No fuel consumption figures are given, primarily because no comprehensive data have been found. This is most probably because in the early flight years, when the Wrights were so meticulously measuring and recording technical information on the important factors affecting their work, the flights were of such short duration that fuel economy was of very minor importance. After success had been achieved, they ceased to keep detailed records on very much except their first interest--the flying machine itself--and when the time of longer flights arrived, the fuel consumption that resulted from their best engine design efforts was simply accepted. The range obtained became mostly a matter of aerodynamic design and weight carried. Orville Wright quotes an early figure of brake thermal efficiency for the 1903 engine that gives a specific fuel consumption of .580 lb of fuel per bhp/hr based on an estimate of the heating value of the fuel they had. This seems low, considering the compression ratio and probable leakage past their rather weak piston rings, but it is possible. In an undated entry, presumably in 1905, Orville Wright's notebook covered fuel consumption in terms of miles of flight; one of the stated assumptions in the entry is, "One horsepower consumes .60 pounds per horsepower hour"--still quite good for the existing conditions. Published figures for the 6-60 engine centered around .67 lb/hp hr for combined fuel and oil consumption. The Wright Shop Engine Despite the fact that the Wright shop engine was not a flight unit, it is interesting both because it was a well designed stationary powerplant with several exceedingly ingenious features, and because its complete success was doubtless a major factor in the Wrights' decision to design and build their own first flight engine. Put in service in their small shop in the fall of 1901, it was utilized in the construction of engine and airframe parts during the vital years from 1902 through 1908 and, in addition, it provided the sole means of determining the power output of all of their early flight engines. By means of a prony brake, its power output was carefully measured and from this the amount of power required for it to turn certain fans or test clubs was determined. These were then fitted to the flight engines and the power developed calculated from the speed at which the engines under test would turn the calibrated clubs. Although a somewhat complex method of using power per explosion of the shop engine was made necessary by the basic governor control of the engine, the final figures calculated by means of the propeller cube law seem to have been surprisingly accurate.[19] Restored under the personal direction of Charles Taylor, it is in the Henry Ford Museum in Dearborn, Michigan, together with the shop machinery it operated. [Footnote 19: _The Papers of Wilbur and Orville Wright_, volume 2, Appendix.]