Is the take off run longer on a moist day (high RH) ?
 

Hi,

I read in other interesting posts that high relative humidity (RH) does not affect significantly the power output of airplanes powered by a gas turbine engine (= turbofans, turbojets,...).

However I was wondering if the runway lenght necessary to get off the ground on a moist day (high RH) is affected !!?

Here goes my reasoning. Could anyone tell me whether I am wrong or right ? Thanks.

* 1st reason : I would think that on a very moist day (= high RH), the air density is smaller than usual so you need to roll at a faster TAS to get the required lift for lift off (Lift = Weight at lift off) since L = 0,5 Rho TAS² S Cl. So you need more runway to accelerate to that higher TAS.

Thus the answer is yes, a jet airplane needs a longer runway length on a day with higher relative humidity to get off the ground.

Do you agree ? Is my assumption right ?

And what would happen if the day is dryer (very low RH) than certification day ? The runway length to get off the ground would be shorter ?

* 2nd reason : Additionally, the pilot rotates its jet airplane at Vr. However Vr is an indicated airspeed. So if there is a higher RH than on certification day, the air is again less dense, so the TAS is higher than usual for a same indicated Vr (IAS). So again, to accelerate to that higher TAS we need more runway.

Is that correct ?

Thank you very much for your answer.

yes you are correct. in an atmosphere with high relative humidity, the take off roll will be longer. The most obvious reason is the reduction in density of the atmosphere and therefore, a reduction of thrust produced by th engines. Add these factors together and a longer take off run will be required.

The short answer to your question is "Yes it will".

But the longer answer is "But not by very much because ..................".

To understand why this is the case you we need to look at the amount by which varying humidity will affect air density.

If we compare the densities of pure water vapour and dry air at the same temperature and pressure we will find that the density of water vapour is about 63% of that of dry air.

If we then look at how much water vapour can be held in air we find that at 20 degrees Celsius at sea level saturated air contains about 1.8% water vapour.

If we combine these two figures we will find that the difference in density between dry air and saturated air is very small.

So the increase in take-off distance is also very small.

Thank you very much Keith !

So basically I consider my whole reasoning correct but to summarize, the change in take off run is not significant.