Condensation and pressure distribution
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Condensation and pressure distribution
I had a nice approach into MUC last week on an A340 with great condensation on the wing right up to touchdown. However, what I was able to see was confusing me a little bit. Condensation only happened aft of the mid chord of the wing, mainly on top of the flaps.
According to the pressure distributions I have seen so far, a modern supercritical airfoil with flaps and slats should have a suction peak in the area of the fixed leading edge at high lift coefficients, and most of the wing upper surface should see less pressure than the upper surface of the flap.
So what I would expect is to see lots of condensation on the forward half of the wing, exactly the opposite of what happened. How can this be explained ?
Does condensation take some time to occur, so does it take a certain time after air pressure has dropped and temperature therefore as well (ideal gas), before tiny droplets do form around seeds (dust particles) in the air, hence visible condensation is produced? Did the air travel along half of the wing before it finally condensates? Does condensation show the pressure which was present a few milliseconds ago?
Or do modern aircraft keep that much margin to Cl max during approach and even during the flare, that indeed the lowest pressure is located on the flap upper surface (where calculation / wind tunnel tests do show the lowest pressure for moderate lift coefficients)? Do we "waste" that much high lift capability to have a safe margin?
Is condensation on a wing visualizing the chordwise pressure distribution? Or just the spanwise lift distribution?
According to the pressure distributions I have seen so far, a modern supercritical airfoil with flaps and slats should have a suction peak in the area of the fixed leading edge at high lift coefficients, and most of the wing upper surface should see less pressure than the upper surface of the flap.
So what I would expect is to see lots of condensation on the forward half of the wing, exactly the opposite of what happened. How can this be explained ?
Does condensation take some time to occur, so does it take a certain time after air pressure has dropped and temperature therefore as well (ideal gas), before tiny droplets do form around seeds (dust particles) in the air, hence visible condensation is produced? Did the air travel along half of the wing before it finally condensates? Does condensation show the pressure which was present a few milliseconds ago?
Or do modern aircraft keep that much margin to Cl max during approach and even during the flare, that indeed the lowest pressure is located on the flap upper surface (where calculation / wind tunnel tests do show the lowest pressure for moderate lift coefficients)? Do we "waste" that much high lift capability to have a safe margin?
Is condensation on a wing visualizing the chordwise pressure distribution? Or just the spanwise lift distribution?
It happens where the local velocity is highest and thus the pressure and temperature is lowest.
Your aeroplane has slotted Fowler flaps, which is why you saw it where it was.
Your aeroplane has slotted Fowler flaps, which is why you saw it where it was.
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"my" airplane also has slats, so for example according to NACA Report 732 I would expect it to have by far the highest velocity at the slat, not at the flap. Especially as the A340 uses single slotted and significantly less deflected fowler flaps than some of the Boeing aircraft.
Does anybody know another source of pressure/velocity distribution around a modern supercritical airfoil with slats ?
Edit: found another source, AIAA 2002-0844, also confirming the lowest pressure / highest velocity at the fixed leading edge just aft of the slat and on the slat itself.
Does anybody know another source of pressure/velocity distribution around a modern supercritical airfoil with slats ?
Edit: found another source, AIAA 2002-0844, also confirming the lowest pressure / highest velocity at the fixed leading edge just aft of the slat and on the slat itself.
Last edited by Volume; 6th Nov 2013 at 12:25.
....around a modern supercritical airfoil with slats
I can supply a diagram if you wish.
A slat, or more accurately, the slot which it forms, energises the boundary layer, which helps to keep the flow attached at high alpha.
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Volume - there is no simple answer. Other factors which may be producing the phenomonen you describe could be
a cold area of wing surface in contact with fuel cooled at altitude
an area of reduced airflow attachment preventing any condensation from 'evaporating'
a cold area of wing surface in contact with fuel cooled at altitude
an area of reduced airflow attachment preventing any condensation from 'evaporating'
