The minimum comfortable cruising speed of an aeroplane depends on the complexity of characteristics for the design in question, which may be broadly grouped under the headings of stability and control, drag and propulsion.

The store of knowledge relating to such speed is composed, in the main, from experience with aeroplanes powered by supercharged reciprocating engines, for which, at the low powers appropriate to the conditions considered, the brake horsepower and drag horsepower both increase with height at roughly the same rate (constant aeroplane weight and equivalent air speed being assumed). For a turbine-engined aeroplane the brake horsepower (or thrust) decreases with altitude and it might be supposed that this difference would result in a difference between the minimum comfortable speeds of similar aeroplanes driven by turbine engines (either jet or propeller) and by reciprocating engines.