johnny comelately wrote: ↑26 Jan 2023, 12:07
Re: the excess weight. What I meant, if I am interpreting your figures right Tommy, is that with 70% excess over lambda 1 for every tonne of fuel carried 410kg would be necessary but a deficit for either range or armaments
eg by mid-WW2 the Merlin is automatically running lean most of the time ie except at high powers ie high boosts
but automatically eg running 60 or 70% rich at emergency powers ie power demand beyond the takeoff gate
(Bristol gave 60+% - maybe for the Pegasus ?)
reduction in demanded power increases prop pitch, reduces rpm and boost, and leans the fuelling
so actual throttling loss is zero or small
weak mixture at 20" manifold uses 17 imp gal/hr at low altitude
richening commenced above 46" manifold and 2700 rpm
Mustang 67" manifold/3000 rpm is WE power using 137 imp gal/hr - 61"/3000 rpm is takeoff using 112 imp gal/hr
WI is heavier/bulkier (its a very poor fuel) and less flexible
presumably less suited to British point-defensive type operations
WI users anyway surely ran ('wasteful') conventionally-rich at high power
btw the USAF P-51D flight handbook says eg
15 min power ratings don't limit to 15 min duration - they're only to help engine life conservation via low rpm & boost