5 ms·

Like the professor in TFA I am also confused: how can the ground be imparting power to the propeller while moving downwind?

efxhoy 5y ago
wind pushes vehicle. wheels spin. wheels drive propeller (via gears). propeller pushes vehicle. I first assumed it was the other way around with wind->propeller->wheels, which it is not.
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 ago
  Wind does not drive the propeller.
  - CarVac 5y ago
    I 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 ago
    The 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 ago
      I 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 ago
        Propeller pitch is not fixed on Blackbird, that is one of the few controls it has.
shagie 5y ago
The original Veritasium video https://youtu.be/jyQwgBAaBag https://youtu.be/jyQwgBAaBag The video of the bet and the further explanations - https://youtu.be/yCsgoLc_fzI https://youtu.be/yCsgoLc_fzI -- at 13:28 is an example without aerodynamics.
LeifCarrotson 5y ago
The wheel friction and gearing allows the vehicle to tie the effective velocity of the sail (actually a propeller) to the velocity of the ground. Energy can be extracted from the interface between the ground at one velocity and the air moving at another velocity. A sail tied to the ground and extending into this wind will exert a force according to the drag equation: Force = 1/2 x drag coefficient x density x area x (differential velocity)^2 Typically, when going downwind, sailors ignore the ground (or the water) and just extract some energy from the difference in speed between the vehicle and the wind. Obviously, as the vehicle accelerates, this velocity difference asymptotically goes to zero where there is no more force and no more acceleration. For a more static example, imagine an enormously long vehicle with a linear bearing running the entire axis from the front to the back. Assume for the sake of argument the vehicle is ultra-light and has very low rolling resistance, some kind of cross between a top-thrill dragster and a train, hypothetically a kilometer long. On the axial bearing, mount a wind turbine with a cable and an anchor that can be fixed to the ground. Start with the wind turbine at the front of the vehicle, then drop anchor to fix the wind turbine in place. There is a velocity differential between the ground-referenced turbine and the air that will impart force on the turbine blades, use this to generate electricity. Use an electric motor to drive the bottom part of the vehicle forwards. When the turbine reaches the back of the vehicle, lift anchor - remember we're in spherical cow territory, so assume it's foldable and super-light so this part takes little energy and can be done arbitrarily fast - and push it back to the front of the vehicle, where you can drop anchor again. In this system, the energy you can extract only depends on the difference between the ground speed and wind speed, not the vehicle speed. Blackbird is just like this repetitively anchored wind turbine concept in that the point of reference for the wind energy extractor is the ground instead of the vehicle. But instead of dropping an anchor, you're tying to the ground speed through tires, and instead of shuttling the turbine from the back to the front, you're continuously advancing it with the gear mechanism, and instead of driving with an electric motor you're directly using the drag force.
- IgorPartola 5y ago
  In your static example how does the turbine get back to the front of the vehicle? Does the force of the wind push it there? If so, if the wind is blowing at 10 knots but the base of the vehicle is moving at 20 then as far as the turbine is concerned it is actually being pushed backwards, no? Or is the idea that when you lift the anchor the vehicle slows to below wind speed while the turbine gets pushed?

efxhoy 5y ago

wind pushes vehicle. wheels spin. wheels drive propeller (via gears). propeller pushes vehicle. I first assumed it was the other way around with wind->propeller->wheels, which it is not.

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 ago
Wind does not drive the propeller.
- CarVac 5y ago
  I 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 ago
  The 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 ago
    I 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 ago
      Propeller pitch is not fixed on Blackbird, that is one of the few controls it has.

shagie 5y ago

The original Veritasium video https://youtu.be/jyQwgBAaBag https://youtu.be/jyQwgBAaBag The video of the bet and the further explanations - https://youtu.be/yCsgoLc_fzI https://youtu.be/yCsgoLc_fzI -- at 13:28 is an example without aerodynamics.

LeifCarrotson 5y ago

The wheel friction and gearing allows the vehicle to tie the effective velocity of the sail (actually a propeller) to the velocity of the ground. Energy can be extracted from the interface between the ground at one velocity and the air moving at another velocity. A sail tied to the ground and extending into this wind will exert a force according to the drag equation: Force = 1/2 x drag coefficient x density x area x (differential velocity)^2 Typically, when going downwind, sailors ignore the ground (or the water) and just extract some energy from the difference in speed between the vehicle and the wind. Obviously, as the vehicle accelerates, this velocity difference asymptotically goes to zero where there is no more force and no more acceleration. For a more static example, imagine an enormously long vehicle with a linear bearing running the entire axis from the front to the back. Assume for the sake of argument the vehicle is ultra-light and has very low rolling resistance, some kind of cross between a top-thrill dragster and a train, hypothetically a kilometer long. On the axial bearing, mount a wind turbine with a cable and an anchor that can be fixed to the ground. Start with the wind turbine at the front of the vehicle, then drop anchor to fix the wind turbine in place. There is a velocity differential between the ground-referenced turbine and the air that will impart force on the turbine blades, use this to generate electricity. Use an electric motor to drive the bottom part of the vehicle forwards. When the turbine reaches the back of the vehicle, lift anchor - remember we're in spherical cow territory, so assume it's foldable and super-light so this part takes little energy and can be done arbitrarily fast - and push it back to the front of the vehicle, where you can drop anchor again. In this system, the energy you can extract only depends on the difference between the ground speed and wind speed, not the vehicle speed. Blackbird is just like this repetitively anchored wind turbine concept in that the point of reference for the wind energy extractor is the ground instead of the vehicle. But instead of dropping an anchor, you're tying to the ground speed through tires, and instead of shuttling the turbine from the back to the front, you're continuously advancing it with the gear mechanism, and instead of driving with an electric motor you're directly using the drag force.

IgorPartola 5y ago
In your static example how does the turbine get back to the front of the vehicle? Does the force of the wind push it there? If so, if the wind is blowing at 10 knots but the base of the vehicle is moving at 20 then as far as the turbine is concerned it is actually being pushed backwards, no? Or is the idea that when you lift the anchor the vehicle slows to below wind speed while the turbine gets pushed?