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How UFO's fly!

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UFO 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 the flat rotating wing of a UFO kite fly is much more complex than a 'normal' kite. Indeed it was only in the last decade that the actual reasons and forces involved were properly 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, slowing the air below the kite, and thus increasing the air pressure below the kite. A normal airfoil wing by contrast increases the wind speed above the wing (due to a longer distance to travel), to produce the same effect but without the extra drag, of slowing the wind down.

The lift generated is not very strong, indeed, for each unit of lift, the same amount of drag will be produced, no matter how fast the air is moving. Lift to drag ratio generally ranges from 1:2, up to a maximum 1:1 in ultra light weight versions in strong winds. Compare this to 3:1 up to 10:1 ratios of a normal airfoil. But that any lift is produced is what makes this a fascinating and impressive 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 erratic 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. A 45 degree flying angle (1:1 lift to drag ratio) is the most efficient you can get. Some of by own UFO kites have achieved a good 40 degree flying angle in a good strong steady wind, and only exceeded the 45 degree angle when caught by an up draft or thermal. The lightness of these kites makes thermal riding a pleasurable, though extremely rare experience.

Also the stronger the wind the faster the kite spins (though also producing more drag). As such kite lines gets a lot of pull at that low angle, with very little line sag for such a small kite. If the wind dies the kite does not just drop, but slowly spins out of the sky. This I have found makes the kite very good for simple, light weight, line messagers and ferries (say for dropping a few minutes).

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

The flat UFO's spin for two reasons. First of all, the turbulence the kite generates behind it is where all the energy to spin is coming 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, as well as the reed in saxophones. The difference between these and a rotor kite is that the rotor kite can continue to rotate in the same direction, due to that vibration.

The second and more important reason rotor kites continue to rotate is the vertical 'ear' of the kite. This ear provides a fly-wheel effect (known as 'Moment of Inertia' or spin mass), 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?

A UFO kite which is spinning upside down, say after an encounter with another kite line, 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.

The 'ear' seems to be the means by which the kite knows which way is up. 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.

Also quite often when attempting to launch a UFO when I am by myself, I have had the ear fail to fold out, The kite then zooms up in a circle to fly back down into the ground. In other words with out any ear, the kite wants to fly upside down!

At one point I thought that adding alittle extra weight to the ear would improve the kite, prehaps make it spin better, by increasing the kites 'moment of inertia' (or spin mass). This however does not improve the kites stability, or spin. All it did was make the kite heaver.

I have also had an 'ear' on some UFO's which had shifted slightly at an angle from the normal center. This UFO's 'leaned' to one side, no matter in which direction the kite is spinning.

As a consequence it is the ears, surface, not it weight that matters. Though how this surface plays its part is still a mystery.

It is likely the 'ear', or vertical stabiliser as it is often called produces some gyroscopic, and 'precedence' effects (the strange forces that keeps spinning tops upright), which along with gravity, keeps the kite spinning the right way up. But exactly why would 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 kite, somehow, which way is up.

More Information

Created: 13 September 1996 (as part of a larger discussion)
Updated: 10 January 2000
Author: Anthony Thyssen, <Anthony.Thyssen@gmail.com>
FYI: How did I create the UFO animation