Bartman
Welcome to MultiRotorForums.com!!
The weather here in New Jersey, USA has been getting warmer and the effect on my copter has been very noticeable. In fact I've reduced my flight times from 11 minutes to roughly 8 after experiencing several brown-outs following level-offs from quick descents. If you've never heard of density altitude, it's worth a quick read as it explains a lot of why the sudden warm weather would have such an effect.
Wings, propellers, and rotor blades all rely on air density to get best performance. When air density goes down they have to work harder (spin faster) to deliver the same performance you've come to expect. It may not be obvious but when it gets hot and there's a lot of humidity in the air, air density goes way down.
In aviation (and elsewhere) it is referred to as Density Altitude. On a hot and humid day, the density of air at sea level will actually be much more like the density of air at a higher altitude on a standard day (standard is 15 deg C (59 F). Pilots refer to this to help determine how much more runway they'll need for takeoff or landing on hot days. Flying multi-rotor helicopters, we can use the same information to realize why our batteries are going dry so much quicker than when it was cold outside (batteries don't take the heat much better than propellers but their reasons are unrelated to air density). Whatever altitude you are located at, on a hot day your copter will feel like it's flying at a much higher altitude. It could be different by thousands of feet depending on how hot and how humid the air is. Thousands of feet make a big difference for an airfoil that uses air density to make lift so the way they make up for it is by increasing velocity. If you could measure your props' hovering RPM's you'd find them to be higher on a hot/humid day. Spinning the props at higher RPM's takes more power so the batteries are being depleted more quickly.
I suppose you could help things out by increasing prop pitch on hot days but there's still a net increase in work being done due to decreased air density so the batteries will feel it. Factoring in the fact that the batteries and motors don't appreciate working harder in higher temps and it just means that flights have to be shorter.
Here's a calculator I found online. Use 0 (sea level) for altitude, 96 deg. F for temp, 29.92 in. Hg. for altimeter setting, and 93 deg F for dewpoint (dewpoint is usually only a few degrees below temp on hot/humid days which makes for a very unstable atmosphere and lots of thunderstorms!). Watch what the Density Altitude is (the altitude the copter thinks it's flying at).
http://wahiduddin.net/calc/calc_da.htm
I've gotta run but I'll check in later. Hope that helps.
Bart
Wings, propellers, and rotor blades all rely on air density to get best performance. When air density goes down they have to work harder (spin faster) to deliver the same performance you've come to expect. It may not be obvious but when it gets hot and there's a lot of humidity in the air, air density goes way down.
In aviation (and elsewhere) it is referred to as Density Altitude. On a hot and humid day, the density of air at sea level will actually be much more like the density of air at a higher altitude on a standard day (standard is 15 deg C (59 F). Pilots refer to this to help determine how much more runway they'll need for takeoff or landing on hot days. Flying multi-rotor helicopters, we can use the same information to realize why our batteries are going dry so much quicker than when it was cold outside (batteries don't take the heat much better than propellers but their reasons are unrelated to air density). Whatever altitude you are located at, on a hot day your copter will feel like it's flying at a much higher altitude. It could be different by thousands of feet depending on how hot and how humid the air is. Thousands of feet make a big difference for an airfoil that uses air density to make lift so the way they make up for it is by increasing velocity. If you could measure your props' hovering RPM's you'd find them to be higher on a hot/humid day. Spinning the props at higher RPM's takes more power so the batteries are being depleted more quickly.
I suppose you could help things out by increasing prop pitch on hot days but there's still a net increase in work being done due to decreased air density so the batteries will feel it. Factoring in the fact that the batteries and motors don't appreciate working harder in higher temps and it just means that flights have to be shorter.
Here's a calculator I found online. Use 0 (sea level) for altitude, 96 deg. F for temp, 29.92 in. Hg. for altimeter setting, and 93 deg F for dewpoint (dewpoint is usually only a few degrees below temp on hot/humid days which makes for a very unstable atmosphere and lots of thunderstorms!). Watch what the Density Altitude is (the altitude the copter thinks it's flying at).
http://wahiduddin.net/calc/calc_da.htm
I've gotta run but I'll check in later. Hope that helps.
Bart