The Wind Powered Sail-less Boat

apologies if this answered in another post.

would the groundspeed above windspeed remain as a constant %age?

lets say the propellor exerts 10% of the vehicles grounsdpeed as thrust

so the tailwind is 10kts, vehicle speed would be 11kts.

tailwind increase to constant 30kts, vehicle speed becomes 33kts.

tailwind drops to 15kts, vehicle slows until the propellor thrust is exerting a force on the tailwind resulting in 16.5kts

i don't know if the above %ages are workable, i just plucked them from the air. i just want to make sure i'm understanding the general idea and what happens as the speed increases and decreases

 

That is roughly right.

You need a certain windspeed to get the prop working in a reasonable regime. Some have suggested that in light wind you start with the wheels declutched until the tailwind builds some speed and then drop it into gear.

In wind around 10kts and more the vehicle should have no trouble self starting. If the gearing is fixed then the % increase over windspeed will not change much with wind strength. THere is some variation because the prop slip reduces as the apparent wind increases.

If it has variable gearing then you can take advantage of this when the prop is in a nice regime. The overall efficiency limits what % is possible.

A streamlined vehicle with low rolling resistance can double windspeed without too much design finesse.

The best I can get a manned boat in my modeling is about 20% above windspeed but its drag rises faster than a road vehicle.

I am finding with the boat design I am playing with that it is very sensitive to gear ratio and only works in a narrow range of gearing. This is unexpected. I have not worked out if this is a fault in the model or a real limitation. It would mean in practice that you would need to be able to fine tune the gearing even to get it to work. The road vehicle is likely to have a wider range of operation for any gear setting.

Rick W

 

A long time friend of mine is retired airforce and used to fly the huge refuelling planes (kc-135) which weigh i forget how many hundred tons.

We had a huge argument over what limited angle of climb. He insisted that the limit was angle of attack, as in as the nose goes up so does angle of attack and eventually stalling! I could not believe he did not understand the difference between angle of attack and pitch attitude.

Yet he was completely unmovable in his misguided ideas , concluding with the classic arrogism ; "well, lets see you fly one and then you'll tell me"

So there you have . Just because you can fly a plane does not mean you understand how it really works.

 

The IRF of the cart is not the only one where case on the treadmill & outside on the road are identical. Any IRF will do, you just have to have both IRF used moving at the same speed related to the cart to make it the same.

 

Have you tried the method of asking them what would be realistic thrust a prop can produce in certain airspeed and at given shaft power, and then ask at what groundspeed a wheel can generate same torque*angularspeed with given drag force less than the prop thrust. And then just combine those answers. Are they going to argue against their own results or just refuce to talk about it ?
That approach won't work on anybody not familiar to calculating the specs but why doesn't it work on aero professors ?

 

All you have to realize is that wind is defined as movement of air related to ground. In the treadmill case define ground as moving belt on treadmill and you have the air moving related to the ground in the treadmill case. Exactly as in the outside road case. Now in both cases moving air relative to ground called as wind provides the power exactly same manner. No differences there at all.
You can consider youself standing on the belt, which we call ground and the situation is the same as standing on the ground on the outside case. Just add curtains to block the view from treadmill case so you don't see the room or the treadmill below the belt. Same thing as blocking your mind from thinking about earth moving around sun while discussing about the outside case. That blocking doesn't bother you so why would the curtain ?

 

Unless in heavy wind conditions structural loads exceed limits before that or if gigantic pitching moment compared to ice-boats with same downwind VMG cause pitch poling or wheels slip. Any of those will limit top speed rather than efficiency if wind speed is increased far enough. And that happens quite soon.

 

Devilment and communication and learning...

After seeing that completely daft conveyor belt plane mythbusters,

What if we put wings on the little trolley cart on the treadmill?

Sorry, couldn't resist. if there was a horned smilie, I'd put it in here...

On a serious note, the method of getting difficult concepts across fascinates me. Often, something that is relatively straightforward is obscured by lots of detail, or lots of unknowns, that appear relevant, but are not.

My maths and physics are too rusty to follow mathematical arguements in any detail, certainly in the time I can squeeze to follow this thread, but what does it for me is finding a useful image, or mental model, or metaphor which
gets the idea over. This is absolutely key. As I ran through in an earlier post, I thought about the DWFTTW boat version without the water 'prop' on, watching it drift downwind at close to windspeed, in still water, then imagined sticking the water 'prop' on and it turbining around in the relatively fast water stream, and thus driving the air prop, which claws its way down the relatively close to stationary air stream, and thus goes faster than the wind. I have been totally happy with this, reinforced by thinking about bicycles, and have never had any doubts whatsoever. However, I can't follow all the maths!

Different strokes and that.

 

As far as ddwfttw is conserned the cart on the treadmill isn't a simulation, but the cart on the road with tailwind is, because tailwind has turbulence & fluctuations in airspeed & fluctuations in direction. It all makes it a simulation of ddwfttw unless you go significantly faster to exceed even instantaneous top windspeeds during gusts. You can accheave that and possibly already have. It still leaves the simulated ddw course which degrades it's performance.

 

Powered by a treadmill/gravity vs. powered only by a free air stream are two different animals.

The problem as stated asks if you can exceed wind speed when moving 'directly' downwind. As your vehicle speed approaches wind speed, the apparent wind velocity drops to 0 leaving you with no energy source. So you hit a wall as you approach the wind speed.

What about ice boats? As I understand it, they are angled to the wind, not moving directly downwind. This allows them to exceed wind speed. But I suspect that if you measured their speed in the direction of the wind rather than their forward speed (take the dot product of the normalized wind vector and the vehicle's velocity vector) you'd find it's still moving less than the wind speed.

 

You don't "hit a wall" at wind speed because this cart exploits the energy available at the ground/wind interface - which still remains regardless of the speed of the cart.

What you suspect is wrong. The downwind velocity component can be as much as 3X to 4X the true wind speed.

 

No difference what so ever, even gravity is the same in both cases.

Apparent wind becoming 0 doesn't remove the energy source. Source is truewind over ground below wheels regardless if the ground is rubberbelt or some othermaterial as long as it doesn't cause wheels to slip.
Apparent wind is never energy source for any kind of vehicle. If you tryed to use that you would simply slow down by eating up your own kinetic energy. That's not how traditional sailing boats work. They use energy from truewind over water and the amount of energy they can extract depend on apparent speed the sails see, or in ddwfttw craft case the apparent speed blades on the prop see, which is never zero as the blades rotate around the axis.

Not so, ask any one who has ever sailed on iceboat and they all tell you are dead wrong on that assumption. Or travel north and see for youself.

 

I'm not worried about the RATE of acceleration, just where the force comes from. An aeroplane is not a useful analogy for that, because it is in contact with only one medium, not bridging two media and deriving power from the relative velocity between them.

I think I'm happy now:

- an increase in tailwind increases the force pushing the cart downwind, so accelerating it

- after the cart has reached a new (greater) downwind velocity, the wheels rotate faster, generating more power.

- the increase in power increases the force supplied by the airscrew.

- the cart speeds up, increasing frictional and aerodynamic drag

- when things balance out again, we have a new steady state with a higher constant velocity

To get the true picture, you have to keep reducing the interval in the description above and see what the limits tend to (i.e. you can only get a proper picture of the dynamics using differential calculus, and I can't be bothered to work it all through once I see where it's leading).

It IS a question of dynamics, not statics. The system only accelerates if the rate of increase in power exceeds the rate of increase in drag. That's why the cart in the video I found needed a push to start it. From rest to slow forward speed, drag rose faster than propulsive force (sticky wheels, bumpy road surface?).

 

You seem to be on the right track now.
But you only need to use dynamics analyses if you want to see how it accelerates, to see the end result you only have to do statical analyses on the top speed case using v as a variable, not a constant and see where it balances out. In addition one should show that there is execss thrus accelerating it in all speeds below top speed by statical analyses.
No complex dynamical analyses needed.

 

Good point, Sailor2. And I've just realised I've made a classic exam mistake of not reading the question carefully. In Slide 2, the battery provides power to turn the prop, but I have not yet been told how the 3 m/s apparent wind provides the remaining power.

One way would be for it to drive another airscrew, which drives the wheels - in which case, the next step would be to increase the size of the airscrew until it can also replace the battery as a power source.

I can't wait to see what Slide 3 brings.