doggie said...

getfunky said...




And just for Doggie

(reckon about 80% of Treckies would pop their phasers when the saw this - the other 20% go more for Kirk)

WOW trekie BOOBS (.)(.) thanks funky I love it WOOF!

And it is Friday after all

frant said...
Is it possible to go upwind without slipping?

Yes. This winter we will modify the cart for upwind operation. We hope to achieve a direct upwind speed of 2X (but that's a bit optimistic - we'll be pretty happy to do more than 1X directly upwind).

Answered my own question (I think)

Look at that - I'm not needed here : )

so why dont they just put up a big sail at the front and turn the fan on?

About the third time this topic's been up. Always a good topic for a bit of mental exercise, so many ways of looking at it. Here's another one.

Start off by thinking about the cart in no wind.

Give it a good push up to speed on level ground and release it.

You'll find the propellor is turning in a direction to continue pushing the cart - like a taxiing model aeroplane.

The push of the propellor, thrust using aeroplane terminology, can be turned into power and transmitted through the gear box. The amount of power that is transmitted though the gear box is equal to thrust times the velocity of the cart over the ground.

So the issue of what happens next gets down to how much thrust is generated by a propellor versus how much power is required to turn a propellor.

The thrust of a propellor is proportional to the lift generated by the propellor blade times the cos of the pitch angle of the blade. The power to turn a propellor is proportional to the lift of the blade times the sin of the pitch angle. (no drag in this thought experiment).

When you go through a trial calculation (use a simplified propellor! one that neatly corkscrews through the air) you find that the thrust that can be converted to power is exactly equal to the power required to turn the propellor. Always impressed at how conservation of energy is embedded in all these mechanical calculations where friction is ignored.

So with friction a cart pushed in still air will of course roll to a stop.

Now add wind.

If instead of zero wind we have a slight tail wind then the propellor blades can be run at less pitch and still deliver the same thrust. With less pitch angle it takes less power to turn the propellor. With a bit of power at hand to overcome friction the cart is away.

(All well and good thinking after the event - so impressed with the fellows who came up with the idea in the first place)

[b]spork said...
You actually don't need a ratchet. If the gearing is such that your cart is designed to go downwind (or down threaded rod), and the losses are low enough, it will start in the correct direction. If the gearing is changed such that it starts in up wind (or up rod) direction, it will in fact be an upwind cart. The max speed ratio happens right at that cut-off of gear ratios. In a sense it should do an infinite multiple of wind speed both upwind and downwind at that ratio. Just to either side of that ratio it will either have a high upwind or downwind multiple depending on which side of the ratio you choose.

Hi Spork, great project.

But don't you also have to change the prop for an upwind cart? I'm thinking that; downwind the propellor thrust is driving the cart - the wheels keep the prop turning. Upwind the propellor thrust opposes the motion of the cart, but the rotation of the prop is geared so the wheels overcome the "unwanted" thrust and drive it upwind.

A prop that pushes air must have blades profiled differently to one that is driven by air?

[b]spork said...
You actually don't need a ratchet. If the gearing is such that your cart is designed to go downwind (or down threaded rod), and the losses are low enough, it will start in the correct direction. If the gearing is changed such that it starts in up wind (or up rod) direction, it will in fact be an upwind cart. The max speed ratio happens right at that cut-off of gear ratios. In a sense it should do an infinite multiple of wind speed both upwind and downwind at that ratio. Just to either side of that ratio it will either have a high upwind or downwind multiple depending on which side of the ratio you choose.

I'm wondering if you played around with the aerodynamics, so the vehicle was much more slippery with wind from the front than from the back and with very careful gear ratio selection, it would work both ways.
Wind from the back, goes forward, wind from the front goes forward.

And what about a boat?? water prop driving an air prop. Or would the friction be too great.

Ok, that's great. Now somebody design something that will sail dead upwind and all our energy problems are solved.

In my stupid mind I imagine something with two sails, both on close hauls, one starboard the other port. Should work just fine.

evlPanda said...

Ok, that's great. Now somebody design something that will sail dead upwind and all our energy problems are solved.

They have !!!!!!!

www.treehugger.com/renewable-energy/windmill-sailboat-sailing-against-the-wind.html





www.seabreeze.com.au/forums/Land-Yacht-Sailing/Construction/Quiz-Time/




Use of the Magnus effect to power a ship was revived by Jacques Cousteau in 1985 with his famous "Alcyone".

best-breezes.squarespace.com/journal/2006/1/7/rotor-kites-move-from-toy-novelty-to-high-tech-power-generation.html