Air Density and Kites
Date: Fall 2011
There is a debate in the local kiteboard forum about the influence of air density in the pull given by a kite. It was referred that the pull given by a kite in Brasil is perceived as different from the pull given by a kite in Lisbon for the same air speed and kite, and this could be explained by the difference in air density between both locations. However my question relates to how a simple anemometer could captures this difference. My understanding is that an anemometer is basically a wing that "sees" the wind - both the velocity and density - without being able to differentiate both. The wind then accelerates the kite up to a certain speed in knots (ie: the anemometer is actually reading: air speed*density).
In this sense if the same anemometer reads the same 20 Knots in Brasil and in Lisbon then the pull of the kite should be the same. Is this true?
Regarding air density, kite flying has a parallel in airplane flight. Airplane pilots concerned about the performance of their aircraft need to know the air density when calculating take-off distance, since air density has a profound effect on lift. The denser the air, the more lift a wing will generate at any given speed. Kites generate lift as well, and denser air will provide more lift to the kite (more string pull) for any given wind speed.
Regarding anemometers, there are many different designs, some of which are more prone to effects of density than others. It sounds like you're referring to the vane type of anemometer which sort of looks like a propellor. For a good vane anemometer, the mass of the vanes is low enough and the rotating friction is so low that the vane speed is fairly insensitive to air density, so it really is only sensing wind speed.
Technically speaking, a 20 knot breeze in Lisbon should more pull than a 20 knot breeze in Brazil because of higher air density in Lisbon. However, the difference in air pressure between the two sites will only be on the order of 5%, and lift is proportional to air density, so I don't think you'd notice any difference in the pull of the kite string.
John C. Strong
I can tell you one distinction.
A spinning-fan-blade anemometer tries to be an unloaded wing.
It spins as freely as possible
so it accelerates to that speed at which
the attack-angle of the wind over the wing is zero
and the wing lift crosses zero.
No lift, no dependence on density.
If the blade is spinning too fast, the wing gets negative lift and slows down.
If too slow, then positive lift speeds it up.
The equilibrium speed is not proportional to any lift or drag forces applied by wind.
It is a geometric angle, a direction, times the wind speed.
Suppose the wing is tilted by 30 degrees in the fan-ring.
Then the blade's tangential speed at equilibrium is equal to
wind-speed times the sine or tangent of 30 degrees.,
just so each wing can exactly follow one stream-line of air.
(Imagine how an airplane in zero-gee would behave.
Whenever flying straight and steady,
the pilot would be holding the angle of attack at "zero"
and the lift would be zero.)
On the other hand, a kite is stationary with respect to the ground,
and due to its weight and string-angle
it imposes substantial load on its "wing".
A kite experiences drag and uses drag and lift.
So there density matters.
An anemometer wing is not dragged through the air like a kite is.
If the blade was stopped and its torque was measured
instead of its free-wheeling speed,
then it would be more equivalent to a kite.
So I think your colleagues might actually have a point.
But I also think that temperature differences would make pretty small density differences.
Large differences in ground altitude could have a strong effect,
but your friends probably thought of that already.
I wonder if the speed they measure where they stand is the same as at kite-altitude,
and whether that ratio of speeds varies between the different locations.
Does anybody ever mount an anemometer on the kite?
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Update: June 2012