Good plan. I should have thought of that long ago when you were insulting our experiments and videos, and whining that we wouldn't do your work for you.
I will not ignore any future ignorance of the basic principles of physics.
DDWFTTW - Tests.
- Thread starter ynot
- Start date
Good plan. I should have thought of that long ago when you were insulting our experiments and videos, and whining that we wouldn't do your work for you.
I will not ignore any future ignorance of the basic principles of physics.
It appears I've been proven wrong. I seem to recall seeing that picture in the main thread. My apologies.
The point I'm making is that when the wheel that powers the prop is held against the moving surface it is forced to reach the speed of the moving surface (wind speed) and the prop is forced to create the maximum thrust created by that speed. A cart that would never reach wind speed in an outside wind test can be forced to reach wind speed with a treadmill or turntable. Holding a cart against a moving surface is the same as pushing a cart up to wind speed in an outside test.
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Accepted.
That's exactly right. But in that situation you would be holding the cart forward on the belt - not holding it back. If you release the cart and see it surge forward it tells you that you were clearly holding it back while it was trying to exceed the steady-state condition you are enforcing. In this case we can be sure it will find an average velocity greater than wind speed.
Michael C
Graduate Poster
That doesn't answer the question: would a single prop cart be better?
As far as I know, a two-bladed prop is more efficient than a four-bladed one.
I have seen airplanes with two bladed props, four bladed props and some with even more bladed props, If two was significantly better why do they ever use four and more? All I know is that two two bladed props together on the same shaft (essentially a four bladed prop) worked fine on my cart and it was able to progress against the moving surface.
Michael C
Graduate Poster
There is always a compromise. For a given diameter, adding more blades should increase the thrust-producing surface but the prop will become less efficient since there will be more unwanted interaction between the blades. For a large aircraft a two blade prop capable of producing sufficient thrust might have to have such a huge diameter that it would become too unwieldy. For small-scale machines such as yours, a two-blade prop should be the most efficient. See for example this information on model aircraft propellers:
ThinAirDesigns
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A counter weight would be counter productive because when fully balanced there would be no normal force between the wheels and the track so no traction. You could always compensate by using magnetic wheels and a steel track but everybody would then know it was the power of magnets that was then moving the cart
A counterweight need not be installed in such a way as to lift the cart off the rolling surface. It can be mounted so that it places a side load on the center hub and no lifting force on the cart -- and still balance the rotation.
JB
ThinAirDesigns
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For a given diameter, more blades will absorb more horsepower but less efficient as a whole.
If I have a lot of power but I don't have the ground clearance (or body clearance) to increase the prop diameter I would need to add blades.
Again, our tests showed better results with one prop but I don't insist that will be true with all props. I expect however it would be true if the correct prop is used.
JB
Dan O.
Banned
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Or, if you fully balance the cart, you can then add a specific force from a spring to control the available traction to investigate how traction affects the operation.
ThinAirDesigns
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I pretty sure I know how traction affects the operation -- it's good for it (but I suppose I should check with the goldfish before I say for certain).
JB
John Freestone
Graduate Poster
Ynot, hi, continuing from your recent success with the cart on a turntable (tt):
I was a little surprised that you plan to do the double-sided cart after such 'amazing' speed, but I guess that suggests that your motives now aren't about proving the principle so much, and have become something else, like just doing something new for the sake of innovation or improving the performance as much as possible.
However, I would have thought it would be well worth your trouble doing tests, even fairly rough and ready ones, on the single cart before you add the second, for comparison purposes. (I'm just curious as to how much better the double is.)
It would also be best to make sure you get the constant turntable drive sorted out first, as that's fairly crucial to correct test results, I think.
I might be going on about things you already know, but you could get a pretty good test result just by counting the revs of your tt, then counting the counter-revolutions of your cart over a certain time. I guess actually the first is more difficult than the cart revs, because its so much faster. You might manage to count a mark on the circumference over a given time. I suppose it can't be taken from 'known revs' quoted on a motor, because that will vary with load. You really need the absolute revs of the tt. Otherwise, if it's too fast to count a mark, one way would be to hire a strobe light with a fairly accurately calibrated and variable pulse with a readout, and adjust the period until it coincides with your edge mark. I'd be interested to know how you decide the revs.
Then there'll be a simple formula involving those figures tt-revs and cart-revs and the radius that will give you the relative speed, or the 'multiple of windspeed' or whatever you want the performance as. Being slightly math-o-phobic, I'll set myself the task of working it out, since it can hardly be that difficult and I really should start my aversion therapy!
Do you expect approximately twice the performance from the double-cart? I can't see any reason why not....or maybe your testing will be more approximate or even 'yes-no' (although you surely can't have any doubts that it's a yes now).
Anyway, well done. I reckon at some point, if this principle continues to attract enthusiasts, there's going to develop a 'world record' in the form of a multiple of windspeed achieved (probably one for 'wild' wind and another for more accurate indoor tests on treadmills and turntables). You never know, your twin cart design might just set it.
I'm working on a DD cart using a completely different approach. It's in the design stages at the moment, top secret, and probably complete rubbish!
I was a little surprised that you plan to do the double-sided cart after such 'amazing' speed, but I guess that suggests that your motives now aren't about proving the principle so much, and have become something else, like just doing something new for the sake of innovation or improving the performance as much as possible.
However, I would have thought it would be well worth your trouble doing tests, even fairly rough and ready ones, on the single cart before you add the second, for comparison purposes. (I'm just curious as to how much better the double is.)
It would also be best to make sure you get the constant turntable drive sorted out first, as that's fairly crucial to correct test results, I think.
I might be going on about things you already know, but you could get a pretty good test result just by counting the revs of your tt, then counting the counter-revolutions of your cart over a certain time. I guess actually the first is more difficult than the cart revs, because its so much faster. You might manage to count a mark on the circumference over a given time. I suppose it can't be taken from 'known revs' quoted on a motor, because that will vary with load. You really need the absolute revs of the tt. Otherwise, if it's too fast to count a mark, one way would be to hire a strobe light with a fairly accurately calibrated and variable pulse with a readout, and adjust the period until it coincides with your edge mark. I'd be interested to know how you decide the revs.
Then there'll be a simple formula involving those figures tt-revs and cart-revs and the radius that will give you the relative speed, or the 'multiple of windspeed' or whatever you want the performance as. Being slightly math-o-phobic, I'll set myself the task of working it out, since it can hardly be that difficult and I really should start my aversion therapy!
Do you expect approximately twice the performance from the double-cart? I can't see any reason why not....or maybe your testing will be more approximate or even 'yes-no' (although you surely can't have any doubts that it's a yes now).
Anyway, well done. I reckon at some point, if this principle continues to attract enthusiasts, there's going to develop a 'world record' in the form of a multiple of windspeed achieved (probably one for 'wild' wind and another for more accurate indoor tests on treadmills and turntables). You never know, your twin cart design might just set it.
I'm working on a DD cart using a completely different approach. It's in the design stages at the moment, top secret, and probably complete rubbish!
You are not thinking clearly, J. The double-cart will be quite the same. If your car goes 100 mph and your friend's car goes 100 mph, you can't do 200 mph just by driving together...
(You can pull twice as much external load though, like a one-horse sled compared to a two horse sled...)
Christian Klippel
Master Poster
Hi John,
to get a constantly driven tt, i would guess that one of these electric drill's (not the ones using batteries, obviously) that come with electronic regulation would be sufficient. From my experience, the better ones maintain a rather constant RPM under slightly differing load conditions.
As to the RPM measurement, well, there is a rather simple way. Just use counter and a stopwatch. Have the counter count each revolution, and let it run for a given period of time. Preferably, for one minute to make calculations easy
A smooth going mechanical counter would be ok, but of course an electronic one would be better. Otherwise there are these hand-held RPM counters, which aren't that expensive.There is another way using an LED, AC supply and a camcorder. If interested i can explain that as well. Just think of the reverse running spoke-wheel effect that you often see on TV......
Ynot, do you have some skills and equipment for doing electronics (using microcontrollers)? If so, building a constant RPM drive is a snap.....
Greetings,
Chris
John Freestone
Graduate Poster
It won't be the first time, huh34.
Ah, yes, after more thought, I see. My prediction couldn't be right without making the props move through the air too fast and not provide thrust, indeed retard motion instead. Thanks.
The main reasons I’m building a double-sided cart are as follows . . .
The cart/turntable combination are always balanced (good thing to have on a turntable I assure you).
Friction forces on the centre tether pivot are less.
It reduces any effects of the turntable surface not being perfectly level.
The combination of props on two opposite carts may give a more stable thrust.
One of the tests I will do is spin the turntable and cart from stationary and see if the created wind alone (cart won’t be restricted in any way) will cause the cart to travel against the turntable (faster than the wind).
The “sail” area of two props/carts will be better for testing when the cart isn’t held against the spin of the turntable (not that my new cart design has much area).
I already know that prop thrust can move a cart against the motion of the surface it’s running on if it is forced to do so by holding the cart against that motion. Hopefully these tests (and others) will show if it is possible when the cart isn’t forced and if it’s sustainable (both the same thing really).
The cart/turntable combination are always balanced (good thing to have on a turntable I assure you).
Friction forces on the centre tether pivot are less.
It reduces any effects of the turntable surface not being perfectly level.
The combination of props on two opposite carts may give a more stable thrust.
One of the tests I will do is spin the turntable and cart from stationary and see if the created wind alone (cart won’t be restricted in any way) will cause the cart to travel against the turntable (faster than the wind).
The “sail” area of two props/carts will be better for testing when the cart isn’t held against the spin of the turntable (not that my new cart design has much area).
I already know that prop thrust can move a cart against the motion of the surface it’s running on if it is forced to do so by holding the cart against that motion. Hopefully these tests (and others) will show if it is possible when the cart isn’t forced and if it’s sustainable (both the same thing really).
Couldn’t resist posting a quick video of my new “Wind Cheetah” cart design prototype to show how well it works even though the turntable is buckled and still turned by hand. This is NOT a test and no claims are made. The prop is driven from the centre of the wheel by a flexible cable. After spinning the turntable and then picking up and switched on the camera to start filming it had slowed down quite a bit so was going faster than is shown. The video is not sped up and the cart is moving purely by the thrust of the prop.
http://www.youtube.com/watch?v=mXhxhPs0qZc
http://www.youtube.com/watch?v=mXhxhPs0qZc
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ThinAirDesigns
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