Improvements to the Spitfire

LisaM said:

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The Mustang's better speed on the same engine power was 66%-75% radiator and 25%-33% better detail efforts and quality control.
The Mustang needed 300bhp for the same speed, NA's own figures showed they radiator gave them, at least,. 200bhp of that.

The Spit III's better radiator was mostly a better position, further forward into less turbulent air and hence more efficient, the photos show that very clearly, bit deeper too.

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Spit III, being a prototype, probably received more attention to the fit & finish than a series produced Spit II or V, that could amount to some gain in speed; most of the gain was probably due to fully covered undercarriage, plus some speed gain due to the clipped wing (that came with a price in RoC).

The radiator placement and layout was probably the thing that gave Mustang some advantage in Cd0, but there were also other things where Mustang was in advantage. Comparing 2-stage Merlin outfitted examples of both designs:
-fully retractable & covered U/C (5+ mph difference?)
-absence of rear wiev mirror (3.5 up to almost 7 mph difference)
-no protruding cannon barrel and bulges (7 up to 10 mph)

Just those 3 things amount to 15-25 mph of difference. The Spitfire with 2-stage engines got better in many regards than Spit V, than main offender, receiving better fit & fininsh, better carb and exhaust.

What Allison Mustang had over Spitfire V was:
-better carb, ram air intake, and exhausts (up to 16 mph combined)
-better fit & finish (up to 11 mph)
-fully covered U/C (5+ mph?)
-absence of external BP glass (almost 8 mph)
-no rear wiev mirror

Some 40+ mph of difference, before we even start cosidering the angle of windscreed or U/C drag.
The P-51A/Mustang II also dispensed with fuselage HMGs, gaining few mph there.

My point is that even if we, in our great wisdom ( ), retain the radiatort placement and basic shape of the wing, the slightly modified and carefully fitted Spitfire will be just a tad slower than the Mustang on similar power.

tomo pauk said:

Some 40+ mph of difference, before we even start cosidering the angle of windscreed or U/C drag.
The P-51A/Mustang II also dispensed with fuselage HMGs, gaining few mph there.

My point is that even if we, in our great wisdom ( ), retain the radiatort placement and basic shape of the wing, the slightly modified and carefully fitted Spitfire will be just a tad slower than the Mustang on similar power.

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I think you are greatly underestimating the Mustang's radiator which, using the Meridith Effect, had only 10% drag due to the offsetting thrust developed. As I said NA claimed that on their numbers it was worth 200bhp.

The Spit probably had close to zero offsetting thrust. The position was terrible, too far back on the wing and thus in turbulent air, it was too small in volume, the shape was wrong and there was no automatic rear shutter to constantly vary according to need. Much later they developed a better one for the Spiteful.

The Mosquito was far closer with also superior radiators and the (light) prototype was almost the same difference in speed on the same engine power as the Mustang had over the Spit (later versions became much heavier), however even with all that a late model XXX was 10mph faster on the same engines as the comparable HF VIII or IX Spit., even though it was pushing 25,000lb by that stage fully decked out and the thrust/weight ratio had deteriorated and was less than the comparable Spits.

I also think you are overestimating the other impacts, the cannon was only 5mph, the windscreen the same, tail wheel maybe 2-3 mph and so on.

What is not so well known was that the allowable tolerance in performance out of the factory was +/- 3%, which means for a LF Mk IX rated at 404mph meant it could be between 392 to 416 mph...smart and senior pilots always tried to get the 'good ones'.

NA's quality control; was far, far better and an 'average' Mustang was probably close to a 'good' Spit in finish.

LisaM said:

I think you are greatly underestimating the Mustang's radiator which, using the Meridith Effect, had only 10% drag due to the offsetting thrust developed. As I said NA claimed that on their numbers it was worth 200bhp.

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It was not NAA that claimed their radiator was 'earning' 200 bhp, it was Atwood who claimed, after Schmued's death?
But, at any rate, I will readily conceed that Spitfire was worse in this regard, the difference in benefit is greater when we compare 2-stage engined versions (Merlin Mustang vs. Spit VII/VIII/IX) than when we compare 1-stage engined versions (Mustang I/II vs. Spit V), because of 2 reasons.
1st reason is that 2-stage Spitfire got the intercooler radiator, along with bigger oil and coolant radiators - those really added to the drag vs. Spit V. In the same time, NAA redesigned the whole cooling system when going from -A to -B, featuring not just the much bigger gap between entrance and fuselage. The final design was a 'winner' between 6 or 7 proposed and tested designs.
Was it a difference of 200 HP? Maybe in the 2-stagers, but far less when we compare 1-stagers.

The Mosquito was far closer with also superior radiators and the (light) prototype was almost the same difference in speed on the same engine power as the Mustang had over the Spit (later versions became much heavier), however even with all that a late model XXX was 10mph faster on the same engines as the comparable HF VIII or IX Spit., even though it was pushing 25,000lb by that stage fully decked out and the thrust/weight ratio had deteriorated and was less than the comparable Spits.

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The Mosquito was featuring a 'flush' skin, no gaps, far less drag from undercarriage, no rear wiev mirrors, prototype had no guns. But yes, the radiators were also a factor.

I also think you are overestimating the other impacts, the cannon was only 5mph, the windscreen the same, tail wheel maybe 2-3 mph and so on.

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Not me The speed loss figures, except the tail wheel, are from the RAE Technical Note that Aozora kindly posted. Check out the table attached.

I agree that NAA quality control was better, the Aozora's file mentions the consequences of bad fit & finish in some Spitfires (up to 11 mph loss).