Foil Assist for Windsurfer

Hi All!
New poster here.
Want to put this question out here as I have read many great threads on foils on this message board.

Background
- formula windsurf is a very fast and controllable sailing craft in 8-25 knots wind (not much improvement needed)
- under 8 knots or in lulls on a racecourse, a formula board is dead-weight and cannot be sailed around a course very effectively
- compromises have included the current Olympic rsx board (mini formula with a centreboard)
- the goal of this board would be to be equal or better than the rsx in 4-8 knots and sail like a regular formula in +8 knots (which will smoke a rsx board)

Idea
- add one foil to the middle of a formula windsurf board
- use as a foil assist - foil takes +/-2/3 of weight, back of board takes 1/3
- retractable
- for use in 4-8 knots of wind
- goal - allow the board to plane in 4-8 knots of wind and be retracted over 8 knots when a formula board is very fast, controllable, etc...
- possible foil ideas
1. canted centreboard made very flexible
2. retractable horizontal foil
3. surface piercing v foils mounted to side of board
4. dss type foil extensions - Protrude out from hull

Questions
- would it work? (would the board plan with a foil assist?)
- what size and shape of foil would be ideal

Thanks
Joe

 

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Welcome to the forum! You might be able to gain something in 5-8 kts of wind but you have to use the formula in this thread for a close approximation: http://www.boatdesign.net/forums/sailboats/hydrofoil-formula-48599.html
Just to try it out use a CL of .4 and then .6. Remember to convert to ft.per second instead of knots.
My gut feeling is that you won't gain much if anything. But try the formula and see what size foil you'd need for the wind range above. I'm thinking a t-foil would be best-a surface piercing foil set would require a lot of area, DSS foils wouldn't work because they need to be submerged at least 1 chord below the surface -and their primary use is to increase righting moment on a hull designed for them. But these are just guesses until you come up with the required foil area. Get the area then try that area in the various configurations you've mentioned and see what looks like it will work best.

 

You might look at what the kite boarders are doing. I think the real answer depends on a lot of factors and needs a velocity prediction program. I believe there's a simple one for foilers by Alan Smith.

lohring Miller

 

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Lohring, Alan sent me this program a ways back-is this the one you were refering to? Set up for a monofoiler:
Jeff, the moderator and owner made it possible for this to be uploaded.

NOTE-this program does not reflect improvements made over the last three years. The newer version may be released before too long.


NOTE: the program below is not to be used for any commercial purpose whatsoever without written permission from Alan Smith.

 

You might want to look into the Miller sailboard. And le Foilboard.

 

Doug, that's the program I was talking about, thanks. I couldn't remember where I downloaded it. You might also read Tom Speer's post on foiler stability that I copied below. The smaller the foiler, the more it can be balanced by crew weight with fixed foils. As the Moth class is proving, it's all in the details.

Lohring Miller

Quote:
Originally Posted by ancient kayaker
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1) Aircraft wings do not operate just below a medium discontinuity (the surface) like boat foils.

2) Boat foils do not have to operate during takeoff and landing just above a medium discontinuity (the ground) like aircraft wings.
Above or below is irrelevant. What matters is the nature of the surface. Solid ground presents one kind of boundary condition, while the compliance of the free surface of water at high speed presents a different boundary condition.

There are two aspects to consider. One is the effect of a free surface, which is mostly a performance issue, although Russian hydrofoils designed to operate on the flat water of rivers have used the drop-off in lift near the surface to help regulate height.

The other is the precision necessary to avoid broaching the surface. Flight path precision is a matter of specifying the performance of the control system, and this is no different from aircraft.

Analytically, both kinds of surface condition can be handled with the method of images. The only difference between the solid surface and the linearized high-speed free surface is the sign of the singularity strengths used in the image system.
Quote:
3) An aircraft can lose far more height than a foiler without coming to grief.
This is another precision of control issue. There is a quantitative difference, but not a qualitative difference.
Quote:
...It has been stated that for static flight stability the center of gravity must be in front of the neutral lift point, which is the center of area of combined main wing and stabilizer. This is true for aircraft, canard or conventional, where the rear flight surface must operate at a lower angle of incidence (alpha) than the front. This ensures the front surface is closer to stalling; then if the nose rises and alpha increases forward lift is reduced and the nose drops...
That's not quite right. The rear surface must be more lightly loaded than the forward surface for static pitch stability. The incidence of the rear surface will depend on the camber of both surfaces, downwash from the forward surface, etc., and could be less than or greater than the forward surface. And stall doesn't enter into it. Even in the linear lift range, far from stall, the rear surface needs to be more lightly loaded for pitch stability. The relative loading of the two surfaces ensures that for a positive change in angle of attack there will be a negative change in the pitching moment.

In addition to the static pitch stability, there is an additional static stability criterion that has to do with pitch-heave coupling. This criterion requires that a positive change in height (at constant angle of attack) generates a negative pitching moment. Another way of stating this is the "heave stiffness" of the forward foil must be greater than the heave stiffness of the aft foil. There are several ways of achieving this. One way is to use a surface piercing forward foil and a fully submerged aft foil. Hydroptre is of this variety. Another way is to use height feedback to the forward foil, ala the Bradfield system used on the Rave and Moth classes.
Quote:
If manual CHAP is intended the rule should probably be obeyed; probably but not certainly, because the theory of foiler flight is not yet mature. However, the rule is ignored in aircraft using fly-by-wire, and a foiling boat that uses feedback from a surface sensor for CHAP has the equivalent of fly-by-wire.
The theory of stability and control of flying vehicles is quite mature. The International Hydrofoil Society publishes CDROMs with hydrofoil technical literature. CD#1 has a hydrofoil design manual by Hydronautics that lays out the theory of hydrofoil stability and control. There are also NACA reports on hydrofoil dynamics. Joddy Chapman has also published their analysis of sailing hydrofoil dynamics. The theory of foiler flight is out there, if one researches the literature.

The difficulty is not in the understanding of the dynamics or solving the equations of motion to determine the stability or transient motion. The hard part is coming up with the right numbers for the stability derivatives that accurately represent the characteristics of the configuration. This is why companies spend big bucks on wind tunnel and tank tests to get these numbers. Despite the expense, it is still cheaper than experimenting at full scale for large vessels. For small sailing hydrofoils, it's just the opposite - it's cheaper and faster to use cut-and-try methods than to use sophisticate analytical methods. Nevertheless, the Chapmans found a simple dynamic analysis gave a lot of insight into the problems and made it much easier to find a solution.

Fly-by-wire control does not ignore the requirement for static stability. Instead, FbW augments the stability using feedback control. The mechanical feedback of height to flap from the wand of the Bradfield control system augments the heave stiffness of the forward foil so that it meets the requirement for stable pitch-heave coupling. Instead of losing lift due to loss of area as the craft rises, as would be the case for a passive surface-piercing foil, the lift is reduced by the flap deflection. The effect of the feedback can be analyzed as an equivalent characteristic of the foil itself
Quote:
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I believe a boat using immersed foils should have a single lifting foil and a single control foil, much like an aircraft.
The problem with this approach is control of height using pitch control has a great deal of time lag. This can destabilize the system and it reduces the bandwidth and precision of the height control. To see why, imagine how the system has to respond to an error in the flying height. In order to return to the correct height using pitch control, the elevator first has to be deflected in response to the height error. Then the craft has to accelerate in pitch, achieve a pitch rate, and finally get the desired change in angle of attack to change the lift on the forward foil. Then the pitch rate has to be arrested and pitch attitude maintained as the craft glides or climbs back to the desired height. The pitch control has to start leveling out before the desired height is reached. If the pitch control isn't changed until the height error goes to zero, the craft will overshoot the desired height until the height error is sufficient in the opposite direction to change the pitch control enough to level out. But now there's a new error in the opposite direction, and the whole process starts over again.

Direct lift control for regulating height does not have to wait for the pitch attitude to change in order to generate a change in lift. The vertical acceleration is immediate, as soon as the feedback of height error moves the flap. Flap deflection is reduced as the craft approaches the desired height, again without having to wait for a change in pitch angle.

Control of height with pitch does have its place. But it is best used for low-frequency trim to optimize performance, rather than high-frequency regulation of height due to waves and disturbances. Direct lift is limited to the flap deflection available, and this is soon saturated. So direct lift should be washed out with time and the steady-state equilibrium achieved with pitch control. This returns the flap to its neutral position, making maximum control power available for transient height regulation. Washout of the flap can happen naturally through the dynamics, as pitch trim changes the lift. Because the lift has to equal the weight in the steady state, as more of the lift is produced by the change in angle of attack, the feedback to the flap will naturally reduce the flap deflection.

This is why it is useful to have a flap or incidence control on the stern foil to adjust pitch trim, in addition to wand feedback to the forward foil. Because it is a low-frequency function, it can be manually controlled.

In a purely automated system, one could get the same effect by using proportional control from height feedback to flap deflection, but integral compensation from height feedback to pitch control. Mounting the height sensor forward of the forward foil adds lead to the pitch control that improves the damping of the system, and anticipates the change in height at the foil as waves approach.
Quote:
...The boat should ideally be balanced on the lifting foil and the control foil should be only large enough to react necessary movement of crew. In a boat perfectly balanced over the lifting foil the movements of the control foil can be very slight, letting the inertia of the boat average out pitch fluctuations and requiring only long-term, slow response control over the average height.
The ideal split between the load carried by the forward foil and the aft foil is a performance issue that depends a great deal on the combined spanloading of the two surfaces. Although the aft foil has to carry less load per unit area than the forward foil to be statically stable in pitch, that doesn't mean it can't carry the majority of the total load if it is larger than the forward foil. Good examples of this approach are Don Nigg's Flying Fish, the Miller foiling sailboard and Sam Sutt's hydrofoils.
Quote:
However, this concept does not accommodate the bow-down moment from the rig. Some changes are required for this and several solutions come to mind.
The pitching moment from the rig is an issue of pitch trim. Most of what we have been discussing has to do with stability. Stability and trim are two separate issues. Stability has to do with the changes in the forces and moments as the craft is disturbed from equilibrium. Trim has to do with the forces and moments at equilibrium.

Trimming the moments from the rig is mainly an issue of having enough control authority to handle the aerodynamic moments and still have enough control left to handle the transient disturbances and maneuver the boat.

Feedback control with integral compensation is also very useful in achieving the control deflections necessary to reach and maintain the equilibrium.
Quote:
...I personally am inclined towards solving the problem of bow-down moment in the air since any such arrangement would also handle gusts.
I think there would be severe performance implications from trying to produce a rig that is pitch-neutral. There are no performance penalties from moving weight aft relative to foil center of lift to trim out the pitching moments from the rig. However, aft centers of gravity make it more difficult to achieve positive static stability.
Quote:
A foiler must transition from displacement (or planing) mode to flying mode.

In displacement mode it is probably easier to raise the bow than to lower the stern for takeoff. The CoG is generally closer to the stern than the bow when sailing so it makes sense to me to use a forward location for the control foil.
Dragging the stern can also lead to unstable pitch-heave coupling. The "stiffness" of the stern buoyancy is high compared to heave stiffness of the forward foil. This is the opposite of what's required of stable pitch-heave coupling. As the foil lifts the craft with the stern dragging, it will tend to rotate about the stern, increasing the pitch attitude and angle of attack, and increasing the lift. Which makes the foil lift the craft up even more. Which increases the pitch more, and the whole process feeds on itself. The completely stable craft would wheel-barrow, lifting the stern first and balancing on top of the foil as it took off.

However, the unstable situation of lifting by the foil and dragging the stern doesn't have to last very long. As soon as the stern lifts out, stability is restored. It's quite possible to exploit this behavior. When sailing hullborne, the foils can be set to produce a low level of lift, minimizing the drag while accelerating to takeoff speed. When the speed is high enough for the forward foil to start lifting the bow (while still carrying part of the weight buoyantly in the stern), the unstable pitch-heave coupling will cause a momentary runaway of the pitch angle, generating enough lift to raise the whole craft. As the stern leaves the water, the normal pitch stability can reduce the pitch attitude and level the craft out. This makes the boat pop out of the water when it reaches takeoff speed. The trick is to manage the transient so that it doesn't become a sudden shoot-for-the-sky-and-crash-back kind of behavior.
Quote:
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I am reminded of a method I once used to automate “liftoff” of a highspeed catapult-launched model glider. It had a delta wing with an elevator that was forced down by air pressure at the high takeoff speed for a smooth climb-out and raised at height by a rubber band to transition into a glide after losing speed. ...
This is the principle behind the spring tab - another aeronautical invention whose theory is well worked out.

You might find it useful to use a spring to hold the flap down on the forward foil. This will cause it to blow up and reduce the lift at high speed, and extend down at low speed when you want more lift.
Quote:
...Once flying, if automated CHAP is used it should be effective but with a minimum of added drag. I am not sure at this point whether the control foil should have a flap or be pivoted. Whichever turns out to be optimum, the sensor and feedback system is critical to success.
Flap control has the advantage that it shifts the drag bucket toward the new operating condition. Pure incidence control does not shift the drag bucket, and the foil may end up operating outside the low-drag region. This can double or triple the profile drag. Depending on the section, size of flap, etc, the optimum may be a combination of incidence and flap deflection. This can be done by gearing the flap deflection to the incidence and controlling the two with a single feedback.

In order to minimize the impact to induced drag, it's important that full-span flaps be used.
Quote:
Most of the successful systems to date seem to use a surface sensing wand with a mechanical linkage. Other than surface piercing foils, there does not seem to be many alternatives in use. I will try to think out of the box and throw out some ideas.
There have been a number of different systems used. Greg Ketterman uses planing floats to rotate the entire ama of the Trifoiler and change the incidence of the foils for height control. The Shutt strut invented by Sam Shutt uses a small planing float attached to a floating forward foil to change the incidence of the foil, and the foil then rotates the entire craft to change the incidence of the aft foil, which is the main lifting foil. The Hook hydrofoil system uses planing surface out ahead of the boat that were linked to the hydrofoils.

If one wants to use electronic sensors and powered actuators, ultrasound sensors (like the ones used in early auto-focus cameras) can look down to measure the height above water. Laser altimeters use two converging beams and the distance between the reflected spots of light is proportional to the height.
Quote:
If the forces required for pitch control can be kept small enough, it may be possible to use the variable buoyancy of a tapered strut for fine pitch control, using the control foil for manual, coarse corrections.
Grumman developed a system in which a vertical strut was attached directly to the flap. Drag on the strut deflected the flap down as the hydrofoil was immersed, balanced against the hinge moment of the flap. It's hard to imagine a simpler, purely mechanical system, as the flap+strut was the only moving part. No buoyancy was involved. It was purely a balance between the hydrodynamic hinge moments - this made it self-compensating for changes in the speed of the craft. There are papers describing this system on the IHS AMV CDs.

[/quote]Another depth sensing arrangement not yet tried as far as I know uses hydrostatic pressure feedback....[/quote]
I think you'll find this has been tried. There are a couple of problems with it. First, there's the amount of control power available from the change in pressure. You're only talking a small force unless you let the pressure act on a very large surface area.

And then there's the change in static pressure due to the speed of the craft. You need to locate the static source at a location that naturally has a pressure coefficient near zero for the range of angles of attack, etc., or else you need to measure total pressure as well and compensate for the position error at the static source.
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Tom Speer

 

Foil Assist

Thanks, Lohring-I have that classic saved as well.

 

This is a popular one with Kite Boarders

 

Great stuff guys!
It does look like i would need a large foil (1 m x .35 m by my untrained calculations) to get even 130lbs of lift at 4 knots speed.
Probably not worth it.
Although the foils on the kiteboards and windsurf boards don't seem to have very much surface area for producing alot of lift at low speeds.
I still would like to try it though - it would be great to even get a bit more subplaning speed.
Joe

 

Joe, don't hesitate to experiment-great things can happen!