Humidity and Helicopter performance

[pacrat]

Humidity correction is not a requirement in certification to my knowledge, yet I recall training on a Brantly B-2, in a past life and the hover ceiling graph showed the marked decrease in hover ceiling at 100% humidity as compared to about 30% I recall. It is without doubt true that high humidity will have a negative impact on performance but is the effect ever calculated by manufacturers at the performance flight testing stage?

In short can you answer these questions for my;
- If performance is always referenced to an ISA datum does ISA have limits on humidity before it becomes a required factor for specific consideration?
- When a correction is applied for humidity is it acceptable to derive its effect on Density Altitude and performance by pure mathematical means?
- Mathematical correction seems problematic as while the comparison of atomic mass of water vapour is easy enough as is that of the principal gases in air which it displaces, humidity as a percentage varies with temperature also. The calculation is therefore rather complex but no doubt possible. So what is the formula?

I would appreciate any guidance on this matter or referral to those that may have tackled the topic in the past.

[rotorfossil]

To my knowledge there is no requirement to schedule the effects of humidity in helicopter flight manuals, so it isn't done.
In days of yore when helicopter performance was sometimes very marginal, it was done. I am thinking of the Naval Westland piston engined Whirlwind HAR Mk4.The pilots' notes had a graph giving the correction of MAUW for 180 ft/min performance.
In the RAF SAR Whirlwinds, the navigator consulted the WET/DRY bulb figures every morning and computed the max weight for the day. From memory, between totally dry air and 100% humidity, the difference was about 5%.
Rather an important consideration in the Far East theatre.

[pacrat]

Thanks RotorFossil. It makes some sense that it was done for those early types with marginal performance. This would capture the Old Brantly B-2 I once flew. The practice of establishing a power margin with a minimum power fly-by and subtracting it from maximum power available gives a more real time prediction of landing performance than plotting graphs before departure anyway. Thanks for you perspective. I would appreciate any other opinions or experiences from pilots with exposure to the practice of applying humidity corrections in performance calculations.

[rotorfossil]

As an afterthought, I believe the question arose recently as to whether single engine power assurance checks on twins were adversely affected if conducted in high humidity conditions.
Logically the torque output of a free power turbine engine will be affected whenever the gas generator section is N1 limited, since high humidity will reduce density and hence the mass flow. This is in addition to any reduction in rotor performance. As there was no easy method of establishing absolute humidity in flight, there was no answer to the question.

[pacrat]

Indeed humidity is a double edged sword. Decreased rotor mass flow increases collective pitch required so Power Required is driven up by the Induced Power component. Obviously lower engine mass flows will bring down the Power available curve.

[JimEli]

see my remarks here:

http://www.pprune.org/rotorheads/318977-effect-humidity-turbine-engine-perf.html

[pacrat]

Many Thanks JimEli for the link to the earlier thread.