May 09, 2012, 08:30 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,420 Posts
Quote:
 Originally Posted by ShoeDLG I created this airfoil in XFLR5 and analyzed it at an AOA of 7 degrees. The results show a Cl of +0.122 (so the force on the airfoil is upward), but if you look at the boundary layer, you can plainly see the air is being deflected upward behind the airfoil. We all know that an airfoil can't be lifting unless it is deflecting air downward, so when are they going to fix XFLR5?
Your comment is appreciated. My thoughts:

1) At sufficient distance behind the airfoil, do you really believe that there is an upwash not a downwash? Really? Really and truly? I am very skeptical of this idea.

2) If so, even if the net momentum transfer is upward, could the net force transfer be downward? So that a scale under the block of air in which the airfoil is flying still feels the aircraft's weight? How could it possibly be otherwise?

Steve
Last edited by aeronaut999; May 09, 2012 at 08:50 PM.
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 May 10, 2012, 01:46 AM Registered User Germany, BW, Stuttgart Joined Mar 2012 1,204 Posts 1) How can what happens behind the wing make any difference? You sometimes see a barrier near the end of a runway to deflect jet exhaust upward. If the exhaust from a jet engine gets deflected upward, does that mean the jet is now pushing downward? Of course not. It's the direction the air is going when it leaves the nozzle that matters. Just look at all those planes using vectored thrust. If the air is pushing up on a wing, certainly Newton's Laws must require the air to leave the wing with downward momentum. Jets and wings use the same mechanisms to create aerodynamic forces after all (look inside a jet engine some time, and I bet you'll see lots of little wings). 2) Surely you aren't suggesting a wing can create lift without deflecting air downward. Next you'll be saying you can stand behind a turning propeller and not feel the momentum. There have been several threads where people have insisted downward momentum is required for airplanes to fly, so it's true. The physics behind lift must be simple (I mean you can actually feel the momentum). Look how many words you've already had to use to suggest otherwise.
 May 10, 2012, 02:07 AM B for Bruce The 'Wack, BC, Canada Joined Oct 2002 14,577 Posts An odd train of thought occurs to me in connection with this. In some other thread a while back the case of a lighter than air ship came up in connection with the "how" of why it floated in the air. The upside was that the envelope filled with the lighter than normal gas in the floatation bag provides lift by being big enough vertically that it uses the natural pressure gradient of the atmosphere to end up with the pressure at the top side being enough less than the pressure from below. So the air simply pushes the balloon or blimp upwards. Let's look at our airplane again. We've got a low pressure zone above the wing and a higher pressure below the wing. Could it be that these pressure changes above and below the wing mimic this same air pressure gradient caused by the balloon? In effect the wing produces a "lighter than air" zone above the plane and a "heavier than air" build up of pressure below that mimics the effect of the pressure gradient found operating on a LTA airship? I don't have the math to show this idea one way or the other but it sort of makes sense. Note that no downwash is needed in this way of looking at the how of producing lift. But it DOES fit in with your conservation of momentum theory. Like with the LTA balloon or blimp the pressures around the wing all work to ensure that the final weight of that particular column of air is pushing down with the same mass as a similar column of air with no airplane or LTA ship within the column. And since the air column doesn't weigh any more with or without the airplane then it's not trying to pull the planet towards the airplane. It also explains why you can stand below the glide path of a big heavy airliner that passes overhead only a couple of wingspans high and not feel a localized pressure pulse or feel any downwash on the ground.
 May 10, 2012, 02:36 AM Registered User Nederland, GE Joined Jan 2010 164 Posts Bruce, that is spot on. that is exactly what i was getting at with my sentence about the possibility of flying, already pushing from within the air. also hence, one can say the planet is *already* pulling down on the air, and pressurizing it.
 May 10, 2012, 05:25 AM Registered User Germany, BW, Stuttgart Joined Mar 2012 1,204 Posts For those with access to Hulu, I think the following clip captures the nature of the "downwash" / "no downwash" debate pretty well... http://www.hulu.com/watch/61320/satu...mmer-floor-wax Is it possible for a wing to fly without adding any net momentum to the air? Absolutely. Just seal a hangar and fly a plane inside it. Because a sealed hangar prevents the CG of the air from translating, the net momentum of the air in the hangar must remain zero at all times. Sealing the hangar won't prevent the wing from flying (I'm speculating... haven't actually tried sealing the hangar). Is it possible for the lift force generated by a wing to be balanced exactly by momentum transfer to the air? Absolutely. I have trouble coming up with a simple arrangement to show this, but imagine a huge, isolated volume of air deep in outer space. Suppose you had a positively-charged airplane flying through the air at a constant distance from a very long, negatively-charged, fixed rail (extremely contrived, but the electric attraction allows the plane to translate steadily while generating a constant lift force, and by balancing the lift force with something other than gravity, any acceleration of the air comes only from interaction with the wing). The air is exerting an aerodynamic force on the wing, so Newton's Third Law suggests the wing must exert an equal and opposite force on the air. In this unique example, there isn't anything else that can exert a force on the air, so Newton's Second Law suggests the air must experience a rate of change of its momentum equal and opposite the lift force. There you have it... a case where a lifting wing flies without transferring any net momentum to the air, and a case where a lifting wing transfers momentum to the air at a rate exactly equal to the lift force acting on it, and in the opposite direction. Turns out a lifting wing is a floor wax AND a dessert topping. Next topic... Last edited by ShoeDLG; May 10, 2012 at 05:37 AM. Reason: changed desert ->dessert and added last line
 May 10, 2012, 10:53 AM Registered User Nederland, GE Joined Jan 2010 164 Posts yes, although, in space a volume of air does not stay, unless pressurised, and/or heated. it would collapse or otherwise dissipate. one could suspend a 3D matrix of marbles, but then the aircraft would shoot the marbles out while deflecting them (thus pressurizing or heating the very cold "gas") furthermore, the density of a actual gas like that in space is very very low. one may say the aircraft must breach the confinement, or else experience a very very low force.
 May 10, 2012, 11:00 AM Registered User Germany, BW, Stuttgart Joined Mar 2012 1,204 Posts So you're saying you REALLY need to stretch to contrive a situation where the lift of a steadily translating wing would be exactly balanced by momentum transfer to the air. Agreed.
 May 10, 2012, 11:12 AM Registered User Nederland, GE Joined Jan 2010 164 Posts i am not sure i understand that yet, but, i did mean to point out certainly the air must have a pressure or density, and if not the aircraft will provide it. the energy needed to pressurise or contract the air beforehand (whichever is applied) is considerable. imagine shutting off gravity on earth and hence unconfining the air. it would give a considerable blast. that energy must be present, for a plane to begin to fly. a balloon is a exception it may seem, since it can suspend in space and zero G, but, it does not actually "float" in air then. likewise a unpropelled airplane would come to a stop after a while, in a pressurised cabin at zero G. i mean to say that a airplane is a way to split random heat/kinetic energy of the air, into a defined pressure gradient, and (almost) back. for a balloon that means lighter gas inside (helium taken out of ambient air say) or heating it. Last edited by m4rc3l; May 10, 2012 at 11:27 AM.
 May 10, 2012, 01:18 PM Registered User Germany, BW, Stuttgart Joined Mar 2012 1,204 Posts I'm no cosmologist (nor cosmetologist for that matter), but I was just thinking that if you put enough air molecules close to each other in outer space, their mutual gravitational attraction would create a blob (sort of like a gas giant planet without the solid core). By adjusting the number of air molecules (i.e. the diameter of the blob) could you make the density at the center about what it is at Earth's sea level? You might have to throw in some heaters to match both temperature and density. The bottom line is that trying to imagine a scenario where you have a lifting wing in steady flight through completely unbounded air is a challenge. In a terrestrial scenario it's difficult to cleanly isolate the wing/air interaction because the pesky ground always seems to get in the way. Perhaps the relative simplcity of the "no downwash" scenario and the complexity of the "full downwash" scenario says something about which mechanism better applies to airplanes flying on Earth?
 May 10, 2012, 01:27 PM Registered User Nederland, GE Joined Jan 2010 164 Posts indeed, in the case of gravity i suppose. that is a planet right there ;p. the ambient airpressure on the surface the threadstarter mentions, is already there this way, and enables terrestrial flying or rather gliding in the same gravity. in space a pressurized cabin is needed, and another source of propulsive force, but no sustained lift or downwash direction necessarily. eventual sustained downwash will be outwards to the walls i guess. then like a vortex core one could fly a core of low pressure amidst of high around. Last edited by m4rc3l; May 10, 2012 at 01:31 PM. Reason: additions
May 10, 2012, 02:06 PM
B for Bruce
The 'Wack, BC, Canada
Joined Oct 2002
14,577 Posts
Shoe, I can't see the video so I can't comment.

Do some planes push air down under some circumstances? Most certainly. Check out this youtube video;

 C-17 Landing on dirt strip in Southern Afghanistan (1 min 5 sec)

The field in this case appears to be very fine and light talcum like dust. So it's easy to spot the pressure disturbance from the ground effect as the wing comes down to within a span or so of the touchdown.

For another option check out this video at some tropic beach. Note how low some of the big heavys are passing over the water and sand. Yet no APPARENT downwash that kicks up the sand. I suspect that again there's some ground effect pressure build but clearly it's not strong enough even at these lower altitudes of only at or just over one wing span;

http://www.youtube.com/user/billschannel?v=ce0MsHMl8GA
May 10, 2012, 02:41 PM
Registered User
Nederland, GE
Joined Jan 2010
164 Posts
nice also nice the reporter with a spitfire, one can see a sortof displacement wave over the grass
 Spitfire low level (0 min 35 sec)
 May 10, 2012, 03:23 PM B for Bruce The 'Wack, BC, Canada Joined Oct 2002 14,577 Posts An oldie but a real goodie..... That Spit was well below one span worth of altitude and actually pulled up as he came up on Allan.... We need to differentiate between the well known pressure effects that extend out above and below the wing by a about two chord lengths and more. This pressure effect that extends above and below the airfoil is part of the ground effect. It's also why there's much to be lost and little to be gained by spacing wings of a biplane closer together than one chord.
 May 10, 2012, 04:04 PM Registered User Germany, BW, Stuttgart Joined Mar 2012 1,204 Posts Bruce, "Do some planes push air down under some circumstances?" Yes, I agree, most certainly, but to identify momentum transfer as THE mechanism responsible for lift, it's not enough to show that some air is being pushed down. You would have to consider all the air being influenced by the plane and show that the net rate of downward momentum transfer to the air is equal (and opposite) to the lift force. When you look at the C-17 landing, you can see that the dust outboard of the wingtips gets drawn upward. The plane is simultaneously accelerating air upward and downward. To understand the role played by momentum transfer, you need to subtract the all upward momentum transfer from all the downward transfer.
May 10, 2012, 05:10 PM
Registered User
United States, UT, Salt Lake City
Joined Oct 2007
9,917 Posts
Quote:
 Originally Posted by ShoeDLG Bruce, "Do some planes push air down under some circumstances?" Yes, I agree, most certainly, but to identify momentum transfer as THE mechanism responsible for lift, it's not enough to show that some air is being pushed down. You would have to consider all the air being influenced by the plane and show that the net rate of downward momentum transfer to the air is equal (and opposite) to the lift force. When you look at the C-17 landing, you can see that the dust outboard of the wingtips gets drawn upward. The plane is simultaneously accelerating air upward and downward. To understand the role played by momentum transfer, you need to subtract the all upward momentum transfer from all the downward transfer.
Nopthing unusual there - push over a sheet of plywood in a dusty room- same thing happens - air is compressed under the sheet - whilst pressure is lowered over the sheet
the air squashed out around the sheet simply fills the low pressure area

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