Directorly Downwind Faster than the Wind

[Yucca]

@ All:

My main concern is that the fellow said you can't just put this on a flat surface like a gym floor and have it work..it works on the treadmill.  This I do not understand.  When learning to fly and also building my own small airboat, I had a chance to learn a little about propellers.  The main principle I see here is that when a propeller is turning, the relative wind sees it as a solid disk.  Now, this only applies when the prop, depending on the pitch and airfoil design, is turning faster than the relative wind, or slower.  There is an equilibrium point for all propellers at a certain rpm in a certain relative wind that adds 0 drag and 0 thrust, but this is a small window.  They use this when they teach you to land simulating an "engine out" experience.  If you just set the engine to idle, the drag produced by the prop would be way higher than in a real engine out scenario, and therefore would be of little use for learning to deal with a real one.

So, I can see where you have a 10 mph tailwind, and the prop is turning via the wheels after an initial push, so the prop then acts as a parachute or sail to catch the tailwind and push the craft faster.  Once it reaches the speed of the tailwind, as Stefan said, through proper gear ratios, the propeller turns at its designed rpm and therefore produces thrust which makes it go faster than the wind.  All of this works for me with what I know except it not running on a flat surface like a gym floor.  This part, I can not explain at all.

Bill

Hi Bill,

The whole things a bit of a mind bender. It´s an impressive device and appears to do something that many people say is impossible which is always a good thing to do!

The only thing I can think of is that on the treadmill, the craft does not see an oncoming airstream and thus sees no air friction. Wheras if you push it along a gym floor it would have to overcome the oncoming air resistance.

Also, I think this craft enables one to move a certain percentage faster than the driving wind, that percentage might vary and have a sweet spot of wind speed where you get the best gain, but in the case of 0mph wind as in a gym then that percentage will be zero so it wont sustain motion. To simulate the gym floor using the treadmill one would have to face the treadmill into a wind that matched the treadmill speed.

P.S.
impressed that you built an airboat! model planes and helis are as far as I´ve gone.

Yucca.

[allcanadian]

I thought this was pretty easy to figure out, on the treadmill the device produces near balanced forces, that is thrust nearly equals drag from the driving components or ground speed nearly equals generated air speed. But if this device is travelling with the wind behind it something really neat happens. To understand this I think you have to concentrate on relative motion and the neutral point of force. If the tail wind speed is 5mph pushing the craft forward then when the craft reaches 5mph ground speed the apparent wind you would "feel" is now zero mph so there is very little air resistance. Also if the ground speed is 5mph or equal to the tail wind speed then directly behind the prop there is a point of zero windspeed which constitutes a pressure boundary.
(Wind) 5mph----->zero<-----5mph(propeller)
One analogy could be standing in an elevator moving upwards at 5mph and jumping upwards at 2mph, you "feel" a 2mph velocity(apparent) but you are moving upwards at 7mph(reality). The reason this can happen is because you are "in an elevator", if you were not in an elevator you would have nothing to push against to jump. We can say that the only reason this craft can exceed the wind speed is because the prop generated wind has something to push against and this something is a wall of "pressure" having an apparent velocity of zero mph but having a real ground speed equal to the craft. A good analogy is having the launch pad of a rocket accelerating behind the rocket but not being attached to it, in this case we are not dealing strictly with reactive mass, we are dealing with a wall of pressure acting in the same direction of acceleration because the gasses ejected from the rocket are pushing against a boundary or obstruction. In this device the forces would look something like this--
(wind) 5mph----->Zero<----------- 7mph(propeller)----->2mph 
Total =  (wind) 5mph---> + 2mph----> = 7mph ground speed = 7 mph thrust
As the ground speed increases so does the thrust generated by the propeller but the propeller does not "push" the craft----the pressure generated behind the craft does and the wind has provided half of the velocity to maintain this pressure wall behind the craft and all of the energy to maintain the velocity of the craft.
Regards
AC

[TechStuf]

I wonder what would happen if you put magnets on that thar perpeller.


lol?

[petersone]

Hi all
Do's this mean an electric motor uses less juice when a fan is fixed to the shaft?
peter

[PhiScience]

Hi all

The principle of this device is very simple and straightforward, the “air car” is getting its energy from the treadmill motor. Friction between the wheels and belt turn a shaft that is attached to an angle miter gear that is attached to a second shaft that drives a propeller, thus producing thrust.
At the proper rpm the propeller will produce enough thrust to overcome the friction gravity, and drag to move the vehicle forward. But it is not producing its own energy it is getting it from the treadmill.

Cool toy though.