Wing profile

[Peter_Frank]

Snip....

The interesting thing that I don't understand is that thickness doesn't seem to play much of a role? if you look at Eppler 817 at 11% and Eppler 818 at 9% the curves (lift and drag) are almost identical - if fact the thicker wing has a wider range where it works better. I'd be most grateful if someone could explain this, it's counter intuitive that a thick wing and thin wing would have the same drag.

If you eyeball the Spotz wing profile and the Sword profile, they're completely different, so obviously there is more than one successful approach.

My thoughts and experiences on above:

For normal freeride the 817 with 11% thickness is mostly superior to the 818
It has much better L/D and also higher L so you can start earlier and also go higher upwind.
So in this respect thickness plays a huge (positive) role.

You can ride just as fast, you have better upwind L/D performance as said, and better low end, and a much stronger and stiffer foil/wing.


Lets say the 818 IS an 817 just thinned out a bit.


There are 3 aspects to be considered, besides the obvious max L and max L/D at almost no cost in terms of drag at low AOA

1. The 817 will have higher pressure changes throughout the surface, as it has steeper surface changes (curved more). This would (could) IMO lead to even lower pressure areas, thus being bad for cavitation thus no good for really high speeds.

2. Who has made the polars ? How precise are they, and do you have a better resolution of the drag at low lift coefficient ?
Because, eventhough they look alike, the 818 could be a few percent lower in drag here, and even if we say just 2% lower (which can not be seen on such a rough polar), it is about 10 seconds you are ahead of number 2, in a 15min race.

3. The Cm is much lower at the "often used" AOA around 1-4 for the 818, which means it will be much more stable and easy controllable at really high speeds - and you can use smaller control surface thus less drag. A win win seen only in this perspective.


Now, the 818 could suddenly be preferred (or not) over the 817, seeing all aspects

Just my view, based on aerodynamics, physics, and cavitation principles.


Apart from this - agree - there are MANY different ways to obtain good results - and some does not look alike whatsoever

PF

[Phezulu1]

This is a much more interesting thread than the dreaded C.


2. Who has made the polars ? the polars are straight off the airfoil tools website- they are for re 1e6 - actual range for a kite foil wing is somewhere between 1e6 and 2e6. I don't know how accurate they are - but probably more accurate than we can make our foils too anyway. I think the point is that they aren't that different, it probably makes a difference when you get to the higher speeds.
3. The Cm is much lower at the "often used" AOA around 1-4 for the 818, which means it will be much more stable and easy controllable at really high speeds - and you can use smaller control surface thus less drag. A win win seen only in this perspective.

- I originally thought the Cm was important, but my current thinking is that it doesn't really matter because the plane of input force (at the the harness) is so far above the foil that these moments are much greater than the wing moments.

Now, the 818 could suddenly be preferred (or not) over the 817, seeing all aspects : yes, I'm not sure either - but I'm beginning to think that a high aspect, fairly highly cambered wing with shortish chord would be preferable over a bigger chord thin wing as the Cd drag is about the same, the wetted surface area is less - that seems to be the thinking on the 2013 Spotz wings. I'm also starting to think that stability might be more important that outright drag reduction - the drag is already pretty low, the reason we can't go faster is that we can't control the foil better at speed.

The other thing that I'm realising is important it is that the wings can shed lift for higher speeds, in other words low Cd at lowish Cl values. We actually have more than enough lift at high speeds, in my view this is more important than high l/d ratios.

It's all good fun, the more you learn the more you realise how complex it is.

[Europ2]

For the builders out there.

Different profiles should disserve our attention. The question is "what can 1 and 2 give in water which presents a higher density compared to air... ? "

1 - Kline–Fogleman (stepped) airfoil for lift increase and easier to manufacture:
-> disapponting L/D ratio in the real world but very successful among the RC community:
Easy to make: http://www.rcgroups.com/forums/showthread.php?t=1117276
http://en.m.wikipedia.org/wiki/Kline%E2 ... an_airfoil
http://www.rexresearch.com/klinfogl/klinfogl.htm#ultim


Investigation of Flow across a Symmetrical Airfoil with Backward Facing Step
http://www.taylors.edu.my/EURECA/2014/downloads/23.pdf

Read More (filtered access) : http://ascelibrary.org/doi/abs/10.1061/ ... 3A1%289%29
Aerodynamic Performance of an Airfoil with Step-Induced Vortex for Lift Augmentation:

The results suggest that incorporation of backward-facing steps on the lower surface that are located at the midchord and extend back to the trailing edge with 50 depth of the airfoil chord may lead to considerable enhancements in lift coefficients and lift-to-drag ratios. The data produced may serve as a reference for future studies on the possible use of separated vortex structures in enhancing the aerodynamic or hydrodynamic performance of vehicles and structures.

2-Coanda nozzle slotted airfoils in "Coanda slotted wings" for lift increase

http://mmut.mec.upt.ro/mh/Conferinta_MH/808_Galeriu.pdf


3- The (natural) laminar flow (NLF) airfoil sections for optimum L/D

http://www.dreesecode.com/primer/airfoil5.html

Ex: The decambered NASA NLF (414 415 416, ...)
NACA 6 digits (66-415, ...) , NASA GAW section (GAW2, )

[zfennell]

My thanks to all.
The personal insight and perspective of everyone is refreshing.

Peter is correct to question the validity of the data presented by airfoil-tools.
( I'm pretty sure they do provide a brief description which component are calculated and which are interpolated from other data sets)

That said , I doubt any hydro calculation is good to +/- 2%
And that would be independent of the rest of the assembly interactions, tolerancing of any parts, test environment and ,of course, the rider.

But when you are in 'design' mode math is a great way to make apples to apples comparisons of potential solutions.
Gone are the days when you would build one of everything and just test it.....too bad.

You guys all seem to have progressed to the point that 'fast' has become the priority.
Everyone acknowledges that operational AOA is low ( 0-4) and required lift is readily available

Correct me if I'm mistaken, but the principal design parameter seems to be to reduce drag in this small area of operational AOA
I'm guessing ease of operation while going warp speed would be a close second

In that regard, making foils thinner and thinner sounds logical, but always at the risk of being too 'twitchy' or sensitive to AOA ( stalling, separation, ventilation, etc.)
Some have already mentioned the relatively low Reynolds numbers of kite hydrofoils (1-2 million)
I never would have guessed this but fully submerged ,high aspect, low speed foils have a good chance of maintaining a laminar boundary-layer over most of their surface.
Even if tripped by bugs, or bubbles, or dirt, etc., the short chord length would prevent much of the turbulence from spreading to other regions of the wing.

Tom Spears ( on his web page) has done a pretty good job of showing performance benefits if the foil design incorporates laminar flow.
Additionally, his design approach, extends to the specific avoidance of laminar separation by allowing the boundary layer to transition to turbulence when laminar can no longer be sustained.

Back to peter's point that math and reality are often quite different. ...
Tom has stated that he has not yet built any of the profiles he has discussed.
But the 'apples to apples' math shows promise when compared with popular Eppler shapes.
It's certainly an approach that sounds worthy of consideration for folks focusing of drag reduction.
Additionally , if any of this works , I believe it would also translate to the strut, with just as much wetted area as the foils.

Too bad I cant take credit for any of this.
It's just the result of listening to a lot of interesting people.
Thanks again,
-bill

[zfennell]

the following is extracted from T.Speers hydrofoil page.
http://www.tspeer.com/Hydrofoils/h105/h105.htm

the plots below compare a Speers (H005) profile and Eppler 817:
boundary layer transition vs angle of attack
and
boundary layer separation vs angle of attack

The interesting point, i think, is that the Eppler profile exhibits BL separation at 4 deg while the H005 does not separate until 10 deg
additionally,
transition from laminar to turbulent occurs at 2.5 deg AOA for the Eppler 817 compared to 4 deg AOA for the H005. at this point, the upper suface of both profiles goes from mostly laminar to 100% turbulent.

for a device that operates at a small AOA , its possible to select a profile that provides a significantly larger sweet -spot for optimal AOA.

overall the optimal lift and drag polars are similar (3rd plot)
but for me the 'take-away' is how slow do you go when AOA is no longer optimal.
we've all experienced stall , spin-out, separation, etc where you go from warp drive to snail speed.

it would seem that proper selection/design of your foil profile could give you greater margin of error while stll going fast.

just a thought,
-bill

[Blackrat]

i know this is an old post, but has anyone used the H005 profiles ?