Downwind faster than the wind. Poll

[RAI]

To go faster than the wind, more air will flow through the propeller, causing the angle of attack to reduce and conversely if the cart moves slower than the wind, the angle of attack will increase. Now change the propeller for one with a greater overall pitch angle and the propeller will generate more thrust from the same speed and will tend to accelerate the cart in the direction it is travelling. But there will be a narrow band of speeds that a given propeller would be able to demonstrate this since movement of the cart relative to the air changes the relative airflow and therefore thrust from the propeller.

Interestingly, the manned cart experiment has a variable pitch propeller.

 

[halfway]

Ubergeekian: "simplistic models don't cut it"

I AM NOT USING A SIMPLISTIC MODEL. I am re-referencing a REAL LIFE DEMO to what it actually means if the cart was moving. I have never used any model except for measurements off a real model plane stationary on the ground with the engine running. The re-referencing is a simple addition of 10kts.


RAI: "Maybe that explains why they call it tacking down wind instead of gybing down wind."

TRY GOING SAILING. Think about windward and downwind sailing. The sails are nearly at 90 degrees to each other. When sailing down wind in a "zig zag", the sail is the side of the boat nearest the centre line of the desired track. When sailing up wind the sail is on the outside of the boat furthest from the desired track.

When sailing down wind you can not see ahead of you to the finish line, but can see perpendicular to it. When sailing up wind you can not see one side, but you can clearly see the finish line.

Sailing down wind the mast leans in the direction of the finish line (Even when faster than the wind)
Sailing up wind the mast leans away from the finish line

I can not believe anyone who sails confused tacking with gybing. Anyone that does has a particular place to stand: The pontoon.


Fireball: "I suggest you go racing .."

I did once, we beat the fleet of 8 Bavarias in a small cruising boat. I could not see the point after that.

My point, in the re-referencing post, was that the wind is killed normally in front OR behind. BUT NOT BOTH. The re-referencing of the real life treadmill demonstration shows that the wind is effected in the same sense both sides, and in the same sense in front and behind.

-----

Sorry for the capitals, but I just can not see how you can keep up this pretence that the prop is "blowing". I have shown that the "proof" assuming it is a normal prop is wrong. I have shown that the thought of wind being push against behind the cart is wrong. I have shown that re-referencing a real life demo makes situations that are impossible. I have sketched out the real proof, Snowleapod has drawn the velocity diagram. (Although we are still missing the force diagram). I have shown examples of cylinder world, staged derivation. I have shown how each element of a boat directly relates to the force transfer in the cart.

The required "bladed thing" on the cart does NOT "Blow" it simply slows the air after it passes by. IT IS A SAIL. Hence removing energy. How hard is that as a concept on a sailing forum. It is not a prop and does not conform to any "blowing" or "sucking" efficiency models used by any optimization system for aeronautical engineering.

IT IS A ROTATING SAIL.


(You are not alone though, those poor chaps building the car have also designed the wrong prop. One section of it will work, but the rest will be just drag. They also seem to think that minimum blades is good. That is the idea behind high speed props and running into the old air from the blade in front. These props are doing nearer to 45 Degree helix movements to reduce the gearbox losses, and so will never meet the air from the blade in front. Big sections of air will go though undiminished.)

 

[fireball]

Fireball: "I suggest you go racing .."

I did once, we beat the fleet of 8 Bavarias in a small cruising boat. I could not see the point after that.

Quite why beating 8 Bavarians in a small cruising boat is significant I don't know - perhaps you looked at my profile and decided that I raced the Bavaria ?? Ever looked at my username? Fireball ?? Give you a clue as to my sailing past? No? Believe me - the Bavaria is tame.

 

[Pierrome]

The required "bladed thing" on the cart does NOT "Blow" it simply slows the air after it passes by. IT IS A SAIL. Hence removing energy. How hard is that as a concept on a sailing forum. It is not a prop and does not conform to any "blowing" or "sucking" efficiency models used by any optimization system for aeronautical engineering.
IT IS A ROTATING SAIL.

If the cart is going faster than the wind, the air does not "pass by" you are still thinking that it is going slower than the wind!
If it is going faster than the wind - it is going through the air like a plane starting along a runway with the wind behind it. It is a propeller, it provides thrust by pushing back air. It is the force that pushes the cart which is opposite to a sail which is a device that collects energy. The equivalent to a sail on the cart is the wheels.
It harvests the apparent road speed and puts this energy into the air, to enable the cart to travel through the air which is downwind faster than the wind. I think I've said this three times now.

 

[xxyyzz]

IT IS A ROTATING SAIL.

I disagree, but it's an interesting viewpoint, what would be the optimum rotational speed of such a sail?

Is rotating as fast as possible still what you're after there?

If you could vary the pitch presumably you think the same thing could be made with a mechanism simply changing the blade pitch as it rotates, with no ground linkage at all?

 

[halfway]

fireball: "Ever looked at my username?"

Nothing to do with you at all. It was a group of charter boats that raced every Friday and cruisers joined in. So we wiped the floor with them. It was the only time I have ever raced in the true sense. It was not my boat.

I have no competitive desire, but I do like speed.


Nature is the one true adversary.

 

[boomerangben]

Ubergeekian: "simplistic models don't cut it"

IT IS A ROTATING SAIL.


(You are not alone though, those poor chaps building the car have also designed the wrong prop. One section of it will work, but the rest will be just drag. They also seem to think that minimum blades is good. That is the idea behind high speed props and running into the old air from the blade in front. These props are doing nearer to 45 Degree helix movements to reduce the gearbox losses, and so will never meet the air from the blade in front. Big sections of air will go though undiminished.)

Sail, propeller blade, turbine blade etc are aerofoils. Sailing is about generating lift and optimising the direction of that lift in the direction you want to go. A propeller is about generating lift and optimising that lift axially. I think trying to use a sail as a corrorally for this cart is very misleading. The only commonality is aerofoil theory, the application is very different.

I am not sure where you get the 45 degrees helix from since an aerofoil will stall at around 18 degrees angle of attack. As a helicopter pilot, I can assure you that no sections of air go through the disc "undiminished" and helicopter rotors go around a lot slower than an aircrarft propeller.

 

[halfway]

xxyyzz: "what would be the optimum rotational speed"

The theoretical optimum is 1:1 with the ground speed. As in a 45 degree helix. Or like a 45 degree zig zag course. Any higher speed just increases the force multiplier back through the gearbox and does not improve anything. In fact the higher forces will produce more friction in the gearbox. Lower and you lose the ability to go faster than the wind earlier for a given aerofoil.

"Is rotating as fast as possible still what you're after there?"

NO. That is like asking is sailing straight down the course better than zig zaging when down wind sailing faster than the wind.

"If you could vary the pitch presumably you think the same thing could be made with a mechanism simply changing the blade pitch as it rotates, with no ground linkage at all?"

Absolutely not. The ground is required to extract the energy needed to counter the drag of the prop. However any cart to demonstrate this effect would have to a have variable pitch in order to start, without a cheating push. It can be on a governor system that varies the pitch as the rotation increases. (Since it is directly related to ground speed)

 

[RAI]

RAI: "Maybe that explains why they call it tacking down wind instead of gybing down wind."

TRY GOING SAILING. Think about windward and downwind sailing. The sails are nearly at 90 degrees to each other. When sailing down wind in a "zig zag", the sail is the side of the boat nearest the centre line of the desired track. When sailing up wind the sail is on the outside of the boat furthest from the desired track.

When sailing down wind you can not see ahead of you to the finish line, but can see perpendicular to it. When sailing up wind you can not see one side, but you can clearly see the finish line.

Sailing down wind the mast leans in the direction of the finish line (Even when faster than the wind)
Sailing up wind the mast leans away from the finish line

I can not believe anyone who sails confused tacking with gybing. Anyone that does has a particular place to stand: The pontoon.

Well I sail differently for you. When I turn my boat such that the apparent wind crosses the bow, I call it tacking. When I turn it and the apparent wind crosses the stern, I call it gybing. I work with the apparent wind and not with the wind on the pontoon.

 

[halfway]

boomerangben: "helicopter rotors go around a lot slower than an aircraft propeller. "

Yes, but you try getting a helicopter to fly directly up at any significant speed.

"The only commonality is aerofoil theory"

Absolutely. I have never said do not use aerofoil theory. However it is not necessary to use it. Just one fixed point real world lift/drag/angle of attack graph is enough to prove the concept. No theory is required when you use real life data. (Prop modelling is a different matter)


"will stall at around 18 degrees angle of attack"

But if I was driving the cart I would not need more than a few degrees of attack angle at any time. The prop pitch would be increased from zero, as the speed of the vehicle increased and so would track (just ahead of it) the apparent angle forward.

 

[Ubergeekian]

Ubergeekian: "simplistic models don't cut it"

I AM NOT USING A SIMPLISTIC MODEL.

I was being kind.

Sorry for the capitals, but I just can not see how you can keep up this pretence that the prop is "blowing".

That's what props do. In this case the prop exerts a forward force.

The required "bladed thing" on the cart does NOT "Blow" it simply slows the air after it passes by.

Ah. You're getting confused by that whole "reference frame" thing, I'm afraid.

Let me ask you three very, very simple questions.

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?

 

[Ubergeekian]

Quite why beating 8 Bavarians in a small cruising boat is significant I don't know ...

Could you fit eight Bavarians in a small cruising boat? Surely the euphoniums would get in the way? There are good reasons, you know, why oompah bands rarely win the Round the Island Race.

 

[halfway]

RAI: Your full quote was:

Not very surprising, since BMW Oracle is close hauled herself. Maybe that explains why they call it tacking down wind instead of gybing down wind.

And was in response to me saying that you would have to swap to tacking suddenly when you got close to Oracle. Which would be impossible.

So your answer was to say it was a normal thing to do. You clearly do not under stand the implications. Would you swap the sail sides at 1 boat length or 2 boat lengths. When would the wind suddenly go the other way.

 

[RAI]

RAI: Your full quote was:

Not very surprising, since BMW Oracle is close hauled herself. Maybe that explains why they call it tacking down wind instead of gybing down wind.

And was in response to me saying that you would have to swap to tacking suddenly when you got close to Oracle. Which would be impossible.

So your answer was to say it was a normal thing to do. You clearly do not under stand the implications. Would you swap the sail sides at 1 boat length or 2 boat lengths. When would the wind suddenly go the other way.

In your boat perhaps (mine too) but watch what BMW Oracle or land/ice yachts get up to.

 

[halfway]

Ubergeekian: "A propeller is turning in still air."

Like all your proofs, shall we start off with an impossibility, and then dig the hole deeper.

There is no prop that spins in still air. Even at zero pitch the air flow is into both sides of the prop and out at the edges.

Please get you head out of the theory and come back to the real world.

If you want to set puzzles, then can we have the steady state before the effect change and the steady state requirements after the effect change.


"That's what props do."

Maybe in you narrow world where props "blow" and aircraft modelling is the universe. But if a prop is a number of aerofoils rotating in a plane around a fix point then there can be situations where they do not "blow".

 

[halfway]

RAI: "In your boat perhaps (mine too) but watch what BMW Oracle or land/ice yachts get up to."

If there was a point away from a boat that had a huge apparent wind change necessary for a gybe to tack swap, then there would need to be a small black hole to remove the surplus wind.

 

[Ubergeekian]

Ubergeekian: "A propeller is turning in still air."

Like all your proofs, shall we start off with an impossibility, and then dig the hole deeper.

There is no prop that spins in still air. Even at zero pitch the air flow is into both sides of the prop and out at the edges.

You know perfectly well, I suspect, what I meant. There is no bulk air movement relative to the mounting point of the prop.

Now, are you going to answer those simple questions?

 

[fireball]

then there would need to be a small black hole to remove the surplus wind.

I think we could probably do with that black hole in places in this thread !!

 

[RAI]

If there was a point away from a boat that had a huge apparent wind change necessary for a gybe to tack swap, then there would need to be a small black hole to remove the surplus wind.

As BMW Oracle goes into a direction change at 25 - 28 knots and comes out at 15 knots on the other tack before accelerating again, with only 6 - 8 knots of wind behind it, its apparent wind is still going passed the bow and not passed the stern as it turns.

The black hole is used to remove the cash needed to win the America's Cup.

 

[xxyyzz]

The theoretical optimum is 1:1 with the ground speed.

They're clearly using a gearing much greater than that. You can see in the videos that when the tip of the prop has moved from the top to the bottom the cart has moved nothing like that distance forward.

"Is rotating as fast as possible still what you're after there?"
NO. That is like asking is sailing straight down the course better than zig zaging when down wind sailing faster than the wind.

It's more like asking if you should tack lots or as little as possible.