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hydrofoil vs. aerofoil

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Kevin Brooker
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hydrofoil vs. aerofoil

Postby Kevin Brooker » Sun Nov 27, 2016 11:52 pm

Some basic research has shown most aerofoils will not work well as hydrofoils because of cavitation problems. I see many posts where builders are using eppler and other foils I've used with rc gliders.

What is the main difference between hydro and aero foils?

I am very experienced making foam and CF wings for RC and would like to build a foilboard.

Is anyone experimenting with trimming the angle of incidence while the foil is in motion? Changing cambers using tabs or flaps. It seems if you can reflex the wing the speed will increase and the ability to add camber will make one foil work under a variety of waterspeeds.

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Re: hydrofoil vs. aerofoil

Postby ronnie » Mon Nov 28, 2016 1:04 am

Kevin Brooker wrote:Some basic research has shown most aerofoils will not work well as hydrofoils because of cavitation problems. I see many posts where builders are using eppler and other foils I've used with rc gliders.

What is the main difference between hydro and aero foils?

I am very experienced making foam and CF wings for RC and would like to build a foilboard.

Is anyone experimenting with trimming the angle of incidence while the foil is in motion? Changing cambers using tabs or flaps. It seems if you can reflex the wing the speed will increase and the ability to add camber will make one foil work under a variety of waterspeeds.
They use a surface-following flap on the Moth hydrofoil.
I did read that a study was done comparing that to changing the AOA of the whole wing, and they found trimming the whole wing was more efficient.

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Re: hydrofoil vs. aerofoil

Postby tahoedirk » Mon Nov 28, 2016 3:34 am

If you use an airfoil in the water , what is it? if you use a hydrofoil profile on a RC glider wing , what is it ? If you use a propeller profile designed for a slow flying bush plane on a electric trolling boat prop what is it? and what if it works really well?

We are all doing the R&D right now. I don't think Mike or Spot or Banga or Mr Sword are going to tell you exactly where or how their profiles evolved or were chosen, or labeled. Everything will cavitate , when and where how fast and for who......

It would be so nice to be able to stretch or thicken or adjust the AOA or add camber on the fly like on my 747 . I think you are onto something for sure Kevin. I say go for it. Some hang gliders have flaps now.

I've been trying for a while and I am still very baffled by the subtleties . Just tuning an existing model is very challenging without an R&D army.

Print some hydrofoil profiles and then lay them on similar air or prop foil profiles and then muse over how similar they are, quickly before you fall asleep or have to make some money.
I'm gonna get me a water tunnel even though today was a powder day. R&D is so much more fun in the summer with bare feet.

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Re: hydrofoil vs. aerofoil

Postby DartBoard » Mon Nov 28, 2016 12:35 pm

If i remember right, its going back 20 years so please chime in if I'm mis-remembering, when selecting or design a foil for air or water the Renolds Number (froude number in hydrodynamics) is a critical design parameter. It completely characterizes the flow environment and includes density, velocity and viscosity of the fluid. From a site I was looking at the other day by a hydrofoil designer named Tom Speer, he mentioned that a 3 inch wide hydrofoil travelling at 12 knots has a Renolds Number of c. 350,000 ish. By comparision, a commercial jet might have a Renolds Number of 110,000,000. So its not that airfoils won't work as hydrofoils its just that many, but not all, of them will be very inefficient because they are designed for hugely different Renold's Numbers.

http://www.foils.org/hysecdes.pdf

The link above is to some design talk by Tom Speer about a few different sections. From an outline point of view the differences look pretty subtle but it seems that performance is quite different.

All that said, I suspect that for DIY jobs like ours the nuances of the designs probably aren't going to be that important unless you're aiming at the high performance end of town. Its only a guess but I suspect that ventilation and flow separation will be a bigger problem for DIY efforts than cavitation at least in the beginner to intermediate range.

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Re: hydrofoil vs. aerofoil

Postby Kevin Brooker » Mon Nov 28, 2016 1:49 pm

Thanks for the replies. I've read the Speer paper and found it very educational and also very confusing. I am familiar with Reynolds numbers ( here's a wiki link for more info https://en.wikipedia.org/wiki/Reynolds_number ) but didn't want to start off with asking about them as it feels very hoity toity and omnipotent. I also understand a foil is a foil and the application and medium have a direct bearing in the nomenclature. I was more curious about the terminology used in the foil board world. I have a few ideas of how to trim on the fly, change camber, AoA, Angle of incidence and reducing induced drag and parasitic drag. I just wonder where the DIY guys are.

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Re: hydrofoil vs. aerofoil

Postby Mossy 757 » Mon Nov 28, 2016 3:13 pm

Kevin Brooker wrote:I have a few ideas of how to trim on the fly, change camber, AoA, Angle of incidence and reducing induced drag and parasitic drag.
They've done some drag studies in the Moth world and some INSANE percentage of total drag comes from the adjustable foil apparatuses on their wings, something like > 90%. That's not to say that there couldn't theoretically be some advantage to a dynamic wing section, but would be your goal? To grab from your original post:
It seems if you can reflex the wing the speed will increase and the ability to add camber will make one foil work under a variety of waterspeeds.
The best of the best foils right now can ride in a wind range from about 6 knots to 30+ knots and can achieve speed through the water of about 7 knots to over 40 knots (not sure what the current outright top speed record is right now). Which waterspeeds are you trying to access that are not currently available with fixed-wing technology? What "regime of flight" merits optimization through a dynamic wing section?

I'm not trying to shit on your thread, but kite hydrofoils are so insanely easy to ride right now and are arguably the fastest way to sail without a multi-million dollar budget, I'm curious where you think they need the most improvement.

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Re: hydrofoil vs. aerofoil

Postby Kevin Brooker » Mon Nov 28, 2016 6:18 pm

No shitting taken. The reason for trying things is just for the sake of discovery. The current foils might work very well and I can buy one which will work as described. Making a foil with variable geometry and the ability to alter the flight parameters has the ability to speed up experimentation. The foil controls will not add any drag and when racing a 1% advantage is huge. By messing about there is nothing to lose. I can build a wing for about $5 US in materials and much of the fun I take from hobbies is trying to see if I can build something as good as what is commercially available. I most likely spend more money screwing around but this is where I find enjoyment with the activity.

A fully adjustable foil might not be for everyone It might not be for me but I will have fun in and with the process.

The areas of improvement are all subjective. Your observation about modern foils is correct but the first kites from the 90s were fun to fly and at the time they were the best there was. Depower was a great improvement and the kites of today are really not much different then those of 5 years ago but they are more fun to fly. Small improvements make for big gains. If designers just stopped playing with kites because they were fine, we;d not have the great kites of today. 5 years from now the kites will be better because someone played with the designs. Many of my ideas might be crap or impractical but as stated above, the fun is in the trying.

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Re: hydrofoil vs. aerofoil

Postby Mossy 757 » Mon Nov 28, 2016 7:22 pm

Kevin Brooker wrote: If designers just stopped playing with kites because they were fine, we;d not have the great kites of today. 5 years from now the kites will be better because someone played with the designs.
Totally on board with that, agree 100%. What performance or design parameters do you think most demand innovation when we consider current state of the art? I had a rare opportunity to pick a top racer's brain a few weeks ago and he said the only thing holding him back from going faster is his body. He thinks the materials and designs are all good enough to go 50+ knots, but that his body as a component in the system is the weak point and that we won't really see anyone making massive gains in performance until that can be addressed. His idea was some kind of hydrofoil kite skiff that anchors the kite power directly to the foil via the board instead of being harnessed to a person. I tend to agree that applying and managing power without the variable human element in the mix is also the right direction to go. Again, in the Moth world the adjustable control surfaces are a huge setback to performance for the sake of usability. Given the stark differences between a kite hydrofoil and a moth, I think it's worth looking elsewhere for incremental improvements, but of course it's up to you how to spend your time, money, and intellectual capital.

Great thread, good luck!

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Re: hydrofoil vs. aerofoil

Postby skyte » Mon Nov 28, 2016 10:04 pm

I think one of the greatest challenges with kites and hydrofoils and any other kiteboard is that it is very difficult to measure performance in any quantitive way... Almost all appraisal is done qualitively and this is always then blurred by whoever is doing the appraisal.

I suppose flat out speed is the ultimate measure but this will always be influenced by the rider and conditions on the day.

Even if you do find a way to measure performance and optimise it, there is always a risk that too much performance is not necessarily a good thing and can give you something that might be great for a very small percentage of riders.

Re: your original question... I had not realised people were using Aerofoils as hydrofoils. i.e. foils designed to flow in air instead of foils designed to flow in water. I thought it was all down to the Reynolds number and Mach number which are selected to suit the medium they will operate in. Tom Speer's explanations are good but it is complicated!

The Eppler 817 is listed as a hydrofoil and so are some of the NACA foils.

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Re: hydrofoil vs. aerofoil

Postby DartBoard » Mon Nov 28, 2016 10:59 pm

I think that being able to adjust some of the parameters is a great idea. It would certainly speed up prototyping and I am sure that lots of people would fine the results really interesting because its an interesting area.

A friend of mine built a foil from cut down moth components and kept the adjustable trailing edge on the main wing which he can tune (on the beach not real time) by adjusting the push rods that he's got running down the mast. We don't get very strong winds at our local so it was mostly looking at the low end performance and performance in the surf where the turbulence and waves makes the relative speed of the water and the AoA very dynamic. At the low end the speed it stalls at and stability were affected quite a lot. I suspect its the same at the top end of the speed range.

Like the other have mentioned the changes in performance are subjective but when you can make the changes quickly enough and ride in the identical conditions it can be more scientific. Please keep us posted on progress!

If your just learning to foil (as I am - just 6 months into it) I'd say make the adjustable one your second foil so you can get out on the water and start getting the stoke for foiling.


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