Thread: downwind vehicle, can it go faster than the wind

just to be clear, you're talking about a vehicle that produces thrust in the opposite direction that it is moving, correct?

I'd say that similar to magnetic braking, you can never provide enough resistance force to stop the vehicle, because stopping the vehicle means removing the resistance force. in the absence of friction and other parasitic forces, it would assume a state of equilibrium, but in the real world, it would come to a halt eventually, unless it was rolling down a sufficiently steep and infinitely long hill. as for moving faster than the wind, it's hard to say. it really depends on how much thrust the propeller generates, relative to wheel speed.

I guess I was a little confused. I thought you meant that the propeller would provide thrust to slow the vehicle down.

in any case, in the real world, this sort of thing has been tried before throughout history. perpetual motion is impossible because of friction and entropy. you can't possibly make a system efficient enough to run forever.

Originally Posted by Elkvis

In the real world, this sort of thing has been tried before throughout history. perpetual motion is impossible because of friction and entropy. you can't possibly make a system efficient enough to run forever.

It's not a perpetual motion machine, like any wind powered device, it extracts energy from a wind by slowing down a portion of that wind (with respect to the ground). In the frame of reference of the vehicle moving directly downwind, as long as there is a wind, then the ground speed (relative to the vehicle) is always greater than the apparent wind speed (relative to the vehicle). Say the wind speed is 15 kph, then the ground speed will always be 15 kph greater than the wind speed (relative to the vehicle). This allows for the effective gearing that divides speed and multiplies force, such that even after losses in the drive train and propeller, there's still more thrust from the propeller than opposing force from the ground that drives the propeller (but at a slower speed). Again this strategy only works if there's a wind, allowing the propeller to produce thrust at a slower speed.

Say the wind speed is 15 kph and the vehicle speed is 20 kph (downwind). Relative to the vehicle, ground speed is -20 kph, and (head) wind speed is -5kph. So the propeller could be effectively geared quite low if it only needed to produce thrust to operate with a 5 kph headwind (power output), using the 20kph relative ground speed (x force = power input).

Originally Posted by Epy

Might be of interest to you:Betz' law - Wikipedia, the free encyclopedia

Betz law is about the maximum efficiency of a turbine, comparing the energy extracted from a gas or fluid versus the energy output by the turbine. For the downwind case, the propeller is being used as a propeller, and Betz limit doesn't apply. Average propellers on aircraft have efficiency around 80%. Speciaized propellers for slower speeds can be more efficient.

your initial problem specification was unclear, and now I'm even more confused than ever. please stop using words like downwind and upwind. please reference everything relative to the direction of the vehicle's movement, and restate the problem so that it's a bit clearer.

is the propeller driven by the wind to drive the wheels, or is it driven by the wheels to produce thrust?

What propels the vehicle? Is it just the wind force against the propeller?
Or is there some other wind driven thing like a sail?

And is this supposed to start up on it's own from the wind, or is it assumed to already be in motion?
In other words does it have to be initially started with an additional force?

Originally Posted by megafiddle

What propels the vehicle? Is it just the wind force against the propeller?
Or is there some other wind driven thing like a sail?

And is this supposed to start up on it's own from the wind, or is it assumed to already be in motion?
In other words does it have to be initially started with an additional force?

The only net external force is from the wind. The vehicle can start up on it's own from the wind, but during the initial phase the propeller acts as a bluff body (like a sail), simply blocking the wind. Eventually as the vehicle and propeller speed up, the propeller transitions into behaving as a propeller, producing thrust against the wind that is blowing from behind the vehicle, allowing the vehicle to travel downwind faster than the wind.

Since I've already pretty much stated that it's possible to do this, here's a video of a turntable model. The wheel is attached via a cable to the propeller axis. The only motor is the one turning the turn table. As mentioned above, initially the "cart" just blocks the "apparent" wind, but as the propeller speeds up, the propeller produces thrust and the "cart" ends up advancing forwards on the backwards spinning turn table. Youtube video:

http://www.youtube.com/watch?v=MCB1Jczysrk&hd=1

and a video of a full scale model. The streamers at the end of the thin poles face downwind when the vehicle is moving slower than the wind and they face upwind when the vehicle is moving faster than the wind. Skip forward to about 1:22 into the video when the vehicle finally starts moving. The wind was blowing at around 15 mph to 18 mph (24 kph to 29 kph) and the vehicle reached a top speed of 37 mph (59 kph) about double the wind speed. On an official run monitored by a sanction land sailing group, it's best "performance" was about 2.8 x wind speed, about 28 mph (45 kph) in a 10 mph (16 kph) wind.

http://www.youtube.com/watch?v=EEuAqq8FINw

A later video of the vehicle now enclosed in a composite shell reaching about 50 mph (80 kph) in a 20 mph (32 kph) wind.

http://www.youtube.com/watch?v=5CcgmpBGSCI

These guys have a web site now. Last I read they were working on an upwind faster than the wind modification to the vehicle. Based on the math, if the vehicle can go 2.8 x downwind, it should be able to go about 1.8 x upwind.

http://www.fasterthanthewind.org

Originally Posted by megafiddle

Suppose you had a variable ratio drive between the wheel axle and propeller.

That would help with start up, but making a variable ratio drive would be complicated, and it would tend to have more losses. As an option, the pitch (angle of attack) on the propeller could be adjusted, but again that's a complicated setup.

Originally Posted by megafiddle

And you start out with the ratio set ...

You want to start off with a negative ratio, which isn't practical unless you accomplish this using an adjustable pitch propeller, and setting the pitch to be negative so that the propeller acts like a turbine that drives the wheels forward. Then the pitch is transitioned back to positive (producing thrust) as the vehicle gains speed.

Originally Posted by megafiddle

So that the propeller would make a large number of revolutions for a small number of wheel revolutions ... starting from a standstill, I would expect the propeller to be driven backward by the wind, and the vehicle to creep slowly backward (toward the wind).

If the effective gearing is propeller speed < wheel speed, it's a downwind vehicle, if it's propeller speed > wheel speed, it's an upwind vehicle. The closer the ratio is to 1:1, the faster the theoretical top speed, but there's a limit as to how efficient the overall vehicle setup can be. I suspect that the effective ratio for the full scale model is around 8 / 10 (propeller thrust speed / wheel ground speed).

Originally Posted by megafiddle

Instead of travelling at 20 kph in a 15 kph wind, why not travel at 5 kph in 0 kph air?

There needs to be a wind or more specifically, a difference between the speed of the air and the speed of the ground. The vehicle is interacting with both the air and the ground and is taking advantage of the fact that the air is moving with respect to the ground. Using the ground as a frame of reference, the vehicle is extracting energy from the air by slowing down some of the air (the air affected by the thrust from the propeller). It wouldn't be possible for the vehicle to slow down the air if it wasn't also interacting with the ground. You could switch the frame of reference to be the air, in which case it's the ground (earth) that is moving, and the vehicle extracts energy from the earth by slowing it down by a very tiny amount.

All "wind" powered devices are actually relying on the fact that the air is moving at a different speed than the ground. This is true of sail craft, the fastest of which are ice boats, where it's possible for the downwind component of speed of an ice boat to exceed the speed of the wind.

On the other hand, if you had a wind blowing over a frictionless surface, then an object on that surface would just end up moving at the same speed as the wind.

For an unusual analogy, there was a short period in history where brennan torpedoes were made. The torpedo had two spools of wires where the axis were connected to propellers. By using land based spools to pull the wires at some specific rate, the torpedoes were propelled forward by retracting the wires backwards. To make the analogy, the wires would be like the ground for the vehicle, and the water would be like the air. The devices worked because the speed of the wires was different than the speed of the water, and the devices interacted with both the wires and the water. Wiki article:

Brennan torpedo - Wikipedia, the free encyclopedia