I recently built a quadcopter, according to the manufacturer each motor has a max pull of 880g with a 3S battery and a 10x4.7 propeller. Can I assume that those specs are at sea level?
I usually fly at around 800m (2600ft) ASL and get 8min with ~1900mAh of a 2250mAh battery.
This is the thing, I'm planning on taking some pictures at a place where my takeoff will take place between 2600m (8500ft) and 3000m (9800ft). Does anyone have experience with the effect of density altitude in the lifting power and flight time on a multicopter?
What have you done, or would do in a situation like this? Go for 4S batteries, change propellers, go for higher capacity batteries or just accept less air time and plan missions accordingly?
Thanks,
Eloy
Replies
Yes, the specs are best at sea level. Without getting too involved in the math, thrust is going to be roughly proportional to air density. I found a neat little calculator here:
http://www.denysschen.com/catalogue/density.aspx
Plugging in 3000 feet of altitude (assuming everything else is the same), the difference in density is about 12%. Slugs per cubic foot is the UOM here, so in the ideal power equation 0.00238 becomes 0.00209 (converting everything to Imperial units, i.e. 880 grams becomes 1.94 lbs):
10" prop copter: square root of (1.94 pounds/2 * 0.00238 * 0.545sqft.) = 27.4 feet/second ^-1 and times our thrust is 53.1 pound feet/sec ^-1. Dividing by 550 yields an ideal power of 0.0965 horsepower or * 746 = 72 watts at sea level.
10" prop copter: square root of (1.94 pounds/2 * 0.00209 * 0.545sqft.) = 29.2 feet/second ^-1 and times our thrust is 56.6 pound feet/sec ^-1. Dividing by 550 yields an ideal power of 0.103 horsepower or * 746 = 77 watts at 3000 feet.
Now...figure your motor is heating up and only giving you 60% efficiency and if your prop has an FM of 50, your real power is approximately 240 watts at sea level and 257 watts at 3000 feet.
I know - a lot of work to walk through to tell you - meh, it doesn't much matter. :-)