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Main wing profile

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mopman365
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Main wing profile

Postby mopman365 » Tue May 23, 2017 2:14 pm

Hi,

Deciding on a profile for my next (3rd) main wing. Seems everywhere the EPPLER 817 profile is suggested as the best for low-speed hydrofoils. My first one was AQUILA 9.3% that has a flat bottom - so less sanding required for the Corecell core. Second one was the EPPLER 817 and it's a bit of a mission to shape (and also makes the allignment more difficult).

My question is, why everyone's using Eppler 817 when a simpler profile like AQUILA 9.3% seems to have better L/D performance? Am i missing something:
Profiles_airfoiltools.JPG
Blue line is Aquila that has better L/D from 2.5 degrees and more:
LiftDrag_airfoiltools.JPG
Lift_airfoiltools.JPG
Lift_airfoiltools.JPG (34.44 KiB) Viewed 959 times

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Re: Main wing profile

Postby rynhardt » Tue May 23, 2017 3:42 pm

I'm no expert, but I believe asymmetrical profiles are more speed sensitive, i.e. a flat-bottom profile will tend to increase lift with speed (given a zero AoA).

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Re: Main wing profile

Postby TomW » Tue May 23, 2017 7:41 pm

Where did you get those curves?

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Re: Main wing profile

Postby mopman365 » Wed May 24, 2017 6:23 am

TomW wrote:
Tue May 23, 2017 7:41 pm
Where did you get those curves?
Sorry, forgot to mention: airfoiltools.com/airfoil/details?airfoil=e817-il
and
http://airfoiltools.com/airfoil/details ... quilasm-il

You can compare profiles using http://airfoiltools.com/compare/index

@Rynhardt, both of these are asymmetrical, but lift increases (exponentially) with velocity for any wing profile shape - 2 times faster = 4 times the lift. https://wright.nasa.gov/airplane/lifteq.html

I'm wondering why the Eppler profile is preferred for hydrofoils when other (flat bottom) profiles have better Cl. So in theory you could get away with a smaller or thinner wing to get the same lift at a given speed. Again, I might be missing something?

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Re: Main wing profile

Postby kostantin » Wed May 24, 2017 7:42 am

Hi,

I think drag will increase on a flat bottom profile with speed. Lift is not everything. Also there is a limitation when it comes to profile thickness. Very thin wing profiles are a serious task to build.

tks

Kosta

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Re: Main wing profile

Postby rynhardt » Wed May 24, 2017 9:19 am

For a zero AoA, on a symmetrical profile there is the same lift force generated both in the up and down directions, resulting in a net zero lift at zero AoA, irrespective of velocity. So lift is determined solely by the AoA.

For a zero AoA, on an asymmetrical profile, the upwards and downwards lift forces are not the same, resulting in a net non-zero lift force at zero AoA. This net non-zero lift increases with velocity, even at zero AoA.

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Re: Main wing profile

Postby mopman365 » Wed May 24, 2017 9:32 am

rynhardt wrote:
Wed May 24, 2017 9:19 am
For a zero AoA, on a symmetrical profile there is the same lift force generated both in the up and down directions, resulting in a net zero lift at zero AoA, irrespective of velocity. So lift is determined solely by the AoA.

For a zero AoA, on an asymmetrical profile, the upwards and downwards lift forces are not the same, resulting in a net non-zero lift force at zero AoA. This net non-zero lift increases with velocity, even at zero AoA.
I agree with the above, but i'm not interested in symmetrical vs asymmetrical. Both Eppler and Aquila are asymmetrical. I'm interested why the more complex (to shape) Eppler profile is preferred over a simpler and more efficient (higher lift, lower drag at various angles) profile like Aquila?

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Re: Main wing profile

Postby chfin » Wed May 24, 2017 12:57 pm

I'm by no means an expert in aerodynamics, but I think your answer lies in the first image you posted: the E818 and E817 are designed for hydrofoils, wile the Aquila was designed for R/C sailplanes. While there are a few similarities, air and water behave very differently (see compressible vs incompressible flow, risk of cavitation in water, different range of Reynolds numbers...) and a foil that works well in one medium might not be optimal for the other. The eppler hydrofoil profiles were specifically designed to reduce cavitation over a wide range of conditions, hence the more complex shape (I found http://www.foils.org/hysecdes.pdf to be a good resource for this subject). Without some testing it will be hard to say how much the choice of one foil shape will influence the performance of a hand shaped wing, but I usually go with the Eppler as at least it takes one unknown out of the many parameters that go into the design of a wing.

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Re: Main wing profile

Postby tahoedirk » Wed May 24, 2017 3:38 pm

I've built and tested numerous profiles now. I can foil well now but I am less of an expert than when I started building 3 years ago. There are so many factors that change with each home built model that it is very difficult to know what the effects of the profile alone are. Build one of each with identical everything , make two identical boards with identical everything and then test them in the exact same conditions at the same time. ????? It all seems to work , even well, when tuned, the tuning is the hard part . R/C, hyrdrofoils, air foils, propeller foils , who knows if there will be a KBHF designation? Pick one and build it though, it is quite a journey, and Have fun!

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Re: Main wing profile

Postby TomW » Wed May 24, 2017 6:50 pm

I was laying here recovering from a back injury, watched 2 hrs of YouTube videos about Americas Cup hydrofoil and boat design.
Only way to nail it for sure is using the CAD and fluid dynamic tools they were using.
Hydrofoil designer says they design 40 on the computer, do CFD on them, choose 10 to make prototypes to 0,1mm tolerances, test them all on the water, choose 4 to refine, more CFD, build and test, then select one. That last one can be tweaked too. Some of this are rules limiting number they can make, but that process was 7-8 months with a whole team and big wallet.
When I saw that, I can't see how anyone can really push the evolution significantly year to year, on various performance needs:
Race
Freeriding
Wave riding

I really get the attraction of the problem. I can't wait to get proficient on the foil so I can start using my CAD skills, 3D printing resources and composite building skills to try it.
But I'm really sceptical how an amateur can figure it out without copying tried and true designs.


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