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Repeatable measuring of lift-to-drag ratio, anybody?

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Horst Sergio
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Repeatable measuring of lift-to-drag ratio, anybody?

Postby Horst Sergio » Fri Jul 06, 2018 9:35 pm

Who should be interested in a good lift-to-drag ratio and therefore to find a possiblity to compare it quantitative?

Low drag and therefore a good l-t-d ratio is the key to not just lowest wind, but also to be able to use the smallest kites.
The less drag you have the less pull you need.
For example ridding 6 m² in about 10 knts with fun: https://www.youtube.com/watch?v=STWet7fLx5Y

Drag is caused by the main three factors:
- surface friction, depending mainly on surface size off all parts in the water
- frontal area, mainly limited by strength and stiffness of material
- induced drag, increasing with low AR wings

My question is, are firms or others testing just this ration in clean conditions for example behind a boat or in a pull test water tank or similar? And what are the values?
For kites like the old speed 1, I remember a test with a pick up truck 10 years ago, resulting it has a ratio of about 6, while tubes of this time had about 5, done by a kite magazine with the help of a aeronautic ingenieur / kite designer.

What's about with foilboard industry, any repeatable test and numbers around?

Motivation for my question is, without knowing its number a few days ago I think I nearly accidentally tried a foil with a close to the best actual possible and ridable ratio, showing the picture below,
but also would be interested in numbers of any anchor foils as old alu or carafino, airchair, etc. or even twintips and doors ...
Image

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Re: Repeatable measuring of lift-to-drag ratio, anybody?

Postby Regis-de-giens » Fri Jul 13, 2018 3:17 am

Very nice topic that may slide to deep technical arguments. I have been investigating a long time on L/D ratio measures, here is my current conclusion :

For a kite , several options on a theoretical point of view :
1- measuring the angle to zenith : The tangant gives you the L/D ratio
2- on the beach, measuring the speed when crossing the middle of the wind window. Ratio of speed gives you the LD ratio.
3- when performing the 360 degree exercise by running on the beach without wind. Duration to complete the 360 degree gives you an estimation (Under-estimation) of the LD ratio.

For a Hydrofoil :
1-You can use a boat to pull the rider , put a load cell (like luggage weight sensor) on the traction rope. Using the speed and weight of the rider you can derive the LD ratio.

2- without sensor you can just let the boat traction rope go and you can derive an from the distance you can do without boat you can get an average LD ratio among a range of AoA variation.

We can discuss the formulas for each if you want. By the way these measure of LD ratio is depending on bar stroke (For kite) and rider'weight (For HF) you need to built a graph LD vs AoA (= vs bar stroke Or Weight) actually.

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Re: Repeatable measuring of lift-to-drag ratio, anybody?

Postby Peter_Frank » Fri Jul 13, 2018 2:54 pm

Regis-de-giens wrote:
Fri Jul 13, 2018 3:17 am
Very nice topic that may slide to deep technical arguments. I have been investigating a long time on L/D ratio measures, here is my current conclusion :

Snip...

For a Hydrofoil :
1-You can use a boat to pull the rider , put a load cell (like luggage weight sensor) on the traction rope. Using the speed and weight of the rider you can derive the LD ratio.

2- without sensor you can just let the boat traction rope go and you can derive an from the distance you can do without boat you can get an average LD ratio among a range of AoA variation.

We can discuss the formulas for each if you want. By the way these measure of LD ratio is depending on bar stroke (For kite) and rider'weight (For HF) you need to built a graph LD vs AoA (= vs bar stroke Or Weight) actually.

Correct, and good thoughts.

But, a few but's regarding number 1:

Must say I dont think it will be a good way, in my experience with measuring (occupation), as I think there are too many variables in this affecting the result, compared to the desired "accuracy" needed to compare foils.

You can make a L/D curve yes, but you can not get the AOA into the equation, unless you somehow know the polar beforehand IMO.

You have to see the full foil assembly front wing, fuselage, stabilizer, part of the mast, connections, somewhat as a "black box".

It will be possible to make rough estimates of the AOA, but thats it.

I think a water tunnel (made for it) is the only way to make comparable tests regarding this, with the benefit you get the AOA measured also, and much easier to plot the behaviour when over the stall point :thumb:

Regarding #2, it will be almost solely dependant on how many tries you got, as I dont believe anyone can ride a foil at max L/D for long. Which you also imply by "a range of AOA variation".

For going max distance you dont need to change AOA, and the speed should be held constant, IF you have let go at max L/D speed or lower, otherwise you are correct.
Remember, max L/D is at a lower AOA than max "time" above the water before you touch down, two different AOA's.

You can ride for longer time before touchdown, at a bit lower L/D (higher AOA), but you will not ride as far when you touch down.

For everything else than best VMG, the full polar is needed if you have to compare speed, as it is close to zero AOA (constellation wing + stabilizer) we ride at high speed.

I might be pleasantly surprised of course, but I doubt any of these methods will work really :roll:

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Re: Repeatable measuring of lift-to-drag ratio, anybody?

Postby ronnie » Wed Jul 18, 2018 11:08 am

I hope someone somewhere is actually using a flow tank to develop hydrofoils and then they would have to measure the lift drag of the various designs. They could also check the pitch stability at differing angles of attack.
Something like this, but with instruments measuring the loads in real time.


Nicolas mentions in the 9th March post that Zeeko were working with a large University on digital simulations and in a pool for the AVS study.
https://translate.google.co.uk/translat ... rev=search

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Re: Repeatable measuring of lift-to-drag ratio, anybody?

Postby tegirinenashi » Wed Jul 18, 2018 10:38 pm

From this video I infer their next gen foil would feature rectangular mast cross section?


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