Can turbines on the front of cars ever pay off? [closed]

Question

This news article is proving to be quite controversial here. Basically it's an electric car with a turbine bolted onto the front. On the face of it sticking a turbine on the front is going to increase drag and the intuitive assumption in that scenario is that the electricity generated in this case won't beat the extra electricity used to counter the increased drag.

However in the light of Blackbird, a land yacht noted for going downwind faster than the wind it seems harder to rule out the possibility that such a system could, at least in theory be more efficient.

What's the link between the two? Is the former as much of a hoax as it seems it ought to be? How does that get reconciled with Blackbird and other similar "dead downwind faster than the wind" vehicles?

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I can see at least one important difference between the two cases that I missed when I first considered this problem. I initially assumed that the case of both vehicles going dead downwind at more than the true windspeed was identical, but that's not the case.

A key feature of the explanations of "downwind faster than the wind"¹ I've seen is the fixed transmission in the case of vehicles like Blackbird. This transmission can run either way - with wind turning the wheels or wheels turning the blades. This is an important difference in the transmission of the two though. For the electric transmission in the car there's only ever transfer from the turbine to the wheels. For Blackbird that transmission can run both ways because it's just a few gears, so the wheels can drive the propeller as well. Without that DWFTTW won't work.

(Even with that the position of the turbine on the car is such that it would just be blowing air at the front of the car which wouldn't be terribly helpful either)

Which means that for the car case as far as I can see the only way it can possibly be a net gain is if the turbine inadvertently has the effect of reducing the drag (which might just be possible if it makes a blunt nose more curvy overall). Either way though there are far better ways of making it more streamlined so it seems incredibly unlikely that there's anything but pure quackery at play.

^{This might be a topic for a question on its own, but my original question presupposed that Blackbird is not a hoax. There is an existing question on this topic on physics.SE but the answers aren't brilliant.}

Answer 1

The blackbird land yacht can go downwind faster than the wind for the same reason that sail ships can sail against the wind at an angle (called "close hauled") and reasonably light sail boats, e.g. lasers and wind surfers, can also go faster than the wind: The wind is not actually pushing the sail/propeller (henceforth called "airfoil"). Rather, the shape of the sail creates a pressure differential between its two sides when the wind passes around it. Essentially, it is similar to how a wing works.

As an aside: Boats cannot go faster than the wind when sailing directly downwind, because in that case, the wind is literally pushing the sail and thus, the boat cannot go faster. However, when the boat goes downwind at an angle (called "broad reach"), the air flows around the sail, and the pressure differential is actually greater than when running straight downwind. If the boat's drag through the water is low enough, it can then go faster than the wind.

More details about how sails work: http://www.sailtheory.com/sail.html (mainly aerodynamics and Newton). Also, there is some explanation of how sailing faster than the wind works on Wikipedia: http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind

The blackbird's propeller is, in effect, just a fancy adjustable sail that, instead of pushing the craft directly, turns the wind's force into torque. Propellers are airfoils, too.

However, and this is the important part: Despite going faster than the wind, the force is still supplied externally. It is this external wind that is powering the craft. If you tow the blackbird up to speed on a totally windless day (so that there is apparent wind due to the blackbird moving through still air) and then release it, it will coast to a stop.

Answer 2

The car with the fan on the front is just a tired old perpetual motion machine.

But the land yacht that travels downwind faster than the wind is not. I was very skeptical of it until I understood how it works. It is a clever application of leverage.

There are some very clear videos showing how that works here, and illustrated by this simple diagram:

In other words, an object can move faster than what is pushing it, provided the part of the object where the pushing happens is not moving faster.

The simplest everyday example I can think of is a baseball bat, where the batter's wrists move at a certain speed, but the end of the bat moves much faster.

The car proper is not being pushed by the wind, because the car is traveling faster than the wind. But part of the car is not traveling faster than the wind, namely the surfaces of the propeller blades. The propeller is geared to the wheels in such a way that when the car moves forward 2 cm (say), the surface of the blades seen by a parcel of air only move forward 1 cm (say), because the blades are pitched at an angle, like this:

I had always wondered how a fast sailboat could travel downwind faster than the wind, but it's got to be the same principle.