Summer heat and humidity can rob your engine of the performance you had in the spring.

Why ? The air is less dense. In the spring the air is cooler and therefore denser, which means more oxygen available for your engine to use.

To "off set" the less dense air of summer increase your nitro content. I suggest increasing the nitro content INITIALLY by 10%. Then, depending on the performance of the plane AND HOW THE ENGINE IS PERFORMING, adjust the amount of nitro from that point in 5% increments. You will most likely also have to adjust your idle and high end also.

This "trick" also works quite well when you have to fly at a higher altitude than you normally fly (go from sea level to a couple of thousand feet in elevation ) since the air is less dense at higher elevations.