A while ago there was a disagreement where some were not convinced that a propeller driving / being driven by the wheels of a vehicle could go faster than wind speed whilst travelling directly downwind.
A better explanation is given in this video:
I'll admit that my money was on Prof Kusenko.
Out of interest, how fast does the car travel when there's no wind?
What is important is the difference in speed.
I thought the treadmill, i.e. still air, was an ideal vehicle to show the concept.
I also thought the best proof would have been to measure torque, and hence the direction of power, from the wheels to the propeller when travelling at an excess speed. And to show the change in sign at some point, from driving to being driven.
I enjoyed the video. Absolutely mind bending.
It doesn't travel at all
You cant take energy out of wind which has none with respect to the ground
According to the treadmill test, in still air, this thing will run faster than the treadmill. Perpetual motion, or there's simply something wrong with the test?
There must be a long corridor somewhere. Why didn't we see one of these machines plonked down on the corridor floor and gaining speed?
The key issue is windspeed with respect to the surface the wheels are on
In essence a treadmill in still air is no different from wind over stationary ground except that in the first case the energy comes from the treadmill
Cant happen, No energy to drive it
Also some more:
Presumably taking some energy from the treadmill.
On the man carrying version, I would like to know how they got it moving initially. Stationary wheels = no drive to the prop.
On 06/07/2021 23:39, Fredxx wrote: <snip>
Yes, but rather than constantly re-directing the thing, a couple of bits of string keeping it on course would have been good, especially if there were some easy way (elastic?) to show the tension.
Well, a stationary prop is still a sail of sorts and gets pushed by the wind.
I think the point is that you cannot get more out of something than is there, but the more efficient it is and the lighter it is the better. After all you can sail a boat into the wind, its the losses and the efficiency, but if you calculate the actual energies involved its obviously not possible to get more out than you put in, you just change its properties, If its speed you want then you can have it but without much pulling power its all a bit academic. Brian
Yes. Won't it rotate in the wrong direction moving the vehicle backwards?
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You could try the Linac building at SLAC. That's two miles long.
No, the prop is geared such that as the vehicle moves forward the wheels turn it so as to blow air backwards. It's a bit like a sail/prop combination.
Um, the prop is driving the wheels. It?s not driven by the prop.
Tim
Um, no.
Tim
A well-reasoned response, but, um yes. As the vehicle moves forward, the prop turns so as to blow air backwards. If that's not obvious, look at the treadmill tests - which way is the prop blowing the air?.
Correct
no.
It is a matter of gearing. If the rotor to wheel gearing is very low it will claw its way upwind. If the rotor to wheel gearing is higher it will get blown downwind and accelerate past the windspeed
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