Foil-Spring Controlled Flapped Foil

This is an idea I may experiment with down the line with my new test model trimaran. In the interests of having a very simple foil system on the fullsize boat(as well on other applications that might benefit) I've come up with a flapped foil where the bottom of the flap is attached to a spring so that at zero speed the spring pulls the flap down to a 30 degree angle for maximum lift. The spring would be in a carbon tube and could extend forward of the leading edge of the foil. As speed increases the pressure would tend to allow the flap to neutralize. This could be augmented by a very small "trim flap".
The application is limited to boat/foil configurations that do not require downforce or altitude control( this eliminates Bradfield type foilers and most monofoilers) But it fits my ama foil to a tee if we find that the foil needs more manual angle of incidence adjustment than I'm willing to go along with. Or if, by increasing low speed lift, AOI adjustment at mid and high speed range could be avoided since the foil could be set to a lower angle of incidence to start with. It could possibly work on the main foil except that , in its role of working with the rudder on pitch control, the mainfoil needs to be able to develop some down force(active altitude control).
Anyone with thoughts on the concept?

Here is a rough sketch and a picture of the ama foils it might be tried on:
click---

 

I think you could also think about making a compliant mechanism, rather than using precision mechanics and external add-ons. A compliant mechanism is the one which transforms forces and motions through the use of elasticity of the material, instead of using mechanical joints. It would reduce the complexity and the drag cost of an external spring attached to the foil.
Cheers

 

Probably beyond my capabilities to test. I can modify the ama foil at very little cost to try this if I become convinced it is worth trying. Do you think the spring would work for my purposes: that is, to increase foil lift at slowest speed while not interfereing with the basic characteristics of the foil at higher speeds?

 

Designing a compliant mechanism must be challenging .

To understand what must be accomplished and then to build to compliance.

The trial and error process must be long

 

Imo it could, at least intuitively. Few calcs would be necessary, in order to get a sharper picture.
First you'd have to calculate the section lift force and flap hinge moment for various flap deflections and for various speeds. It will allow you to establish the desired value of the flap angle at each speed. From there, you can determine the spring rigidity which will give you the flap motion close to what you need.

Is this the ultimate dimensional data of your boat?

 

Slavi, after testing the foil as is I could make an experimental test setup for the flap/spring in about an hour. It would probably be the best to just test it against the foil w/o the spring since it's so easy to do.

PS- that is the correct info, but don't spend any time on it because, now that I've had more time to think about it, testing the idea is fairly simple and quick to do.......

 

For your boat, the spring could be as simple as a piece of thin rubber band pulling the flap down and slowly extending as the speed goes up. The only problem (if it is a problem for you) is that rubber bands behave in a less deterministic manner, because they have a considerable hysteresis. Check this page: http://www.wired.com/wiredscience/2012/08/do-rubber-bands-act-like-springs/ . You can imo expect a rapid oscillatory flap movement in your case, due to the turbulence of the inflowing water and due to rapid moving through the waves.
Cheers

 

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I was thinking that a small tab flap(on the trailing edge of the main foil flap) designed to keep a bit of tension on the spring when the flap was in neutral might work to heavily dampen or eliminate oscillation of the flap. But it may not matter since the main foil on a Moth oscilliates rapidly due to the wand movement over waves and that boat is extremely fast. But eliminating the oscillation would be something I would try to do

 

Oscillations mean fatigue, wear and tear of mechanical parts and also increased drag. So it is always better to try to minimize it.

I am not sure that I'm visualizing this idea correctly. If you post a rapid sketch of it, the vision might become more clear.

 

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Maybe this will help-click for larger. These are two separate ideas. Both attempt to load the spring even when the flap is in neutral.

Note drawing has been corrected thanks to Slavi!

 

Doug, please note that the way they are shown in the drawing these tabs will do the contrary - they will unload the spring at zero flap deflection.

Unless the spring is on the upper side of the foil, which is not advisable.

When the flap is set at positive angles (which means downwards) the simple tab shown in that pic will generate a downwards-rotating moment, while the extended tab will generate an upwards-rotation moment. But you already have it done by the flap itself.

I would leave that idea aside for a while and first see how the flaps behave without tabs. Perhaps you won't need them at all.

Cheers

 

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Your're 100% right-I drew it backwards- you said you wanted it quick! I'll fix it immediately* thanks for pointing it out! Check back in a few minutes and check the sketch again.
* Done!
The good news is that the spring system and the "dampeners" are exceedingly simple to experiment with.

 

Doug, in aero, that setup would be termed "controls free", as opposed to controls fixed. In controls fixed the elevators etc. are considered fixed in position and the craft's stability coefficients are derived for that condition. There is a neutral point for the CG location that is a global reference once all the calcs are done. Then the whole process can be repeated with controls free, meaning fixed moments are applied to the control surface but it can rotate freely. A completely different set of stability parameters result from this analysis, and a second neutral point is calculated for the CG. So for a stable aircraft, there is an aero center, then a static CG, then a controls fixed CG, then a controls free CG, where all the CG's identify projected points of neutral stability.

Here is a doc I stumbled across recently. https://courses.cit.cornell.edu/mae5070/
It shows how the analysis is conducted, although it is only a bunch of supplementary notes to an actual text. Ch3 is the part I'm referring to.

 

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Thanks for the info Phil. This spring set up is only to create a large amount of lift at slow speed only on a surfacepiercing/fullysubmerged hybrid foil. It will not control altitude or ride angle of the foil or boat. Ride angle is controlled by the interaction of the wand controlled main foil and the rudder foil. Ride height is controlled by foil immersion/speed. The spring, if it works, will allow a greater range of speed that won't require manual adjustment of the angle of incidence of the foil since it would be able to be set up much lower to start with.

 

You have to run the maths to decide if that's what it will do. And you have to evaluate the drag such an arrangement produces and compare that to more sophisticated, but less draggy active controls and see if the costs are justified. I'm currently looking for methods to help with evaluating the gains or losses in drag when a gadget such as a foil, along with its control mechanism, is added to a concept boat. In history, there haven't been very many new concepts for hull improvements that couldn't just be extrapolated from previous boats. But now, I think there is a need for a pretty solid tool for assessing a concept architecture and having the control power and propulsive power projected for different sea states. The key is looking at dynamic stability coefficients and knowing how they affect one another and thus being able to say what is the best way to use 1 extra Watt if you have one. Because boats and planes developed from manual controlled craft, there doesn't seem to be an analytic framework that brings control systems in at the conceptual phase. Nowadays, that's just not good enough.