Down wind faster than the wind

[technoextreme]

Yes, it will stop accelerating at some point. But that point is at a speed faster than the wind - downwind. This is clearly true and clearly demonstrated, and the only point we set out to make.

No it isn't. You never provide any measurements.

 

[mhaze]

MHaze,
Well, what I was saying is that is not an inertial frame of reference. This platform is not notional in any way. It's rolling-road model. It's not the wheels that are the ground it's the treadmill belt. Everything is literal. The wheels are wheels, the air is the air, the propeller is the propeller. Just like on Mythbusters.

So, only the labels are changed. But, the body of the treadmill is connected to real ground, so the belt is also real ground. Call the air the ground if you like. No matter, as long as you realise what that means.

There is little impetus. The impetus between an operator, and the cart is the same that would be felt in the real world. Because, it is the real world.
Friction aside, the bigger one is as easy to move around as the little one. The only impetus lies in the difference between the forces controlling the cart itself, and how they are coupled to the belt by friction through the wheels. Moreover, the impetus is the momentum stored in the propeller.

I am not saying that a windblown model cannot be, of course not, but that this model is not an indicator of the potential for greater then wind-speed vehicles, other than flywheel driven, of course.

Brian-M, Mhaze. At least 3 of us understand.

Yes.

This example should be used in college Aerodynamics 101 as an example of thrust, momentum, lift and drag. But at the end of the course, not by any means when these concepts were introduced.

 

[humber]

Tell me, please, just before you release the cart, do you feel wind coming from the direction of the propeller? No. Then neither does the cart. No tail wind.

Wind from the direction of the front of the cart? No head wind. Just the scarcest pull of the difference between those two "winds". The one that could be, should be, generated were it to be moving along with the belt, and an imagined one. The difference of that imagined calculation is the difference of those two. It's movement is an indicator of it's inability to remain at zero velocity. It's a cat trying to climb up a slippery pole, and barely succeeding.

 

[spork]

No it isn't. You never provide any measurements.

You don't need a digital readout to tell whether the cart is advancing or retreating on the belt - at least I don't.

 

[sol invictus]

No, think Mythbusters.

I have no idea what you're talking about. There was a thread here about planes on treadmills, in which many people said wrong things. I'm guessing mythbusters did the experiment? Why did they bother - the answer was completely obvious.

Traveling against a treadmill on a windless day.
Apparently, he thinks it should accelerate to the left.

The claim is that the cart will accelerate against the direction the treadmill is trying to carry it (at least if you start it at rest with respect to the air). The mechanism is easy to understand - the belt spins the wheels, which spin a propeller, which moves the plane. It could move it either way depending on how you connect and orient the propeller. Big deal.

Really. Put the cart on the belt as usual and get it moving forward.
Kick the underside of the treadmill, so as to make the cart jump.
At what speed is it traveling, at the moment it leaves the belt?

Answer: Very slowly.

Relative to the air and ground, yes.

What's the 'windspeed'? Answer: Belt speed, -belt speed.

Huh?

You seem to be profoundly confused about reference frames. There is no such thing as absolute velocity - get that out of your head. There is only relative velocity. The speed of the cart with respect to the ground is totally irrelevant in the treadmill experiment. All that matters are the relative speeds of air, cart, and belt.

 

[technoextreme]

You don't need a digital readout to tell whether the cart is advancing or retreating on the belt - at least I don't.

That doesn't prove anything. Give me the velocity that proves your little cart works.

I have no idea what you're talking about. There was a thread here about planes on treadmills, in which many people said wrong things. I'm guessing mythbusters did the experiment? Why did they bother - the answer was completely obvious.

The claim is that the cart will accelerate against the direction the treadmill is trying to carry it (at least if you start it at rest with respect to the air). The mechanism is easy to understand - the belt spins the wheels, which spin a propeller, which moves the plane. It could move it either way depending on how you connect and orient the propeller. Big deal.

I love how you complain about people getting stuff wrong when you were not even concerned enough to know what we are talking about. No one is disputing the cart won't accelerate. And if they are well they are really horribly wrong. The claim is that the acceleration means that it's traveling faster than the wind which is a big no.

 

[humber]

You seem to be profoundly confused about reference frames. There is no such thing as absolute velocity - get that out of your head. There is only relative velocity. The speed of the cart with respect to the ground is totally irrelevant in the treadmill experiment. All that matters are the relative speeds of air, cart, and belt.

Mythbusters example was not appropriate then. I have another.

What are the wind and ground speeds of your hand. Both zero.
That's odd, because the cart barely deviates from this when on the belt.
There is no wind to be felt upon the hand, from either direction.
Uuum...no wind at all.

It's just the model's reaction to being driven by a belt.

 

[spork]

What are the wind and ground speeds of your hand. Both zero.

What if I'm driving down the road at 20 mph and I happen to have a 20 mph tailwind. I stick my hand out the window. What wind does my hand feel? Zero.

 

[technoextreme]

What if I'm driving down the road at 20 mph and I happen to have a 20 mph tailwind. I stick my hand out the window. What wind does my hand feel? Zero.

Right. Now go faster. You have now have a head wind which will now case the propeller to act like a giant break.

 

[spork]

Right. Now go faster. You have now have a head wind which will now case the propeller to act like a giant break.

Ahhh - that explains why our cart always fails to advance on the treadmill. There's our problem right there.

 

[technoextreme]

Ahhh - that explains why our cart always fails to advance on the treadmill. There's our problem right there.

No. A basic understanding of physics is your problem. The cart in all scenarios never fails to go up the treadmill. It will stop accelerating which your movies never actually prove except when it's on an incline but that's the wrong force.

 

[sol invictus]

I love how you complain about people getting stuff wrong when you were not even concerned enough to know what we are talking about. No one is disputing the cart won't accelerate. And if they are well they are really horribly wrong. The claim is that the acceleration means that it's traveling faster than the wind which is a big no.

No. Either you didn't read what I wrote, or you don't understand the concept of relative motion.

Hold the cart in place at the center of the treadmill, letting its wheels spin. That situation is identical to one in which the cart is rolling across the ground at precisely the wind speed.

Now let the cart go. If it accelerates against the direction of the treadmill, as it does in the video, that is identical to the cart accelerating in the wind direction, so that it ends up going faster than the wind.

 

[sol invictus]

What are the wind and ground speeds of your hand. Both zero.
That's odd, because the cart barely deviates from this when on the belt.
There is no wind to be felt upon the hand, from either direction.
Uuum...no wind at all.

Yes - that's the precisely the point.

 

[spork]

No. A basic understanding of physics is your problem. The cart in all scenarios never fails to go up the treadmill. It will stop accelerating which your movies never actually prove except when it's on an incline but that's the wrong force.

Excellent. Then you should take me up on my $100K wager. We'll do the test your way.

 

[humber]

What if I'm driving down the road at 20 mph and I happen to have a 20 mph tailwind. I stick my hand out the window. What wind does my hand feel? Zero.

Yes, but then you are already at windspeed. The premise is that, when placed on the belt, the cart is already at windspeed - at launch.

I only 'realised' this, when thinking about a test for JB. It's not only
incapable of doing any real work, but that there are no real winds other than a very slight wind blowing over the propeller.
It all comes from the belt, as we have being trying to tell you. It got lost in detailed discussions about oscillations and so forth. It creates its own drag.

The Mythbuster's example is of a propeller driven vehicle travelling on a belt that is moving moving backwards or forwards, but it actually has wind blowing across it, no matter what direction you choose as wind.

If you put the cart on a belt, and have an external wind generator, then that is a different story.

ETA:
But we have told you that we fully expect that to happen. It has no relation to wind, but demonstrates that a propeller is capable of creating enough drag to stop itself on a belt, in vague manner.

 

[technoextreme]

Hold the cart in place at the center of the treadmill, letting its wheels spin. That situation is identical to one in which the cart is rolling across the ground at precisely the wind speed.

Nope. Nice try though but that cart aint moving at all with respect to the air. Hence no drag which is present when the car actually moves.
D = Cd * A * .5 * r * V^2
Thus they are not equivalent frames of reference when you let the car go. Admittedly, you do get to that point eventually which is the point of zero acceleration.

 

[humber]

The wind from the propeller, cannot be equated to wind. There is never any full-speed wind over any part of the cart. It responds to all sorts of thrusts, resistances and torques as a reaction to being driven, but nothing else.

Air can be thought of as viscous fluid, certainly for small objects. Replace the air with an imagined syrup, what drags on what? You will see a cart immersed in syrup, with the propeller making a local disturbance, but the cart remains essentially fixed to that disturbance. It does not travel through the bulk of the syrup, but moves about a fixed point, determined by the propeller's local drag 'hook'.
Whatever claims you have for faster than windspeed travel, it does not relate to this experiment.
If you could build such cart, it would essentially stand still in the wind.

 

[spork]

This is an absolute train wreck. I just don't understand. You guys haven't designed, built, or tested one of these. All you've seen are videos of them - the very videos that you claim don't prove anything - because who could possibly tell what's happening off screen and who knows what trickery is being played. You clearly have no training in physics or aero. But somehow from absolutely no evidence at all (this is what you tell me) you're able to correct JB, theBigGuy, Sol, and myself.

I can't speak for the others, but I have an M.S. in aero, and MANY years of flying everything that flies from R/C models to hang gliders, to small planes, sailplanes, helis... I know JB's background with flight is very impressive as well. I strongly suspect TheBigGuy and Sol have training in physics or Aero.

But you three can correct us all with no evidence or background whatever - based only on truly bizarre "theories" you make up on the fly?

Build the stupid cart. I posted the parts and directions. You could have built 10 of them in the time you spewed all this misinformation on this forum. Having done absolutely nothing at all - your words carry absolutely no weight at all. Knowwhatimean?

 

[JWideman]

You guys almost had me.
Work = thrust - drag
If thrust equals drag, no work is being done. The treadmill is merely a misdirection.
Nice try.

 

[Brian-M]

Normally I'd assume you're joking, but after 3 days on this forum I'm not going to make such assumptions.

Search on spork33 on youtube. I've got a pile of videos on this.

Sorry, I'd forgotten you'd mentioned the videos before.
The one in this video is clearly accelerating as you said it should...




I think I owe apologies all around. After thinking about what was happening in the video and the forces involved, I realized where I was going wrong in my mind.

Early on in the thread, I mentioned it would be possible for a wind-powered device to travel upwind, but would have to gear-down to achieve this, and so would be limited to travelling at less than wind-speed.

When I started looking at this from the viewpoint of a treadmill, with the air as the stationary reference-point things suddenly flipped around in my mind, and I realized the same principle would work in this situation too, but with ground and air reversed.

So, instead of pushing against the ground to move at less than wind-speed against the wind, this device would be pushing against the air to move at less than ground-speed against the ground.

So, converting this back to a fixed ground reference, the maximum speed of an idealized version of device (including 100% power transfer at the propeller, no wind resistance, no friction, etc) would be up to twice wind speed.

So, I guess I was wrong after all.

(I doubt anyone else would think of it in exactly this way... I suspect I'm operating with a non-standard brain. )