Author Topic: Prop driven craft that can drive downwind 2.8x (at least) faster than windspeed  (Read 352 times)

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Offline jr2

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Prop driven craft that can drive downwind 2.8x (at least) faster than windspeed
I'll put the details first then the video:

Veritasium - 2 weeks ago:

If you want more detail on the explanation here it is:
1. The car is powered only by the wind. There is no motor or batteries of any kind.
2. The propeller does NOT spin like a windmill. The wind does NOT push it and make it turn.
3. Instead the wheels are geared to the propeller to turn it the opposite way, like a fan, so it pushes air backwards.
4. To start the vehicle the wind simply pushes on the whole vehicle (like a block of styrofoam) and gets it moving.
5. The wheels are turning so they turn the propeller in the opposite direction to how the wind is pushing it.
6. The prop is pushing air back so air pushes the prop forwards, accelerating the car.
7. Once you get up to wind speed there is no apparent wind on the vehicle. If the prop were spun like a windmill this would mean no more thrust. But, since the prop is operating like a fan, it still accelerates air backwards, generating thrust.
8. You can go faster than wind speed continuously because even when going faster than the wind, the prop can still accelerate air backwards (in the car's frame of reference) generating thrust. In a stationary frame of reference you would see that the wind behind the propellor is slower than the surrounding air. So it's clear that the energy is coming from the wind.

FAQ: If power is coming from the wheels to turn the prop, why doesn't that slow down the wheels more than it gets the prop to push back?
A: Because the wheels are moving over the ground much faster than the prop is moving through the air (because there's a tailwind).

Let's say the car is going 12m/s in a 10m/s tailwind, so faster than the wind (note the prop will be moving through an apparent headwind of 2m/s).

Power = Force x Velocity

Let's say the chain applies a drag force of 100N on the wheels to drive the prop. This means we're taking power from the wheels = FxV = 100N x 12m/s = 1200W

If we apply this power to the fan, it can create a force of F = P/V = 1200W / 2m/s = 600N

Admittedly I've assumed no losses, but even if we waste half the power, we'd still get 300N of thrust which is more than the 100N of drag the prop adds to the wheels. The key is that we're harvesting power at higher speed, lower force, and deploying it at lower speed, higher force (which is only possible because we have a tailwind - in still air this wouldn't work because the relative velocity of the wheels over the ground would be exactly the same as the relative velocity of the prop through the air).

Re: Prop driven craft that can drive downwind 2.8x (at least) faster than windspeed
Yeah I watched that, and it kind of hurt my brain trying to think of it intuitively. Which is kind of a fun feeling actually.