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Faro
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Pressure Above Vs Below Wing

Thu Sep 15, 2011 11:44 am


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Compared to standard atmospheric pressure of 1013 Hectopascals, how low would the lowest pressure in the condensation in the above pic be?

Is wing lift caused more by low pressure on the upper wing surface or high pressure on the lower wing surface? Would wing ribs/spars (not horizontal wing surfaces) be in a state of net vertical tension or compression when airborne (or is it more complex than this)? What about on the ground?

Faro
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Starlionblue
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RE: Pressure Above Vs Below Wing

Thu Sep 15, 2011 12:03 pm

Quoting faro (Thread starter):
Is wing lift caused more by low pressure on the upper wing surface or high pressure on the lower wing surface?

In essence those are the same thing. That is, lift is generated by the pressure difference between the upper and lower surface.

[Edited 2011-09-15 05:04:14]
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Fabo
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RE: Pressure Above Vs Below Wing

Thu Sep 15, 2011 1:25 pm

Quoting Starlionblue (Reply 1):
That is, lift is generated by the pressure difference between the upper and lower surface.

Indeed, but compared to static pressure outside the airplane and its wake, how does it fare?
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tdscanuck
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RE: Pressure Above Vs Below Wing

Thu Sep 15, 2011 1:54 pm

Quoting faro (Thread starter):
Compared to standard atmospheric pressure of 1013 Hectopascals, how low would the lowest pressure in the condensation in the above pic be?

About 970 (depends a lot on the speed, I wagged it at 150 knots and a minimum pressure coefficient of -1.5).

Quoting faro (Thread starter):
Is wing lift caused more by low pressure on the upper wing surface or high pressure on the lower wing surface?

As noted, it's the differential that matters. However, the upper surface pressure is considerably more below freestream than the lower surface pressure is above. In that sense, most of the differential is coming from the upper surface.

This is what the pressure distribution looks like around a modern supercritical airfoil:


Aerodynamicists usually convert absolute pressure to pressure coefficient for apples-to-apples comparrison. You usually invert the vertical axis (negative up) so that it corresponds to the upper surface and normalize the horizontal axis by the chord length:


Note: these two pictures aren't for the same airfoil, they're just good examples of how this type of data is presented.

Quoting faro (Thread starter):
Would wing ribs/spars (not horizontal wing surfaces) be in a state of net vertical tension or compression when airborne (or is it more complex than this)?

It's more complex than this. Most of the vertical load is being carried by the spars, which are in compression on top and tension on the bottom due to the bending. The ribs are mostly there to maintain the airfoil profile and their individual loading will depend on what the upper surface is doing and the fore/aft loading...I'd expect most of them to be in shear.

Tom.
 
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RE: Pressure Above Vs Below Wing

Thu Sep 15, 2011 8:42 pm

I always wondered how the pressure difference would work at extreme speeds, since the pressure on the top of the wing is limited to zero, while there wouldn't seem to be any limit to the pressure on the bottom.
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tdscanuck
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RE: Pressure Above Vs Below Wing

Fri Sep 16, 2011 5:16 am

Quoting nomadd22 (Reply 4):
I always wondered how the pressure difference would work at extreme speeds, since the pressure on the top of the wing is limited to zero, while there wouldn't seem to be any limit to the pressure on the bottom.

Once you go supersonic the aerodynamics qualitatively and quantitatively change. The exact shape of the airfoil becomes much less a factor and it boils down, almost entirely, to the shock wave structure. The way the shock relations play out you basically can't get an infinite pressure differential in real-world situations (temperature limits you long before you get to Mach numbers where you get absurd pressure differences).

If you get into really thin air you can get very high Mach numbers (the Space Shuttle went over M=25) where wierd things happen, but the density is so low in those cases that the actual forces are smaller than the speeds might make you think.

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ferpe
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RE: Pressure Above Vs Below Wing

Sat Sep 17, 2011 7:50 pm

Quoting tdscanuck (Reply 3):
t's more complex than this. Most of the vertical load is being carried by the spars, which are in compression on top and tension on the bottom due to the bending. The ribs are mostly there to maintain the airfoil profile and their individual loading will depend on what the upper surface is doing and the fore/aft loading...I'd expect most of them to be in shear.

Tom, I am a bit surprised the the wing box skins are not mentioned, sure the spars carry a lot of the load but they would buckle had you not the skins taking a substantial part of the load. (What you describe is a structure before stressed skin technology, i.e. a pre WW2 frame, covered by not load-carrying material for aerodynamic shape).

The total case is that the spars and the skins, formed like a box, carry the loads, both bending as before and the torsional ones coming from the aerodynamic pressure distribution over the profile.

The upper skin would be in compression and the lower in tension, the stringers that run the length of the skin helps the upper skin not to buckle under the compression load.
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tdscanuck
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RE: Pressure Above Vs Below Wing

Sun Sep 18, 2011 4:31 am

Quoting ferpe (Reply 6):
Tom, I am a bit surprised the the wing box skins are not mentioned, sure the spars carry a lot of the load but they would buckle had you not the skins taking a substantial part of the load.

The question I was responding to was about ribs/spars:

Quoting faro (Thread starter):
Would wing ribs/spars (not horizontal wing surfaces) be in a state of net vertical tension or compression when airborne

The upper and lower skins are, as you say, integral to the whole structure and carry huge tension/compression loads. However, they're carrying relatively little of the lift (shear) load to the fuselage because they're constantly "shedding" load onto the spars.

Tom.

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