend to
have very wide rudders compared to newer gliders); perhaps it has a
differential aileron deflection that is poor; perhaps the airfoil at the
aileron is high drag with down aileron. I'm really skeptical that
shortening the tail boom would help matters!

In general, the farther the rudder
hinge line is from the wing spar - the bigger the rudder needs to be for
proper yaw response.

If this were true, why do gliders have such long tail booms? Why aren't
they all as short as the Genesis? If being closer meant smaller
surfaces, we'd see all gliders with the surfaces mounted close to the
wing. I'm also skeptical that this idea has been missed by best glider
designers in the world.

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Eric Greenwell
Washington State
USA