One subtle point I should have made in the video: Newton's second law (NSL) in angular form says that the torque on a body equals the rate of change of the body's angular momentum. Now, Newton's laws hold in inertial frames of reference. Nevertheless, the angular form of NSL holds as long as the point P about which the torque and angular momentum are calculated satisfies either (1) P is at rest in an inertial frame, or (2) P coincides with the body's center of mass. In the video I use the result (2) to justify applying NSL in angular form to the fan and wheel.
Thanks a lot for this $10K explanation. It is definitely a lot more satisfying than the purely geometric explanation where the wind force is replaced by a solid contact.
You are making so great videos with clear algebraic(calculus) explanations in General Relativity playlist as well. I am waiting for Fluid Mechanics. Best Regards :D
All this experiment proves is that good old sails work better than modern propellers at harnessing wind energy. 😄 Think of the propeller's surface as a sail, when it turns in the forward direction it converts the air resistance in front of it into additional thrust. 🎉🎉🎉
// "Blackbird is a wind powered vehicle consisting of a wind turbine linked to its wheels."// When traveling downwind as in the Veritasium videos, the Blackbird is configured with propeller blades, not turbine blades. If it were a turbine, it would be unable to reach the speed of the wind, let alone exceed it. When the Blackbird was reconfigured to set the upwind record, the blades were indeed replaced with turbine blades and they drove the wheels as the craft advanced upwind at >2x windspeed.
@@PhysicsUnsimplified Correct, and the mode of operation totally changes with direction. Upwind, it's turbine blades which drive the wheels. Downwind, the wheels drive propeller blades.
@@PhysicsUnsimplified Compromises could be made which allow the blades to operate has either turbine or propeller blades depending on the situation. However, those compromises would impact performance in one or both directions depending on the choices. Since our goal was to maximize performance for record purposes, we opted for specialty blades for each direction.
I wrote a comment in the main thread expanding on the change of mode of operation between upwind and downwind. You may find it interesting (To avoid misunderstanding: I haven't done calculations. I present reasoning in terms of the underlying physics.)
On June 30 Derek Muller released a follow-up video, one section in that video shows a device that is the simplest instance of the operating principle of the Blackbird vehicle. I use the name 'difference cart' for this device: th-cam.com/video/yCsgoLc_fzI/w-d-xo.html The difference cart is in contact with two different surfaces that have a velocity with respect to each other. The difference cart is harvesting propulsion from that velocity difference. In order to harvest propulsion the cart needs equal *traction* on both surfaces that it is in contact with. If the cart loses traction on either of the two surfaces the cart loses propulsion altogether. The thing about the Blackbird vehicle that is confounding people is that its operation is dependent on traction, but blades have poor traction. I believe it is that mismatch that is keeping people from recognizing what is going on. For your calculation you have set up the case as a cross-flow fan case I propose to see a cross-flow fan as a form of paddle wheel. (As we know, before the development of water propellers engine driven ships had paddle wheel propulsion.) Here is a variation on the difference cart setup: A narrow channel with flowing water, on either side of the channel there is grippy surface, for wheels to have traction. The vehicle has a paddle wheel in the water. Then the wheels must turn the paddle wheel, and the paddle wheel must turn fast enough such that when the whole vehicle is overtaking the water the paddle wheel is still pumping the water to the rear of the vehicle fast enough so that a surplus of water at the rear of the vehicle is maintained. That surplus of water is then pushing the paddle wheel vehicle forward. Returning to the Blackbird vehicle: Blades have poor traction, and because of that there is an asymmetry between going upwind and going downwind. When going upwind the blades are driving the wheels, so the blades must be optimized for acting as a turbine. When going downwind the wheels are driving the blades, and the blades must be optimized for acting as a propellor. Again, this asymmetry is only there because of the difference in traction. If you would set things up such that the traction comes out roughly equal then the propulsion harvesting becomes more like the directionally symmetric process of the difference cart. (I describe blades as having 'poor traction'. Please read that as shorthand for 'not as much traction as a wheel on a solid surface'. A propellor does have traction, of course, otherwise airplanes with propellors would not be able to fly.)
You are correct -- you can show an example of two solid surfaces of different speeds and the device utilizing those solid surfaces and people don't have much problem figuring out how the device moves or at least believing that it does. The air confounds them however. Like the board and little blue wheeled cart in the last Derek video they say "BUT THAT'S NOT AIR", not realizing that an 80% efficient propeller (easy to achieve) is *exactly* like that board pushing, only with 20% slip of the wheels.
@@johnborton4522 Perhaps Derek Muller can be persuaded to ask Xyla Foxlin to built a setup for a tabletop paddle wheel version. Air is invisible, but with a water-and-paddle-wheel version people will be able to witness the paddle wheel actually getting traction, allowing the vehicle to harvest propulsion from the difference in velocity between the water and the channel banks. I had no idea that a propeller can be that efficient. If someone would have said 50% tops I would have believed that.
@@cleon_teunissen 80% is very common on airplane propellers, but for the same reason sailplane wings are long, longer propeller blades turning slow improve efficiency. The Blackbird is likely closer to 90%, and the small treadmill models are likely in the 70s. This device is very sensitive to propeller efficiency. I suspect you would have a very hard time getting the rest of the system efficiencies high enough to be able to survive the gross inefficiency of the paddle wheel design, but it could be fun to find out.
@@johnborton4522 Yeah, in open water a paddle wheel has low efficiency. I'm imagining a tabletop setup with the channel so narrow and shallow that the paddle wheel almost scrapes the sides and bottom. The vehicle then needs to be on tracks, so as to avoid actually scraping.
@@johnborton4522 I asked one of the "NOT AIR" guys to imagine the top wheel of the trolley was made geared, and drove a worm screw, which engaged into a rifled semicircular channel track above. It didn't, um, "engage."
Why call the "fan" a turbine! It's a PROPELLER! this will just confuse people - a propeller propels things - a turbine collects energy - on Blackbird its a propeller in the air
The professor was right. If he was wrong then the vehicle should keep traveling when no wind is present after an initial push. There should be a chart where at wind speed of 1 mph the cart moves at 2.7 mph, at 2 mph the cart moves at 5.4mph and so on. But this chart will never be made because it brakes the 1st law of thermal dynamics. As for the treadmill test is ludacris!if anything it is simulating a head wind. this should be able to chart on a graph as well. What do you think will happen when the treadmill goes at a walking speed? The answer is nothing will happen. To really test it they should take the most optimal small scale modle that they have created, make a wind tunnel the length of a football field where every bit of air in the tunnel flows exactly the same speed (as close as possible), have multiple readings take place throughout the tunnel to get average wind speed and then set the unit down and see if it can beat the average wind speed th get to then end. But it can't and won't. It would be cheaper to build this than that large unit but they can fudge the numbers with the large unit how fast should it go at 1mph, 2mph, 3mph. There should be a graph charting this. Wind tunnel and unit on a track is the best way to test. Keep it small like the treadmill units. Have steady wind speed. But this won't happen because it would ruin the theory. If it runs faster than the wind @ a wind speed then it should be able to run faster than the wind at 0 mph. Let me explain: If it's the wheels powering the propeller which pushes the vehicle faster than the wind then after starting the unit with a push the wheels should be able to power the propeller and push it at the same rate as when it has wind. A vehicle moving at 11 mph with a 10 mph tailwind is moving upwind at 1mph. The wind in perspective to the vehicle is now a head wind. A vehicle traveling 1 mph in no wind has a 1mph head wind. It's the same thing. Get the vehicle started with a push and it should beable to power itself. I don't think I'm missing anything but if I am please explain just rewatched the blackbird professor bet video. @12:05 the claim is made "the propeller thrust force is greater than the drag on the wheels". If this statement is true we can expect the unit to produce greater thrust force in a no wind test. Just get the vehicle started with a push and then the propeller will overpower the drag that wheels produce Im not into all the math equations but just coming from a practical standpoint imo the black bird is a hoax
The energy is coming from the ground and wind speed differential. 10mph vehicle, 10mph wind, 0mph ground is equivalent to 0mph wind, 0mph vehicle, -10 mph ground. In fact this exact scenario is shown with the treadmill demonstrations. The vehicle doesn't require wind, just a difference in airspeed and groundspeed.
One subtle point I should have made in the video: Newton's second law (NSL) in angular form says that the torque on a body equals the rate of change of the body's angular momentum. Now, Newton's laws hold in inertial frames of reference. Nevertheless, the angular form of NSL
holds as long as the point P about which the torque and angular momentum are calculated satisfies either (1) P is at rest in an inertial frame, or (2) P coincides with the body's center of mass. In the video I use the result (2) to justify applying NSL in angular form to the fan and wheel.
Thanks a lot for this $10K explanation. It is definitely a lot more satisfying than the purely geometric explanation where the wind force is replaced by a solid contact.
Extremely interesting! Great stimulus for the mind!
You are making so great videos with clear algebraic(calculus) explanations in General Relativity playlist as well. I am waiting for Fluid Mechanics. Best Regards :D
All this experiment proves is that good old sails work better than modern propellers at harnessing wind energy. 😄
Think of the propeller's surface as a sail, when it turns in the forward direction it converts the air resistance in front of it into additional thrust. 🎉🎉🎉
// "Blackbird is a wind powered vehicle consisting of a wind turbine linked to its wheels."//
When traveling downwind as in the Veritasium videos, the Blackbird is configured with propeller blades, not turbine blades. If it were a turbine, it would be unable to reach the speed of the wind, let alone exceed it.
When the Blackbird was reconfigured to set the upwind record, the blades were indeed replaced with turbine blades and they drove the wheels as the craft advanced upwind at >2x windspeed.
Thanks, that's interesting. So blackbird uses different types of blades depending on the direction of travel...
@@PhysicsUnsimplified
Correct, and the mode of operation totally changes with direction. Upwind, it's turbine blades which drive the wheels. Downwind, the wheels drive propeller blades.
@@PhysicsUnsimplified
Compromises could be made which allow the blades to operate has either turbine or propeller blades depending on the situation. However, those compromises would impact performance in one or both directions depending on the choices.
Since our goal was to maximize performance for record purposes, we opted for specialty blades for each direction.
I wrote a comment in the main thread expanding on the change of mode of operation between upwind and downwind. You may find it interesting
(To avoid misunderstanding: I haven't done calculations. I present reasoning in terms of the underlying physics.)
@@johnborton4522 incorrect. the propeller actually always produces thrust. Never acts as a turbine
On June 30 Derek Muller released a follow-up video, one section in that video shows a device that is the simplest instance of the operating principle of the Blackbird vehicle. I use the name 'difference cart' for this device:
th-cam.com/video/yCsgoLc_fzI/w-d-xo.html
The difference cart is in contact with two different surfaces that have a velocity with respect to each other. The difference cart is harvesting propulsion from that velocity difference.
In order to harvest propulsion the cart needs equal *traction* on both surfaces that it is in contact with. If the cart loses traction on either of the two surfaces the cart loses propulsion altogether.
The thing about the Blackbird vehicle that is confounding people is that its operation is dependent on traction, but blades have poor traction. I believe it is that mismatch that is keeping people from recognizing what is going on.
For your calculation you have set up the case as a cross-flow fan case
I propose to see a cross-flow fan as a form of paddle wheel.
(As we know, before the development of water propellers engine driven ships had paddle wheel propulsion.)
Here is a variation on the difference cart setup:
A narrow channel with flowing water, on either side of the channel there is grippy surface, for wheels to have traction. The vehicle has a paddle wheel in the water. Then the wheels must turn the paddle wheel, and the paddle wheel must turn fast enough such that when the whole vehicle is overtaking the water the paddle wheel is still pumping the water to the rear of the vehicle fast enough so that a surplus of water at the rear of the vehicle is maintained. That surplus of water is then pushing the paddle wheel vehicle forward.
Returning to the Blackbird vehicle:
Blades have poor traction, and because of that there is an asymmetry between going upwind and going downwind.
When going upwind the blades are driving the wheels, so the blades must be optimized for acting as a turbine.
When going downwind the wheels are driving the blades, and the blades must be optimized for acting as a propellor.
Again, this asymmetry is only there because of the difference in traction. If you would set things up such that the traction comes out roughly equal then the propulsion harvesting becomes more like the directionally symmetric process of the difference cart.
(I describe blades as having 'poor traction'. Please read that as shorthand for 'not as much traction as a wheel on a solid surface'. A propellor does have traction, of course, otherwise airplanes with propellors would not be able to fly.)
You are correct -- you can show an example of two solid surfaces of different speeds and the device utilizing those solid surfaces and people don't have much problem figuring out how the device moves or at least believing that it does. The air confounds them however.
Like the board and little blue wheeled cart in the last Derek video they say "BUT THAT'S NOT AIR", not realizing that an 80% efficient propeller (easy to achieve) is *exactly* like that board pushing, only with 20% slip of the wheels.
@@johnborton4522 Perhaps Derek Muller can be persuaded to ask Xyla Foxlin to built a setup for a tabletop paddle wheel version. Air is invisible, but with a water-and-paddle-wheel version people will be able to witness the paddle wheel actually getting traction, allowing the vehicle to harvest propulsion from the difference in velocity between the water and the channel banks.
I had no idea that a propeller can be that efficient. If someone would have said 50% tops I would have believed that.
@@cleon_teunissen
80% is very common on airplane propellers, but for the same reason sailplane wings are long, longer propeller blades turning slow improve efficiency. The Blackbird is likely closer to 90%, and the small treadmill models are likely in the 70s.
This device is very sensitive to propeller efficiency. I suspect you would have a very hard time getting the rest of the system efficiencies high enough to be able to survive the gross inefficiency of the paddle wheel design, but it could be fun to find out.
@@johnborton4522 Yeah, in open water a paddle wheel has low efficiency. I'm imagining a tabletop setup with the channel so narrow and shallow that the paddle wheel almost scrapes the sides and bottom. The vehicle then needs to be on tracks, so as to avoid actually scraping.
@@johnborton4522 I asked one of the "NOT AIR" guys to imagine the top wheel of the trolley was made geared, and drove a worm screw, which engaged into a rifled semicircular channel track above. It didn't, um, "engage."
Why call the "fan" a turbine! It's a PROPELLER! this will just confuse people - a propeller propels things - a turbine collects energy - on Blackbird its a propeller in the air
yep, this explanation is for a completely different machine
Sir I love your videos
Thank you!
🥰🥰🥰🥰🥰😍🥰🥰😍🥰🥰
Sir I am from India
I drive a Honda
I drive a Hyundai
I drive a cow
I am under the water
It's a hoax.
The professor was right.
If he was wrong then the vehicle should keep traveling when no wind is present after an initial push.
There should be a chart where at wind speed of 1 mph the cart moves at 2.7 mph, at 2 mph the cart moves at 5.4mph and so on.
But this chart will never be made because it brakes the 1st law of thermal dynamics.
As for the treadmill test is ludacris!if anything it is simulating a head wind. this should be able to chart on a graph as well. What do you think will happen when the treadmill goes at a walking speed? The answer is nothing will happen.
To really test it they should take the most optimal small scale modle that they have created, make a wind tunnel the length of a football field where every bit of air in the tunnel flows exactly the same speed (as close as possible), have multiple readings take place throughout the tunnel to get average wind speed and then set the unit down and see if it can beat the average wind speed th get to then end. But it can't and won't. It would be cheaper to build this than that large unit but they can fudge the numbers with the large unit
how fast should it go at 1mph, 2mph, 3mph. There should be a graph charting this.
Wind tunnel and unit on a track is the best way to test. Keep it small like the treadmill units. Have steady wind speed. But this won't happen because it would ruin the theory.
If it runs faster than the wind @ a wind speed then it should be able to run faster than the wind at 0 mph. Let me explain:
If it's the wheels powering the propeller which pushes the vehicle faster than the wind then after starting the unit with a push the wheels should be able to power the propeller and push it at the same rate as when it has wind.
A vehicle moving at 11 mph with a 10 mph tailwind is moving upwind at 1mph. The wind in perspective to the vehicle is now a head wind. A vehicle traveling 1 mph in no wind has a 1mph head wind. It's the same thing. Get the vehicle started with a push and it should beable to power itself. I don't think I'm missing anything but if I am please explain
just rewatched the blackbird professor bet video. @12:05 the claim is made "the propeller thrust force is greater than the drag on the wheels".
If this statement is true we can expect the unit to produce greater thrust force in a no wind test. Just get the vehicle started with a push and then the propeller will overpower the drag that wheels produce
Im not into all the math equations but just coming from a practical standpoint imo the black bird is a hoax
The energy is coming from the ground and wind speed differential. 10mph vehicle, 10mph wind, 0mph ground is equivalent to 0mph wind, 0mph vehicle, -10 mph ground. In fact this exact scenario is shown with the treadmill demonstrations. The vehicle doesn't require wind, just a difference in airspeed and groundspeed.
As for where the energy comes from, the propeller can hit wind particles from the correct direction even when the cart is going faster than the wind.
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