I don't understand how this car works.

That image makes it clear where the mechanical advantage is coming from....

I remember the spool demonstration from college.

I didn't at first either, but after a bit of virtual ice boat training it became clear.

Along the way we got to see some cool treadmill videos and finally a reminder of the yoyo principle (DownStringFasterThanTheString)

This has been a fun and educational thread DD. Thanks.

In principle a sail boat (on water or ice) with a mainsail & moveable boom CAN still generate forward thrust from the sail even when the boat is moving at speeds faster than the wind. This can be verified in a simple thought experiment. Envision such a boat heading 45 degrees to port of the direction from which the wind is blowing and travelling at the wind velocity. Properly positioned, the boom will be on the starboard side something greater (by an amout equal to the optimum angle of attack for the mainsail) than 112.5 degrees clockwise from the ship's head - say about 120 degrees. In this configuration the relative wind velocity will be about 75% of the wind velocity and the sail will produce a lift force proportional to the square of the relative wind speed and roughly perpindicular to the sail plane (thus about 30 degrees starboard of the bow. This lift force will have a thrust component (aligned with the boat's axis) of about 82% of the lift generated by the sail.

The significant question here is the relationship between the ship's speed and the thrust required to overcome its drag. For a submarine or an aircraft in one fluid medium the drag is proportional to the square of the vehicle speed. For a boat, creating wave drag (between the air and the water) the total drag is proportional to the cube of the vehicle speed. For a wheeled vehicle the drag is comprised of wheel friction and aerodynamic drag. In the case at hand, with a fixed mechanical connection between the wheels and the rotating propeller, the vehicle's aerodynamic drag is the sum of the profile drag of the vehicle itself and the wind force on the propeller. Assuming the propeller disc can be steered relative to the vehicle axis (like the plane of a mainsail) , we then have a complex geometric situation somewhat analogous to the sailboat situation described above.

The bottom line is that an iceboat involves very little drag and can easily sustain much higher speed than the windspeed. It is, however, limited by the tipping moment generated by the sail. A sail boat involves much greater surface drag, but the fastest ones, such as the 12 meter America's Cup designs of a decade ago, could travel slightly faster than the wind speed - but they had very little payload and were designed only for speed.

In terms of total drag, a surface vehicle is somewhere between an ice boat and one that travels in the water. The big issue is the wheel friction which creates drag proportional to a power of speed that itself is proportional to the weight and wheel loading of the vehicle. The key point here is that such a vehicle can indeed be designed, but it too will be limited by the propeller tipping moment, and itwon't go much faster than the wind speed. Even that is severely limited by its weight - certainly no significant payload could be carried.

I suspect that they are using what are essenitally very, very high pressure bicycle tires in order to cut rolling resistance to an absolute minimum.

You're right that no real payload could be carried by this! I wonder if there's even a really, really light person driving it

Cycloptichorn

If by "propeller tipping moment" you are referring to the torque reaction on the vehicle from the power applied to the prop shaft, this can be countered in a myriad of ways -- one of which is counter rotating propellers.



Is 3X the speed of the wind "much faster than the wind"?

It's pretty clear that with 3X available from a 'garage built' vehicle on a very tight budget, 4x or even 5x could likely be achieved if this were an effort important enough to do so (it's likely not).



When it comes to top speeds (as opposed to acceleration) added weight only effects top speed through the Crr (rolling resistance) numbers which are quite low. Our current vehicle weighs ~400lbs and is operated with an ~210lb pilot. Using ordinary Crr numbers for a car tire on concrete (o.01) we could tow a trailer with over 4,000lbs onboard behind the Blackbird and still maintain a speed just over that of the wind.

JB

I'm sure you are correct. High speed bycicle tires operate at about 60 psi: the tires on commercial aircraft at about the same pressure and carrier aircraft at about 110psi (however these get VERY little mileage). In general the coefficient of friction is a function of the tire material & pressure and the contact footprint of the tires in square inches: stated another way, of the vehicle weight and tire material.

The "tipping moment I referred to above involves the capsizing of the vehicle. High speed sdailing boata have very large, deep keels precisely to enable them to maintaain high speed on a large reach relative to the wind; ice boats & road vehicles rely on the distance between the runners or wheels.

The PSI on modern bike tires is even higher; I pump mine up to 100 PSI or higher regularly.

Cycloptichorn

I do not think that we got to the moon by magic or by breaking every law of physics!!!!!!!!!!!

How in the hell can you people listen to this nonsense and not take note that Mr. Thinsairdesign go from referring to situations that have nothing to do with his group claims to then going back to so call experts who know far better then Newton what the laws of nature happen to be.

There are times when it would be so nice to be a con-man as it would seem that the vast majority of people are willing to throw all commonsense and all understanding of how the world work if someone can play a good word game.

No wonder that the human race have been enslaved by one made up gods one after another, less alone investing their lives savings times after times in fool gold of one kind or another.

Yes indeed there is a way of getting energy out of a wind that does not flow over a vehicle and the secret is chaining the wheels to the prop and then killing a virgin and oiling the chain with her blood.

Rolling resistance is not the coefficient of friction.

Coefficient of friction indicates how reliable the interface is between the tire and the road. (i.e., what it will take to make the tire skid.)

Rolling resistance is how much energy is lost when the tire rolls over the ground. (Such as when the tire flexes, causing friction.)

DrewDad -- is there some reason you're choosing not to answer the last question from last night (that I posted again this morning)?

It's the question about how many HP the 100% efficient generator can produce when being pulled by the propeller in the tailwind consuming 1/2hp.

Thanks
JB

It slipped past me. I'll address it when I get a chance.

No problem -- here is it.

http://able2know.org/topic/145626-18#post-4167782

[translate]I can't for the life of me follow the step by step explanations and equations that have demonstrated conclusively for so many that there are no physical laws of the universe violated here so I'll instead I'll continue to throw out strawmen and cry "WITCH!!". [/translate]

JB

It's fun watching BillRM -- he claims that he wants folk with "advanced degrees" involved in the testing and yet when one of the most advanced degreed (and respected) aerodynamicists in the world (Dr. Drela, MIT) states that EXACTLY what we are doing can be done "without too much difficulty" he ignores it completely.

LOL

JB

Yes, your math is of the same class as your words and I love you confusing a scalar quantity with a vector quantity time after time. Speed is not the same as velocity and velocity is not the same as speed. Or bringing up iceboats that do go to great trouble to maintain a relative wind force something your car does not do and can not do going directly downwind.

In any case, there is no need for higher math or even high school science math as when you do not have a relative wind you do not have a driving force from that wind end of subject no matter how you wave your hands in the air.

Of course the energy you had spend on this nonsense here is enough to drive that car of your to the moon and back.

Good thing, 'cause you apparently cannot do either those or high-school level physics, Bill. All you have are assertions and an angry old man attitude.

I still have not heard you commit to manning up and admitting you are wrong, if tests come back showing that you are in fact wrong. Will you do this?

Cycloptichorn

I would love to see people with advance degrees not in bed with your magical car project helping keep an eye on all tests of your claims along with the conditions of those tests.

If it later would be found out for example that some unknown group was able to pull a hoax on some hobbies sport body the damage to that sport would be large indeed and a damn shame.

The risk seem high given the long history of "experts" being fool in one manner or another.

If you disagree with Bill, you are in bed with the people Bill disagrees with.

Yes, I will man up and you are right indeed, you can get energy out of still air and if I were you, I would invest my life savings on such a wonderful technology that this car is just demonstration of.

Damn it you are right Robert how dare I assume that the people who are part of the project are in bed with the project!