How does the hydrothopter work ?

[Externet]

Hello.

This contraption keeps its flotation and its forward motion from the operator's movement; what mechanical actions and reactions are involved, and how does each element performs ?

Miguel

[swansont]

You might want to provide a link to a description/picture of the device.

[Externet]

There is several versions of the contraption; here is one:

 

http://www.instructables.com/id/EW28Z3LF18DW901/

 

http://www.instructables.com/id/EW28Z3LF18DW901/?ALLSTEPS

 

And youtube shows these:

 

 

 

 

 

Miguel

[Glider]

Hello.

This contraption keeps its flotation and its forward motion from the operator's movement; what mechanical actions and reactions are involved, and how does each element performs ?

Miguel

I have seen these before. The mail element is the 'wing', the main cross member that is submerged. This is a hydroplane and acts like a dolphin's tail. The contraption doesn't actually float with a rider on board.

 

Downward force from the rider cause forward thrust and lift (even the force of getting on board) in the same way as the downward force ('falling') of a glider causes air to pass over the wing providing lift.

 

The hydroplane moves forward and rises in the water then the rider pushes down again. The thing essentially 'porpoises' through the water. As with any wing, it requires a minimum airspeed (or in this case, water speed) to provide sufficient lift. Below that, the thing will stall and sink.

[Externet]

"...The hydroplane moves forward and rises in the water then the rider pushes down again..."

 

Cannot comprehend how a rigid narrow fin can propel anything upwards when pushed downwards, and from observing the motion, the thing rises at the 'upward' stroke, -not as you say- without having flotation. The forward speed has to be the one creating the force pushing up under the wing, as on an airplane.

Without forward motion, it just sinks no matter how much jumping on it.

 

Guessing, the downstroke only provides forward speed somehow, and later in the cycle the forward speed pushes the underside of the wing back up.

To happen, the angle of the fin should be changing from 'leading edge down' at the beginning of downstroke and changing to 'leading edge up' to lift at the end of the downstroke... That, I can visualize.

 

For that, the 'wing' should be horizontal at about half submersion.

Better said still guessing, the first half of the downstroke provides forward motion, the second half starts providing lift.

 

Now the front canard is pivoted, its angle appears to barely change; going 'leading edge up'? when downstroking. There is some key cadence to keep it going, and calibrated elastics.

 

What would be the effect of a wider wing? Perhaps would not synchronize to the natural human cadence (resonate to jumping) of ~1.5Hz ???

 

What is the energetic calculation to keep a ~60Kg human above surface, plus propel forward at >1m/s, against drag during for a couple of minutes ? That is the sum of vectors up and forward.

 

Still lost in space...

Miguel

[Glider]

"...The hydroplane moves forward and rises in the water then the rider pushes down again..."

 

Cannot comprehend how a rigid narrow fin can propel anything upwards when pushed downwards,

Look at a glider in flight. The weight of the glider provides a downward force, the airofoil translates this into forward motion (airspeed) which provides lift. If a glider weighed nothing, it would go nowhere.

 

and from observing the motion, the thing rises at the 'upward' stroke, -not as you say- without having flotation.

There is no 'upward stroke'. Gravity acting on the rider, and the force generated by the rider 'jumping' dictate a downward force only. The hydroplane (which has to be moving forward anyway, hence the 'kick off' given by the rider at launch) translates the downward force into further forward motion (acceleration) which increases the speed of water over the 'plane increasing lift and resulting in the upward motion. At the top of the arc, the rider pushes down again, providing the energy that is tranlated by the 'plane into forward motion, and so it cyles.

 

The hydroplane describes a sinusoid motion through the water ('porpoising'). This is also done by parascenders to gain forward speed in dead air. Your own body weight provided the downward force, and if you pull the brakes to make the wing almost stall, then release the brakes, the canopy dives (accelerating also). Then, you pull the brakes and the canopy flares (makes use of the increased airspeed (lift) to regain a bit of altitude), then you release the brakes and the canopy dives and so-on. If you get the frequency right, this increases your airspeed and the canopy 'porpoises' through the air faster and losing a lot less altitude than if you just pointed at the target and waited.

 

The forward speed has to be the one creating the force pushing up under the wing, as on an airplane.

Without forward motion, it just sinks no matter how much jumping on it.

Yes, that's why the rider has to give an initial 'kick off' to create forward motion. The downward thrust provided by the rider translates into acceleration, but without that initial forward motion, it would just be downward force and the thing would sink (same as a airofoil stalling).

 

Guessing, the downstroke only provides forward speed somehow, and later in the cycle the forward speed pushes the underside of the wing back up.

Yes, see above.

To happen, the angle of the fin should be changing from 'leading edge down' at the beginning of downstroke and changing to 'leading edge up' to lift at the end of the downstroke... That, I can visualize.

It does to a degree. The contraption pivots on the forward plane(s) so the hydrofoil describes a vertical arc as it moves through the water. Hold your forearm in front of your face parallel to the ground. Now. keeping your elbow still, move your forearm up and down and notice the angle of your hand change from leading edge up to leading edge down. That's how it works.

 

For that, the 'wing' should be horizontal at about half submersion.

Better said still guessing, the first half of the downstroke provides forward motion, the second half starts providing lift.

Not quite. That implies an 'active' upward stroke. The downstroke provides acceleration and increased lift. The increased lift results in the upstroke (which is passive). The rider bending his/her knees preparing for the next downstroke facilitates the upstroke. If the rider kept their knees straight, the upstroke would be impaired and the thing would sink.

[Rocket Man]

nice, so the hydro foil angle comes up higher than it does down allowing an easier upward motion and points down only a little so as to support the riders weight better while applying forward thrust.

i need to make me one of those.

[Glider]

You could probably download plans. I'm too far from water for it to matter. Althouh, maybe I could scare the ducks on Hampsted ponds?

[grifter]

maybe I could scare the ducks on Hampsted ponds?

 

not to mention the human inhabitants...

[Rocket Man]

how would you make an autonomous model?

[Glider]

I think you'd need a motor spinning an eccentric weight or flywheel (something like the one below, but smaller) to provide the kind of up and down motion the rider does on the full sized thing (like riding a one pedalled bicycle).

 

You'd need to experiment with the RPM to find the optimal cycle rate. The full sized versions seem to cycle at 100/120 per minute (approximately).

 

Standard RC servos would serve for the steering.

 

[edit]Actually, I'm not sure the eccentric weight idea would work. The rider pushes down and rides up, the weight would provide a rotary force (i.e. backwards and forwards too). I'm not an engineer though.[/edit]

[Glider]

Ok, new idea. Forget the eccentric weight. I don't think it'd work. I think you need something like a piston and crankshaft design, but in this case, a motor drives the crankshaft which drives a weight up and down (like a piston). I think that would more accurately replicate the forces the rider provides.

[Rocket Man]

the eccentric should work. if all else fails, you can use counter rotating ones to nullify the lateral motion.

 

i noticed the down stroke was longer than the upstroke. if you chain drive it, you could use a shaped sprocket to get the right motion.

 

the hydrothopter has a lot of flex in it, that's what gives the strokes their effect. the tilt on the blade passes horisontal between each stroke. steeper upward but reasonably shallow downward. a rubber band and a stop gap for the down would allow a simpler design.

you'd be fiddling with ballast for hours to get the skimmer going...