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A puzzler!

E

elippse

Well Known Member
Here I am clipping along at just a couple of feet above 1000' where the outside air pressure is 2040 psf. Some instrumentation tells me that the average pressure on the bottom of my 63-series airfoil is 2020 psf, while the average pressure on the top of this wing is 2000 psf.
The wing, except for some ribs and spars, is basically hollow, and is filled with air at the ambient pressure of 2040 psf. Looking at a diagram of the pressure acting on the upper and lower wing surfaces, I see that there is 2020 psf pushing up on the bottom surface, and 2000 psf pushing down on the upper surface.
But the air in the wing is pushing down on the bottom surface with a pressure diffential of 20 psf, and at the same time is pushing up on the top surface with a pressure differnential of 40 psf.
So, could it be said that the wing's lift is really a result of the air in the wing pushing up on the top surface 20 psf harder than it is on the bottom? What say you? And please don't invoke the idea that there is a force called suction! Air doesn't suck, it only blows!
And yes, airfoils with curved lower surfaces have lower pressure than ambient in flightl:confused:
 
What if your wing was solid....

with no "air" inside? Would it not still fly?
I say it flies because there is more pressure on the bottom than the top thereby creating a differential.
 
The air inside the wing, whatever its pressure might be, pushes down just as hard on the bottom skin as it pushes up on the top skin. It therefore has a zero net effect on lift. Sorry to spoil the fun :)

Your observation about the higher pressure inside the wing does raise an interesting thought though. Is there a way that we really could use it to our aerodynamic advantage?

One idea that comes to mind is to port it to points along the top surface of the wing where it can be used to accelerate the flow with sort of a "jet" effect (but not enough of it to significantly raise the pressure above the wing and therefore spoil the lift). Know what I mean? Serve a purpose similar to vortex generators, to accelerate the flow to help keep it attached at high alpha, but do it by means of small jets of high pressure air. In its simplest form, you could just drill tiny holes through the top skin at a very shallow angle facing aft. And unlike VG's, you could deploy these selectively during flight, only at high alpha when they're needed, by simply opening or closing the ports from the inside of the wing. Now that I think about it, the idea is pretty similar to a slat. But much lighter and simpler mechanically. Hmm...
 
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Air pressure inside the wing is a constant, it presses equally on the top, and bottom of the wing.

Thus it cancels itself, and only the differential on the exterior of the wing is a factor in generating lift.

Next Q?
 
how accurate is your instrumentation? is it proven to be accurate enough to tell the difference in 20 psf? is your instrumentation able to get a true average of the wing surfaces? wouldn't certain portions of the lower wing have different readings? how are those averaged?
 
roee said:
The air inside the wing, whatever its pressure might be, pushes down just as hard on the bottom skin as it pushes up on the top skin. It therefore has a zero net effect on lift. Sorry to spoil the fun :)

.....
Click to expand...

what if there is air movement inside the wing? suppose air enters from a lower portion of the wing, even if from the rear of the wing and is deflected against the upper wing surface. this would have to produce some upward force wouldn't it?

forgot we are talking about a lancair wing, i doubt there is much air movement inside that wing. where is your fuel tank, you have no fuel in the wings?
 
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roee said:
Is there a way that we really could use it to our aerodynamic advantage?

One idea that comes to mind is to port it to points along the top surface of the wing where it can be used to accelerate the flow with sort of a "jet" effect (but not enough of it to significantly raise the pressure above the wing and therefore spoil the lift). Know what I mean? Serve a purpose similar to vortex generators, to accelerate the flow to help keep it attached at high alpha, but do it by means of small jets of high pressure air. In its simplest form, you could just drill tiny holes through the top skin at a very shallow angle facing aft. .
Click to expand...

This was done in the 50s or60s, when they were working to optimize laminar flow-----as I recall, this was a jet aircraft, and they used bypass air ducted to a series of small holes just aft of the airflow separation point.

This kept the airflow attached longer, reduced drag, and increased lift.

I dont remember why it is not a commonly seen thing these days??
 
elementary my dear watson

A quick and dirty free-body diagram would demonstrate the forces acting are external to the wing. Now, heat up that internal air so that you no longer disregard bouyancy effects ... ;)

I recall reading a similar argument made decades ago by intelligent folk that spacecraft could never work becase there was noting in space for the exhaust to push on :eek:

in jest,
 
Well there you go!

Scherminator said:
I recall reading a similar argument made decades ago by intelligent folk that spacecraft could never work becase there was noting in space for the exhaust to push on :eek:
in jest,
Click to expand...

More proof of the "smoke & mirrors" theory.
 
Pressurized fuselage

And what external aerodynamic effect does a pressurized fuselage at 40,000ft. have? And if the fuselage is meeting the relative wind at a positive angle of attack? And then what happens if the pressurization is lost at the same altitude?

Answer: nothing, nothing and nothing.
 
More important question

Of much broader importance;

If I say something in the forest and my wife isn't there to hear it, am I still wrong?
 
In the 70s, Ed Johnstone made and patented a wingtip that used air pressure inside the wingtip to improve lift. The tip (sealed from the wing) had a NACA vent on the bottom and a slit along the back edge. It smoothed out the vortex and, on a Cherokee, improved the rate of climb considerably, something like 30%. It had no effect on cruise. Ed was an engineer on the 747 and a terrible aircraft builder. He had great ideas, however. I have no idea if this is relevant to anything. Actually, I'm pretty sure it isn't.

Bob Kelly
 
This was done in the 50s or60s, when they were working to optimize laminar flow-----as I recall, this was a jet aircraft, and they used bypass air ducted to a series of small holes just aft of the airflow separation point.

This kept the airflow attached longer, reduced drag, and increased lift.




This was BLC on F-4s using 16th stage compressor air.
John
 
F=MA

Assume a wing is solid aluminum. Like in a propeller. Or a rotor. Wings and propellers and rotors are all airfoils that move through the air. This assumption eliminates any internal pressure distractions.

As these airfoils move through the air, they have an angle-of-attack. This angle forces air downward in the case of the wing and rotor, and rearward in the case of the propeller.

Using the famous equation: Force equals Mass times Acceleration (F=MA) you multiply the mass of the air times the downward acceleration of that mass of air, and presto: you get lift (or thrust).

There may be a pressure differential between the top and bottom of the wing, but it is the downward airflow that creates the lift. Pressure ain't got nuttin' to do with it.

A flat board will create lift if it has an angle of attack as it moves through the air. Try it. Stick a small board out the window of your car at 60 mph, and then twist it. You can feel the lift change as you change the AoA of the board.

F=MA is all you need to know.

Flame suit on, and zipped up! :p
 
Pete is absolutely right. For every pound of weight in your airplane, you have to push enough air down to counteract the force of gravity. Has nothing to do with what we were all taught (and are still taught) about venturi effect etc. How do you know? Stand under a helicopter or behind a prop, and the air is being pushed one way while your aircraft is being pushed the other (action & reaction, F=ma). That's why the spaceship works as well, gases going one direction and spaceship going the other.

greg
 
Mel said:
with no "air" inside? Would it not still fly?
I say it flies because there is more pressure on the bottom than the top thereby creating a differential.
Click to expand...
But Mel, I noted that there was a pressure differential between the air inside and that outside. So if the air inside has no effect, wouldn't tha mean that all of the lift force is applied only to the lower surface?
And as far as a solid wing is concerned, both surfaces act together so that has nothing to do with the hollow wing.
 
PCHunt said:
There may be a pressure differential between the top and bottom of the wing, but it is the downward airflow that creates the lift. Pressure ain't got nuttin' to do with it.
Click to expand...

The downward airflow is the result of the lift, not the cause. That's like saying the reaction generates the force, not the action. Pressure is the only thing that counts.
If the latches on your forward opening canopy come open in flight, it is the air inside the cockpit that will push it open. That is the same effect that took place with Steve Wittman; it was the air inside his wing, pushing out against the lower pressure on top of the wing that caused his upper surface to peel off. It is also what causes the rubber sealing strip from the wing to the fuselage to get pushed out. Let's all repeat together: suction is not a force!
 
Another thing to consider. Why is it necessary to lace the fabric on a wing to the ribs. If the fabric is just along for the ride, providing only streamlining, then there shouldn't be any force on it other than drag. So again, is all of the lift only on the lower surface?
C'mon, guys! Really put your thinking caps on and quit writing all of this action-reaction stuff. I know all about downwash. Divide LIFT by MDOT and you have VD! My equations solve for it in my propeller programs so that I can converge on the induced angle of attack. Think about and draw the pressure differences I wrote about in the first posting and figure out what exactly do they mean.
 
And as far as rocket engines are concerned, I worked on the guidance system for the Atlas Space Launch Vehicle at Vandenberg AFB for 28 years. I wrote a computer model of the Atlas autopilot system for use in a total vehicle simulator I designed. I can tell you all about the chamber pressure of the booster and sustainer engines on the Atlas, and how the supersonic flow in the diverging engine nozzle causes the gases to speed up to C*, the effective exhaust velocity, and the pressure to decrease, and under-and- over expanding nozzles, so please, let's not go there.
 
elippse said:
The downward airflow is the result of the lift, not the cause. That's like saying the reaction generates the force, not the action. Pressure is the only thing that counts.
Click to expand...

I'm gonna stick my neck out for Paul on this one (and open the door for a debate with my neighbor Greg...)

I'm in the Pressure (differential, that is) camp on this. That whole Bernoulli thing! ;) Now I don't know whether it's little "lifters" on top of the wings, or little "pushers" on the bottom (or both), but "the gospel" according to my aerodynamic instructors was acceleration of air past an airfoil resulted in differential pressure between the top and bottom (or front and back in the case of a prop). That either lifts the wing, or pulls the prop forward (with the plane in tow...remember, they can tell in a prop strike whether power was applied to a prop and thrust was being generated by whether the blades curl forward...power on, or aft...power off). Same thing can be seen when a helicopter takes off. Watch what happens to the rotor "disc"...it goes up, and then pulls the helo up with it (seen in long wings too..they go up, and pull the airplane up).

I don't think there is much downwash from airflow over the wings (except in the vortices), as most wind tunnel video I have seen shows the smoke continuing back, not turning down. I will admit I'm scratching my head on why prop wash goes backwards and rotor wash goes downwhile wing wash does not go down (I believe)...but maybe Paul can 'splain that one...I figure it has to do with the prop/rotor traveling in a fixed arc, while the wings travel through (mostly) undisturbed air (and I'm sure I'm not 'splainin' that right either!)

Now I'm no PhD Greg, but what's this heresy about pushing air down for an equal and opposite reaction? (and I'm kiddin' around, in case you can't see my tongue firmly in my cheek ;), Greg will tell ya!)

Now, as for the air inside the wing...isn't it pretty static (pressure-wise)? There may be some movement withing the wing (leakage), but overall, wouldn't the differential (that Paul mentioned in the OP) between inner top and inner bottom really be the same as the differential between outer bottom and top? I'd have to liken it to the solid prop in that case (as another poster did) and say the forces (pressure...ahem :p) are similar, whether solid or hollow. And I don't think you could harness it for boundary layer control, due to the "energy can't be created or destroyed" thing (which Newtonian Law was that?). No free lunches (usually), and that's why BLC has to be pumped from a turbine or the like...at least I think that's true...always willing to learn!

Now that leakage in and out...that's another story. Bad stuff that is, me thinks! I'm trying to find ways to eliminate that speed-robbing dastardly stuff. It may not be suction, per se, but it sucks, so Paul, sorry, I have to disagree with your quote below :D:

elippse said:
Let's all repeat together: suction is not a force!
Click to expand...

And yes, this post does count as midnight madness! :rolleyes:

Cheers,
Bob
 
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Let's all repeat together: suction is not a force!
Click to expand...
Now I don't know whether it's little "lifters" on top of the wings...
Click to expand...

The pic below starts to answer some of your questions ;) There is clearly a pressure differential across the top skin to produce the "shape" seen - and in fact, just look at your RV wing in cruise to see the effect. Take that pressure differential x skin area = quite a lot of lift! I am sure the bottom skin would show the same effect, albeit in reverse i.e. actual pressure in wing = atmospheric, above wing <Atmospheric, below wing >.

Lift v Wing Upper Surface

Andy
RV-8 G-HILZ

PS it's not we're pulling large amounts of 'g' :eek: - just the angle of the sunlight v metallic paintwork ;)
 
Paul
Assuming that your instrumentation is correct then yes 20 psf would be enough to push, by the top skin, our aircraft upwards. My rocket has 104 square feet of wing and so this would be 2080 pounds of force upwards. You can not see the bulging of the wing skins on my EVO wing but you could certainly see this on my RV4 wing. And when you add in the Coanda effect....http://en.wikipedia.org/wiki/Coandă_effect
 
OK, I think I have this all figured out now. But, how about in the southern hemisphere? Seems like everything said would have to be re-thought??>>:p

Does one need to roll inverted when crossing the equator?
 
I don't know if your question is a legit question or a quiz, but the answer gets confusing because it gets down to the load path rather than worrying about just the aerodynamic pressures.

The pressure inside the wing is dependent on the static and dynamic pressures on the outside of the plane, as well as leak paths for air into and out of the cavity. In any case, though, it doesn't matter what the pressure is, it has no effect on the lift. If you assume that the cavity is perfectly sealed and pump it up to 10,000 psf, then the wing skins will "pooch out", obvoiously, but the lift has not been effected other than the fact that you are now carring around more mass of air. Conversely, if you sucked it "dry" and turned it into a vacuum, the wing skins would "pooch in", but again the lift on the airplane would not be effected exept for the lack of mass of air, which is commonly described as buoyancy. So the original question is really "What is the load path for the lift from the air to the airframe?"

Well, regardless of the pressure insided the wing, the air on the bottom of the wing is pushing up in the skin, into the ribs, the load then flows to the spar, then into the airframe. On the top of the wing, it is also pushing down, just not as hard as the air on the bottom pushing up, onto the wing skin, into the ribs, then to the spar, and to the airframe. The net effect, obviously, is the air on the bottom of the wing is pushing harder, so the airplane goes up, not down.

The bottom line is that the lift is caused from the differential pressure outside the wing. The wing shape can be effected, particularly in your fabric wing example, by the pressure, or lack thereof, inside the wing, but the pressure inside the wing will have no effect on the net lift (outside of shape changes). It will, however, have a structural effect on the design details of how the wing skin is held to the airframe structure.

Make sense? Probably not, it is complicated and I am probably not articulating very well.

Another, realted, interesting question would be, exactly how do the high velocity gases excaping from a rocket motor, solid or liquid fueled, transfer the thrust load from the high velocity gas to the rocket itself? Or, how do the gases accellerating through a jet engine transfer the thrust load through to the airframe? Complicated questions, for sure, but the net effect can be easily calculated by not worrying about any of that detail and just calculating the momentum change of the mass flow. You only really need to worry about all of those internal forces if you are designing or developing components that have to deal with these loads directly.

Tim
 
Hey Andy; thanks for the picture! I have a picture of a Bonanza shot in flight in the late afternoon which shows the skin in between the ribs bulging slightly up, and back when I was on a flight in a twin-turboprop Metroliner I could look out and see the very slight upward bulge of the wingskin between the ribs.
BTW, I didn't really measure the pressure on my wings; that was a hypothetical problem that I was trying to present. 'Sorry if I made it seem like it was an actual happening.
At 200 mph, my 27' wingspan intercepts about 168,000 cu.ft./sec. or about 400 lb mass flow/sec. At 1350 lb, the downwash velocity is about 3.4 ft/sec, so that the induced angle of attack is about 0.66 degrees. The wing area is 77 sq. ft., so the average wing loading will be 17.5 psf. Since the pressure is not the same at every square foot, on some regions the differential pressure is much higher than this, and on others it's lower, especially since the pressure is lower across the lower wing surface, so that the pressure on the upper surface must make up for that.
In the macro sense, the lift on a wing is due to the pressure difference below the wing to that above the wing, but the air inside the wing also plays a part, as the photo shows; it definitely pushes out on both skins. Now can anyone provide a really good analysis of what that role is?
 
Tim, too often the thrust of a jet or rocket engine is spoken of only in terms of the mass flow, but that doesn't really tell where these loads actually are taken up in the engine, such as the nacelle thrust of a jet engine. Speaking of the thrust in terms of the reaction instead of the action is like saying that it is the kick of a rifle that propels the bullet. Without action there is no reaction!
 
Ahh...

elippse said:
Hey Andy; thanks for the picture! I have a picture of a Bonanza shot in flight in the late afternoon which shows the skin in between the ribs bulging slightly up, and back when I was on a flight in a twin-turboprop Metroliner I could look out and see the very slight upward bulge of the wingskin between the ribs.
......
Click to expand...

...but is it "suck" from the top of the wing or "blowing up" from the inside of the wing that is making the skins bulge...:)
 
Andy Hill said:
The pic below starts to answer some of your questions ;) There is clearly a pressure differential across the top skin to produce the "shape" seen - and in fact, just look at your RV wing in cruise to see the effect.

Lift v Wing Upper Surface

Andy
Click to expand...

That's a very cool photo, and gorgeous planes, by the way! So is the pooching of the skins from pressure inside the wings (as I believe Paul postulates), or from little Bernoulli's "pulling" on the top skin. tjo's discussion would seem to say both top and bottom surfaces would have positive pressure, just differential, so lift is a "push" from the bottom. My aero instructors and I would tend to agree, which lends credence to Paul's position that the pooching comes from air in the wings...hmmmm...

Edit...just saw Gil's post...he said it in far less words than I!

Tom Martin said:
Paul
You can not see the bulging of the wing skins on my EVO wing but you could certainly see this on my RV4 wing. And when you add in the Coanda effect... [/url]
Click to expand...

Is that because the ribs are closer together on the EVO wing Tom, or because the wings are stiffer and have a higher aspect ratio? Just wonderin'. And oh yeah, that coanda effect...;)

AlexPeterson said:
OK, I think I have this all figured out now. But, how about in the southern hemisphere? Seems like everything said would have to be re-thought??>>:p

Does one need to roll inverted when crossing the equator?
Click to expand...

No, you have to turn to the right...or is it the left?...:D

tjo said:
Make sense? Probably not, it is complicated and I am probably not articulating very well.

Tim
Click to expand...

I thought you did a great job!!

elippse said:
At 200 mph, my 27' wingspan intercepts about 168,000 cu.ft./sec. or about 400 lb mass flow/sec. At 1350 lb, the downwash velocity is about 3.4 ft/sec, so that the induced angle of attack is about 0.66 degrees. The wing area is 77 sq. ft., so the average wing loading will be 17.5 psf. Since the pressure is not the same at every square foot, on some regions the differential pressure is much higher than this, and on others it's lower, especially since the pressure is lower across the lower wing surface, so that the pressure on the upper surface must make up for that.
In the macro sense, the lift on a wing is due to the pressure difference below the wing to that above the wing, but the air inside the wing also plays a part, as the photo shows; it definitely pushes out on both skins. Now can anyone provide a really good analysis of what that role is?
Click to expand...

Numbers Paul...we need more numbers! :p And I need another cup of coffee before engaging in aerodynamic discussions in which I come to an intellectual gun fight armed with a knife :p

Cheers,
Bob
 
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Sailplanes are in production....

jjglennon said:
This was done in the 50s or60s, when they were working to optimize laminar flow-----as I recall, this was a jet aircraft, and they used bypass air ducted to a series of small holes just aft of the airflow separation point.

This kept the airflow attached longer, reduced drag, and increased lift.

This was BLC on F-4s using 16th stage compressor air.
John
Click to expand...

...with "blown" wings for boundary layer control.

The German translation seems to come out as "blast turbulators" but I would call them "turbulator holes" - one references around 400 holes of 0.022 inch diameter. No power involved, just higher pressure ducted air from the leading edge or similar location.

The ASH-26 and ASW-27 are examples.
 
After many years of water skiing, I decided that a wing is more like a ski on water, than anything else............for the main force of lift. Just like sticking your hand out the car window. Skis & hands stall too! And then the various shapes of the top of the wing just make the difference in the effectiveness of the push from below...........as they'll effect pressures,... in which below the wing pressures have to fight against.

One thing for sure though............."theory" of lift, still continues after all these years. It isn't a known and absolute science, no matter what a few say & believe! :D

L.Adamson -- RV6A
 
Ah, so it is a quiz, not a question. The problem is it can be described about a thousand different ways in english, and as a result can be understood, or misunderstood, in many different ways. If the discussion is, "Is it static or dynamic pressure differentials that cause lift on a wing (or airplane for that matter)?", then the answerr is both, IMHO, but in your original example, it is clearly the difference in the external pressure and has nothing to do with the internal wing pressure.

A flat plate can generate lift, like your hand out the window of your car, with little help from Bernoulli, just the effect of the mass flow of air hitting the bottom side of your hand, versus the top. The result, of course will be upward force on your hand and downward flow of air. This is fairly inefficient lift, but it works. You could put an airfoil shape out the same window at essentially zero degree angle of attack and it would generate lift due to the static pressure differential and the dynamic pressure of the air on the airfoil would be only in the horizontal direction, the vertical componoent would be zero. The result would again be, upward force (lift) on the airfoil and downward flow of the air. This is relatively effecient lift, but again, it works.

In the end, in the real world an airplane experiences both forms of lift, in different proportions, during different phases of flight. In either case, the pilot doesn't know the difference, or care, as long as the plane flies like he expects it to and the fuel burn is not excessive.

Tim
 
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Very interesting discussion - causes me to rethink the entire thing. I (like most everybody) learned about the venturi effect way back and that's how lift was always explained. Later on, I read some articles that argued that it was not the venturi/bernoulli effect that caused lift, but rather the downwash. So now, I'm not so sure of either....

With regard to the bulging of wing skins, IF they are bulging on both top and bottom, then there must be pressure above ambient (exterior to the wing) on both sides. To me this would argue that there is some other source of air pressure in the wing. The obvious largest source for this air is (in the case of the RV), the pushrod hole, which allows air from the cockpit (at static pressure) into the wing. If one accepts the venturi effect as operating, then there would be relatively lower pressure on both sides of the wing as compared to static pressure within the wing, because of the movement of the wing through the air and consequent speeding up of the air as it goes by the wing.

I'm still thinking about whether the venturi effect is still the major cause of lift. As several have noted, if a flat (symmetrical) piece of wood is held into the wind and twisted (and I hope I'm not twisting in the wind here :D), there will be increased pressure on the upwind side of the piece of wood, relative to the downwind side (i.e., why a fan blade moves air). This increased pressure is because the wood is compressing the air molecules in front of it and rarefying the molecules behind it. Certainly this effect also contributes to the pressure differential across a wing shape as well, as it is angled into the relative wind. If one only considers this effect, an airplane could theoretically fly with no need of those italian guys.

I don't think this is the only thing going on, but is part of the explanation. Here's a link to an article describing causes of lift that explains better than I could ever do:

http://www.allstar.fiu.edu/aero/airflylvl3.htm

All very interesting stuff....

greg
 
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The downwash is a consequence of lift, not a cause. If I stand on a wagon and throw a brick off the back the wagon rolls forward. It is my throw that caused the motion, not the brick.

P.S. Bernoulli was Swiss.:D
 
The downwash is a consequence of the engine shoving the airplane forward through the air. Due to the forward motion of the plane, the air exerts upward force on the plane, the plane downward force on the air, the result is the plane stays aloft, the air flows downward, and fuel was burned to supply the power that makes it all possible.

Tim
 
szicree said:
The downwash is a consequence of lift, not a cause. If I stand on a wagon and throw a brick off the back the wagon rolls forward. It is my throw that caused the motion, not the brick.
Click to expand...

Not so fast. What is a throw without a brick? Try the same thought experiment again, but this time with a slight variation. Stand on your wagon and make the same throwing motion toward the back, but this time without a brick. Will the wagon still roll forward? :D
 
So if I may focus on one square foot of lower skin for a moment, there is a force of 2040 lbs directed downward and 2020 upward. So why don't these forces cause the piece of skin to accelerate downward? There must be something pulling up on that skin and I would say it has to be the rivets! The same would then be true of the upper skin (only down). If this is true, then the ribs are under tension in the vertical direction. If I've got this right it means the lower half of the rib is pulling down on the spar, and the upper half is pulling up on the spar (harder than the lower half). This differential tension keeps the plane in the air. Feel free to tell me I'm crazy cuz it just doesn't feel right.
 
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How do you explain how foam/fiberglass wings work???

There is no air cavity inside them to be pressurized, like a metal wing.

Of course, the foam has a million tiny cavities, but they are not subject to pressure being induced from the action of flight.

Only thing I can think of that internal air pressure would do is to strengthen the outer skin, to help it maintain the airfoil shape.
 
So, I think Tim has it most succinctly and correct. Good old Newton's laws. If you're going to push the airplane upward (or even keep it at the same elevation) , then there has to be something that goes the other way (the old action-reaction dictum). That stuff going the other way is the air.

greg
 
So have we come to a concurrence on which is the chicken and which is the egg (lift or downward air pressure/movement)? ;) (I'm still having fun...I do not think I have all the answers..."they" only issued me the lower left corner of the big picture! :)) I'm just musing here again, and wondering...

I'm still mulling over the downwash thing. Seems to me we feel it from a helo, and we feel it (in a horizontal sense) from the prop, but I don't think it exists in the same manner with a wing moving through an air mass. I've been on the ground right below a 747 landing (running at LAX & MSP...one of those exciting airline layover moments :p), and didn't feel downwash. I heard the wingtip vortex go by (that was kinda cool), but felt no downwash like from a helo. Having mentioned the flow of smoke in wind tunnel past a wing (which IMHO shows no massive downwash towards the ground either), is it just that the forces acting on the wing, and the resultant (Newtonian ;)) reactions within the surrounding air are all occurring within the small span of air across the wing, and are not seen (in smoke) or felt (as downwash) past where the air mass is undisturbed by and unaware of the passing wing? To illustrate, I tried to find a good smoke tunnel pic, but could not, so I grabbed this from the web:

lift.jpg


So perhaps all of the force and reaction is taking place in the vertical space where the lines are displaced, and are unfelt where the lines are once again flat (above and below the wings).

And if that is true, then why is it not the same for a helo rotor or a prop from which we feel the downwash/propwash (not the stuff in the bucket :p). Inquiring minds want to know, 'cause I'd have a hard time proving to Greg that the wing doesn't do the same thing to air that a prop, or a rotor...or the fan blade he mentioned earlier, does to the air. Any thoughts?

As for the pooching of the top skins, that still puzzles me. I wonder if a hermetically sealed wing would exibit the same pooching. Heck, I wonder if that wing would be faster than its pourous, leaky cousin! Hmmmm...I wonder how many tubes of RTV it would take to find out! :eek::)

Cheers,
Bob
 
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Bob,

Propwash twists, right? I bet if one were to examine the downwash under a helo, it would twist as well. The difference with the wing is that the wing is not moving in a rotary fashion, rather it is moving through the air forward, so the downwash occurs, but is left behind as the wing moves forward. If you had taken a second lap around LAX, you might have felt that downwash as it finally got to the ground:D. (although it would dissipate somewhat due to friction with the surrounding air). Have a look at the classic photo of the downwash (and wingtip vortices) from that jet flying just above the clouds, and you'll see the result of downwash, which is behind the wing.

I think we need a breakfast discussion of this next weekend....

cheers,
greg
 
Try this: The upper surface lift is 2 X sin(2 X alpha), 0.07/degree, and the lower surface lift is sin(2 X alpha), 0.035/degree, with a combined CL of 0.105/degree. 'Look familiar? The upper surface lift is due to Coanda effect, and drops off rapidly when the adverse pressure gradient at high CL causes flow back up the surface which destroys the Coanda effect, which depends upon the fact that there is no air acting normally to the surface to allow the reduced pressure. The lift on the lower surface follows a sinusoid which peaks at 45 degrees, then rolls-off to zero at 90 degrees. This is the pressure-lift you experience on a kite, water skis, and your hand held out of the car window.
Since the upper surface lift equation is similar to the equation of the side thrust produced by a bend in a hose carrying a flow of water, and the lower surface lift equation is similar to the equation of a jet of flow deflected by a flat plate, you could properly call this version of the lift produced by a wing the "hose and kite" lift hypothesis! ;)
You resolve the lift on a propeller blade by its induced angle of attack plus its pitch angle into its forward, thrust, and tangential, torque, components. Since the downwash velocity is exactly opposite to the direction of the lift, it is the lift divided by the mass-flow, it to can be resolved into its axial flow component and its tangential component; this is the swirl behind the prop or rotor. However, keep in mind that in propeller theory, the velocity at the disc is doubled downstream of the blade in its "vena contracta" flow.
Hasn't this been an interesting discussion? It's always good to challenge the the proposed hypotheses of how things take place for so many are founded not on scientific principals but instead on observations that are flawed. For instance, when I tried to explain to my wife that the heat pump in a refrigerator took the heat inside and pumped it to the outside, she replied "How can that be since it's colder inside than outside!". 'Sorta like "suck"!
 
Greg Arehart said:
Bob,

Propwash twists, right? I bet if one were to examine the downwash under a helo, it would twist as well. The difference with the wing is that the wing is not moving in a rotary fashion, rather it is moving through the air forward, so the downwash occurs, but is left behind as the wing moves forward. If you had taken a second lap around LAX, you might have felt that downwash as it finally got to the ground:D. (although it would dissipate somewhat due to friction with the surrounding air). Have a look at the classic photo of the downwash (and wingtip vortices) from that jet flying just above the clouds, and you'll see the result of downwash, which is behind the wing.

I think we need a breakfast discussion of this next weekend....

cheers,
greg
Click to expand...

I'm sure there was some downwash, but my line of thought is that it doesn't persist very far downward (as does rotor wash). It has to be there to some extent, as that is perhaps what we know as ground effect (I know its more complex than that, but downwash is probably a component. In LAX and MSP, I stopped when I saw I was going to get flown over. Didn't feel any wash (7-4 maybe at 100-150'). No wash, just the whoosh-snap of the vortices. Trees didn't even show a marked effect, even after the plane passed, as far as I could tell...where as a much smaller smaller helo at that altitude might be felt...dunno.

And Paul, I got kinda lost in your last post...soes that explain why wings don't make downwash and props/rotors do? Can you dumb it down for me? :p

All in fun, no slept lost last night! ;) We'll see what my FO thinks of all this...good banter for any long legs we might have (cruise, non-critical phase only, of course! ;))

Cheers,
Bob
 
Everbody knows where the internal wing air goes.

On an RV, it comes rushing into the aileron pushrod holes in the fuse and freezes your a..

I guess I will change my stall speed by installing pushrod boots. :)

The internal body pressure is irrelevant to calculation of lift generated by a particular shape unless that pressure changes the shape of the body.

But, I'll put my two cents in for a test. Since the Coanda (top wing surface) effect is dependant on viscosity and boundary layer attachment, who is going to volunteer to take a ball peen hammer to their rear upper wing skins to see if drag in the turbulent region can be reduced ala golf ball dimple style boundary control ????

Ok.Ok so that may be silly but I'm half way serious given the reports on the irregular geometry of whale fins and some remarkable fluid dynamics related to them. I'm looking forward to a homebuilt with wings like THAT.
 
elippse said:
Try this: The upper surface lift is 2 X sin(2 X alpha), 0.07/degree, and the lower surface lift is sin(2 X alpha), 0.035/degree, with a combined CL of 0.105/degree. 'Look familiar? The upper surface lift is due to Coanda effect, and drops off rapidly when the adverse pressure gradient at high CL causes flow back up the surface which destroys the Coanda effect, which depends upon the fact that there is no air acting normally to the surface to allow the reduced pressure. The lift on the lower surface follows a sinusoid which peaks at 45 degrees, then rolls-off to zero at 90 degrees. This is the pressure-lift you experience on a kite, water skis, and your hand held out of the car window.
Click to expand...

My head hurts:p (but its a good kind of hurt!).

rzbill said:
who is going to volunteer to take a ball peen hammer to their rear upper wing skins to see if drag in the turbulent region can be reduced ala golf ball dimple style boundary control ????
Click to expand...

There must be a few VAF members in Kansas or Oklahoma. I volunteer them for doing the test by leaving their airplane outside next summer.:D

greg
 
Try this, RV. The lift of an airplane has to equal the weight to stay at a given altitude in the air, which is the mass-flow rate times the downwash velocity. When a plane is moving rapidly through the air, it intercepts a very large volume, mass, which is approximately a circle having the diameter of the wingspan times the forward speed. So the downwash velocity isn't very high, especially for a large jet landing at 140k or so.
But a helo in hover has only the mass-flow which it induces which is not very much and so the downwash velocity must be high. It is similar to the high velocity of the air behind your plane when it's at static full throttle. That is also why the helo experiences translational lift as it begins to move forward and more air mass flows through the rotor disc.
My little 125 HP plane delivers a measured 220 lb static thrust at WOT and 2200 rpm vs 2800 rpm rated or 98 HP. That means that the average velocity, Ve, downstream of the 62" prop is about 54 fps, 37 mph, at sea-level, but at 200 mph at sea-level, with drag=thrust of about 164 lb, it's about 11 fps.
 
rzbill said:
...who is going to volunteer to take a ball peen hammer to their rear upper wing skins to see if drag in the turbulent region can be reduced ala golf ball dimple style boundary control ????
As you probably know, the golf-ball effect only occurs at a narrow range of Reynolds numbers.
Click to expand...
 
elippse said:
Try this: The upper surface lift is 2 X sin(2 X alpha), 0.07/degree, and the lower surface lift is sin(2 X alpha), 0.035/degree, with a combined CL of 0.105/degree. 'Look familiar? The upper surface lift is due to Coanda effect, and drops off rapidly when the adverse pressure gradient at high CL causes flow back up the surface which destroys the Coanda effect, which depends upon the fact that there is no air acting normally to the surface to allow the reduced pressure. The lift on the lower surface follows a sinusoid which peaks at 45 degrees, then rolls-off to zero at 90 degrees. This is the pressure-lift you experience on a kite, water skis, and your hand held out of the car window.
Since the upper surface lift equation is similar to the equation of the side thrust produced by a bend in a hose carrying a flow of water, and the lower surface lift equation is similar to the equation of a jet of flow deflected by a flat plate, you could properly call this version of the lift produced by a wing the "hose and kite" lift hypothesis! ;)
You resolve the lift on a propeller blade by its induced angle of attack plus its pitch angle into its forward, thrust, and tangential, torque, components. Since the downwash velocity is exactly opposite to the direction of the lift, it is the lift divided by the mass-flow, it to can be resolved into its axial flow component and its tangential component; this is the swirl behind the prop or rotor. However, keep in mind that in propeller theory, the velocity at the disc is doubled downstream of the blade in its "vena contracta" flow.
Hasn't this been an interesting discussion? It's always good to challenge the the proposed hypotheses of how things take place for so many are founded not on scientific principals but instead on observations that are flawed. For instance, when I tried to explain to my wife that the heat pump in a refrigerator took the heat inside and pumped it to the outside, she replied "How can that be since it's colder inside than outside!". 'Sorta like "suck"!
Click to expand...

elippse said:
Try this, RV. The lift of an airplane has to equal the weight to stay at a given altitude in the air, which is the mass-flow rate times the downwash velocity. When a plane is moving rapidly through the air, it intercepts a very large volume, mass, which is approximately a circle having the diameter of the wingspan times the forward speed. So the downwash velocity isn't very high, especially for a large jet landing at 140k or so.
But a helo in hover has only the mass-flow which it induces which is not very much and so the downwash velocity must be high. It is similar to the high velocity of the air behind your plane when it's at static full throttle. That is also why the helo experiences translational lift as it begins to move forward and more air mass flows through the rotor disc.
My little 125 HP plane delivers a measured 220 lb static thrust at WOT and 2200 rpm vs 2800 rpm rated or 98 HP. That means that the average velocity, Ve, downstream of the 62" prop is about 54 fps, 37 mph, at sea-level, but at 200 mph at sea-level, with drag=thrust of about 164 lb, it's about 11 fps.
Click to expand...

Scary, but the more I re-read these, the more I (almost) understand it...sort of! The mass flow vs. velocity makes sense, and explains the wing downwash vs. rotor downwash (and propwash). At least I can accept it on a TSAR basis, even if I could never do the math! :rolleyes: Thanks for the effort to write all that down.

We still haven't solved the puzzle of air in the wings though, have we? Still thinking that finding a way to stop all the leakage will make us faster (as long as the fix isn't heavier that the airplane itself!)

elippse said:
rzbill said:
...who is going to volunteer to take a ball peen hammer to their rear upper wing skins to see if drag in the turbulent region can be reduced ala golf ball dimple style boundary control ????
As you probably know, the golf-ball effect only occurs at a narrow range of Reynolds numbers.
Click to expand...

So Paul, would that be low Reynold's numbers (viscous, laminar flow) or high Reynold's numbers (high inertia, turbulent flow) that work with golf ball dimples? And where on the wing would you put the dimples...forward or aft, upper or lower surface?

How many ways can we hijack this thread! :p

Cheers,
Bob
Click to expand...
 
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Bug Bunny says "I never studied law"

elippse said:
rzbill said:
...who is going to volunteer to take a ball peen hammer to their rear upper wing skins to see if drag in the turbulent region can be reduced ala golf ball dimple style boundary control ????
As you probably know, the golf-ball effect only occurs at a narrow range of Reynolds numbers.
Click to expand...

Yep, sure. Post was mostly kidding.
It likely does not apply to our wings anyway since the dimple purpose is to trip boundary into turbulence before the more flow damaging laminar separation occurs. Smaller negative pressure gradients on the back of our wings probably don't need it as much as the severe curvature of a sphere does.

It sure would be nice to use the higher internal pressure for some sort of bleed or blow augmentation over the flaps but the mass would have to be replaced by an inlet in the freestream. When that happens, my mental picture starts thinking about violating first and second law of thermo (paraphrased "You can't win and you can't break even"). Smarter minds than mine have already thought about this for many years. Fun thread though.
Click to expand...
 
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