centroid was Re: micro lugger!
> From about 1939 to the 1970s, no one1) I read somewhere that NASA did more research on fabric foils
> looked at it.
looking into paraglider-like alternates to parachutes for returning
space craft than all the previous research on the topic combined.
2) YOU may not have negative pressure on your sails, but I have
backwind just aft the luff of the main.
3) With respect to centers of pressure that are completely off the
foil: I think this is more a matter of engineers wanting to continue
using a simple formula rather than moving to a more complex one.
Obviously, all the forces have to be acting on the foil, not on some
spot of clear air. However, there might well be multiple forces
acting on the foil that resolve to a single vector acting at a point
that is not on the foil.
Peter
But thanks anyway Roger - I really enjoyed your
so-called 'disjointed' ramble. I'm just a lurker, but
I like to follow these threads...to read of others'
gentle cynicism re the 'experts'..
Andrew Wallace.
--- Roger Derby <derbyrm@...> wrote:
http://au.movies.yahoo.com
so-called 'disjointed' ramble. I'm just a lurker, but
I like to follow these threads...to read of others'
gentle cynicism re the 'experts'..
Andrew Wallace.
--- Roger Derby <derbyrm@...> wrote:
> A popular myth anyway. Who knows the basis, but it......Somewhere I have a treatise on hull design from
> stems from the 1930s so
> ... Maybe a bit after the time when it was
> recommended that the Patent
> Office be closed because everything had been
> invented.
> ...I like the explanation
> that the bee and mosquito are really climbing around
> in molasses rather than
> "flying" like a man-sized aircraft.
> .....but only a
> jib without roller furling can have a well defined
> leading edge, and
> typically those have the same scrawny edge that
> makes the Wright Flyer such a bear to control.
> about 1700 which devotesFind local movie times and trailers on Yahoo! Movies.
> excruciating amounts of space to the laying out of
> ellipses, circular arcs,
> etc. when defining the sections of a large sailing
> ship. All futile, but
> wonderful for convincing the unwashed that those
> naval architects were the
> experts and should be paid high wages.
>
> A disjointed ramble, sorry.
>
> Roger
>derbyrm@...
> derbyrm.mystarband.net/default.htm
http://au.movies.yahoo.com
A popular myth anyway. Who knows the basis, but it stems from the 1930s so
... Maybe a bit after the time when it was recommended that the Patent
Office be closed because everything had been invented. Yes, the problem was
a lack of familiarity with Reynold's number effects. I like the explanation
that the bee and mosquito are really climbing around in molasses rather than
"flying" like a man-sized aircraft.
One of the problems with applying airfoil research (doing one's homework) is
that a lot of it just doesn't apply. Most aircraft foils have regions of
negative pressure. Sails don't. The fabric moves to eliminate it. The
leading edge is critical for most foils to keep flow attached, but only a
jib without roller furling can have a well defined leading edge, and
typically those have the same scrawny edge that makes the Wright Flyer such
a bear to control. Aircraft airfoil just don't do well if the flow detaches
near the leading edge. I suspect that most sails have attached turbulent
flow rather than laminar flow in that first critical 25%.
John Ronz was an interesting fellow to talk to. He did lots of airfoil
research in the 1980s with significant improvements since the
"professionals" were all addressing the higher speed regimes and no one was
working on the sub 200 mph area. From about 1939 to the 1970s, no one
looked at it. I doubt many are working on the five or ten knot region even
now. It takes lots of expensive computer time to do a proper CFD analysis
and I'm not convinced they've got turbulent flow well modeled. (John had
built his own "super computer" by lashing together many PC motherboards
spread out across the kind of folding tables one finds at the local church
picnic. He then wrote some very impressive software for it/them.) His was
the original airfoil for the VariEze canard that got in trouble with the
bugs you mentioned, and with rain.
Somewhere I have a treatise on hull design from about 1700 which devotes
excruciating amounts of space to the laying out of ellipses, circular arcs,
etc. when defining the sections of a large sailing ship. All futile, but
wonderful for convincing the unwashed that those naval architects were the
experts and should be paid high wages.
I like Annie Hill's cartoon showing a rowboat with rags flying and the
caption "How bad does a sail have to be before it won't work at all?" She
and Pete put many thousands of miles on Badger with its junk rig.
A disjointed ramble, sorry.
Roger
derbyrm@...
derbyrm.mystarband.net/default.htm
... Maybe a bit after the time when it was recommended that the Patent
Office be closed because everything had been invented. Yes, the problem was
a lack of familiarity with Reynold's number effects. I like the explanation
that the bee and mosquito are really climbing around in molasses rather than
"flying" like a man-sized aircraft.
One of the problems with applying airfoil research (doing one's homework) is
that a lot of it just doesn't apply. Most aircraft foils have regions of
negative pressure. Sails don't. The fabric moves to eliminate it. The
leading edge is critical for most foils to keep flow attached, but only a
jib without roller furling can have a well defined leading edge, and
typically those have the same scrawny edge that makes the Wright Flyer such
a bear to control. Aircraft airfoil just don't do well if the flow detaches
near the leading edge. I suspect that most sails have attached turbulent
flow rather than laminar flow in that first critical 25%.
John Ronz was an interesting fellow to talk to. He did lots of airfoil
research in the 1980s with significant improvements since the
"professionals" were all addressing the higher speed regimes and no one was
working on the sub 200 mph area. From about 1939 to the 1970s, no one
looked at it. I doubt many are working on the five or ten knot region even
now. It takes lots of expensive computer time to do a proper CFD analysis
and I'm not convinced they've got turbulent flow well modeled. (John had
built his own "super computer" by lashing together many PC motherboards
spread out across the kind of folding tables one finds at the local church
picnic. He then wrote some very impressive software for it/them.) His was
the original airfoil for the VariEze canard that got in trouble with the
bugs you mentioned, and with rain.
Somewhere I have a treatise on hull design from about 1700 which devotes
excruciating amounts of space to the laying out of ellipses, circular arcs,
etc. when defining the sections of a large sailing ship. All futile, but
wonderful for convincing the unwashed that those naval architects were the
experts and should be paid high wages.
I like Annie Hill's cartoon showing a rowboat with rags flying and the
caption "How bad does a sail have to be before it won't work at all?" She
and Pete put many thousands of miles on Badger with its junk rig.
A disjointed ramble, sorry.
Roger
derbyrm@...
derbyrm.mystarband.net/default.htm
----- Original Message -----
From: "Lincoln Ross" <lincolnr@...>
> I'll bet that that's a myth. I'll bet that what really happened is that
> they proved the aero theory didn't apply well in cases with really low
> Reynold's number and unsteady flow, and some "journalist" distorted the
> results. Most times a prof, or at least the one I respected most, put
> this sort of thing in front of me, it would be preceded by 5 or 10
> qualifying statements. For anyone who doesn't want to do their homework
> and achieve a sufficient understanding of what's going on, or if there
> are lots of unknown variables, I suppose a SWAG is a better way to go.
> However, something I've noticed is that if people start with a bogus
> theory they will only notice those parts of reality which support it,
> and they will hammer the rest to fit.
>
> Sails are well within the Reynold's number of the usual theory (though
> that theory is now being extended to cover bugs) , but I suppose the
> shapes are not well determined, and then part of it is probably commonly
> stalled, particularly with triangular sails.
>
> >Roger Derby wrote:
> >Just remember that these are the same aero-heads that proved a bumble bee
> >couldn't fly.
> >
> >Take a SWAG and then test, test, test.
> >
> >Roger (I like vector calculus. Divergence and curl are fun things, but
then
> >there is chaos.)
I'll bet that that's a myth. I'll bet that what really happened is that
they proved the aero theory didn't apply well in cases with really low
Reynold's number and unsteady flow, and some "journalist" distorted the
results. Most times a prof, or at least the one I respected most, put
this sort of thing in front of me, it would be preceded by 5 or 10
qualifying statements. For anyone who doesn't want to do their homework
and achieve a sufficient understanding of what's going on, or if there
are lots of unknown variables, I suppose a SWAG is a better way to go.
However, something I've noticed is that if people start with a bogus
theory they will only notice those parts of reality which support it,
and they will hammer the rest to fit.
Sails are well within the Reynold's number of the usual theory (though
that theory is now being extended to cover bugs) , but I suppose the
shapes are not well determined, and then part of it is probably commonly
stalled, particularly with triangular sails.
they proved the aero theory didn't apply well in cases with really low
Reynold's number and unsteady flow, and some "journalist" distorted the
results. Most times a prof, or at least the one I respected most, put
this sort of thing in front of me, it would be preceded by 5 or 10
qualifying statements. For anyone who doesn't want to do their homework
and achieve a sufficient understanding of what's going on, or if there
are lots of unknown variables, I suppose a SWAG is a better way to go.
However, something I've noticed is that if people start with a bogus
theory they will only notice those parts of reality which support it,
and they will hammer the rest to fit.
Sails are well within the Reynold's number of the usual theory (though
that theory is now being extended to cover bugs) , but I suppose the
shapes are not well determined, and then part of it is probably commonly
stalled, particularly with triangular sails.
>Roger Derby wrote:
>Just remember that these are the same aero-heads that proved a bumble bee
>couldn't fly.
>
>Take a SWAG and then test, test, test.
>
>Roger (I like vector calculus. Divergence and curl are fun things, but then
>there is chaos.)
>derbyrm@...
>derbyrm.mystarband.net/default.htm
>
Modern treatments use aerodynamic center and pitching moment (in our
case more like a twisting moment, but this theory is oriented toward
airplanes). THis is done because in some cases you can actually end up
with a "center of pressure" that is an infinite distance behind the foil
(sail, bedsheet, whatever), although I'm not sure this can happen with a
sail, since it might collapse. In other words, you can have twist with
no lift. The aerodynamic center is at the quarter chord point, i.e. sort
of like a centroid biased halfway toward the leading edge. Highly
cambered shapes, such as sails have, have a high pitching moment. I
guess it gets complicated enough that the sail centroid is a reasonable
approximation, but it's only an approximation. If you set up your sail
with a lot of camber in the front and not much in the back, it will
twist less and act at a more forward point than if you set it up with a
lot of camber (curve, more or less) in the back. If you have a totally
symmetrical shape the forces will act at that quarter chord point. In
the case of rigid "sails" of known shape, such as in some of those c
cats, there is probably no excuse for absolutely nailing the placement
right off, unless the hull shape is doing weird things.
case more like a twisting moment, but this theory is oriented toward
airplanes). THis is done because in some cases you can actually end up
with a "center of pressure" that is an infinite distance behind the foil
(sail, bedsheet, whatever), although I'm not sure this can happen with a
sail, since it might collapse. In other words, you can have twist with
no lift. The aerodynamic center is at the quarter chord point, i.e. sort
of like a centroid biased halfway toward the leading edge. Highly
cambered shapes, such as sails have, have a high pitching moment. I
guess it gets complicated enough that the sail centroid is a reasonable
approximation, but it's only an approximation. If you set up your sail
with a lot of camber in the front and not much in the back, it will
twist less and act at a more forward point than if you set it up with a
lot of camber (curve, more or less) in the back. If you have a totally
symmetrical shape the forces will act at that quarter chord point. In
the case of rigid "sails" of known shape, such as in some of those c
cats, there is probably no excuse for absolutely nailing the placement
right off, unless the hull shape is doing weird things.
>pvanderwaart and someone else wrote;
>
>
>>> So are you saying even though the centriod of the sails are
>>> the same the thrust is in a diff. location? Please explain.
>>
>>
>
>Yes, that is what I am saying. The book I have on hand
>(http://www.duckworksmagazine.com/s/reviews/curry/index.cfm)dates
>from 1925 or so, but it was based on experiment. Curry, page 41: The
>Center of Pressure. With the changes in relation of length to
>breadth of a plane, not only do the pressure and its distribution
>change, but the center of pressure does as well, shifting either
>toward the fore edge, that is opposed to the wind, or toward the
>rear edge, where the wind passes off the plane.
>
>page 42: These curves show that the center of pressure on the high
>sail for the small angles of opposition - that concern us on a beat
>to the windward - lies farther forward, nearer its luff, than on the
>low, gaff rigged sail.
>
>The designer doesn't know where the center of pressure (thrust) is
>since the actual center of pressure changes depending on the precise
>trim of the sail. He is positioning the sail by rule of thumb based
>on the centroid. Many designs from major designers have their rig
>adjusted after trials of a prototype.
>
>Peter
>
Just remember that these are the same aero-heads that proved a bumble bee
couldn't fly.
Take a SWAG and then test, test, test.
Roger (I like vector calculus. Divergence and curl are fun things, but then
there is chaos.)
derbyrm@...
derbyrm.mystarband.net/default.htm
couldn't fly.
Take a SWAG and then test, test, test.
Roger (I like vector calculus. Divergence and curl are fun things, but then
there is chaos.)
derbyrm@...
derbyrm.mystarband.net/default.htm
----- Original Message -----
From: "Lincoln Ross" <lincolnr@...>
> Modern treatments use aerodynamic center and pitching moment (in our
> case more like a twisting moment, but this theory is oriented toward
> airplanes). THis is done because in some cases you can actually end up
> with a "center of pressure" that is an infinite distance behind the foil
> (sail, bedsheet, whatever), although I'm not sure this can happen with a
> sail, since it might collapse. In other words, you can have twist with
> no lift. The aerodynamic center is at the quarter chord point, i.e. sort
> of like a centroid biased halfway toward the leading edge. Highly
> cambered shapes, such as sails have, have a high pitching moment. I
> guess it gets complicated enough that the sail centroid is a reasonable
> approximation, but it's only an approximation. If you set up your sail
> with a lot of camber in the front and not much in the back, it will
> twist less and act at a more forward point than if you set it up with a
> lot of camber (curve, more or less) in the back. If you have a totally
> symmetrical shape the forces will act at that quarter chord point. In
> the case of rigid "sails" of known shape, such as in some of those c
> cats, there is probably no excuse for absolutely nailing the placement
> right off, unless the hull shape is doing weird things.
>
> >pvanderwaart and someone else wrote;
> >
> >
> >>> So are you saying even though the centriod of the sails are
> >>> the same the thrust is in a diff. location? Please explain.
> >>
> >>
> >
> >Yes, that is what I am saying. The book I have on hand
> >(http://www.duckworksmagazine.com/s/reviews/curry/index.cfm)dates
> >from 1925 or so, but it was based on experiment. Curry, page 41: The
> >Center of Pressure. With the changes in relation of length to
> >breadth of a plane, not only do the pressure and its distribution
> >change, but the center of pressure does as well, shifting either
> >toward the fore edge, that is opposed to the wind, or toward the
> >rear edge, where the wind passes off the plane.
> >
> >page 42: These curves show that the center of pressure on the high
> >sail for the small angles of opposition - that concern us on a beat
> >to the windward - lies farther forward, nearer its luff, than on the
> >low, gaff rigged sail.
> >
> >The designer doesn't know where the center of pressure (thrust) is
> >since the actual center of pressure changes depending on the precise
> >trim of the sail. He is positioning the sail by rule of thumb based
> >on the centroid. Many designs from major designers have their rig
> >adjusted after trials of a prototype.