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Humidity and aircraft performance

Very little is out there about this…

I believe that a high humidity results in the air being thinner and is thus equivalent to a high temperature.

Administrator
Shoreham EGKA, United Kingdom

Peter I think you are correct but the effect on aircraft performance so small that any calculation of this would have no practical application.

For aerodynamics, isn’t that equivalent to the change of the air intial air density parameter at ISA sea level (which everybody assumes constant to start with)?

I guess you can convert that impact into height, temperature or speed and use whatever existing tools and all should be good with limited impact as long as wing temp is high than 0C

For engine performance, I don’t know what is the real impacts but again all should be good with limited impact as long as carburator temp is higher than 0C

I don’t think the presence of water vapour in fuel air mix lead to low thermic return as much as N2 in the air does…

Paris/Essex, France/UK, United Kingdom

According to this: https://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html and https://hypertextbook.com/facts/2000/RachelChu.shtml

At 15C and 1013 hPa, the maximum water content of air is 12.3 g/m3. At the same time, the mass of 1 m3 of air is about 1.2 kg. This means that the amount of water in air can be at most 1% by mass until it’s fully saturated. And some of this water mass displaces air, so the mass difference between 1 m3 dry air and 1 m3 fully saturated air is likely to be even less than 1%.

I agree with others that that 1% will not have any significant effect on aerodynamics or engine performance. (It may have other effects such as icing and reduced visibility though.)

On a standard pressure day in 5°C, the difference between 0 and 100% humidity is around 110 ft density alt. Alter ambient temp to 30°C, the difference becomes around 540 ft. In FL100, on a standard day, there is 70 ft difference in density alt and on an ISA+25-day, density alt differs just short of 400ft.

mh
Aufwind GmbH
EKPB, Germany

A_and_C wrote:

Peter I think you are correct but the effect on aircraft performance so small that any calculation of this would have no practical application.

Counterexample: fly a carb’ed aircraft at 30 F and 100 percent humidity. I guess you will see an impact on performance ;-).

Still true, the impact on glide performance (engine OFF) or om flying performance under same power (RPM*MP) is small between 0% and 100% humidity ;)

Paris/Essex, France/UK, United Kingdom
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