Lifting Vortices On Kites

by Al
(USA)

Or, 'vortex generators'. Recently I have improved the flight characteristics of several different kinds of kites by modifying them with what I call FLOWTHRU SLOTS. Last year I conducted some simple wind tunnel tests and have determined that the enhanced performance of all the kites I tested was derived by inducing 'lifting vortices' and changing the lift-drag ratio.

Rectangular flaps or slots cut in the sail that are positioned close to the center of gravity and the center line of the kite have increased the flight angle and provided greater stability in high and turbulent wind conditions.

Whitold Kasper back in the 70's designed and patented an Airplane wing that incorporated variable wing configurations that produced 'lifting vortices' (a vortex generator) thus, allowing airplanes to generate lift at very slow airspeeds.

Many kites by their design features have the ability to direct a large amount of air flow thru slots in the sail that can produce 'lifting vortices'. I first encountered this phenomenon when I designed a flat kite called a TEECA back in 2002. I was pleasantly surprised by its high angle of flight and its stability in high and gusty wind conditions.

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Lifting Vortices
by: Tony Sangster

I commend you Al on your work and dedication.
In Adelaide several of us are measuring line tension with fishing and luggage scales and one of our number, Steve. A model yacht and sail maker from way back attaches pieces of old videotape to his kites to see what the airflow is doing.

We are most impressed with the performance of Tim's carbon diamond with its slots.
In the pipeline for some time now is experimenting with horizontal slots cut in a Genki kite, the plan is to have a usual patterned Genki and compare it to one with slots cut in front of and another with slots cut behind the cross spar.. This style of kite is wider than long, so the results should be interesting to compare to other styles you are working on.

We will contact you once we have some solid data and again congratulate you on the work you are doing.

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Experimenting
by: Tim Parish

Thanks - it's clearer now: what you have done is closer to 'leading edge slots' as used on heavy jets, including large passenger jets.

So, span-wise slots... (I was thinking one longitudinal one!)

I think I *will* fiddle a little with my Dowel Delta. Or perhaps make 2 new ones from scratch with one as a control. Or, since I'm re-designing the 2-Skewer Delta anyway, it would be quicker to make a couple of those and fly them side-by-side on identical lines. Far enough apart not to tangle though.

Should be fun! A minute or so of continuous video should show up any differences in flight characteristics.

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Clarifying...
by: Al

Concerning your comment, please consider the following:

1. The location of the slots have no effect on tip vortices.

2. The slots do not negatively disrupt the air over the upper surface of the sail but in effect create a larger and smoother air airfoil section above the slots.

I am presently testing different single line kites that have a span of 38" to 48" and I buy them in pairs (base and test) at the Dollar Tree and Toys R Us.

What I have determined is that two rectangular sail openings 1 1/2" x 6" positioned 1/2" from the center line of the kite and 1/2" below the spreader stick have had some very promising results. The rectangular openings cut in the sail can be flaps, slots or floppy strips, and have increased the kite's lift and reduced its drag. Also, kite oscillations are dampened in gusty and turbulent winds.

I would ask but not insist that you would experiment with some cheap single line kites using the basic information above. I think it would give you a better understanding concerning the overall effectiveness of FLOWTHRU SLOTS.

I would like to know about any of your findings should you do any experimenting.

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Slots / holes in sails
by: Tim Parish

Thanks for that interesting info! It brings to mind a couple of snippets I have come across while researching for this site over the years.

1) A NASA site which does a great job of explaining the physics of kites mentioned that tip vortices play a much more significant role in the behavior of kites than for full-size aircraft. One of the results of this is that aspect ratio counts for less with kites. That is, trying to increase the performance of a kite by increasing its aspect ratio doesn't work nearly as well as it does for larger aircraft. Sailplanes for example.

This hasn't stopped me from designing a high-aspect ratio Delta - mainly for looks!

2) Some traditional makers of kites have discovered, centuries ago (!), that putting a hole in the middle of a kite can increase its stability. For example, some of the rectangular designs from Korea.

I'm sure I've come across other references too, to more modern kite makers who have stumbled across the stability-inducing effects of a slot or hole near the middle of the sail.

You are fortunate to have access to a wind tunnel! I imagined how your idea might be working... Assuming your slot was a longitudinal one, it might be counter-acting or just suppressing the tip vortices and thus reducing induced drag. Perhaps the sheet of air flowing through is disrupting the inward rotation of air over the upper surface of the sail, near the trailing edge in particular.

I guess anything that can inhibit that rotation will have the desired effect. The latest sailplanes have vertical bits on the wing tips. Same thing - the tip vortices are damped down, bumping up the Lift/Drag ratio.

Now, at this point, we might have lost a lot of the faithful readers of this blog page. Hopefully just until the next post! But others will really enjoy it ;-) Watch them come out of the woodwork in further comments... Maybe.

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