Aerodynamics?

[beaky]

This will keep the academics arguing for years.

LOL! It has... and will... and you better not be calling me an academic- them's fightin' words, mister.

You Colonials usually go the opposite way (I suspect just for the sake of being different) but I don't think so in this case. I see angle of incidence being the angle the wing is fitted to the aircraft. This is usually fixed. Angle of attack can be changed in flight with the elevators.

Whew, that's a relief... I heard somewhere that you Brits have it all backwards... . I actually own a copy of the very dry but fascinating An Introduction to aeronautical Engineering, Vol.1 : the Mechanics of Flight by A.C Kermode, B.A., Fellow of the Royal Aeronautical Society (printed 1941), I was thinking I got that impression there... let's see... flip, flip... but no, he's got it right, too. Probably just one of those pernicious myths you hear hanging around airport bums.

And for what it's worth, here's an excerpt that shows an even more interesting view of how a wing produces lift, which integrates the pressure and downwash theories:
Another way of looking at it is to consider the curvature of the streamlines.  In order that any particular particle of air may be deflected on this curved path, a force must act upon it towards the centre of the curve, so that it follows that the pressure on the outside of the particle must be greater than that on the inside; in other words, the pressure decreases as we approach the inner streamlines, i.e. the ones near the top of the aerofoil. This point of view is interesting because it emphasizes the importance of curving the streamlines downwards, which is the essence of the whole matter.
Hmm. I have only skimmed this little book, mostly to admire the nice photos of vintage airplanes, but I should really hunker down and read it thoroughly some time. ..

PS. Surely this is the whole crux of the argument. The wing is moving through the air, not the other way round.

You're absolutely right, but the "down and back" I keep referring to describes only the air's movement relative to the path of the airplane... although when you consider how the air following the curve of the airfoil (and the zone of higher pressure ahead of its leading edge  produced by it as it moves throught the air) apparently accelerates, there's a little more to it... it all looks so simple, but man, is it complicated!!
Whatever- all I can say at this point is that if I start thinking about fluid dynamics on the molecular level while flying, I'm going to end up in a tree somewhere, but if I remember that the wing must keep that downwash going, I'll be OK.
If I ever want to design an airplane, though...  ;)
 

[Hagar]

Just in case you missed my PPS. Check this out. http://www.dfrc.nasa.gov/Gallery/Photo/B-747/Medium/ECN-4242.jpg

[Brett_Henderson]

I like Rotty's reconcilliation (and so does my engineering pal, as he's cursing with each email for making him think about this stuff).

My last thought is comforting and confusing at the same time. I read a formula showing the total pressure-differential lift for a Cessna at 100kias as 60 pounds.. and it makes sense. Where 60 pounds is obviously a fraction of the lift needed to keep a 2300 pound Cessna airborne.. my pal pointed out that it's like a thrust itself. It's not just 60 pounds in a single impulse.. it's like 60lbs/sec/sec.. (acknowledging the fact that mass and weight are ony interchangeable when gravity is a constant) and will have a cumulative effect storing the lift along with stored inertia (both as altitude and velocity)..

[beaky]

I like Rotty's reconcilliation (and so does my engineering pal, as he's cursing with each email for making him think about this stuff).

My last thought is comforting and confusing at the same time. I read a formula showing the total pressure-differential lift for a Cessna at 100kias as 60 pounds.. and it makes sense. Where 60 pounds is obviously a fraction of the lift needed to keep a 2300 pound Cessna airborne.. my pal pointed out that it's like a thrust itself. It's not just 60 pounds in a single impulse.. it's like 60lbs/sec/sec.. (acknowledging the fact that mass and weight are ony interchangeable when gravity is a constant) and will have a cumulative effect storing the lift along with stored inertia (both as altitude and velocity)..  i.e.   1 lb of thrust can eventually accelerate a 1,000,000 lb object to near the speed of light when drag is anything less than 1 lb.

I need a beer too.. and I don't drink  lol

Hooboy, that's a lot to absorb... but only at first glance. A second careful reading shows it's pretty simple (gravity is expressed in ft per second per second, so...); what would have me jumping out the window would be seeing it expressed mathematically... :-/ Aerodynamics is full of surprises for us; we may fly, but we weren't made to know air the way birds do.
But as we should have known, we were all on the same page all along.

Poor  Theis... he just wanted a simple explanation, and we've dragged him to academic hell and back, with the grandpappy of all hangar-debate topics...

[beaky]

Just in case you missed my PPS. Check this out. http://www.dfrc.nasa.gov/Gallery/Photo/B-747/Medium/ECN-4242.jpg

That's interesting... what's with the vapor trail off the right stab, I wonder? Is it slipping?

[Brett_Henderson]

Yeah.. Gravity is 32ft/sec/sec and when you factor that out of pounds you have the English unit for mass.. the Slug ..  Or when you plug it into Grams.. you end up with Newtons (the true metric equivalent to pounds).

The "/sec/sec" stuff just points out that it's a force, like gravity that accelerates not just moves an object.

[Hagar]

Poor Theis indeed. I'm sure he will find this discussion more interesting than the average crusty lecture. It's given me something to think about too. Rotty almost had me convinced.

I'm afraid all these complex formulae make my head spin. I'm just an 'umble fitter, a practical sort of chap. I once had a brilliant maths teacher who demonstrated that maths is like statistics. A skilled mathemetician can prove almost anything depending on how it's presented & will enjoy doing so.

That's interesting... what's with the vapor trail off the right stab, I wonder? Is it slipping?

Unfortunately the caption doesn't give the conditions at the time the photo was taken or the reason for the test. That break in the smoke could be something simple like a minor blip with the smoke generator. However, I think it serves its purpose by showing no significant downward movement of air (except behind the lowered flaps) which I would expect if the Newton/Coanda theory is correct.

I was looking for something to demonstrate the airflow behind the wing of an aircraft in normal flight without resorting to all this scientific gobbledegook. A picture can say more than a thousand words. Sadly I can't find the well-known photo of a B747 streaming contrails or the B-17 formation I was looking for. If I recall correctly both conclusively show the airflow behind the wings being deflected upwards rather than down. Then I suddenly realised I've seen the proof many times over the years at various air displays. I've even taken photos myself. I think this one of a B-17 streaming smoke from the engine exhausts demonstrates that there's no significant downward deflection of the air below or behind the wing in normal straight & level flight.


I'm not sure what these prove but thought I might as well post them anyway. Despite the attitude the aircraft was moving horizontally under full control.

[Theis]

OMG!!

You ask for a finger, and you get the whole hand!!

*Faints because of too much info!*

[beaky]

Hagar has definitely rattled me with those pictures, but I'm still on my feet.
Gimme a minute...
Okay, have a look at these:

I was trying to find a pic of a plane passing through smoke or mist; I think part of the reason so little deflection is seen in your pix is because of the forward movement of the plane, and the plane being the source of the smoke or vapor. It's not like the downwash effect is going to produce a pronounced S-curve in smoke trailing from a plane... it's more subtle in that case. But ag planes  provide a good study, as they are moving fairly slowly and in a level attitude...granted, the nozzles point downward a bit, and of course the spray is heavier than air, but...I think these pictures show downwash. Maybe I'm wrong.
This pilot could not have just been flying lower- is it just the spray settling, or does it seem to also be flattened behind the plane? This pic really shows the lateral component of airflow along the wing, something that just complicates this debate... But I will say that the obvious energy of the tip vortices indicates a much higher-than-ambient pressure situation below (and just aft of?) the wing...


This pic shows that the spray may not just be settling, causing it to be lower than the flight path (although it obviously will, eventually- Newton again). Look at the streams of spray nearest the vortice off the right wing. Is the vortice "carrying" it upwards, or what? It looks like the spray is actually higher than the plane well beyond what could be the "downwash zone" but this front view makes it hard to say.


this one supports my case pretty well, I think. If that downward trail of spray is due only to the velocity of the spray as it exits the nozzles, that would be some pressure in that system! I don't know how high the pressure would be, but... I'll grant also that the spray is expanding in a conical shape, so again this doesn't prove very much.


Here's a closer look at the spray exiting the nozzles. It's really not spraying downward much, but look at its path immediately after. It goes down. Not much; in fact, it's  as if it's only being deflected just beyond the flight path... which would make sense in level flight, as all that's required for that is, well, enough lift for level flight. Brett's earlier post about the forces required helps explain this. I can't find a pic of a plane starting to climb in a relatively flat attitude (yet! ); I think that might tell the tale. And look again at this one: about a plane-length behind the agplane, the mist is still fairly coherent, but hasn't sunk much due to gravity... from the trailing edge of the wing to the edge of the picture, you see a long , gentle curve followed by a flat area several feet below the flight path (assuming it didn't climb at all). Of course I'm reading a little into this , but... if it was all about a high forming above the wing due to air accelerating on a curved path, would gravity  alone bend the mist this way?



Now, everybody's seen this nice picture; often used to illustrate wingtip vortices (another thing we could discuss forever). But it shows something else... look closely at the trough carved by the plane. If there were no downwash, wouldn't the vortices just be flung up over the level of the cloud deck? And if the air were just slipping under the wing, practically undisturbed, would there be such a deep trough in the cloud? Of course, the plane is just flying off the edge of the cloud, and possibly descending slightly, but I believe that the part of the cloud nearest the camera, where the cloud top is sloping down and not providing much vapor for good vortice-viewing, shows the dimensions of the depression left by the downwash before the cloud moves in to close the gap as the vortices dissipate. Not proof of anything, really, but...

[Hagar]

Ah, I see you're not giving in without a fight.

I don't think using a cropduster to demonstrate airflow gives the true picture as the spray is intended to fall as evenly distributed as possible. I'm sure there would have to be a reasonable pressure to form the spray & the nozzles would be directed downward. I really can't understand where this downward deflection is supposed to come from on a conventional wing that's flying straight & level.

I found that B747 photo I was looking for. http://www.airliners.net/open.file/0239080/M/
Lots more amazing contrail photos here. [url]http://www.airliners.net/search/photo.search?album=6740
[/url]

You're free to think what you like. These are more than enough to convince me.

[Theis]

You two just WONT give up a good fight, huh?

[Brett_Henderson]

I still can't picture the downwash both coming from the wing AND being a thrust-like lifting force..ESPECIALLY in level flight.

I can however picture a downwash when AoA lift is added to the picture while climbing (and sacrificing airspeed).. that (to me ) is no differrent than flap downwash.. or a boat's wake.. It's evidence that lift is occuring, but not the source of the lift.. any more than a boat's wake (in a turn) is helping propel the boat..

[Brett_Henderson]

Now I'm trying to picture a venturi-like tube attached to an aircraft and trying to explain to myself that because the air leaves the tube faster than when it entered, there's at least enough thrust created to cancel the drag that the tube creates.. let alone have some left over to do any type of work,, such as lift.. It aint addin' up  

[Nexus]

Guys, aeronautical engineers aren't even 100% sure why aircrafts fly.
All of you are correct, more or less.
Differential pressure
Coanda effect
Bernoulli and Newton etc.

But the air leaving the wing is indeed deflected DOWNWARDS, thats why you'll get in trouble should you decide to choose a lower glidepath than the heavy infront of you.

[Brett_Henderson]

I think you're talking about wake turbulence and/or wing-tip vortices. There's no disputing that they travel downward. And it's not just heavies..

I can tell you first-hand about it, and turning a low base-leg just after a Citation had touched down after a long, straight-in final.