Canard Hydrofoil Configuration Design Questions

I am trying to design hydrofoils for my boat. Right now I'm considering a canard configuration. Also, I'm thinking about making the canard foil a fully submerged foil and the main foil a surface piercing foil. Does anyone know where I can find any research on this type of design? Also, if I have this configuration, does anyone know much about the interaction between the two foils (such as downwash)? Thanks.

 

Pitch control foil forward is a common set up in certain boats. Typically a very small foil relative to the main lifting foil and far enough away that it does not interfere much.

The big factors to cater for compared with a wing is the hugh strut drag and wave drag. There is an optimum depth of design operation where the strut drag or surface piercing section drag, combined with the foil L/D results in a drag minimum.

It would probably help if you gave some idea of the weight of craft, intended power and target speed.

There is a nice bit of software called JavaFoil that helps with foil design. There are papers around on the wave drag with foils. The linked video shows how significant the wave making is if the foil is shallow relative to chord length.

http://poisson.me.dal.ca/~dp_04_5/images/testing/successful-hydrofoil-test-march-14-2005.wmv

The Flyak uses a very efficient foil system that does not require active pitch control. This is a matter of have an inherently stable system. It also means the foils would need to be preadjusted to get the most efficient set up for any particular speed.
http://www.foilkayak.com/faq/theory/

Rick W

 

As far as the some of the information you requested (weight, power, speed) the boat weighs approximately 450 pounds, it will have 40 pounds of thrust, and I am looking to get up to around 30 mph.

That JavaFoil software, is that available somewhere on the internet?

When you say the small foil needs to be far enough away, do you know of any calculations or formulas that could approximate the correct distance where the two foils would begin to have the least interaction?

Thanks for your help.

 

This will get you to the JavaFoil applet:
http://www.mh-aerotools.de/airfoils/jf_applet.htm

The default options are for air so you need to set parameters for water. You can duplicate a foil and then scale one and place it in the required configuration to see the interaction. It will not give the whole story because it is 2D but it will give you an idea of how large the flow fields are. You can set the aspect of the foil though so it will determine the induced drag.

You can use JafaFoil to determine strut drag. It will not give you near surface impact due to wave drag or loss of lift.

Rick W

 

Attached shows pressure field and streamlines for two well-spaced foils. You can see the influence at this distance is quite small. To get actual detail you would need to get data for a foil individually and then get the data from the combined flow field. The actual pressure fields in the attached suggests there will be very little interference in these proportions. I selected the foils to have an aspect of 10. You can see you could expect an L/D of 20:1 in this configuration without allowing for wave drag and strut drag.

Rick W

 

That's an extraordinarily bad idea, unless you have a really reliable control system for the forward foil. The reason is because it causes unstable pitch-heave coupling.

Imagine the craft is flying in trim. Now give it a small bow-down disturbance. The lift on both foils will be reduced as the angle of attack is reduced. The bow will start to descend. The stern will also start to descend, but this will be slowed by the surface-piercing foil picking up additional area. So the bow will go down more than the stern. Which will increase the bow-down attitude. Which will make the bow go down faster. And you have a spectacular crash-dive, probably faster than you can react. Or the disturbance will be bow-up, and it will leap out of the water, then crash back down.

The forward foil must change its lift faster than the rear foil as the height varies if the craft is to have stable pitch-heave coupling. This can be accomplished with a feedback control system that actively changes the lift on the forward foil faster than it changes naturally on the rear foil. A surface piercing foil forward with a fully submerged foil aft does it naturally, because there's very little change in lift with height for the fully submerged foil.

There have been craft with surface piercing foils both fore and aft - Don Nigg's Flying Fish being one of them.

BTW, surface-piercing foils can benefit from feedback control, too. And the ability to alter the pitch trim (typically by a flap or incidence control of the aft foil) is important to their performance.

To go the International Hydofoil Society and order their AMV CD's. That will give you a whole technical library of hydrofoil literature, including design manuals.