Shortround6
Major General
While reading the online account of Spitfires over Darwin (located here: http://darwinspitfires.com/index.php?page=the-vokes-air-filter-controversy ) I ran across this error.
"Norwood's emphasis upon height relates to the fact that the RAAF's Spitfires were fitted with the high altitude Merlin 46, which produced its maximum power output at 21,500 feet, rather than the 11 000 feet rated altitude of the Merlin 45 fitted to most Spitfire V aircraft in the UK. The Merlin 46 produced a modest 1115 hp at take-off, but thanks to its supercharger was still producing 1150 hp at its rated altitude of 19 000 feet, and with a maximum power output of 1210 hp at 21 500 feet (both outputs were achieved at 3000 rpm and plus 9 pounds boost."
Now all of this is true as far as it goes, however, the Merlin 45 was using 16lbs of boost at 11,000ft not 9lbs and it did make 1515hp at that altitude.
Source is "The Merlin in perspective-the combat years written by Alec Harvey-Bailey and published by the Rolls-Royce Heritage Trust.
This means the Melrin 45 has over 500hp more than a Merlin 46 running at 9lbs boost at 11,000ft.
Of course if allowed the Merlin 46 to run at 16lbs of boost it would make almost 1400hp at 11,000ft.
The Merlin 45s rated altitude at 12lbs of boost was 18,000ft (source Lumsden), It used a 9.089 supercharger drive ratio (same one as the Merlin 46) compared to the 9.49 gear ratio for the high (or full) supercharger gear for the Merlin XX series. Impeller was spinning 95.7% as fast as the Merlin XX.
The Merlin 45 was not a low altitude engine. It used the standard impeller with the highest gear ratio used in a single speed engine.
Low altitude engines used either a cropped (smaller diameter impeller) or a lower gear ratio or both.
The Merlin 46 & 47 used a larger diameter impeller (10.85in instead of 10.25in) , improved guide vanes, diffuser vane ring and larger chokes in the carburetor.
The Merlin 46/47 had the highest critical altitude of any single speed supercharger used in WW II.
that passage, as written, gives a misleading impression of the abilities of the two engines.
"Norwood's emphasis upon height relates to the fact that the RAAF's Spitfires were fitted with the high altitude Merlin 46, which produced its maximum power output at 21,500 feet, rather than the 11 000 feet rated altitude of the Merlin 45 fitted to most Spitfire V aircraft in the UK. The Merlin 46 produced a modest 1115 hp at take-off, but thanks to its supercharger was still producing 1150 hp at its rated altitude of 19 000 feet, and with a maximum power output of 1210 hp at 21 500 feet (both outputs were achieved at 3000 rpm and plus 9 pounds boost."
Now all of this is true as far as it goes, however, the Merlin 45 was using 16lbs of boost at 11,000ft not 9lbs and it did make 1515hp at that altitude.
Source is "The Merlin in perspective-the combat years written by Alec Harvey-Bailey and published by the Rolls-Royce Heritage Trust.
This means the Melrin 45 has over 500hp more than a Merlin 46 running at 9lbs boost at 11,000ft.
Of course if allowed the Merlin 46 to run at 16lbs of boost it would make almost 1400hp at 11,000ft.
The Merlin 45s rated altitude at 12lbs of boost was 18,000ft (source Lumsden), It used a 9.089 supercharger drive ratio (same one as the Merlin 46) compared to the 9.49 gear ratio for the high (or full) supercharger gear for the Merlin XX series. Impeller was spinning 95.7% as fast as the Merlin XX.
The Merlin 45 was not a low altitude engine. It used the standard impeller with the highest gear ratio used in a single speed engine.
Low altitude engines used either a cropped (smaller diameter impeller) or a lower gear ratio or both.
The Merlin 46 & 47 used a larger diameter impeller (10.85in instead of 10.25in) , improved guide vanes, diffuser vane ring and larger chokes in the carburetor.
The Merlin 46/47 had the highest critical altitude of any single speed supercharger used in WW II.
that passage, as written, gives a misleading impression of the abilities of the two engines.
