Yes, the availability of the Merlin 60 series in quantity required was one of the limiting factors in the transition from the Spitfire V to the Spitfire VIII and Spitfire IX. Consider that later build Spitfire XVI, which was essentially a Spitfire IX but powered by a US built Packard Merlin didn't go into production until late 1944 when sufficient quantities of the Packard Merlins were available to meet both Mustang and Spitfire production requirements. Sholto-Douglas in his correspondence in 1943 and within the Air Staff meeting minutes and MAP production minutes, were all looking at the balance between keeping what they had flying and introducing the later and better variants into service. Sholto-Douglas in particular was very concerned about the diversion of 60 series Merlin Spitfire variants overseas, which meant he had to keep what he considered a disproportionate number of Squadrons still equipped with the Spitfire V. When you look at the number of Squadrons in the UK still equipped with the Spitfire V by D-Day, it says something about the planning and manufacturing of all types required by the RAF at home and overseas. In particular, the percentage of resources split between Fighter Command and Bomber Command was a continual source of tension.
Spitfire Combat Radius (range) evolution, limitations? (4 Viewers)
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Shortround6
Lieutenant General
The resources split between Merlin aircraft engines and Meteor tank engines was not the easy and smooth story it is often portrayed as.
Resources include casting facilities, not all RR engine factories made their own crankcases, engine blocks and heads casting. It was not the simple RR swaps the Meteor to Rover for the Jet engine project that is often told. That part may have been simple. It is further out in the supply chain that some factories/facilities had to go from aircraft production control to tank production control.
ThomasP
Chief Master Sergeant
re
The center of mass of the real timeline 29 Impgal tank was located about 81" behind the aircraft CG. This gives a moment of 81 x 238 = 19,278 lb-in.
The center of mass of the engine/propeller combination was about 42" in front of the aircraft CG. So if you wanted to restore the aircraft CG with a full tank you would have to add 19,278 ÷ 42 = 420 lbs of ballast weights to the engine bearers with an average distance of 42" in front of the aircraft CG. As you suggest above this would be a significant weight increase and the weight alone would reduce the performance of the aircraft.
But then the problem arises of what happens to the aircraft CG when the tank is empty. When empty and weighing only 30 lbs the aft moment due to the tank is 81 x 30 = 2430 lb-in, while the forward moment due to the 420 lbs of ballast weights is still 19,278 lb-in, yielding a net moment of 16,848 lb-in forward of the aircraft CG.
In both of the above cases the aircraft CG is out by a factor of 2 or more (I think, if I did my math right) from what was acceptable - either forward or aft.
The example I used upthread was the PR Mk IV, most of which used the Merlin 45 or 46 and the associated propeller(s).
It should be noted, however, that some of the earlier PR Mk I variants did use a rear fuselage tank similar to the 29 Impgal ferry tank, but in order to balance out the weight of the ferry tank (empty and full) the radios and armament had to be removed. The aft moment of the radios was between 6,200 and 9,600 lb-in depending on the outfit (ie TR.9D, TR.1133, TR.1143, etc), while removal of the armament (8x MG, ammo, and associated bits) reduced the aft momment by about another 4,000 lb-in. So a net rear aircraft CG moment of 5.678 to 9,078 ib-in, still requiring a significant 135 to 216 lbs of ballast weight to return to the original aircraft CG with the ferry tank full, but allowing more acceptable CG limits.
It should also be noted that the PR Mk IA thru F and PR Mk IV series were not intended to engage in ACM, relying on speed and height (for the high altitude missions at least) and evasion in order to survive.
I would also like to reiterate that I am not saying that a significantly useful amount of fuel could not have been added to the Spitfire Mk V series, just that it would not have been anywhere near as easy as some are suggesting, and it would have required significant redesign and/or modifications and testing to achieve an acceptable result.
NOTE to any aeronautical engineers out there reading the above - if I am phrasing something wrong please let me know.
It is a significant CG issue. The empty 29 Impgal non-self-sealing ferry tank is not too big a deal as it would only weight around 30 lbs including the mounting bits, fuel line, etc. But when full it would weigh about 238 lbs.
The center of mass of the real timeline 29 Impgal tank was located about 81" behind the aircraft CG. This gives a moment of 81 x 238 = 19,278 lb-in.
The center of mass of the engine/propeller combination was about 42" in front of the aircraft CG. So if you wanted to restore the aircraft CG with a full tank you would have to add 19,278 ÷ 42 = 420 lbs of ballast weights to the engine bearers with an average distance of 42" in front of the aircraft CG. As you suggest above this would be a significant weight increase and the weight alone would reduce the performance of the aircraft.
But then the problem arises of what happens to the aircraft CG when the tank is empty. When empty and weighing only 30 lbs the aft moment due to the tank is 81 x 30 = 2430 lb-in, while the forward moment due to the 420 lbs of ballast weights is still 19,278 lb-in, yielding a net moment of 16,848 lb-in forward of the aircraft CG.
In both of the above cases the aircraft CG is out by a factor of 2 or more (I think, if I did my math right) from what was acceptable - either forward or aft.
The example I used upthread was the PR Mk IV, most of which used the Merlin 45 or 46 and the associated propeller(s).
It should be noted, however, that some of the earlier PR Mk I variants did use a rear fuselage tank similar to the 29 Impgal ferry tank, but in order to balance out the weight of the ferry tank (empty and full) the radios and armament had to be removed. The aft moment of the radios was between 6,200 and 9,600 lb-in depending on the outfit (ie TR.9D, TR.1133, TR.1143, etc), while removal of the armament (8x MG, ammo, and associated bits) reduced the aft momment by about another 4,000 lb-in. So a net rear aircraft CG moment of 5.678 to 9,078 ib-in, still requiring a significant 135 to 216 lbs of ballast weight to return to the original aircraft CG with the ferry tank full, but allowing more acceptable CG limits.
It should also be noted that the PR Mk IA thru F and PR Mk IV series were not intended to engage in ACM, relying on speed and height (for the high altitude missions at least) and evasion in order to survive.
I would also like to reiterate that I am not saying that a significantly useful amount of fuel could not have been added to the Spitfire Mk V series, just that it would not have been anywhere near as easy as some are suggesting, and it would have required significant redesign and/or modifications and testing to achieve an acceptable result.
NOTE to any aeronautical engineers out there reading the above - if I am phrasing something wrong please let me know.
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wuzak
Captain
The longer and heavier 2 stage Merlin required the use of ballast in the tail to get the CoG correct.
Given that, I would think that the effect of adding the fuel in the rear fuselage would be as bad, or nearly as bad, for the VIII as it would be for the V.
drgondog
Major
FWIIW the Merlin engine imposed 65 pounds of ballast on the P-51B. The ballast was removed when the 85-gal fuselage tank was installed.
