8
$\begingroup$

To start off, yes this question was asked before, yet I don't find the answers satisfying and here is why:

It appears to me that this phenomenon only appears during the approach phase of a flight and during low speed turns, or rather when the angle of attack is rather high. Also, usually the flaps are deployed when it happens, yet I'm not sure if that makes a difference.

Considering this, the answers wouldn't necessarily answer my question. They all talk about how there is a lower pressure above the wing, which causes the dewpoint to drop and thus makes the water condense. But is there a "lower" lower pressure when the angle of attack is higher and therefore making this phenomenon occur more often? Or is the point at which the laminar airflow gets turbulent (which moves forward as the aoa is increased) an important factor here?

Here is a picture to show what condensation I mean: shallow yet clearly visible clouds form on the wing

And a more extreme one:
This 757 (I think) is doing it right!

$\endgroup$
8
$\begingroup$

Yes, speed and flaps do make a difference.

The lift created by the wings is approximately constant over the duration of a flight. With full tanks, it is higher at the beginning of the flight, and when the aircraft banks, it is again higher to create the centripetal force for changing its course. But the difference is small¹.

Lift is caused by a pressure difference between the two sides of the wing. In cruise, this difference is low but extends over much of the wing's chord. At low speed, however, the suction on the top surface has a peak right aft of the leading edge, so the maximum suction is bigger than it is in cruise. Exactly this changed pressure distribution causes the humidity to condense, because it lets the air's relative humidity rise above the condensation point. In cruise, the pressure change is lower and the dew point is not exceeded. Also, since the airplane flies slower when close to the ground, the air has more time for the condensation to develop.

See below for a color-coded plot of the speed distribution over a typical airliner airfoil in the landing configuration. Red symbolizes the lowest speed (highest pressure), while purple stands for the highest speeds with the lowest pressure. Such suction peaks must be avoided in cruise to enable the aircraft to fly at a high cruise Mach number.

Airliner airfoil with high lift devices in landing configuration

Airliner airfoil with high lift devices in landing configuration (picture source)

¹ The fuel fraction of airliners rarely exceeds 40%, and the added lift in a 30° bank is only 15%.

$\endgroup$
  • $\begingroup$ There are two more reasons for why this happens mostly during take-off and landing: 1: You can most often see it just below the clouds, or close to the ground, in some places which also often have fog because that's where humidity is highest. 2: The required lift during approach is not higher than in cruise, so the absolute pressure difference isn't, either -- however, the pressure distribution is less even. For example, you'll often see water condensing in the very strong flap tip vortices (which are closed in cruise, of course). $\endgroup$ – Zak Aug 1 at 23:07
7
$\begingroup$

As the angle of attack increases, the pressure on the upper surface of the wing becomes lower. The following figure shows this. Note that the pressure on top is suction.

Pressure change with AoA

Source: avstop.com

During the landing phase, the angle of attack of the aircraft is higher compared to to other regimes. This is the reason condensation clouds are formed during the landing phase.

However, the clouds are formed during any subsonic maneuver that reduced pressure over the upper surface of the wings, like high-g turns.

F 18 Condensation Cloud Source:www.telegraph.co.uk

$\endgroup$
  • 6
    $\begingroup$ Moisture is more prevalent in the lower troposphere, which contributes to seeing the phenomena more commonly at low altitudes. $\endgroup$ – casey Aug 23 '15 at 20:26
  • $\begingroup$ +1 great picture $\endgroup$ – PJNoes Jul 12 '18 at 18:02
2
$\begingroup$

Let's remember for a second that on the upper side of the wings there is lower pressure than the free stream (air far from the aircraft).

If the aircraft is travelling through humid air, the adiabatic process (which all packets of air that end up near the wing have to "suffer") dictates that:

$$P_{\infty} ^{(1-\gamma)} \cdot T_{\infty} ^\gamma = P_{wing} ^{(1-\gamma)} \cdot T_{wing} ^\gamma $$

and since we just remembered that $P_{wing} < P_{\infty}$, it means that $T_{wing} < T_{\infty}$.

Still from the wikipedia link above:

Adiabatic cooling occurs when the pressure on an adiabatically isolated system is decreased, allowing it to expand, thus causing it to do work on its surroundings. When the pressure applied on a parcel of air is reduced, the air in the parcel is allowed to expand; as the volume increases, the temperature falls as internal energy decreases.

If the initial $T_{\infty}$ was already low enough, or if there was enough umidity in the air, the packets of air above the wings finds themselves below the dew point, producing the condensation that prompted your question.


But is there a "lower" lower pressure when the angle of attack is higher and therefore making this phenomenon occur more often?

Yes, higher angles of attack mean higher lift, that is generated by higher differential pressure between upside and underside of the wing. Remember that the lift is roughly

$$L \propto \alpha \cdot V^2$$

and since the weight of the aircraft is not reduced, to keep the lift constant at lower speeds (such as the speeds just before landing) you need an higher $\alpha$.

To prevent the need of extreme angle of attacks, the effect of the flaps is to increase the $C_{l\alpha}$, meaning that for the same angle of attack you get more lift, changing the proportional relationship above.

$\endgroup$
1
$\begingroup$

When you next snap open a can of fizzy drink watch carefully at what first comes out of the hole. A tiny cloud of condensation that quickly disperses. This is your personal adiabatic decompression demonstrator. It's exactly the same Physics as what's going on in the photos above.

$\endgroup$
-4
$\begingroup$

it appears to me that this phenomenon only appears during the approach phase of a flight.

Flying at high altitudes where it is -40°F fuel in the wings cold soaks.

That is my answer for the first two pics of the commercial aircraft above, the one pic of the vapor bubble or whatever about the fighter jet I would remove from the discussion too many unknowns about the picture regarding speed and the maneuver. Commercial Jet aircraft flying > 30,000 feet altitude for X amount of time the wing becomes a nice cold plate increasing the likelihood and making it more common to see during the approach/landing phase of a commercial flight.

$\endgroup$
  • 2
    $\begingroup$ It's unrelated to the temperature of the wing. The cause of condensation is temperature drop due to adiabatic expansion of air over the wing. $\endgroup$ – DeltaLima Jul 12 '18 at 21:20
  • $\begingroup$ if it had nothing to do with temperature of the wing/fuselage then "phenomenon" would be seen at same amount during climb to altitude as it does on descent. It does not. Temperature of the fuselage is a driving factor in addition to the air temp and humidity on descent. water vaporizes easier under lower pressure, approach of 200+ knots = vapor cloud under appropriate conditions: colder aircraft or more humidity, or greater pressure drop about the wing (flaps) will increase the likelihood of the vapor cloud. $\endgroup$ – ron Jul 16 '18 at 17:26
  • $\begingroup$ The lift coefficient during climb to altitude is lower that during final approach, that is why you see more condensation during the approach / landing phase, but not exclusively. The temperature has no effect; on approach, the airspeed is approximately 90 m/s. The wing chord is approximately 4 meters. The exposure time of the air to the wing is thus 44ms; way to short for any significant heat transfer. Also note that the condensation is not only at the wing surface, but also further away; another pointer that it is not due to heat transfer from the wing. $\endgroup$ – DeltaLima Jul 16 '18 at 17:46
  • $\begingroup$ I think you forgot to mention the speed of N1 and N2. I'm out. $\endgroup$ – ron Jul 19 '18 at 14:13
  • $\begingroup$ I have a video of a 777 taking off out of ORD on a 70Deg F day and there’s plenty of condensation above the wing. As mentioned above temperatures of wing surface due to fuel would have no effect in this case. It would be a result of pressure differences. $\endgroup$ – user33273 Jul 23 '18 at 3:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.