Directorly Downwind Faster than the Wind

[nightlife]

Goat, "Does this theory mean that we could apply it to a windmill in reverse to get more energy out of the wind ?"

I would think that they have already tested both and they are nor designed to work off what is best but you never know, it may have been over looked.

I dont have a wind mill to test but if some one else does, please check this out and see and then let us know.

[hartiberlin]

It's counter-intuitive, but physics is OK with this device.  This possibility has been discussed at length on Steorn here:

http://www.steorn.com/forum/comments.php?DiscussionID=60059&page=5&Focus=2265999

My contribution:

Given access to two streams with parallel velocities (relative to you)

What 2 streams do you exactly mean ?
Can you explain it some more ?

It seems from the videos, that it only depends how fast the wheels
are rotating and this is geared up to the propeller.

So from the test videos on the threadmill
it seems, that this device only needs to be pushed up to 10 miles/hour speed
and then it will accelerate itsself without needing any blowing wind itsself to speed up.
All is probably done via the right combination of the right wheel to propeller gear ratio.

Then it seems to overcome friction and seems to selfaccelerate.

So it seems one could build cars like this, that just only
need a motor to push them over the 10 miles/hour threshold
and then you could switch off the motor and the car would
selfaccelerate. Amazing !

Regards, Stefan.

[Mr.Entropy]

What 2 streams do you exactly mean ?
Can you explain it some more ?

The original thread at steorn was about sailing.
In this case, the 'streams' are the treadmill and the air at rest.  Or the moving wind and the road at rest.

It seems from the videos, that it only depends how fast the wheels
are rotating and this is geared up to the propeller.

The air and road have to be moving at different speeds.

So it seems one could build cars like this, that just only
need a motor to push them over the 10 miles/hour threshold
and then you could switch off the motor and the car would
selfaccelerate. Amazing !

Sure.  They'd still be wind powered, though, so you'd have to keep them very light, and the speed at which you could practically expect to move is very limited.

[TechStuf]

Even a propellor with zero pitch, once started, should continue turning, as the 'virtual surface area' is increased with velocity.  The fact that the prop still works when pitched to provide a modicum of thrust against the tail wind, demonstrates how effective a small surface area can be at creating said increase in 'virtual surface area' when moving perpendicular to the flow of wind.

This still does not fully account for the slight force asymmetry expressed in the higher vehicle velocity, which is why I propose duplicating the experiment with winglets on the prop tips.

It seems plausible that VRS (vortex ring state) may be involved....producing a small area of low pressure ahead of the craft.  If such is the case, winglets of sufficient size at the prop tips will serve to modify velocity differentials.

TS

[Pirate88179]

@ 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