> Fly lower to save fuel? How does that make sense?
Propellers like lower speeds and altitudes; fans like higher speeds and altitudes [1]. This is because propellers hit the air at the true air speed. Compressors in a turbofan modify the incident pressure and air speed, letting the engine purr away without concern for creating shock waves.
> Turboprop engines are most efficient at speeds between 250
and 400 mph and altitudes between 18,000 and 30,000 feet
Ignoring the detail that your source is talking about turboprop (so, turbine engine with a big fan attached) and not a small piston engine, that's still not low altitude. That's class A airspace!
Even in a piston aircraft, you have to adjust your mixture as you climb, right? You have to add _less_ fuel per unit of air. And there's still all the drag from the aircraft itself, it's not just about propellers.
Sure, I doubt this drone would have a turbocharger, so performance (or rather, horsepower) is expected to drop above 10000ft. But not efficiency.
Reducing the fuel/air mixture doesn't increase efficiency, yes you need less fuel per unit of air, but you need more units of air too.
The problem is the air gets thinner as you go higher, meaning you need to fly at higher speeds, which means you need to fight more drag. Really it's not flying at low altitude to save fuel, it's flying slow to save fuel.
Jet engines become more efficient at higher speeds which compensates for the increased drag, and thus jet aircraft fuel efficiency goes up at higher altitudes. Piston engines are unaffected by airspeed, so at higher speeds they are doing more work at the same engine efficiency. Thus the aircraft fuel efficiency goes down.
> you need to fly at higher speeds, which means you need to fight more drag
I could not find a non-technical source for this, thank for explaining well. In summary, drag increases quadratically with mass flow while thrust increases linearly to density. So a slower-turning prop in denser air moving the air frame slower keeps the frame aloft and works more efficiently than the same frame running higher and (necessarily) faster. The power plant (turboprop or piston prop) is independent of all this.
The distance that needs covering to fly from northern Morocco to Spain is just a few km... probably would be a waste of energy to get it to 2km altitude
Probably mostly by not expending energy to climb and then descend.
Helicopter lift efficiency varies heavily with altitude, but how it varies depends on the weight of the helicopter. Heavy helicopters require quadratically more torque to hover at higher altitudes, but lightly-loaded helicopters can actually require less torque to hover at higher altitudes. Here's a (complicated but really cool) diagram to calculate on page 7, figure 7-6. https://www.faa.gov/regulations_policies/handbooks_manuals/a...
Since electric motor current draw is approximately linearly proportional to their torque, heavy-lift electric helicopters/drones would almost certainly conserve energy by flying lower. I'm not sure how that applies to this drone, which probably weighs below the lowest line on that diagram, and also has more, smaller, higher-speed rotors.
This vehicle probably spend most time traveling as a fixed-wing aircraft, and for those, efficiency almost always increases with altitude. (Except that their combustion-powered engines may require compressors to continue operating at higher altitudes. This aircraft's ICE may actually suffer from this, if it is an inexpensive engine not designed for aircraft.) So if the statement is correct, it's probably referring to the energy saved by not having to climb in the first place.
Fly lower to save fuel? How does that make sense?