The Wind Powered Sail-less Boat

It is helpful here to imagine the plane is a keel at the bottom of the loop and a sail at the top. It uses momentum to transfer the energy between the fast and slow moving parts of the fluid. Kind of like 'bouncing' back and forth. The whole plane is acting as a unit , the wings are vertical purely because that is the angle it must be at to counter the centripetal acceleration.

 

True enough. I stand by my statement that a plane flying in the jetstream is not taking advantage of the differential velocity between two media. However, dynamic soaring does do exactly that.

You beat me to the punch. The only thing I would add is that dynamic soaring is almost exactly like sailing except that it uses the plane's inertia to connect the keel and sail rather than static structure as is done with a sailboat.

 

This is to an observer hovering in the bottom airmass.
To an observer hovering in the top airmass, the wing is a sail at the bottom and a keel at the top.

The same type of viewpoint symmetry applies to a sailboat.

1) To a swimmer treading water:
The sail has a propelling force along the boat's path through the water.
The keel only provides a sideforce.

2) To a guy in a hot-air balloon:
The keel has a propelling force along the boat's path through the air.
The sail only provides a sideforce.

I'm sure lots of people are familiar with this air/water symmetry, but it's useful to point out here, because it's somewhat more obvious with the DS glider.

 

A variable pitch prop would be very useful, but not necessary. With a fixed pitch, the prop will definitely not have an autorotating moment at low speed while the air is blowing through the disk from back to front. But it doesn't need to, because the drag will cause it to roll forward and power the prop with the wheels. It is stalled in that condition and not very effective. But it doesn't need to be. With variable pitch, the initial acceleration would be greater. But that is only a difference in degree, not a difference in kind.

If you are restricted to fixed pitch, the pitch angle may have to be flatter than would be the case if it were optimized for high speed at cruising. This is the same as with aircraft. If you have a fixed pitch propeller, you have to choose between a "climb prop" and a "cruise prop". The former has flatter pitch and is optimized for specific excess power at low speed for takeoff and climb. The latter has a higher pitch, optimized for cruise performance, at some sacrifice to takeoff distance and climb.

A Bauer machine with negative pitch will have the most acceleration from a standing start, but won't be able to exceed wind speed. A Bauer machine with positive pitch may have very poor initial acceleration. In an extreme case, the prop may even try to rotate backwards under static conditions, driving the machine upwind instead of downwind. But there will be a positive pitch flat enough that the rotor will be stalled and not able to resist the torque from the wheels as drag moves the craft downwind.

Then the prop starts rotating the right way, developing thrust, and it's off to the races from there. Until the angle of attack on the blade elements drops to the point where the thrust equals the drag and resistance from the wheels an it settles into equilibrium.

That equilibrium may be slower than the wind, or faster than the wind, depending on the details of the gearing, prop pitch, rolling resistance, internal losses, etc. But again, that is only a difference of degree and there's no inherent reason for the craft to be limited to wind speed.

So I agree that the prop of a typical fixed-pitch Bauer machine does produce thrust all the time. And a variable pitch prop could augment the drag and produce drive through the wheels at low speed, transitioning to being thrusting as the craft accelerated. Just like a helicopter can transition from autorotating to climbing.

 

I think you're preaching to the choir Tom. I'm not sure if I've made all those points on this forum. But I've certainly made them a number of times. On the upside, it seems we're in complete agreement.

 

I'm slow answering, because these days there's a lot of inspecting fishing boats work at my office (630 boats!) on top of a new business developing, so my spare time is scarce and when I get it, I'm quite tired.

Let me explain how I see the rotored cart/boat thing with some more detail.

As we all know, the ice boat gets energy continuously by 'cutting' through new not disturbed air all the time. Even when repeatedly tacking downwind it can do it if it has developed already a projected downwind speed greater than the wind's.

To reach such a projected downwind speed, the cart needs first to accelerate in a not tacking course through undisturbed air, for enough time to get the needed energy. Once it reaches it, there's no problem in keep on going faster than the wind.

But with the rotored cart/boat the (powered by the wheels/turbine) air rotor turns in disturbed wind until the cart reaches an speed (and the rotor a turning speed) such that the blades see a 'usable' apparent wind (this may happen before the cart reaches wind speed, if pitch is adequate). So, to accelerate in the initial stage only drag of the wind over the cart structure and the blades of the rotor is the source of power. The problem is precisely to know if it can get enough energy from such drag alone to accelerate it to the point where the rotor begins to see usable apparent wind at its front, and begin to send a flow of air backwards. This is the first point where I have doubts.

And there is an extra difficulty: when the rotor approaches this point, that I've called the 'stagnation' point (not a good name, I have to recognize), the air coming back from it hits against the still faster incoming wind, is inverted by it, pushed ahead of the rotor and then sucked into the rotor again, thus developing a a vortex ring. I do not have clear if this happens, but if it does it can be a barrier which may by itself avoid the cart to reach and overpass wind speed.

Not easy to me to solve it with mental speculation only. We (I) need some good maths here, or then a properly performed test (not the treadmill, of course).

We (at least me) need to keep on thinking.

Cheers.

 

Ever sail an iceboat or landyacht? That's not how it works at all.

There's no need for cutting through undisturbed air. It's purely a matter of reducing drag to get a higher lift/drag ratio than a watercraft. There's little drag penalty to the "hull" for going faster with an iceboat or landyacht. You'd say they were easily driven if they were on (liquid) water. The only reason your typical cruiser doesn't get up and go 30 kt is because the drag rises rapidly as the speed increases.

Sailing an on land is no different than sailing on water. The apparent wind is just a whole lot farther forward. Just like a decent racer-cruiser multihull goes downwind best with the apparent wind on the beam, an iceboat or landyacht goes downwind best with the apparent wind well forward of the beam. Closehauled, in fact. But it's still the same kind of sailing any other sailcraft does with the wind ahead of the beam.

Rubbish. Pilots on land jibe as they need to even while accelerating down wind if they're running out of space or have another yacht coming at them. The fastest way to accelerate is to head off on a beam reach to build speed and then head down, but that's true of almost any sailing craft on water or land.

Really, it's just like normal sailing. Only 4 times faster.

There is, indeed, a vortex ring state. If you think about it, you'll realize that it can only exist when the craft is traveling well below wind speed, when the relative wind is strong enough blowing from the back to be greater than the induced velocity over part of the disk. Near and above wind speed, the air is flowing through the rotor disk from the front of the craft to the rear all across the disk.

Why not the treadmill? It's no different than observing the craft from a chase vehicle running along side at wind speed. The whole controversy has to do with the transition from traveling slower than the wind, through wind speed, to faster than the wind. Nobody is saying that if one can travel, say, 1.1 times wind speed, one can't go 1.2 times wind speed. It's all about whether one can go from (to pick some numbers) 0.9 times wind speed to 1.1 times wind speed. Wind speed is where it's at, and a treadmill is perfect for observing what's happening at wind speed.

 

Back to the model sailplane for a moment.

Sailplanes use gravity as the driving force. In still air, they descend to the ground. They stay up (or even rise, quite quickly at times) by flying in an air mass whose velocity has an upward component. In this case, the hill is diverting the wind upwards.

In still air, a sailplane can perform a vertical loop. In doing so, it transforms gravitational potential energy into kinetic energy as it dives and transforms it back to potential energy as it climbs back up. It does not reach its original height because some energy is lost to overcoming various forms of drag as the sailplane moves through the air, so within a few loops it will hit the ground.

A sailplane in a current of air moving vertically can loop continuously, maintaining or gaining mean altitude.

The track of the sailplane in the video can be split into two components - a vertical loop and a descending spiral - with mean altitude maintained by the upward component of the wind velocity.

There is no need for any wind shear to make this flight possible.

 

There is no need for wind shear to make the flight you described, but there is a need for wind shear to make the flight in the video. The speeds achieved in the flight pattern followed can't be done on just a rising air mass.

 

I have to admit the speeds are high. Now you've made me go away and think again. Is nothing as simple as it looks in this world? =>

 

OK. Didn't take long this time. From my (limited, but including helping ground crew at a world championship event) experience of sailplanes, I admit that the flight pattern and the speed at various points in the loop don't tally with the wind strength experienced by the guys on the ridge. And I accept the transfer of momentum explanation. Fascinating.

 

Thanks for the explanations - really helpful. And the plane using its momentum to connect 'sail' and 'keel' behaviour - wild! yet when you watch the video, it intuitively makes perfect sense. And Mike K-H, Spork, re reading my first post on this - the way I baldly quoted you seems confrontational- my apologies, the last thing I intended- It was just your posts got me thinking and reminded me of the video.

 

That's been done to death on this thread -- just because you insist on seeing math that says "it can't be done" while excellent math minds produce math saying "It can" doesn't make their math bad.

The folks with the math have asked you repeatedly to show them exactly where their math is flawed, but as you have stated, responding to such questions just isn't your M.O.

Once again the flaw is yours -- the treadmill is by far the most controlled, reliable and thus "proper" place to perform the tests. Your lack of understanding of Galilean relativity simply puts you at a disadvantage in grasping this.

JB

 

Guillermo

Not sure what you are saying about the standard of my calculations here. The numbers I have provided are within 2% on each element of the boat - hull drag, turbine performance and propeller performance.

If you want to look at the situation at half wind speed. By that stage the turbine is in a much better operating regime than the prop. (Consider the energy in the respective streams) At this stage the mechanical advantage of the turbine of 1.6:1 will guarantee the turbine spins the prop and it creates forward thrust.

There would be some advantage in reversing the pitch of the prop to use it to accelerate the boat downwind but it is not necessary. It still gets there but not as fast as it would by actually powering itself downwind rather than the prop being forced through a stall condition at some point.

Rick W

 

I thought you were being quite polite. I don't go out of my way to confront people, but I am very used to people doing it to me. I have an intelligent but arrogant son whose company I enjoy until I get exhausted - and I've worked with one or two really aggressive characters in my lifetime.

In fact, I was once appointed to hold the fort for my absent manager largely on the grounds that at least one person hated the guts of each of the other three candidates...