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Old 07-04-2021, 05:10 PM   #111 (permalink)
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Remember the blue roller that goes faster than the plank moving it?
If you put it upside down it would still go faster than the plank in the opposite direction.

Just like Blackbird would do if you set the gearing of the propeller to move the air faster than the car (instead of slower than the car)
In that case the forces trying to turn the propeller with the wind would be stronger than the force on the wheels trying to move the propeller against the wind. So the propeller starts to turn with the wind, and the gearing forces the wheels to move the car toward the wind.

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Old 07-05-2021, 01:06 AM   #112 (permalink)
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Wind can't blow something downwind faster than the wind speed, but that's not what's happening. The wind is generating energy through the ground into the wheels and the wheels transmit it back into the propeller which is able to add it back into the air. It doesn't take much to add.

This wouldn't be possible without the ground. You couldn't do it on the water with a sail boat or in the air with a glider (assuming level flight).

Side note, now thinking about that boat. I wonder if this same principle could be used to propel a boat down a river faster than the current flows?

The way it works in my mind (meaning I'm not doing that math which shows it works) is thinking of it more as a wind energy concentrator than a sail. Energy is still conserved.
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Old 07-05-2021, 06:36 AM   #113 (permalink)
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I think it should be possible to make an aquatic version of the Blackbird, f.i. by replacing the wheels with paddle wheels for simplicity. But the drag on the hull and losses on the paddle wheels due to the viscosity of the water will make it hard to exceed the wind speed by much, if at all.

You'd need a very big propeller and a fair gear reduction anyway, and with a big reduction you won't get far beyond wind speed if there's no friction of any kind.

A setup with one small high-angled propeller in the water and one almost flat propeller in the air on the same (long) axle would be very simple in construction. Someone must have built something like that already.
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Old 07-05-2021, 07:11 AM   #114 (permalink)
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Quote:
Originally Posted by RedDevil View Post
I think it should be possible to make an aquatic version of the Blackbird, f.i. by replacing the wheels with paddle wheels for simplicity. But the drag on the hull and losses on the paddle wheels due to the viscosity of the water will make it hard to exceed the wind speed by much, if at all.

You'd need a very big propeller and a fair gear reduction anyway, and with a big reduction you won't get far beyond wind speed if there's no friction of any kind.

A setup with one small high-angled propeller in the water and one almost flat propeller in the air on the same (long) axle would be very simple in construction. Someone must have built something like that already.
Search for push me pull you boat.

I think you would need a hydrofoil to get low enough drag, but then you need to get up to foiling speeds. Theoretically possible, realistically practicable? I doubt it.
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Old 07-05-2021, 09:53 AM   #115 (permalink)
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Found it!
http://202.38.64.11/~cxyu/AJP_pushmepullyouboat.pdf

Quote:
... We have demonstrated
such a boat at recent pvblic displays in our junior level
physics laboratory. A typical nonphysicist is suitably impressed by the novelty of the boat, whereas the initial response of people with· some physics background is often
one of disbelief. Being familiar with the laws of energy and
momentum conservation, they feel that somehow these laws
are being violated. Indeed, a detailed analysis of the device
is an instructive application of these laws, suitable as an
undergraduate physics exercise. (The topic also aroused
considerable interest when given to a graduate physics
problems class.) We present such an analysis here. ...
Note that in the PMPYB the air propeller is used to power the water propeller to push the boat against the wind.

The distance the air moves through the propeller is greater than the propeller covers in the water, therefore the wind will force the propeller to turn with the wind despite the water propeller resisting it, as the latter covers less distance while the force on both propellers must be about equal.

It could be inverted by using a wind propeller with almost flat blades and a water propeller with an aggressive angle of attack, so the latter covers more distance for each rotation.
In that case the force on the water propeller produces more torque than the force on the wind propeller, making that turn against the wind.
The boat then would run with the wind faster than the wind (at least theoretically).
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Old 07-08-2021, 06:12 PM   #116 (permalink)
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FWIW, Xyla Foxlin's side of the story: building a working treadmill model.


She could have saved herself a whole lot of trouble by getting the math right
Her first couple of models had the prop moving air faster than the wheels were going. Those models don't go faster than the wind downwind; if the prop moves faster than the wheels it takes more force to turn the prop against the wind than the wheels can provide. Instead it will turn the propeller with the wind and the wheels force the car against it. They may even go faster than the wind upwind, but they did not test that.
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Old 07-08-2021, 06:25 PM   #117 (permalink)
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It's like deja vu all over again.
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Old 07-09-2021, 09:29 AM   #118 (permalink)
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Hackaday had a decent explanation on how it worked: the prop blades are crosswind to the airflow so they work like sails thus enabling faster than wind. Roncz did design an airfoil that worked thusly for the Americas cup.
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Old 07-09-2021, 02:34 PM   #119 (permalink)
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Quote:
Originally Posted by freebeard View Post
My apologies. I missed that video here and chanced upon it from my feed, then I did a text search on 'Foxlin' and found a link to her models here but not that post.
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Old 07-09-2021, 03:05 PM   #120 (permalink)
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Quote:
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Hackaday had a decent explanation on how it worked: the prop blades are crosswind to the airflow so they work like sails thus enabling faster than wind. Roncz did design an airfoil that worked thusly for the Americas cup.
A sail has to be angled in between the direction of the wind and the direction of the boat to be able to generate a forward force.

Likewise, the blades of the propeller have to be in between the direction of the rotation (e.g. at a square angle to the direction of the car and wind) and an angle that makes them push the wind back just as fast as the car is moving forward.

With the blades flat the prop works like a sail and the wheels will make the propeller spin, as the blades will not meet much resistance; but it will of course never go as fast as the wind.

If the propeller is geared to push back at wind speed nothing happens.
If you push the car forward in a calm the blades would not move the air, neither would wind be able to move the car. The force needed to turn the propeller against the wind is equal to the force moving the car forward, canceling each other out.

Just like the sail on a sail boat the best effect is in between; a slighter angle or a lower gearing.
As the propeller turns against the wind it is able to still endure pressure from the wind, even if that does blow no faster than the car is moving already.
It needs to turn slow enough to move the wind back slower than the car, which means in the direction of the wind respective to the ground, but of course slower than the wind.

It would take almost no power to spin the propeller when flat, and just as much as it receives when it is geared to push the air back as fast as the car moves, but somewhere in between if the gearing is in between.

Which means it will spin slowly against the wind when the car rolls with the wind, and it requires less power to spin the propeller slowly against the wind than the wheels generate by rolling forward faster than that.


There are two forces working against each other and that they operate at a different scale.

Just like a seesaw where two kids of the same weight are in imbalance because one of them (propeller gril) is sitting closer to the pivot point than the other (wheely boy). Wheely boy covers a bigger distance when the seesaw flips, so he gets pulled down by gravity while the other moves a smaller distance up.

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Investors woes:
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"In hindsight, I should have bet more on that horse"

Last edited by RedDevil; 07-09-2021 at 03:14 PM..
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