Aspect ratio

I've included a chopped version of the original posting at the end of
this note for reference. I hope that I haven't chopped the mail up too

Please also note that I am going to make some broad generalizations
and that these are not pointed at Grant or Philippe. Also note that my
training is in software engineering, so there are probably points here
where I am wrong, but I think I'm right enough to warrant this post.

I'm always surprised at how little people know about aerodynamics. I
know only a little, yet people often come out with "facts" that I know
are wrong.

For instance, what is the aspect ratio of a kite? How can you tell if
one kite has a higher aspect ratio than another? What does the aspect
ratio of a kite have to do with it's performance?

The aspect ratio of a wing is the ratio of the wing span and the wing
chord. For an unswept, constant chord wing you can just measure the
span and divide it by the chord.

        aspect_ratio = span/chord

Most wings are swept and have non-constant chord, so measuring the
chord is difficult. Instead use:

        aspect_ratio = span*span/area

Note that the span and the area are the *projected* span and area. 
The best way to measure this is by measuring this while the kite is in

Okay, what does the aspect ratio tell you? In general, with all else
remaining constant, the higher the aspect ratio of a wing, the higher
it's lift/drag ratio.

Let's say that the wing ("===" in the ascii drawing below) is
traveling through the air from left to right. The upward force is the
"lift" being generated by the wing. There is also a force to the left
that is the drag of the wing.

                  | lift
     drag         |
   <------   ==========   ---> direction of travel

The lift/drag ratio is a measurement of the efficiency of the wing.
Aircraft (full size and model) that need to be efficient have high
aspect ratio wings. When efficiency isn't that much of an issue then
wings tend to be lower aspect ratio. Sailplanes need high lift in
order to remain aloft, so have high aspect ratio wings. Jet fighters,
have powerful engines to overcome drag, so have low aspect ratio

A kite with a longer span will generally have more drag than a kite
with a shorter span, which might result in lower forward speed. The
reason that I qualify this is that the higher aspect ratio kite will
have more efficiency and this will generate more lift which might end
up with higher forward speed.
Tight turning is an interesting thing to talk about too. How do you
measure this? Is it measured relative to the wingspan of the kite?

Here's an example: Most people feel that diamond stunters don't turn
very tight, the center of the turn is outside of the wingtip. Most
large delta stunters turn inside the wingtip, so they are considered
tight turning kites.

But try this, take a big wing delta and do a ground pass about 3 feet
off of the ground and do a down turn to fly the kite in the opposite
direction. You can't do it. But try the same stunt with a diamond
stunter, say a Mirage, this is really easy. Which kite turned the

Relating this to aspect ratio doesn't shed any light. The lower aspect
ratio kite (the diamond) may have a larger turning radius when
compared to the wingspan (the ratio is lower) than the higher aspect
ratio kite (the diamond), but in terms of absolute measurements, the
smaller diamond turns tighter.

Actually, if you look at the highly acrobatic airplanes (both full
size and model) you will find that the wings have lower aspect ratios,
both for aerodynamic and structural reasons.
Two types of lift are generated by a flying kite. One is plate lift.
The other is Bernoulli-effect lift.

Imagine a sheet of plywood that is bridled so that it sits at 45
degrees to the "flying" line. Now imagine that you have a tennis ball
gun (a device that will shoot tennis balls at high velocity). The gun
is lined up to shoot balls parallel to the flying line. 

When a tennis ball strikes the plywood, it goes downward. Some of the
force of the tennis ball is translated into an upward force. If you
fired tennis balls fast enough, the plywood would rise into the air.

Of course, this is a simplification, but much of the lift on a kite,
especially when it is near the ground with nearly zero velocity, is
plate lift. The momentum of air molecules result in an upward force.

The other type of lift is Bernoulli. It happens when the airflow
around an object is uneven, one side has a higher velocity than the
other. A force is generated to the side that has the higher velocity
of air.

One of the myths about Bernoulli lift is that you have to have a
curved upper surface in order to generate lift. This is false. a
completely flat surface will generate lift although the mechanisms are
a lot more complicated, and the lift generated isn't as great.

In stunt kites, Bernoulli lift requires a bit of forward motion, this
is why it often takes a few steps, or a sharp tug to get a kite going.
Once a kite is in motion the Bernoulli lift component dominates. When
a kite is stationary, and near the ground, the plate lift component
I'm on a roll, so I'll talk about one more thing, the angle of attack
of a kite and the effects of bridling on the angle of attack.

The angle of attack is the angle between the apparent wind that the
kite sees and the mean camber line. A wing's camber is the shape of
the wing parallel to the body (perpendicular to the span).

If a kite was a flat rigid rectangle the mean camber line would be
easy to calculate, with the typical wing, it can get pretty complicated.
The mean camber line is an integral function of the camber at a given
place on the wing and the chord of the wing at the point, along the
span of the wing.

Obviously, the mean camber line is a function of the shape of a kite.
A kite with straight spreaders will have a different mean camber line
than the same kite with the spreaders bowed.

This is where it can be really interesting. If you move the pick
points away from the spine, you can create a kite that has two
different shapes, one when it is standing still and one when it is
moving forward. The standing still shape is with the straight
spreaders, and the moving forward shape is with bowed spreaders.

If the kite in question is very flat, i.e. little billow, the effects
can be startling. You can set the bridles so that the angle of attack
(down wind, near the ground) with the kite flat is less than 90
degress, but when the kites is bowed it will have an angle of attack
greater than 90 degrees. If the kite is pointing upward, this would
mean that you can have the kite go either up or down.

Fly this kite in a low ground pass, and do a sharp snap stall and it
will travel downward into the ground. High Performance had their North
Shore Radicals tuned this way and used to have contests to see who
could break the most spine nocks.

In article <9309030929.AA18592@is1e.vub.ac.be>, plepez@ulb.ac.be (Lepez Philippe) writes:
>I have the book from Marck Cotrell (also sp?) : it gives some informations, 
>but not enough. For instance, I still don't know how does the aspect ratio
>affect flying carracteristics. Oh yes there was the answer of Bert tanaka on my
>query for nose width, but what he describe is it related to the change in 
>aspect ratio or to the change in the billow of the sail ? 
><< start of Grant's answer ==
>From: grantm@syacus.acus.oz.au (Grant Mc)
>       From my experiences, these are the differences between high and low
>aspect ratio kites:
>High Aspect: Turns very sharply; slow; doesn't track that well.
>Low Aspect : Turns through large arcs; fast; track well.
>Obviously you want something in between which is why a majority of kites have
>a nose angle of 90-100 degrees. It becomes very interesting when you try and
>reason why the kite behaves the way it does. Here are some observations.
>A jet fighter sweeps its wings back to increase speed. Why? To reduce air
>resistance along the leading edge. Problem with this? Cannot turn well! Can
>you see the relationship with a low aspect ratio kite?
>I remember reading the thread about billow. My opinion is that the billow is
>the contributer to the pull (not the forward motion). Are you ever amazed that
>when your kite picks up in speed how it pulls alot harder? The induced camber
>is what causes this. The camber creates a force away from you and is why it
>pulls. A flexifoil uses the camber for generating lift making the kite pull
>less and fly faster in strong winds.
>Building kites does not necessarily need to be expensive. As spars are the most
>expensive part, I always reuse them. I make my kites to the lengths of arrow
>shafts, thus no need to cut and waste. I use cheap ugly looking sail cloth for
>the sails I prototype and use glue sparingly.
>Anymore questions just let me know! If you find my posts helpful, feel free to
>put them one the net. Happy flying!
Marty Sasaki            Harvard University           Sasaki Kite Fabrications

Return to Kite Fliers's Site