I assume the issue is not the energy density but the support density.
I have a 3DR Solo and it can carry up to 1 lb of extra weight for 20 minutes or so on a full charge, and run for 30ish minutes while only carrying its gimbal.
The battery in the 3DR Solo weighs about 1.3 lbs itself, and the frame is by no means lightweight. Assembled it weighs at least 70% more than the Mavic 2 Pro that I occasionally get to use at work.
To fly my drone, the 3dr solo draws an average of 14.2 volts at 15 amps (so close to 200 watts on average) but up to 800 watts at peak.
You would need an ICE and Generator combo that can put out an average 200 watts of constant power but that can peak at 800 for short periods that weighs less than 2.3 lbs including fuel that runs for more than 20 minutes to equal that 1 battery.
For instance, this is one of the most well known and light RC ICE machines:
It weighs 9.5 oz and runs for 45 minutes on 6 oz of diesel (probably not at maximum output, but still.)
So you're at roughly 1 lb (assuming 1/2 oz of screws and tubes to connect the motor to the fuel and both to the frame.
However, there is an issue here. This motor is rated at .35 hp. 1 hp is roughly 746 watts of power. That means max rev, full bore, this 1 ICE only has enough power before conversion losses to hover my 3dr solo in calm winds.
This one is much better. 1.8 hp at max revs, so 1300+ watts. But, it weighs 1.87 lbs.
Add in fuel and you're at the max, so good to go, right?
Well, the next problem is that even though we have an engine, we don't have a generator to convert from fuel to electric.
You would also need some supercapacitors or batteries to store the charge, some system to integrate the control of the motor into the drone, more mounting materials.
And, you are already over the weight limit. That doesn't mean it can't be solved, for instance you could use a hexacopter or octocopter to increase how much carrying capacity you have.
Basically, you can do it, but there are better ways to do it. You would have to design an ICE RC Drone from scratch accounting for all of the intricacies and difficulties of using combustion engines in a drone and controlling them safely just to get what will probably be a slower, more expensive and more fragile drone that breaks down more often and needs frequent servicing intervals.
I have a 3DR Solo and it can carry up to 1 lb of extra weight for 20 minutes or so on a full charge, and run for 30ish minutes while only carrying its gimbal.
The battery in the 3DR Solo weighs about 1.3 lbs itself, and the frame is by no means lightweight. Assembled it weighs at least 70% more than the Mavic 2 Pro that I occasionally get to use at work.
To fly my drone, the 3dr solo draws an average of 14.2 volts at 15 amps (so close to 200 watts on average) but up to 800 watts at peak.
You would need an ICE and Generator combo that can put out an average 200 watts of constant power but that can peak at 800 for short periods that weighs less than 2.3 lbs including fuel that runs for more than 20 minutes to equal that 1 battery.
For instance, this is one of the most well known and light RC ICE machines:
http://www.mecoa.com/hp/vt/21.htm
It weighs 9.5 oz and runs for 45 minutes on 6 oz of diesel (probably not at maximum output, but still.)
So you're at roughly 1 lb (assuming 1/2 oz of screws and tubes to connect the motor to the fuel and both to the frame.
However, there is an issue here. This motor is rated at .35 hp. 1 hp is roughly 746 watts of power. That means max rev, full bore, this 1 ICE only has enough power before conversion losses to hover my 3dr solo in calm winds.
So, let's find a bigger motor.
https://hobbyking.com/en_us/ngh-gt17-17cc-gas-engine-with-rc...
This one is much better. 1.8 hp at max revs, so 1300+ watts. But, it weighs 1.87 lbs.
Add in fuel and you're at the max, so good to go, right?
Well, the next problem is that even though we have an engine, we don't have a generator to convert from fuel to electric.
You would also need some supercapacitors or batteries to store the charge, some system to integrate the control of the motor into the drone, more mounting materials.
And, you are already over the weight limit. That doesn't mean it can't be solved, for instance you could use a hexacopter or octocopter to increase how much carrying capacity you have.
Basically, you can do it, but there are better ways to do it. You would have to design an ICE RC Drone from scratch accounting for all of the intricacies and difficulties of using combustion engines in a drone and controlling them safely just to get what will probably be a slower, more expensive and more fragile drone that breaks down more often and needs frequent servicing intervals.