Downwind Faster than the Wind - Successful Run by manned cart

[ThinAirDesigns]

If others have questions I'll be happy to answer them, but going forward I'll be pretty much content to watch from the sidelines with 'halfway' until he produces a craft worthy of competing for our record.

Since he clearly doesn't understand the basic physics of the vehicle and has no concept of engineering tradeoffs (or is just a foul and jealous person which is always an option), I won't be holding my breath that he'll ever be a competitor rather than mere peanut gallery.

JB

 

[spork]

If others have questions I'll be happy to answer them, but going forward I'll be pretty much content to watch from the sidelines with 'halfway' until he produces a craft worthy of competing for our record.

Since he clearly doesn't understand the basic physics of the vehicle and has no concept of engineering tradeoffs (or is just a foul and jealous person which is always an option), I won't be holding my breath that he'll ever be a competitor rather than mere peanut gallery.

JB

I'll be happy to discuss (and explain) the topic to halfway even if he doesn't care to compete in the DDWFTTW category. But not just for the abuse. I'll discuss it with anyone that actually cares to discuss it.

 

[snowleopard]

Note that claim: the apparent angle bends forward of the craft. If that was the case, the propeller would be pushing the plane backwards

I have to pick up on that one. It is quite feasible for the apparent wind angle to be forward of the beam provided the prop is pitched to still give a positive angle of attack, in fact it would be surprising if higher multiples of wind speed could be achieved without.

 

[Ubergeekian]

Ubergeekian: "Note that claim: the apparent angle bends forward of the craft. If that was the case, the propeller would be pushing the plane backwards"

NO, not if the aerofoil is forward of the apparent wind. If the apparent wind is 4 degrees forward and the aerofoil is 10 degrees pitch the air flow is still on the correct side.

But you've already said that that there is zero angle of attack when the plane is not moving!

Ubergeekian: "A propeller is turning in still air"

This is still impossible, however many times you ask it.

If you don't understand what's meant you probably shouldn't be playing in this sandpit.

It is also irrelevant as all these props are "sucking" and "blowing". I've never seen Oracle sucking!

Reduced pressure (ie "sucking") on the upper side of an aerofoil contributes about 2/3 of the lift and that increased pressure ("blowing") on the underside contributes about 1/3.

If you want people to play games, then do not set questions that can be interpreted different ways.

The questions are very simple and straightforward. I would expect a first year engineering student to answer them with little difficulty.

 

[Ubergeekian]

I have to pick up on that one. It is quite feasible for the apparent wind angle to be forward of the beam provided the prop is pitched to still give a positive angle of attack, in fact it would be surprising if higher multiples of wind speed could be achieved without.

Oh I know. But our friend Mr Halfway has already said the propeller has zero AoA when the plane is stationary...

 

[spork]

Reduced pressure (ie "sucking") on the upper side of an aerofoil contributes about 2/3 of the lift and that increased pressure ("blowing") on the underside contributes about 1/3.

That's an interesting debate in itself. If you want to get down to the real pedantic nitty gritty, there is no such thing as "sucking" on the low pressure side of the sail. The low-pressure side is exactly that - lower pressure. In other words, it pushes back less than the high-pressure side pushes. It's just convenient to think of all pressures relative to ambient atmospheric pressure. But at the lowest level there is positive force pushing on the wing from every direction.

That being said, I agree that we normally think of the upper surface of the wing being responsible for about 2/3rds of the lift. Unfortunately, nothing about wings is simple when you try to think about them very hard. You just can't separate what the top would do if there were no bottom for example.


On a separate pointless (but interesting) derail... I think we generally think of sailboat sails acting as turbine blades - being worked on by the wind. But there are definitely courses of sail in which they act as prop blades - doing work on the wind. Whenever your downwind VMG is greater than windspeed for example.

 

[Ubergeekian]

That's an interesting debate in itself. If you want to get down to the real pedantic nitty gritty, there is no such thing as "sucking" on the low pressure side of the sail. The low-pressure side is exactly that - lower pressure. In other words, it pushes back less than the high-pressure side pushes. It's just convenient to think of all pressures relative to ambient atmospheric pressure. But at the lowest level there is positive force pushing on the wing from every direction.

That being said, I agree that we normally think of the upper surface of the wing being responsible for about 2/3rds of the lift.

I used to fly gliders a lot, and the sucking/blowing model can be quite useful there because there is a source of static pressure - the fuselage interior - available as well. Air will generally be sucked out of openings in the top of the wing and be blown in through openings in the bottom. The 2/3 - 1/3 business explains why keeping the top surface of your wings clean is much more important than the bottom, too.

As an engineer I also enjoy tickling up physicists with the term "sucking". I worked in cryogenics for a bit - boy, do some people hate the term "coolth" ...

 

[spork]

I used to fly gliders a lot, and the sucking/blowing model can be quite useful there because there is a source of static pressure - the fuselage interior - available as well. Air will generally be sucked out of openings in the top of the wing and be blown in through openings in the bottom. The 2/3 - 1/3 business explains why keeping the top surface of your wings clean is much more important than the bottom, too.

I fly sailplanes as well. Not so much these days. More kitesurfing, hang gliding and paragliding now. But yes, the model you use is fine (and certainly common enough), but what is an internet forum for if not for getting painfully pedantic sometimes.

As an engineer I also enjoy tickling up physicists with the term "sucking". I worked in cryogenics for a bit - boy, do some people hate the term "coolth" ...

Yup, I'm sure that would do it. It's like when I used to do work in a dark room and had to remind people not to leave the door open and let all the dark out.

 

[Ubergeekian]

It's like when I used to do work in a dark room and had to remind people not to leave the door open and let all the dark out.

I like that so much that I shall probably steal it.

 

[RAI]

Talking of the aerofoil profile spork/ThinAirDesigns, would you still choose the same profile and blade shape now that you are aiming at more than 2x wind speed?

(I can see the need to make a future BUFC into a lower friction, slippery beautifully unique flowing competitor. - Well someone is going to try and beat your record)

 

[halfway]

Spork: "But not just for the abuse."

My views were plainly stated BEFORE you arrived. I never abused anybody. It was one of your team that said "and yes loves to talks s*** through a keyboard". Which begs the question why enter a new conversation, a long way from home, where it is clear that critics of your WORK reside. Especially if you can not take criticism. If however you are genuine Spork, and will discuss the issue, then I appreciate that.

Consider one of my first lines to you was about "marketing". I feel that you have, by your team's out bursts, that are now permanent, done more harm to your cause. There is a trait in people (More in the British, but still present in Californians) that labels a person by their inability to cope with criticism. You can ignore it. You can counter it. But people that just preen and display and then write abuse are treated differently.

I have discussed it. I have put forward a number of proofs. From "Cylinder World" to the difference of 2 vectors proof, where the prop is a sail. I notice that none of your team have actually said anything about those at all.

Spork:"Incidentally, I haven't read most of his stuff."

Well that is a bad position to start. I suggest you do some research before you criticise. I have done you the honour of reading everything I can find from your project. As you can see I have even looked at all your pictures. So IF you want a discussion then I suggest until you have done your background work we stick to engineering issues that are in plain sight.

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ubergeekian: "But you've already said that that there is zero angle of attack when the plane is not moving!"

I have reread the original post about how a prop can have an apparent wind angle limit. At NO time do I say that stationary prop has a zero angle of attack. I instead say; the apparent wind is "close" to the prop plane. This is in comparison to the case that I was trying to describe of when the air speed increases in a gravity dive. So the full quote below shows that I was trying to show that the apparent angle moves forward in the dive and can in a death limiting case go past the zero angle of attack of the prop. Imagine that yellow canvas and wood plane going into a dive in front of you and what happens as it accelerates.

The full quote: "On the ground the rotational speed makes the apparent wind angle close to the plane of the prop. As the air speed increases the apparent angle bends forward of the craft. When it gets bigger than the minimum angle of the attack the prop it is useless."

Adding back in the last sentence (which you left out) makes more sense. As the description is trying to show the progressive movement forward of the apparent wind. At infinite velocity the apparent angle is directly ahead.

So is it clear now that I NEVER said that on the ground it was IN the plane of the prop and I NEVER said anything about the angle of attack, at all, for the ground situation. The angle of attack was only mentioned AFTER the plane was in the dive "as the airspeed increases". The dive was the easiest way to show how a prop works (or not) outside its normal usage. It also demonstrated the apparent angle change that a prop sees when air is forced over it.

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Spork: "I think we generally think of sailboat sails acting as turbine blades - being worked on by the wind. But there are definitely courses of sail in which they act as prop blades - doing work on the wind. Whenever your downwind VMG is greater than windspeed for example. "

Now that is the nub of the problem. A sail NEVER does work on the wind. (Assuming the Iron sail is silent). A sail is moved by the keel/boat motion, through the wind and the wind does work on the sail. If there were cases that suddenly changed the way the sail worked, then how would the cloth of the sail know when the situation was different. It always just sees the apparent wind. It has no eyes to see where the water is going. The wind and the sail cloth have no way to see the difference between a downwind faster than the wind gybing course and a tacking upwind. Only the skipper knows the difference!

If it was doing work, then it would cause a pressure difference one boat length ahead and behind the boat. Do not confuse the surface pressure difference of a sail or aerofoil with the gross "far field" effects. If a prop "blows" then the work done on the air causes long distance pressure changes and wind circulation. A sail however can only "deviate" the air flow and extracting energy and produce forces. It is limited to extracting only the relative velocity from the wind. If the sail is moved in an approximate 45 degree direction to the wind, using the keel, then the apparent wind energy can be extracted using a sail. But only up to a point determined by a given sail or aerofoil.

If you break off the keel (assuming for now it is a flat bottomed trimaran and will not just fall over) the lateral resistance of the keel would disappear and the boat would bob along sideways, straight down wind, UNDER the speed of the wind. That is exactly the case when the chain breaks on the cart. It just blows along IN the wind not faster than it. There is no discontinuities in sailing, the apparent wind is well behaved and ALWAYS moves forward as the boat speed increases. Since the sail "sees" nothing except the apparent wind it can not know when to change its behaviour.

 

[[32511]]

Very politely put, Halfway- the Talkrational site is submerged in profanities!
Just one point, though, if you are going downwind in a yacht/ land yacht faster than the wind on a broad reach, you do not need to gybe. The apparent wind is coming from the front, at an angle, so you tack, just like tacking when going upwind.

 

[halfway]

tobermoryphil: "so you tack, just like tacking when going upwind"

Please do not start that again. There is a whole thread on showing a gybe is always a gybe. I put up 10 reasons and then someone mentioned that you can go around the "wrong way" and tack. So that made 11 reasons. Maybe more have arrived since.

That is why I put "Only the skipper knows the difference!" with an exclamation mark.

 

[snowleopard]

Just one point, though, if you are going downwind in a yacht/ land yacht faster than the wind on a broad reach, you do not need to gybe. The apparent wind is coming from the front, at an angle, so you tack, just like tacking when going upwind.

For pity's sake don't resurrect that one or we'll have another 100 posts going round in ever-decreasing circles!

 

[Ubergeekian]

The full quote: "On the ground the rotational speed makes the apparent wind angle close to the plane of the prop. As the air speed increases the apparent angle bends forward of the craft. When it gets bigger than the minimum angle of the attack the prop it is useless."

Adding back in the last sentence (which you left out) makes more sense.

No, it still makes no sense at all. The reference to minimum angle of attack is meaningless: aerofoils stop working - by stalling - at a maximum angle, typically about 18 degrees.

At what forward speed do you think propellers stop working?

A sail NEVER does work on the wind.

That's an interesting claim. Is your contention (a) that the presence of a working sail has no effect on the flow of air around it or (b) that despite the flow of air being changed, no work is being done on the air?

 

[spork]

Talking of the aerofoil profile spork/ThinAirDesigns, would you still choose the same profile and blade shape now that you are aiming at more than 2x wind speed?

Good question. We optimized the prop and vehicle speed ratio for 2X wind speed in an 18 mph wind. With our assumed values for rolling resistance, transmission efficiency, etc. we concluded that the design should take us a little over 3X (although we thought that was a bit optimistic at the time). If we now set out with that goal in mind, it would be interesting to see how different the prop would look. I suspect the differences would be small.

Spork: "But not just for the abuse."

My views were plainly stated BEFORE you arrived. I never abused anybody. It was one of your team that said "and yes loves to talks s*** through a keyboard". Which begs the question why enter a new conversation, a long way from home, where it is clear that critics of your WORK reside.

First of all, "one of my team" ain't ME. Secondly, I'm not a long way from home. I'm a member here just like you. Finally, I'm here because "one of your team" showed up on talk rational describing the interesting work he'd done in the field, and described other interesting conversations taking place (here).

Especially if you can not take criticism.

In my opinion your criticism is intended as anything but constructive. It's also completely unfounded in most cases. Yes, I should probably read the entirety of all DDWFTTW threads here before commenting, but you should get at least a passing familiarity with the people and the vehicle before making ridiculous claims about either.

people that just preen and display and then write abuse are treated differently.

Sorry, we aren't the sort to just sit and take the abuse.

I have discussed it. I have put forward a number of proofs. From "Cylinder World" to the difference of 2 vectors proof, where the prop is a sail. I notice that none of your team have actually said anything about those at all.

You claim to have read everthing I've written on the topic. If so, you'd see I've posted on those perhaps hundreds of times.

Spork:"Incidentally, I haven't read most of his stuff."

Well that is a bad position to start.

Perhaps so, but I don't need to know your entire history to see what you think currently, or how you behave.

I suggest you do some research before you criticise.

Wow - you just destroyed my very last irony meter.

So IF you want a discussion then I suggest until you have done your background work we stick to engineering issues that are in plain sight.

And here I am with no irony meter. You tell us all about the myriad bad choices made in the design and construction of a vehicle that you know nothing about - and have demonstrated the same. That's on record as well. Do we really have to review that?

Spork: "I think we generally think of sailboat sails acting as turbine blades - being worked on by the wind. But there are definitely courses of sail in which they act as prop blades - doing work on the wind. Whenever your downwind VMG is greater than windspeed for example. "

Now that is the nub of the problem. A sail NEVER does work on the wind.

I assure you you're wrong. But it's 4:30 am here and I have an early morning. So this will have to wait.

 

[halfway]

Spork: "You claim to have read everything I've written on the topic."

No I actually said: "of reading everything I can find from your project." Yes there are things I can not find, and there are things that are outside your project references, or reference your project.

----

Spork: ""one of my team" ain't ME"

But you clearly state:

Spork: "It is. So far I've Bogarted the driver's seat. JB is pretty keen on keeping a close eye on various aspects of the cart from the chase vehicle. But he will undoubtely take a spin soon."

Sounds like a team to me.

----

Spork: "Sorry, we aren't the sort to just sit and take the abuse."

So do not. Counter it will real arguments. I notice you only countered on the brake and missed all the safety issues. I acknowledge that recognising them would put you in a sticky position.

----

Spork "If so, you'd see I've posted on those perhaps hundreds of times."

Your dry land "Cylinder world" in the posts I found, missed out a few issues. The first is that you derive it based on the assumption of a land yacht can do it. Then it is a thought experiment to get to the prop version. However, you then do not recognise that the characteristics of a yacht and the small and big carts do not match. A yacht does not alter the wind 1 boat length in front, but the treadmill cart demonstration clearly does. The yacht (performance types) uses a sail, that is as wide as the rig can support, at the mast top, but the prop on the big cart is a sharp point. Yes, the relative tack angle is different because of the circle but that does not mean the blade must taper to nothing. Only the pitch must flatten. It is an approximate 45 degree helix path so there is no blade interference. That is the reason for me supporting the original prop over the one on the big cart you drive.

But then he did it with technology from 40 years ago.

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Spork "I assure you you're wrong."

I will wait.

----

Ubergeekian: "that despite the flow of air being changed, no work is being done on the air"

If I put a 4x8 sheet of plywood on the pontoon now, and fix it with a brace against the wind, but slightly angled, is it doing work on the wind?

Hardly, as it has no source of energy to transfer to the wind. It is a piece of plywood. However the wind is now going a different direction. The wind is probably a little warmer. The brace is being compressed. Did the plywood supply the energy to make this happen.

Or are you going to change the reference again and say the world bumped into the wind. As your proofs only seem to work from one reference point.

 

[Ubergeekian]

Ubergeekian: "that despite the flow of air being changed, no work is being done on the air"

If I put a 4x8 sheet of plywood on the pontoon now, and fix it with a brace against the wind, but slightly angled, is it doing work on the wind?

Hardly, as it has no source of energy to transfer to the wind. It is a piece of plywood. However the wind is now going a different direction. The wind is probably a little warmer. The brace is being compressed. Did the plywood supply the energy to make this happen.

Oh dear oh dear oh dear. Let me give you a clue: action and reaction are equal and ... ?

See, this is the problem. You're sounding off about a fairly complicated problem and yet it's clear that you don't really understand terms like reference frame or doing work. It doesn't make you a bad person, of course, but in the absence of a very basic understanding of physics you are going to struggle to make more sophisticated arguments and risk looking rather silly when you try.

Or are you going to change the reference again and say the world bumped into the wind. As your proofs only seem to work from one reference point.

My models all work in any reference frame. They are just slightly easier to apply in some reference frames than in others.

 

[ThinAirDesigns]

Talking of the aerofoil profile spork/ThinAirDesigns, would you still choose the same profile and blade shape now that you are aiming at more than 2x wind speed?

I don't believe the changes would be enough to attract notice if a pictures was taken of the propellers and not displayed side by side. You might not even notice if they were side by side. Very subtle.

I can see the need to make a future BUFC into a lower friction, slippery beautifully unique flowing competitor.

A conceptual drawing from quite a while ago ...

Not exactly what we would build -- likely we would go with twin props and their profile wouldn't look like those, but enough to give you an idea. We call this one the BBFC.

JB

 

[ThinAirDesigns]

A sail NEVER does work on the wind.

This giant tidbit of wrongness is why halfway's ddwfttw vehicle could never exceed windspeed and why he thinks our prop is the wrong shape.

The only relevent difference between the blade on BMWO as it reaches at 2x WS (downwind vmg) and the blade on the BUFC at 2x WS (vehicle on DDW path) is the radius of the circle.

A: in a sailboat, it's impossible to achieve downwind VMGs of > 1.0 without the airfoil operating as a propeller (adding energy to the wind wrt to the airfoil).

B: in a sailboat (while reaching DW with VMGs < WS) one *can* use the sail as a turbine (removing energy from the wind wrt the airfoil), but even in the simplest modern sailboat the sail used properly is acting as a propeller when reaching.

C: let's be clear, there's not some 'magical' setting that only the 'pros' know that has the sail working as a prop when reaching -- *everyone* that learns the craft in it's basic form sets the sail this way.

D: as spork once said "if the other guy doesn't reach using a sail setting where the sail is acting as a propeller, I want to race him for money."

JB