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Old 10-24-2010
otaga05 otaga05 is offline
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Quote:
Originally Posted by tomaz_423 View Post

I have problems understanding this part:

Let us say the vehicle is traveling DDW.
Slower then the wind. It accelerates.
It reaches the speed of the wind.
What apparent wind does it have at that point? 0. Nada. Zero.
So, what the hell is turning the prop at that moment?
If there is no force from the wind it must come from somewhere else or we could make wind turbines to generate electricity in no wind.
I must take a closer look at this. At first glance it seems not possible.
The wheels are pushing the prop. It is the thrust from the prop that is pushing the whole thing forward, not the wind as such.

Consider the example they give of the device operating on a treadmill with no wind. The treadmill turns the wheels, the wheels in turn operate the prop and the prop pushes the device forwards against the direction of the treadmill. In order for this device to work the forwards thrust from the prop has to be larger than the backwards force of static friction on the wheels. (If there is a hoax here, whether or not that is possible is at the crux of it.) The increased kinetic energy of the device as it accelerates forward comes from the treadmill motor.

Imagine that you are sitting in the device and it is moving downwind at wind speed. You sense no apparent wind, just like on the treadmill and the ground is running backwards underneath you just like the treadmill belt. The prop takes air that is at rest in front of you and gives it a backwards velocity from your perspective and the air gains kinetic energy. Where does the energy to do this come from? The Earth. When your wheels are pushed backwards by friction, you exert a forward force of friction on the earth slowing it down a tiny bit and reducing its kinetic energy. (Remember that the earth is moving backwards in a frame of reference where there is no apparent wind.) You don't notice any significant change in the motion of the earth because it is so massive.

From the point of view of a person on the ground the air that was moving faster in front of you has now been slowed down behind you by the prop. (Think of what an oar does to the water in a river as you row downstream.) The air has lost kinetic energy, and this kinetic energy loss is where the increased kinetic energy of the device comes from, and the ability to offset the energy loss due to drag. Where the energy appears to come from depends on your frame of reference.

Another way to visualize how this works is to take a spool of thread and set it up so that the thread unwinds from the bottom. Pull on the string and you will find that the larger force of friction on the edge of the spool (the thrust of the prop) causes the spool to move forward against your pull on the thread (the friction with the ground.)
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