Sustainable Human powered foiling

I've been obsessed with human powered foiling for the last 25 years, but not the Decavitator style 100m dash at (nearly) 20 kn. I don't have the check book or the maths to back that stuff up.

There's plenty of craft been designed, from Flyak to rowing shells on foils to the latest fad of pumped surf, wind or SUP foils and many an hpb, that can only remain foil bourne at AN or TR (heart rate zones equating to sprinting).

What do folk on here know about successful implementations of longer duration foiling, necessarily at UT1 or even UT2?

I've got to what I think is a good design for a fast displacement hull that flips its foils down (from above the waterline) once its at terminal sprint velocity for the displacement hull. (Circa 8kn). But the drop off speed for the foil, once foiling, is likely more like 5 kn. So quick sprint, pull a (very long) lever to very rapidly drop foils (or more accurately lift athlete, hull, mechanism etc. onto foils, then relax (slightly) and continue for an hour on foils.

Because foils always have a higher ascent speed than their drop off speed this foil swapping MUST have been tried. Anyone seen anything I need to look at before I start working up my plans further.

My mechanism allows me to have foils that can be optimised for a narrower window of angles of attack, so for the given power input they can be more efficient. At no point do they have to be able to drag a slow moving boat uphill and out of the water (which requires a thicker foil that must not have breakdown of laminar flow even at relatively high angles of attack).

I probably don't need to state this obvious and sadly well proven point (but will state it anyway); the commercial possibilities of short duration very high speed sprint foiling only relevant to elite athletes are infinitesimally small.

But, an on-water exercise device that allows the average gym-fit guy or girl to fly up their local river 6 inches above the water under human power alone about 4 knots faster than they would otherwise be going...

I can't be the only one thinking about this stuff.

Ps, I've seen the NZ motor augmented foiling bike that's currently in production in case anybody's about to send that link.

It's not quite doing what I'm trying to do. I first started thinking of using a brushless motor and Lipos and a feathering prop to get on foil, with a lifting mechanism completely removing all motor shaft, prop etc from the water once foiling. But I realized some years ago that it still needs to get up on foil from low, draggy non foiling speed, so still needs those thicker foils mentioned above to get it 'uphill'. And if you're going to have a motor, well that's not human power and you might as well have an electric foil board.
Anyway the NZ foiling bike (which I'm very pleased to see on the market I must add) needs its motor assistance all the time it's foiling. So sort of exercising, and sort of cheating in my view. I don't want to build an electric foiler with pedals on it now that's been done.

Interestingly, I note the NZ foiling bike started out without a motor. It acquired the motor late on in development ( I guess when they realised only Mark Cavendish and his mates would be able to pedal it hard enough to get it on foils). However, my design, I guess ultimately has this backup option too!

 

The only human powered craft with a secondary foils for getting out of the water I have seen was "Decavitator" for a speed record. That seems like too much trouble for an exercise craft.
You have lots of information but nothing the least bit calculable. You need to give some hard info for anyone to help.
what is the weight of the user?
how will they be generating the thrust?
what is their power output max for 1 minute, and sustained for whatever your flight duration.
how is flight height controlled?

 

It's at the drawing board at the moment, just a concept.
Main foil is just behind the c of g in 2 parts, with L-shaped hinged foil components that flip down and thier tips almost meet in the centre. It effectively makes a wide, horizontal foil once the two components are flipped down.

Front foil is a Shutt strut that the mechanism ( of dyneema lines low friction blocks etc will pull down to foiling height at the same rate as the main foil.

Because of the Shutt strut, I shouldn't have to worry too much about pitch control.

I'm aware I need to do the maths on available power, foil sizes and section etc etc.

My question is on the general premise of arranging flip down foils. The principle of having a fast displacement human powered boat that is comfortably capable of going faster than the foil drop off speed of its foils, and using a great big lever or tackle to force it up onto its foils once a fast sprinting displacement speed is achieved. And my question about this is " has anyone done it?"

As I'm just trying to foil at the his stage, rather than beat Drela, or anyone's speed record, I of course also have the option of using larger foils. Foils that would be too draggy at slow speed to ever achieve lift off in a traditional fixed foil design, but once on foil, capable of quite slow foiling speeds. It's only by having a slow foiling speed that it would be possible to get the power requirement right down to where I need it.

This is somewhat different to what Drela did with takeoff foils and cruising foils for Decavitator.

If mine only goes half a knot faster on its foils than it did in displacement mode (for the same energy input), it's still going to be a considerable triumph.

 

It's been done, for short times and pedaled by trained athletes. The limiting factor is power to weight ratio. How much power can you input? Until you can answer that accurately, the rest is irrelevant.

 

And what is the value of that limiting factor? Even if @jakeeeef knows the weight and the power he can develop, without knowing the value of that factor he will not be able to do anything. I'm wrong?

 

Yes

 

Please be nice and give us that value.

 

It is whatever the OP is capable of generating. In English, that would be the limiting factor.

 

And some minutes later :

Was the first sentence written in a language other than English?

 

I'm curious why the op thinks take-off occurs at a higher speed than sustainable flight. That isn't true. Sustainable flight is at a lower lift coefficient than takeoff. And the lift slope is effected by the free surface. Heave stability and other dynamics should be optimized for speeds faster than take-off. You normally want to climb the hull out as you hit hull speed, then accelerate to cruise. The hull provides the necessary stability during lift off phase and acceleration phase, then the foils must be inherently stable at flying speeds. Stability is draggy. It is always draggy. To get out of the hole, you can't afford decently stable foils, you have to skim the hull during acceleration. Once you have doubled your speed and dropped your CoL to maybe 0.2, you can afford the drag associated with dynamic stability. Until then, the induced drag dominates. So lift-off at 7 knots, accelerate to 14 knots, then fly. You can't scale back to lower speeds and still fly, at least not unless you put more power into the system than at 14 knots.

Foil systems seem to add about 30% to the weight of small craft dry weight. So figure a 200 pound guy on a 100 pound boat is 300 pounds. And the best L/D ratio you should expect when flying is about 14:1 for a one person boat, maybe 16:1 for a two person boat. So you need 21.4 pounds of thrust at cruise.

100 Watts input would be 93 Watts at the prop. Prop efficiency won't be more than 60% at the speed I'm about to show you - so 56 Watts propulsive. 56W/1.36 = 41.2 ft lb/sec. @ 21.4 lbs thrust, that gets you 1.92 ft/sec. That's 1.3 mph. See the problem?

 

My answer to you, "on the general premise of arranging flip down foils", is that you don't need extra flip up foils for takeoff because "I'm just trying to foil at the his stage, rather than beat Drela, or anyone's speed record". Dr Drela only did it because he was going for top speed which required getting to the smallest foils capable of supporting the craft at that high speed.

You just want to foil. The Shutt patent shows a viable configuration with a large surface piecing foil that can get you up. You just need to do the math to get the dimensions.

 

Not sure what the point is of the "when pigs fly" scenario. From what I have seen in the past is ~170lb pilot capable of 300w sprint for minutes in 30-40 lb craft at speeds in the teens and I can't imagine they produce a thrust over 12lbf average. There are foiling standup paddleboards for God's sake. 200lb craft at your 16:1 is 12.5 thrust (until we do real foil calc). 200W is a reasonable workout for our athlete and 75% is a more reasonable prop. eff. 200w is 186w at prop. 139.5w propulsion or 102.6 ft lbf/s over the 12.5 lbf gets 8.2 ft/s or 5.6 mph. Disappointing but it is reasonable to foil.

20:1 L/D, 83% prop eff. ~7.7mph. This would be a very good design and it just gets to what my Pax 20 kayak will do.

 

So, I am only going to ask a question. Does a foil really afford backing off much?

I would have assumed there is a hump speed and when you hit that speed, up you go, on plane. But I would not think you can take much off? Because there isn't much keeping you up other than speed.

I don't think phil was at all negative. But just talking about some realities. My hip joints are all shot, so the idea of me flying is not a reality either.

Where can I go read about foiling, as in foiling 101?

 

I re- read Ray Vellinga's hydrofoil book last night. Curiously I had forgotten it has a whole chapter about human powered hydros in it. Somehow I completely missed this chapter first time round. And the configuration of Shutt strut up front and high aspect foil just aft of the C of g is a tried and tested plan for hpbs.

So, it also turns out that part of what I'm proposing ( in terms of input power in and getting on foil) it's already been done.

Steve Ball's Dragonfly takes off at 4 mph, and has allegedly cruised for 45 mins on foil.
I'm unsure how old Ball was at the time, but his sprint takeoff power is quoted at 450 watts, and his sustained is 120.

So there are, in existence, foiling personal exercise devices. Ray's book provides a menu for building one. BUT, they are custom built, incredibly delicate, insect- like craft.

I wonder if my system of keeping the foils within the perimeter of the craft and folding them down when needed could be a worthwhile development or not. It might be a step towards making something marketable.

The main foils, as L foils would be lower aspect than the flat mast supported foils on most successful hpbs, and I need to do some maths on that one.
But (again thanks to Ray Vellinga - P38, 39) I note that the necessary angle of attack and hence efficiency of a hydrofoil only drops off a cliff below an aspect ratio of about 6. With my ( limited) composite skills and equipment I can do L foils that will be strong enough at an aspect ratio of about 8. Just not the AR of 20 or so that many hpb main foils have.

In answer to some of the questions posed above.
Power output- 500w sprint, 180 sustained. (Before prop/ transmission losses). These are conservative estimates. Many a 25 year old club cyclist can squeeze out 600w in a sprint, more on a recumbent due to the backrest. An elite stage race sprinter can throw down 1100w, at the end of a tour de France stage, 1500 miles into a 3 week race. ( Although that's irrelevant here, I just like the idea of that.)

Not sure where people are getting 100w sustained from though. I said 'gym fit' and did not mean a seventy year old.

Total weight, rider 85 kg, displacement hull 14 kgs, foils, amas, crossbeam and mechanisms 15 kgs. Let's say 125 total.

As for drop off speed being different from ascent speed, I'm basing this on conversations I've had with foiling moth sailors. They say they ascend at about 8 kn but can cruise at 6 or even 5, remaining on foil through a wind lull. Part of this might be due to thier adjustable foil trailing edge though and I stand to be corrected.
On a simplistic basis, doesn't a foil have to be 'driven uphill' to get on foil, but it is only having to drive on the flat (or more accurately a less steep hill) when it's in level glide?

 

We can break drag into three categories that relate to our geometry. There is friction from wetted surface area that increases linearly with speed. There is wave resistance that increases exponentially after the quarter wavelength. There is induced drag that declines as Cl is replaced with V^2. There are eddy losses, but if you make those you have no business worrying about the rest.
A displacement hull has friction and wave resistance that gets limiting at higher speeds. A foil can't lift the craft at low speeds, has too much induced drag when it begins to lift, then as it gets to having extra lift it can be trimmed to reduce induced drag and speed up to the new friction dominated drag. There is a resistance 'hump', but power needed might still increase because the same force at twice the speed is twice the power. Getting this craft up out of the water and to controlled stable foiling is far from trivial as Phil pointed out. The power calculations quantify what it takes to make this work ~1W/lb and a very high quality build just to equal a racing plyak.