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Old 11-02-2013, 11:49   #31
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Re: Settle an argument

A wing works because of a pressure differential between the two surfaces. The more time the air flow has to change direction keeping the laminar flow, the greater that pressure differential will be. If the air hits a sudden 15 degree turn at the leading edge, then there is greater turbulence than if the air hits a 2 degree change at the leading edge. A curve allows for a more gradual change in the direction of the air flow. A more gradual change means less turbulence and more laminar flow.

A flat sail will work but the pressure differential will not be as great because of the greater turbulence on the low pressure side of the sail.
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Old 11-02-2013, 12:03   #32
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Re: Settle an argument

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Originally Posted by Adelie View Post
Two posts further along: Settle an argument
Negative, Sir. They are talking shape of the object, not forces exerted on two objects of same area.

But keep on trying.

Makes me think why all racers are so stupid and try and flatten their sails going downwind.

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Old 11-02-2013, 12:33   #33
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Re: Settle an argument

<<He claims that a perfectly workable sail could be made out of a flat solid sheet of material with no curve and you would get decent performance out of it. Not as good a a curved sail, but almost as good.>>

It all hinges on what is meant by "almost"

I think a useful thought experiment is to think of a child's kite, the very basic type.

If this was clad with infinitely stiff paper, would it still develop a similar amount of lift, at a similar angle of attack? I think it's pretty clear that it would.

If you look sideways at such a kite at a realistic angle of attack in a wind tunnel, injecting streamlines of smoke upstream, I imagine you'll see an eddy forming behind the leading edge on the leeward (top) surface, which effectively (and not particularly efficiently) creates, for the laminar stream flowing around it, a crude likeness of an asymmetrical, teardrop shape. (eg approximating the sectional profile of an aircraft wing)

That's my untutored guess at why it, kind of, works. Noelex's flat plate rudder is a more familiar and convincing example, but I mention the kite because it operates in the right sort of fluid, and we see it hanging there, demonstrating steady-state lift.

I think the FiL was closer to the truth than the OP, based on the OP's account of their respective positions.

Perhaps the FiL was influenced by knowing that you can fly a plane upside down semi-indefinitely (for as long as the fuel refrains from leaking out the "filler cap" !), which is a worse case again.

ON EDIT: Oooops, sorry, Cal40John - ; note to self, do not skim thread so carelessly !
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Old 11-02-2013, 12:39   #34
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Re: Settle an argument

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Originally Posted by barnakiel View Post
Negative, Sir. They are talking shape of the object, not forces exerted on two objects of same area.

But keep on trying.

Makes me think why all racers are so stupid and try and flatten their sails going downwind.

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Drag Coefficient

Top of the page, the formula is Fd = Cd * 1/2 * ρ * v^2 * A

If the area A is the same for both the flat and cupped shapes, and the air has the same density (ρ) and speed (v) then the differences in force generated is proportional to differences in the Coefficients of Drag for the two shapes.

Admittedly the numbers I have are for circular shapes as viewed from ahead or behind, but they are qualitatively indicative of how other shapes such as squares and triangles will behave if they are flat or cupped
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Old 11-02-2013, 12:44   #35
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Re: Settle an argument

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Originally Posted by Andrew Troup View Post

Perhaps the FiL was influenced by knowing that you can fly a plane upside down semi-indefinitely
Flying upside down requires flying at a much greater angle of attack by the wing in order to compensate for the wing being upside down. This greater angle of attack leads to much greater drag and much decreased range, 50% maybe.
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Old 11-02-2013, 12:59   #36
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Re: Settle an argument

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Originally Posted by Adelie View Post
Flying upside down requires flying at a much greater angle of attack by the wing in order to compensate for the wing being upside down. This greater angle of attack leads to much greater drag and much decreased range, 50% maybe.
It depends on the construction.
Many aerobatic aircraft have a fully symmetric wing section.

Inverted is great fun. When flying inverted I never great thought to the range, but in a fully aerobatic, purpose built aircraft I suspect it would be similar.
A long trip inverted would scare the bejesus out of the passengers, however.
Which is why, I suppose, most are single seaters
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Old 11-02-2013, 13:04   #37
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Re: Settle an argument

Quote:
Originally Posted by Andrew Troup View Post
<<He claims that a perfectly workable sail could be made out of a flat solid sheet of material with no curve and you would get decent performance out of it. Not as good a a curved sail, but almost as good.>>

It all hinges on what is meant by "almost"

I think a useful thought experiment is to think of a child's kite, the very basic type.

If this was clad with infinitely stiff paper, would it still develop a similar amount of lift, at a similar angle of attack? I think it's pretty clear that it would.

If you look sideways at such a kite at a realistic angle of attack in a wind tunnel, injecting streamlines of smoke upstream, I imagine you'll see an eddy forming behind the leading edge on the leeward (top) surface, which effectively (and not particularly efficiently) creates, for the laminar stream flowing around it, a crude likeness of an asymmetrical, teardrop shape. (eg approximating the sectional profile of an aircraft wing)

That's my untutored guess at why it, kind of, works. Noelex's flat plate rudder is a more familiar and convincing example, but I mention the kite because it operates in the right sort of fluid, and we see it hanging there, demonstrating steady-state lift.

I think the FiL was closer to the truth than the OP, based on the OP's account of their respective positions.

Perhaps the FiL was influenced by knowing that you can fly a plane upside down semi-indefinitely (for as long as the fuel refrains from leaking out the "filler cap" !), which is a worse case again.

ON EDIT: Oooops, sorry, Cal40John - ; note to self, do not skim thread so carelessly !
Wait, so you say the sail is not a wing? Every bit of research I have done describes the sail as just that. A wing on its side. Even using your kite example, lift is involved. The eddy swirling behind the kite causes a low pressure zone creating the needed pressure differential for lift to occur. I assume a flat sail would behave in a similar manner and would be no more than half as effective as a traditional sail. A plane can only fly upside down due to a change in angle of attack. Stunt planes wings are basically symmetrical top to bottom. With wind perpendicular to the wing, they generate little to no lift, but if you change the angle of attack they generate a bunch, in either way which allows them to fly upside down near as efficiently as they do right side up.

Tom
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Old 11-02-2013, 13:30   #38
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Re: Settle an argument

Now here is some food for thought,
Durring the racing.of the AC 45s , after rounding a mark on an up wind leg and heading down wind, the speed is such that even thou they are going, down wind, they are exceeding wind speed and trimming as they were going to weather...
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Old 11-02-2013, 13:40   #39
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Re: Settle an argument

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Originally Posted by Randyonr3 View Post
Now here is some food for thought,
Durring the racing.of the AC 45s , after rounding a mark on an up wind leg and heading down wind, the speed is such that even thou they are going, down wind, they are exceeding wind speed and trimming as they were going to weather...
Even my crappy old Hobie SX18 with a fairly full assym. had best downwind speed angles with the apparent wind at 70 degrees from the bow, close reaching. Newer F18 type beach cats don't even lower the traveller on the main to go downwind is how close the apparent wind is for them to go down. Aussie 18s had to go to asymms that were flatter and shorter foot a long time ago because the apparent wind is so far ahead of the boat to make it a better upwind sail to go downwind.

My Hobie 20 doesn't even use a chute and in around 12+ knots of wind it's time to head up to a close reach, pop a hull out and let the boat speed take you down.
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Old 11-02-2013, 13:43   #40
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Re: Settle an argument

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Originally Posted by cal40john View Post
If a curved wing surface is required to make lift in one direction explain to me how planes fly upside down.

Arvel Gentry, Boeing Engineer, sailor, using a flat plate to explain how lift is generated.

The Origins of Lift

Proper wing shape important, yes. Proper trim important, yes. But we're underpowered half the time due to lack of wind. Also, trying to sail upwind most of the force of the sail is pulling us in a direction we don't want to go, so doing everything possible to maximize the lift to drag ratio makes a big difference in how well you go to weather. You would still go to weather with a flat plate, just slower.
Buernulles law is bunk!
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Old 11-02-2013, 13:44   #41
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Re: Settle an argument

A shaped wing - ie, Bernoulli's principle - accounts for only a small portion of the lift required for an aircraft to fly. The lion's share of the lift is generated by angle of attack. So the FIL isn't entirely wrong.
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Old 11-02-2013, 13:47   #42
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Re: Settle an argument

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Originally Posted by noelex 77 View Post
It depends on the construction.
Many aerobatic aircraft have a fully symmetric wing section.

Inverted is great fun. When flying inverted I never great thought to the range, but in a fully aerobatic, purpose built aircraft I suspect it would be similar.
A long trip inverted would scare the bejesus out of the passengers, however.
Which is why, I suppose, most are single seaters
Even in a aerobatic plane the range would be significantly decreased, the angle of attack for the fuselage and empenage would be significantly changed.

For a normal aircraft the range reduction would be even greater due to increased drag.

Going back to the root of this thread, what is possible is not necessarily efficient. Father-in-Law is full of it going up-wind, and even down wind not very right.
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Old 11-02-2013, 13:52   #43
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Re: Settle an argument

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Even in a aerobatic plane the range would be significantly decreased, the angle of attack for the fuselage and empenage would be significantly changed.
True, but most sailboats are symmetrical. Ie, they are not designed to create greater lift on one tack than the other. So, FIL is mostly correct, after all!
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Old 11-02-2013, 13:58   #44
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Re: Settle an argument

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Buernulles law is bunk!
I am very confused. Are you saying the link I posted had anything to do with Bernoulli, because it doesn't.
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Old 11-02-2013, 14:03   #45
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Never argue with the FIL, even when he's wrong as in this case!
+1

Clearly a man who has stayed at peace with his in-laws!

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