There is an apparent contradiction in all Merlin ground running notes prior to about 1940, which probably means I don`t understand fully what they mean, hence this post:
They always specify that the engine is warmed up at 1200 rpm prior to opening up, which is what I always do. In fact they specify 70 degrees for the early types but maybe realised that this would not always give fighters with rads under the wings sufficient time to take off, and this figure was subsequently reduced to 40.
Anyway, two paragraphs further on it states “Running at the gear (meaning reduction gear I assume) or crankshaft periods must be avoided, ie for the former 820 to 1080 rpm, and for the latter about 1200 rpm”
So my point is, to warm the engine up to 70 degrees is going to take up to 5 minutes depending on the ambient conditions, which means running it for 5 minutes at a speed they then tell you not to run it at.:confused:
The 1200 rpm thing was obviously never an issue in reality because the paragraph does not appear in the Merlin XX notes (1941) and any later ones I have seen, and 1200 is always specified as the warm-up rpm as far as I know.
Pete
By: MerlinPete - 11th June 2007 at 19:16
Many thanks chaps.
I have got a bit more info from Dave Piggot at the RRHT. As far as the contradiction is concerned, that can just be down to the fact that different aspects of these manuals were compiled by different staff, and there is nothing to say that any of it should be taken as gospel!
Clearly the crankshaft torsional vibrations were never an issue in service, and these figures were printed early on when there was less operational experience.
Rather than using crankshaft dampers as were fitted to the Allison V-1710, Rolls-Royce instead tuned the front and rear quill drive and spring drive shafts to orders of 1.5 and 2.5 away from the crank frequency, and it was difficult to get the 1.5 order far below 1200 rpm.
Still, an interesting subject for total engine anoraks such as myself, and I would agree that the type of propeller fitted would no doubt have a bearing on how the engine behaves as there is a massive difference in the inertia of a Watts two-blader and a Ham. Standard 23EX as fitted to Lancs, weighing in at about 300lb.
Pete
By: Camlobe - 8th June 2007 at 14:49
Pete,
I’m dragging the brain cells here, so might be off the mark slightly.
A long time ago, I can remember a concern about ‘bell resonance’ on the reduction gear of Merlins, more of a concern on wooden prop blade models than metal types (e.g. Spit vs Lanc). Again, can’t remember specific rpm’s but do remember it was sub-1200 crank rpm.
This is not to be confused with rpm-avoid ranges that affected metal props due to harmonic frequencies of the metal blades themselves.
Don’t know if this helps.
camlobe
By: stuart gowans - 8th June 2007 at 08:15
I believe the pilots notes for the Spitfire mention a specific minimum temp, before take off, in order that the guns don’t freeze, as the gun heating obviously comes from the engine; nothing to do with crankshafts I know, but I’m a firm believer in that just because you don’t know the answer, doesn’t mean you shouldn’t contribute something….
Sign In
June 8, 2007 at 8:01 am