Why do kites of same size have different pull?

[Jeff Burns]

I have a Prism Illusion. It has been discussed in rec.kites that this
kite pulls hard for its size, I agree (though I like the feedback). Last
weekend I flew a Jam session. Its pull was dramatically less than the
Illusion, though it was approximately the same size (I found the lack of
feedback disconcerting after getting used to the Illusion).

I've read and heard, but not experienced, that AirMaster kites are
notorious for gentle pull.

What is causing such dramatic differences? It would appear that wing
size (area) and attack angle would be the main forces, but kites with
similar size and flying characteristics seem to have dramatically
different pulls. What's the deal?

I read the archive discussion on pull, but it really didn't answer this
question.

Thanks, Jeff

[Mark Reed (Prism Designs Inc.)]

Jeff-
Here are a few things that make for differences in pull between kites of
similar sail area:

1).How much lift the kite generates relative to its drag
Two sails of similar size can generate very different amounts of lift
depending on their design. Lift is what drives your kite around in the sky;
it's also what creates pull on your lines. Drag stabilizes the kite,
helping it to track straight and hold a stall. The different planes in a
kite sail contribute to either lift or drag in flight. It's the
relationship between the two that affects how much the kite will pull, and,
incidentally, how easily it will drive upwards in light winds. When I
designed the ILLUSION, I set things up so that you can change this
relationship with moveable standoffs. Moving the inner standoffs changes
the size of the keel (drag surface) relative to the rest of the wing
(mostly lifting surface). Move them inwards and you reduce the drag, giving
you more speed, more lift in light winds, and more pull. Move them outwards
and you get slower speed, more precision, easier stalls, and less pull. The
difference in pull can be pretty dramatic from this adjustment alone.
Another contributor to lift is the profile of the lifting surfaces of the
wing. Generally speaking, an airfoil with a deeper profile up forward will
generate more lift and more pull. I'm not talking about the overall depth
of the kite here, just the "thickness" of the wing when you look at it in
section. A cambered leading edge gives a deeper profile up at the leading
edge and also creates less drag than a straight one. Many kites with deeply
cambered LE's pull harder than older style kites with straight ones. A
primary reason to create a deeper profile when designing a kite is to give
more lift at the slow speeds a kite experiences in light winds. Go too far,
however, and the kite will pull like a truck and be reluctant to hold a
stall (try the Macro Ion in 20+ mph!).

2).Frame flexibility
In many circumstances, a more flexible frame allows a kite to deflect under
load and reduces its pull. As the frame flexes, the lifting surfaces of the
sail go to a shallower angle of attack and "spill" the wind more. The
tradeoff with a flexier frame is usually a sloppier, less responsive feel
and a lower top end wind range. If a frame gets too stiff, on the other
hand, it can start to feel harsh and unforgiving.

3).Position of bridle tow points relative to the centers of pressure in the
sail.
Each wing of your kite has an aerodynamic center of pressure that's
determined by the distribution of area in the wing. Most kites are bridled
to put the tow points slightly outboard of the centers of pressure. The
farther outboard your bridle tow points go from the CP, the more your frame
will bow like a Flexifoil as it comes under load. As the frame flexes like
this, the sail goes to a lower angle of attack and spills wind, reducing
pull. Generally speaking, tow points farther outboard will make a kite
track straighter, increase stall stability, and reduce oversteer as well as
pull. The tradeoff is often a less responsive, spongy feel. Tow points
farther inboard give you tighter spins, more responsiveness, and more pull,
but less "locked in" tracking.
Some kites allow you to adjust the position of your tow points. On the
Illusion this is done at the center T with the knotted adjustment line. Set
to the last knot in strong winds, the kite will have dramatically less pull
than on the first knot.

4).Angle of attack in flight
Your kite's angle of attack is set by adjusting your bridle as well. Little
changes usually make a big difference to pull and performance. On the
dynamic bridle of the ILLUSION, your angle of attack is set primarily by
the position of the bridle along the red outer yoke. Move it upwards
towards the nose from the black mark and you'll create a shallower angle of
attack, reducing pull. Move it downwards and you'll increase pull up until
a point where the kite becomes reluctant to fly. From this point on, you'll
reduce pull if you move the knot lower. To reduce pull on the ILLUSION, you
can move the bridle adjustment point either up from the mark or down to the
point where the kite starts to slow down.

Hope this isn't too much techno-babble. As you can see, the design process
is all about choosing compromises. The more you understand the compromises,
the more control you have as a designer. Given a kite that tugs too hard,
there are a dozen different things you could do to reduce the pull. The
choices you make determine the overall personality of the kite.

-Mark Reed, Prism Designs Inc.

Jeff Burns <gre...@charlotte.infi.net> wrote in article
<337475...@charlotte.infi.net>...