Flapping Propulsion.

Hi all.
This is a new thread where you can explain your opinion of what is flapping propulsion. Is it better than propeller propulsion? What is the difference?

 

I'm game.

Flapping Propulsion is that caused by moving a solid blade or a flexible fin side to side or up and down. It must go either up or down, or to one side or the other, then stop, then go the opposite direction.

This is at least theoretically less efficient than a propeller which is nothing but a rotating hub with two or more fixed blades mounted to it. Since it usually spins in one direction, it enjoys the advantage of momentum. On a large vessel such as a tug or a ship, this has to be seen to be appreciated. I was once on a large tug when the skipper decided to go from full speed ahead to full speed astern just to see how long it would take. It took several minutes. The heavy, 10ft dia. prop had to lose momentum before it could be reversed.

With a 'flapping' propulsion system, this has to be done with every stroke (the equivelent of one propeller revolution). So now you are not only losing the advantage of momentum but fighting kinetic energy as well, meaning you now need four times as much energy to stop and restart something that has the same mass that is moving merely twice as fast. Maybe this is why fish have such huge tails. Imagine a prop with a diameter of at least the beam of the boat. (The tug I was on would then have to have at least a twenty foot dia. prop.)

This, of course, is not the whole story.

To put things in ProPper perspective, a prop too has its disadvantages. It must be on a spinning shaft that has to be below the waterline. This shaft must lead to some kind of torsion generating device that usually does not like to be immersed in water. So now you need a seal. And this seal is, as I understand it, never completely water tight. Not only that, but this shaft must be as parrallel to the waterline as possible or the thrust is downward as well as foreward and only the forward portion counts. And a third problem is that the prop shaft has to be perfectly straight and the prop perfectly ballanced or it will shake itself apart. I have heard of people making their own paddles. I have heard of them making their own oars. I have even heard of them making their own paddle wheels. But I have never heard of anyone making their own propeller.

But if all those conditons are met, the prop is most likly superior to a tail of the same surface area due to its momentum and even 'stroke' (rotating speed).

I suppose if God were an engineer (she's perfect, after all) she would give fish counter rotating propellers. But, perhaps, God had other design objectives to meet. If fish had propellers, angels probably wouldn't have wings. Imagine Satan with a beeny cap and counter rotating rotors. Wouldn't look menacing at all. In fact, as far as I'm concerned, he would look down right silly.

Bob

 

it's a good thing that bearings weren't around when fish were invented... Imagine a fish with a prop shaft, we would never have learned about flapping propulsion systems...

bye
Hans

 

Could you inmaginate the disaster if all this fishes was equipped with propellers.

 

Perhaps global warming by prop-equipped fish?

 

Fish don't go in reverse. Sam

 

Kjell,

If all fish were equipped with propellers there would be no disaster since they would all suffer from a similar disadvantage...
If all fish but one were equipped with propellers and that special fish was equipped with a 'regular' fish tail it's possible to assume there would have been a mass extinction of the 'propeller fish'

Yoav

 

Flapping foil propulsion

For those interested in designing/building your own flapping foil propelling machine I suggest you use 2 peaucellier cells (with bearings at all articulations) per foil to turn rotational movement into the desired pitch and heave motion of the foil. Stainless slides could also be employed as well.

 

You are right. The disaster I was thinking about was the result of thousand sardines swimming close together with a “Turmix” connected at the rear end.

 

If you like to learn about flapping propulsion have look to my web pages.
http://www.dahlberg-sa.com/kd/index.html

 

Kjell:
From your pages:
"The Tail propulsion on the Model Boat was moving without making waves, when the oscillating frequency was in harmony with the wave frequency"

Could you further please explain this, please?
Froude's number for the tests?

Interesting sites on these matters:
http://www.ece.eps.hw.ac.uk/Research/oceans/people/Michael_Sfakiotakis/IEEEJOE_99.pdf
http://www.rbbi.com/folders/tech/product/penguin.htm
http://web.mit.edu/towtank/www/Tuna/tuna.html

 

Methinks one could devise a simple propulsive fin for something like a canoe (I select this because of it's long, narrow efficient design, and that it flows water easily to the fin). Consider attaching a SCUBA fin vertically to a narrow shaft out the back of the canoe. The pivot point on the shaft can be at the 'transom' of the canoe, and the opposite end of the shaft can be driven back and forth in a lateral motion.

While the idea of the peaucellier cell is unique, it is more complicated than we need here, and contains more inefficiencies. If we look at a basic internal combustion piston engine, we see that we are converting linear motion to circular motion via a crankshaft. Since in this application we do not need circular motion, why not just stick with the linear motion and keep the engine at peak efficiency? Consider a two-cylinder engine, with each cylinder opposing the other. At the end of the piston rod that would normally be attached to a crankshaft, we instead attach it to the shaft that drives the SCUBA fin. Same goes for the opposing piston. The cylinders are fired in 180-degree fashion, so that they cause the shaft to swing back and forth.

In this case, the engine can be quite small and does not need to create an abundance of power, since the shaft can be used as a lever depending on where the pivot point lies on the shaft. Perhaps something the size of a glow engine like those used in model airplanes would suffice. The speed of the engine would be dependent on the needs of the propulsion system (although we are no longer talking rpm's, since there is no conversion to circular motion). I'll leave that for the engineers to figure out... I'm just a biologist.

 

[ I had the luck to eliminate the waves on one of my Model Boat. You can se the same boat with and without waves, using the same drive and tail. I think that the change of frequency was producing this change. The wave phenomena just happened during one of the test I was recording with my camera.[/QUOTE]
I would think that if this phenomema was not the result of outside forces such as water current or a puff of wind, that it would be easily repeatable either by the same model at a certain frequency or any other shaped model at it's own certain frequency. Was the drive tail the same size/shape? Perhaps the depth of the water had something to do with it, a boat in shallow water has more drag than a boat in deeper water, I think the water being compressed between the boat bottom and the pond/river bottom creates an invisible third, vertical "wave" that the boat has to overcome. Maybe the "frequency" of the third wave cancelled out the two horizontal waves. For the tail at the back of the boat to eliminate wavemaking at the front of the boat almost seems like predicting the future, a time machine. Maybe the pond was not level and the model was going downhill? ;o) As for your sail drive, do they give the same speed as regular sails? Would there be any advantage of adding extendable 'flaps' like jets use in takeoffs/landings? Would there be any advantage ( or would it even work) to substitute a fabric 'wing', like from a para-wing/parachute, for the solid wing you use? Sam

 

Well, displacement boats use to create two trains of waves, one at the bow and the other at the stern. The waves length and height depends on speed (so Froude's number) for a certain hull and, as far as I know this fenomena has basically nothing to do with the propulsion sytem. I'm most surprised.

I recognise I have no experience with fins propulsion, but I could think of some kind of interference between the superficial wake waves created by an horizontal fin close to it and the stern train of waves from the hull, so creating a zero amplitude (or almost zero) interference wave. But... interference with the bow train? Sounds weird...Bow trains are minimized creating interference by means of a bow bulb, not stern mechanisms.

You say you changed frecuencies, so probably this was affecting the model speed. Maybe I'm wrong, but from my point of view the model was not running at the same speed in both pictures (in your web page).

To my understanding, in the 'With Waves' photo there are two conspicuous waves trains, at the bow and at the stern, and it seems like there is a third 'wake train' of waves created by the fin, interfering with the stern train (There is an apparent conspicuous more or less triangular zone of steeper waves at the center of the stern train. One of these waves close to the model is even breaking).

In the 'Without waves' photo there is not bow train but also it seems there is not either an stern train. To me the waves that can be seen are rather the wake waves created by the fin, not the hull's stern train. So most probably the model was not running at the same speed in both photos. The number of crests along the hull suggests a Froude's number around 1 for the 'With Waves' photo and under 0,5 for the 'Without Waves' one.

But I may be wrong, because....

...Something also weird to me is the pattern of the bow train in the 'With Waves' photo. It has not the typical configuration of these trains, usually in the form of several separated and diverging 'Vees'. The pattern looks more like an stern one, so...Maybe the in the 'With Waves' situation the model was oscilating (pitching) due to the effect of a higher frecuency (and power) at the fin...? May this pitching movement create this unusual pattern of bow train...? So, may it be possible that what really was happening is that with a higher frecuency-fin power, a bow train was rather being created instead of eliminating it...?

I'm most curious....