Warm air is less dense. There are fewer molecules of air in a given volume when it is warmer. With fewer molecules of air moving past a flier, there is not as much aerodynamic drag and therefore a flier experiences
less lift. We haven't had enough experience to nail this down precisely, but it seems like the difference between 50 and 80 degrees F. is aproximately 30 RPM. For example, to have the same sensation of lift we have
at 570 RPM at 50 degrees F., at 80 degrees, we must increase the RPM to 600.
Also, a wind tunnel powered by a non-turbocharged internal combustion engine will produce less power at higher temperatures because there is less oxygen to mix with fuel. A turbocharger uses engine exhaust which to
turn a pump that increases the air the engine can use.