Numerator: aM

The passage around this formula

Aircraft range is not a free parameter; it is fixed by a single classical relation once cruise speed, aerodynamic efficiency, and fuel fraction are set. In its standard Mach-number form, the range an aircraft can fly in cruise is: $R = \frac{aM}{g\,c_T}\cdot\frac{L}{D}\cdot\ln\!\left(\frac{W_1}{W_2}\right)$. where a is the local speed of sound, M is cruise Mach number, g is standard gravity, $c_T$ is thrust-specific fuel consumption, L/D is the lift-to-drag ratio, and $W_1$ and $W_2$ are the aircraft’s weight at the start and end of the cruise segment [ 1 ] . Three of those four factors — speed, engine efficiency, and aerodynamic efficiency — are where a century of aeronautical engineering has already done most of its work and where further gains now…

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