Aspect ratio
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Aspect ratio: what is it?
Simple answer
In simple terms, a high aspect ratio kite is long and thin, and a low aspect ratio kite is fat and dumpy.
More complicated answer
The best definition of aspect ratio I have found is the square of the wingspan divided by the area. Mathematically, this can be written as:
d * d / A
where:
d is the wingspan of the kite and A is the area of the kite.
This formula is really good as it allows non-rectangular kites to have their aspect ratio (or AR) calculated in a consistent manner.
Why does aspect ratio matter to different sorts of kites?
Why are race kites all high aspect ratio? Why are high aspect ratio kites faster through the air than low aspect ratio? Whay aren't Little Devils race kites?.... etc...
Well, the answer is to do with the drag of the kite. The drag (the force pulling backwards on the kite and slowing it down as it goes through the air) of the kite is made up of two components: one related to the physical size, shape and speed the kite is travelling through the air and the other is related to the fact that a wing doesn't work as well towards the edges as the air spills over the end of the wing (due to the fact that the air above the wing is at lower pressure than that below the wing and the higher pressure wants to move towards the lower pressure). This effect can sometimes be seen on humid days when aircraft come into land and you get vorticies (swirling air vapour) forming just behind the wingtips. So, the total drag seen on the wing is:
Cd = Cds + Cdi
where:
Cds - coefficient of drag due to wing properties Cdi - coefficient of drag induced
The drag coefficient from the size and shape of the wing is incredibly like the lift coefficient.
Cds = D * (A * .5 * r * V * V)
where:
D - drag on the wing A - Area of the wing .5 - err... a half r - air density V - "apparent" speed the wing is travelling through the air.
D increases as the intrinsic lift of the wing goes up (as the wing is thicker - ever tried pushing a door open against the wind...). Whilst V increases as the wing goes faster.
The (even more boring) equation that determines the drag at the wingtips (also known as induced drag) is:
Cdi = (Cl * Cl) / (pi * AR * e)
Where:
Cl - Coefficient of lift (different for different wings) pi - 3.14159 - that number you did at skool AR - Aspect ratio of the kite e - a measure of the efficiency of the kite
So, the key to lowering this effect is getting the multiple AR * e as high as possible. e varies between 0 and 1 and is 1 for a perfect ellipse - so that's why modern kites are shaped like they are. It also explains why the Spitfire (one of the most beautiful aircraft that ever flew) was so fast compared to the competitors at the time. So the only "real" variable you have to play with is the aspect ratio. Increasing it reduces the wingtip effect of drag.
Now, the interesting thing is that you can also reduce the drag by lowering the top of the equation (i.e. Cl * Cl) so there is a balance to play here. High aspect ratio kites that have a high amount of lift (err... anyone mention Blades...) will also have more induced drag from this equation. So you just can't make the wing incredibly lifty otherwise the wingtip effect will slow the kite down reducing the lift (as the apparent lift you feel on the lines depends on how fast the kite is flying...)
So, race kite designers play with several variables. Getting a race kite to generate pull means getting it to move quickly as you can generate proportionately more power by moving quicker. BUT, drag is acting against you slowing you down so you want to minimise this by getting D low (make the wing thin) and reducing the effects at the end of the wing (by increasing aspect ratio).
So, why is the Flexifoil Blade a good "all round" kite. Because it doesn't have the highest speed, but it has a lot of lift and it is fairly efficient - bang in the middle of all the variables.
(This was copied from the Flexifoil Forum on Aspect ratios - Credit to and permission of Andy S))
