Downwind Faster than the Wind - Successful Run by manned cart

fireball said:
Just been reading through a bit of this thread ... and I'm probably wrong ... but ....

The truck initially just blows along the ground - ala Avon dinghy you forgot to tie off ... (or with a nudge for the big models)

The movement forward rotates the wheels which turns the prop - which pushes air backwards - but you've still got air coming forwards - so you're effectively creating a mini high pressure behind the cart and it's this cushion of air/high pressure that pushes the cart onwards ....

Take away the true wind and the truck will stop ...

??
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Exactly.

Or think of it this way:

You're an ant, who landed on one of the propellor blades as the paint was drying. So you're stuck there.

You're stationary, and the cart is pointing downwind. Once the brakes are released, the cart starts to blow forward. Because the wheels are driving the prop, the prop starts to turn.

So the apparent wind you feel on the prop is mostly in line with the cart, but a little bit at right angles to it - from the rotation. The apparent is at such a high AoA that the propellor blades are stalled out - not generating any lift.

But the cart has been well designed and build, with little friction. So the wind pushes it a bit faster. So the propellor rotates faster. So the apparent you feel - as a fly, stuck to the propellor, moves round further.

Eventually, the apparent moves around far enough that the propellor starts to develop some lift. As this lift is along the axis of the propellor, it increases the speed of the cart.

At some point, the apparent moves from coming behind the 90 degree to the direction of travel line to in front of it. You're travelling downwind faster than the wind.

Eventually, the apparent moves as far forward as it can with the propellor still generating optimum lift. In the same way that there's an optimum pointing angle for sailing upwind.
 
Keen_Ed said:
Exactly.

Or think of it this way:

You're an ant, who landed on one of the propellor blades as the paint was drying. So you're stuck there.

You're stationary, and the cart is pointing downwind. Once the brakes are released, the cart starts to blow forward. Because the wheels are driving the prop, the prop starts to turn.

So the apparent wind you feel on the prop is mostly in line with the cart, but a little bit at right angles to it - from the rotation. The apparent is at such a high AoA that the propellor blades are stalled out - not generating any lift.

But the cart has been well designed and build, with little friction. So the wind pushes it a bit faster. So the propellor rotates faster. So the apparent you feel - as a fly, stuck to the propellor, moves round further.

Eventually, the apparent moves around far enough that the propellor starts to develop some lift. As this lift is along the axis of the propellor, it increases the speed of the cart.

At some point, the apparent moves from coming behind the 90 degree to the direction of travel line to in front of it. You're travelling downwind faster than the wind.

Eventually, the apparent moves as far forward as it can with the propellor still generating optimum lift. In the same way that there's an optimum pointing angle for sailing upwind.
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Did the ant die in the end?
 
Woodlouse said:
The difference between this and BMWO is that it's claiming that it can go dead downwind at twice the true wind speed. BMWO would do 3x the wind speed, but on a broad reach angle.
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Woodlouse, please look up VMG when the destination is direct downwind. BMWO apparently achieves a velocity component direct downwind of 3x windspeed. So it is almost exactly the same thing and has the exact same conceptual 'issues'. Just implemented differently.

md
 
mobeydick said:
Woodlouse, please look up VMG when the destination is direct downwind. BMWO apparently achieves a velocity component direct downwind of 3x windspeed. So it is almost exactly the same thing and has the exact same conceptual 'issues'. Just implemented differently.

md
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Maybe so, but to do that BMWO did not sail directly down wind, but broad reached. I see that as being quite different to trying to go faster than the wind whilst running clean away from it.
 
shaunksb said:
What limits it to "about 3 times wind speed"?
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The lift to drag ratio of the propeller blades. Refer to SL's vector diagrams.

Although the blades can be trimmed to continue to present their optimum incidence angle to the apparent wind with increasing cart speed, trimming them at ever increasing pitch also rotates the lift and drag vectors so that the lift starts to oppose the propeller's rotation and the drag increasingly opposes the cart's further acceleration. This limit will occur even if there is very little other friction. Of course, with the normal friction, the cart stops accelerating at a lower speed.

The parallel with BMWO is obvious, it too has an optimum reaching angle (alias pitch) and an optimum incidence angle of its wing to the apparent wind.

There is nothing firm about 3 x wind speed DDWFTTW. A yet more optimises design might yield a little more, just as ice and sand yachts continue to improve their performance. That just seems to be the figure that is currently attainable.
 
Woodlouse said:
Maybe so, but to do that BMWO did not sail directly down wind, but broad reached. I see that as being quite different to trying to go faster than the wind whilst running clean away from it.
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The tips of the propeller blades are "broad reaching" in a spiral down wind, even though its axle is travelling dead down wind in a straight line.
 
Another thing, how are they going to get any of the speeds they do ratified? Don't land speeds need two runs in opposite directions to make sure there are no elements in the course helping them (like a downhill gradient)?
 
Woodlouse said:
Another thing, how are they going to get any of the speeds they do ratified? Don't land speeds need two runs in opposite directions to make sure there are no elements in the course helping them (like a downhill gradient)?
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Ha! Yes, and the reason for the two directions within a short period of turn round time is to cancel out the effect of the wind.

I think the current land speed records for wind powered vehicles probably drop this requirement.
 
tobermoryphil said:
I still believe that, at low speeds, the wheels drive the propellor. At some point, whilst there is still an apparent tailwind for the cart, there is an apparent headwind for the prop, and the prop then starts driving the wheels.
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For any propeller, with given rotational speed and pitch, there is a bulk fluid speed at which it will do no work. Stick it in fluid moving slower and it pulls forwards: stick it in fluid moving faster and it moves back.

However, that doesn't apply in this case, because the propeller is geared to the wheels and therefore always has enough surplus speed to push back against the wind.
 
obnoxious

Ok Ok

I've been in a bit of an obnoxious mood today.


I'm sorry if I have been a bit sharp with one or two of you.


I respect that we have a lot of combined wisdom out there that I will one day no doubt need to call on (No not you Ubergeekian)


and.......................I might have even said earlier that this carty thing was powered by this kids mum's washing machine belt.


Well I've had a good look at the other video's now and listened to what the project leaders have to say for themselves.


It's obviously apparent that a great deal of time and effort has gone in to this subject and the kids have built quite a sophisticated device.


So no....................It's not powered by a washing machine belt.



Because of the height generated by the design to ensure that the propeller gets a good flow of undisturbed air he must have used his sisters tights.


You would need quite a thick weave - I reckon a least a 40 denier in order to obtain the required stretch whilst maintaining enough grip to not loose any drive to the prop. This would also minimise any traction lost by fluctuations in the wind speed.


Allthough when his sister finds out he is going to be in big big trouble.
 
I can see how it is supposed to function, and seen from the video that the telltales show that it does go downwind faster than the true wind speed. What I have difficulty with is that even for a vehicle with low friction, running on a smooth surface, and with low wind resistance, there are still forces of friction and wind resistance tending to retard it. For it to work the prop thrust must be greater than the sum of those forces, yet the power required to generate that thrust has to come from the forward speed of the vehicle driving its wheels to drive the prop via a chain, with the only external input of power being the wind. I know that a vehicle like that only needs a small amount or power to drive it, and it may be true that the wind does so through a combination of low friction, low drag and clever gearing but I await seeing details of independent verification, not just of wind and vehicle speeds but close up physical examination to prove that there is not a small electric motor helping it along.

As an aside to make it quicker to self start I wonder why they did not fit a small sail to get it going, then furl the sail away as it approached wind speed.
 
Ubergeekian said:
I'm hurt. But maybe it just goes to show how much more difficult it is to forgive someone for being right than it is to forgive them for doing wrong.
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I know I'm sorry.........................and i'm thru now.

I will agree to the possibility that the cart can travel DDWFTTW if

you will agree to the possibility it's powered by his sisters tights.


Deal?
 
Norman_E said:
As an aside to make it quicker to self start I wonder why they did not fit a small sail to get it going, then furl the sail away as it approached wind speed.
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As it has a variable pitch prop I think setting the blades to fully fine would do the job. If they are set square to the wind they would act like a square sail then, once they are turning at a reasonable speed, they could be pitched up to give some drive.
 
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