## Directly UPWIND faster than the wind

 Quote by rcgldr The ice boats seem to get their best ratio of boat speed versus wind speed with a wind speed around 10 mph == 16 kph.
I don't know if that is true in the absolute sense, but in the case of their downwind VMG, the multiple definitely does get higher with higher wind speeds. And their limit is also determined by their structural strength.

 Quote by A.T. http://www.instructables.com/id/Down...The-Wind-Cart/
Ah thanks - that looks not too difficult indeed!

 Quote by harrylin Ah thanks - that looks not too difficult indeed!

If you build one you definitely have to let us know how it goes. You too can play a part in the grand hoax. :)

 Quote by ozone Can someone link me to where I can learn more about the physics behind this.. I find this quite intriguing.
This paper handles both cases directly up & down wind faster than the wind:
http://orbit.dtu.dk/fedora/objects/o...748519/content

This video explains the basic mechanical principle, also for both directions:

 These may be silly questions, but that never stopped me before. When going upwind, is the speed measured in multiples of the wind? So if you are doing twice the wind speed, the ground speed is twice the wind? So does that mean the wind felt by the craft is three times wind speed? 10mph wind. 20 mph up wind. Wind from the crafts POV is 30 mph?

 Quote by r-j These may be silly questions, but that never stopped me before. When going upwind, is the speed measured in multiples of the wind? So if you are doing twice the wind speed, the ground speed is twice the wind? So does that mean the wind felt by the craft is three times wind speed? 10mph wind. 20 mph up wind. Wind from the crafts POV is 30 mph?
You are correct.

Some questions were posed via PM that I'd like to respond to on the thread (I've already responded by PM as well)...

 First I had to make sense of what a propeller even does, and from what I gather it produces propulsion by pulling the vehicle into the air in front of it.
That's essentially correct. It's basically a couple of wings swung around an axis. The lift generated pulls the vehicle forward.

 The wheel/propeller are connected through some sort of transmission.
That's correct. There is a fixed ratio transmission between the propeller and wheels in the downwind case, or between the turbine and wheels in the upwind case.

In both cases we use ratchets to let the prop or turbine spin freely if it gets ahead of the wheels. This relates to practical matters and allows us to prove we're not using stored energy. Ratchets are not a necessary part of the design.

 Next the gearing is set so that the propeller has to spin faster than the wheels turn (not too sure of the exact ratio here but I think it needs to be greater than 2).
Not exactly. It's less about the rotational rate of the wheels and propeller, and more about the rate each advances through (or on) its medium. For the downwind cart, the prop needs to advance through the air more slowly than the wheels advance over the ground. It's exactly the opposite for the upwind vehicle.

 In the initial accelerating phase i.e. when the cart is slower than the wind you are simply drawing power from the wind until you reach some cutoff point.
In the downwind case, the whole vehicle (prop included) is simply a bluff body being pushed by the wind initially. As it begins to move, the prop starts to turn (since it's geared to the wheels). This of course begins to produce thrust. There's a smooth transition between the bluff body phase and the thrust phase. Prop thrust is the sole contributor by the time we reach wind speed.

In the upwind case, the turbine is turned by the wind, and that in turn turns the wheels. There is no change in phase in this case.

 Once you hit this cutoff point the cart then actually begins to draw power from its own forward movement...
Yes and no. We have to consider the relative wind over the vehicle when doing the analysis, but we never use any stored energy or momentum of the vehicle to our advantage.

Whether a wind powered vehicle can "make its own wind" is a bit more of a philosophical question, but my inclination (without going into a lot of detail) is to say that it really can't. You could argue either side of this, but you don't get something for nothing as it might otherwise imply.

 ...and the potential difference between the velocity of the wind at the propeller and the wind speed at ground level (this part is probably the most confusing aspect of the whole machine, and I am not entirely sure how this process works).
It sounds like you're talking about wind gradient. While it's possible to design a vehicle that exploits wind gradient, this one does not. This vehicle exploits the energy available from the relative velocity of the two media (ground and air).

Recognitions:
Homework Help
 In the initial accelerating phase i.e. when the cart is slower than the wind you are simply drawing power from the wind until you reach some cutoff point. Once you hit this cutoff point the cart then actually begins to draw power from its own forward movement.
In both cases, the cart draws power from the wind by slowing the wind, extracting kinetic energy from the wind (using the earth as a frame of reference). Even when the cart itself is moving faster than the wind, the thrust from the prop is slowing the wind (wrt earth).

 Quote by rcgldr In both cases, the cart draws power from the wind by slowing the wind, extracting kinetic energy from the wind (using the earth as a frame of reference). Even when the cart itself is moving faster than the wind, the thrust from the prop is slowing the wind (wrt earth).
Absolutely right. It's also worth mentioning that, while there's a sort of change in operation mode for the downwind cart, the change is not fundamental. For the downwind cart, the wheels turn the propeller - always - at all speeds both above and below wind speed.

For the upwind cart, the turbine always turns the wheels.
 There is an easy way to understand the feasibility of the upwind cart.. That's to understand that individual blades of the prop do not move directly upwind. They are "tacking" in 3D.
 Just wishful thinking. Then all the physics laws will violated by Youtube.

 Quote by CWatters There is an easy way to understand the feasibility of the upwind cart.. That's to understand that individual blades of the prop do not move directly upwind. They are "tacking" in 3D.
This is true as well - and is true of both the upwind and downwind cart. In fact this is how I originally conceived of the downwind cart (though it turns out I was not the first to do so).

To understand that the blades are on a continuous helical tack, it's of course important to understand that their gearing to the wheels replaces the constraint that would otherwise be provided by the keel of a sailboat.

 Quote by jduffy77 You are correct.
There's something I rarely see. ;)

What causes the cart to reach a maximum speed? If the faster it goes the more wind it 'feels', is there a limit to how fast a craft could eventually reach?

 Quote by r-j There's something I rarely see. ;) What causes the cart to reach a maximum speed? If the faster it goes the more wind it 'feels', is there a limit to how fast a craft could eventually reach?
The advance ratio between the rotor and the wheels sets the hard theoretical limit.

For instance, in the downwind case, a propeller pitched and geared to advance half as far through the air as the wheels go over the ground, will be limited by that to twice wind speed. Of course, since a propeller can never achieve "hard gearing" with the air to do that (there will always be "slip"), the limit will be less than that. An example of a very similar situation is:

But in real terms, it is mostly aerodynamic drag force, as well as the other efficiencies, that catches up to the the thrust force and prevents further acceleration.
Attached Thumbnails

 Quote by r-j There's something I rarely see. ;) What causes the cart to reach a maximum speed? If the faster it goes the more wind it 'feels', is there a limit to how fast a craft could eventually reach?
The vehicle has a "design speed" given by the turbine pitch and gearing. This is what we call the "vehicle speed ratio" (VSR). It's the rate at which the turbine would advance through the air divided by the rate at which the wheels advance over the ground - keeping in mind the two are connected by a transmission.

If that number is greater than 1.0, you have a vehicle that can theoretically go directly into the wind faster than the wind. If it's less than 1.0, you have a vehicle that can go directly downwind faster than the wind. The closer you get to 1.0 from either side, the greater a multiple of wind speed you can theoretically achieve (this can be worked out through simple kinematics).

But... the closer you get to 1.0, the greater the required efficiency needed to make the vehicle work at all at that VSR. You can change the VSR by changing the gearing or the prop/turbine pitch. With the Blackbird, we do have the ability to change the prop/turbine pitch on the fly. We can change the gearing, but only while stopped.

That being said, there is no theoretical limitation on maximum multiple of wind speed. It's just a matter of building a vehicle with high enough efficiency - and operating in a low enough wind.

The reason you need to operate in a low wind if you want to achieve very high multiple of wind speed is that you start fighting compressibility effects of air at higher speeds. This will inherently reduce your efficiency, and therefore limit your multiple of wind speed.

 Tags directly upwind

 Similar discussions for: Directly UPWIND faster than the wind Thread Forum Replies General Physics 305 General Discussion 1 General Physics 172 General Physics 73 General Physics 175