Hydrofoil Profile Question [Archive]

tspeer

10-21-2008, 09:58 PM

Does your craft tack or shunt? That would influence what kind of symmetry you seek - right/left or fore/aft. The right/left symmetric version of the NACA 4312 is the NACA 0012, and it's a good choice for centerboards.

For fore/aft symmetry, you have two choices: sharp-edged, like ogival sections, or rounded edged, like these (http://www.basiliscus.com/ProaSections/ProaIndex.html). CFD says the round-edged sections can be as efficient as a conventional section, but I have my doubts as to whether that will work out in practice. The sharp-edged sections suffer from leading edge separation for all but a narrow range of angles of attack, so both approaches have their problems.

ancient kayaker

10-21-2008, 11:50 PM

tspeer

10-22-2008, 10:27 AM

I am looking for a shunter type profile, that is, with fore-aft symmetry, which I cannot get with the NACA formula. I am thinking of using a section with a circular arc on one surface and flat on the other, about 12% thickness, with about 2% radius on the edges. Does that sound about right?

What you've described is an ogival section. You can find data on them in Hoerner's "Fluid Dynamic Drag".

In Abott & VonDoenhoff's "Theory of Wing Sections" (Dover Books), you'll find the NACA m=1 camber line. That might be another suitable shape for a sharp-edged section, either with a flat surface on the leeward side, or scaling the m=1 camber line for both the leeward and windward sides, so as to have some droop for both the leading and trailing edges.

Who or what is CFD?
CFD stands for computational fluid dynamics. Numerically calculating the flow to predict the pressures, boundary layer development, lift and drag. Unfortunately, numerical approximations do not necessarily capture all the detailed flow physics and can sometimes be misleading.

I've recently gained access to a more sophisticated program than the one used to analyze the Proa 3 series (http://www.basiliscus.com/ProaSections/AppendixC/AppendixC.htm) sections. It might do a better job of calculating the flow at the trailing edge, which is critical to the performance of these sections.

ancient kayaker

10-22-2008, 11:22 AM

Thanks. I have taken the liberty of copying the P-30208 data for off-line study, as that seems closest to my needs; hope that is OK with you. It will take me a little time to come sufficiently up to speed on CFD to understand what you have provided. Assuming the P-30208 is a good choice it can be made in a fabricated foil since the upper surface appears to be circular between 10% to 90% of the chord and the lower surface appears parabolic. I can easily get those shapes by bending sheet material, with carved LE/TE.

tspeer

10-23-2008, 12:27 AM

Thanks. I have taken the liberty of copying the P-30208 data for off-line study, as that seems closest to my needs; hope that is OK with you. It will take me a little time to come sufficiently up to speed on CFD to understand what you have provided.
I'll reiterate my original caution: the computer says it performs well, but that doesn't mean it will work well in practice. It's quite possible that the flow will separate from the trailing edge in a way that will decrease the lift and increase the drag.

Based on what you've described about your methods of building the foil, I think a sharp-edged section would have a better chance of performing the way you want.
Assuming the P-30208 is a good choice it can be made in a fabricated foil since the upper surface appears to be circular between 10% to 90% of the chord and the lower surface appears parabolic. I can easily get those shapes by bending sheet material, with carved LE/TE.

There is no part of the section that is either circular or parabolic. It's very important that the foil be built as accurately as possible to the coordinates as given. And that you put a turbulator, such as strip of zig-zag tape or something like a length of fishing line glued to the surface, down the mid point on the windward side. This section was designed by defining the hydrodynamic characteristics and then computing the shape that would have those characteristics.

What you are describing is essentially the same as the Proa 1 series. That series did have circular arcs with radiused edges. And it was predicted to perform poorly.

Guest625101138

10-23-2008, 02:40 AM

ancient kayaker

10-23-2008, 09:37 AM

... There is no part of the section that is either circular or parabolic. It's very important that the foil be built as accurately as possible ... what you are describing is essentially the same as the Proa 1 series ...

Tom: If the profile is too sensitive to errors I may not be able to get it close enough to get the predicted performance.

Rick: I need to study your data, haven't had time yet. I am rethinking the fore/aft symmetry. Perhaps I should go for the NACA 4 digit series; I used those way back in my model aircraft days and they did not seem too sensitive to errors. The old Clark-Y was an old standby, flat one side, easy to build, not sure how it behaves in an incompressible fluid though.

Guest625101138

10-23-2008, 04:30 PM

......... The old Clark-Y was an old standby, flat one side, easy to build, not sure how it behaves in an incompressible fluid though.

The attached is the Clark-Y data on a comparable basis to the ones above. It is probably much thicker than you need as it has a wide working range. It does not perform as well as the other foils from a drag perspective. They have L/D as high as 17 whereas this one only gets 12.

Rick W

ancient kayaker

10-23-2008, 07:21 PM

Guest625101138

10-23-2008, 09:36 PM

I think the old Clark Y was around 12% which is around what I planned to use. It looks similar to the profile I get for NACA5312 but the NACA profile is (more or less) flat over a greater portion of the chord.

I didn't understand the results in your table: Cl does not become negative and Cd doesn't pass through a minimum, at least for the range of alphas shown.

AK
You are correct. I loaded the coordinates straight from a data file I picked up on the web. It drew the section correctly but JavaFoil did not like analysing in reverse. The attached is the correct data and it is now much closer to the others. The foil is very close to 12% thickness.

Rick W.

ancient kayaker

10-23-2008, 11:51 PM

We may have a winner, but I want to study all the numbers a bit more. Thanks for all your help!

There is very little difference between the Clark Y and the 4512 despite what looks like a radical difference in profile. That suggests that minor build errors would not have drastic results.

It also reflects my model airplane building experience, which was that hand-drawn "looks OK" profiles worked about as well as carefully plotted ones, at least outside of the laboratory. However, that experience may also be due to the inability to accurately control the shape of a fabric covered wing between ribs.

The profiles that have fore/aft symmetry might be more picky.

Guest625101138

10-24-2008, 12:55 AM

AK
I am not certain about the detail of TS's P foil but I believe it aims to achieve a drag "bucket" and this is likely where the sensitivity to shape may be.

Generally results do not differ a huge amount if the profile is smooth without unintended bumps, the trailing edge is thin and the leading edge is nicely curved.

The numbers I have given you are for an aspect of 4 at Re# of 200,000.

If you get into more detail design then you can use numbers closer to where you would like to operate. You may do a little better with a NACA 16-series designed for the required lift but it will likely be harder to make.

The attached image is the Clark Y rotated 2 degrees to give the horizontal base. I have made templates from these images by scaling them to suit in powerpoint and cutting them out from the printout. Works for any section smaller than your printer.

Rick W

Tcubed

10-24-2008, 08:45 AM

http://www.basiliscus.com/ProaSections/ProaIndex.html

Try this for more info.

Myself, i've made a number of sculling oars. These must fulfill the same symmetry requirements as you are after.

In the case of a sculling oar the camber and thickness vary along the length, but i would make the edges parabolic and pretty "sharp". These foils would exhibit remarkably high efficiencies, no separation problems or anything.

In the case of your daggerboard, why are you aiming for 12% thickness? It seems needlessly thick. Myself, i'd favour 10 or even 9 per cent. Camber shouldn't be too high either. I'd say around 2 % is good for optimum L/D ratios. Then figure out at what speed you need best performance and calculate area accordingly, so as to operate at design coefficient of lift.

Tcubed

10-24-2008, 08:58 AM

<<<<<<<I am rethinking the fore/aft symmetry. Perhaps I should go for the NACA 4 digit series; I used those way back in my model aircraft days and they did not seem too sensitive to errors. The old Clark-Y was an old standby, flat one side, easy to build, not sure how it behaves in an incompressible fluid though.>>>>>>

The clark Y foil is good general purpose and easy to build foil for aircraft wing, but it is far from impressive for performance. Could do better, in other words.

Air, as long as you are travelling significantly slower than mach1, is for all intents and purposes incompressible. I know, i studied incompressible fluid flows. So take any notions of compressible flow and throw it out the window, no sailboat ever needs be concerned about compressibility.

The fabric sag you mention between ribs-didn't you use sheathing? In my model gliders i would always sheath from the spar to the leading edge, upper and lower side, and quite often from the spar to the trailing edge but only on upper side, the lower side there is so near flat it's not worth it. This would completely eliminate sag.

ancient kayaker

10-24-2008, 01:37 PM

Rick: thanks again, the Clark Y profile and others will be useful to me.

Tcubed: "In the case of your daggerboard, why are you aiming for 12% thickness? It seems needlessly thick. Myself, i'd favour 10 or even 9 per cent"

-my intent is still evolving, it is now a Bruce foil which, for a given depth requires greater aspect and also greater force than a daggerboard, both of which lead to greater moment (double at 45 deg) to be supported by the structure, hence the increased thickness which was originally 9%. I thought the fore-aft symmetry would allow me to flip it to either side; it's doable but it doesn't look safe. I'll use 2 foils. A foil with lateral symmetry will work on either lee- or weather side but I've done that already.

Honesty forces me to admit that I still haven't quite made up my mind on the final outcome, but I have some ideas on sailboats that I want to put together. Don't we all?

It's good to know about the incompressible fluid thing; it seems logical now but so do a lot of things when someone points them out.

The gliders I was making were super-lightweight, barely strong enough for outdoor use. Sink rates were ridiculously low. I lost all of them to thermals, even a few that kept going up after the dethermaliser popped up. I didn't care as they were cheap and I could turn them out at a great rate; I was doing what I do now I guess, varying one parameter at a time and looking to see what happened. It kept me off the streets.

Tcubed

10-24-2008, 05:12 PM

That makes sense. You were into the minimum sink speed thing, and consequently keen for extreme lightness. Myself, i did slope soaring and a lot of time would use reflex in the ailerons/flaps as well as ballast to get my flight speed up, so could build in a lot of stuff. In fact needed to. The last wing i made i tested on the ground to withstand up to 30G, and still worried sometimes about breaking it. Previously i had bust a wing, being a bit too enthusiastic in my manoeuvring...

By the way, i'm still not sure what kind of boat you're intending this foil for. It's not a proa?

ancient kayaker

10-24-2008, 11:15 PM

Not a proa. I was planning on a symmetrical profile for the kayak sailing kit I am designing for next season, to build over the winter. I mentioned proa because I didn't know how to define what I was looking for until Tom defined it correctly as fore-aft symmetry. I no longer think the idea of using the board on either side is workable, although I know a single leeboard can be deployed on either side using a tether, but it has been great exercise that has taught me a lot about profiles I didn't know before. I am going back to an earlier concept with a Bruce foil on each side. The Bruce foils will cancel the heeling moment of the sail, so I don't need the lifting sail I was considering using. These are the threads that address what I am thinking:

http://www.boatdesign.net/forums/sailboats/articulated-sailboat-24598.html
http://www.boatdesign.net/forums/sailboats/sailing-kayak-project-24541.html

I haven't given up on wild ideas althogether though, I am trying to fit a wing sail into this concept somehow, so the profiles may get more than one application.

ancient kayaker

10-24-2008, 11:29 PM

... i did slope soaring and a lot of time would use reflex in the ailerons/flaps ...

I used a reflex profile once on a tailess Jetex model which flew really well. Or did you mean undercamber? I used that a lot on gliders and once on a performance power plane. That also disappeared through the clouds at about 4,000 ft despite only having enough gas for 15 seconds; I thought I wouldn't need a dethermaliser on its first powered flight. Damned thing probably reached 500 ft or more in that time under power, and then kept going up. I Got it back months later.

Tcubed

10-25-2008, 09:51 PM

Interesting.
I would adjust reflex into the trailing edges just to get sufficient penetration on windy days. Normally i would avoid that of course, because it's not so efficient. But when you have that much lift you can afford a drop in efficiency just so you can keep up with the wind...The only plane i designed and built with a reflexed camber line (zero pitching moment foil) as the normal operating mode was a three Metre flying wing, which handled amazingly well.

The sailing kayak is very interesting. I personally would research kite sails for that. These seem the ideal-no heeling moment and ultra stowable. The foils then can just be straight up and down. Front foil/daggerboard (you probably already have a pedal operated rudder?) can just be "clipped" on to one side. That way no holes in hull. Don't have to switch it over, the slight asymmetry won't make a bit of difference, especially since there will be no heeling. (there are other very neat and efficient arrangements as well) The only drawback to kites i can see, is you must wear, not tack, always. I don't think that's a big deal, unless you're in narrow water, in which case you'de be paddling anyways.

ancient kayaker

10-25-2008, 11:54 PM

My model was a flying wing too, straight wings with lifting fuselage. Tiny, about 18 inches, but flew well and very stable. I was very careful to get the profile exactly according to instructions on that one. It seemed quite efficient although that always surprised me.

I have not been able to get a kite sail to work. Where I kayak is surrounded by tall trees and full of tree-covered islands. The air currents are too chopped up close to water level for a kite.

My last design had one Bruce foil only and a lifting sail offset from center and also offset from vertical. Conventional foot-operated stern rudder. Sail folded umbrella fashion; the whole thing could be put up or taken down in 30 seconds. Before that I was experimenting with a square-rigged sail and leeboard.

As the current design is evolving, there will be two foils which will rotate to steer with foot pedals, and a stern skeg to keep the hull in line. The whole sail/foil assembly will be removable from the hull, a belly band around the forward hull will hold it down, taut but not tight, and a much tighter line will pass around the cockpit coaming. A quick release on the pivot will allow the rotating part of the assembly to detach and the cockpit band will have a backup quick release so the assembly can just slide off the side if I get into a situation.

I will probably build it in two versions, one with a lifting lateen sail and one with a sail wing as mentioned previously. The hardest part will be to make it rigid enough and still build it in what is just a woodshop; I can cut and drill metal but no welding. I think I have all I need to go ahead now, except for some more Aluminum tubing.

Tcubed

10-27-2008, 01:43 AM

I'm sure you'll come up with viable design solutions, just make sure you let us know what you achieved..

I don't understand what your conditions are, but surely kites are good for catching steady wind above the surface turbulence caused by geological and biological perturabations?

joakimhansson

11-06-2008, 05:06 PM

http://www.foilkayak.com/

Have you seen this page...

ancient kayaker

11-06-2008, 11:25 PM

Yep. The flyak. Bin told about that. Needs at least 225 watt just to beat the regular kayak. To do that you would need to look like the guy in their picture for a start, and have olympic level performance. I doubt if I ever got close to that output; right now at 69 years old I would guess I'm a 75 watt kind of guy. At that output their own numbers show that I can go twice as fast in a regular kayak. However, it's still an amazing piece of engineering.

My intention at this point is a home-made sail rig for a kayak, I need the profile, subject of this thread, for a Bruce foil which will give me a non-heeling rig since a kayak is not stable enough for a regular centerboard or leeboard. However, I was also thinking of a lifting foil as a down-the-road exercise, so mounting the sail rig on the flyak is a fascinating possibility.

The flyak would have a speed of 10k at 225 W which translates to about 11 lb thrust from the sail if I have done my arithmetic right, a bit less as the inefficiency of the paddles is no longer a factor. However, the Bruce foils would create additional drag so I should allow for their drag. A sail of the size I have planned (25 sq ft) should provide enough power. Of course, there would be the question of weight; the displacement of 90 kg given in the web site would need me to lose some flab.

I have another thread going in which I am discussing the shape of hull that would be best. From its shape the flyak is at it best in a straight line, like the kayak I will be using. I have yet another thread discussing a rig in which the Bruce foils and sail rotate separately from the hull for steering; this allows the possibility of almost instant tacking; anyone who has tried to tack in a sailing kayak knows this can be a challenge since they are slow to turn and lack momentum, so one can easily run out of forward motion before the tack is completed. I thought a less directional hull would do a better job than a kayak, possibly a skimming dish shape, like an racing keelboat less the keel and with a flat aft for planing.

Lot of thinking and design work left to do ...

Guest625101138

11-07-2008, 12:07 AM

.......
Despit all my envious and crapulous cynicism, however, it's still an amazing piece of engineering. I do wonder how many of their customers will actually get it up on the foils and for how long.

AK
I understand that if you buy one you get a training course thrown in and it seems to be quite extensive; not just a 5 minute get in and see how you go.

With hydrofoils it is not only power that counts. It is also weight. Hence you need to look at both aspects of the equation. Maybe a fellow weighing 65kg could keep it flying at 225W but if he weighs 80kg he will need proportionally more power.

The flyak should outperform a single scull over a 2000m course as the power levels required to keep it flying are within the potential of an Olympic athlete for the 4 to 5 minutes it would take to cover 2000m.

I have tried foiling with pedals and can just get the boat to lift clear of the water but there are lots of control factors that need to be considered and I do not have the power reserves to be bothered with the effort. It simply takes all my concentration to get enough power to fly. You can fly at lower power levels but you go slower than a displacement boat. The break even for better speed is around 250W if you can build it really light and make really good foils.

Rick W

ancient kayaker

11-07-2008, 10:11 AM

I agree.

I don't have human power levels for activities of a few minutes but according to a Wikipedia article an elite athlete can achieve 6W/kg for more than an hour. You can half that for a regular, in-shape male off the street. The Flyak figures are given for 90 kg displacement so deducting 15 kg for the boat we have a 75 kg paddler achieving 225 kw. At that output the Flyak's speed is given as 3 m/s or about 6 k, not enough to beat sculls or a decent kayak; but our paddler could certainly do better for only 4 minutes and the flyak's speed increases linearly, according to the graph given.

I doubt if the foils I would be able to build would be good enough, but I can get more power from a sail so foiling is still a possibility; not my prime objective though.

sigurd

01-29-2009, 04:34 AM

Ancient, What sort of kite did you use? There are big differences in low wind usability and stability. The development rate for them in the recent past has been insane. It is ofcourse a downer if you have to pull back the kite when the wind dies for a minute. If you can paddle during that minute you may keep it aloft on the apparent wind. The bridled soft foil kites are the lightest and can have a high CL together with efficiency. Dropping them in the drink and relaunching without pulling them back in is also possible with some models (closed leading edge), though they will be wet and a bit heavier.

EDIT: By the way, can't you please make a rigid wing kite, like a sailplane? You said your planes all dissapeared in thermals, if you can make them strong enough too, it could be the most efficient kites yet.

"The only drawback to kites i can see, is you must wear, not tack, always. "

It's not necerily true.

tohid padashi

01-29-2009, 04:48 AM

I need inrormations aboat design of high speed craft.tanke you

ancient kayaker

01-29-2009, 11:03 AM

Sigurd: it was a very small kite, fabric over a rigid frame. The real problem was I had to launch from inside the boat rather than on the beach as I believe is normal, due to wind patterns. It got dunked first time and just didn't want to go back up there again. For me kiting was an interesting theory and an unsatisfactory practise, something I don't really want to do again. I accept that a better kite, with proper training would do a better job but ...

Tohid: I think you are in the wrong thread. I see you joined recently, welcome to the forum. To start a new thread for your topic click on the "Boat Desing" link at the top of this page then click on "New Thread" at the top of the new page. State the topic clearly in the title to attract attention from the memebrs.

Guest625101138

01-29-2009, 03:14 PM

I watched a fellow trying to launch a kite from a canoe in flukey conditions. He only managed to get very wet and ended up on the lee shore with a long walk back carrying the canoe. He did not have a paddle.

It was entertaining.

Like many things it no doubt takes practice. Steady light breeze would be nice as well during initial attempts.

Rick W

ancient kayaker

01-30-2009, 12:38 AM

Steady light breeze would be nice as well during initial attempts.

Rick W

That's what I don't ever get at my local pond. I can sail Lake Simcoe at Barrie where there's usually a good breeze, but the beach that has an offshore prevailing breeze is swimmers only except for a tiny stretch that is masked by buildings and a big stretch with nasty rocks.

There's also a couple of high-rises that have monstrous wind shadows, I couldn't believe how far and how much they killed the wind. Sail a kite board behind those and its a long paddle home with a wet kite!

I forgot the other factor I have to deal with locally: two bridges with about 6 ft clearance. Maybe I should just move!

Guest625101138

01-30-2009, 01:15 AM

I have been tempted to buy a small kite to play with on one of my pedal boats but it has such limited scope that I think it would be a waste of money.

I think they offer value in the open ocean and I am not inclined to take a small boat into that situation.

Rick W

Munter

01-30-2009, 02:49 AM

Not exactly hydrofoil profiles but seeing as we're wandering...
I have kited from a canoe. I took a small (4m) ram air on a trip with my girl to see if I couldn't get a bit of a free ride along some section. See photo attached.
http://picasaweb.google.com/lh/photo/Kr8utxoExKWOznd_bmAXTQ?authkey=Gf4SiMLXL38&feat=directlink
The kite flew off handles and despite a relatively steady breeze I had big control issues. I had to fly the kite from the mid-point of the canoe and found directional control much harder than on a kite board. Mostly it was a trawling exercise. I take some pride in the small bow wave shown in the pic though - proof I did get it going (a bit...).