I thought I would give a try to the DARPA Lift Challenge. My understanding of the specifications that DARPA wants include a drone that can lift 4 times its weight. The max drone weight is set at 55lbs. Thus (4×55 = 220lbs) so this would be a 55lb drone taking off with a 220lb payload. The takeoff weight then would be (55 + 220 =275lbs). With four propellers (it is a drone and I am pretty sure DARPA is wanting it to have VTOL, probably also ability to transition to fixed wing). So… 4 propellers doing a static lift of 275 lbs is (275/4 = 69lbs and 69 lbs is 306 Newtons) so 306 Newtons per propeller.
I have a propeller design that I call a “fat chord” propeller, it is optimized for static thrust at zero airspeed (before a drone can do anything as a flying drone, it must first get itself off the ground with static thrust at zero airspeed).
I prepared a pdf that has screenshot images of what would be the parts for this design. I think the correct motors for these military drones need to be hydraulic motors. I think electric motors leak or radiate a small amount of radio frequency energy (RF) as part of their process to convert electricity into rotary force. I think the amount of RF radiated is probably a percentage of their total electrical power input. In the electric motor propeller power values needed for these larger drones, this small percentage of the input electrical power will yield an RF radiated power output that will be detectable for miles. This means an electric heavy lift military drone will be detectable by enemy forces. I think the military will not want their drones to be immediately and constantly detectable. Hydraulic motors and pumps do not radiate RF energy. Thus, for a military heavy lift drone, I think DARPA will figure out the propeller motors need to be hydraulic. The configuration will be hybrid-hydraulic.
I think the military will have zero interest in a heavy lift drone with almost no range. To me, this range requirement will wipe out the hybrid-electric approach. Fuel engines (for the hp per unit of engine weight value) give much better range abilities after drone transition to fixed wing because the energy per unit of weight for fuel is so much higher than what current batteries can supply.
Thus the hybrid-hydraulic approach.
I created a pdf to address all this starting with the propeller and its input power needs, then a hydraulic motor to yield the propeller torque and rpm needed, then a small turboshaft jet engine to run the hydraulic pumps.
I could not find any way to get 275lbs of payload in the air with a drone that weighs 55lbs.
I will place here a free for download pdf of a set of screenshot images that I made as I tried to get this amount of payload up in the air.
Download 15mb pdf This is a set of images about the DARPA Lift Challenge Discussion (9 downloads )