It was actually not at all unusual for aircraft to start out just on the edge of enough engine power, but due to the dance between manufacturer and military, not yet equipped with all the 'stuff' needed for actual combat...and by the time that stuff was put in the plane became too heavy for the engine or the airframe. In the case of the P-40, my point was simply that the issue was the engine not the airframe.
In reverse.
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They were using external fuel tanks on P-40E from early on. Not sure about the D but a lot of the D models only had four guns."
ALL of the Ds had 4 guns, but according to some sources there were only about 30 Ds (or 30 built for the US) built before the contracts were amended to E models so the point is rather moot.
Some (all?) of the US Ds saw use in Hawaii.
With full internal tanks the D/E was good for 148 gallons, with drop tank the total capacity was 200 gallons. Point is that even with drop tank gone you can't fill the internal tanks and keep the plane under 8100lbs unless you take out guns/ammo. Filling the internal tank/s means an additional 168lbs.
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However changes to the engine and experiments with stripping weight were going on from almost the first day of combat. They did take out guns, they took off with half fuel for intercept missions and so on during the debacle at Java and at Port Morseby / Milne Bay. The AVG also mentioned tinkering with their engines and doing all kinds of things (waxing, filling holes, sanding etc.) to speed up their Tomahawks and later Kittyhawks. Not sure about Darwin but I would be surprised if they didn't do it there as well as one of the commanders of the 49th FG had been at Java."
I love the bit about
tinkering the engines. Nobody ever says what was done. You can't do much with the timing. Allison ignition timing was fixed, there was no variable advance. You can either set it up for a little more advance or retard it a little but then either your starting and idle/slow speed cruise are affected. or the top end is affected. There were a variety of approved spark plugs and perhaps one or two degrees timing change worked better with plug han another?
You can't change the valve timing. You can change the valve clearance but why?
You can change the jets in the carburetor (assuming you have different jets or spares to drill out) but there again you have very narrow window of possibilities and what works at one temperature/air pressure situation is worse than standard at a different temperature/air pressure situation.
The big thing that could be done and was done once WEP power settings were approved was to hacksaw off the ends of the exhaust stacks to open them up and reduce back pressure at the higher power settings. please note that this may have lowered the exhaust gas velocity at standard power settings (climb and cruise) for loss of power/thrust under those conditions.
Working on supercharged aircraft engines is a lot different than working on a 1940s Chevy six or Ford Flat head V-8.
Taking weight out helped but please look at the factory strippers. The Early P-40L and the Early P-40N. the chances of any squadron mechanics getting a P-40 pf any type down to the weight of an an early P-40N are about zero unless they had access to aluminium radiators and oil coolers, maganismum wheels, lighter tires, used four guns and restricted the four guns to 200 rounds per gun, pulled out the electric starter for the engine (leaving hand crank only) and took out the battery or substituted much smaller one ( a mark of desperation for a plane that used an electric propeller. If the generator fails how long before the prop becomes a fixed pitch prop at best or runs away at worst.
1/2 fuel mans what? 100 US gallons if you count the drop tank? 74 US gallons if you don't ? 60 US gallons (50 imp gallons) if it is 1/2 the "book" combat fuel load?
A P-40E was going to use over 40 US gallons just starting, taking off and climbing to 20,000ft. Figure another 8-13 gallons to make it to 25,000ft.
In combat the engine might use 1 to 1/2 gallons a minute at high altitude. (much more at lower altitudes).
Sorry, gliding back to the airfield after an intercept doesn't sound like that great of an idea. Reduced fuel yes but 1/2???
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The official engine settings (not just boost) for the earlier Tomahawk P-40 B/C and for the P-40D /E were too low. And there was a little too much weight in it. However this was changed in the field almost as soon as the aircraft went into combat. "
Now this needs a bit of looking at too as there were several changes to the engines used in the Tomahawk P-40 B/C that do NOT show up as changes in the engine type.
Allison had to call back 277 (?) of the original -33 engines and rework them at company expense because they were failing well before the expected overhaul life. Changes included but were not limited to a new crankshaft and new crankcase. Until the rework was done the planes continued to fly with the engine restricted to 2770 rpm and with restricted boost pressure. I don't know if the British got any of these engines or if all went into early US production. In any case it may well have made both Allison and the US army at bit hesitant to approve high boost levels on that "2nd" generation -33 engine until the service life was proved out (and it did have considerably more life than the originals, not just meeting the original goal).
You also run smack into the the fuel situation. British (and commonwealth) fuel in 1940 and 1941 was the equivalent to 100/115-120 so a fair amount of over boosting could be done as shown by the Merlins in the BoB going from 6lbs (42in) 12lbs (54in) however US fuel at this time was 100/100. Over boost this stuff at your own risk. The US did specify/but and issue (for testing?) some 100/125 and many US engines of this time period (1940-41) were rated to use this grade but information on use in service is limited (nearly no existent one way or the other). The British and the US coming up with the first of many specifications for 100/130 fuel (1st specification limited the lead use to 3cc per imp gallon). The 100/130 fuel showing up about the time many of the planes were (although the amount of 100/130 fuel in the Philippines might be subject to question).
Back to the engines, the first Allisons used a "plain" steel crankshaft. I don't know when it was changed, Plain steel means a high grade steel alloy suitable heat treated. At some point they went to shot peening the crankshaft. I don't know if this was part of the
fix for the early engines or if it came latter. These crankshafts had much improved fatigue life. By the end of 1941 ( and after over 1000 P-40Es had been built) the crankshaft was changed to one that was both nitrided and shot peened for another very significant increase in both strength and fatigue life. It was this crankshaft that allowed the 1325hp take of rating in the P-40K (after hundreds if not thousands of hours of testing, both factory and operational).
Saying that early P-40Es could have been over boosted like the P-40Ks were given the different crankshaft and possibly different fuels is pushing things.
They were were overboosted but squadron records don't seem to show the failure rate. And the failure rate has to take into account the availability rate (number of planes grounded with metal bits in the oil waiting for an engine change) and not just the number of crashes due to engine failure. How many minutes/hours at how much over boost before engine failure?
compounded by the crappy operating conditions. does the crankshaft fail from fatigue before or after the engine bearings are wrecked by sand/grit in the oil?
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It was actually not at all unusual for aircraft to start out just on the edge of enough engine power, but due to the dance between manufacturer and military, not yet equipped with all the 'stuff' needed for actual combat...and by the time that stuff was put in the plane became too heavy for the engine or the airframe. In the case of the P-40, my point was simply that the issue was the engine not the airframe."
There was no "dance" between the manufactures and the military. In 1939-40 there was a change in what was required in military aircraft in many countries. Curtiss lost over $14,000 when the 2nd XP-46 with full military equipment failed to come close to the projected performance figures. Please note that the P-40D/E was a back up to the XP-46 using the same engine as a low risk----low loss of production alternative.
Armor, self sealing tanks, good radios and more than a handful of rifle caliber machine guns were all just starting to be fitted to most fighter aircraft in 1939/early 1940.
Early Bf 109Es having just four 7.9mm machine guns, little or no armor and no self sealing tanks during the attack on Poland. Trying to say that aircraft companies/designers were using smaller than needed engines and getting by by skimping on military equipment is not an accurate picture. There simply weren't any more powerful engines in a suitable stage of development using the fuel available to each country at the time. In 1939/40 the US didn't have any liquid cooled engine anywhere near production status except the Allison and there was no good air cooled radial alternative. For instance the 1700hp version of the Wright R-2600 didn't start design work until the same month the P-40D/E was ordered. The first prototype engine ran a few months later (about the start of the BoB) but the 5th production engine was not built until over a year later. Oct of 1941
The issue was not the engine, it was the airframe/service load.
Please compare the Allison to the Merlin 45. You could drop a Merlin 45 into a P-40 with not a lot of change in performance until you get to around 18,000ft. You could drop an Allison into a Spitfire MK V and if you allow over boosting probably get similar performance up until about 18,000ft.
The Allison is about 3.6% bigger in displacement and uses about 10% more compression so at the same rpm and boost pressure it should make a bit more power.
from this test :
Spitfire F. Mk.VB Climb and level speed performance
we can see that a Spitfire V with a cropped impeller Merlin 50 engine could hold 6lbs boost (42in ) to 16,000ft with the RAM supplied by climb speed. At max level speed it could hold 6.2 pounds to 18,000ft. it can only hold 18lbs/66in (18.2 in the test airplane) to 3800ft when climbing so it isn't that different from an Allison with 8.80 supercharger gears.