Testing system for foils

I'm intending to try to make some small model foiling boats and to experiment with some different types of foils. To develop the foils I've been trying to design a test system to better understand the optimal profiles, angles, foil types etc. This is a hobby, so I'm working with limited resources, so the system needs to be pretty simple. I'd appreciate a sanity check on the system I am currently considering. It consists of a small motorised cat with the test foils mounted vertically so that they act like dagger boards (see attached image - view from above).

The foils are identical but are configured so that their "lift" forces (actually horizontal to the water surface) oppose one another. One foil is fixed while the other can move on tracks and a device joins the two to measure the stretching force. The power of the motor can be controlled and the speed measured. Am I correct in thinking that the stretching force will reflect the "lifting" force, while the relationship between the power and the speed will represent the drag? Please keep in mind that I'm not trying to measure the lift and drag coefficients, but just trying to find a way of comparing things like: different foils; the angle of incidence of a particular foil; the lift at different speeds etc.

 

I would consider an even simpler system. Build a hull with the proposed foils and tow it. The drag can be measured by a scale on the rope and you will get an idea about the stability of the system as well.

Lohring Miller

 

Well a couple of thoughtss. Lohring's point about drag is spot on that's how Frank Bethwaite did it.

He actually did it even more simply. He had two identical hulls on each end of a 2m stick and the stick had a damped pivot in the center to which the tow line attached.

Then he would tow two foils and the higher drag one would fall behind.


But that doesn't give you lift and neither does your platform since your AoA is zero. now you could set it up so that both foils are at an angle to the direction of travel, but then you will need some bigger foils in the hulls to keep thngs from getting pulled off center.

And you will need to angle both foils so that they are generating lift "to the outside" otherwise the stretch approach won't work.

Lastly the hard part is that you need to have some way to get the boat up to "steady state" speed before you "release" the foils for their test, because otherwise the low speed initial conditions could adversely affect your test

 

You might want to try a little different arrangement, in which the two foils oppose each other but are also allowed to trail. Toeing the foils in would set their angle of attack. The opposing force would be twice the lift and the drag would also be doubled, making it easier to measure. The angle of trail would be a sensitive measure of any difference in lift between the two foils.

Similar test rigs have been used for testing tires. Take a look at:
http://edwardstilson.com/wp-content/uploads/2010/07/tire-testing-rig.jpg
http://www.davidpublishing.com/DownLoad/?id=2211
They might give you some ideas.

Measuring the lift is important, but what's really important is measuring the drag. The speed of your cat will be affected by the drag on the foils, but you would have to have a very well calibrated power train to be able to discern the drag of the foils this way because the drag of the foil, and especially the difference in drag between the two foils, will be a small part of the total drag of the cat.

Frank Bethwaite's "High Performance Sailing" describes how he measured the drag on dinghy hulls by towing them from a balance beam on a power boat. You might be able to adapt something like that.

 

Yes, towing is a more elegant solution. The boat attached to a pulley on a wire, and the pulley pulled by a winch might be a good approach. Both the drag force and the lift force could be measured on the boat.

 

It was pointed out to me that the opposing lift forces would not be doubled. The opposing force from the opposite foil would be the same as the force between a single foil and a fixed frame. However, the drag will be doubled with the two-foil arrangement, and that will help with the measurements.

You could have the frame supporting the foils be entirely above the water so you don't have to deal with the drag of a floating support.

Actually, you'd want four frames. The first one is attached to the tow boat. The second frame is attached to the first frame using flexures. The flexures would allow the frame to move fore and aft, but not sideways. The fore-aft movement of the frame would be restrained by a spring and the deflection measured to provide the twice the drag force. The third and fourth frames would each hold one foil and be attached with flexures to the second frame, such that they were only allowed to move in a transverse direction. A spring between the two frames would restrain the movement and the deflection measured to provide the lift force. The mounting of each foil to their respective frame would provide for varying the toe-in, cant, and rake (pitch) angles of the foils.

The kind of flexure bearings I'm talking about might be like the ones shown in these figures:


They have the advantages of having no friction, slop or backlash, and being very stiff in the desired direction while providing very little resistance in the desired direction.

 

the issue with the flex approach is calibrating it. making sure each side has the same flex force aint trivial. OTOH a dampened balance beam works quite well. And calibration isn't hard - you start with two floating hulls that have a slot for you CB and drag them with no CB and adjust their positions (use a screw thread of a padeye at each end of the "beam")

then insert the CBs and compare. if you want you can put a strain/force gauge in each side

have the beam right on the stern of the tow boat. so that you can get the tow boat up to steady state speed before releasing the beam.

 

Is this the sort of arrangement you are proposing (see attached)? The flexures are a great idea.

To keep the foils at a constant depth it might be best to mount this frame on small cat and tow the cat?

 

Yes, the first part is what I had in mind. You can cascade a second platform from the first, but with the flexures oriented at right angles in order to measure the other component.

The problem with towing the cat is it's going to be hard for you to tend to the apparatus. I suppose you could have a small cat trailing from the front beam of a larger cat so you can access it readily from all directions.

 

Something like this?

 

Yes, that's it. The height of the flexures is probably a bit exaggerated in the diagram, but that's the geometry I had in mind.

It might be possible for both of the foils to have their own lift platform, like the foil on the left. A spring between them would measure the average lift, and a spring between one and the middle level platform would measure the difference in lift between them. That might make it possible to get a more sensitive measurement, or to rotate the foils so that the difference in lift was nulled out in order to compensate for any errors in flow angle.

Flexures can be made with a thin sheet, as shown, or they can be machined from a metal bar. The bar would be necked down at two locations so it bends in just those areas, forming two hour-glass shapes. So there are lots of ways to implement the same concept. Multiple parallel sheets can minimize the resistance to motion while still carrying large loads.

Here are some commercial products that can give you some ideas. These lecture slides cover the engineering of flexure bearings.

 

Thanks again for the info. I like the idea of each foil being on its own moving platform. I think that I will go for the thin sheet flexure approach. I think that this will work nicely with the extruded Al profile that I intend to use. It will be easy to clamp the sheets to the profile and experiment with material, thickness, height, width, #, etc.

I am planning to keep the test device as simple as possible. The deviation of springs will be used to measure force, with the springs simply pre-calibrated using weights. The deviation will simply be measured using small video cameras.

I'm currently planning to have the profiles 3D printed in polymer, and then if necessary reinforced with carbon rod. I anticipate that they will need to be finished in some way to provide a smooth surface.

One thing that I am wondering is what shape tips to use? I want to to try to minimise the effect of the tip shape of the foil on the comparison of different profiles.