3 ms·
The wind pushes the propeller, the gearing of the wheels to the propeller cause the overall vehicle to advance faster than the blades on the propeller (as seen
by CarVac 5y ago
The wind pushes the propeller, the gearing of the wheels to the propeller cause the overall vehicle to advance faster than the blades on the propeller (as seen by the wind).
It's an overdrive—the wind pushes the vehicle faster than the wind itself, but the vehicle gets less net propulsion force when you sum up the tractive force and the wind force.
- formerly_proven 5y agoWind does not drive the propeller.
- CarVac 5y agoI did not say that. I wrote "Wind pushes the propeller." The vehicle is basically extracting energy from the difference in velocity between the ground and the air. The wind pushes the propeller forward, the ground pushes the wheels backwards.
- IgorPartola 5y ago> Downwind, when the vehicle is traveling faster than the windspeed, the ground is the fastest-moving medium relative to the vehicle, so the wheels harvest the power and impart it to the rotor, which propels the vehicle. So the wind pushes the vehicle faster than wind speed to start with? How?
- CarVac 5y agoThe angle of attack of the propeller blades and the gearing relative to the wheels cause the blades to be seen by the wind as moving upwind relative to the ground. Alternatively, think of it as merely harvesting energy from the velocity difference between the ground and the air. It's connected to the ground by wheels, and to the air by a big prop. It doesn't matter how fast the vehicle itself is moving as long as the gearing is right. Ultimately, the top speed is limited to when the friction the vehicle encounters from drivetrain losses and aero drag exceeds the power it can extract from the wind/ground speed difference.
- sopooneo 5y agoI agree that theoretic top speed is limited only be drag (and other resistive forces) if you are able to adjust propeller pitch, gear ratio, or wheel diameter on the fly. But if those values are fixed, as with Blackbird, then you are of course limited to a top speed that is some constant multiple of wind speed.
- vel0city 5y agoPropeller pitch is not fixed on Blackbird, that is one of the few controls it has.