WPIAeroGuy
Topic Author
Posts: 261
Joined: Tue Aug 21, 2007 11:52 pm

### Air Powered Cart- Is It Real?

http://www.ayrs.org/DWFTTW_from_Catalyst_N23_Jan_2006.pdf

Usually I can pick apart these things right away, but after reading 20 pages of posts on another forum I got so confused I don't know what to think.

I know energy has to be lost somewhere, but I can't put my finger on it. My best guess is that it can be figured out by finding the torque at the prop hub and the torque needed at the wheels to overcome inertia and the front pressure differential.

What do you guys think?
-WPIAeroGuy

rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

### RE: Air Powered Cart- Is It Real?

It’s cute, and quite real.

There is energy in wind, the trick is extracting it. In a sailboat is running downwind, the mechanism (a big old hunk of cloth providing drag) limits you to not going any faster than the wind speed. On a tack, OTOH, you *can* go faster than the wind speed because the sail is a wing and generating lift. It’s basically the difference between a stalled wing and one that’s not stalled. Note that actual sailboats only rarely manage to exceed the wind speed because of the amount of drag from the hull, but that ice boats and land yachts (which run on blades or wheels, respectively) quite regularly manage three to four times the wind velocity on a good tack.

What he’s done is simply provided a mechanism that allows the “sail” to actually be flying unstalled during the downwind run. Basically the rotating “sail” is tacking sideways through the relative wind at all times.

jetmech
Posts: 2320
Joined: Wed Mar 29, 2006 2:14 am

### RE: Air Powered Cart- Is It Real?

 Quoting Rwessel (Reply 1): There is energy in wind, the trick is extracting it.

Sounds about right. The energy available from wind varies as the cube of the wind speed. There are two ways to extract energy from the wind. You can use a drag device or lift device. As mentioned previously, drag devices cannot exceed the speed of the wind. A lift device however, especially one configured as a propeller, can easily be made to have a tip speed that greatly exceeds the wind speed.

As with anything in nature, as you gain one thing you lose another. Although drag devices cannot exceed the wind velocity, they have a much higher starting torque compared with an equivalent lift device.

A good reference for information on drag and lift devices is pages 215-218 of the following,

Spera, D. A. (1994), "Wind Turbine Technology, Fundamental Concepts of Wind Turbine Engineering", ASME press, New York.

The following also illustrates the point. The device labelled "A", is an example of a drag device. The device labelled "E1" is a three bladed lift device. The top graph shows the power coefficient vs. tip speed ratio for the various devices. The second graph shows the axial thrust coefficient vs. tip speed ratio for the various devices.

The tip speed ratio (lambda) is defined as the ratio of the tangential velocity of the device, divided by the wind speed. IIRC, the higher power coefficient achievable from lift devices is due to the fact that the tip speed ratio (lambda) can be easily made to exceed one.

Regards, JetMech

[Edited 2008-07-09 18:37:06]

[Edited 2008-07-09 18:37:43]
JetMech split the back of his pants. He can feel the wind in his hair .

WPIAeroGuy
Topic Author
Posts: 261
Joined: Tue Aug 21, 2007 11:52 pm

### RE: Air Powered Cart- Is It Real?

JetMech, thanks for that graph, that even answers some other questions I had. I understand that the tip speed can far exceed the wind speed, however the part I'm not getting is how this system keeps running.

Would this device not be the same as having perfectly still air and giving it a push? Would it then continue on its own?

Let's assume the drive is at a ratio where tip speed equals wheel speed for simplicities sake. The tip speed is faster than the relative air, meaning the wheels will be spinning fast enough to propel the cart forward faster than the air. I have it up to that point. Where I lose it is when you put this cart on the ground. Is there not a huge energy loss when the wheels actually have to propel the vehicle forward, AND spin the prop at the same time?

Man it's been a long day today, hopefully I'll wake up tomorrow and understand this, but its just not clicking right now.
-WPIAeroGuy

rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

### RE: Air Powered Cart- Is It Real?

 Quoting WPIAeroGuy (Reply 3):Let's assume the drive is at a ratio where tip speed equals wheel speed for simplicities sake. The tip speed is faster than the relative air, meaning the wheels will be spinning fast enough to propel the cart forward faster than the air. I have it up to that point. Where I lose it is when you put this cart on the ground. Is there not a huge energy loss when the wheels actually have to propel the vehicle forward, AND spin the prop at the same time?

Think of it this way. The turbine (let's call it that, since calling it a propeller is a bit misleading, since it's extracting power from the wind, rather than pushing air around), is being spun up to a speed compensating for the carts forward velocity. At that point any wind from will tend to turn the turbine in (more or less) the same way it would if the turbine were motionless and the card standing still.

That energy is what's then used to generate torque to drive the wheels and the turbine to its base "zero equivalent" velocity. The latter is actually a fairly small amount of energy, since it corresponds to no work being done. If the wind suddenly died (while the cart continued to roll), this arrangement would leave the turbine slicing neatly through the air with little drag - unlike a stopped turbine which would have a great deal of drag.

Any left over energy will accelerate the cart, and that can happen until the drag of the entire cart matches the amount of energy that the turbine can extract from the relative wind.

WPIAeroGuy
Topic Author
Posts: 261
Joined: Tue Aug 21, 2007 11:52 pm

### RE: Air Powered Cart- Is It Real?

What do you mean by "zero equivalent"? Is that the speed where the turbine is extracting as much energy from the wind as it take to move the cart into the wind

So if there's a 10 knot tailwind, according to the article, this should be able to 14 knots over the ground. Thats a relative wind over the cart of 4 knots, but as a headwind. That means that "wind" is being created the cart driving itself forward, and its getting power for that from the wind turning the turbine. Its cyclical energy; you can't hook a generator up to a motor and have it run forever.

But now as I type that, I'm realizing that there may be no relation between speed of the vehicle and speed of the oncoming wind. Looks like its not any clearer today
-WPIAeroGuy

rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

### RE: Air Powered Cart- Is It Real?

 Quoting WPIAeroGuy (Reply 5):What do you mean by "zero equivalent"? Is that the speed where the turbine is extracting as much energy from the wind as it take to move the cart into the wind

The turbine is being spun so that it's effectively at rest in the airflow created by the speed of the cart. Imagine the cart going 10kts on a completely calm day. It's being spun so that it's creating a nominally 10kt forward bite into the airstream, which is exactly canceled out by the 10kt forward speed of the cart. So the turbine is spinning at a 10kt rate, but it's producing zero thrust or drag because its also being moved through the air at exactly that speed.

IOW, you've factored out (more or less) the airspeed caused by the forward motion of the cart, and the airspeed the turbine is seeing at that point is effectively zero.

Put another way, the screw that is the turbine, is advancing into its medium (the air) at exactly the same rate as the threads (aka blades) are advancing, and so is generating no force at all (thus requiring little energy).

Now toss in a tailwind, and the turbine will see a force from the tailwind. Assume the tailwind is 5kts. If the cart is at rest, the turbine (which is also be stationary), will directly see the 5kt breeze, and extract energy from the wind based on that. If the cart is moving at 10kts, because the turbine has been pre-spun to cancel out the 10kts of the cart, the turbine will *still* see a tailwind of 5kts.

You can thing of that as (sort of ) producing two forces on the turbine, one is basically torque in the wrong direction (since it’s a tailwind it will be trying to spin the turbine in the wrong direction, thus slowing down the wheels), and a forward force from the wind blowing on the turbine blades. So long as the later is greater than the former, you get net energy input.

In truth you cannot separate the two forces like that, but these are the lift and drag components of the airflow over the turbine blades – the drag trying to slow down the rotation of the turbine, and the lift pushing the cart forwards. With calm winds the turbine is at zero degrees angle of attack (and zero lift), once the tailwind hit, the AoA increases, increasing lift. Just like pitching up in an airplane increases the lift from the wings. So long as the L/D is over 1.0, you’ll be getting a push from the turbine.

There’s no perpetual motion here – the cart’s airspeed is not driving the turbine, the cart’s speed is only used to zero out the effective airflow through the turbine (and to cancel out the drag component from the turbine – but so long as that, plus the drag of the cart itself, are less than the lift, you’ll get acceleration).

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