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Old Mar 05, 2012, 08:16 PM
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The Willamette Valley, Oregon
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Originally Posted by dave1993 View Post
the f3p guys i was referring to on the other hand go to some extremes to include parasitic drag for reasons more related to control. for them things like loading and lift are secondary.
Well, sure. Like, a draggy biplane with lots of struts and wires to keep the speed low in vertical dives, and a powerful engine to overcome the drag in climbs (with a big prop that acts as a brake at low power settings) is a great choice for certain kinds of aerobatics. Pretty far off topic from a discussion of airfoils and lift!

xxxxxxxxxxxx enough for today! xxxxxxxxxxxx
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Old Mar 05, 2012, 08:30 PM
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Off topic

Off topic--

Is landing on the ceiling, taxiing on the ceiling, a common maneuver in F3P yet? If not why not? Ditto walls, going up down and sideways....

Maybe need longer gear legs to let wing be at needed negative angle-of-attack without striking prop....

Randomly googled F3P video
Qualification F3P-AM Mülheim 02/2006 (2 min 30 sec)
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Old Mar 05, 2012, 08:31 PM
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Originally Posted by BMatthews View Post
Dave,air is a rather.... well, airy substance. We can't grab it like we can a solid. So we take physical solids and push them through the air which builds up areas of pressure change near the surface of the solid because the air can't move out of the way fast enough. This area of pressure change next to the surface of the solid shape then acts on the air near the pressure change and so on so we see the effect respond on the air some distance out from the surface which moves the air around.
not as "airy" as most think. closer to water for us. so i wonder how that principle applies to submarine planes.

in any case nearly everyone in this thread has now agreed, grudgingly and/or in moments of weakness, that there is no lift w/o downwash. then arguments drifted into etiology and phenomenology which caused more than a few eyes to glaze over. then came the formulas leaving only diehards. ignoring personal attacks it all seems to boil down to what causes what and how much is due to this or that.

im still convinced we are dog paddling our way through the air and that cupped hands work better than flat ones. so for now im giving more credit to the undersides. sticking to the water analogy im also going to mention that it dont matter too much what shape the back side of the buckets are on a water wheel. how come they dont take advantage of all powerful bernoulli there?

Quote:
Originally Posted by BMatthews View Post
This was why I posted the pressure distribution diagram for a wing section.
id pay a lot more attention to those diagrams if everybody stopped depicting the flow as compressable. it just makes me wonder what else they got wrong.

Quote:
Originally Posted by BMatthews View Post
If you want to see more of how the angle of attack and camber all interact and affect the pressure distribution around a wing check out Foilsim.
i do have a copy and it is definitely easier to use and more fun than profili. maybe its because profili was used in class or because i inherited a copy worth a small fortune but i find myself using it more often. or maybe im just trying to justify the pain of that horrible learning curve.
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Old Mar 05, 2012, 08:58 PM
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noncompressible flow

Quote:
Originally Posted by dave1993 View Post
id pay a lot more attention to those diagrams if everybody stopped depicting the flow as compressable. it just makes me wonder what else they got wrong.
What gives you this idea that the flow is being depicted as compressible?

Is it that the streamlines sometimes get closer together?

...sigh...

http://dev.physicslab.org/Document.a...s_Dynamics.xml

No wonder you don't understand Bernoulli / tops of wings. Case closed.

Quote:
Originally Posted by dave1993 View Post
not as "airy" as most think. closer to water for us. so i wonder how that principle applies to submarine planes.
Exactly the same as to airplane wings.

Bernoulli primarily studied liquids:

http://plus.maths.org/content/daniel...fluid-equation

Water tunnels are often used to model flow over aircraft:

http://www.rollinghillsresearch.com/...xperiments.htm

This idea that the common wind tunnel pictures with varying distance between streamlines are showing compressible not incompressible flow is causing you much confusion.

Quote:
Originally Posted by dave1993 View Post

it just makes me wonder what else they got wrong.
This thread has been "off the rails" for a long long time-- as already have been often noted -- see for example post 91 back in November...
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Old Mar 05, 2012, 10:07 PM
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Steve, I had the exact same thought about an hour ago as I was in town getting groceries. I also think this is where Dave is confused.

Dave, we all agree that air is considered "incompressible" at subsonic speeds. But, all that the technical definition of incompressible says is that the density remains constant. In an air compressor, say, for filling tires on your car, lots of air is packed into a small can to increase the pressure. This kind of compression increases the density of the air.

When we talk about the pressure present around a wing, we are not talking about the same kind of pressure. We are talking about pressure gradients that are naturally occuring. For example, close to a thunderstorm there will be a low pressure area, whereas 20 miles away it will be higher air pressure. This does not mean there is "less air" at the thunderstorm, it is just that the pressure is lower.

When you look at a pressure gradient chart (an aviation weather map for example) you see the isobar lines, indicting the areas of higher and lower pressure. The air density between those points is the same, eg: the higher pressure area does not have "more air" than the lower pressure area.

What we are doing with a wing, is manipulating those naturally occuring pressure gradients. Here is how it happens. You can agree that the air going over the top is accelerated and the air passing the bottom of the wing is slightly slowed. We can see that in wind tunnel tests with pulsed smoke, there was a video of this a page or three back. Now, this is where Bernoulli's equation comes in to play, and all we really care about here is the fact that as the air speeds up, the conservation of mass says that the pressure goes down, and as the air slows down, the pressure goes up. NOTE: you will notice we are not packing more air into the same space (compressing the air), the amount of air stays the same. So, the pressure on top the wing is reduced, and below the wing is increased, resulting in a net force on the wing, upwards.

Please do not confuse air pressure with compressed air. They are two different things.
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Old Mar 05, 2012, 10:39 PM
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Incompressible

Well --actually--

High -pressure days really do have more air per unit volume at the surface-- for a given temperature and humidity-- hence a lower "density altitude"-- hence wings and engines work better -- the effect of pressure changes on density may be quite small compared to the effect of temperature and humidity changes on density, but is not zero -- http://wahiduddin.net/calc/calc_da.htm --

On the meteorological scale, air pressure can be thought of having to do with the weight of the air stacked up above a given point...

In the ocean there are similar pressure differences caused by differences in the weight of water above a given point or level, due to differences in the height of the ocean surface on the scale of inches or feet-- but because water is so much less compressible, the resulting density differences are much smaller--

The definition of an "incompressible" fluid, is that pressure changes don't affect density at all.

In an absolutely "incompressible" fluid, pressure would have no effect on density, but temperature still would.

I don't fully understand in what circumstances we need to think of the flow around our aircraft as being compressible, or how it can be justified to think of the flow as being incompressible. But as far as wind tunnels and water tunnels are concerned, one core idea is that even in an absolutely incompressible fluid-- which water is very much closer to being than air-- it is totally expected that the streamlines get closer together or further apart due to changes in velocity, even though the resulting pressure changes will have absolutely no effect on the fluid's density.

But yes, in the idealized case where we think of air as being incompressible (density does not change in response to pressure changes), we still can have changes in air pressure due to changes in velocity. That's what Bernoulli studied, except that he was focused on liquids-- blood, etc.
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Old Mar 05, 2012, 11:17 PM
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As others have stated, compressibility only starts to matter for Mach numbers past 0.3, which is 334 ft/s at standard sea level.

Keep in mind the perfect gas law (good assumption here)
P = rho*R*T. P is static pressure, R is the gas constant for air.
So for still air P is at atmospheric pressure, about 14 psi. If you look at how much the pressure changes over an airfoil, at our speeds we are dealing with changes in pressure that are very small relative to atmospheric pressure.

Pulling from a calculation Bruce made earlier, that the wing loading on a Boeing 747 is 0.93 psi.
For some more perspective, the Boeing 767 cabin altitude of 6900 feet. This gives the passengers a pressure that is 77% of sea level pressure. You certainly feel that with your ears but no other part of your body (not me at least).
Similarly, if you dive 10 feet under water, your body feels an increase in pressure of about 31%, and you can feel that with your ears too.

Back to aerodynamics. Dynamic pressure for a 100 ft/s flow at sea level is
q = 1/2*rho*v^2 = 1/2*0.002377*100^2 = about 12 lbs/ft^2 or 0.084 psi. This is about 0.6% of atmospheric pressure.

So when you stick your hand out the window and feel the air pushing it back, the pressure is a small fraction of what is already pushing on your hand, but it doesn't seem this way because you don't notice atmospheric pressure.

Sometimes is can be hard to feel ALL of the physics.
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Old Mar 06, 2012, 09:41 AM
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Quote:
Well --actually--

High -pressure days really do have more air per unit volume at the surface-- for a given temperature and humidity-- hence a lower "density altitude"-- hence wings and engines work better -- the effect of pressure changes on density may be quite small compared to the effect of temperature and humidity changes on density, but is not zero..............................
I know all that from my pilots licence training, I was just intending to keep it simple and basic regarding normal atmospheric pressures and such. Without getting into all the hows and whys of weather, all that really needs to be understood is that the atmosphere (air) has a certain amount of pressure, and that when flying, we are using our wing to play with those pressures to make them do work for us. And remembering that air pressure, and compressed air are two different animals.

Quote:
Pulling from a calculation Bruce made earlier, that the wing loading on a Boeing 747 is 0.93 psi.
Just for fun, I dug up that post....here is the important part

Quote:
Posted by BMatthews......
As a point of very general intrest have you looked at the average pressure differential over the wing area of a 747 to see what the pressure change is that is holding up the airplane?

Wiki lists one model has a wing area of 5500 sq ft. The max takeoff for this model is 735,000 lbs. This reduces to a wing loading of 134 lbs/sq ft. Which comes down to 0.93 lbs/sq inch. The average human adult can generate just barely 2psi to get the basic party ballon started. So we're talking about an average pressure over the wing area of a moderate breathing puff or a heavy sigh to lift a jumbo jet.
Some time ago, I did a fun little calculation myself. Being that we are on an R/C forum, I figured out the pressure difference required to keep a Sig Kadet LT-40 trainer airborne. And it may surprise some how little pressure change is required.

Kadet LT-40 manual says 5.5-6lbs all up, so lets use 6. Wing area is 900 sq in. So, that means that we have a wing loading of....ready for this 6/900=0.00666 lbs per sq inch.
So, to lift a Kadet LT-40, we only need to generate a difference in pressure between top and bottom of the wing of 0.0067psi!!

Now tell me, knowing what we do about about aerodynamics, does the air have to be accelerated much to reduce the pressure by that small of an amount? Especially when you consider that chart Bruce Matthews posted that we can learn roughly 25-30% of lift comes from an increase in pressure below the wing!! (For that particular airfoil anyway. I am just using it as an example, I'm sure most airfoils are close to the same) Just for fun, lets say as much as 1/3 of the lift comes from teh increase in pressure on the bottom. That means that of that 0.0067psi required to lift the LT-40, 0.0022psi is produced on the bottom, and then we only need 0.0044psi on top!! Shouldn't be too hard to get that with the nice thick, flat bottom airfoil the Kadet uses
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Old Mar 06, 2012, 09:55 AM
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I believe that the idea of the air being incompressable from a low speed aerodynamics standpoint is to make the calculations easier. In reality air is one of the very best springs you can find. As such it most definetly does alter it's local density in response to pressure changes of even the smallest amount.

"We", being aerodynamicists, just choose to ignore this for calculations done around a wing. I'm not sure of the reasons for this as I'm afraid my brain goes into overload at the depth of the math needed to justify it. But I'm sure that it's done because the small amount of density change due to the pressure changes around a wing is probably so minimal as to not be significant in comparison to the other effects.
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Old Mar 06, 2012, 10:25 AM
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Quote:
I believe that the idea of the air being incompressable from a low speed aerodynamics standpoint is to make the calculations easier. In reality air is one of the very best springs you can find. As such it most definetly does alter it's local density in response to pressure changes of even the smallest amount.
There ya go, some much smarter than me have clarified the whole "compressibility" issue
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Old Mar 06, 2012, 12:36 PM
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Careful with tossing around names like "much smarter". You'd be surprised at how many topics I'm pretty ignorant of....

I'm a firm believer in the old adage "the more I learn the dumber I realize I am".
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Old Mar 06, 2012, 02:55 PM
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Ahh, c'mon Bruce, I have read a lot of your posts, don't be so modest I quite often find that I know the material, but to type it out so that it makes sense is a problem That is a problem you don't have!! And, like you said, the more a guy learns, the more you realize how much you really don't know. I sure have learned a lot from guys like yourself since I got into the forums, and I thank you for it.
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Old Mar 06, 2012, 04:03 PM
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Quote:
Originally Posted by Tom Harper View Post
Tim,

Clarification:

Are you saying that the weight of the air that is pushed down by the wing has a weight equal to that of the airplane being supported?

Tom
No - the speed of the air and the mass of the air and the air's direction all come into play. Less air at faster speeds can equal the force of more air at lower speeds.
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Old Mar 06, 2012, 04:05 PM
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Originally Posted by Tom Harper View Post
Then the downwash would have to contain an amount of air equivalent to the weight of the airplane each second.
Tom
You have to account for the velocity and mass of the air - not just the amount of air.
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Old Mar 06, 2012, 04:09 PM
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[QUOTE=BMatthews;20938703]This reply of yours just illustrates how out of touch you are with so much of what has gone on in this thread. Lift does not have any connection at all with time. It is simply the force needed to hold up the airplane.
/QUOTE]

Lift is a reaction to the action of forcing a lot of air downward while also accelerating it.

Nothing about time in the above that I can see.
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