Mad Genius - Wind powered vehicle travels 2.8x speed of the wind

[Rameses]

change pitch of the blades to have the oncoming wind of it's forward motion power it once it exceeds tailwind?
friction losses would negate something I'd thing though otherwise it's a perpetual motion machine  

I don't know if anybody else picked up on this, but when it first starts to move, the propeller is spinning backwards.

The wind isn't spinning the propeller in the beginning.  The propeller is spinning because it's being driven by the wheels turning.  The propeller at this point is acting like a sail and the wind it simply pushing the cart along.  The propeller turning is merely a by-product of being geared to the wheels.

Then as soon as there's a headwind, the wind is now driving the propeller and the cart is now wheel driven.  At this point the cart starts accelerating much much faster.

It's not perpetual motion because eventually air resistance and rolling resistance equal the gear magnified force that wind is transferring to the rear wheels and it reaches a constant speed.


What I haven't worked out yet is how the transition is made from tailwind to headwind.  It seems like there needs to be some sort of stored energy to push the cart through this zero relative airspeed period.

Unless by this point the propeller is spinning fast enough that it's producing thrust and has enough rotational momentum (which I guess would be the stored energy I mentioned) to continue turning fast enough and long enough to drive it into a headwind state.

[aguacate]

What I haven't worked out yet is how the transition is made from tailwind to headwind.  It seems like there needs to be some sort of stored energy to push the cart through this zero relative airspeed period.

The turbulence of the wind causes it to pop.

Just like an electric motor.

[SacDuc]

The turbulence of the wind causes it to pop.

Just like an electric motor.

Wouldn't the cart still have to be going at least 101% of the wind speed before there is this turbulence you speak of. How does that happen by a mere push from a rear wind?

sac

[lethe]

maybe the driver ate Mexican for lunch and is providing some added wind thrust

[NFG]

[retracted]

Bah.  Put me down for skepticism.

----

I can't put my finger on why, but I have trouble reading any of this without feeling like I'm being snowed.  It's like someone's asking you a riddle and the careful phrasing of the question misleads you into going down the wrong mental path.  "Why is the horizontal reflection of a mirror reversed, but the vertical reflection is not?"  Someone's being tricky and too many not-quite-smart-enough internet wizards are claiming to know the answers.

[kopfjäger]

If the Hydroptere can reach speeds in excess of the wind, why can't it be done on land? (I know jack nor shit about this stuff)


Hydrofoil Speed record

[El Matador]

If the Hydroptere can reach speeds in excess of the wind, why can't it be done on land? (I know jack nor shit about this stuff)


Hydrofoil Speed record

The Hydroptere is not going directly downwind. Nevertheless, I'm a believer, I have my theories, but I'll wait to discuss them with my aero professor tomorrow before making any bold claims. It does seem physically plausible though.

[derby]

If the Hydroptere can reach speeds in excess of the wind, why can't it be done on land? (I know jack nor shit about this stuff)

that's at an angle to the wind. this "directly downwind" challenge is a specifically different set of circumstances.

[El Matador]

The Hydroptere is not going directly downwind.

that's at an angle to the wind. this "directly downwind" challenge is a specifically different set of circumstances.

Derby 

[pennyrobber]

I too remain skeptical. Time to break out the pad and pen and think about it in more detail.

[aguacate]

Wouldn't the cart still have to be going at least 101% of the wind speed before there is this turbulence you speak of. How does that happen by a mere push from a rear wind?

sac

Real Wind force is not a smooth theoretically consistent vector  force that pushes the cart along according to some equation.  The air molecules are smashing around everywhere, and they build up in groups.  It is very chaotic.  When you talk about velocity and acceleration of the cart, fortunately you can simplify it with the average force that the wind makes; however, in this case I think the chaos plays an important role in helping the cart transition stages.

Let's say the cart is going the same speed as the average wind speed.  The cart moves smoother and more consistently than the wind because 1) it is more dense mass-wise, and 2) the rotational momentum built up in the mechanics.  So the cart follows the average wind speed more closely than the local wind surrounding the cart does.  A strong cluster of wind molecules builds up and acts on the "sail" to drive the cart forward.  The cluster disappears, and there is a vacuum behind it.  Or better yet there is another strong cluster of wind moving the opposite direction.  This causes a negative pressure on the air stream.  The cart has too much momentum built up in its gearing/mechanical system to be very effected by the negative pressure, plus it was just pushed ahead by that forward moving force cluster that has left a "force hole" behind.  The air stream, localized around the cart, slows down significantly.  The cart was going the same speed as the average wind and hasn't slowed down, but the local wind slowed down for the moment, so the cart is now going faster than the local wind.  Now the cart is moving downwind faster than the local wind.  We know the local wind speed will center around the average flow over time, so the local wind is going to pick up again.  If the local wind picks up, it will bring the cart with it.  As the wind picks up again, the cart remains in the downwind-faster-than-wind stage, and it will continue to accelerate into that stage until the power it harvests from the wind is equal to the power required to  keep it moving.

This is how you make money in a failing stock market.

[reksio]

I don't know if anybody else picked up on this, but when it first starts to move, the propeller is spinning backwards.

It is a propeller that spins like propellers do: opposite to the torque from the air. If it would spin in accordance with the torque from the air it would be a turbine.

The wind isn't spinning the propeller in the beginning.  The propeller is spinning because it's being driven by the wheels turning.  The propeller at this point is acting like a sail and the wind it simply pushing the cart along.  The propeller turning is merely a by-product of being geared to the wheels.

Correct so far

Then as soon as there's a headwind, the wind is now driving the propeller and the cart is now wheel driven.

No, that wouldn't work. The rotor would push the air forwards and accelerate the true wind. To extract wind energy you have to slow down the true wind. When going downwind that means always push the air backwards with the propeller.

It's not perpetual motion because eventually air resistance and rolling resistance equal the gear magnified force that wind is transferring to the rear wheels and it reaches a constant speed.

The idea with a turbine driven by the apparent headwind above wind-speed would in fact create more true wind and thus energy out of nothing. It cannot work continuously.

What I haven't worked out yet is how the transition is made from tailwind to headwind.

There is no transition. Nothing changes in the power transmission at wind-speed. The wheels always turn the propeller.

[reksio]

Nevertheless, I'm a believer, I have my theories, but I'll wait to discuss them with my aero professor tomorrow before making any bold claims.

What did the aero professor say?

[Rameses]

There is no transition. Nothing changes in the power transmission at wind-speed. The wheels always turn the propeller.

No, there is clearly a transition.

The cart goes from traveling slower than the windspeed to faster than the windspeed.

You can't argue that no change from a tailwind state to a headwind state occurs.

[reksio]

You can't argue that no change from a tailwind state to a headwind state occurs.

The direction of the power transmission doesn't change at windspeed. The wheels always turn the propeller.

The switch of apparent tailwind though zero to apparent headwind at the chassis is not that important (just for drag). What really matters is the apparent wind at the rotating airfoils, which never drops to zero. At the airfoils the apparent wind changes it's angle to the travel direction right from the start continuously while the vehicle accelerates. At wind-speed it comes at 90deg to travel direction, which easily allows acceleration with the right orientation of the airfoil.