Update from 11-25-25:
Hello everyone, when I first was thinking about this DARPA Lift Challenge, it did not seem to me there would be any method to get a drone that weighs only 55lbs to have a power source that could generate enough kW to run a group of propellers where this group of propellers together created enough static thrust to lift off the ground a take off weight of (55lb drone +220lb payload) 275lbs. It is still not clear to me that this is possible.
Of interest moving a weight up in the air, if a time interval and distance are specified, this becomes a definition of energy. DARPA states they want the device to fly along at 350 feet off the ground. If 275lbs went straight up at 1 ft/sec, then this would be 275 ft-lbs/sec. And 275 ft-lbs/sec is 0.372 kW. To get up to an altitude of 350 feet at 1 ft/sec would require 350 seconds, and at 0.372 kW x 350 seconds, this becomes an energy expenditure of (0.372 kW x 350 sec) = 130.2 kW-sec which is 130,200 Joules. Looking at JP-9 fuel, it has an energy density of 39.6 million Joules per liter. So, (130,200 Joules) / (39,600,000 Joules/Liter) is .00328 liters (3.28 ml). Google Ai states normal energy conversion efficiency of small turbine jet engines is in the range 15-30%, I will choose 20%, meaning one would need (3.28ml) x 5 = 16.4 ml of JP-9 fuel just to satisfy the energy equation for getting 275 lbs up in the air by 350 feet in 350 seconds.
For propellers the issue will be essentially static thrust since the device is moving up so slowly. Google Ai gives this example for calculating propeller thrust vs energy expenditure for a propeller moving through the air at 1 ft/sec. Google Ai does make the point, that for the aerodynamics community, the propeller efficiency for static (zero airspeed) thrust is defined as zero, and Google Ai uses a work-around to get the thrust efficiency.
Here is my actual Google Ai question, “energy conversion efficiency of modern propellers at 1 ft/sec velocity thrust output in lbs vs energy input in Joules”
I think you could type this question into Google Ai and ask Google Ai to go into the “more depth discussion” option and you should get Google Ai’s discussion of the work-around that Google Ai used.
Well… I did not know this. The webpage editor will not allow copy and paste from a Google Ai discussion. Weird.
So I will try to summarize the discussion.
Google Ai is addressing the issue of an approach to a propeller running at low airspeed. It makes an assumption of low propeller efficiency (10% efficient).
1 ft/lb x 1 ft/sec = 1 ft-lb/sec. Power input (because of the 10% efficiency) will need to be 10 ft-lbs/sec
10 ft-lb/sec x (1.356 Watts/ft-lb/sec) is about 13.56 watts,
this is because 1 ft-lb/sec is equal to 1.355 Watts
So, thus, the propeller to produce 1 lb of thrust at 1 ft/sec, this would require that 13.56 Joules of energy would need to be put into the propeller every second.
So, for a 275lb drone, we would need (13.56 Joules/sec) x (275lbs, for a 275lb drone) = 3729 Joules per sec for the propeller related thrust. This needs to last for 350 seconds to get the drone to an altitude of 350 feet, thus (3729 Joules) x (350 seconds) = 1,305,150 Joules to get the drone to 350 feet. If the energy conversion efficiency of the jet turbine engine is 20%, then we will need the jet engine to be using (1,305,150 Joules needed) x (5, energy efficiency penalty) = 6,525,750 Joules to get the drone to altitude.
Looking at JP-9 fuel, it has an energy density of 39.6 million Joules per liter. This would mean, in order to get the 6,525,750 Joules of energy input the engine would need to consume (6,525,750 Joules)/(39,600,000 Joules/Liter) = 0.164 Liters or 164 ml of JP-9 to get the drone up to 350 feet altitude.
DARPA also wants the drone to fly 5 nautical miles (30,380 ft) mostly at this 350 ft altitude. Thus the propellers will need to keep creating the thrust and the energy input rate will be 164 ml JP-9 per second. If the drone flies along at 30mph (a fast pace for a loaded drone) which is 44 feet/sec, the drone will be flying for (30,380 ft)/(44 ft/sec) = 690 seconds. If it is using 164 ml JP-9/sec, then it will need (690 seconds) x (164 ml/sec fuel input flow rate) = 113,160 ml of JP-9 fuel. The density of JP-9 fuel is 0.8gm/ml, so it will need in order to fly the 5 nautical miles (113,160 ml needed) x (0.8 gm/ml density) = 90,528 grams of JP-9 fuel, which is 90.5 kg of fuel, and at 2.2 lbs/kg, this is 199.1 lbs of JP-9.
So…. because of these calculations, I decided to just walk away from a 275lb drone and instead do calculations on a 55lb drone using JP-9 fuel turboshaft jet engines to power a distributed hydraulic drone propeller system and see how much (drone wt + payload wt) I could get to.
I plan some videos about this. More to come 🙂 pg