Challenge: MythBusters and Cruisers Forum

[ThinAirDesigns]

Originally Posted by anotherT34C

Did you do the narration on the video? Seems pretty grandiose for demonstrating the principle of "motor and propeller".

Call it what you wish, but something like 95% of folk conclude that DDWFTTW is impossible. Is spite of the fact that it's a simple sailing device, that percentage holds true even on a sailing forum such as this. It's a physics brainteaser that confounds people from every walk of life, education and interest level.

You can call the video "grandiose" if you wish, but remember that that video was created and submitted as a 'sales pitch' to the MythBusters and was successful in it's purpose. There's no question that marketing is different from education -- that's a point I would absolutely concede.


Just to be clear on the "discovery" accusation you leveled, we've been flawless in our credit to others.

Pop quiz from our MythBusters Video --

-- What's the first picture you see on the screen? (Bauer Cart from '60s)

-- How many seconds into our video is it before we state that others have done it? (75)

-- In the video which cart is first shown operation? (not ours)

-- Which cart do we say inspired us to build ours? (Goodman's)

-- Do we say we designed our small cart or do we toss credit to someone else? (someone else)

We throw credit around like Christmas candy and again, make no claim of "discovery".


[/quote]As for different principles, what's the direction of propeller rotation on the street cart relative to the treadmill cart (essentially a windmill)? What does this tell you about the forces involved?[/quote]

You've really got to get this 'there's a difference between the cart on a treadmill and the cart on the street' out of your head. I don't know how else to say it other than ***THEY OPERATE IDENTICALLY***

Same gearing, same prop, same prop rotation, same physics. Every design and every cart we've build (over a dozen different ones now) and each one others have built have been run inside and out with NO changes.

JB

[Pblais]

Once you come to the realization that a traditional sailing rig is capable of downwind VMGs much higher than the wind,

That is true so long as you are not on a dead run. At a say a 45 degree angle the force applied due to the wind becomes constant as a function of the angle to the wind. The force applied adds an acceleration component offset by the friction and all the rest of the imperfections of the system. deceleration forces such as friction and air resistance increase with the velocity of what ever it is and so you can gain speeds greater than the wind speed so long as the acceleration force is applied.

Should the angle to the wind become downwind the force applied by the wind decreases as the velocity of the object increases. The cart on the road can go faster than the wind so long as the cart is not directly downwind. Even a small angle could propel the cart faster so long as the drag was low enough relative tot the constant force applied to the cart due to the degree of being off the wind. the only way the object can travel at all is if there is a constant force to accelerate the rig.

The cart on the treadmill test is meaningless. If you dropped a brick on the treadmill it would easily pass the cart and fly off the front of the treadmill. Thus proving that a brick can travel relatively faster than the cart.

For even more ridiculous experiment place the treadmill in a truck facing backward and then accelerate the truck to 50 knots and then turn on the treadmill, then drop the brick. How fast will the brick be traveling when it hits the back door of the truck?

[ThinAirDesigns]

Originally Posted by Tropic Cat

Early on in the video are photos of boats rigged with this type of propulsion system. Where are the videos of these boats successfully propelled by this rig against the real world of water resistance?

Hi Tropic Cat.

There are numerous sailboats that can achieve downwind VMGs greater than 1.0 while on a broad reach -- I know that PlayStation/Cheyenne is one and others in that class (Phillips, etc) have also achieved that regularly. We have been unable however to find anyone aware of *direct* downwind faster than the wind on water -- *yet*.

We do know that the Australian gentleman who designed this record breaking HPB is working on a design as we speak:

Most distance travelled on water by human power-world record set by Greg Kolodziejzyk

Certainly the challenges of hull drag etc are greater than that of a land vessel where it turns out to be rather trivial (though apparently mindbending).

JB

[ThinAirDesigns]

Originally Posted by Pblais

That is true so long as you are not on a dead run.

Hello Pblais.

With a traditional sailing rig fixed to a hull you are quite right.

The cart on the road can go faster than the wind so long as the cart is not directly downwind.

This is where your statement and reality part. Even when the chassis of the cart is going DDW, the sails are still on a continuous broad reach just as the traditional sailing rig is -- and thus the continued thrust. On a sailboat, the sail and hull must travel the same path ... not so with the sail and chassis of the cart. This is a point which confuses many folks.

The cart on the treadmill test is meaningless. If you dropped a brick on the treadmill it would easily pass the cart and fly off the front of the treadmill. Thus proving that a brick can travel relatively faster than the cart.

I urge you to drop a brick onto a treadmill yourself and see which direction the brick travels when it hits the belt (hint: the same direction as the belt.) The cart in the video is traveling and advanceing in the *opposite* direction of the belt. Again, the brick will travel one way and the cart travels in the other.

If you truly have any questions regarding which direction the treadmill belt is traveling vs the cart, I'm quite happy to create and post a video just for you where we drop objects (probably not bricks LOL) onto the treadmill belt with the cart and you will be able to see where your hypothesis is flawed. Let me know -- I can have that video posted before the day is out if you wish.

For even more ridiculous experiment place the treadmill in a truck facing backward and then accelerate the truck to 50 knots and then turn on the treadmill, then drop the brick. How fast will the brick be traveling when it hits the back door of the truck?

Happy to answer your question, but you have not included enough information for it to be done.

JB

[anotherT34C]

Sorry TAD, but they are not exactly the same. In which direction does the propeller rotate in each setup? What mechanism provides the forward force on each? To illustrate (intuit) the differences, consider what would happen with arbitrarily large propellers on each. How would the motions and forces differ?

[ThinAirDesigns]

Originally Posted by anotherT34C

Sorry TAD, but they are not exactly the same.

Happy for you to demonstrate the difference -- I'll refrain from the "yes it is, no it isn't" and give you a chance to do so.

In which direction does the propeller rotate in each setup?

The same direction in each -- CW as viewed from the rear. Unless you claim we faked our videos, they support this claim without fail.

What mechanism provides the forward force on each?

The same on each -- sails on a broad reach, each with a VMG greater than 1.0.

To illustrate (intuit) the differences, consider what would happen with arbitrarily large propellers on each. How would the motions and forces differ?

I have now considered it -- the motions and forces would not differ. I'm hoping you'll explain why you feel otherwise.

JB

[anotherT34C]

No, I don't think you faked your video. No need, really. Look at the longer version:

YouTube - DDFTTW

You can see the cart pinching and falling. Your treadmill cart is only demonstrating thrust. The "wind" is irrelevant on the indoor setup.

[ThinAirDesigns]

Originally Posted by anotherT34C

No, I don't think you faked your video. No need, really. Look at the longer version:

You can see the cart pinching and falling.

Tell me, what does your claim of "pinching and falling" have to do with your concerns from your previous posts? This is what you asked in the post to which I responded:

In which direction does the propeller rotate in each setup? What mechanism provides the forward force on each? To illustrate (intuit) the differences, consider what would happen with arbitrarily large propellers on each. How would the motions and forces differ?

How does the "pinching and falling" claim add any support to your assertion of propeller direction in each, mechanism of each, and an "arbitrarily large propeller"?

I'm simply not following your argument here.

Your treadmill cart is only demonstrating thrust. The "wind" is irrelevant on the indoor setup

Wind is the relative motion between air and surface. There are two ways to make "wind", move the air over the surface (eg: wind tunnel) or move the surface under the air (treadmill). You keep telling me that they are different, but you won't tell me how.

JB

[anotherT34C]

Imagining an arbitrarily large propeller powered from the treadmill should make it more clear (intuitively) that the indoor cart is demonstrating thrust only.

The difference in forward motion:
1. Outdoor cart: apparently pinching and falling to the wind
2. Indoor cart: forward thrust powered by an electric motor

If you had a longer treadmill, you might be able to show pinching and falling. Instead, you ONLY demonstrate thrust. When the cart moves forward, you (or whoever) pushes it back, repowering the craft.

The propeller direction question came from my visualizing the forces on a foil backwards. Pinching and falling can't occur with a backwards propeller.

[ThinAirDesigns]

Originally Posted by anotherT34C

Imagining an arbitrarily large propeller powered from the treadmill should make it more clear (intuitively) that the indoor cart is demonstrating thrust only.

Well, it doesn't make it any more clear -- a larger propeller, turning the exact same speed and the same direction will do exactly the same thing indoors or out.

When the cart moves forward, you (or whoever) pushes it back, repowering the craft.

Sorry, but when I push back on the cart, that's the same direction as the belt is already going so it can't be "repowering" the craft. It's like someone pushing down on a hovering ball and you saying "of course it hovers ... you're pushing *down* on it".

Additionally, if we slow the treadmill down, someone doesn't have to push back on the cart to keep it from climbing up and off the treadmill.

YouTube - Downwind Faster Than the Wind #6 (DWFTTW)

The propeller direction question came from my visualizing the forces on a foil backwards. Pinching and falling can't occur with a backwards propeller

Not sure how valid that assertion is, but it's irrelevant ... the prop spins as a prop and not as a turbine all the time in operational mode.

JB

[anotherT34C]

Originally Posted by ThinAirDesigns

Well, it doesn't make it any more clear -- a larger propeller, turning the exact same speed and the same direction will do exactly the same thing indoors or out.

You're missing the point. A larger propeller on the indoor cart would make it obvious that you are observing thrust, because the force would be much larger.

Sorry, but when I push back on the cart, that's the same direction as the belt is already going so it can't be "repowering" the craft.

You're right, you're not. But you're still not observing pinching. You're just pushing back against the thrust.

If your goal with the indoor cart is to show that thrust exists, then ok. But, if your goal is to show that you can backwind a propeller until it pinches, pushing the craft faster than the downwind speed until it falls, it doesn't work, as far as I can see.

[Ex-Calif]

This is the greatest stuff going. It really whacks everyone out.

I contend that the treadmill demonstrates nothing about the behavior under wind power. Because there is no wind. The power in the treadmill equation comes from the treadmill belt and that in itself invalidates the test as pertains to water craft.

You can show me the math but the test will validate the math.

You've demonstrated low drag, gearing and the fact that a propeller will develop the thrust needed to overcome the drag of the wheels and gearbox on a treadmill.

At best it is a mechanical-aero test and is not an aerodynamic test - cannot be.

I am not surprised at the result at all.

What I am surprised about is that no one in the 2 years or whatever that this has been around has actually done a water based scientific test.

That is rig your propeller powered model boat. Put it in a hydro tank with water flowing 6 knots and blow six knots of air at the back of the propeller.

If the theory is true, the vehicle should cross the tank upstream at some speed greater than 6 knots.

Until then the debate is useless.

I am OK with the cart on the road test but quite frankly the environment is too uncontrolled for any useful conclusion or data.

Yoiu have to be able to control the boundary specifications in order to validate research.

A geared cart on a treadmill is not a sailboat.

BTW a propellor driven craft can have the vehicle travelling DDWFTTW as long as the propellers are not stalled.

[ThinAirDesigns]

Originally Posted by Ex-Calif

I contend that the treadmill demonstrates nothing about the behavior under wind power. Because there is no wind.

There's wind to *anything* on the treadmill belt.

Stand on the deck of an aircraft carrier in the dense fog -- stiff breeze flowing from bow to stern. Fly a kite. Set up a turbine powered generator. Listen to the repeated crack of the flag. Run a small land-yacht around on the deck. Test a DDWFTTW cart. Blow drifting bubbles with the kids. Hell, flood the deck and hold a tiny regatta with tiny sailboats.

Is there wind? Every wind powered device in the world says yes and behaves normally. How can you know?

Is the carrier:

A: sitting still in the ocean with a breeze blowing across it?
B: cruising on it's engines on a calm day?
C: engines off and drifting with a strong water current on a calm day?
D: sitting on a rollers with large electric motors winching it across the ground?
E: floating down a man made recirculating river where large electric pumps move the water?
F: inside a foggy wind tunnel where large electric fans create the wind?

It's a serious question guys/gals.

Let's invert that -- same deck, same fog, no wind. Flags don't flap. Bubbles don't drift. Wind turbine doesn't rotate. Is it calm, or is the carrier steaming with the wind? Or is the water drifting with the wind? Or is the wind tunnel turned off? Or is the carrier on rollers and being winched with the wind. You get the idea.

That's the entire point of Galilean relativity, something established without fail for four centuries now -- of two objects moving relative to each other, one can't tell which is 'moving' and which is 'still' without external reference -- reference wind carts and sailboats don't have.

JB

[Tropic Cat]

Originally Posted by Ex-Calif

This is the greatest stuff going. It really whacks everyone out.....

What I am surprised about is that no one in the 2 years or whatever that this has been around has actually done a water based scientific test.

That is rig your propeller powered model boat. Put it in a hydro tank with water flowing 6 knots and blow six knots of air at the back of the propeller.

I'd agree with this post completely except that the video shows boats with this rig already installed. Some folks must have done the modeling before spending the big bucks on their boats.

Where's the model data? Heck, where's the data on these already rigged boats?

I suspect that the data will show that the concept is great on a 2 pound model but there isn't enough power available to push (pull) a 20,000 lb boat through water unless the propellor is gigantic.

Great for toys but not ready for prime time.

[ThinAirDesigns]

I think it's been pretty well established that traditional sailing rigs are by far more practical on a boat than an air prop or turbine.

JB