Merged DDWFTTW in a “Real” Wind

[RossFW]

Firstly, it will show the contrary, and ddw is a specific claim. There is no violation of any laws when tacking. The best outcome for you on that basis, given that you deny to role of the pylons, is that props make good sails.
Already known, but of limited practical use. Sail carts need to meet all sorts of demands made by the market place.

So, as long as the props are at an angle to the Relative Wind, they can cause the cart to accelerate, right?

 

[Jack by the hedge]

The math says I am right.

Only if you are using the wrong maths. garbage in, garbage out.

Despite what you think, the reel accelerates faster than the hand, but friction dampens that as it wind up the thread.

Rolling without slipping, the reel moves a distance which is a fixed multiple of the distance moved by the hand. When the hand moves at a constant speed, the reel moves at a higher but still constant speed. (This is the same principle which stumped you back at the start of 2010 when you wrongly insisted that a cart pulled by another cart with a rope running over a pulley must be accelerating.)

The apparent speed gain is the difference of the reel and the winding of the string. Take the under reel, which moves 6" with each turn, and keep tension in the thread. Keep the hand where it is and move, but do not roll the reel 6" towards the hand. Now you have 6'' of slack thread, 3 of which go on the reel, and three taken up by the hand. Hey presto the reel is "faster" than the hand. A nice trick that works only with thread.

So you believe that the fact that the hand travels 3" and the reel travels 6" does not mean that the reel travels futher than the hand? i.e. 6" is not more than 3"? I shall avoid the obvious crass joke. One bad joke is enough.

 

[humber]

So, as long as the props are at an angle to the Relative Wind, they can cause the cart to accelerate, right?

The prop does not tack, RossFW. Sails are limited by Betz's limit. Rotating sail can overcome that to some degree, but not ddw. The cart is not ddw, so the question is hypothetical regarding performance ddw.

Props can make good sails, but at the expense of power. Boats need a lot of power to work against the water. But, why did Jenkins not use a prop if they are so good? He needs power to go 128mph. 50mph takes a lot less power, and may even yield a higher WS ratio than his Greenbird, but he wants the 128 land speed record, and so needs that power. Note note not ddw. The BB and Greenbird are NOT ddw.

The prop is perhaps a "better" sail, but that has nothing to do with it "tacking the wind". More like this, and yes, there is more to it, and that will address any nit-picking.

 

[humber]

Only if you are using the wrong maths. garbage in, garbage out.

I will make a plaque of that, and hang it in the lab.

Rolling without slipping, the reel moves a distance which is a fixed multiple of the distance moved by the hand. When the hand moves at a constant speed, the reel moves at a higher but still constant speed. (This is the same principle which stumped you back at the start of 2010 when you wrongly insisted that a cart pulled by another cart with a rope running over a pulley must be accelerating.)
So you believe that the fact that the hand travels 3" and the reel travels 6" does not mean that the reel travels futher than the hand? i.e. 6" is not more than 3"? I shall avoid the obvious crass joke. One bad joke is enough.

The gearing doubles the applied force. The work is F*d, so any distance covered by the reel means that twice the work is done on it, than on a same simple mass over the same distance. That appears as twice the acceleration.
The rest is winding.

 

[RossFW]

So, the prop can generate thrust as long as the wind is at an angle to the carts path, even though that is a different direction than the path of the prop?

Clever of the prop to know.....

 

[Jack by the hedge]

The prop does not tack...

Nitpick: Please explain what you mean by this.

Please will you explain as clearly as you can what it is that you say the sail does which the propeller blade does not do.

 

[ThinAirDesigns]

So, the prop can generate thrust as long as the wind is at an angle to the carts path, even though that is a different direction than the path of the prop?

Clever of the prop to know.....

"Smart Props" -- gotta love'm.

JB

 

[jadebox]

There are no vehicles that accelerate when braked.

I haven't finished the thread, so excuse me if someone else has already pointed out the obvious ....

All vehicles accelerate when braked. That's the whole point of brakes!

-- Roger

 

[Jack by the hedge]

I will make a plaque of that, and hang it in the lab.

I was thinking of a tee shirt. You can be wrong out of the lab too.

The gearing doubles the applied force. The work is F*d, so any distance covered by the reel means that twice the work is done on it, than on a same simple mass over the same distance. That appears as twice the acceleration.

I beg you to read this back to yourself and see if you can work out what you thought it meant when you typed it. If you can, please try again in earth-language.

The rest is winding.

Don't bother with this bit. Nobody will ever know what it means.

If all you're saying is that the acceleration of the reel is double the acceleration of the hand, then can you see that this means the velocity of the reel is double the velocity of the hand, and the displacement of the reel is double the displacement of the hand?

 

[humber]

So, the prop can generate thrust as long as the wind is at an angle to the carts path, even though that is a different direction than the path of the prop?

Clever of the prop to know.....

Ddw is ddw, and a unique state amongst the many other non-ddw paths.
Efficiency and power are not the same thing. And that is why sails are used on boats, but not props, and vice versa.


But, your argument, false as it is, is also self-defeating. You are claiming "tacking" for cart ddw, so it would do better when not ddw, as that would serve to increase or reduce the effective angle as required.
The failed up wind cart, is now up for the consolation prize of the best speed and power limited, temperamental, sluggishly accelerating, inefficient, enormous, impractical and short distance sail boat ever made.

 

[humber]

I was thinking of a tee shirt. You can be wrong out of the lab too.

beg[/I] you to read this back to yourself and see if you can work out what you thought it meant when you typed it. If you can, please try again in earth-language.
Don't bother with this bit. Nobody will ever know what it means.
If all you're saying is that the acceleration of the reel is double the acceleration of the hand, then can you see that this means the velocity of the reel is double the velocity of the hand, and the displacement of the reel is double the displacement of the hand?

The work goes in two ways. It is lost to heat of friction, or stored as KE in the object. Applying twice the force, a constraint of the gearing, means that twice the work is done over the same distance, as if the reel were rolled by the center, like-a-this:
http://www.internationalskeptics.com/forums/attachment.php?attachmentid=21632&stc=1&d=1301670914
If the work goes as KE, that will accelerate the reel. It rolls up the string faster than the hand, and winds up the difference produced by the difference of acceleration. Do that for more than a short run, and the reel will travel the same average distance as the cart. You can see the effect of intermittent but damped acceleration when towing a soap box cart. The rope can become taught or slack, as the cart accelerates a bit now and again.
The average speed is the same as the hand. Winding and the increased rate of acceleration, just adds some visual confusion.

 

[humber]

I haven't finished the thread, so excuse me if someone else has already pointed out the obvious ....

All vehicles accelerate when braked. That's the whole point of brakes!

-- Roger

All vehicles lose KE when braked. It is pedantry to invoke acceleration to include +ve acceleration, when the word "braked" is employed.

 

[Jack by the hedge]

As the string gradually winds onto the reel, the distance between the hand and the reel becomes shorter and shorter. Do that for more than a short run and the reel will have caught up with the hand. It is not possible for the reel to do that if it travels at the same average speed as the hand.

The winding may cause you some visual confusion, but one trivial observation is that when you wind the string onto the reel you eventually run out of string.

 

[Jack by the hedge]

All vehicles lose KE when braked. It is pedantry to invoke acceleration to include +ve acceleration, when the word "braked" is employed.

A vehicle which harvests the braking effort and invests it in gaining greater forward thrust from an available power source which more than compensates for the drag of the brakes will accelerate when braked.

 

[humber]

As the string gradually winds onto the reel, the distance between the hand and the reel becomes shorter and shorter. Do that for more than a short run and the reel will have caught up with the hand. It is not possible for the reel to do that if it travels at the same average speed as the hand.

The winding may cause you some visual confusion, but one trivial observation is that when you wind the string onto the reel you eventually run out of string.

Try it with a large reel, long thread. The reel will dance about, as it trails behind you. Average speed, your speed.

 

[Jack by the hedge]

But, your argument, false as it is, is also self-defeating. You are claiming "tacking" for cart ddw, so it would do better when not ddw, as that would serve to increase or reduce the effective angle as required.

Mistaken again. The claim is that when the cart is facing DDW, the propeller blades are each equivalent to a tacking sail. If either blade or sail is not properly aligned with the incident airflow then they lose efficiency. When not DDW, the prop blade cannot be at its optimum angle of attack for all points on its sweep.

 

[Jack by the hedge]

Try it with a large reel, long thread. The reel will dance about, as it trails behind you. Average speed, your speed.

You are saying the reel has to become unstable and slip in order not to catch up. That is a different scenario. There is nothing in the model which requires the reel to slip. Your "explanation" does require this, so it fails.

 

[I Ratant]

You mean his spool that climbed the incline, faster than the string pulling it?

.
It was better than a doodle with the wrong lever arm.
I wondered why, when the spool of thread drops on the floor and rolls underneath the couch, it never comes out when you pull on it! You just get a lot more loose thread, and have to poke it out with a stick.

 

[spork]

I wondered why, when the spool of thread drops on the floor and rolls underneath the couch, it never comes out when you pull on it! You just get a lot more loose thread, and have to poke it out with a stick.

wrong.

 

[I Ratant]

As the string gradually winds onto the reel, the distance between the hand and the reel becomes shorter and shorter. Do that for more than a short run and the reel will have caught up with the hand. It is not possible for the reel to do that if it travels at the same average speed as the hand.

The winding may cause you some visual confusion, but one trivial observation is that when you wind the string onto the reel you eventually run out of string.

.
Bottom pull.
Pulling from the top, the thread gets longer.
2 different conditions, same stimulus.