With larger blades, why does the drone have less thrust
 

I recently upgraded my six-rotor UAV.In order to improve flight efficiency, I specially refitted larger blades. But when it was installed, I tested it and found a series of problems. Firstly, the electric power is very hot, and secondly, it is obvious that the thrust of propeller is getting smaller and smaller. Can someone tell me why?

with the larger blade the motors rpm decrese.so less thrust

Propellers are rated by diameter and pitch. If you went to a lower pitch, then yes, you can have less thrust.

What made you think that changing diameter would increase efficiency?

lack of knowledge!

Your motor and prop combo didn't match. I use a lot of large blades on multirotor, the are indeed very efficient but the most important thing is to make sure the motor KV value and torque must match the prop blades requirements. High KV motor (fast RPM) with smaller blades suits best, low KV (slow but high torque) work best with bigger blades...its a similar concept applied to R/C car/truck when try to spin bigger or smaller wheel or gear ratio on particular motor. I even fitted bigger blades 4 blade on my FPV plane running on 700KV brushless. Manufacturer always stated on their motor specification on max limit prop size it could support, so check your motor specification.

In this video i used 850KV (low RPM motor) to spin GWS (SF type) 10x4.5 prop as documented in this page: https://www.supermotoxl.com/projects...tor-450mm.html

Video:

FYI "Bell" type or known as mushroom motor design have more inertial torque to spin big propeller due to its bigger outer casing and magnet size.

Simply put, with the same motor and battery, the big KV value uses the small propeller, and the small KV value USES the big propeller. In principle, smaller KV values and larger blades can show better power efficiency. In the case of MMC's drone, Skylle power system kit uses a KV115 motor with 28-inch blades. The Notuzi power system kit uses a KV270 motor with 21-inch blades.