[RedDevil]

Originally Posted by aerohead

This would have been helpful from square one, on June 11.
https://en.wikipedia.org/wiki/Blackb...dStreamers.JPG
27.7-mph from a 10-mph wind.
Finally some numbers! In July.

In order to go 27.7 mph when the wind is only 10 mph it needs to push the air back at at least 2/3 of the ground speed, or the wind would not be able to push against the blades.
Most likely though the gearing was closer to 4/5th, making the propeller push the air to a speed about midway between ground speed and wind speed.

So the propeller has a speed of about 22.2 mph relative to the car which is 5.5 mph downwind relative to the ground. The difference with the wind speed is 4.4 mph, that's providing the load on the blades.
There's some slippage for sure, just like a glider plane has; just like the load on the wings of a glider plane descending at 4.4 mph, producing the lift to move the plane forward and keep that rate constant.

We don't know the exact forward component of that load, but let's take a guess here. Cavallaro said he needed to extend the left axle to prevent the torque on the propeller from tipping it over. The force on the propeller must be substantial for it to lift the driver...

Say the forward load on the propeller is just under 22 pounds; that would clearly not be enough to tip it over, but then we have only a 10 mph wind, so it seems realistic.
As luck will have it that is equal to exactly 100 Newton.

There we have it: the wind pushes against the propeller blades with a force of 100 Newton.
There's some friction on the car body, there's rolling resistance to overcome, but let's assume 90 Newton remains to turn the wheels.

The wheels rotate at 27.7 mph, that is about 45 km/h or 12.5 meter per second producing 1125 Watt (or about 1.5 hp).

The wheel axle drives a chain that drives the propeller axle.
We lose 5% there so about 1070 Watt of power remains to turn the propeller against the wind.

The propeller is supposed to move at 22.2 mph, which is 36 km/h or 10 meter per second. Say we lose 7% to friction as our propeller has a measly 15:1 load/drag ratio where 60:1 is possible, but hey.
So instead of 107 Newton the 1070 Watt of power turning the propeller against the wind only provides 100 Newton of thrust.
That is exactly what is needed to maintain the speed of the propeller and the thrust moving the car forward, but not more. No wonder the car does not accelerate any faster - but it does not slow down either.

There you have it.
The numbers go round, slippage and friction are taken into account (about as much power is lost as one would need to ride a bicycle) yet the car moves 2.77 times faster than the wind despite wasting some power on friction.