Design of flaps on a curved foil.

[MechaNik]

If the flap was made of a flexible material would a drawstring cable down the inside of the tail give sufficient deflection?
Only one direction of deflection into the curve but this seems like a simple solution to gain some control factor.
Are tail flap even very efficient? Should you not be looking at total fin control? If so would it not be better to look at a pivoting stock that the sliding foil would be mounted inside of. Then you would be free to develop/replace foils separately without making them individually controllable. Perhaps just change the profile inside of the stock.

[Doug Lord]

Originally Posted by MechaNik

If the flap was made of a flexible material would a drawstring cable down the inside of the tail give sufficient deflection?
Only one direction of deflection into the curve but this seems like a simple solution to gain some control factor.
Are tail flap even very efficient? Should you not be looking at total fin control? If so would it not be better to look at a pivoting stock that the sliding foil would be mounted inside of. Then you would be free to develop/replace foils separately without making them individually controllable. Perhaps just change the profile inside of the stock.

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The Hobie Trifoiler foil system works by moving the whole foil using "feelers" forward to sense the waters surface. A system like that could potentially be adapted to use a retractable curved foil mounted in a trunk. And there are now at least one variation on the Trifoiler system that uses a trailing "paddle" for lack of a better description to do the same thing.
I'm interested in adapting a curved foil w/flap to a wand altitude sensor which I feel is simple and robust should the main foils ever strike anything.
I'm building some fairly big model foils(20" X 3") that I will experiment with.
I appreciate the suggestions!

[tspeer]

Originally Posted by bscally

The challenge is the implementation of flaps on a hydrofoil.

This has been performed on foils of regular section T and V, but too our knowledge never on J or C foils.

J and C foils offer higher efficiency and are attractive for choices where this is the driving design goal.

First of all, I'm not convinced that a J or C foil is necessarily more efficient than a T foil. The J and C foils are often used to meet class rule restrictions, like staying within a maximum beam, being able to insert the board from the top, or fully retract it. They may be more appropriate for a non-flying boat that uses a combination of foil lift and buoyancy, but does not lift the hull clear of the water.

However, the first question to answer is, "What are the requirements?" The flap might be implemented a number of ways, depending on what the purpose of the flap is. A flap may not be the best way to meet the requirements in the first place - control of the foil rake or cant may be more appropriate to a curved foil.

Is the purpose to control the vertical force for heave control or to optimize the vertical and horizontal lift distributions for minimum drag? If it's the former, then maybe you only need to flatten the horizontal part of the J and use a conventional flap. If it's the latter, then you may need a number of flaps distributed along the span, or a flexible system that can be twisted or morphed. The range of trailing edge deflection may be less for the latter case, too.

What kind of control system is available? A fly-by-wire control system can operate a multitude of small flaps, which could be easier to implement on a C foil. I wouldn't rule out doing this for a mechanical control system, either.

In any event, the first step is to define the hydrodynamic objectives with regard to the magnitude and spanwise distribution of the changes in lift you want to achieve with the flaps.

[ancient kayaker]

I recall suggesting a pressure sensor as an alternative to a wand some time back on another thread. I don't remember how it was received but it seems to me more robust, with less wiring and probably more powerful. I am thinking more alng the lines of a bulb than a cylinder/piston system.

[tspeer]

Originally Posted by ancient kayaker

I recall suggesting a pressure sensor as an alternative to a wand some time back on another thread. I don't remember how it was received but it seems to me more robust, with less wiring and probably more powerful. I am thinking more alng the lines of a bulb than a cylinder/piston system.

Pressure sensing has its own problems. The purpose of a wand is to measure height above the water and this is complicated to do with pressure measurements. The local pressure at a sensor is going to be a function of the static pressure at the depth of the pressure port, dynamic pressure from the speed of the boat, shape of the foil/bulb and position of the pressure sensor, angle of attack, flap deflection, and leeway angle. In order to figure out what the depth is, you'd need to measure and accound for all those other influences.

[ancient kayaker]

Originally Posted by tspeer

Pressure sensing has its own problems. The purpose of a wand is to measure height above the water and this is complicated to do with pressure measurements. The local pressure at a sensor is going to be a function of the static pressure at the depth of the pressure port, dynamic pressure from the speed of the boat, shape of the foil/bulb and position of the pressure sensor, angle of attack, flap deflection, and leeway angle. In order to figure out what the depth is, you'd need to measure and accound for all those other influences.

It sounds like a nice compact, even self-contained system, and height above water is going to vary the same as depth of foil. The pressure sensor should even out the rapid fluctuations of a surface-following sensor, and a narrow tube connecting the sensor to the actuator would act as an additional filter. Some changes may be needed on a prototype to optimize bulb design and find the best sensor location to minimize hydrodynamic interference, but I suspect that would not be a problem in practice. Has it been tried?

[tspeer]

A pressure sensor will typically respond rapidly to pressure changes, and will "see" every wave that passes. The measurements can be filtered to average them out, but that will add phase lag that will destabilize the control system. The boat will be responding to "yesterday's" altitude.

Pitot-static probes can be designed to minimize position error, but will need to project well ahead of the foil to do so. Which will make them vulnerable to damage, not to mention even more of a weed catcher than a foil already is. But even a properly designed probe will still be subject to some speed, angle of attack and leeway dependent position error that must be accounted for with calibrations. This is the same problem as determining barometric altitude for an airplane.

Another type of height sensor that can be used is ultrasonic. Some time ago, experimenters used the ultrasound range finders from Polaroid cameras that were used to drive the camera focus. Some WIG craft have also used ultrasonic altimeters. In principle, an ultrasound range sensor could look down at the water or up at the surface.

Regardless of the type of sensor, if it is electronic it will need to be combined with a powered actuator of some sort, and this runs afoul of the prohibition in the racing rules of sailing against the use of stored energy. One advantage of the wand is it not only senses the water's surface, it also transmits the force necessary to move the flap for control of the boat.

[ancient kayaker]

Another advantage of the surface sensing wand is it can be arranged to "look ahead" a upcoming conditions.