[Bletchley]

Possibly Akaalias, although the difference doesn't look that significant to me. The Spad XIII was a later type and so it probably did have an altitude compensating carburettor, but then some of the later versions of the Spad VII may also - it is actually very difficult to say, as these carburettors were, I believe, retro-fitted in the field as they became available (just as the higher compression pistons may have been on the H-S). The climb figures between 2000m and 6000m provide the best clue, as the manually operated altitude compensating device maintained a more nearly correct fuel/air ratio as the aircraft gained altitude and reduced the loss of power due to the over-enrichment of the mixture in the thinner air at altitude, although a good photograph of the cockpit that showed a second lever on or near the throttle quadrant is also a clear indication. On the later 300 hp H-S engines the mixture lever was mounted on the quadrant next to the throttle lever, and the two were calibrated together so that the pilot could grasp them both with one hand and pull them back together to open the throttle and close the fuel supply as the aircraft gained altitude (as the late war over dimensioned or higher compression engines were generally throttled back slightly at the lower altitudes), but this was with the later 65 D.C. Zenith carburettor - on the earlier ones the mixture lever may have been separate, but would most likely nevertheless be located somewhere near the throttle quadrant. I think a small consistent difference in the climb performance may have had more to do with weight differences, or aerodynamics, than with carburettor type.

This additional info. on the 300 hp H-S comes from: Ministry of Munitions. The 300 hp Hispano-Suiza aero engine (direct drive), December 1918.

Bletchley