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Down wind faster than the wind

wbeaty said:
Perhaps our gut-level thought experiments would become simpler if we split these two functions. Use a cart with both a large flat sail held perpendicular to the wind, and also a smaller ducted fan driven by the cart's wheels.
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As soon as the cart is running faster than the wind, the large perpendicular sail will only be a hindrance.
 
spork said:
When going downwind faster than the wind, the propeller acts exactly as any propeller does.
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Definitely. And I strongly suspect that this is the typical source of misunderstanding and outrage. All propellers are pumps which provide a pressure difference, but they also exhibit internal resistance to air motion. Their "pump" behavior is superposed upon their "air friction" behavior (somewhat like an ideal voltage source having internal resistance.)

spork said:
It does not at that point act as a flat sail catching the wind in any way. It does perform that funtion to some extent when starting from a stand-still and then approaching wind speed.
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I disagree: a propeller *always* adds a frictional component against moving air, although I doubt the friction is constant. It would act like a flat sail where the size of the sail varies with relative air speed. (Don't mistake air resistance for energy losses: the resistance could be large even at zero relative air speed. Yet at zero speed there would be no losses.)

But at the same time the propeller acts as a fluid pump. Just as "battery" is a separate concept from "internal resistance," the "pump" concept is separate from the "sail" or "air friction" concept. Mixing them together would prevent us from realizing that a simpler verbal explanation might be possible.
 
wbeaty said:
I disagree: a propeller *always* adds a frictional component against moving air, although I doubt the friction is constant. It would act like a flat sail where the size of the sail varies with relative air speed.
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I took what you were saying above to mean that the prop in part acts as a flat sail, capturing the force of the tailwind while the cart goes DDWFTTW. This is not the case.

If you want to talk about the "frictional component against moving air" that's fine with respect to the drag of the airfoil moving through the air (and thus creating a reaction torque on the prop shaft), but it doesn't make sense to talk about a "flat sail" component when the prop is creating thrust. A given prop turning at a given rate, and moving through the air at a given speed, creates a certain amount of thrust - and that can be measured. There's no benefit that I can see from trying to decompose that into an ideal thrust and some amount of flat-sail force - largely because I don't think there's any physical rationale behind such a model.

But at the same time the propeller acts as a fluid pump. Just as "battery" is a separate concept from "internal resistance," the "pump" concept is separate from the "sail" or "air friction" concept. Mixing them together would prevent us from realizing that a simpler verbal explanation might be possible.
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I'm all ears. What's the simpler verbal explanation that describes the performance of a prop?
 
The simplest explanation for me comes from thinking of the cart running in flat-calm conditions.

Power is extracted from the wheels, which tends to slow the cart down, and fed to the propeller which pushes the cart forward.

If we imagine a cart with a 100% efficient propeller and transmission system, then the drag from the wheels would exactly balance the thrust from the prop. So (neglecting other drag and friction) the cart could run at any speed in any direction and would neither gain nor lose speed.

Of course, any real cart doesn't have a 100% efficient anything, so a real cart would quickly come to a standstill in calm conditions.

But any wind blowing adds to the thrust of the propeller - it does this because the prop can expend its power moving air at a lower speed - the power consumed by an ideal prop is the product of thrust and speed.

When the additional thrust generated by the prop in a wind is greater than the extra drag caused by all the real-world inefficiencies of the cart, then the cart accelerates until the thrust and drag once more become equal.
 
ceptimus said:
The simplest explanation for me comes from thinking of the cart running in flat-calm conditions.

Power is extracted from the wheels, which tends to slow the cart down, and fed to the propeller which pushes the cart forward.

If we imagine a cart with a 100% efficient propeller and transmission system, then the drag from the wheels would exactly balance the thrust from the prop. So (neglecting other drag and friction) the cart could run at any speed in any direction and would neither gain nor lose speed.

Of course, any real cart doesn't have a 100% efficient anything, so a real cart would quickly come to a standstill in calm conditions.

But any wind blowing adds to the thrust of the propeller - it does this because the prop can expend its power moving air at a lower speed - the power consumed by an ideal prop is the product of thrust and speed.

When the additional thrust generated by the prop in a wind is greater than the extra drag caused by all the real-world inefficiencies of the cart, then the cart accelerates until the thrust and drag once more become equal.
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That's pretty darn accurate as far as I'm concerned. The only slight clarification I'd add is that, with a fixed pitch prop, the vehicle has a specific design speed as a multiple of wind speed. You're right that it will occur when thrust equals drag, but that will be somewhat due to the fact that thrust will decrease as the speed increases (and become negative beyond the "design" speed).
 
Rather than use a variable pitch prop, wouldn't it be easier to alter the gear ratio between the wheel axle and the propellor shaft? I'm thinking derailier gears like those on bicycles.
 
ceptimus said:
Rather than use a variable pitch prop, wouldn't it be easier to alter the gear ratio between the wheel axle and the propellor shaft? I'm thinking derailier gears like those on bicycles.
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We do have a gear cluster on the axle, but we can't change gears on the fly. That requires varying the chain length, which is tougher to do when you have 13' of free span chain and want to keep it tensioned. Also, gear changes come in discreet increments. Changing pitch on the fly has its own downsides, but we chose what we thought was the lesser of two evils.
 
spork said:
We do have a gear cluster on the axle, but we can't change gears on the fly. That requires varying the chain length, which is tougher to do when you have 13' of free span chain and want to keep it tensioned. Also, gear changes come in discreet increments. Changing pitch on the fly has its own downsides, but we chose what we thought was the lesser of two evils.
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Think CVT.
 
Well, it's good to see this got the backing it needed to demonstrate the practicality. Good job :)
 
In a quazi-related hypothetical, I'm trying to picture a bicycle (no pedals or drive mechanism) wherein the wheels are 3-vaned propellors, connected at the ends by a rim.
Assuming that it didn't blow over, and was set perpendicular to the wind, it should start to roll. Would this be a combo of prop/turbine? And what hypothetical speed could be reached, in relation to the wind speed?
 
quarky said:
In a quazi-related hypothetical, I'm trying to picture a bicycle (no pedals or drive mechanism) wherein the wheels are 3-vaned propellors, connected at the ends by a rim.
Assuming that it didn't blow over, and was set perpendicular to the wind, it should start to roll. Would this be a combo of prop/turbine? And what hypothetical speed could be reached, in relation to the wind speed?
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The propellers would convert the wind energy to mechanical energy in the bicycle, so I would call them turbines. Theoretical speed limit? c? :p

A semi-related follow-up: What about using bicycle wheels shaped like Wells turbines in races with a lot of side wind? Is it allowed?
 
Ririon said:
The propellers would convert the wind energy to mechanical energy in the bicycle, so I would call them turbines. Theoretical speed limit? c? :p

A semi-related follow-up: What about using bicycle wheels shaped like Wells turbines in races with a lot of side wind? Is it allowed?
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From what I've heard, even disced wheels on bikes are trouble in cross winds.
I have some wind spinners made of light bike wheels with duct tape vanes, connected to opposing spokes. Its been spinning for ten years.
That's what got me pondering the query.
 
spork said:
We do have a gear cluster on the axle, but we can't change gears on the fly. That requires varying the chain length, which is tougher to do when you have 13' of free span chain and want to keep it tensioned. Also, gear changes come in discreet increments. Changing pitch on the fly has its own downsides, but we chose what we thought was the lesser of two evils.
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A bicycle derailleur ought to handle that. Perhaps an idler shaft in the system would make the length easier to handle. It would also tell us whether the prop drive the wheel of vice or versa, by which side of the chain is under tension. That is back to my idea that the wheel acts as a governor to the prop, keeping it going in a narrow rpm/apparent wind speed. Two months since I posted a sketch of my theoretical aerodynamics of the prop?
 
casebro said:
A bicycle derailleur ought to handle that. Perhaps an idler shaft in the system would make the length easier to handle. It would also tell us whether the prop drive the wheel of vice or versa, by which side of the chain is under tension. That is back to my idea that the wheel acts as a governor to the prop, keeping it going in a narrow rpm/apparent wind speed. Two months since I posted a sketch of my theoretical aerodynamics of the prop?
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I woulds think so, too. I have a recumbent bike with a very long chain and an idler pulley wheel. It shifts fine. Terra Cycle makes nice idlers.
 
I posted a new topic weeks ago to share the info we obtained at TR from Ken Bauer, son of aerodynamics pioneer Andrew Bauer. He passed away a few months back. I’m sure some who were involved in the discussion here will find it of interest, and may have missed my earlier thread.

This first link is to Ken Bauer's post at TR, and includes the letter his father sent to Paul MacCready. If you scroll a bit there are also photos of it and of the cart he made for a treadmill. If there is interest I could find those and post them here. http://talkrational.org/showthread.php?p=1098420#post1098420

Here is Andrew Bauer 1969 Wind Machine Video- http://www.youtube.com/watch?v=dAkJ_QVbloQ&feature=player_embedded

This last link is to my earlier thread about the Blackbird’s march toward establishing the first land speed record in going directly downwind; 2.8 times faster than the wind: http://www.internationalskeptics.com/forums/showthread.php?t=173124 Those not familiar with this topic might find this useful as a summary and finale of this 2 year long debate that really started to get traction with my_wan's post here at JREF.
 
I just did some math in my head, and must ask this question:

Hypothetically, would the cart exceed wind speed by 3x if it could handle 100mph wind?
Is the concept limited by the lack of craft that could handle going 300 mph?
Or does the need for friction (wind drives wheels that shove off the ground) neutralize this graph of gain with wind-speed that my head suggested?
 
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