Quadrotors and helicopters in general use a lot of energy compared to planes and every other vehicle. To make them stay in the air as long as possible we make them as light as we can, removing all excessive weight and try to find the right battery. But a underrated parameter is the motor efficiency. This is usually measured by how many grams lift pr watt the motor gives.
I made a test using a kitchen scale and the OSD, transmitting the ampere usage to my monitor as I changed the throttle and gaining yet more thrust. I added some weight to the quad, giving a total weight of 2200 grams. The thrust was then simply 2200 minus what the scale showed. For example 1200 grams on the scale means 1000 grams of thrust or lift capacity. Here are the numbers I got for my four cheap Emax CF2822 1200 kv on Naza 10×4.5″ propellers, using 3 cell 11.1 volt fully charged 2200mAh battery:
I measured the efficiency between 500 grams and 1700 grams in total, meaning 125-425 grams thrust per motor.
Quad or hexa?
The result is showing quite clearly that the more throttle and thrust, the less efficiency. This might speak in direction for using six or eight motors. Will the extra weight for two arms and motors eat up the efficiency gain you get? Here is the math: My quad with 5000mAh battery and 400 gram camera is about 1700 grams. Adding two motors will put on 180 grams for motors, arms, speed controllers and other. Lifting 1700 grams with Quadrotor setup will take 30 amps, giving 57 grams/amp (5.2 grams/watt). That´s 14.2 amps per motor. Lifting 1880 grams with Hexarotor setup will give the efficiency equal to 1250 grams on the graph. That´s about 65 grams/amp. This means 29.2 amps in total. That´s almost one amp saved. Not great, but it shows that adding more motors doesn´t necessarily take more energy.
Another calculation: If the quad total weight is 1200 grams, it uses 18 amps. Going for hexa is adding 180 grams. The thrust number for 1380 grams with four motors equals 920 grams with six motors. That´s 75 grams/amp. Giving 18.4 amps in total. A little bit more for the hexa then.