Spend fuel on distance or time

Which speed gets the most miles from a kilogram?

For a turbojet, thrust-specific fuel consumption relates fuel flow to thrust: FF ≈ TSFC × T. For a piston/shaft-power model, brake-specific fuel consumption relates fuel flow to shaft power: FF ≈ BSFC × Pshaft. At fixed engine efficiency and propeller efficiency, jet SR is proportional to V/D and propeller SR to V/Prequired, or approximately 1/D.

Begin with fuel flow

For a turbojet, thrust-specific fuel consumption relates fuel flow to thrust: FF ≈ TSFC × T. For a piston/shaft-power model, brake-specific fuel consumption relates fuel flow to shaft power: FF ≈ BSFC × Pshaft. At fixed engine efficiency and propeller efficiency, jet SR is proportional to V/D and propeller SR to V/Prequired, or approximately 1/D.

Thus ideal propeller maximum range occurs near minimum drag; ideal jet maximum range lies faster than minimum drag, where the line from the origin is tangent to the drag curve. These are teaching assumptions, not an AFM speed schedule. For a given engine, colder inlet conditions generally improve jet TSFC over the relevant normal operating range, and very low thrust can give poorer TSFC because fixed engine losses remain. The actual schedule is engine-specific: TSFC is not constant across altitude, temperature and thrust, and lower thrust does not imply a proportional reduction in fuel flow.

Mass, height, CG and weather

As mass rises, best-range speed rises and the optimum altitude usually falls. An aft CG within approved limits often reduces trim drag and fuel consumption; forward CG can do the opposite. Jet range often benefits from climbing to colder, less dense air until thrust, Mach or buffet constraints dominate. Propeller SRG also depends on engine efficiency, TAS and wind; higher is not automatically better.

SRG uses groundspeed. A headwind generally moves the economical ground-range speed upward; a tailwind moves it downward relative to the still-air optimum, within the approved schedule. Compare fuel used over the whole route, including climb to a new level. A stronger headwind aloft can erase an aerodynamic benefit.

Which speed keeps you airborne longest?

Endurance = available fuel/fuel flow. In an ideal jet with constant TSFC, minimum fuel flow occurs at minimum required thrust, hence minimum drag. In an ideal propeller model with constant efficiency and BSFC, it occurs at minimum required power, at a slower speed. This is why the jet/propeller answers for endurance and range differ.

Minimise fuel per hour

Endurance = available fuel/fuel flow. In an ideal jet with constant TSFC, minimum fuel flow occurs at minimum required thrust, hence minimum drag. In an ideal propeller model with constant efficiency and BSFC, it occurs at minimum required power, at a slower speed. This is why the jet/propeller answers for endurance and range differ.

At minimum-power speed a propeller aeroplane may have high AoA, poor speed stability and little manoeuvre margin. Published holding speeds can therefore be faster than the theoretical endurance optimum. Protect stall and buffet margins; there is no licence to slow to an analytical minimum regardless of turbulence or configuration.

Wind does not run the fuel pump

In a constant atmospheric and engine condition, a steady wind changes your ground track and distance, not endurance through the air. Altitude changes engine efficiency and the IAS/TAS relationship: jet endurance may improve at altitude over part of its envelope, while a normally aspirated propeller aircraft has competing engine and power requirements. Use the aircraft data for the actual optimum.

When a known arrival delay is expected, reducing en-route speed under an approved and coordinated plan can turn holding time into useful distance. It can save fuel by avoiding a separate hold and descent/climb penalties, but ATC constraints, minimum speeds, weather and reserve requirements remain binding.

Can you make the performance decision?

Four original questions. Work from the stated conditions and review the reasoning.