Briefly about flapping hydrofoil propulsion.

It is well known birds can fly not only in the air but also under water. The best underwater flying birds are penguins. When I’ve seen Japanese RC robot flying under water (web site: http://www.imasy.or.jp/~imae/kagaku/ Some videos: 1 2 3) I decided to create my own toy boat with a flapping hydrofoil propulsion and carry out some investigations. My toy boat swam perfectly but I wanted to improve its characteristics. You can see the dragonflies' flight capabilities are prodigious. It is very hard to call a flying maneuver, which dragonflies cannot do. I decided to investigate just one question: How does dragonfly achieve a top speed using two pair of flapping wings? Most of the dragonflies when flying about use phased-stroking (where the hind-wings cycle about 90° - a quarter cycle - before the fore-wings). I was interested in this flight style. It generates more thrust than others. There were no problems to put into practice this stile. But I did not know what distance must be between wings in my model. There are two variants of investigations. I can establish a phase shift exactly 90° and change the distance at the time of measurements or I can lock in a distance and change a phase shift. It was easy for me to do second one. I fastened the boat inside a sloping gutter with water and can change a water level. The result was unexpected. When the phase shift was right wings provided the boat with a top speed but power consumption diminished (I used one electric motor for both pair of the wings and measured a general consumption current). In the presence of top speeds the water ran over the gutter.

 

Why does the second picture only show one wing? What was the phase angle for maximunm speed with minimum power?

 

Jonathan,
I am sorry. The second picture must be first.
I can’t say my investigations are perfect. I’ve made amateur measuring equipment on my desktop. To declare my results with absolute confidence I must reiterate measurements more definitely. I am afraid that a shape, dimensions and elasticity characteristic of the hydrofoils introduce amendments into result but I believe I found out a rule. A dragonfly passes some distance during complete flapping cycle (360°). This distance is a wave’s length. If a distance between pair of wings is a quarter of the wave’s length the phase difference must be 90°; if a distance is a half of the wave’s length the phase difference must be 180° and so on. In my model the phase difference at top speed was about 120°. The increase and decrease in phase difference decreased the speed.

 

With flapping hydrofoil propulsion your power is enough to fly…

Here's the Trampofoil (http://www.trampofoil.info/video.html). It is a human-powered flapping hydrofoil without the pontoons (see pictures 2, 3, 4). The hydrofoil weighs around 12 kg and its speed is around 8 mph. On this device you should jump all the way, because when you get tired out you go under. The hydrofoil assembly floats itself, but the start in the water is impossible, you must swim to the nearest shore and pull the Trampofoil with. Start from jetty is a simple as it is to walk, you just step on the Trampofoil! The hard thing to do is to learn how to jump. You have to try it a few times, before you figure out how to do it. The main problem lies in the position of your body; so you must coordinate your position according to the center of gravitation of the boat. Simply, you must lean forward that much, that the front mechanism stays in water. The steering is just like a riding a bike.
The Pogo Foil (http://www.ocean.washington.edu/people/faculty/parker/pogo_foil.htm) is an improved human-powered boat with flapping hydrofoil propulsion. It has pontoons. In the photo 1 below you can see Molly Knox. She is riding/flying this boat on Green Lake in Seattle. It weighs about 50 lbs, goes over 11 mph, but is only good for a few hundred meters before the rider/pilot gets tired out. The boat rests on pontoons at the start, and then the rider starts hopping up and down. The vertical motion causes the main hydrofoil below the pilot to start to make thrust. This main hydrofoil is made of a carbon/epoxy composite, is about 2 m in span, uses a NACA 4415 airfoil section, and pivots just ahead of the quarter chord. A lever arm to a nonlinear spring aboveboard controls the pivoting. The springiness controls how much thrust you get. As you gain speed the front, smaller foil pushes the nose of the boat up. When 'flying' the depth of the front foil is controlled by a little spatula on a lever arm, which rides along the surface of the water and actuates the elevator flat on the front foil. The front foil is steered by the handlebars, and when you are flying you balance the craft just like a bike. As you gain speed you lean forward, and then in a magical and difficult-to-master moment the craft rotates up, the hulls pull clear of the water, and you are flying. Then your weight is fully carried by the hydrofoils, which remain 10-30 cm below the water surface. It is an amazing feeling to fly the craft.
The Trampofoil and Pogo Foil are not very efficient but there are lots of rooms for improvement by motivated inventors and scientists! Nature has much to teach us.

 

The trampofoil and flapping hydrofoils are nice but what should bring hydrofoils propulsion into practical relevance is powered reciprocating hydrofoils. There could be a set of four, left & right front foils and left & right rear foils. The left front & right rear foils would be pushed down while the right front & left rear foils were pulled up. Computer control of the angle of attack would be necessary to maintain lift equal to the weight of the craft. While the foils pushed down would be generating high lift to compensate for the negative lift of the other pair, that pair being pulled up would be angled to limit the negative lift and balance vertical forces. Forward thrust would be generated in up and down strokes. Since foils need to be in the water for lift and attitude and roll control, using them for thrust also would take an unneeded rotary propeller out of the water and simplify things. The computer should allow control of the foils so they can ride near the surface unpowered for a short time to glide over an obstacle near the surface. Computers are capable of flying by wire in water as much as they can fly by wire in the air. Please, someone, build a model.

 

'Pumping' foils work very well indeed, although it is fairly hard work. These foils are set up like an aeroplane with the stabilizer providing downforce, its surprising how much a small change in shape or angle on the stabilizer can affect pitch stability and pumping power.

I have found i can propel a small inflatable dinghy by holding the foil off the back and replicating this pumping motion

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I do not have in front of me a picture of the human powered hydrofoil that you write about, but the stabilizer is important because it controls the angle of attack of the main foil. Angle of attack can make the difference between a sail boat skimming the ocean on its foils and a boat going bow under and flipping end over end. Some accidents can be very bad. I seek to develop some mechanism by which a computer can sense the position and orientation of the foils with respect to the surface and make adjustments in the angle of attack to ride in the attitude directed by the pilot. It is a difficult problem for me, but I am surprised that designers have not done better than having foils fixed in orientation to the hull or adjusted in orientation by a stabilizer such as the one to which you refer. I hope to have some drawings eventually to inspire model makers. Most likely they will be inspired to avoid the impractical and unfinished nature of my drawings.

 

Sorry i cant embed the instagram video here, this is the foil. The stabilizer is fixed, adjustment is manual with shims