Foiler Design

Hi Tom,

So would you think, your H105 (or H106?) suits better to a foiling moth than for example NACA63412? What would be the benefits?

Thanks

berthold

 

Probably the most important difference is it would be less susceptible to leading edge stall at low speeds. It also has a wider drag bucket.

 

News and Pictures of PStevo's new built foiler Moth can be found at.
http://scott.projectsomewhere.com/10039,02,2-0-Phil Stevo's new foiler.html

Congrats to Phil who's building process is a lot faster than mine!

 

Hydrogyro

If this was posted on April 1st you might think it was a spoof. But it is intended as a brain teaser which might actually work.

An autogyro is a type of aircraft that gains its lift from an unpowered rotor. It has to be driven forward by a conventional prop and before takeoff the rotor has to be started (usually manually) as the prop give forward velocity. From then on it freewheels.

Well, you can guess what I am going to say can't you!

What about putting an autorotating rotor on the bottom of a centreboard and foiling on it? You would need to start it rotating when you need lift (pull starter from mower or outboard motor?).

The key advantage I can see is that when not rotating it is low in drag and could even be aligned with the foil if 2 bladed.

Apparently autogyros suffer from the problem that if you allow the rotor to experience negative incidence, the rotor decelerates and tries to reverse rotation, lift goes to zero and it falls like a brick. In the flying variety they are careful to train this trick out of the pilots. But would this effect in a hydrofoil be any worse than the ever present risk of taking negative incidence on a 'normal' foil and going down the mine?

Maybe someone has thought of or done this before. If not, you heard it on the boatdesign foiler thread first!
(Why tell the world? To stop some bright spark trying to patent it and spoil the fun.)

 

There's been at least one rotary hydrofoil I know of - the hydrocopter.




I don't see any way a rotating hydrofoil would avoid being so draggy that the boat would be better off without it. There's a reason why you don't see autogyro sailplanes...

 

Nothing New Under The Sun

Well, I am glad that the 'Nothing New Under The Sun' principle is still firmly in operation!

Autogyros have been used as sails with 'success' but practical diffucutlies; see http://www.users.globalnet.co.uk/~fsinc/yachts/auto/hist1.htm for the Brabazon Redwing.

One little point in favour of an auto gyro foil: it would work equally well in either direction and hence may be of interest on a proa configuration.

Cheers

 

hi alan and tom,

thnx a lot for copying us in on your interesting discussion.
i am really curious when you are going to fill us in on details about how exactly you are planning to control ride height [even with a stable configuration] and other "real world" factors. where did you get your data from regarding drag and efficiency of sails or rather rigs at speeds of +40kn? you might want to contact someone like barry spanier [Gaastra Sails for windsurfers and he made the sails for Long Shot too], who must have made and designed more sails than anyone else that have gone +40kn.
i really like your approach. it's all nice and well thought trough on a pice of paper [or rather computer screen], but...

how close are you to actually realising this project?

what is the formula behind the statement in the quote above?

thnx
boogie

 

New Buzzword

Just received the new issue of Yachting World and found a few pages and pictures devoted to "Foiling is the new buzzword" ......pages 110-114, June issue.

 

Simple sweep theory says the spanwise component of the flow doesn't count and it's the component normal to the foil's axis that matters. You can see this if you imagine a very long slender foil moving through the water. Now start sliding it toward one end while still moving it forward. In the middle of the foil, the flow remains the same. But this is the same situation you'd have if the foil were swept and moving in the combination of the original direction plus the slip towards one end.

This is why you see swept wings on jet aircraft. With enough sweep, the wing still performs as though it were in subsonic flow, even if the aircraft is flying at supersonic speed. At the root, the symmetry of a typical aft-swept wing dictates that the pressure isobars curve so they are perpendicular to the flow. So it helps to extend the chord at the root and get a lower thickness ratio for the same physical thickness. This, plus some other considerations (like where to stow the landing gear), drives the classical jet transport wing planform.

So the velocity component normal to the foil at 45 degrees of sweep is equivalent to a straight foil moving at 70% of the speed. At 50 kt, the swept foil will "think" it's at 35 kt [Veffective = V * cos(sweep)].

Of course, for a sailing hydrofoil, one doesn't have to make each foil symmetric. The oblique foil would be a logical choice.

 

hi tom,
thnx. informative and to the point as ever.

hi alan,

looking at these two quotes above i have a suggestion how to achieve moving the CoG forward.
once you reach the minimum drag speed and weight plays a smaller role [you still have to accelerate it] you could fill a water ballast tank in the font of the vessel by the motion through the water. i don't know how much weight you need up front to have a sufficient shift in CoG. with large exit vents for the ballast tank you could get rid of it pretty quickly. sure, filling the tank will create drag, but at you point of minimm drag you might have a few percentage points to spare to get to the next level.

i like your idea of having large foils at low CL, but as tom said before i'm sceptical about their drag at higher speed. it seems you have done the numbers though.

if you are going to "fly" kooee manually i could imagine a crew of two.
one to control the sail and a pilot with foot pedals for steering the rudder of the craft and a true pilots steering collum with the wheel to control the roll and pulling on it to lift and pushing on it to get the nose down. maybe with power steering assist.....


all good stuff. keep up the good work.

cheers
boogie

 

Tip vortex reduction

I want to thank you everybody contributing to this forum!!!

There has been great stuff above about effective foildesign. However I wonder if the vortex at the tips of a hydrofoil is a problem and if it could be reduced.

/Niklas Lundberg (Sweden)

 

Everythng one does to reduce induced drag reduces the tip vortices. This inlcudes increasing the span (most effective way), shaping the planform, and adding tip devices (like winglets). The vortex is not shed just at the tip - vorticity is actually shed wherever there's a change in the spanwise lift distribution. (There aren't many hdyrofoil illustrations of this, so I'll have to resort to using aircraft as hydrofoil analogs.)


If the condensation over the wing in the photo above roughly represents the spanwise lift distribution, one can see that its slope is greatest at the ends where the lift has to go to zero. So this is where the shed vorticity is the greatest. If there's a rapid change in lift elsewhere, a vortex will be shed there, too, like at the ends of partial-span flaps:



In between the vortices is the downwash region, in which the flow has been deflected by the foil:


But this downwash affects the foil, too, acting as a downward current that the foil has to continually "climb" - this is the origin of the induced drag.

The best way to reduce the downwash and cut the drag for a given amount of lift is to make the span greater. This deflects a greater amount of fluid a lesser amount, which is a more efficient way of producing the same force.

If the span can't be increased, then the next best thing to do is to optimize the shedding of the vorticity along the span. It turns out the induced drag is mimized when the downwash velocity is constant along the span, and proportional to the dihedral angle of the foil. This applies to struts as well as horizontal foils, and it's possible using computational fluid dynamics to design a planform shape and twist to accomplish this, taking into account the interference between the various components.

It's possible various tip treatments can reduce drag, although it's not easy to design something that doesn't add as much parasite drag as it saves in induced drag. Otherwise, there's no net benefit. The best improvement with the least parasite drag typically comes from extending the physical span.

 

What about this Minix method?
http://www.boatdesign.net/forums/attachment.php?attachmentid=1408&stc=1

 

Indeed, what about it? Show us the data!

According to the Minix website (http://minix.free.fr/redirected/en/index2.htm) the device cuts induced drag by 8%. The same drag reduction could be had by extending the wing span 4%. So the question becomes, is the parasite drag of the Minix less than a 4% wing extension? He doesn't address parasite drag at all that I can see. The 22 July 2003 Flight International article says he's tested it in the wind tunnel, but there are no test data shown on the site.