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

[RedDevil]

In hindsight I really like the treadmill experiment.
Because it is so simple, and because it so obviously seems to violate 'logic'.

Here is a thought.
If you put the car model on the treadmill but remove the pusher propeller, what would happen?

The cars wheels would turn (given it is held in a steady position) but once released it would gradually lose speed and move to the end of the treadmill.
However, assuming internal friction is low, it would stay on the tread for a few seconds.
If you hold the car steady it would require very little force to do so.

If we re-attach the fan and still hold it by the axle, then the fan will spin. But with the fan spinning it pushes the air backwards, creating a force that pushes the car forward.

In order to move the car forward, the force on the propeller must be larger than the drag on the wheels.
So it needs its gearing set up in a way that the propeller moves the air backwards slower than the speed of the treadmill.

As the treadmill moves the wheels over a greater distance than the prop moves the air, it requires less energy to create the same force that allows the wheels to generate it.
It is a negative feedback loop, but not a full negative feedback loop.
The air is pushed over a smaller distance than the tread travels in the same time.

Suppose the propeller generates 1 Newton of force blowing the air aside at 1 meter per second, and the tread is moving at 2 meter per second.
The propeller would require 1 Watt of energy to maintain the airflow and force (bar losses).
It derives that power from the wheels that move at 2 meter per second.

It only takes half a Newton to generate 1 Watt worth of energy over 2 meter per second.
So the wheels only feel half the resistance on the thread that the propeller provides by milling the air.

Hence, strange as it may seem, the propeller pushes the car forward against the rolling tread because the wheels offer less resistance than the thrust of the propeller!

[AeroMcAeroFace]

Originally Posted by California98Civic

Yes, but to the professor's credit, he admitted he was wrong and conceded the bet. That's integrity. An interest in learning. We can't be right all the time.

Yes, that is correct, I agree. But maybe stay away from "debunking" things without full knowledge. Provide counter arguments, but debunking?

[California98Civic]

Originally Posted by AeroMcAeroFace

Yes, that is correct, I agree. But maybe stay away from "debunking" things without full knowledge. Provide counter arguments, but debunking?

The critics were too confident. Even Bill Nye the science guy got a little confused. The counter intuitive nature of the car... seems to boggle lots of minds. Cool story. Great youtubers.

[freebeard]

I thought that wrapped things up, but now I see that Xyla Foxlin has posted her downwind faster car to MakerBot Thingiverse: https://www.thingiverse.com/thing:4897423



Complete with a Bill of Materials from McMaster-Carr:

[aerohead]

Originally Posted by California98Civic

The $10,000 bet with a UCLA Prof that the project doesn't work. The video is pretty remarkable. I don't want to spoil anything.

I'll play Devil's Advocate and suggest:
1) that the math is incomplete.
2) we still lack incomplete data for the runs presented in the video.
3) the treadmill models in no way represent the dynamics of Blackbird on the dry lakebed.
-------------------------------------------------------------------------------------
That said:
4) a 19.2-foot International Tornado catamaran, with 235-square-feet of mainsail and jib was clocked at 19.2-mph, in a 13.9-mph wind, with course angle= 90-degrees relative to true wind.
5) Windbuggy-Class ice-racers achieve velocities which are 4X the true wind velocity.
6) A.B. Bauer, mentioned in the video, achieved approximately 14-mph into an approximately 12-mph headwind, for 40-seconds, with his propeller-powered car. ( see ' Faster than the Wind,' A. B. Bauer, 1st AIAA Symposium on Sailing, California, 1969 ).
--------------------------------------------------------------------------------------
From Hucho's 2nd-Edition, p. 215 - 216, he gets into Earth boundary layer Turbulence level, then ' wind speed gradient' ( as mentioned in the video ).
From Figure 5.3, showing a natural sidewind, the wind velocity at Blackbird's propeller could be 3.5 X the wind velocity measure at the yarn/ Mylar tattle, indicating that the while the tattle was indicating forward velocity near the ground, the propeller would still be in a tailwind.
No mast for a propeller-elevation tattle or wind speed indicator was provided to us in the video at El Mirage. Upper wind velocity was an unknown quantity.
-------------------------------------------------------------------------------------
While propeller efficiency was mentioned, no coefficient was provided to us.
We have no mechanical efficiency values for:
1) rolling resistance of three tires on dirt
2) wheel bearing losses
3) differential gear losses
4) lower sprocket-to-chain losses
5) internal multi-link chain losses
6) upper chain-to-sprocket losses
7) propeller jackshaft bearing losses
8) propulsive efficiency of the propeller-to-atmosphere coupling ( a maximum of 93.3% as of 1935 NACA testing ).
9) we have no projected frontal area for Blackbird
10) we have no coefficient of aerodynamic drag for Blackbird, 'forwards' and 'backwards.'
11) we have no test weight with driver for Blackbird
12) sub-critical Reynolds number was never mentioned by anyone
13) we have no physical exterior dimensions for Blackbird.
--------------------------------------------------------------------------------------
At sub-critical Reynolds number, Blackbird is a golf ball without dimples, whether going 'forwards' or 'backwards.'
Blackbird will experience terrific flow separation, casting a large wake.
If it turned out that windspeed at the propeller was indeed higher than down below at the tattle, then according to laminar boundary layer aerodynamics, the entire body of Blackbird would be in separated flow, with a low-pressure turbulent wake ( ahead of the superstructure ).
Any yarn tuft of Mylar streamer would be impelled to seek this low pressure regime, providing the 'illusion' of extra wind velocity ground speed.
--------------------------------------------------------------------------------------
The instability of the propeller, inducing cyclic torsional displacement and deformation of the upper body, is a classic phenomena associated with 'downwind', two-bladed propeller asymmetrical wind loading of the propeller disc, from body interference. We need to pay attention to this, as it's a default wind direction indicator.
None of this was in the calculus.
--------------------------------------------------------------------------------------
I have no skin in the game and am not convinced that Blackbird cannot do what has been purported. I would just like to see a fully-instrumented test, and all data and specifications.
It's a book keeping issue. Accounting. Complete energy balance on the spreadsheet.

[freebeard]

The instability of the propeller, inducing cyclic torsional displacement and deformation of the upper body, is a classic phenomena associated with 'downwind', two-bladed propeller asymmetrical wind loading of the propeller disc, from body interference.

Fair point.

Would it be faster backwards?

[aerohead]

Originally Posted by RedDevil

related to THE CAR. the plank will move in the same direction as the car related to THE GROUND at half the speed of the car. Just like in Veritasum's video.

The whole point of that demo is to prove that you can make an object move faster than the speed of the two media it interacts with.

I don't know where it says that, but I would agree to it if you replace 'body' with 'propeller'. Als long as the car moves slower than the wind the minimal drag on the body helps a bit though.

Exactly! The car does not break the law of conservation of energy (thermodynamics are not relevant here, but not violated either).

If anything, it makes clever use of it.
There are two media at work here: the wind and the ground.
The wind pushes against the propeller, the ground pushes against the wheels.

The forward force on the propeller and the wheels are roughly equal.
But the distance the wheels move over the ground is larger than the distance the propeller moves the air relative to the car. And energy is force times distance.

So the energy needed to spin the propeller to withstand that force is less than the energy obtained by pushing the wheels over the ground with that same force. No broken laws, just a well used one.
Ground speed and air speed are equal in a windstill. The car obviously has no power source then. I think you meant to say something else, but I'm not guessing.Indeed.Negative to the car you mean? Because that is exactly what we try to achieve.
There's no such thing as negative wind speed compared to the ground.
There's no such thing as negative wind speed compared to the surface of the propeller blades.
Obviously the wind needs to keep pushing against the propeller. It has to turn to allow for that.

Again, energy is force times distance. And as the propeller is forcing the air backwards slower than the wheels move over the ground, it needs less energy to do so than the wheels provide from the same force.No need for tacking if you rotate around an axle for an 'infinite tack'. The propeller is nothing but two sails on an axle.
As for lift to drag ratio, that can go all the way to 60:1. Seems adequate.
Ha! What would anyone have to do to convince you?
When building the damn thing, having sceptics inspect it, having an official registration of a record 2.8 times wind speed and all, and winning a 10,000 dollar bet with an UCLA professor does not even convince you?

No proof will ever be good enough for you, because the only thing you'd do with it is trying to debunk it.

Because deep in your heart you KNOW it cannot work.

Because you make a mistake in interpreting its physics, but you cannot admit that somehow.

1) the little model is not air driven.
2) the little model does not mimic El Mirage Dry Lake conditions
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 )
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.
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'
6) the tires resist rolling on the dirt as a function of their coefficient of rolling force ( which must be quantified for dirt )
7) along with the body, the air flow impacts the thrusting face of the propeller.
8) I'm uncertain how you could quantify what the forces would be on the propeller and wheels.
9) the distance the wheels travel will be whatever it is.
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.
11) we don't want to confuse gross available energy with net useful energy. Blackbird lives on the margin.
12) do you have the propeller specifications for Blackbird?
13) do you have the aerodynamic 'push' data for blackbird?
14) yes, when airspeed and ground speed are equal, there is no more impulse, and momentum is constant.
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.
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.
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.
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 ).
19) the ground just experiences, from a fixed position, whatever the wind is doing.
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.
21) the wind will just blow.
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.
23 ) I'd prefer we use 'power' rather than energy.
24) Blackbird will have a ROAD LOAD Horsepower requirement under any given set of circumstances.
25) this power is associated only with aerodynamic drag and rolling resistance.
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.
27) everyone ( I believe ) accepts that the propeller is 'tacking' around its shaft and capable of imparting thrust.
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.

[oil pan 4]

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

[aerohead]

Originally Posted by jakobnev

The plank will move half speed in the same direction as the car.

The upper wheels turn half as fast as the bottom wheels.

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!

[aerohead]

Originally Posted by freebeard

Fair point.

Would it be faster backwards?

Depends.
Blown from behind, the prop is in the wind shadow of its A-Frame mast support, which will induce the asymmetric loading. Not so good.
Exceeding the velocity of the tailwind though, the prop is now looking at the undisturbed air of a headwind. Favored by many aeronautical engineers. Some of the prop wash will impact the A-frame, compromising thrust efficiency.
Placing the prop in the rear gets rid of any structure impeding its slipstream, however you'd want it in a 'clean' environment, free of oncoming turbulence.
Jack Northrop tried to address this in his flying wings.
https://www.airway1.com/northrops-hi...crashes-in-us/