$$L/D$$

Equation 6 · Technological Evolution

Flight at the Energy Floor: Transport Selection to 2100

This mathematical expression combines the displayed quantities; its precise role follows from the surrounding article text.

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 come in single-digit percentages per generation, the flattened tail of aviation’s S-curve. The fourth factor, the mass-ratio term $\ln(W_1/W_2)$ , is where the aviation lineage’s entire battery-versus-kerosene argument actually lives, because $W_1$/$W_2$ is set…

Meanings in this article

• $D$: the lift-to-drag ratio, and $W_1$.

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Equation 13 · Technological Evolution

Flight at the Energy Floor: Transport Selection to 2100

This mathematical expression combines the displayed quantities; its precise role follows from the surrounding article text.

Concorde is the clearest case in transport history of a regulatory filter, not an economic one, capping a technology’s addressable market. The aircraft cruised at Mach 2.02, carried between 92 and 128 passengers, and burned roughly 4,800 US gallons of fuel per hour at cruise, delivering about 15.8 passenger-miles per gallon against 33.3 for the Boeing 707, 46.4 for the 747, and 53.6 for the McDonnell Douglas DC-10 — the direct fuel-efficiency cost of pushing lift-to-drag ratio through the transonic and supersonic regime, exactly the L/D term in the Breguet equation collapsing even as speed M rises [ 11 ] . Its program cost ballooned from an initial 1962 estimate of 70 million pounds to…

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