Precipitation affecting helicopter performance

[sebbo]

Hey there

I have always been wondering why percipitation (even heavy rain) doesn't seem to affect performance of helicopters (even the smallest ones) at all. On the other hand, a little bit of sticky pollen residue on the MR blade has a huge impact on power available (talking about small piston helicopters).
Why is rain not affecting performance?

Thanks for any (hopefully scientifical) explanations.

[griffothefog]

Precipitation used to rip the **** out of the plastic blade protection on my 269a model back in 1982..:eek:

[Savoia]

.
No real mystery (as far as I am aware). If we are talking about aerofoils (in this case the MRBs) then reduced performance is almost always symptomatic of 'dirty' airflows caused by changes to the shape of the aerofoil; usually either by damage (eg: hailstones) or accumulated properties such as frozen rain and perhaps even your Swiss pollen - wouldn't know about the latter because I've never really had pollen-related rotor issues!

Rain (unless frozen or getting close to being frozen) does not interrupt the shape of the aerofoil but disperses on impact or passes around the blade (often as 'mist' as a result of being partially vaporised by the surrounding air pressure). It also depends on the intemsity of the rain!

Sav

[rotorfossil]

Most light helicopter aerofoils, typically symmetrical 12% sections are far from the modern semi laminar flow sections which are appearing on light fixed wing types which can be noticeably affected by rain. Anyway, rain washes the bugs and muck off unless it is heavy rain which erodes the leading edge!

[[email protected]]

A major factor in the CL (coefficient of lift) on an aerofoil is its shape so if you alter that by sticking pollen to it, it won't work as well and your CD (coefficient of drag will increase markedly).

[whoateallthepies]

Griffo
The Wessex blade tape was always bloody splitting at the first sight of rain as well. Used to cause some lovely out-of-balance vibrations. Once had to throw one down in a field in NI when I was being chucked about the cockpit!
https://lh4.googleusercontent.com/_6sDYRTvw520/TaVOAlq8kZI/AAAAAAAAAEs/5mDPSLMHHG8/pie.jpg