Formula for loss of efficiency at higher altitude for heli?

[Johnnz]

Just wondering if there is a formula out there that can calculate just how much more power (and hence higher battery drain and drop in performance and flight time) is required to fly at different altitudes/ air pressures?

In the past I had mostly flown at sea level, probably about 15m asl. Now I am up in the mountains at about 320m asl and am noticing that even with brand new batteries I am getting noticibly shorter flight times with my electric micro heli.

I would just like to know how much of a factor the extra altitude is in this effect.

I used to get a good 10 minutes before the Lipo battery alarm came on, but now I guess I get about only two-thirds that. Probably I am damaging the batteries since they are only rated at 8C.

The other thing is that the temperature is rather cooler here than where I was before. Here it stays around 22 degrees C, whereas I was previously flying in 30 degree C weather.

Thanks for any information...

[markdrela]

Flight power, both in hover or forward flight, varies as 1 / sqrt(air density). At 5000ft ASL, the air density is about 86% that of sea level. So the power increase factor at 5000ft is
1 / sqrt(0.86) = 1.08
or an 8% increase.

[Johnnz]

Thanks for that. Well, I am nowhere near 5000ft (I think maybe I am at about 1000ft) so that doesnt account for my shortened flight times... maybe the cooler temperature might cause the batteries to be less efficient? I cant think what else would do it. Everything is running smooth, perhaps my brushed tail motor is sucking more juice than it was when new? Oh well, thanks anyway for your information.

[archiebald]

No formulas but a real life example I can quote.

I live in Japan - One of our club members was involved in the R&D on the XRB Lama RC version with the LiPo battery.
Hirobo's factory at Hiroshima is at sea level and gave around 15 minutes on a full charge but here in the mountains of Nagano at 700m above sea level this dropped to about 12 minutes.