Rotor Kites -- How do they fly!

Rotor kites are very, very different to any other sort of flying kite. It does NOT have a fixed `wing' like an aeroplane or an other type of kite to produce a lift in the `normal' way.

The reasons a rotor kite and the more interesting flat winged UFO kites fly is also much more complex than a `normal' kite. Indeed it was only in the last decade that the actual reasons and forces involved were known and understood.

What produces the lift?

Rotor kites are very, very different to any other sort of flying kite. Basically instead of using a fixed wing, a rotating one generates a horizontal vortex which slows the wind underneath and speeds it up on top, thus generating a slight lift. This `vortex' spreads out from the kite to either side, like the huge vortices you see over the wings of a plane landing on a dusty or very hot runway. The rotor kite just generates these same vortices more directly.

[Diagram] These vortices produce what is called the ``Magnus Effect'', and creates a `complex' non-wing to slow the air below the kite, instead of increasing the wind speed (due to a longer distance to travel) in a normal airfoil wing.

The lift however is not very strong, indeed, for each unit of lift, the same amount of drag will be produced, no mather how fast the air is moving. Lift to drag ratio generally ranges from 1:2, up to at maximum 1:1 in ultra light weight versions. Compare this to 3:1 up to 10:1 ratios of a normal airfoil. In any case, that any lift is produced makes for a impresive kite.

Just for those who don't know, the magus effect is also the reason `curve balls' work in base ball, and ping pong balls with a lot of spin on them seem to follow eratic paths. It is also why when you throw a thin wooden ruler up into the air, or watch a thin strip of paper fall, they tend to follow curving paths all over the place, landing somewhere completely unexpected.

As the lift is not very strong, and the drag is also just as strong (or stronger) they do not fly very high. 45 degrees (1:1 lift to drag ratio) is the most efficent you can get. Some on by own UFO kites have achieved a good 40 degree fling angle in a good strong steady wind.

Also the stronger the wind the faster the kite spins (also the more drag) as such the kite line gets a lot of pull at that low angle. very little line sag for this kite, unless the wind dies and the kite slowly spins out of the sky. This I have found makes the kite very good for simple line messagers and ferrys.

How does it keep spinning?

The classical rotor kites have `S' shaped wings which `cup' the wind like the rotating cups of a wind meter. For a long time this was thought to be a requirement for the kite to fly. However the modern UFO kite uses a completely flat `wing' for lift!

This rasies the second question everyone asks me after the above explaination. ``How does it keep spinning?''. Well it doesn't have `cups' to force it to spin, so how does it keep turning?

Well the flat UFO's keep spinning for two reasons. First of all, the turbulence the kite generates behind it is where all the energy to spin is comming from. If you look at a construction ribbon, which is streched between two poles on a windy day, you will see that it vibrates up and down rather rapidly. In fact it is this same effect which makes your vocal cords vibrate, and the reed in saxaphones. The difference between these and a rotor kite is that the rotor kite can continue to rotate in the same direction.

The second and more impoartant reason rotor kites continue to rotate is the vertical `ear' of the kite. This ear provides a fly-wheel effect which keeps the kite spinning in the same direction. Without it the kite will become unstable and occasionally reverse its spin, generally causing it to plow into the ground, hard!

How does it know which way is up?

The `ear' also seems to provide the kite means of telling which way is up. A UFO kite which is spinning the wrong way up and is heading toward the ground will, if given enough space, right itself and circle so it is again the right way up, and heading skyward.

I know it is the `ear' surface which does this has I have build UFO's with an `ear' which was too small. This kite just seemed to roll around the sky a lot more than usual and eventually hit the ground, the same as a normal kite loops around a lot when it does not have a long enough tail. Adding a bit of extra weight, to increase the ``monent of inertia'', does not seem help much either, just making the kite heaver.

I have also had the `ear' shift slightly on some of by UFO's making the kite want to `lean' to one side, no matter in which direction the kite is spinning. The `ear' surface is very important. Though how this surface plays a part is still a mystery.

It is likely the `ear', or vertical stabiliser as it called technically, produces some gryoscopic, and `precedence' effects which keep the kite up right. But exactly why this is so probably would involve a whole blackboard of equations, and we'd still be no wiser. Lets just take it that the ear does tell the kiet which way is up.

Created: 13 September 1996 (as part of a larger discussion)
Updated: 29 March 1999
Author: Anthony Thyssen, <anthony@cit.gu.edu.au>