LONGITUDINAL STABILITY OF FLEXIFOIL?

• Subject: LONGITUDINAL STABILITY OF FLEXIFOIL?
• From: byang@magnus.acs.ohio-state.edu (Been-Der Yang)
• Date: Wed, 26 May 1993 08:09:58 -1000

Hi:

I am not an enthusiastic kite builder and flyer, and only fly some
simple home-built stunt kites for fun in summer time.  But, I am
interested in the history and secret of flight. 

I saw one person flew a "flexifoil" in one of our local kite festivals
last year.  That was my first time to see a kite ballet-dancing over
the sky.  Then, the memory just left behind.  Recently, I am studying
the theory of the flight stability.  It occured to me that why the
fexifoil with the flying lines attached to the leading edge can
maintain its longitudinal stability?  I went to our local kite shop to
make sure of the structure of the flexifoil and see a video show of
the demonstration of flying the flexifoil.  But I still can not have a
clear picture of the stablilty of the flexifoil.  Theoretically, the
pitching moment of the airfoil is negative (keeping the nose down)
with respect to the leading edge.  It would not fly nose-up to the
sky, but keep sinking nose-down to the ground once you pull the flying
lines.  However, it does fly!!!  Rec.kites might be the place to find
the answer.  So, I raise this question:

WHY THE FLEXIFOIL CAN MAINTAIN THE LONGITUDINAL STABILITY?

Acturally, I am thinking one possibility: the bending of the spar (I
kept looking at the flexifoil picture in one kite book to catch this
answer.).  When the flexifoil faces the wind, the spar (leading edge)
bends.  But, the trailing edge bends less because of less stress on it.
The difference of the bendings provides a twist over the airfoil.  The
angles of attack outboard is less (maybe go the negative value) than
those at the center of the flexifoil.  Therefore, the center of the
flexifoil with postive angle of attack provides the life, and the
outboard of the flexifoil with negative angle of attack provides the
positive pitching moment.  When you pull the flying lines, the angle of
attack at the outboard becomes more negative and the pitching moment
increases to overcome the your pulling.  It is stable!!!

I saw someone mentioned the "lightweight" spar. Does it mean the
weight or the flexibility?  In which situation we should use the
lightweight spar?  The quality of the spar seems to have great
influence in the flying of the flexifoil.  Would it be weight or
flexibility?

Since I never have the experience in flying the flexifoil, all my
explanation is based on the theory I know.  But, no matter what the
answer is, I admire the person who invented the flexifoil.  From the
history of human flight, man can fly always after he really understand
the reason.   

I appreciate any answer to my question.

Been


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