Return to Winged Keels

weightless

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 I did like the scene with the balloon, which had sails to propel it.....  that must be from the Jon Favreau universe of physics. Not this one. 
How tall was the balloon? ;)

image.png

 

DDW

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Yeah, not that tall. 

There is a Gedanken experiment where you float a balloon over water, and stick a foil - like a centerboard - down in it. Doing so (in theory) you can float faster than the wind, just as you can drift faster than the current on a zero wind day in a sailboat. 

 

weightless

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Not sure water is needed. For instance, there's a wind gradient between 600 and 500 mb on that sounding that looks workable. Fair sailing if you can get foils between the winds, don't mind the rarefied air, potential for high speed jetliners, a wee chill and are, for some ineffable reason, motivated to give it a try, perhaps?

 

DDW

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It is also referred to a couple of times as a "hot air balloon" which it clearly is not. 

But yeah, anytime two fluids are moving relatively, energy can be extracted. 

 

pschwenn

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Only from a distant point of view. Every inch of that wing is making lift, but the forces are resolved within the structure so the net lift on the whole is zero. Make that ring out of something crushable, and it will crush itself, then you won't say there's no lift. 

Weightless above has the generally accepted definitions of lift and drag. You can make up your own definitions, but then no one will know what you are talking about. "Lift" and "drag" are simply the overall force vector resolved into two equivalent vectors, which are easy to measure and convenient to use in the design of real things. 
DDW,

I'm not saying that there is only drag, or that there is no lift; but that components of the overall forces don't have inherent properties of lift, or drag.

Before resolving the overall force vector into lift and drag, you need to know (and you usually do) what the intent is in the situation you're looking at, what you want to happen.  So for example, a computer program cannot resolve lift and drag without information about the desired direction(s) for one or the other.

regards

 

DDW

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Intent has nothing to do with it. No matter what you intend, there is exactly one resultant vector due to the physics. Resolving it into Lift and Drag, or Drive and Heeling vectors as sometimes done in sailing is a completely arbitrary choice. There is no intent, or information, or moral judgement contained therein. To aid communication, and make the math easier, we have (otherwise arbitrarily) defined Lift and Drag to be what it is. 

 

weightless

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components of the overall forces don't have inherent properties of lift, or drag.
It is just a convention but lift and drag are unambiguously defined. To be sure results from the definition don't always answer the question that is being asked. Knowing the question that is being asked is an important step in answering it and intentionality is usually involved in formulating the question.

If you want to know how much a body in a flow at some arbitrary orientation is preventing leeway or holding an airplane up against gravity then knowing the lift and drag of that body might not answer the question. Regardless of how useful they are to the task at hand the body has properties that by convention are called lift and drag.

 
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pschwenn

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It is just a convention but lift and drag are unambiguously defined. To be sure results from the definition don't always answer the question that is being asked. Knowing the question that is being asked is an important step in answering it and intentionality is usually involved in formulating the question.

If you want to know how much a body in a flow at some arbitrary orientation is preventing leeway or holding an airplane up against gravity then knowing the lift and drag of that body might not answer the question. Regardless of how useful they are to the task at hand the body has properties that by convention are called lift and drag.
Weightless,

They are usually unambiguously for us, but that unambiguous definition depends on the intent of the vehicle and on the situation, which a formal system like a computer program cannot determine by itself however unambiguous the definition is.  When the intent or requirement changes, what part of the overall force is lift or drag may change, even when nothing in the physical situation changed.

I'm not trying to question anyone's determination or measurement of lift, or whether lift exists; but that the force you're looking at might not be lift in a different situation, a force is not inherently lift or drag, which we identify usually without any difficulty or conscious attention.  This can make a difference it takes attention away from the other forces in the whole - e.g. away from the drag(s) of a trim tab or winglet.

regards,

 

weightless

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When the intent or requirement changes, what part of the overall force is lift or drag may change, even when nothing in the physical situation changed.
No, the usually agreed on definitions of lift and drag do not change when the requirements change.

 

DDW

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This is why I use Lift instead of lift and Drag instead of drag. In lay terms they might mean anything. In fluid dynamics they mean a very specific thing. Regardless of intent or requirements.

 

pschwenn

New member
This is why I use Lift instead of lift and Drag instead of drag. In lay terms they might mean anything. In fluid dynamics they mean a very specific thing. Regardless of intent or requirements.
DDW,

The 1/4 chord line convention and the associated NA convention for the Lift and Drag vectors were introduced to simplify the description of the real force distributions.  They are conventions without a physical basis other than the foil geometry.  I don't think we're back and forth about the underlying forces, but about the naming. 

For example if a tack is sudden, your Lift soon changes sides of the keel, because the AOA has been reversed, by the flow direction due to the speed of rotation, before head-to-wind, and that Lift is pushing you away from your next mark, and possibly torqueing the boat opposite to your tack.  In a smooth tack maintaining momentum and the keel AOA smoothly reversing, that Lift will go to zero and reverse but at or after you pass head-to-wind. The convention definition of Lift naming may obscure what's resulting from the underlying forces (a slower mark rounding).  But it's a naming thing, not an argument about what is happening at the keel.

regards,

 
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DDW

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Yes, it is a naming thing. Some of the things you mention contribute to why some use Drive and Heeling vectors instead of Lift and Drag, when discussing sailing physics (Marchaj as an example). Lift and Drag are convenient vectors on aircraft and in wind tunnels, less convenient on a sailboat, where the Lift of the keel is in almost the opposite direction as the Lift of the sail, nor do the Drag vectors align being typically at right angles. 

 

weightless

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Lift and Drag ... are conventions without a physical basis other than the foil geometry. 
If I understand what you're getting at I don't think that it's quite right. There is some net force on the object or part of it that is of interest from the fluid flow and lift and drag are a way of representing that force in terms that can be useful. Thinking of sailboats, particularly ones that are doing things other than sailing in a steady state, probably confuses the issue. I wanted to draw a diagram but all I've got on this computer for drawing is paint and I don't know how to use it. Apologies then for the horror of it but here's what I got out of paint:

image.png

It's supposed to show a kind of idealized boat with just the keel and sail. The lift and drag components of the the force vector are shown for the sail. In 3d space the plane that they end up on and the orientation of the axes is probably not convenient for answering a lot of the questions. The keel forces work out in a different place and orientation. Chances are that at the very least some transformations, projections and thoughts about moments are gonna go into getting useful answers. However, useful or not, the lift and drag of the bits are well defined and represent real forces.

 
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pschwenn

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Weightless,

(I promise never to post on this again).

I would agree with you that on an individual sail or keel or any more or less wing-like part, the conventional definition of lift and drag may be useful at some level.  But can obscure what resolutions to make of the net force into forces that matter to your purpose.  Even "net force", truly unambiguous in definition, is not necessarily helpful, and it does not suggest any particular lift/drag convention. 

I would also agree that if you don't accept or think in terms of Circulation for the wing-like parts, you may have to depend on the convention.  Because then lift and drag for wing-like parts are inherently different, not part of a single phenomenon in which lift and drag are not naturally distinct.

And agreed there are good alternative ways of looking at foil dynamics - for calculation and simplicity.

(Good place to skip to "regards," below.)

But they don't include flow bouncing off the windward face (a favorite of many physicists).  Nor flow has further to go (so faster, lower pressure) on leeward side [keels are symmetric as are wings for acrobatics]. Nor faster from a tube based Bernoulli effect [no tube].  Nor any "faster, lower pressure" explanation in which, at the trailing edge the flow continues aft without heavy spin (otherwise some air or water has gone missing.)

The hull, strakes, superstructure, propeller, ... as parts; and buckling, crushing, significant bending, ... for force study, don't fall happily into the convention. The choice of the 1/4 chord and perpendicularity to the a chord-line (or parallel/perpendicular to the free-stream), for the frame of the convention, is somewhat arbitrary (foil shapes are so diverse that neither the chord-line nor the quarter-chord have any natural link to any notion of lift (and if the free-stream is not level with respect to gravity, it may lift you into terrain - but again a convention that may be useful and convenient.)

The British aeronautics design authority got through WWII without it (the discoverer was a Brit, studied long before by the Germans, from the wrong Public-School) but at great cost (the waste of time, effort and aluminum on the Spitfire, puzzlement over swept wings and other German aeronautic innovations, ...)  Without radar, the doubling of RR Merlin engine power (saved the P-51 also with a laminar-flow wing that couldn't), early warning organization and Goering's alcohol and lying, the Battle of Britain would probably have been lost.  Once one starts to analyze with Circulation in mind, the hard things, the contentious things, start to fall into place, like why an elliptical wing will rarely have an elliptical lift pattern (non-laminar flow) and why a rectangular wing might have it (about a particular AOA).

If you need to see it to believe it, watch it initiate and develop in a flow chamber, visualized with smoke or other.

regards,

 
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weightless

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Because then lift and drag for wing-like parts are inherently different, not part of a single phenomenon in which lift and drag are not naturally distinct.
I don't think so. You could measure the lift and drag of a crumpled up bit of paper in a wind tunnel, add them up and determine the total force. There's nothing particular to wingness about it.

(I promise never to post on this again).


That's one more reason not to focus on them.
That was quick ;)

 
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pschwenn

New member
I don't think so. You could measure the lift and drag of a crumpled up bit of paper in a wind tunnel, add them up and determine the total force. There's nothing particular to wingness about it.

That was quick ;)
It would be unusual to be able to directly measure what you would lik         {oops]

 
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