Does a sail act as a turbine blade or a propeller blade?

[ThinAirDesigns]

For the purposes of the above poll, here are the definitions:

"Sail" excludes spinnaker but includes any typical/traditional sailing rig on any typical/traditional sailing vessel.

A turbine blade has work done on it by the air.

A propeller blade does work on the air.

JB

[PAR]

Both . . .

[robherc]

Both simultaneously. The sail uses the inertia of the air to effect the direction of travel of the air. So, in effect, it harvests energy from the air to move that very same air in a different direction & produce thrust.

Turbine, and propeller, in one piece & with no moving parts! How's that for ingenious?

[PAR]

Did I win a prize?

[pkoken]

Sails act in two fashions.

#1 - as a lift producing device (wing). Sails act to generate lift on MOST points of sail. Lift from the sails works in conjunction with lift from the keel (centerboard or hull) to produce forward motion.

#2 - as a drag producing device. Sails act to produce drag once the wind is so far astern that you no longer have attached flow, and your sails (wings) are stalled.

[Frosty]

There is two types of turbine blades that cover your 2 examples.

[Knut Sand]

Most sailboats i normally see, have sails stacked down/ wrapped up, probably to dampen the blow you get on your head when you pass the boom or when the boom passes you....:p

[Deadeye]

Hey all
Call it new-guy syndrome...but I don't really get the question:

An exhaust turbine acts like a pinwheel: the blades tend to be shaped more like a scoop or a bucket than a foil. When the exhaust gasses hit it, Bernoulli's principle isn't the primary force responsible for turning the shaft (more Newtonian I guess). Compressor blades on the other hand are foil shaped, but compressor design maximizes pressure buildup by the use of alternating rotor and stator blades to minimize the velocity of the compressed air being fed into the can. The fan blades (in a bypass turbofan like an airliner) act as many propellor blades taking small 'bites' of the air vs a conventional propellor taking fewer, bigger 'bites (per revolution).

My question is here:
When you say 'turbine blades' I assume you mean fan blades because that is what most people see on the type of aircraft most people see. What difference between fan blades and propellor blades are you trying to compare a sail to ?
Not trying to be preachy or argumentative, just curious...

[robherc]

Deadeye

By "turbine blades" he means the actual turbine stages, not the fan/compressor stages as those are "propeller blades" if you read his first post:
For the purposes of the above poll, here are the definitions:

"Sail" excludes spinnaker but includes any typical/traditional sailing rig on any typical/traditional sailing vessel.

A turbine blade has work done on it by the air.

A propeller blade does work on the air.

JB
Read: Turbine blades harvest energy from the air, Prop. blades use energy to move the air.

PAR-
Did I win a prize?
I'll say you did...first one with the truly correct answer ;)

[ancient kayaker]

I can't vote until there is a fourth choice "none of the above"

A turbine compresses air using mechanical source of energy or maybe not if it is extracting mechanical energy from a stream of moving gas. A propeller uses mechanical energy to move a vessel through a fluid.

A sail has very little to do with mechanical energy unless it is on a windmill in which case it is part of a turbine. It would be just as sensible to compare a sail with a windshield, since both deflect air.

[Deadeye]

...he means the actual turbine stages, not the fan/compressor stages as those are "propeller blades"...
Rob, I'm not sure what you mean by 'actual turbine stages' but your response did definitely answer my question.

WIND turbines, i.e. 'aeroelectric' generators...I was thinking generally about turbofan engines and specifically about the Allison 250 turboshaft engine - that's why I wasn't sure.

Neglecting apparent wind, I'd say turbine. Once apparent wind is factored in, it's both. But I guess that's what you meant, Paul?

Never thought about like that...interesting.

[markdrela]

I can't vote until there is a fourth choice "none of the above" It's worse. To answer rationally one also needs to:
A) Define a "propeller" and "turbine".
Are they defined in terms of propulsive force? (propeller has force with motion, turbine has force against motion).
Or are they defined in terms of fluid energy? (propeller puts energy into fluid, turbine takes energy out of fluid).

B) Define which observer we're talking about.
The sailor on the boat? A swimmer treading water? An observer in a hot-air balloon?

Different observers and different propeller/turbine definitions will have different correct answers to the poll.

[spork]

Mark, I agree with your concerns about definition - particularly when it relates to work done on air by the blade of vice versa. It occurs to me that that spinning device on the back of the prop cart is almost necessarily a propeller in the cases we discuss, while the overall cart could be considered a turbine. Afterall, the cart has the effect of extracting energy from the air in the frame of the ground (although adding energy to the air from the frame of the cart).

For the less ambiguous case of a bladed structure spinning on an axis I'd propose one or both of the following definitions:

1) if the structure is turning in the same direction which its shaft is torquing it, that structure is a propeller. If it's turning opposite that direction (as a result of flow over the structure) it's a turbine.

2) If we consider the flow through the actuator disk a propeller will always have higher pressure downstream of the disk, while a turbine will have higher pressure upstream of the disk.

A possible 3rd definition (perhaps redundant with the second), a turbine is subject to the Betz limitation, while a propeller is not.

Considering only the limited case of a structure spinning on a shaft would you agree with these definitions? Are there cases (for such a structure) where those definitions would still be ambiguous?

[markdrela]

Afterall, the cart has the effect of extracting energy from the air in the frame of the ground (although adding energy to the air from the frame of the cart).
1) if the structure is turning in the same direction which its shaft is torquing it, that structure is a propeller. If it's turning opposite that direction (as a result of flow over the structure) it's a turbine.
2) If we consider the flow through the actuator disk a propeller will always have higher pressure downstream of the disk, while a turbine will have higher pressure upstream of the disk. OK, that works for the cart or a similar water vehicle.
But this is about a sail, to which these do concepts not apply. Here, instead of the cart's torque and rotation, you can try to use force and velocity. But rotation is the same for all inertial observers, while velocity is not. To some observers the sail is giving a thrust force along the apparent velocity (its a propeller blade?), and to other observers it's giving a drag force (it's a turbine blade?).

[sailor2]

1) if the structure is turning in the same direction which its shaft is torquing it, that structure is a propeller. If it's turning opposite that direction (as a result of flow over the structure) it's a turbine.

2) If we consider the flow through the actuator disk a propeller will always have higher pressure downstream of the disk, while a turbine will have higher pressure upstream of the disk.
I agree with these definitions.
However the original posts was about common sail, and these definitions tell us nothing about that as far as I can see, as there is no axis & torque.

One could possibly compare pressures with apparent airflow directions, and conclude there is a higher pressure in front on average, but not necessarily on leading edge. Thinking about aerofoil here. Jib hasn't this effect having so thin LE, so, it has always high pressure upstream like actuator disk of a turbine, but not like turbineblade alone which has suction peak at LE.
Confusing really, bnetter leave the sail out of this.

My concluson would therefore be that a sail is neither a turbineblade nor a propeller blade as it doesn't fit these sensible definitions at all.
As far as the poll is conserned I still answered assuming the original post definitions as given regardless.

[chabrenas]

Under the conditions that rulebooks used to try to define as fair sailing, a sail transfers energy from the moving air mass to the boat (but not very efficiently when it is stalled, as in running before the wind).

In roll tacking a dinghy in light airs, work done by the crew swings the mast and sail through the air, 'pumping' it backwards - in effect, you have just rotated a propeller through about 50 degrees.

In the natural roll of a larger boat in moderate seas, there is an element of using the energy of the roll to drive the sail as a propeller as the boat rolls to windward.

[spork]

OK, that works for the cart or a similar water vehicle.
But this is about a sail, to which these do concepts not apply. Here, instead of the cart's torque and rotation, you can try to use force and velocity....

Agreed. This is why I wanted to start with a less ambiguous case where I expected we could agree on definitions. Having thought about the case of the sail, my recollection is that there were cases where the sail itself (not necessarily the craft) seemed to fall pretty neatly into the category "turbine" while other courses of sail fall into the category "prop".

An example would be an ice-boat on a 45 degree downwind course with downwind VMG greater than wind speed. In this case it seems we could picture a long row of these ice-boats running abreast, and all on the same tack. We could think of their sails as forming the actuator disk. The flow would clearly go from low-pressure to high pressure as it passes our virtual disk. In this case it seems clear to me that the sails are acting as propellers. But I haven't scrutinized it closely. Will do so at lunch.

I'd be curious though if you can think of a way or frame in which to look at this scenario in which the sails (not the entire craft) could be thought of as turbines.

[ThinAirDesigns]

An example would be an ice-boat on a 45 degree downwind course with downwind VMG greater than wind speed. In this case it seems we could picture a long row of these ice-boats running abreast, and all on the same tack. We could think of their sails as forming the actuator disk. The flow would clearly go from low-pressure to high pressure as it passes our virtual disk. In this case it seems clear to me that the sails are acting as propellers. But I haven't scrutinized it closely. Will do so at lunch.


Sort of like this drawing?

JB

[spork]

Sort of like this drawing?

Yes, that's exactly the configuration I had in mind. Unfortunately the left to right arrows refer to the direction the sail moves in the cross-wind sense. The fact that the whole craft is moving downwind as well may complicate matters. That's what we should scrutinize over some In & Out burgers.

[robherc]

Deadeye,

I think this illustration from Jack L. Kerrebrock's (MIT Professor) "Aircraft Engines and Gas Turbines" (Second Edition) will illustrate what I'm trying to say better than I could explain it.


Mark, Spork, et al:

I think if we use the (somewhat oversimplified) constraints that were applied in the original post, we can all agree (after an appropriate amount of thought) that my first post here quite well describes the situation in all but DDW useage of a spinnaker. Even in "Wing on Wing" DDW sailing, the sails are both harvesting energy from the wind, and exerting energy on the wind to alter its course...so both (and spinnakers were disqualified in the orig. post anywise).

[sailor2]

It's worse. To answer rationally one also needs to:
A) Define a "propeller" and "turbine".
Are they defined in terms of propulsive force? (propeller has force with motion, turbine has force against motion).
Sorry, but consider case where a powerboat is moving backwards, and then suddenly puts all ahead. The prop would produce large force forward against motion (of boat related to free stream water outside prop stream) if you define that as a turbine , I'll have to think engine would disagree and so would most boaters.
Or are they defined in terms of fluid energy? (propeller puts energy into fluid, turbine takes energy out of fluid).
That sounds good, if fluid energy is defined in IRF of free stream by definition, is it ?

[Zappi]

It is of my opinion that Spork and TAD are one in the same. The question really is... If one person is portraying two people and both theoretical people agree on the subject, is he a split personality?

[spork]

It is of my opinion that Spork and TAD are one in the same. The question really is... If one person is portraying two people and both theoretical people agree on the subject, is he a split personality?

Very astute. I assume your logic is that we're just plain too smart for a single personality - and you're right. But I'm the better looking one.

Nonsense! I am.

[spork]

After some more consideration I'm planning to stick with my notion for the moment that the practical definition of prop or turbine relates to whether the pressure is high on the upwind or downwind side of the actuator disk. If we use this definition I think we can say that a sail DOES act as a turbine in certain situations and as a propeller in others.

I think saying it can be thought of a turbine or a prop depending simply on the frame chosen only makes sense if we define it based on work done. And to me that's a less satisfying definition to begin with since "work done" is a far less tangible notion than forces and flows that can be measured.

[Zappi]

Obviously that was my logic.

[sailor2]

After some more consideration I'm planning to stick with my notion for the moment that the practical definition of prop or turbine relates to whether the pressure is high on the upwind or downwind side of the actuator disk. Now you still have to define upwind side of the actuator disk to make this definition clear. Is it defined as related to flow thrue the disk or related to freestream direction ?
Think about a case where a powerboat is moving backwards and then putting engine on forward ...
1) with very low power setting so that the flow does not reverse thrue the disk, while still going forward. The prop only tryes to work as a prop, but within this definition it acts like a turbine. However the torque on the shaft can well be same as with a prop due to inefficient prop pitch & twist for this operating condition outside of it's design envelope.

2)with high powersetting so that flow is reversed, but boat still moving backwards at this time due to inertia. Which side of the actuatordisk is now a highstreamside ? Pressure is clearly high behind the disk & boat in slipstream. Is this the turbine case or a prop case ?

if the structure is turning in the same direction which its shaft is torquing it, that structure is a propeller. If it's turning opposite that direction (as a result of flow over the structure) it's a turbine.
With this definition both cases above are propellers, not turbines.

These 2 definitions are therefore in contradiction, unless I misinterpreted your definition. Please clarify the issue. The second definition makes more sense for me, but tells nothing about a sail.

[spork]

Now you still have to define upwind side of the actuator disk to make this definition clear. Is it defined as related to flow thrue the disk or related to freestream direction ?

I define it as the flow through the actuator disk - not the free stream. In the case where the blades are stalled, I don't consider it a propeller or turbine. In this case it's just an object I'm dragging through the water.

If it's not stalled, but the net flow through the disk is reversed, it's simply a case in which the model of abstracting the spinning prop as a disk breaks down. The flow in the region of the blade is still in the correct direction if the blade is not stalled.

[jehardiman]

For the purposes of the above poll, here are the definitions:

"Sail" excludes spinnaker but includes any typical/traditional sailing rig on any typical/traditional sailing vessel.

A turbine blade has work done on it by the air.

A propeller blade does work on the air.

JB

The question and poll are meaningless as it is a frame of reference-energy boundary issue, not an absolute. All (prop, turbine, sail) are just degenerate equations from different vantage points of the total energy equation.

FWIW, all are the same, as they all do "work" on the air as work is a vector and only frame of reference places it as positive or negative, work in or work out. The same is true of "lift" and "drag" forces...i.e. it is all entropy as far as the fluid is concerned, it just depends on if it is going in the direction you want it to go. Do not be confused by sophomoric mathematical tricks.

JB, I would suggest you hit the books more; a good thermo and fluids book with historical perspective will show you just how foolish your poll was.

[ThinAirDesigns]

JB, I would suggest you hit the books more; a good thermo and fluids book with historical perspective will show you just how foolish your poll was.

Rather than "foolish", I prefer the term "ignorant". I don't find any discussion which elicits interesting information "foolish".

Many place negative connotation to the term "ignorant", but I find that acknowledging ignorance of any topic allows learning.

JB

[chabrenas]

jehardiman: true, but perhaps a bit pedantic. I took the poll in the only way it makes sense - using the craft on which the sail was mounted as the frame of reference.

However, I do have a question: why rule out DDW spinnaker? Airflow is more complex, but it isn't the equivalent of a flat plate or anemometer cup normal to the wind. A mainsail is probably more completely stalled DDW than a spinnaker.

[spork]

The question and poll are meaningless as it is a frame of reference-energy boundary issue, not an absolute. All (prop, turbine, sail) are just degenerate equations from different vantage points of the total energy equation.

FWIW, all are the same, as they all do "work" on the air as work is a vector and only frame of reference places it as positive or negative, work in or work out.

Sorry, but parts of what you say can be true depending on how you define a turbine or a prop. Parts are absolutely false. To say "all are the same" is definitely wrong. All you have to do is look at the flow through an engine driven prop vs. the blades of a windmill. Look at the stream tube upstream and downstream of the disk. They differ in very material ways. I personally think the best and most practical definition states that a turbine has high pressure on the upstream side of the disk (upstream relating to relative flow through the disk), and a prop has the high pressure on the downstream side of the disk.

The same is true of "lift" and "drag" forces...i.e. it is all entropy as far as the fluid is concerned, it just depends on if it is going in the direction you want it to go.

This is absolutely not true. Induced drag can be said to be similar to lift, but parasite drag is qualitatively a completely different animal.

Do not be confused by sophomoric mathematical tricks.

JB, I would suggest you hit the books more; a good thermo and fluids book with historical perspective will show you just how foolish your poll was.

And I suggest you take your own advice.

[alex folen]

This is fun! For what it’s worth and if I’m understanding the “simple” and quick question, and with the help of "Akums Razors" theory, the simplest explanation is usually correct” It’s the turbine.

[spork]

This is fun! For what it’s worth and if I’m understanding the “simple” and quick question, and with the help of "Akums Razors" theory, the simplest explanation is usually correct” It’s the turbine.

As Albert Einstein said "Everything should be made as simple as possible, but no simpler."

I suppose it would be great if it were so simple that we could just say "it's a turbine". But it's not.

[alex folen]

I guess we’ll have to split up the prize PAR.
A Propeller is designed to produce thrust using some sort of power. Turbines, the wind. Hello? A two question question. …Put you calc books up… like I did years ago.

[alex folen]

"So many people today — and even professional scientists — seem to me like someone who has seen thousands of trees but has never seen a forest."


Auhhh, This is Einstine also.

[jehardiman]

JB: Foolish was perhaps a poor word choice but it was the one that came to hand and I did reject "ignorant" for the reasons you stated. Realisticly, regardless of what some might argue, there is no difference in the mathematical formulation between work in (props) and work out (turbines) units, and as was pointed out, a sail could be either. The very attempt to compare a sail to one of the other is the fault in your premise, it is neither because they are just specific cases of the general situation, just like wings, kites, dynamic catenarys, jets, venturi, manometers, Flettlner Rotor, etc. A sail is a sail, and should be approached from the general equation, discarding all the constant or trivial terms rather than being compared to a degenerate form of the equation that needs to be added to. For some basics see the following websites that I was quickly able to google-up. For more detailed information, and the full expansion of the equations, get a good fluids text as I proposed.
http://en.wikipedia.org/wiki/Euler's_equations_of_inviscid_motion
http://www.roymech.co.uk/Related/Fluids/Fluids_Flow.html

Mike: Pedantical? Yeah, well, but "flow" over foils is one of those things, like "hull speed" calculations, that most people misunderstand because they only learn the contrived equations to get a useful engineering answer, not the correct analysis on which other development can be based. That is one of my hot buttons. For insight into a mathematical contrivince that is now a believed "truth", read Lanchester's and Prandtl's original works for a glimpse into how Gamma was determined.

Spork: I have neither the time, the inclination, nor is it my place to instruct you to be a better fluid dynamicist. All analysis of fluid devices start from the same equation, and wether "+ W" ends up as positive or negative depends on how you draw the flow, energy boundries, and assumptions on what to ignore (viscious heating perhaps?). Even your comments admit to that with "upstream" and "downstream" references that show you understand that basic. If you don't belive that a prop and a turbine are the same devices just carry out a prop calculation from J <<-J(opt) to J>>J(opt) or a centrifical pump calculation for negative specific speeds (i.e. negative omega). The fact that a turbine makes a poor prop and a prop makes a poor turbine and they both make poor glider wings does not make them different things than a sail.

[alex folen]

“Does a sail act as a Turbine blade or a propeller blade”
...If you were stranded on an island and your sails blew away, which would you use if available, a turbine prop blade or an airplane prop blade, assuming they were the same length and weight. I'd grab the turbine blade. …BUT! If winds should exceed 400 MPH and I had some coconuts to counterbalance the weight of the other end of the propeller blade I might choose the prop blade. Am I not understanding the poll question above? I might prefer foolish over ignorant in this response.

[chabrenas]

most people misunderstand because they only learn the contrived equations to get a useful engineering answermost people misunderstand because they only learn the contrived equations to get a useful engineering answer

OK. I see where you're coming from - and given your profession I can perhaps guess at some of the encounters that have hardened your attitude (:=>

This forum really does take me back to university lounge and coffee bar days - for which I'm grateful. And, as others have pointed out, its seems to be frequented by more mathematicians and scientists than certain 'physics' fora.

[chabrenas]

The fact that a turbine makes a poor prop and a prop makes a poor turbine and they both make poor glider wings does not make them different things than a sail.

Which suggests that the design of each is optimised to work efficiently in that range of values of the variables in the equations you mentioned which defines its designed working environment.

So we can reasonably interpret the poll as asking whether the normal operating environment of a sail is defined by a set of values which is contained within the set defining the designed working environment of a turbine, or within the set defining the designed working environment of a propeller, or within one that intersects both sets.

[spork]

Spork: I have neither the time, the inclination, nor is it my place to instruct you to be a better fluid dynamicist.

Fortunately for you I do have the time and inclination to correct your flawed ideas about fluid dynamics. Unfortunately, I don't have that time at this very moment. I will say that you may be misunderstanding the question. The question is not whether an object such as a propeller CAN act as either a propeller or a turbine - it's a question of whether and when it acts as one or the other.

If you consider a windmill planted in the ground, and operating in the normal way, are you actually suggesting that we can say it's vanes are acting as a propeller if we just look at it the right way!?

[jehardiman]

Which suggests that the design of each is optimised to work efficiently in that range of values of the variables in the equations you mentioned which defines its designed working environment.

So we can reasonably interpret the poll as asking whether the normal operating environment of a sail is defined by a set of values which is contained within the set defining the designed working environment of a turbine, or within the set defining the designed working environment of a propeller, or within one that intersects both sets.

Yes, useful devices are optimized for thier intent and operating conditions, but are described by constant equations.

Think of a 1 kilo mass having a velocity (including zero) in a gravitational field. It always has mass, but it only has "weight" (an unbalanced force acting towards the center of the larger body) in certian conditions. In a balanced energy stable orbit "weight" = 0 only becuase the net force vector is perpendicular to the direction that "weight" is measured. In a negative energy unstable orbit it has positive "weight" force and '"falls" towards the larger mass, and we have the ability to extract some useful work from it or it is balanced by an outside force (the mass is supported), and no apparent work is done though the mass may have significant energy. In a positive energy unstable orbit it has "negative weight" or "centrifugal force" and moves away from the larger mass and requires work input or it is balanced by a "centripetal force", again doing no work though it may have significant energy. So from a semantic point of view the mass only has "weight" when we try to prevent if from going to a lower energy state, and only has "centripetal force" when we attempt to hold it at lower energy state.

Most people would use Newtons equations to slove the above example. Now assume the mass is a fluid. All same conditions apply except now "weight" becomes "hydrostatic pressure" and "centripetal force" becomes "lift". It is all point of view

Fortunately for you I do have the time and inclination to correct your flawed ideas about fluid dynamics. Unfortunately, I don't have that time at this very moment. I will say that you may be misunderstanding the question. The question is not whether an object such as a propeller CAN act as either a propeller or a turbine - it's a question of whether and when it acts as one or the other.

If you consider a windmill planted in the ground, and operating in the normal way, are you actually suggesting that we can say it's vanes are acting as a propeller if we just look at it the right way!?

Make the time, I will, as this ought to be good.

IMHO it is rather obvious you have no real experience in applied fluid dynamics because of the example you chose. In real life applications the windmills blades will act as a propeller fairly often with any -del V. This is because there cannot be an instantainous change in momentum of the blade and work will be extracted from the blade into the air. A ships engineer would laugh in your face and point to the speed to turns warning placards if you told him that a propeller will never backdrive the main reduction gear. Or all the shaft generators sold.

There is no massless, steady, inviscid, irrotational flow in the real world. The fact that those simplifing assumptions can lead to a plethora of degenerate situationaly specific equations does not make them or that simplifing process correct, regardless that the answers you get are within real world ability to resolve.

Anyway, to get back on topic. It is rather obvious to the most casual observer that a sail can be either, and it can be demonstrated in a relatively simple way. If you cannot use the sail as an energy in propulsion device, why is there Rule 42? FWIW, I used to propell my 26' sail boat at about 2 knots in 0 wind by sallying her with the main sheeted well home.

[ThinAirDesigns]

Spork:
If you consider a windmill planted in the ground, and operating in the normal way, are you actually suggesting that we can say it's vanes are acting as a propeller if we just look at it the right way!?

jehardiman:
IMHO it is rather obvious you have no real experience in applied fluid dynamics because of the example you chose.

Wow, just wow.

In real life applications the windmills blades will act as a propeller fairly often with any -del V. This is because there cannot be an instantainous change in momentum of the blade and work will be extracted from the blade into the air.

You do see the "operating in the normal way" in sporks assertion don't you? -- we don't "normally" place wind generators up on windy hills just so they can be occasionally backdriven. I'm pretty sure you'll find that by "normal way", spork means in a steady state wind.

At any rate, even by your own irrelevent example of a turbine being backdriven you are acknowledging that there is a fundamental difference between an airfoil being used as a prop and being used as a turbine -- a difference that isn't just related to frame of reference. It is this difference to which the OP/poll refers.

Anyway, to get back on topic. It is rather obvious to the most casual observer that a sail can be either,

Since that is one of the poll options, but yet not everyone picked it ... what makes the poll "foolish" in your mind?

JB

[jehardiman]

You do see the "operating in the normal way" in sporks assertion don't you? -- we don't "normally" place wind generators up on windy hills just so they can be occasionally backdriven. I'm pretty sure you'll find that by "normal way", spork means in a steady state wind.

At any rate, even by your own irrelevent example of a turbine being backdriven you are acknowledging that there is a fundamental difference between an airfoil being used as a prop and being used as a turbine -- a difference that isn't just related to frame of reference. It is this difference to which the OP/poll refers.

Since that is one of the poll options, but yet not everyone picked it ... what makes the poll "foolish" in your mind?

JB

Actually I read "in the normal way" to imply passive, i.e. without power input. The problem is that the windmill stores energy like a flywheel, and bleeds that energy back into the airflow "in the normal way" in certian conditions. As you say it is an airfoil, neither a turbine or a propeller. Those are just transient states, not what the blade is.

As far as the OP/poll is concerned the operative word is "always". That is what makes it "foolish". I believe the old saw is that "absolutes are for tyrants and dullards".

[ThinAirDesigns]

As far as the OP/poll is concerned the operative word is "always". That is what makes it "foolish". I believe the old saw is that "absolutes are for tyrants and dullards".

There's three options there ... two "always" and one "depends". No one is being a tyrant forcing anyone to choose "always".

I myself fell squarely in the "depends" camp well before posting the poll. I suspected that there would be some on the "always turbine" camp and hoped to engage in some interesting conversation in that regard. I hardly find that desire "foolish".

JB

[ThinAirDesigns]

As you say it is an airfoil, neither a turbine or a propeller. Those are just transient states, not what the blade is.

You're apparently misreading the poll and OP question ... it doesn't ask if a sail *is* a turbine or prop. It asks what it *acts as*. In other words, which of the transient states you describe above and under what circumstances.

Since we seem to agree that there truly is a way to determine when the airfoil (sail) is acting in one state or the other, perhaps you could use your understanding of this to educate me as to when a sail acts as a prop -- that is leaving a stream of air behind that is accelerated relative to the free stream.

Please?

JB

[jehardiman]

You're apparently misreading the poll and OP question ... it doesn't ask if a sail *is* a turbine or prop. It asks what it *acts as*. In other words, which of the transient states you describe above and under what circumstances.

Since we seem to agree that there truly is a way to determine when the airfoil (sail) is acting in one state or the other, perhaps you could use your understanding of this to educate me as to when a sail acts as a prop -- that is leaving a stream of air behind that is accelerated relative to the free stream.

Please?

JB

FWIW, "absolutes are for tyrants" is becuase tyrants are absolute rulers and are the only ones who can make absolute laws, not that you or the poll is tyrantical.

Anyway, you seem to be caught up in the idea that that a propeller must cause a constant increase stream velocity (a vector it is important to remember). In truth that is not the case, though for most propulsor theory it is. Rather a propulsor must increase the energy in the fluid to generate a useful force, not necessarly increase any free stream velocity relative to the direction of travel (there are some cute toys based on some tricks to that).

Lets take Rule 42 as an example. You are sitting in a el Toro on a windless day. You sheet out the sail slowly and then pull back rapidly, the boat moves forward and you get protested under 42.2 and DSQ'd. Why? When the sheet came back it pushed (compressed) the mass air aft of the sail and pulled (suctioned by having the same mass now have to fill a larger volume) at the mass in front of it. Remember when I said there is no massless inviscid flow? The mass of air in direct contact with the sail now has to move, and moving a mass from rest requires an acceleration and a consequently a force. The sail does work on the air and consequently a force is generated on the sail which is transferd by the rig to the hull, which is free to move in the horizontal plane, and subsequently to the water as the hull does move (you pulling on the sail also pushes on the hull but that is cancled buy the hull pushing back so it is a null effect)---the sail was used as a propulsor.

Now getting back to what happens in the air, you have added energy to the fluid by creating two zones of del energy, the high and low pressure areas. This energy is dispersed to the surrounding air at the speed of sound (kenimatic viscosity limited). As the fluid must maintain a constant pressure limited only the ability of monemtum to disperse it, it uses a pressure wave and a viscious shear flow to distribute energy in the form of knetic energy to a greater and greater volume of mass until finaly the velocity of the infinte mass of air has been given an infintessimal velocity, thereby rasing the energy in the entire fluid. Conversely, the exact same thing is occuring in the water as the hull moves forward, except there is much less compressability and much more mass and viscious effect.

Edit to add:

If you want me to describe sallying a large boat I can, but it easily can be shown to be similiar to the discussion above.

[ThinAirDesigns]

Anyway, you seem to be caught up in the idea that that a propeller must cause a constant increase stream velocity (a vector it is important to remember).

No, I'm not caught up with any "absolute". I'm giving examples that I find relevent. For the sake of this discussion, I've as comforable with your definition : "a propulsor must increase the energy in the fluid to generate a useful force" as I am with my "accelerated relative to the free stream" definition -- we'll use yours.

Lets take Rule 42 as an example.

Let's not. No offense, but while it may not be explicitly excluded from the OP wording, we're not interested in an obscure sailing example of taking a sail and flapping it like a wing. While everything you say about that scenario may be true, it doesn't relate to the OP.

Let's stick to steady state scenarios -- traditional points of sail, fixed and steady wind, fixed boat course, fixed sail position, etc.

Is there a point of sail and sail position that will always result in the sail acting to "increase the energy in the (air)"? And is there an inversion position?

As poorly worded or as "foolish" as you may find the poll, the above is at the core OP intent.

I'm guessing it should be really easy for someone of your knowledge to educate me as to the steady state scenarios that determine these transient states to which you referred.

JB

[ThinAirDesigns]

jehardiman, I'll ask a more precise question through pictures:

Could you tell me for the following examples, whether the sail on the boat is acting as a "propulsor" by our previously established definition, or the inverse:

First a downwind example with a VMG at or near 1.0 TWS

http://www.mediafire.com/imgbnc.php/3d6b32101e865d0fc85e824e52aed0455g.jpg
-
-
and then an upwind example where I chose VMG of .5 TWS:

http://www.mediafire.com/imgbnc.php/0db0e9702ab4421e695dfa5a6140a6405g.jpg

Thanks

JB

PS: I do apologize for my crappy drafting skills

[jehardiman]

Let's stick to steady state scenarios -- traditional points of sail, fixed and steady wind, fixed boat course, fixed sail position, etc.

Is there a point of sail and sail position that will always result in the sail acting to "increase the energy in the (air)"? And is there an inversion position?

JB

Yes, I can....a boat traditionally being carried downstream by a steady current on a windless day, however I expect that you will apparently find fault with that and require the additional condition of static water and flowing air instead of the reverse...;)

As the "steady state" system you want is a closed contrivence (as much as the situation above) which can only lose energy to entropy, overall energy in the air stream will always be reduced, though axis systems can be contrived to show an apparent increase in energy along one of those axis. The Laws of Thermodynamics prevent anything else.

Edit: X-post with your drawings:

Both situations result in loss of energy from the air to the water and loss of overall energy in the system to entropy

[ThinAirDesigns]

Me:Is there a point of sail and sail position that will always result in the sail acting to "increase the energy in the (air)"?

Yes, I can....a boat traditionally being carried downstream by a steady current on a windless day, however I expect that you will apparently find fault with that and require the additional condition of static water and flowing air instead of the reverse...;)

So If I understand your response, you're saying that when a boat sails upwind, its sail is acting as a propeller and "increasing the energy in the air"?

Is that an accurate representation of your position?

Final answer?

JB

[jehardiman]

Me:



So If I understand your response, you're saying that when a boat sails upwind, its sail is acting as a propeller and "increasing the energy in the air"?

Is that an accurate representation of your position?

Final answer?

JB

The final answer will always be "it depends".

When a boat is sailing in steady state with still air and moving water, the "keel" is extracting energy from the water and transfering it to the sail to propel the boat in the desired direction, so the sail functions as a "propeller" to you. It uses the air to generate a force in the direction you want to go, like the rudder on an airplane or a the wing of a glider.

In the more common steady state case of still water and moving air , the sail is extracting energy from the air and transfering it to the keel to propel the boat in the desired direction, so the sail functions as a "turbine". It pulls energy from the air, but cannot direct that energy, similar to a kite whos string is cut. To work aganist the wind, a keel is needed.

Without a keel in the water, the boat would only drift down wind. Conversely, with out a sail, the boat would only drift down current. To sail a course, the boat needs both a sail and a keel, as to which is extracting energy and which is outputting energy depends of the point of reference (water, air, or boat), the relative speeds of the three items to each other, and the relative geometrical relationships between the sail and the keel. See the first part of Aero-hydrodynamics of Sailing.

[ThinAirDesigns]

The final answer will always be "it depends".

You appear to be contradicting yourself:

First you say relative to the drawings:
Both situations result in loss of energy from the air to the water and loss of overall energy in the system to entropy

Then you say:
The final answer will always be "it depends.

There's no "depends" in your above answer. Your above answer has the sails in both pictured scenarios acting as the inverse of propulsors and thus acting as turbines.


JB

[ThinAirDesigns]

When a boat is sailing in steady state with still air and moving water, the "keel" is extracting energy from the water and transfering it to the sail to propel the boat in the desired direction, so the sail functions as a "propeller" to you. It uses the air to generate a force in the direction you want to go, like the rudder on an airplane or a the wing of a glider.

In the more common steady state case of still water and moving air , the sail is extracting energy from the air and transfering it to the keel to propel the boat in the desired direction, so the sail functions as a "turbine". It pulls energy from the air, but cannot direct that energy, similar to a kite whos string is cut. To work aganist the wind, a keel is needed.


I not asking you to go into the details JE, but is there someway to instrument a sail to know when it's acting as a prop or a turbine? Could we perhaps 'smoke' it and film the results and know? Some other way perhaps?

JB

[jehardiman]

You appear to be contradicting yourself:

First you say relative to the drawings:

Then you say:

There's no "depends" in your above answer. Your above answer has the sails in both pictured scenarios acting as the inverse of propulsors and thus acting as turbines.

JB

This is a cross-post issue. As you put no current in your drawings, I assumed you wanted still water. In that case my original statement stands. If you accept the possible case of moving water and still air, then it depends.

I not asking you to go into the details JE, but is there someway to instrument a sail to know when it's acting as a prop or a turbine? Could we perhaps 'smoke' it and film the results and know? Some other way perhaps?

JB

I'd have to think about it, but most likely an experimental result would need to be a net vector force determination of both hull/keel and sail which could be difficult. But is that really useful? As I started this discussion off with, wether it is work in or work out is irrelevent to the equations, they are the same in both cases, only the input relationships would vary.

[ThinAirDesigns]

This is a cross-post issue. As you put no current in your drawings, I assumed you wanted still water. In that case my original statement stands. If you accept the possible case of moving water and still air, then it depends.

Ahh, ok. That clears up the contradition. Thanks.

Me:
... is there someway to instrument a sail to know when it's acting as a prop or a turbine?

jehardiman:
I'd have to think about it, but most likely an experimental result would need to be a net vector force determination of both hull/keel and sail which could be difficult. But is that really useful?

Well, if your "moving water and still air" vs "moving air and still water" assertions are true, it would be more useful than you could ever imagine.



I've always believed that through even simple visual means one could tell if a blade is acting as a turbine or prop, but if your above assertion is true, then I'm thinking I must be wrong.

As a test of my above belief, let me try this on you:

Here's another image I created. You can imagine the below blades in one of two scenarios:

1: Top view of a windmill/fan with the axle running the same direction as the wind.

2: Top view of a set of vertical blades with a constraint such that they can only slide to the left and right.

For the purposes of this discussion, either one you pick will work for me.

http://www.mediafire.com/imgbnc.php/13c816524110aa45ab79d245ad6948524g.jpg


If we set the above blades out in the wind per the above drawing, can we agree that:

A: If the blades move to the left in the steady state wind, they are acting as turbine blades and each blade is "removing energy from the air"?

B: If the blades move to the right in the steady state wind, each blade is resisting the force of the wind, is acting as propeller blade and is "increasing the energy in the air"? (this of course requires some other force to be acting on the blades other than the direct force of the wind)

Do you agree?

JB

[spork]

You're apparently misreading the poll and OP question ... it doesn't ask if a sail *is* a turbine or prop. It asks what it *acts as*.

Yes - I've pointed that out to him as well. He doesn't care what the O.P. means nor what it says. He's starting with a conclusion and coming up with bizarre transient cases and such to try and shore up what's left of his losing argument.

perhaps you could use your understanding of this to educate me as to when a sail acts as a prop -- that is leaving a stream of air behind that is accelerated relative to the free stream.

I am plenty familiar with your level of knowledge on the topic that I can assure you he won't be teaching you anything. If he's half as smart as he thinks he is, he may let you teach him something however.

I claimed that I did have the time and inclination to correct his flawed understanding of fluid dynamics, but I now have to confess... I can't possibly keep pace with his rate of making errors.

jehardiman decided to show up and insult me without the first clue as to who I am, what I do, or what level of understanding I have of the subject.

I doubt the discussion you have with him will hold much interest beyond the sociological level. Good luck.

[jehardiman]

Well, if your "moving water and still air" vs "moving air and still water" assertions are true, it would be more useful than you could ever imagine.

Not really, waterwheels have been used for centuries to extract power from moving water (indeed many low head hydroelectric plants use propeller-like turbines (http://www.dmg-intertrade.com/images/kaplan_turbine.jpg)) and the need for large side force to drag to go forward in a conventional sailing craft is well documented. There are even power plants, boats, and toys (www.greenoptimistic.com/category/wave-power/) that use the wave orbital velocity for power generation. There is nothing mystic going on here that hasn't been better explained by half a dozen sailing theory books or other fluid mechanic texts.

I've always believed that through even simple visual means one could tell if a blade is acting as a turbine or prop, but if your above assertion is true, then I'm thinking I must be wrong.

As a test of my above belief, let me try this on you:

Here's another image I created. You can imagine the below blades in one of two scenarios:

1: Top view of a windmill/fan with the axle running the same direction as the wind.

2: Top view of a set of vertical blades with a constraint such that they can only slide to the left and right.

For the purposes of this discussion, either one you pick will work for me.

http://www.mediafire.com/imgbnc.php/13c816524110aa45ab79d245ad6948524g.jpg


If we set the above blades out in the wind per the above drawing, can we agree that:

A: If the blades move to the left in the steady state wind, they are acting as turbine blades and each blade is "removing energy from the air"?

B: If the blades move to the right in the steady state wind, each blade is resisting the force of the wind, is acting as propeller blade and is "increasing the energy in the air"? (this of course requires some other force to be acting on the blades other than the direct force of the wind)

Do you agree?

JB

Ok, assume simple frictionless model with no initial transverse velocity. If you constrain the axis of movement to perpendicular to the wind and transverse forces are only generated wind velocity (i.e. no input forces) then airfoils will always move to the left decreasing the total energy in the flow, because in the absence of any other force in the transverse direction, the foil cannot move to the right and is easy to show on any high-school physics air track with a fan and some simple setup. It is important to note that the foils will not continue to accelerate to the left, they will eventually reach a transverse velocity where the net force on them is all in the direction of V and no work is generated out of the system given the constraint in the flow direction, but the drag of the foil extracts energy from the flow in the form of viscous entropy.

Big whorls have little whorls
That feed on their velocity,
And little whorls have lesser whorls
And so on to viscosity.

-- Lewis F. Richardson

However, if there was a transverse energy gradient such as an EM field, or gravity, then whether it moved left or right would depend on the effect of that field also. Think of your drawing as left up and right down. If the foil has weight, then there would be V below which it would move right (down) putting energy into the air, at exactly V it would not move right or left ("lift" = weight) doing 0 work, and above V it would move left (up) extracting energy from the air. This is just the same a kite on a string.

But this is a simple 2 DOF frictionless example: moving to 3D and 6 DOFs complicates the matter and we must get into multiple constraints.

[jehardiman]

Yes - I've pointed that out to him as well. He doesn't care what the O.P. means nor what it says. He's starting with a conclusion and coming up with bizarre transient cases and such to try and shore up what's left of his losing argument.



I am plenty familiar with your level of knowledge on the topic that I can assure you he won't be teaching you anything. If he's half as smart as he thinks he is, he may let you teach him something however.

I claimed that I did have the time and inclination to correct his flawed understanding of fluid dynamics, but I now have to confess... I can't possibly keep pace with his rate of making errors.

jehardiman decided to show up and insult me without the first clue as to who I am, what I do, or what level of understanding I have of the subject.

I doubt the discussion you have with him will hold much interest beyond the sociological level. Good luck.

If all that JB has learned from this discussion is that opinions you get on the internet are worth what you pay for them, then he has learned a significant thing. The best that I hope for is that he follows my original advice and get a good text and some real design experience and finds his own answer to his poll question.

The problem that we appear to have is that the original 4 lines of this thread allowed us to play in the whole sandbox and not be limited to classical theory and unrealistic constraints. There was nothing to restrict the sceanairos, and as an engineer that deals with some really off the wall hydrodynamic situations, I have to deal with those ragged edges.

If JB wanted to start from the classic steady state, invisicid, irrotational, isodense, static water point then, yes, it is easy to prove that a "sail" in the classic sense always extracts energy from the air and would be a "trubine" as defined. On the other hand, as any sailor that has significant sea time under sail in waves knows, it is obvious that there are situations where the interaction between the rig, the hull , and the environment, cause the rig to impart work into the air, which by the polls defination is a "propeller". And you even agree to that (post #17). Both cases are contrived, so my statement that the poll is foolish stands.

Pity that when the first stone landed in my yard (post #31) and I tossed it back, it reflects poorly on both of us. I am sorry that you feel slighted, but your question about the windmill really was off if you had though about it. Of course no real wind is going to always be constant and depending on the situation AOA will be jumping all around. It would have been interesting to hear your views on some of the process of actually selecting modeling techniques to solve a real world problem. Come on, proposing to use disk theory on a sail (post #17 and 31)? Blade element theory would have been a much better choice, and energy methods an even better one. And the fact of the matter is that the development of the equations of state and the total energy equation are the same all flows (unless you want to dig into some of the weirder behaving fluids), only the limit constraints are different leading to different solutions. You do not reformulate the equations if del P is positive or negative, the math handles that. And steady state never exists in the real world, especially when dealing with two fluids at thier interface, so it would be interesting to see if you are familiar with Beck's and Newman's work of flow in waves these last few decades. The problem exists in all fluid modeling that some assumptions and limiting conditions must be made to allow viable solution. In the real world, only solving the steady state condition is often viable, but sometimes not. The question is to chose 20 hours of Cray time to run a Multi-Vortex solution on a regular body that only gives the loads for steady flow, or will the first principles panel model that handles non-steady flow give a better answer at slow speeds of advance in a seaway. Even RANS has coefficients to "calibrate" itself to the total energy equation that can be argued over given the starting Rn or the presence of large eddies.

FWIW, I've seen lots of discussions like this my job, and some real knock down drag out fights much bitter than this. Opinions about and solutions to hydrodynamic problems are as varied as the individuals who have to solve them. If two people independently solve the same problem and get within ~20% of each other, I have usually considered the problem solved. The problem exists when only one solution is proposed and the assumptions to that solution are flawed.

[spork]

Pity that when the first stone landed in my yard (post #31) and I tossed it back, it reflects poorly on both of us.

Actually, I think if we want to find the first projectile, we'd have to go back just a bit further to your very first post in this thread when you offered: "JB, I would suggest you hit the books more; a good thermo and fluids book with historical perspective will show you just how foolish your poll was."

If you really do think a civil discussion on the topic would be interesting it certainly isn't too late. Unfortunately, my schedule will have me only checking in on occassion in the short term. But I will say this. You've taken the question in entirely the wrong way. It's not intended to ask whether a specific lifting device (i.e. sail, wing, prop, keel, etc.) CAN be employed as a prop or turbine; nor is it intended to investigate transient cases of gusts, turbulence, etc. I also doubt the discussion takes us down the path of Navier-Stokes, Reynolds numbers, inviscid flow, etc. but I'm open minded to that possibility.

I know JB well enough, and have been involved in this discussion long enough, to know that he's asking whether a sail is behaving as the blade of a prop or the blade of a turbine at any given point of sail and steady state condition (e.g. downwind VMG faster than the wind).

[ThinAirDesigns]

Well, if your "moving water and still air" vs "moving air and still water" assertions are true, it would be more useful than you could ever imagine.

Not really, ...

Yes, really.

... waterwheels have been used for centuries to extract power from moving water (indeed many low head hydroelectric plants use propeller-like turbines (http://www.dmg-intertrade.com/images/kaplan_turbine.jpg)) and the need for large side force to drag to go forward in a conventional sailing craft is well documented. There are even power plants, boats, and toys (www.greenoptimistic.com/category/wave-power/) that use the wave orbital velocity for power generation. There is nothing mystic going on here that hasn't been better explained by half a dozen sailing theory books or other fluid mechanic texts.

You're a hard man to keep on track ... I can't tell if that's a "Yes we can." or a "No we can't." as it related to my original question: 'Can we learn from instrumenting or examining a sail somehow whether at any given point it's adding energy to the air, or the inverse?"

When I first asked the question, you responded:
I'd have to think about it, ...

If you're still thinking about it, that's fair -- but it's a more relevent question than you might think and I'd greatly appreciate a "Yes" or "No".

JB

[ThinAirDesigns]

http://www.mediafire.com/imgbnc.php/13c816524110aa45ab79d245ad6948524g.jpg
-

Ok, assume simple frictionless model with no initial transverse velocity. If you constrain the axis of movement to perpendicular to the wind and transverse forces are only generated wind velocity (i.e. no input forces) then airfoils will always move to the left decreasing the total energy in the flow, because in the absence of any other force in the transverse direction, the foil cannot move to the right and is easy to show on any high-school physics air track with a fan and some simple setup.

Perfect, we are in complete agreement.

But this is a simple 2 DOF frictionless example:

That may be the way you imagined it, but it is not necessary to imagine it that way -- If you imagine that drawing with axle constraint as I described it, I have merely given you a quick and reasonable (though not perfect) top view drawing of typical childs pinwheel -- 3D and real world friction on the shaft will not change the result you predict ... the pictured blade will still move to the right for all the reasons you describe.

The entire point of that exercise was to demonstrate that if we were to take a simple child's pinwheel and stick it in front of a fan and those blades were to move to the *right* ... you and I would look at each other and say "WTF???". We would likely procede to then investigate the source and delivery method of the the external force that we both know must be acting against the direct force of the wind.

This simple example leads me to believe that there must be any number of ways (some not a simple as the above to be sure) to actually determine whether an airfoil is being used to extract or add energy to the air.

But this is just my current conclusion ... I still very interested in yours.

JB

[jehardiman]

You're a hard man to keep on track ... I can't tell if that's a "Yes we can." or a "No we can't." as it related to my original question: 'Can we learn from instrumenting or examining a sail somehow whether at any given point it's adding energy to the air, or the inverse?"

When I first asked the question, you responded:


If you're still thinking about it, that's fair -- but it's a more relevent question than you might think and I'd greatly appreciate a "Yes" or "No".

JB

"Can we learn...?" <shrug> That all depends. I've taken reams of hydro data over the years that shows scatter so large as to make even generalizations suspect. Often, the harder you try to measure a specific item, the less "real" the value becomes because of testing constraints.

FWIW, I think it is possible, maybe feasible, to devise a test configuration to determine if a sail rig is work in or work out, but I doubt that any useful data can be extracted full scale. The sensor technology is not mature enough.

Also, you understand the engineering differences between possible, feasible, and practical? Possible means that there is a valid operational theory and no violation of natural laws prohibiting it, but major technical development needs to be accomplished before you can start the project. Feasible means that with enough time and money you could develop and build everything you need from existing technology. Practical means I can right now buy or build all the components I need to complete the project.

[jehardiman]

That may be the way you imagined it, but it is not necessary to imagine it that way -- If you imagine that drawing with axle constraint as I described it, I have merely given you a quick and reasonable (though not perfect) top view drawing of typical childs pinwheel -- 3D and real world friction on the shaft will not change the result you predict ... the pictured blade will still move to the right for all the reasons you describe.

The entire point of that exercise was to demonstrate that if we were to take a simple child's pinwheel and stick it in front of a fan and those blades were to move to the *right* ... you and I would look at each other and say "WTF???". We would likely procede to then investigate the source and delivery method of the the external force that we both know must be acting against the direct force of the wind.

This simple example leads me to believe that there must be any number of ways (some not a simple as the above to be sure) to actually determine whether an airfoil is being used to extract or add energy to the air.

But this is just my current conclusion ... I still very interested in yours.

JB

As for 2D modeling vs 6 DOF testing, do not underestimate the effects of roll and pitch on real experimental foils. Look at a fairly moderate pitch or roll motion of +/- 5 degrees over 10 sec. Maximum angular velocity will be +/- .11 rad/sec so del V would be +/- .11R or ~ +/- 1m sec for your average 25 footer masthead and stiffer boats are much worse. Much of the data you would be trying to collect will be lost in the noise.

[spork]

Also, you understand the engineering differences between possible, feasible, and practical? Possible means that there is a valid operational theory and no violation of natural laws prohibiting it, but major technical development needs to be accomplished before you can start the project. Feasible means that with enough time and money you could develop and build everything you need from existing technology. Practical means I can right now buy or build all the components I need to complete the project.


I've been a genuine aerospace engineer for a lot of years and I'd have to disagree with your "engineering definitions". While I disagree with all three definitions, I'll just take one for example. You say:

"Practical means I can right now buy or build all the components I need to complete the project"

I could talk to you all day long about things we could do in my current line of work with off-the-shelf technology that still couldn't in any way be considered practical for any number of reasons (cost, size, logistics...).

As to the other business, you're really getting bogged down in questions of how you would take practical measurements, flow anomalies, etc. That is definitely not the point of this question. I think we're all very well aware that measurement and quanitification of real-world tests can be challenging to say the least; but that doesn't mean all aero questions are defined by that challenge.

[chabrenas]

I used to propell my 26' sail boat at about 2 knots in 0 wind by sallying her with the main sheeted well home.

... and several of us managed to go fast enough for the self-bailers to work on our Lasers in zero wind - standing on the edges of the cockpit, kicker slacked off, mainsheet in one hand and tiller in the other. Just as well, because it was an evening race, the finish line was a mile and a half away, and the rescue boat couldn't have towed all of us at once.

I also witnessed a dinghy race on the Upper Thames which was won by the helmsman who executed the most efficient and most frequent gybes...

I built a model ornithopter once. I guess its wings act as propellers on both up and down strokes. But that was before I'd even heard of differential equations.

I'm rambling off topic, but I'm in that kind of mood. Forgive an old man.

[chabrenas]

JB: One or two of JEH's observations and Spork's interpretations suggest that I may have misunderstood the original poll question, and I may not be alone.

Perhaps the results reflect how our preconceived notions have caused us to interpret it. I'm beginning to suspect that I know your real goal in setting up the poll - if you tell us, it may change the results.

Hopefully, it would make them more useful, but that's not a foregone conclusion.

[jehardiman]

I've been a genuine aerospace engineer for a lot of years and I'd have to disagree with your "engineering definitions". While I disagree with all three definitions, I'll just take one for example. You say:

"Practical means I can right now buy or build all the components I need to complete the project"

I could talk to you all day long about things we could do in my current line of work with off-the-shelf technology that still couldn't in any way be considered practical for any number of reasons (cost, size, logistics...).

As to the other business, you're really getting bogged down in questions of how you would take practical measurements, flow anomalies, etc. That is definitely not the point of this question. I think we're all very well aware that measurement and quanitification of real-world tests can be challenging to say the least; but that doesn't mean all aero questions are defined by that challenge.

Meh.. Just how I think and use the words, you are free to insert your own if other issues drive your definitions.

But don't you agree that...

With enough time and money, and a few tech developments, it is possible to build a fusion powered ion drive starcraft.

Within the foreseeable future and within funding limits it is feasible to build a manned Mars mission spacecraft.

Today it is practical to launch a LIDAR satellite to measure ~meter grid winds.

It is feasable to mount cm grid scale LASER air velocity meter on a rig instrumented sailboat within a realistic budget.

Given the history of technology development, and a few tech developments and tons of cash, it is possible to develop a realitime measurement of a km scale volume to a sub-mm grid scale that is portable enough to be mounted on a rig instrumented sailboat.

It has been my experience (NAME PE for the US Navy Deep Submergence for 25 years) that areospace engineers, based upon the ones I've worked with (and there is a fair amount of crossover given what companies work in deep submergence), have always over-relied on constrained analysis and have under-estimated the effects of the marine environment. I just roll my eyes every time the Navy is delivered another system that never considered wave slap or bolts Al components up with Ti (just read the whole sorry saga of ASDS). It is not a wholesale general condemnation, but I do believe it is an effect of the enviromental conditions and restraints that, in their experiences, they work under. FWIW, a deep submergence vehicle/submarine is effectively equalevent to a launch capsule/shuttle except that they have to work in a much harsher environment. The US Navy has done such a good job of managing the risks with these complex systems, that NASA is looking at copying our SUBSAFE/DSS-SOC program for future manned development.

[spork]

But don't you agree that...

With enough time and money, and a few tech developments, it is possible to build a fusion powered ion drive starcraft.

I wouldn't take the contract to deliver on that. We've felt controlled fusion power was 10 to 20 years off for over 40 years now. I hope we get there - and soon. But I wouldn't begin to predict it.

It has been my experience (NAME PE for the US Navy Deep Submergence for 25 years) that areospace engineers, based upon the ones I've worked with (and there is a fair amount of crossover given what companies work in deep submergence), have always over-relied on constrained analysis and have under-estimated the effects of the marine environment. I just roll my eyes every time the Navy is delivered another system that never considered wave slap or bolts Al components up with Ti...

Your disdain for aerospace engineers aside, I can't begin to tell you how far off in left field you are on this question. This is NOT a question about practical engineering methods. The question relates to the definition of what is a propeller and what is a turbine, and whether a sail acts as one or the other under various steady state conditions. It seems you are simply unwilling to venture an answer to that, and instead feel you have to tell us how complex the business of naval engineering really is in the trenches.

[jehardiman]

This is NOT a question about practical engineering methods. The question relates to the definition of what is a propeller and what is a turbine, and whether a sail acts as one or the other under various steady state conditions. It seems you are simply unwilling to venture an answer to that.

I guess we will have to agree to disagree.

In actual practice, there is never any true steady state condition, especially in a marine environment where a "sail" functions. Therefor, any steady state model is a contrived condition. I can therefor contrive any set of initial assumptions I want as long as they are steady state, as long as that is the only limitation. I have pointed out that depending on the initial assumptions, a "sail", in the marine sense of mounted on a vessel floating in the water with 6 DOFs, can either extract energy or input energy to the air flow dending on the relationships between the air and water flows and vessel/sail shape/trim. As a Naval Architect I choose not to ingore the effect of the water as those effects are my stock in trade.

FWIW, I did answer JB's question about the "sail", but there are a lot of other limitations beyond "steady state" buried in that answer. One should ever be wary of contrived models, otherwise Winston Churchill will be turned into a carrot.

[ThinAirDesigns]

jehardiman, I only have a few moments, but remember that traditional sailing rigs are used on land as well as water. Afrozen lake surface won't induce pitching and rolling after all.

Please, please stop trying to complicate a simple question. If you don't know what the term "steady state" means, do as you have suggested of me and go look it up a textbook.

Once covered by the "steady state" control that I have asserted, all of your flapping and pitching and rolling and, and , and ... appears to be nothing more than obfuscation of the fact that you can't or don't wish to answer the straigtforward question:

Can you know by physical examination of an airfoil if it is acting as a prop or a turbine? Can you know if it is being used to remove or add energy to the air?

A "Yes" or "No" would suffice -- I don't expect you to detail the method for me unless you wish to.

JB

[jehardiman]

jehardiman, I only have a few moments, but remember that traditional sailing rigs are used on land as well as water. Afrozen lake surface won't induce pitching and rolling after all.

Please, please stop trying to complicate a simple question. If you don't know what the term "steady state" means, do as you have suggested of me and go look it up a textbook.

Once covered by the "steady state" control that I have asserted, all of your flapping and pitching and rolling and, and , and ... appears to be nothing more than obfuscation of the fact that you can't or don't wish to answer the straigtforward question:

Can you know by physical examination of an airfoil if it is acting as a prop or a turbine? Can you know if it is being used to remove or add energy to the air?

A "Yes" or "No" would suffice -- I don't expect you to detail the method for me unless you wish to.

JB

If I am guilty of obfuscation, then you are guilty of an unreasoning desire for an unqualified single answer when none exists in the way you wish to have the question framed.

I have already answered both your questions (Can you know by physical examination of an airfoil if it is acting as a prop or a turbine? Can you know if it is being used to remove or add energy to the air?) several times and the answer has always been yes to both, but only over the limited scope of your frame of reference and the limiting conditions of initial flows and measurement ability. (FWIW if you really want to get picky, the answer to the first question is NO if you restrict "physical examination of an airfoil" to the airfoil itself because it is a internally static situation. Either flow or the airfoils movement in the reference frame, both external to the foil itself, must be measured to determine any work flow....anyway.)

We should not continue to take up band width if we cannot agree on the point of whether I answered your question. From my point of view I did, and the fact that it is not an answer you are satisified with is an issue that is not mine to resolve.

Fluid dynamics rarely gives an unqualified answer. I wish you well in your journey where ever it may take you.

[ThinAirDesigns]

Thanks je (I think).

One quick question if you don't mind:

You have asserted clearly that if the water is moving and the air is still, the sail acts as a prop and the keel a turbine. You have asserted that if the air is moving and the water is still, the sail acts as a turbine and the keel a prop.

Which is given which responsibility if the water is moving North at 5 knots and the air is moving South at 5 knots? Who decides?

JB

[jehardiman]

Which is given which responsibility of the water is moving North at 5 knots and the air is moving South at 5 knots? Who decides?

JB

As I stated before, it will depend upon having the frame of reference (external to the foils) and dependent of the geometry of the flow and foils.

I think you will agree that if I have a "keel" of 100m^2 and a "sail" of 0.1m^2 both of normal L/D and a 5 knt north current verctor with a 5knt south wind vector that the boat would be hard pressed to sail "up current" and will have significantly more than a 5knt wind over the sail. Conversely, if the foils sizes are reversed, you may be able to make headway against the current (remembering that seawater is ~800 times denser than air so with a planform difference of only 1000 and losses, that is not a given). The issue here is wether the water or the air is producing the net force in the direction of travel on the vessel.

Remember that work and energy are scalers, if an external force is not moving in the frame of reference, no work is done though the energy may change from useful energy to non-useful energy. A free falling brick in a vacuum does no work, but rather transfers potential energy into kenetic energy without (..simplisticaly..) changing it's total energy.

Here then is the question for you to ponder: in steady state conditions, does the air flow do any work on a "airfoil" rig rigidly fixed to the wall in a wind tunnel? Does the foil do any work on the air? Conversly, in steady state conditions does a "hydrofoil" do any work on the water when towed in a still water towing tank?

[spork]

He doesn't understand the question JB.