Novel Propeller propulsion, going faster downwind than the wind

[RaymondR]

That must be the same wind which is going to allow us to shut down all the nuclear and coal fired power plants and give us reliable 24/7/365 electrical power from wind generators. Damn, that's a relief I thought renewables was all smoke and mirrors.

[skipperpete]

Originally Posted by waterman46

I'm sure it is tempting to think that once the vehicle reaches a speed equal to true wind DDW, and apparent wind is then zero, that the wheels, being in contact with the ground, can somehow extract power from that motion that can spin the propeller and draw the vehicle forward. As I understand it, this is your argument and there seem to be many who tend to agree with this.


However it is not possible. To generate forward thrust, the prop would need to extract power from the wheels. When you extract power from a moving object, that object has to slow down. This is extremely basic physics and I am ashamed to think that some Professor of Physics graduated from a UC engineering school who doesn't know this fact.

Besides, when switching from speed less than true wind to an imaginary speed greater than true wind, will require a reversal of pitch of the propeller. Did any of the video's mention a reversible pitch prop?

So, did you actually watch the video with the inventor (not the driver) explaining his side of the story? It was very entertaining, neither the inventor nor the owner wanted to drive the damned thing.
Your view on this is what cost the professor 10 large.

[Dave_S]

Originally Posted by waterman46

I'm sure it is tempting to think that once the vehicle reaches a speed equal to true wind DDW, and apparent wind is then zero, that the wheels, being in contact with the ground, can somehow extract power from that motion that can spin the propeller and draw the vehicle forward. As I understand it, this is your argument and there seem to be many who tend to agree with this.


However it is not possible. To generate forward thrust, the prop would need to extract power from the wheels. When you extract power from a moving object, that object has to slow down. This is extremely basic physics and I am ashamed to think that some Professor of Physics graduated from a UC engineering school who doesn't know this fact.

Besides, when switching from speed less than true wind to an imaginary speed greater than true wind, will require a reversal of pitch of the propeller. Did any of the video's mention a reversible pitch prop?

I'm sure it is tempting to think that once the vehicle reaches a speed equal to true wind DDW, and apparent wind is then zero, that the wheels, being in contact with the ground, can somehow extract power from that motion that can spin the propeller and draw the vehicle forward. As I understand it, this is your argument and there seem to be many who tend to agree with this.


However it is not possible. To generate forward thrust, the prop would need to extract power from the wheels. When you extract power from a moving object, that object has to slow down. This is extremely basic physics and I am ashamed to think that some Professor of Physics graduated from a UC engineering school who doesn't know this fact.

Besides, when switching from speed less than true wind to an imaginary speed greater than true wind, will require a reversal of pitch of the propeller. Did any of the video's mention a reversible pitch prop?

I'm beyond my pay grade but this is how I see it.

I think you are thinking that the propeller sees a reduction in effect as the vehicle approaches wind speed like a sail would.

If when stationary the vehicle has 10N of force applied by TW and the propeller blades have 10N applied by TW the assembly has 20N of force applied to the vehicle.

Point the vehicle DDW and release the brake. The 20N is sufficient force to move the assembly DDW. The grounded wheel rotates and through a drive drives the propeller in such a way to make more thrust than the losses felt through changes to AW. The thrust is working to move air in the apposing direction to the TW.

Because the propeller is overdriven it is increasing drag/thrust at a greater rate than the reduction of drag felt by the vehicle.

• The vehicle feels a 1:1 reduction in ground speed/AW, decreasing as vehicle speed increases.
• The propeller is driven by the wheels in such a way that it increases force at 3:1, ground speed/thrust (I made up 3:1, I don't know what the magic number is)
• At wind speed the vehicle sees no apparent wind but the propeller is now generating 30N of force, a total of 30N. 50% up from 20N when the vehicle is at rest.
• So at wind speed the propeller is producing more thrust than at rest. Each new speed is just a sum of forces which progress unaffected if it matches wind speed or not.
• It makes no or little difference if the thrust from the propeller is apposing or running with the apparent wind it is just a differential in flow like drag.

[wholybee]

I keep reading explanations of this, and it is surprising that the professor didn't understand it, and why so many people can't seem to grasp a simple idea.

Wind velocity isn't the factor that determines how fast a vehicle can go DDW. It's the force the wind imparts on a vehicle. So you have a vehicle, and a drag device. You make the drag device larger, and the vehicle goes faster, until wind velocity is reached. But you can keep making the drag device larger, to capture more energy. If the drag device is large enough that it could push a vehicle twice the mass of the actual vehicle to the velocity of the wind, there is a lot of extra power to capture, and the drag on the vehicle from turning wheels and mechanics will not slow it down. Then you need to convert that extra energy into velocity to keep going faster.

The wheels are not converting forward motion of the vehicle into energy to move the vehicle. If that were happening, that conversion would slow the vehicle. They are capturing that extra energy from a drag device capably of pushing more massive vehicle, allowing the rotating propeller to convert extra unused energy into velocity.

[waterman46]

I now understand that the prop pitch vs. rotation is configured to provide thrust when moving forward and rotated by connection to the wheels. I had assumed the configuration was to extract power as a wind turbine at vehicle speeds less than true wind.
Conservation of energy arguments are easier for me to understand. I like Wholybees explanation that a large and well designed propeller does the job of converting excess energy into increasing velocity. In effect it creates a more optimum gear ratio to favor velocity over power - the sail is 1:1, but the propeller can be designed to provide a more optimum gear ratio of the system to increase velocity by using excess energy. The excess energy is provided by the velocity gradient between ground and true wind and is always present and can be cleverly used even when the vehicle travels faster than the wind.

[johnn33]

This short film may be of interest

https://www.youtube.com/watch?v=0wcLjgW_x_8

John

[johnn33]

and a little more about Lord Brabazon's Redwing dinghy with an autogyro
AUTOGIRO BOATS - History 1870-1933
John

[johnn33]

and again

Rotary Sailing Homepage
John

[Chris31415]

Originally Posted by wholybee

The wheels are not converting forward motion of the vehicle into energy to move the vehicle. If that were happening, that conversion would slow the vehicle. They are capturing that extra energy from a drag device capably of pushing more massive vehicle, allowing the rotating propeller to convert extra unused energy into velocity.

I don’t think this is a correct explanation. When you get to the wind speed going DDW, the apparent wind is zero and there is no energy from the drag device (sail). The claim appears to be that you would then take some of the kinetic energy from the moving vehicle to increase it’s kinetic energy. That’s seems like a clear violation of energy conservation (1st law of thermodynamics) and why I think most people with a physics background are skeptical if this.

[CyKlop]

Soooo...

I had a multi-week argument about this in the comments section of the original video. I was taking the position that this vehicle (blackbird) was not possible.

However...

There have been multiple papers on this subject showing that it is theoretically possible. It was a question on one of the science decathlons.

My argument hinged on the fact that at some point the car without a propeller will reach energy equilibrium - the drag and friction forces would exactly equal the energy being input to the system by the wind. If we then require more energy by making the wheels turn a prop, that removes energy from the vehicle, thereby slowing it down.

But I was wrong. Oh, and yes, I used to teach physics, though I'm by no means a PhD in physics!

Not sure if anyone wants the pages of math, but it comes down to work equations. The prop is doing almost the same work as the wheels (assuming perfect bearings and stuff for a moment). However, since the wind is moving with the vehicle, the prop is covering less "distance" relative to the wind than the wheels are doing relative to the ground. Since Work = force x distance and Work remains the same for both objects (prop and wheel) and distance decreases for the prop, the prop must, to keep things balanced, be exerting more _force_ on the air mass, which is related to thrust, of course. Thus, the prop is able to accelerate the vehicle past wind speed.

The increase in that force has to be only _slightly_ more than all the frictions and aerodynamic drags on the vehicle in order for the vehicle to continue to accelerate. The inventors claim a factor of 1.1 for their vehicle which is, apparently, enough. it also took a lot of careful tuning of the pitch of the prop blades.


This is a completely different effect than a sailboat sailing DDW, which is relying purely on flat-plate drag for its drive and it can not exceed the wind speed.


Also, due to slippage of the paddlewheel in the fluid, which takes even more energy away from the system, I doubt that a sailboat using these principles actually can exceed wind speed even with a similar set up - paddlewheels are just too inefficient for this. Maybe if you were floating in liquid mercury or something it might work, though I have my doubts about that. I also think that the wind turbine powered vessel that was quoted is a fish of a different mother as I believe the wind turbine drives the water prop rather than the other way around, and so it's not applicable to the discussion.

I have yet to study the math for the opposite direction - direct upwind - for the vehicle, but the claim is that it can work in that regime as well.

[Chris31415]

So I’m in my frictionless vehicle traveling down wind at the speed of the wind. I have not connected the turbine to the wheels yet. You are saying that when I connect them I will get some energy that will cause my vehicle to speed up (accelerate).

Where is the energy coming from that speeds up the vehicle?

[JoeRobertJr]

Originally Posted by Chris31415

So I’m in my frictionless vehicle traveling down wind at the speed of the wind. I have not connected the turbine to the wheels yet. You are saying that when I connect them I will get some energy that will cause my vehicle to speed up (accelerate).

Where is the energy coming from that speeds up the vehicle?

The push prop is slowing down the air behind the car. The air in the wake of the car has less kinetic energy, as that energy was transferred to the car.

[Dockhead]

Originally Posted by TrentePieds

When I wuz a lad, way, WAAAAY before telly, let alone "social media" and YouTube, you often heard it said "It was in the paper - it MUST be true!"

Plus ça change, plus c'est pareil

TP

Before telly? But Zworykin was broadcasting in New York in 1939. You were a lad in 1939?

[EngineerRetired]

Your sail is a propeller of sorts. It why racing hulls can go three times wind speed

[TrentePieds]

Quote: "You were a lad in 1939?"

Ooh, ahr! I wuz indeed. And by 1940 MySaintedMother had conniptions cuz I was wading out to a fisherman's skiff lying on the hook off our home. Next time you come northbound through the Lillebælt look to starbord. You may still see our house at the place called Strib, now a suburb of Middelfart :-)

My first exposure to telly was a physics class in about 1953 when we were drug off to witness a special demonstration of it. Cameras in one room at the art galley in my home town. Receiver in the very next room. Wildly exciting! Telly came to Canada that same year, if I recall, and I was not long after it. Two stations via "rabbit ears": Channel 2, CBC Vancouver and channel 12, KVOS Bellingham (WA). Ch 2 "Juliette", Ch 12 Tennessee Ernie Ford. Each as bad as the other :-)!

But that was all way, way before your time:-)!

Cheers

TP