Directly UPWIND faster than the wind

Llyricist

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.

harrylin

Ah thanks - that looks not too difficult indeed!

spork

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

ozone

Can someone link me to where I can learn more about the physics behind this.. I find this quite intriguing.

A.T.

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:

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?

jduffy77

You are correct.

CWatters

Google "Apparent wind"

spork

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

That's essentially correct. It's basically a couple of wings swung around an axis. The lift generated pulls the vehicle forward.

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.

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 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.

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.

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).

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).

spork

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.

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.

azizlwl

Just wishful thinking. Then all the physics laws will violated by Youtube.

spork

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.

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?

Llyricist

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.

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spork

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.