Humidity Effects on Aircraft Drag 1

[pmac798]

I am trying to determine how the drag on an aircraft changes as the absolute humidity changes. Does anyone know a good reference on this subject?

 

[Ori]

I don't have a good reference for you but, qualititavely, I would think that the effect of humidity on the aerodynamics of the vehicle would be through changes in density... and drag will be proportional to the density. Hope this helps.

Ori

 

[WKTaylor]

pmac798...

Humidity affects "densify" the atmosphere [IE: Rho increases] thereby increasing drag... up-to 100% humidity and condensing/raining atmosphere which can dramatically increase the drag of a body or airfoil!

... and don't forget that humidity also affects engine performance by reducing available oxygen!

"OLD PILOTS" remember the "3-Hells" of aviation which have caused many accidents!

3Hells = High, Hot & Humid



Regards, Wil Taylor

 

[RonFreimuth]

Effect of Relative Humidity:

Two replies contradicted each other. In fact, higher RH reduces air density.

This is because the molecular weight of water vapor, H2O is lower than that of O2 and N2, the main components of air.

At a given temperature and pressure, there are a specific number of molecules per cubic ft of air. Water vapor displaces N2 and O2, so the density decreases.

The effect on AC performance during TO is mainly through the effect on reciprocating engine HP. The effect appears to only become significant around 90% RH at 85F or higher.

Turbine AC have virtually no problem with high humidy at normal runway altitudes. They are generally flat rated up to considerable altitude and ambient temperatures.

Ron Freimuth

 

[WKTaylor]

Folks..

I had a long talk with our aero folks...

They DO factor-in humidity for engine performance reductions, especially large transports operating at maximum power out-put.

They generaly DO not factor humidity into aerodynamics calculations... since they use a "standard atmosphere" model. However, the aero guys are aware that humidity leads to increases in viscosity ... up to a certain RH point when moisture condenses-out in the low-pressure zones of the wing-stab-body ... typically visible as local vorticies and entrained visible moisture-vapor over the [low-pressure] surface of the wing/stab. Most large transports are unaffected by this condition, since it is transient... especially in warm climates. HOWEVER: during heavy-weight and slow approaches in freezing conditions, rough/cold skin surfaces will entrap these condensed moisture particles... and there will be a serious drag-rise as the moisture "freezes" to the skin. Regards, Wil Taylor