Steering foil craft

Actually..it matters a good deal apparently, since I've never seen any recreational foil craft that were computer stabilized.

What often gets lost in the discussion of automatic 'flight' controls is the superior motions and handling one can get from an inherently unstable platform that is flown by computer and can only be flown by computer. Military aircraft designers were the first to figure all that out..and long ago now. Laypersons leap to the very mistaken assumption that such crft require automatic control systems to 'correct' some design deficiency when the fact is quite the opposite.

But for a recreational toy and smaller passenger craft?....not very many can afford the cost and complexity of automatic control and fewer still would maintain it in good working order. The Russian Katran's (a prolific builder..the craft are everywhere) had automatic flight control in all that were built before the collaps of the Union. A beautifully crafted system, with most of the hardware and electronics 'borrowed' straight from their sub control systems. I've yet to see one of the existing craft operate with the auto system functional. I've been asked to try and replace a couple...

 

The motor Industry is taking away from motorists a lot of the decision making that realy matters and are predicting "Flying Cars " well in use in 30 years. It does make you wonder how this will be achieved without bodies everywhere. Computer technology is fairly cheap and everyone is getting into making their own software to do just about anything.The young generation seem to be able to handle any challenge and come up with fairly simple solutions to "fly" all sorts of machines with out immediate human attention.

 

This isn't directly related but the control technology it represents will eventually do a lot for small "Peoples Foilers" power or sail-watch the video:


http://world.honda.com/news/2010/c100406U3-X-Prototype-New-York/video/

 

There are three axes that need to be considered for controlling a hydrofoil. The yaw axis for directional steering is one. Longitudinal control is all about controlling the pitch angle and heave (flying height). And roll control is important when turning - for the same reason it is on a bicycle.

The means for accomplishing all these depends a lot on the configuration of the foils. For the "airplane" configuration that has the main foil(s) forward and a stabilizing foil aft, it's common for the stern foil to pivot for directional control. The stern foil also controls the pitch trim of the vehicle. Heave is controlled using flaps, incidence control, or area reduction (surface piercing foils). Surface piercing foils often use the sideslip-roll coupling to control the bank angle when turning. Fully submerged foils control roll by differential control of the flaps or foil incidence.

Canard configured hydrofoils that have the main foil in the stern use the forward foil to control the pitch attitude and indirectly control the flying height. The forward foil is usually pivoted so it controls the directional steering, too. Flaps on the stern foil can act as ailerons to control roll. Roll control from the canard is not very effective because the canard has a much smaller span and because downwash from the canard on the stern foil causes a rolling moment in the opposite direction.

Fully submerged foils need some kind of feedback control system because the change in lift as the foil approaches the surface is not strong enough to act quickly enough to adequately stabilize the craft. Surface piercing foils can benefit from feedback control, too, although many designers select surface piercing foils with an eye to minimizing the need for feedback control. The feedback can be electronic or mechanical.

Electronic feedback implies a computer and powered control surfaces, using either hydraulic (most common) or electromechanical actuators. This requires a complex system to power everything and connect the whole system together, but it provides the most flexibility for augmenting the natural dynamics of the boat and achieving the required dynamic response.

Mechanical feedback can be in the form of a float or planing surface ahead of a foil that rides on the surface and is linked to the forward foil to maintain a constant flying height above the water. The "Shutt strut" invented by Sidd Shutt is a good example. Sam Bradfield invented the use of a wand that trails on the water surface and is linked to flaps on the foils - a configuration that has had great success in the Moth sailboat class. (It is important that it is the forward foil that flies at a constant height rather than the stern foil in order to have stable pitch-heave coupling. This is true for both the airplane and canard configurations.)

Another mechanical system that was demonstrated by Grumman on a small hydrofoil was a vertical post sticking up from the flap, forming a feedback control system with only one moving part. Water drag on the post deflected the flap down, causing the foil to fly up. As the post came out of the water, the hinge moment on the flap overcame the drag on the post, causing the flap to move up and reduce the lift on the foil. So it's possible for feedback control to be very simple if one is very clever.

A lot of attention needs to be paid to engineering the stability and control of a hydrofoil. A lot of different design conditions need to be considered to define the requirements for the control system. And even if one has a good theoretical basis for analyzing the stability and control, it takes a lot of work to come up with the numbers for the stability derivatives (constants) to plug into the equations. I think this is an aspect that requires more engineering analysis for hydrofoil design compared to typical small boat design.

 

Neat stuff, Doug. There are also amazing developments in digital 'flight control' technology driven by the RC market. Cheap but accurate and sensitive MEMS gyros, microcontrollers, capable servos....etc. Heck, even the age-old problem (old as in I'm going back to PHM development days) of obtaining a reliable 'flying height' signal has been knocked in the head; only a decade ago we were still using 30 thousand dollar radar units..but now get the same sensing packages for less than one thousand dollars each.

Taken all together, the potential for having some really neat but otherwise difficult to operate 'water toys' is here already. There is still that pesky matter of product liability to deal with.... What happens to a craft that is fast and yet dependent on an electronics package and servos for stable flight? I know the answer to that pretty well..although the bruises have healed nicely.

 

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It's all very intriguing-I'm looking forward to being able to do a sailing bi-foiler with automatic stability control that not only keeps the boat level when you want it level but allows the boat to be sailed upwind with 15 degrees of "veal heel"(a method of upwind sailing developed by Rohan Veal that adds righting moment while unloading the vertical fin and other neat stuff). I've been experimenting with this but a heli gyro isn't enough-it needs to have switchable programing. I've been able to sail a bi-foiler with RC but not repeatably with veel heel w/o crashing. But sooner or later they'll be a whole slew of baby RC bi-foilers passing Hobie cats and any RC multihull there ever was! Exciting stuff ahead.....
The simplest RC sailing hydrofoil that uses the foils for RM(dual independent wands) was the F3 that I designed with a lot of help from Dr. Sam-it was the first production RC sailing hydrofoill as far as I know:

 

NOT so fast you Guys..there is a problem...the earth is due for another polarity reversal...What is going to happen? Are all our electric motors going to start running backwards or are we just going to go back in time? It could be a bigger problem than Global warming.

 

A lasting impression was made on me in the late 1940`s by a Gaumont News Reel of a boat (big model) which had skis fitted and being towed beside a towing boat over the wake and a very choppy sea (for a small boat). The model handled the conditions very well.I think it was made by a New Zealand boat builder Carl Augustin. I have often thought about the idea but have never seen anything like it since. It could be great for a cheap recreational boat,with improvments.

 

I get the impression that the skis are attached at single point and are supposed to pivot. At first glance, pivoting skis might seem to decouple the hull from the waves, resulting in a smoother ride. At closer inspection, I don't think that would work out in a beneficial way.

Since the force pushing the skis forward at the pivot is located well above the drag of the ski in water, a significant couple is formed and the ski would tend to rotate aft, with the nose coming down. This rotation would increase as the speed increased, possibly ending with the skis nose diving. Of course, if you saw a model that was performing well, maybe I am missing something.

I had been working on a similar concept with a powerboat supported by slender hulls similar to skis. I had devised a way to mechanically extend the skis down from the hull, while keeping them parallel with the hull, effectively lifting the hull clear of the waves once planing speed had been achieved. Unlike this sketch, the skis were never able to rotate relative to the hull through their range of motion. The slender, wave piercing shape of the skis should in themselves provide a far smoother ride than a conventional planing hull. I got as far as towing a scale prototype, with fairly positive results, but never went as far as to build a full working prototype.

I am now finding hydrofoils a more interesting approach in my quest to design a ultra-smooth-riding small powerboat. Hydrofoils seem pretty finicky, so I am keeping the ski idea in my back pocket if the hydrofoils don't pan out.

 

My interest has been in small personal craft with a leaning towards saftey and comfort for my wife and my self to go exploring in rivers and estuaries etc., Home built as much as possible,and a throw away cost if it does not work out,or modified to a different form. I needed shallow water operation to go ashore for picknics.Jet skis are not much use if you can not easily paddle them home or carry a bit of camping equipment.

 

Shallow water and hydrofoils can be a dangerous combination. One hydrofoil runabout had a stern foil that swung up behind the transom. They hit a sandbar, and the hydrofoil dug into the bottom. It ripped the transom right off the boat!

I suggest the foils be designed to break away from the hull in a controlled manner. A cable attached to the foil would allow you to recover the foil after paying out enough distance for the boat to come to a stop. It would be a good application for the anchor rode when the anchor is not in use.

 

I was the pilot for the three years we flew the 43' hydrofoil demonstrator. I 'parked' her on the bottom once, from 25 knots at full flying height to zero in about 20 feet. Nobody was seriously hurt (had 12 pax on board) and the craft was not damaged in the slightest (grass-covered mud bottom..).

Luck was with me..flew her aground at dead low tide and therefore we got her back off at peak high tide with nothing more than a gentle tug from our rescue craft.

The draft is always a concern with foil-supported and foil-stabilized craft. Nothing quite so 'cheek clenching' as flying along while watching the depth meter read 3' under the keel and knowing that yr foils hang 4' under the keel when not flying.