One thing is extremely clear, though, after Modification 44 in June 1941 (the engine's conrod bolts), Vulture's reliability problems were mostly fixed. At this point, this is perfectly fine as a fighter engine it cannot be improved. Therefore, in FFO, this has to go one step further, with a redesign of the star to eliminate the offset diagonal split and the replacement of the two-size conrod bolts.
The reason that the acft was declared obsolete had nothing to do with the engine or airframe. It was ONLY due to lack of spares. There were 202 Manchesters plus one B-20 and eight Tornados to which engines were committed. That's a demand for 414 Vultures. Only 538 Vultures were ever built.
Cheers: Mark
Report for the Air Ministry - Vulture Engine
Report Prepared by:
Mr Lovesay
Chief Development Engineer
Rolls Royce Defect Investigation Department
30 July 1941
Introduction
The Vulture engine is still a semi-experimental engine, which was placed into operational service prematurely. Many problems with this engine have been corrected during the 43 modifications carried out so far. However, recent investigations have shown that a fundamental problem remains with these engines. As the Air Ministry has decided to redeploy the Manchester force to the Far East without tropicalising the existing engines, this is a decision to retain the Manchester in service. Furthermore, the decision to continue with Tornado in service now means that the problems with Vulture must be corrected.
The major problem with this engine has now been clearly identified. Fortunately, much of the necessary redesign work to correct this problem has already been done.
Events of June
On 2nd June the port engine of Manchester L7318 207 Squadron failed during ground testing. Investigation by Derby[1] indicated that the two short big end bolts had sheared, leading to catastrophic failure of the star rod design big end bearing. On 29 June, L7315 of 61 Squadron suffered an uncontrollable engine fire and crashed near Grantham with the unfortunate loss of the pilot. The investigation showed also that the fire was caused by failure of the con-rod bolts. After such a failure, the unconstrained con rods flail, smashing the crank-casing and nearly cutting the engine in half. Worse, this failure normally cut the hydraulic lines to the feathering mechanism, making it impossible to feather the engine. The Manchester normally cannot maintain altitude following the loss of half its propulsion when the drag of a windmilling propeller is added.
Results
An intensive flying program involving four aircraft each from 97, 61 and 207 Squadrons has confirmed Derby's findings. Worse, tensions of these bolts were found to be uneven, worsening the problem. The metallurgical department has also reported that the high Brinell metal of the bolts lacks sufficient ductility for this function. Unusually for Rolls Royce, Chief Designer Elliot used S65 steel, a high Brinell nickel-chrome steel, for the bolts. This design was required due to the decision not to stagger the cylinders. This forced the design away from the Kestrel's two pairs of fork and blade rods on a common crank pin to a design with a master and three articulated rods, of which one was on the master and two on the cap, forcing the uneven diagonal split line. This is a secondary problem, (the uneven diagonal split line of the master con rod and cap) as it causes the saw teeth to dog-leg the connecting bolts, also affecting their tension.
The crank-case is in two parts, and it is not connected with the usual Company means, but with diagonal bolts and small dowels. The use of larger cheese dowels in either side of the main bearing bore in the crank case panels has corrected the crank case distortion problems. Still, this is another major design issue which should be fully corrected in the redesign of this engine. This redesign is required due to the identification of a problem with the star-rods.
A fundamental error has been revealed. We have recalculated the bolt loads at maximum rpm, and they are higher than was thought in the engine design calculations.
The implication of this is obvious, the star rod design of the big end bearing is fundamentally flawed and cannot be corrected. It requires full redesign and replacement. The good news is that Modification 44 (replacement of all conrod bolts and their tensioning regime) has finally made the engine fully reliable for fighter applications. It remains tolerable as an interim correction for bomber applications, but the engine, in this configuration, cannot be improved. Indeed, rpm de-rating is necessary to protect the short con rod bolts. The rpm has to be reduced from 3,200 to 2,850. Boost has been increased to 9 lb, restoring power. The engine actually runs better at this rating and overhaul at 120 hours is now appropriate. As of 28 July Modification 44 was found to have corrected existing problems and Manchesters in 5 Group are again now cleared for operations. The B-20 and Tornado Squadrons have also been cleared.
Corrections
As three types are now dependent on this engine, and production is accelerating from the very low 1:1.2 in-service to engine production ratio, the star rod unit design of the big end bearing has to be corrected. This is a major redesign, but it will mean that the power growth of this engine (presently capped) will again become practicable.
The Manchester is still somewhat under-powered by Vulture, although the star unit
redesign will assist in increasing the power available from this engine. The basic limitation on Vulture's power is that it was designed using
Kestrel-sized pistons. Discussions with the designers show that this was a risk minimisation strategy for a novel design. However, this does limit the swept
volume of the cylinders, the ultimate arbiter of engine power. The designers also noted that the spacing of the cylinders is virtually the same as Merlin and
that consideration was originally given to future use of Merlin sized cylinders. They also note that experience with the Vulture confirms that the use of
two-piece construction as used on merlin, with the addition of a flanged close fitting collar trapped between the cylinder block and the crankcase will deal
satisfactorily with the problem of shuffling of the liner flange on the crankcase casing.
As the Vulture is now to be attended by a major redesign of the star unit, it
makes considerable sense to move to Merlin sized cylinders and to adopt Merlin-style two-piece construction. All three of these modifications can be conducted
concurrently and they represent such a major change to Vulture that it may warrant reference either under a new name (the name Bustard has been suggested
somewhat tongue-in-cheek) or as Vulture II. These modifications will correct all major flaws while opening a flood-gate in terms of power increase for this
engine. That additional power (perhaps an additional amount in the 500-900hp range can be considered indicative) will greatly assist with the present state of
Manchester, in particular.
It is this point that makes the project worthwhile for the extant Vulture alone, as it allows the future development of Manchester, Tornado and B-20 Brisbane. The time for the star unit redesign will be shortened by the work already done on redesign of the star rod bearing.
