Foiler Design

[Berth]

Bear in mind the H105 was designed as a low speed hydrofoil section, and cavitation wasn't a design consideration. It was intended to operate in the under-30 kt range, and also work well at the Reynolds numbers of a half-scale model. So it's a good match for a beach cat or Moth, but it's no record-breaker. By 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

 

[tspeer]

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.

 

[National3434]

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!

 

[National3434]

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.)

 

[alans]

That sounds about right, although the foil loading may be on the low side. I scaled down the E817 from its normal thickness of 11% to 7% and 5%. Camber was scaled along with the thickness.
Follow the above post by Tom Speer, he and I have continued a dialogue off line. Some of the content of that communication may be of interest to other foilers and is posted below.
ALANS "I am emailing you external to the forum because I have not figured out how to insert graphs/.gif's/.jpg's into replies. Attached are plots from Xfoil for a foil section (7% thick symmetrical 1 deg flap) that I have been working up as part of the "Kooee" design package. Overlaid on your "Incipient Cavitation" plot these suggests a speed of 55 knots is possible, or am I miss using the Xfoil data.
At maximum estimated speed (about 55 knots) Kooee requires a leeward foil with a Cl of plus 0.07 and a windward foil with a Cl of minus 0.06; hence my decision to rely on flap angle to generate the lift. The section is symmetrical with the forward section of the foil at zero angle of attack."
TSPEER "I think the thin flapped section is a perfectly reasonable approach. The E817 is practically a symmetrical section with a flap when you look at its
camber line. I'd be interesting to see lines of constant flap deflection on your avitation diagram. I wonder if there'd be an envelope of flap/angle of attack for best cavitation performance, or if the flapped results would pretty much lie on top of the unflapped results."
ALANS "Re flap angle - angle of attack trade off; to reduce possibility of cavitation.
I have fiddled with it, basically the "Kooee" configuration does not permit any variation of alignment of the front section of the main foils, except by pitching her nose up as part of lift out.
However at very small Cl's there does tend to be a very small improvement in using a slight nose down ( negative alpha) attitude in conjunction with flap. 1.5 degrees of flap plus alpha = -0.2 or there a bouts is a miniscule improvement."

ALANS "For interest I have attached some acceleration plots of Kooee through the lift out phase. ..... These plots ignore the cavitation boundary. The slow acceleration is my simulations "autopilot" keeping the Cl on the front foil from stalling. Initially all three foils lift and progressively the windward foil pulls down. Once Kooee is above minimum drag speed all up weight has no significant effect on top speed!!"
TSPEER "How do you plan to stabilize the boat at zero speed?"
ALANS "The main cross beam (which carries the main foils) would have two small buoyant pontoons, sufficient to stabilize her for easily getting in and out, once under way the foils will do the job even when she is not actually foiling."
TSPEER "You want to go 50 kt, which means you will
need winds of, say, 15 - 20 kt. I'm skeptical that tiny pontoons are going to do the job for you.
....as time goes on the amas get longer, and longer..."
ALANS "When I speak of small pontoon I meant small hulls of a suitable size to get to a lift out speed of a round 8 knots. Windrider Rave demonstrates the feasibility of this aspect of foiling and if Kooee needs outrig hulls the size of Rave's so be it."
ALANS (Having looked at the numbers)"The "pontoon" needs 400 newtons of buoyancy to get kooee underway. At 4 knots the foils can provide all the balancing moment needed to continue to accelerate. At 8 to 9 knots she will lift out on the foils.
A pontoon volume of 400*2/9.8/1000 m^3 or just under 3 cubic feet is all that is required."
TSPEER "But your boat will need to have a fairly high takeoff speed
or its fully submerged foils will have too much wetted area for the ...
speeds you want to hit. Whenever I've talked to Greg Ketterman he's
impressed upon me the importance of erring on the small side when it comes
to sizing the hydrofoils."
ALANS "Re pontoon sizes I am well aware of the issues you raise...The main cross beam (wing) is well clear of the water and about 10 degrees of heel are necessary to put a pontoon in the water. Once above 4 knots the foils easily manage the heeling moment and both pontoons will be well clear of the water. The trimaran diagonal pitching problem does not exist. Our experience in foiling with the skiff (6.5 foot tail arm) has shown that accurate height holding is not difficult even in choppy conditions (20 knots, 0.5 to 1 nm from a windward shore). The main issue is to get the c of g forward and the c of p aft to the point were the boat is dynamically stable in pitch, directly analogous to longitudinal stability of an aircraft. The roll characteristics of kooee are much more placid than an aircraft and holding "wings level" will not be difficult. Significantly easier than holding a constant heel angle on a sail board and that is certainly not difficult. My pontoons are more akin to wing tip floats on a seaplane than the two amas of a trimaran. As with float planes I do not dispute that there is trauma if you put a wing tip in the water at any significant speed.

I believe that if Kooee's contributes to the issues, it is in a change of approach in regard to height keeping, the surface piercing struts that operate at zero Cl, a configuration that makes much greater use of the available wind power than can be achieved with configurations which generate the majority of their righting moment from all up weight and dynamic stability. At speed Kooee can tolerate more than 6 times the side force of a conventional trimaran of the same size and weight. Both passive "ladder" and deep running foils are legitimate approaches to foiler design and you can expect a significant success with Basiliscus. I also think most designers are forced to ladder foils (and surface piercing foils)because they do not understand how to stabilize deep running foils. In kooee's case, as with aircraft, the human endurance factor limits the use of un-augmented man in the loop control. An ocean going Kooee would be a monster requiring a significant auto pilot with hydraulic actuation of the control surfaces, although the majority of the hydraulic pressure require could actually be derived from the dynamic head generated by the speed though the water.

The issues you raised re foil sizes, lift out speed and maximum speed are also real, but the beauty of today's situation is that these issues yield to simulation and sensitivity/tradeoff studies point the way forward. Kooee could be configured to achieve a total L/D of nearly 4 but would not lift out easily, she would hit cavitation problem at lower speeds and could only foil in a very limited wind speed range. At a L/D of 2.5 she is close to a best compromise, lift out at about 8 knots and an estimated maximum speed in perfect circumstance of over 50 knots." (I failed to state that the above L/D was a total system figure not sideforce divided by water drag)

TSPEER "These sound like very low L/D's, even if they only concern the side force vector divided by the drag. I'd think you'd be aiming at something twice that.


ALANS ( To clarify the lift drag numbers) "listed below is a summary of data from Kooee's near best (on paper) performance point. The foils are sized to achieve very low Cl's as previously discussed hence the "poor" L/D of 8.6



Kooee performance point
splay angle 30 degrees
Gama 90 degrees
Vs 27.2 m/s
Va 28.2 m/s
Beta 20.7 degrees
lee foil load 6260 n
wind foil load -4500 n
fwd foil load 2008 n
sail lift 5562 n
air drag 514 n
Fxs 1480 n
Fys 5280 n
drag water 1480 n
induced drag water 76 n
L/D air 10.8
(Sum foil load)/drag 8.63
Fys/drag water 5.3
Cl lee foil 0.07
Cl sail 0.7
system L/D 2.72
1/beta 2.77

Our private exchange of email has been very useful to me, should we put an edited version up in the boatdesign forum?"
TSPEER "L/D of 8+ sounds more reasonable. Even Alexander Graham Bell got 8 on his
HD-4 in the 1920's.
Sure - feel free to post."
ALANS "Just to complete the picture, using the same model as for the previous set of data but ignoring the incipient cavitation boundary and reducing the foil area, and only the foil area, to achieve a hypothetical maximum speed, yields the following numbers. Unfortunately with this size foils the IC boundary is at about 35 knots! and lift out would be delayed to about 20 knots. Large amas would certainly be necessary!

Kooee performance point
splay angle 30.00 degrees
Gamma 95.00 degrees
Vs 41.58 m/s
Va 41.91 m/s
Beta 13.75 degrees
lee foil load 14452 n
wind foil load -11759 n
fwd foil load 1392 n
sail lift 13112 n
air drag 1549 n
Fxs 1480 n
Fys 13105 n
drag water 1611 n
induced drag water 703 n
L/D air 8.47
(Sum foil load)/drag 17.13
Fys/drag water 8.13
Cl lee foil 0.24
Cl sail 0.71
system L/D 4.23
1/beta 4.17
TSPEER "4x wind speed - not too shabby! That's like a typical landsailer.
Curious that nobody's talking about sweep angle to extend the cavitation range. Your 35 kt IC might get you 50 kt with a 45 deg. sweep angle...

 

[tspeer]

...What about putting an autorotating rotor on the bottom of a centreboard and foiling on it? ...
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...

 

[National3434]

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

 

[boogie]

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?

Curious that nobody's talking about sweep angle to extend the cavitation range. Your 35 kt IC might get you 50 kt with a 45 deg. sweep angle...

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

thnx
boogie

 

[brian eiland]

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.

 

[tspeer]

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.

 

[boogie]

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

hi alan,

Once Kooee is above minimum drag speed all up weight has no significant effect on top speed!!"

The main issue is to get the c of g forward and the c of p aft to the point were the boat is dynamically stable in pitch, directly analogous to longitudinal stability of an aircraft.

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

 

[NiklasL]

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)

 

[tspeer]

...I wonder if the vortex at the tips of a hydrofoil is a problem and if it could be reduced.

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.

 

[Wardi]

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

 

[tspeer]

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.