Japanese Zero vs Spitfire vs FW 190
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Shortround6
Major General
simple, they cut the impeller down from 10.25in to 9.5 in and left most (or all) of the rest of the supercharger alone.
This meant it took less power to turn the impeller but also meant the supercharger provided less boost at a given altitude and engine rpm.
Centrifugal superchargers are also not positive displacement devices. too much clearance between the impeller tips and the housing means some of the air (or a higher percentage) near the housing walls isn't moving with the air near the impeller tips.
The Merlin 45M (and the other cropped impeller engines) took less power from the crankshaft, heated the intake air less and ran at low altitudes with the Throttle plate open wider, all of which gave more power at low altitudes for the same manifold pressure, but the cropped impeller and large housing could not provide high pressure at altitude.
How do you get that I suggest that there were no aces flying Tomahawks in the Western Desert ????
IIRC, you earlier have stated that a 3-1 overclaim ratio was the norm in general, with which I agree; so why would a 2-3 overclaim ratio in the Western Desert be surprising?
Nov 22 is hardly repesentive of the whole period; the picture is very different if you look at 29 Nov and 5 + 12 Dec, and the norm is generally somewhere in between those outliers.
Btw, if you do want to use aces as a measure of fighter type success, you should try verifying the victories of the top scoring Allied ace over the desert with known Axis losses.
To add to what Sr6 said: the engines with suffix M (45M, 50M) were also operating on greater maximum boost: +18 psi vs. +16 psi. All of this meant perhaps 150-200 HP more power under ~5 thousand feet (with ram) for the Spitfires with the 45M or 50M. Take off power was also modestly increased, the take off boost still being limited to +12 psi.
Merlins 45M and 50M were the closest equivalents of the V-1710 of mid-war, engine powers maxing out at almost 1600 HP down low.
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No problems with that - my main remark was that a Spitfire V flying beyond 380 mph was not a service aircraft, most of the Mk.Vs were lucky to do beat 370 mph mark. There was still a fair number of Mk.Vs produced in 1943, still with float carbs, draggy exhausts, sloppy fit & finish - all of what contributed to them being lucky to attain 370 mph, too.
A Spitfire V with better carb, exhausts, mirror and fit&finish, fully closed U/C and internal BP glass would've probably went 390+ mph at 20000 ft. People at Boscombe Down made it go 380+ mph without the two last mods I've listed.
Does a similar table exist for the Merlin 46? Looking around for the past little while I have a feeling the Merlin 46 curve in my chart should be knocked back about 15 mph ...
The Australians got their knickers in a twist about the loss of performance which they ascribed to the Vokes filter. Their tests showed a loss of speed in the order of 30 mph, which was quite different from official figures. This led to the idea that the Australians might produce an alternative filter.
They did try to manufacture a copy of the temperate air intake locally, but it proved rather more difficult than expected and was a failure. Eventually they sent to the UK for drawings of the temperate intake, and clarification of the performance differences expected between the tropicalised and temperate aircraft.
If the Australians were losing 30 mph, as they claimed, it was not due to the Vokes filter, though the 17 mph given above is nearer the 20 mph claimed in one of the Australian reports than the 8 mph originally given by the British.
There were other ways of improving speed. For example, smoothing the leading edge of the wing where the double row of rivets ran span wise by stopping, rubbing down and painting gave 8 mph. This became part of the manufacturing process in 1942.
Given that an aircraft is operating at the same RPM, how does the extra power get translated into torque or forward motion? Via the prop pitch?
Stig, obviously we communicate in different ways. I frankly do not understand how you failed to notice the relevance of the post you were replying to in 887, i.e.
" Schweik said:
And the Tomahawks did quite well with the nose guns too, with the AVG, with the Russians (who sometimes removed the wing guns altogether) and in the Western Desert where quite a few Commonwealth pilots made Ace flying them."
This is the tie in. Maybe you are more literal minded or something, but it is very clear to me. If you still don't see it, lets agree to disagree on this particular line of debate, as I don't see further discussion being fruitful - and I don't want another thread to get closed because of it. I have started the new thread allowing analysis of the day to day combat results in the Western Desert and that hard data at least we should be able to agree on. Others can draw their own conclusions as to interpretation.
2-3-1 overclaim was an estimated average across the whole war (in terms of time) and all fronts. In different Theaters there were different conditions - for example whether enemy aircraft can or cannot be recovered / examined is very different over the sea or heavy jungle / mountains vs. steppe or desert - and over time and in different Air Forces different standards for verification. Starting out with just another pilot saying he saw the kill, ending with gun camera footage, crash recovery, and intel intercepts. So for example in the utter chaos at Java in 1942, or Russia in the first three weeks of Barbarossa, I think you can expect less accurate claims. Later on and in places like the desert, better accuracy.
To that last point, one interesting thing that shows up in Shores is that some of the verification of Axis losses came from Ultra intercepts and did not appear in the Axis records.
That is why I started the other thread, so we can avoid debating our interpretations and focus instead on the data.
I believe this has already been done actually. Russel Brown started the ball rolling with his critique of Joachim Marseilles record (which appears to be about 60% verified, with many more Hurricanes than he thought) and some of the other JG 27 experten, which was followed by close looks at the records of the top British Aces, who range from 60%-80% verified, with some actually a bit over (James Stocky Edwards - who was credited with 16 victories in the Desert, appears to have actually gotten between 19 and 22 depending on who you believe, Bobby Gibbes also apparently underclaimed, being credited with 10 he apparently got 12). There will always be some doubt of course since on a given day if 10 were claimed and 8 were actually downed, it can be impossible to determine which Allied pilot actually shot down each of the lost enemy planes. All you can say for sure is that there were enemy losses that correspond with the claims, or not.
But seeing as how most of the top twenty British P-40 Aces generally had 8-10 victories or more, a 60-80% verification rate still keeps them as Aces. Needless to say it's the same on the other side. So I don't actually think it's an issue. If you do feel free to open another thread on it.
On that one day (Nov 22) no less four victories claimed by three British Aces seem to have been verified. Considering that an Ace is just five victories it's fairly significant. We should be through posting almost all of the Tomahawk victories within a few days from now so hopefully it will be clearer.
Yes, prop pitch and/or a different prop better able to handle the extra output. Aircraft with the Merlin 32 usually used a 4 bladed prop.
AIUI, the larger impellers were actually capable of higher boosts than the engine could accept, so by cropping the impeller the engine/supercharger was better matched for low altitudes, where the SC could provide max boost with less energy wasted to drive the SC.
The extra power would be absorbed by increasing the prop pitch. Less power to run supercharger means more to the prop, essentially it's like a small engine car not running the air conditioning
That is really interesting since it seems the British were able to get the P-40 F or L ("Kittyhawk II") to 370 mph at 20k ft in testing (though other tests show down around 355 at lower boost). P-40K modified to P-40N standards also famously got to 378 during testing though that too depended a lot on configuration and boost settings. That plus the Australian test makes me wonder if the later model P-40s (F, K and L) were faster than the Spit V generally speaking.
I would expect the Spitfire was better streamlined and has a thinner and slightly shorter wing, a generally more powerful engine (at least at higher altitudes) and is also considerably lighter so that drag is surprising to me. Was that from production issues from rapid ramping up?
I also have a mechanics diary from the 33rd FG who mentioned specifically sanding and polishing the wing leading edges on the fighters. My thought was "why just the leading edges?" But I guess now I know.
It wasn't just the leading edges.
This is just from a production perspective:
There was also a continuous stream of information to the various RAF commands, but particularly Fighter Command, emphasising the need for careful maintenance of the aircraft and its surface finish when in service.
Some of the bulletins are quite alarming, dealing with anything from bent elevator or aileron shrouds, to use of incorrect fasteners, to simple bent or ill fitting panels. It makes you wonder just how bad maintenance was, and some of the accompanying examples suggest it could be very poor indeed.
The P-40F and L were the versions with Packard Merlin V-1650-1.
Advantages vs. plain vanilla Spitfire V might be: better carb, ram air intake (no ice guard) & exhausts, fit&finish (although the P-40 probably was not as good as P-51 in that regard - that's IMO), retractable tailwheel, possibly the windscreen. Not sure about cooling system, by 1942 neither was in world class.
Disadvantages: thicker wing, main U/C sticking out when retracted a bit.
(I've listed the stuff that should matter with drag, probably some other details can also be found)
P-40N received a new version of V-1710, with improved altitude power (still not up the V-1650-1 standard, though) , so it should be faster than P-40K and earlier. The P-40N that clocked 378 mph was a lighter & less draggy version, with 2 HMGs deleted (less weapon-related drag) and one fuel tank also removed, as well as other bits and pieces - weight can influence the speed a bit.
WRT speed - the Spitfire V and later P-40s (P-40M, N, plus F and L) were probably as evenly matched as one can imagine, typically between 360 to 370 mph. The P-39N/Q was faster than either, BTW, and P-51A was still faster.
Desire to have as many Spitfires as possible certainly meant that some corners were cut. That is not just a thing of fit&finish, but also some other choices that got to be made - BP glass is easier to fit and retrofit to the outside rather than to inside (corrected with Mk-VII and on, but not retroactively on previous examples), draggy undercarriage, no decision to copy exhausts from Bf 109E, no streamlined rearview mirror etc. Not having good carbs on Merlin already before the ww2 was an unfortunate oversight (impacted not just top speed, but also ceiling and negative-G use).
Spitfire VII and later corrected a lot of this (carb, exhausts, BP glass installation), meaning that Mk.IX was measured to have same Cd0 as the Mk.V, despite receiving much bigger radiators.
Reluctant Poster
Tech Sergeant
- 1,669
- Dec 6, 2006
A paper on Spitfire aerodynamics is attached.
Most production Mk VIIs had SU carbs as did early Mk VIIIs and IXs and most XIs. By this time the the SU carbs had the negative G mods.
One of the reasons Rolls Royce fell in love with the P-51 was its superior fit and finish. Hives complaining to Freeman 8 Nov 1942:
"The manufacturing finish of the Spitfire is bad. This is shown up by the variation from machine to machine in performance, and the controls. We do not think it is sufficient to improve the paintwork, although this is the last thing we should want to stop, but given a continuity of production we should expect the aircraft to be manufactured to a higher quality."
The value of the constant speed prop and its governor are often not appreciated or understood by those who don't work with them regularly. Essentially, they constantly vary the pitch of the propeller to keep the load on the engine matched to its torque at the selected RPM and manifold pressure, thus keeping RPM constant. In combat or other acrobatic flight this is a godsend, as the pilot doesn't have to monitor the tach and jockey throttle to keep revs within limits. Throttle can be adjusted to desired thrust throughout maneuvers without worrying about revs, and the engine can be kept at its most efficient RPM through all changes of airspeed, attitude, and G load.
(Personal prejudice here), but IMO, double acting hydromatic-style props are the best there are. Fast acting, reliable, and relatively immune to cold induced sluggishness, as they are constantly pushing hot engine oil out into the dome. Good down to -40°C/F, where most lubricants start to fail, anyways.
Electric props are a bucket of worms, and the higher you go, the worse they get.
Cheers,
Wes
The Spitfires life can be summed up by the phrase, ''good enough now is better than perfect later''. The desperate need for them meant it's development always took a second seat to production resulting in it never maturing into the fighter it should have been.
