"Faster than the wind," impossible they say but the Blackbird settles the debate

[aerohead]

Originally Posted by oil pan 4

The "it can't be possible camp" (aerohead) is not looking so good.

Possibility was never the issue. I already knew for decades of propeller-driven craft exceeding airspeed. With the wind was a new wrinkle.
I wanted to stress the importance of scientific rigor.
Science is about mathematics.
We weren't given enough information to calculate for success, one way or another.
It's my opinion that a better mathematical argument is deserving, regardless of outcome.

[Vman455]

Originally Posted by aerohead

1) since there were no arrows provided in the illustration, I did a napkin sketch.
2) the car is moving from left to right.
3) the wheels in ground contact are rotating clockwise.
4) the upper wheels are powered by their contact with the reduced-radius axle of the ground contact wheels.
5) as the lower wheels rotate clockwise, as in meshed gears, the upper wheel spins counterclockwise.
6) a plank, resting on the upper wheels will be pushed off the cart, falling behind it.
That, or the Kool-Aid's been real good!

You're half right. The plank will move backward relative to the car and slide off the back. But it will also move forward relative to the ground, at a velocity less than that of the car. No Kool-Aid required.

[RedDevil]

Oh boy. Bear with me. Deep dive!!!!

Originally Posted by aerohead

1) the little model is not air driven.

Air driven? You miss the essential part.
it has absolutely NOTHING to do with the speed of the air!
The only thing that matters is the difference between the wind and ground speed.
It should work on a treadmill in still air just the same as with wind on solid ground, or salt, or whatever.

Originally Posted by aerohead

2) the little model does not mimic El Mirage Dry Lake conditions

The only thing that matters is a difference between wind and ground speed.

Originally Posted by aerohead

3) it's Rick Cavallero who says in the vodeo that it's the body of Blackbird which is reacting to the wind impulse to move it ( disregarding the propeller thrust )

The air resistance of the body is negligible in comparison to the pressure on the propeller. When the car is not at speed the wind pushes against the body (which does give way by rolling away) and the propeller blades (which start to turn against the wind, so they give way less quickly)

Originally Posted by aerohead

4) we know from Hoerner that, Blackbird's drag will be over 3X higher with the wind from behind, than from the front, so it is a 'square-rigger sail' so to speak.

Bear in mind that the drag on the propeller has a diffeerent dynamic than the drag on the rest of the car, because it is virtually moving slower (by slowly turning against the wind) and because its surface is much bigger.

Originally Posted by aerohead

5) the Second Law of Thermodynamics is operating at:
* wind-to-body aerodynamic coupling surface friction
* flow separation induced turbulence viscous shearing forces.
* dirt ground-to-tire tread interface
* internal hysteresis loss and heating within the tires
* all wheel bearings
* all gears
* all sprockets
* all power transfers from component to component
* all other bearings
* within every chain link swivel
* at the propeller-to-atmosphere coupling heating
( if you had the proper infrared imaging equipment, you'd see Blackbird raising the temperature of the air around it at all contact points, and within all it mechanical components ) That's all 'entropy'

The second law of thermodynamics is not what you think it is, I fear:

The first law of thermodynamics provides the definition of the internal energy of a thermodynamic system, and expresses the law of conservation of energy.[5][6] The second law is concerned with the direction of natural processes.[7] It asserts that a natural process runs only in one sense, and is not reversible. For example, when a path for conduction and radiation is made available, heat always flows spontaneously from a hotter to a colder body. Such phenomena are accounted for in terms of entropy.[8][9] If an isolated system is held initially in internal thermodynamic equilibrium by internal partitioning impermeable walls, and then some operation makes the walls more permeable, then the system spontaneously evolves to reach a final new internal thermodynamic equilibrium, and its total entropy, S, increases.

In other words, the second law describes entropy in an isolated system. Here we have two entities that move relative to one another; these provide energy to the system, so it is far removed from an isolated system.

Originally Posted by aerohead

6) the tires resist rolling on the dirt as a function of their coefficient of rolling force ( which must be quantified for dirt )

The only thing the ground needs to do is provide enough grip for the wheels to roll and be flat enough to not cause too much friction. Be it dirt, salt, ice, whatever. Tarmac and salt should do fine.

Originally Posted by aerohead

7) along with the body, the air flow impacts the thrusting face of the propeller.

It has to, otherwise it could not push the wheels over whatever they are resting on.

Originally Posted by aerohead

8) I'm uncertain how you could quantify what the forces would be on the propeller and wheels.

What about 'equal'? The wheels touch the ground, the propeller faces the wind; there are no other points on the car that endure outside forces.

Originally Posted by aerohead

9) the distance the wheels travel will be whatever it is.

Originally Posted by aerohead

10) the distance the slipstream moves through the propeller disc will be a function of its diameter and nominal pitch, along its axis of rotation as if it were moving along a helix of slope equal to it's blade angle. Which would need to well exceed the ground velocity in order for Blackbird to exceed the local wind velocity at the prop.

Ground velocity? OK, we're looking at the car now.
The prop does NOT push the air back faster than the ground moves.
It needs to move the air back SLOWER than the ground, otherwise the wind cannot push it forward as then it would require more energy to turn the prop than the wheels provide.
Because it is moving slower than the ground, it requires less energy to produce the same force the wheels get over a bigger distance.

Originally Posted by aerohead

11) we don't want to confuse gross available energy with net useful energy. Blackbird lives on the margin.

In a 1/2 ratio you can lose half the energy to all kinds of friction and still move at wind speed. That's enough margin for me.

Originally Posted by aerohead

12) do you have the propeller specifications for Blackbird?

Not relevant Any decent prop will do. Blackbird's officially goes 2.8 times the wind speed; there's enough margin whatsoever.

Originally Posted by aerohead

13) do you have the aerodynamic 'push' data for blackbird?

Again not relevant but it is directly opposed and equal to the force on the wheels. Pick a reasonable force to work with and do the math from there.

Originally Posted by aerohead

14) yes, when airspeed and ground speed are equal, there is no more impulse, and momentum is constant.

Indeed it does nothing when the wind does not blow and the ground does not move.

Originally Posted by aerohead

15) if the car is exceeding the windspeed, it has a negative impulse acting on it, it's running away from its energy source, while for the first time, experiencing positive aerodynamic drag.

But the main aerodynamic forces don't apply to the car, but the propeller (which is turning backwards slower than the ground speed so still moving forward related to the ground, and still enduring pressure from the wind.

Originally Posted by aerohead

16) impulse from the rear was experienced from a 'tailwind' up until the point where air velocity nulls, at zero apparent air velocity, measured only at the car.

You forget about the propeller turning backwards, which is needed to maintain pressure to move the car forward.

Originally Posted by aerohead

17) if the ground speed of the car exceeds wind speed, its now in a headwind, while the ground is always experiencing some true wind.

The ground endures the wind at full speed, the propeller at maybe half speed, the car faces a headwind.

Originally Posted by aerohead

18) the car is in an ocean of moving air, like a commercial airliner in the Jet Stream, going supersonic by ground measurement, however, only 600-mph by pitot tube/ manometer aboard the plane ).

Replace car with propeller and then you've nailed it!

Originally Posted by aerohead

19) the ground just experiences, from a fixed position, whatever the wind is doing.

Yes, the wind and the ground are the two main entities and they move relative to one another.

Originally Posted by aerohead

20) there would be a point where the propeller experienced zero wind speed, however we'd need to know everything about the prop. to calculate that.

No! Then it does not work.
If the propeller endures zero wind speed there is no pressure on the propeller, no force to drive the car forwards, to push the wheels over the ground and to turn the propeller. It would come to a halt.
The propeller needs to move the air passing through at a speed just shy of the speed of the car. Then the wind can still put pressure on it, forcing the car forwards.

Originally Posted by aerohead

21) the wind will just blow.

Yes, the wind and ground form a dynamic duo moving relative to each other.

Originally Posted by aerohead

22) if the propeller is turning, and it has a certain degree of slipstream coming off it, it will be acted upon by the wind, even if exceeding the wind speed.

There's a margin to compensate for any bad effect from that (apparently).

Originally Posted by aerohead

23 ) I'd prefer we use 'power' rather than energy.

Fair enough, will do.

Originally Posted by aerohead

24) Blackbird will have a ROAD LOAD Horsepower requirement under any given set of circumstances.

Road load horsepower definition
The amount of power at the driving wheels needed to move a vehicle down the road at a steady speed . This power varies according to the vehicle's speed, aerodynamic drag, mechanical friction, and the tires' rolling resistance . Road-load horsepower is distinct from engine power because the output of the engine is sapped by various mechanical losses between the engine's output at its flywheel and the driving wheels.

Assuming friction on the wheels, gears and car body is relatively low compared to the load on the propeller, there is very little power needed to move the car other than overcoming the resistance from the propeller being driven against the wind.

Originally Posted by aerohead

25) this power is associated only with aerodynamic drag and rolling resistance.

Not so much just drag but load on the blades of the propeller, and not so much friction as the force that friction provides to the wheels to make them turn.

Originally Posted by aerohead

26) if the propeller can impart horsepower to the atmosphere in excess of the road load, it will continue to accelerate. That's neve been in question.

It obviously cannot provide more power to the propeller than the wheels provide. But then it does not need to, as it pushes the air backwards at a slower speed than the car is moving relative to the ground.
Power is force times distance, and the propeller endures the same force over a smaller distance.

Originally Posted by aerohead

27) everyone ( I believe ) accepts that the propeller is 'tacking' around its shaft and capable of imparting thrust.

Originally Posted by aerohead

28) given a true wind speed spectra, a terminal ground velocity at that specific wind velocity, and all the necessary particulars of Blackbird, things will be what they are. I'm happy just to have the facts, without all the drama.
And it's the shortcomings of the video presentation which appears to have created all the controversy.
The treadmill tests proved nothing.
The test at El Mirage is lacking in information.
The $10,000 bet means nothing to me.
I'm just asking for algebra. I want the left side of the = sign to match the right side.

I too see a shortcoming, but not in the video.
Funny how easy it is to spot the shortcomings in others and how hard it is to see your own. A man cannot look beyond his own horizon, but easily see what behind the horizon of others.

[aerohead]

Originally Posted by Vman455

You're half right. The plank will move backward relative to the car and slide off the back. But it will also move forward relative to the ground, at a velocity less than that of the car. No Kool-Aid required.

Yes, due to the gear reduction.

[aerohead]

Originally Posted by RedDevil

Oh boy. Bear with me. Deep dive!!!!


Air driven? You miss the essential part.
it has absolutely NOTHING to do with the speed of the air!
The only thing that matters is the difference between the wind and ground speed.
It should work on a treadmill in still air just the same as with wind on solid ground, or salt, or whatever.
The only thing that matters is a difference between wind and ground speed.The air resistance of the body is negligible in comparison to the pressure on the propeller. When the car is not at speed the wind pushes against the body (which does give way by rolling away) and the propeller blades (which start to turn against the wind, so they give way less quickly)Bear in mind that the drag on the propeller has a diffeerent dynamic than the drag on the rest of the car, because it is virtually moving slower (by slowly turning against the wind) and because its surface is much bigger.The second law of thermodynamics is not what you think it is, I fear:
In other words, the second law describes entropy in an isolated system. Here we have two entities that move relative to one another; these provide energy to the system, so it is far removed from an isolated system.
The only thing the ground needs to do is provide enough grip for the wheels to roll and be flat enough to not cause too much friction. Be it dirt, salt, ice, whatever. Tarmac and salt should do fine.
It has to, otherwise it could not push the wheels over whatever they are resting on.What about 'equal'? The wheels touch the ground, the propeller faces the wind; there are no other points on the car that endure outside forces.Ground velocity? OK, we're looking at the car now.
The prop does NOT push the air back faster than the ground moves.
It needs to move the air back SLOWER than the ground, otherwise the wind cannot push it forward as then it would require more energy to turn the prop than the wheels provide.

Because it is moving slower than the ground, it requires less energy to produce the same force the wheels get over a bigger distance.In a 1/2 ratio you can lose half the energy to all kinds of friction and still move at wind speed. That's enough margin for me.Not relevant Any decent prop will do. Blackbird's officially goes 2.8 times the wind speed; there's enough margin whatsoever.Again not relevant but it is directly opposed and equal to the force on the wheels. Pick a reasonable force to work with and do the math from there.Indeed it does nothing when the wind does not blow and the ground does not move.But the main aerodynamic forces don't apply to the car, but the propeller (which is turning backwards slower than the ground speed so still moving forward related to the ground, and still enduring pressure from the wind.You forget about the propeller turning backwards, which is needed to maintain pressure to move the car forward.The ground endures the wind at full speed, the propeller at maybe half speed, the car faces a headwind.Replace car with propeller and then you've nailed it!Yes, the wind and the ground are the two main entities and they move relative to one another.No! Then it does not work.
If the propeller endures zero wind speed there is no pressure on the propeller, no force to drive the car forwards, to push the wheels over the ground and to turn the propeller. It would come to a halt.
The propeller needs to move the air passing through at a speed just shy of the speed of the car. Then the wind can still put pressure on it, forcing the car forwards.
Yes, the wind and ground form a dynamic duo moving relative to each other.There's a margin to compensate for any bad effect from that (apparently).Fair enough, will do.
Assuming friction on the wheels, gears and car body is relatively low compared to the load on the propeller, there is very little power needed to move the car other than overcoming the resistance from the propeller being driven against the wind.
Not so much just drag but load on the blades of the propeller, and not so much friction as the force that friction provides to the wheels to make them turn.It obviously cannot provide more power to the propeller than the wheels provide. But then it does not need to, as it pushes the air backwards at a slower speed than the car is moving relative to the ground.
Power is force times distance, and the propeller endures the same force over a smaller distance.
I too see a shortcoming, but not in the video.
Funny how easy it is to spot the shortcomings in others and how hard it is to see your own. A man cannot look beyond his own horizon, but easily see what behind the horizon of others.

1) the blue-wheeled car is 'mechanically' coupled to 2X4.
2) Blackbird is aerodynamically coupled to the impulse which pushes it, as well as the aerodynamic coupling between the prop. and the surrounding air mass.
3) there is aerodynamic slippage at both couplings, something not present in the blue-wheeled car/ 2X4.
That's what I was referring to.
While all energy from the 2X4 will transfer to the blue car, only a fraction of the wind energy will transfer to Blackbird, and from Blackbird's propeller to the air.
This puts the two vehicles into two separate worlds.
--------------------------------------------------------------------------------------
The mass of Blackbird, with driver, will exhibit rolling resistance challenges not experienced by the low-mass model on a treadmill. Blackbird is already challenged with respect to energy capture through a viscous-shear fluid coupling, then we pile on the resistance of tires rolling on dirt.
Entropy is any irreversible low grade heat generated within a system which cannot be harvested for useful work.( it will show on an infrared thermograph ). The lakebed, car, and atmosphere is the system. Just like any car.
Until the ground velocity matches air velocity, Blackbird's entire body is a sail. It's a non-dimpled golf ball at sub-critical Reynolds number, with high separation, high drag. All of it.
You might be amazed at the increase in rolling resistance on dirt. And this is robbing energy from the system.( not represented on a treadmill ).
For Blackbird to go more than two times faster than the air it's in, the propeller will have to move air very fast to provide that kind of thrust.
We can't quantify anything yet, as to 'equal' or anything, until we get more data.
The propeller diameter determines the 'disc' area and mass of air associated with the prop., as a function of rpm, and pitch of the 'screw' on its helical pathway, as it pulls air through it faster than the air outside the disc.
Other forces the car experiences are cannibalistic losses at capture, every step of power transfer, and finally at thrust.
I can't address how much energy we can lose, as we can't so far quantify anything about the car.
My book on propellers is pretty clear that the choice of a propeller is an enormous decision and made only after much consideration.
The propeller needs to transfer a net amount of horsepower to the air which exceeds the road load horsepower of the car at the point where ground speed equals wind speed.
It would be good to establish an airspeed and a ground speed for Blackbird. It's very difficult leaving this open-ended. My training doesn't allow for it.
And remember, power is force, distance, and time, like lb-ft/second.

[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.

[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.

[RedDevil]

Oh, it may well not work in dirt. I suggest using tarmac or a well treated salt pan.

The body is a non-dimpled golf ball? Just like an aircraft hull?
Planes don't fly and gliders drop like bricks... /s
Viscous shear etc. is present but at these speeds it is not as dominant as you pretend. It isn't even prohibitive at aircraft speeds, obviously.
This is more of a physics event than an aerodynamical one.

The blue wheeled car is just to demonstrate how it is possible to move faster than the difference in speed between the two media touching it.
The Blackbird is another example, but the execution is different even though the principle is the same.

It seems you still don't get that the propeller is moving the air slower than the car moves itself, and it is absolutely essential to grasp just that to understand how it works.

You are presented with something you really don't want to believe.
You better make sure you are not a victim of cognitive dissonance. That turns an enlightening challenge into a horrible one.

[freebeard]

You better make sure you are not a victim of cognitive dissonance.

There's a certain amount of perceptual context involved.

www.nytimes.com: Is That Dress White and Gold or Blue and Black?

[RedDevil]

Yes, but it is quite far removed from the notion that you can sail faster downwind than the wind.

Rick Cavallaro presented his ideas in a yacht club, where most sailors are all too aware that you can indeed sail downwind faster than the wind by tacking (provided the yacht has low drag, like a catamaran or an America's Cup racer).

But even a really plump boat like the barges we used in the yearly Water Scouts races is faster if you don't go straight downwind but tack mildly; just not faster than the wind, but they never are.
I could hardly make up places tacking upwind, but I did win some positions doing just this whenever the next buoy was straight downwind.
A sail is so much more effective when the wind blows over it like a wing rather than just acting as a chute.