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From your link: > Turboprop engines are most efficient at speeds between 250 and 400 mph and altitudes between 18,000 and 30,000 feet Ignoring the detail tha

From your link: > 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.

jjk166 5y ago
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.
- JumpCrisscross 5y ago
  > 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.

jjk166 5y ago

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.

JumpCrisscross 5y ago
> 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.