Foiler Design

hey, been following the forum, this stuff is very juicy.
I built some foils last year for my moth, similar to John Ilett’s early models, as i thought that was a good starting point and a good way to get my head around all the possible problems and solutions etc.

I like the current T foil configuration for its overall underwater simplicity. I have some ideas that could add to its functionality, maybe its pace.

First of all if you can sail on one centreboard foil, why do we have such big foils on the rudder? Could the rudder T foil provide the stability that the submerged canard (or planing canard) provides in the Y foil configuration? I have found my rudder foil to generally be more of a hindrance than a help, as I find myself sitting further aft in the boat when foiling compared to displacement sailing. The lift on the rudder also prevents me from sinking the stern in order to give the foils positive angle of attack when taking off in light airs, and even though it is now smaller than the fastacraft ones I find myself sailing with the flap down on the centreboard and up on the rudder. My foils are both set at +1 degree angle of attack. I gather fastacraft rudders are set at a negative angle to the static waterline? Why is this? And why did they get bigger over the years?

I propose the rudder foil be about half the size, and possibly half the camber, with its sole purpose being attitude adjustment and stability.
It also seems logical to adjust the angle of attack of the rudder foil, and not have a flap, and to do this I guess you would use a shaft as was mentioned pages ago. I see there being a major structural problem with this, not so much the shaft but the bearing, as there is no meat in a 14mm thick foil and the moments are too great, I’m guessing those break a lot?
Could you incorporate a lifting body into the T joint, big enough to provide some structure and improve the flow around the inboard ends of the foil when its pitched?
Have lifting bodies been used before in such a small application? I don’t know that much about them are they efficient? It seems having a large volume at the T joint would provide all sorts of opportunities, such as partially (or fully) retractable foils that escaped inside the lifting body mass, or independently adjustable foils on the centreboard, which could be coupled to a pair of wands, and provide extra righting moment by gaining more lift from the leeward foil and feathering the windward one?

Also since we can now change the angle of attack of the whole foil + - 3 or 4 or 5 degrees, what about some sort of ‘lift lever’ whereby you pull a rope for a momentary burst of lift, maybe to help get you foil-borne in light airs. Works in the same way CDTF yachts can get momentary bursts of windward lateral force. Would the drag caused by that slow you down so that you fall straight back into the water?

 

Foil

Dear

I prepared all airfoil drawing on dxf Format.
http://www.cembercidenizcilik.com/modeldxf/foil.htm

www.cembercidenizcilik.com
oktay@cembercidenizcilik.com
Oktay Çemberci
İstanbul/turkey

 

Is this vortex lift taken account for in the normal calculation of 3d lift curve slope, like you showed it in message #521?

 

When we tried a large wing on the Decavitator to greatly reduce the takeoff speed, the problem wasn't so much the classical induced drag, but the 2-D wave drag. The effect is quantified in Figure 30 of Hoerner's Fluid Dynamic Drag. It shows the 2D value of CD/CL^2 versus the depth-based Froude number Frh = V/sqrt(gh). The peak drag is at Frh = 1.414, where there is a large standing wave over the wing -- almost a hydraulic jump. Here'a photo:
http://lancet.mit.edu/decavitator/images/tsunami.gif
With a chord/depth ratio of about 1.0, the 2D drag in this case is about CD/CL^2 ~ 0.09 which is pretty huge, and made takeoff almost impossible. When we reduced the wing area by roughly a factor of 3 (with a similar aspect ratio), the Frh increased enough to reduce this 2D wave drag, to roughly CD/CL^2 = 0.035, which made takeoff relatively easy. The classical induced drag had little to do with this.

Correct. Something else happens which helps to reduce the required wing size:
As the boat speeds up, the foils lift the hull(s) partially out of the water, which allows more speed, which lifts them out even more... etc. This snowball effect reduces the size of the "takeoff drag hump" and lets you lift off with smaller wings than you'd think at first.

 

What I presented was for a high-aspect ratio (>5) foil operating in the linear lift range.

Low (<3) aspect ratio planforms have a lower lift-curve slope but can go to higher angles of attack and still produce increasing amounts of lift until their maximum lift is greater than high-aspect ratio case.

But the drag is so much higher that the increased lift is really not relevant to the design requirements of a sailing hydrofoil. Getting more lift is easy - just add more area. But reducing the drag is hard. So phenomena like vortex lift that provide a modest increment in lift at the cost of a large increase in drag is not worth pursuing.

 

I think you're on the right track. The main purpose of the aft foil is pitch stability and trim. A down-loaded tail is pretty much the rule in aircraft. There's a natural tendency to think that if the surface isn't lifting upward, then the whole configuration is less efficient because the forward surface has to carry the weight plus the download from the aft surface.

But for the same reason that positive lift in the presence of downwash causes induced drag, negative lift in the presence of downwash actually has a thrust component, so some of the penalty for the tail down-load is recovered. What matters is the spanwise distribution of the combined lift from the forward and aft foils.

Ilan Kroo's research has shown the aft surface should be about one-third the span of the forward surface, and the center of gravity located to provide about a 5% positive static margin. This implies a small down-load on the aft surface for trim. The area of this optimal tail would be < 15% of the area of the wing, so it's definitely in the small-tail/long-arm school of design for providing the required pitch stability.

It's unlikely you could achieve adequate pitch stability with such a surface within the constraints of the Moth class rule, but it may indicate a direction to go. As you compromise on the tail arm, the span of tail needs to grow, but even when the two foils are the same span, the aft foil should be half the chord.

Whether the lift change is done with a flap or with a change in incidence is immaterial from the standpoint of induced drag. A full-span flap can provide a decent shape to the spanloading as it is deflected.

The flap will move the profile drag "bucket". The minimum profile drag can be maintained over the widest range of lift coefficients if there is a combination of flap and incidence change. But I would be inclined to stick with the flap over the all-moving surface as long as the trim deflections were modest. If you need more than about 10 -15 degrees of flap deflection, then the all-moving surface would be the way to go.

I think what you have in mind is not so much a lifting body as a bullet fairing. And they have been used on a lot of hydrofoils as well as aircraft.

A lifting body by itself is very inefficient because of its small span. But in conjuntion with the rest of the foil, it may help to provide carry-over of the lift distribution so you don't have such a dip in the spanwise lift distribution as you would have with an axisymmetric body as the fairing. So I would tend to make it look more like a blended wing-body in shape and design philosophy.

I think there's a lot of merit in adding volume at the joint to provide structural strength and room for control mechanisms. I've taken a first cut at such a fairing, as shown in Figure 22 of my Basiliscus paper. The idea of these fairings was to area rule them in an attempt to reduce the local increase in velocity in the junction between the foils, especially when the foils joined at an acute angle.

If you consider an axisymmetric shape with ogive nose and tail, and cylindrical centerbody, you get a somewhat dumbbell-shaped pressure distribution that has a maximum velocity near the joints with the cylinder. If you neck down the centerbody so the body is dumbbell-shaped, too, you get an even more pronounced set of low-pressure peaks and an increase in pressure in the center.

Now consider the flow in the junction between the hydrofoil and strut. There is an increase in velocity (decrease in pressure) near the maximum thicknesses, a stagnation line along the leading edges (high pressure), and an increase in pressure at the trailing edge. The velocities due to the thickness distributions of the two foils add together near the joint to exaggerate the high-speed/low-pressure region, leading to a lower incipient cavitation speed and steeper pressure increases to the trailing edge that promote boundary layer separation.

These effects can be partially cancelled out by adding a dumbbell-shaped body that is sized so that the low-pressure peaks correspond to the high-pressure regions at the edges of the foil and strut, while the high-pressure region in the middle overlays the low-pressure region in the joint.

The final shaping of the fairing was done by pinching the aft end to flare it out into concave fairings with sharp trailing edges, but which still maintained the increased volume of the dumbbell near the trailing edge. This was done specifically to try to maintain the lift through the junction as you suggest.

I've not done much CFD work, yet, to see how well this concept really works and to optimize the shaping. But I think it has some promise.

This is basically how an aircraft takes off. I accelerates with the nose down, producing little lift and maintaining minimum drag. Then when the speed is fast enough to fly, it rotates the pitch attitude to increase the lift and fly away.

You might want to change the lift on both forward and aft foils simultaneously. Presumably, the wand has already deflected the flap down on the forward foil because the hull is sitting in the water. The lift produced by the foil at low speeds no doubt causes more drag than the buoyancy it offsets, so the acceleration would be better if the flap weren't deflected. If both foils were kept in a low-drag configuration, it would be easier to get to takeoff speed. If you knew accurately what that was!

The big problem, however, is coming off the water with the craft in trim. That is, with the moments balanced. If the moments aren't balanced, it's likely that when you activated the takeoff configuration, the craft would pitch up, zoom out of the water, and crash back down.

 

Tom, I think I didn't quite grasp how you would setup the viscous damper and bobweight thing.

I thought it might be sensible to use a cam or something between the wand and mainfoil flap, so that there were more gain at the ends of travel?
Unlike a lowpass filter there is no delay in the reaction.

For a given speed and AUW, you could center the dead region around the desired ride height by pitching the mainfoil (either directly or by pitching the boat with the rudder foil):

 

just reading miller's pdf on his sailbord foils - am curious if instead of the rolling inverted T canard a fixed U foil could be used.

 

heres an idea for a moth foil incorporating a blended wing and dumbell shaped bullet fairing at the t joint, with one example of a possible method of managing lift, in retractable port and starboard foils sliding on tracks. the benefit is that you can de-power the foil without creating extra drag.
pnumatics would be an alternative to heavy batteries, incorporating a light carbon compressed air cylinder and regulators.

 

Neither do I!

I would figure out what dynamic effects I wanted to achieve, then try to come up with some mechanization that would have those characteristics.

I was thinking of something in parallel with the wand, so the two were added together, instead of being in series with the wand. A simple way of adding the input from two linkages is to have a cross-bar with the two inputs going to the ends and the output coming off the middle. The output then becomes the average of the position of the two input linkages.

You can gear the main foil and the flap together so they move with a fixed ratio. The problem with that is if you do it so as to increase the control authority, which is also the way to do it so as to stay in the drag bucket, it increases the hinge moments so it takes more force from the wand.

 

U shaped canard

I thought about this too. It would make things much simpler. Unfortunately it turns out you need the lateral lift from the canard to be directly proportional to roll angle at all times for a windsurfer . It doesn't matter if this is offset from zero (in the case of millers setup) or zero lift at zero heel, but any change in response makes sailing much harder.

If you have a straight U shape it will provide no lateral force at any angle of heel. This means if the sail or wave resultant doesn't pass straight through the rear foil, the craft will yaw.

"Ok", you say, "what if we put fences on the u foil to act as skids and provide some lateral resistance?" Now when you heel the board, the rear foil will provide a large lateral force proportional to roll angle, while the canard provides a force proportional only to yaw angle. so now the board steers the 'wrong' way when you heel the board. it's now even more difficult to sail.

been there - fallen off!

It might work for a boat but I think submerged foils will have less drag.

keep the ideas coming though

Stability, or more specifically, controllability is more important than efficiency or simplicity for windsurfers.

I think the bobweight and damper idea is great for boats. I've been pondering the same thing for a while. I think someone would have tried it by now if the Moth class allowed electronic control systems.

Mark Mellors
http://www.hyperdynamic.co.uk/hydrofoils.htm

 

I agree that must be why circular shape is not used. but could it instead have "chines"?
I I
V
that way the lateral force would be proportional to roll, at least at some parts of the roll.

I think the problem with the damper and bobeight thing, as I understand it, is that it will not react as fast when it is really needed because of a sudden large wave or something. That is why I suggested the "variable gearing" by a cam.

 

I tried something like that (http://www.hyperdynamic.co.uk/images/hydrofoils/irelan0004.JPG) the problem with the chines was they were correctly proportional to roll at some angles but proportional the wrong way at other angles, so you couldn't predict what the board was going to do.

The bobweight is geared the other way to what you're thinking. If the boat moves quickly, the foil will respond faster and more than if the boat moves slowly.

Mark
http://www.hyperdynamic.co.uk/hydrofoils.htm

 

but then it is selfreinforcing feedback. if the wand sees a wave, the foil will react by altering the pitch of the boat, and this movement cause the bobweight to amplify the reaction, etc. haven't thought it through well though.

 

The bobweight isn't attached to the wand. It is a separate feedback.

In a way the bobweight is kind of like a wand itself, following an "inertial surface". If the boat moves up and down very fast, the bobweight will tend to stay fixed in height as the boat moves up and down relative to it. Kind of like what a gyroscope does for rotation. The gyroscope has a lot of angular inertia, so things rotate about the gyroscope while it stays fixed in its orientation. A massive bobweight that was supported by a very weak but long spring (so as to maintain a near constant force) would move only very slowly, and would provide a level reference for a short time to oppose quick motions of the boat. A practical bobweight wouldn't act quite that way, but I hope this illustrates the principal.

A fully submerged foil without feedback is only weakly affected by depth, but the feedback from the wand makes it act like it has much higher heave stiffness. A bobweight would effectively add to the inertia of the boat, making it act like it was more massive and therefore less affected by the waves.

As the craft pitches up, the bow accelerates upward before the increased angle of attack increases the lift and accelerates the center of gravity upward. So a bobweight at the bow adds lead to the wave motion, like putting the wand even farther forward.

In harmonic motion, like the regular up and down of waves, the acceleration is proportional to the amplitude times the frequency squared. So an accelerometer (of which the bobweight is one example) is more sensitive to high-frequency motion than it is to low frequency motion.

The bobweight feedback would help to oppose disturbances from level, and the wand could be designed to skip over short waves that the boat can't follow and feed back more of the average height as a low-frequency correction to the bobweight feedback.

There is a definite possibility of the bobweight amplifying the motion. It all has to do with the relative phasing of the bobweight feedback, wand feedback and boat response at different wave frequencies. Getting it right is somewhat tricky. This is where a dynamic simulation model is really useful.

A dynamic model can be built up from first principles, taking into account the various forces and moments, masses, and moments of inerita. Or you can sail (or be towed) and add a controlled disturbance (like a sine sweep) to the controls and measure the dynamic response. By transforming the input and output to the frequency domain, you can get a numerical transfer function that relates the input to the output.

Once you have the transfer function in hand, you can wrap various feedback loops around it and tune the loops to get the overall dynamics you want. Finally, you have to then build sensors and a controller that matches the specification from the analytical model.

Of course, it may be faster and cheaper to just try it and tweak it until it works!