Methods for measuring:

• • Lift to drag ratio, L/D, of traction kites and sailing crafts.

  • Pull and Cl, coefficient of lift, of traction kites.

Content.

• • Summary.

  • Comments on other methods for measuring L/D.

  • How to measure L/D.

  • An addition in July 2000 regarding measuring low wind speeds.

  • Measuring the pull and calculating Cl for a traction kite.

  • A L/D measurement report form for a traction kite can look like this.

  • The L/D ratio for other sailing crafts can be measured in an equivalent way.

Summary

The lift to drag, L/D, for a traction kite can be measured by having it flown back and forth close to the ground by a stationary person. Speed of the kite and the actual wind speed is measured. The L/D of the kite is equal to the speed of the kite divided by the actual wind speed. Or if you like, the L/D says how many times faster than the wind the kite is capable of traveling.

When a traction kite pulls for example a buggy the drag of the kite is added to the total drag. A lower drag kite means higher speed.

It is also described how you can measure pull and coefficient of lift for a traction kite.

I hope this method can be of help to designers of traction kites especially when they want to improve the L/D ratio of their creations.

If kite fliers and designers agree on a common method of measuring L/D we can compare different designs by correspondence. And maybe we can arrive quicker at more efficient traction kites.

L/D of sailing crafts can be measured in an equivalent way.

Comments on other methods for measuring L/D

Fly kite from moving car. After reading this page Dave Culp suggested the method of flying the kite from a moving car and measure the angle between line and direction of travel. It is good method specially as it accounts for the lines as when the kite is really used for traction.

But you need a suitable car and suitable place for the measurements. My method requires less equipment and can be performed in any place where you can fly a traction kite.

Fly kite overhead. One way of measuring the L/D ratio of a traction kite could be just flying it stationary overhead and measure the angle between lines (or line to kite?) and the vertical. I have a feeling this is difficult to do with enough accuracy as the kite will not be stationary because of fluctuating winds and the angle has to be measured within degrees.

Also as you want to measure in typical apparent wind speeds and pull of kite when sailing (L/D varies with those) you need a pretty strong (not common) wind, of some 10 m/s (20 knots).

How to measure

Choose a place were the wind has free access. We usually use the leeward side of a local field which is some 2000 m in diameter. A stationary person flies the kite back and forth at low level.

On the ground, under the track of the kite and straight downwind from the person flying, a some 20 meters long course is marked with clearly visible markers. The lines of the kite are long enough to allow the person flying to smoothly fly it in a steady state over the course (a typical distance between flyer and kite for us is 70 meters). This means that the turn and following acceleration are well outside the course.

Before, over and after the passage of the course the kite is flown parallel to the ground and with the lower end one half span from the ground.

At least ten passages, half of them going one way, the other half going the other way, (to compensate for the course not being right downwind from the person flying the kite) are timed by a person with a stopwatch. We have this person standing near the person flying the kite.

The speed for each passage is calculated.

Passages which are not representative are discarded. Reasons can be: Kite flies too high. Kite dives or climbs over the course. Kite was turned to close to course to get up to speed. Major height adjustment was made over the course.

But you don't discard a passage just because it seems unusually slow because this would give a systematic error to the final result.

For each passage the actual wind speed is measured. This speed should for good results be measured at about the same height above the ground as the mid span of the kite flies at. If you measure at a lower level you will measure a wind speed lower than that the kite flies in (because the speed of the wind increases with height) resulting in a too high L/D ratio. We mount our wind speedometer on a pole (the longest we have managed is one 4.5 meter long).

A suggested location for measuring the actual wind speed is some ten meters to leeward of the center of the course. To get the wind speed the kite is just flying in take the measurement just before the wind is disturbed by the passing kite.

An addition in July 2000 regarding measuring low wind speeds

Last winter a friend and I tried to measure the L/D ratio of his very large stand inside 7 sq. m wing skate sail. The wind speed was around 1.5 m/s, 3 knots, which is just about right for a sail of this size on a beam reach, more wind and you become over powered. I had three instruments for measuring the speed of the wind.

A Ventimeter, which has a circular plate which is lifted in a conical tube. Graduation starts at 2 m/s (around 4 knots). Stated accuracy: 0.5 m/s. An accuracy of 0.5 m/s at 2 m/s means the error can be 25 %. which is too much as this error directly gives an 25 % error in the result of the L/D measurement.

A Dwyer Wind Meter, which has a very light small ball in a conical tube. Graduation starts at 2 MPH, which is just under 1 m/s (around 2 knots). No stated accuracy.

Finally I had a vintage Dr Östman's Wind Meter, which has a turbine driving a pointer on a circular scale, via reduction gears. With the pointer reset you check its indication after on minute. No accuracy figure available, documentation missing

Measuring wind speeds below some 1.5 m/s with accuracy is probably a problem. I just checked the www.silva.se site and their turbine wind speed meter. They didn't give a figure of accuracy.

One way to measure low wind speed with simple means (at least on ice and land) is to have a man equipped with a very sensitive air movement indicator, e.g. a one meter long very thin sheet of e.g. plastic foil which hangs from his sideways outstretched hand.

Propelling himself in the direction of the wind he will be able to adjust his speed to equal the speed of the wind by observing the angle of the thin sheet with respect to the vertical formed by a weight at the end of a thread. If it deflects backwards he is going to fast and vice versa. I just checked the sensitivity with a thin foil and it seems to be in the order of 0.1 m/s, which gives an error of some 7 % at 1,5 m/s. Not quite OK but better than some 25 %.

If he travels between two markers with know separation in the direction of the wind his speed can be determined with a stopwatch.

To measure the wind speed higher up the foil and the vertical line can be placed on a pole.

An alternative to the foil can be a lit cigarette or a stick of incense.

Other maybe more complicated ways of doing a similar measurement is to time a puff of smoke or a balloon weighted to float in midair.

(Speed of kite)/(wind speed) is calculated for the passages. This is the same as the L/D ratio of the kite (including the drag of the lines to an extent, see more below). Mean value of the L/D for all passages are calculated and you have a fair value for the L/D.

L/D of a kite is dependent of the apparent wind speed. With this method of testing you get typical apparent wind speeds for sailing even when testing in light winds.

If you have measurements with a wide variation in wind speed you can group your results according this. Or better according to apparent wind speed. To get apparent wind speed for a passage take the square root of the sum of the square of the speed of the wind and the square of the speed of the kite.

The drag of the lines in this test is a quarter of that when actually sailing (because the apparent wind speed over the lines decreases towards the stationary person flying the kite.) If you want to get the L/D of the kite including the lines you use when sailing (suggested as the lines is a necessary part of the traction kite) you can use lines with four times larger diameter when testing, compared to the ones you use when sailing. Or lines four times as long. Or a combination of larger diameter and longer lines.

Note. To get a proper L/D result you must be honest, not trying to get a high L/D. If you for example measure wind speed low and fly high your result will be useless. (This way we have seen a kite (with L/D around 5) fly 17 times faster than the speed of the wind at the ground. That is some 300 percent off!)

Measuring the pull and calculating Cl for a traction kite

Two traction kites of the same area can have different pull in the same apparent wind speed. They develop a different force per unit area.

When you are measuring the L/D of a traction kite in the way described above it is quite easy to also measure the pull of the kite.

You need something to measure force with. We use a spring balance which at one end is connected to the waist of the person flying the kite and at the other end is connected to the lines going to the kite. See picture.

In the middle of each run over the course you note the reading of the balance.

One way to relate the Pull developed per unit area for different kites is to calculate their Cl, coefficient of lift, which is independent of area and apparent wind speed, Vapp.

Cl=(Pull x 1.6)/(Area x Vapp x Vapp)

Pull in N. Area (projected) in sq. m. Vapp (apparent wind speed) in m/s.

Vapp=sq. root of ((Speed of kite)(Speed of kite) + (wind speed)(wind speed))

Typical Cl for traction kites is around 0.4 - 1.5.

As the Cl is calculated from measurements of Pull, Area, Speed of kite, Wind speed, which all have some error the accuracy of the result is not good.

NOTE. As described above, between the horizontal lines, it may be a bit difficult to determine low wind speeds accurately. An error in the wind speed will of course give an error in the calculated Cl.

When you have the Cl of a traction kite you can calculate how much pull it develops in a particular apparent wind speed. (Doesn't apply if kite changes, for example by self reefing.)

Pull=(Cl x Area x Vapp x Vapp)/1.6 (Units as above.)

A L/D measurement report form for a traction kite can look like this:

Below I have entered information from a test we have done. The figures in brackets are approximate as they were not noted at the time of the test.