Downwind Faster than the Wind - Successful Run by manned cart

halfway said:
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I'll try to refrain from posting again until the official NALSA record has been set.... :)
 
halfway said:
ubergeekian: "I'm sorry that Halfway hasn't felt able to answer the three simple questions I asked. Nevertheless, here's a simple one for you: relative to a reference frame fixed to the cart (or boat) how much kinetic energy does it gain as its ground (or water) speed increases?"

What an arrogant unforgiving, and completely wrong person you are. How can you assume I was even reading the forum.
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You responded to my post with the three questions (so obviously saw them) ... and ignored them. Which I thought was a shame.

As for your question. a fixed reference point on a cart means the cart CANNOT have any energy due to its momentum. Since it can not have any momentum. That is what you are missing all along. Your arguments are dogmatic and wrong. It can not have any linear or rotational momentum as it is fixed to the reference. So no energy can be gained or lost by the cart.
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Reasonable. References to momentum a little dodgy though - it's perfectly possible for something without momentum to have kinetic energy.

ubergeekian then goes on to confirm that he has not altered from the energy relative to the ground theory by saying "Air resistance increasing with the square of the apparent headwind, to a large extent. " So confirming his belief in friction being the fundamental limit.
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It doesn't matter where you fix your reference frame: if there's relative movement between cart and wind there's an energy loss there

Back to real world, so Ubergeekian can insult me again in my absence. (I notice he never answered the better specified situations either, I did actually answer his as being impossible.
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Here are my questions. What's so impossible about them? Or did you mean that you found them impossible to answer?

ubergeekian said:
  1. A propeller is turning in still air. The thrust produced is measured. With the rotational speed and pitch held steady, a tail wind (ie from the direction of thrust) starts blowing. Does the thrust produced increase, decrease or stay the same?
  2. A helicopter is hovering in still air. Thermal activity starts and the helicopter starts to rise with the air around it. Rotor pitch and speed stay the same. Does the shaft power required to turn the rotor increase, decrease or stay the same?
  3. In the above case does the work done against gravity increase, decrease or stay the same?
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In case you are wondering, or care, you made some very interesting claims about how propellers work. I wondered what results your models would give, applied to these systems.

Incidentally, do you still claim that the angle of attack for a propeller blade producing thrust is in front of the chord?
 
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shaunksb said:
Is this a Troll?

So the drag off the little carty thing is more than the drag of the great big propeller? So much more its actually has enough power to turn it the wrong way when moving off. That's a lot of drag!

The cart is "aerodynamic" because in the back he has got his dads starter moter hooked up to all hi mates ipod batteries with his mothers washing machine belt driving the prop forwards.

THE EMPEROR HAS NO CLOTHES.
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Actually he's right. The vehicle starts to move simply by being blown downwind. The drag on those stationary prop blades must be quite large.As the vehicle starts to move the gearing off the wheels turns the propeller which eventually starts to develop thrust through the lift off its blades.

My guess is that a variable pitch prop would allow a faster start, by setting the blades fully fine ( flat ) to get the maximum 'spinnaker' effect to overcome the start friction then feed in coarser and coarser pitch to accelerate away. A simple centrifugal pitch control would do it.

I can do the math at 0 kph. Drag downwind gets the vehicle going.
at 1 kph the prop will not be producing much thrust but the overall efficiency of the vehicle is improving. As it improves the vehicle accelerates, as it does so it gets even more efficient to a point where the thrust is actually contributing to the forward motion. By now my maths has got lost!
 
nimbusgb said:
My guess is that a variable pitch prop would allow a faster start, by setting the blades fully fine ( flat ) to get the maximum 'spinnaker' effect to overcome the start friction then feed in coarser and coarser pitch to accelerate away. A simple centrifugal pitch control would do it.
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I PM'd one of the build team. They're planning on sorting out the VPP.

For the fastest start, you would pitch the prop the other way - so that the wind turned the prop and drove the wheels - aka a turbine cart.
 
Snowleopard: So we are agreed they are not worth reading for anyone that needs to get over the initial concept barrier. Both poppycock.

shaunksb: I am sorry but I'm traveling in a vehicle on a tiny system I can not reference all the posts, but just look through the last posts I have sent. Most are with regard to this issue. I think it was in snowleopards original poll thread.

Tommyrot: They WILL post a record. If they do not kill the cart or driver. But it will not prove that the energy,friction,wind relative to the ground explanation is valid. In the same way it will not prove my explanation. However, if they were to measure the wind speed at 1 prop diameter in front of the prop then it would disprove the treadmill analogy.

ubergeekian: I have already answered the questions. They are impossible. I restated them in a possible way and you ignored them. I even proved that a prop can not ever rotate in stationary air. Once you are over that hurdle and restate the problem properly or agree to my scenarios we can progress. As I said above, the key is the wind speed in front of the prop.

ubergeekian: "it's perfectly possible for something without momentum to have kinetic energy" give me an example of how an object can have any motion related energy if it is the reference point of any motion. It does not make sense. However, I do note it is vital for your false proof of this down wind faster than the wind concept.

ubergeekian " if there's relative movement between cart and wind "
Now it looks like you are abandoning your wind and ground relative theory. There has been no objection ever from me about wind and cart. In fact that is the basis for the apparent wind explanation.

ubergeekian: "Incidentally, do you still claim that the angle of attack for a propeller blade producing thrust is in front of the chord? "
Now you are inventing straw men. A straw man is where you create a situation that the opponent never made and then shoot it down. It seems that you are the winner then. But actually you are just a con man.


Why do you not see that a prop can not ever be in still air. On the treadmill the prop effects are on both sides of the prop. But in reality the down wind vehicle has not altered the air in front Answering a situation about stationary props in a flow does not come close to the situation when sailing or driving down wind faster than the wind. So restate it in a relevant context or ignore it. This prop does not blow or suck. It is taking energy from the wind.
 
Porthandbuoy said:
Ubergeek and SL, an aplogy.


Good, though lofty explanation here:

http://projects.m-qp-m.us/donkeypuss/wp-content/uploads/2009/05/drela_efficiency.pdf
faster-than.html[/url]

Its a nice piece of mathematics but as is often the case , if the basis is wrong the whole analysis is wrong. Early in the alaysis there is the line which states that the shaft power of the air propellor is:

Pp = Fp (V −W)/np

Where V is the wind velocity and W is the vehicle velocity. If they are the same then the apparent wind is zero and the shaft power is zero.. so where do we get the drive energy from?
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If the vehicle speed and the wind speed coincide, the apparent wind to the vehicle is indeed zero, so no aerodynamic drag from the vehicle. The whole point is that the rotating propellor blade is definitely not in a zero apparent wind situation- it's whirling around with a considerable apparent wind, generating lift. It is making a spiral path travelling AT AN ANGLE to the directly downwind motion of the cart.
Your ice yacht, land yacht, cat or whatever doesn't stop generating lift when its speed over ground or water equals the wind speed. As it is again at an angle to the wind direction, there is an apparent wind over the sail still producing a lift component to allow the yacht to then travel faster than the wind speed.
The vector diagrams in the posts way back show the triangle of wind speed, sail/prop motion, and the resulting apparent wind speed and direction.
Same theory; same result.
 
saltwater_gypsy said:
Its a nice piece of mathematics but as is often the case , if the basis is wrong the whole analysis is wrong. Early in the alaysis there is the line which states that the shaft power of the air propellor is:

Pp = Fp (V −W)/np

Where V is the wind velocity and W is the vehicle velocity. If they are the same then the apparent wind is zero and the shaft power is zero.. so where do we get the drive energy from?
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I believe you are correct (except that you have reversed W and V). What he appears to have done is taken an equation for a turbine driven by apparent wind, inverted the efficiency and called it a prop.

If you look in his other paper you'll see he calculates power in terms of delta W which makes a lot more sense.
 
halfway said:
On the treadmill the prop effects are on both sides of the prop. But in reality the down wind vehicle has not altered the air in front Answering a situation about stationary props in a flow does not come close to the situation when sailing or driving down wind faster than the wind. So restate it in a relevant context or ignore it.
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Are we all agreed now that the treadmill demo is just a well known high school physics trick.
 
shaunksb said:
Are we all agreed now that the treadmill demo is just a well known high school physics trick.
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No, it's a filter to sort out those who didn't get past the first term of GCSE physics and wouldn't understand a frame of reference if it came up and bit them on the bum.
 
Not really fred drift....

A pal of mine is a retired mechanical/aeronautical engineer who worked in academia after designing and building gyrocopters (more money and better security). He also happens to be an open minded kind of guy....
I saw him earlier today, ran this project past him and pointed him at the DDWFTTW site. Just in conversation - before he looked at the site - he opines that he obviously hadn't ever thought about it before but sees how it might be possible... OK?
Conversation then turns to the curious phenomenum he observed while flying his (various) gyrocopters. Air speed is measurably higher when flying downwind than when flying upwind. Air speed! Curious, huh?

He's off to view the site and have a think about it.
 
"Any sufficiently advanced technology is indistinguishable from magic."

Arthur C. Clarke,

It doesn't always have to be that advanced.
 
shaunksb said:
Are we all agreed now that the treadmill demo is just a well known high school physics trick.
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Oh, absolutely. In the same way that sailing upwind is a high school physics trick.

"Sail against the wind faster than it's blowing you downwind? No. Absolutely not. Can't be done. It's a trick - it's the work of the devil, I tell you!"
 
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