In short, a high wing (or parasol) fighter lacks the benefit of the wing's spars passing through the fuselage for strength.
The cowling of LaGG-3M-82 (LaG-5, future La-5) had nothing to do with the Su-2 one. The Gu-82 - a Gudkov's attempt to improve LaGG by powering with the M-82 - had a Su-2-like cowling, but this airplane was just an experimental one. The best variant of M-82 cowling at that time was developed by Polikarpov, who was obliged to transfer his drawings to other design bureaus. There are confirmations of the transfer of drawings to Yakovlev, Mikoyan/Gurevich (reliably), and posibly Gorbunov. But there is no confirmation that the documents were transferred to Lavochkin - the latter developed the cowling rather independently. Seems, that this version of the cowling was less optimal than the Polikarpov's one.
The spars can most certainly pass through the fuselage in a high-wing fighter, or any other high-winged aircraft. Struts are not needed; note that they don't seem present on many current high-winged aircraft or some aircraft of the WWII era, like the DC-5.
So you're implying that Gorbunov of did not mate an Su-2's engine and cowling to the nose of a LaGG-3 airframe during early 1942?
It was Mikhail Goudkov (Gudkov) who tried to install M-82 on the LaGG-3 in the early 1942 - I mentioned it already. No documents on transfer of the cowling design from Soukhoi (Sukhoi) to Goudkov have been found until now. The design of the Gu-82 cowling was a bit similar to the Su-2 one, but in no way exactly the same.
While gives different structural benefits that save weight overall.
The wet area of struts is very small and they can be optimized even further than historical.
The weight of DC-5 was concentrated in the wings: they contained landing gear, engines, fuel tanks.
An all-metal high-wing monoplane was ca. 100 km/h slower than a low-wing monoplane (even a partially wooden one!) with a similar engine. Low-wing monoplane fighters outperformed high-wing fighters even with less powerful, larger diameter engines (e.g., the I-16 with its single-row Wright). Ok, you can point to the retractable landing gear as the major reason for the difference in performance, but we can compare P.24 to Japanese low-wing monoplane fighters of the mid-1930s with non-retractable landing gear. The latter outperformed it completely (except firepower) even with much less powerful engines. The P.24 was an archaism, suitable only for countries with very limited military budgets. The high wing had no design advantages if used on _piston-engined_ _fighters_ already in the early 30s, although Polish designers created excellent airplanes at the time of their adoption. These airplanes, however, had no future and very quickly became obsolete in contrast to the low-wing monoplanes which had much more reserves for further development.
There are many, many examples showing negligible difference in the performance of well-designed high and low wing aircraft created for the same mission. There are advantages to low wings on fighters, mostly as a) better landing gear placement and b) better pilot visibility.
What exactly aerodynamic flaws in the P.24 caused such a loss in speed? It's really not quite clear to me. For example, I could refer to the discussion between Polikarpov and TsAGI (Central Aerohydrodynamic Institute, the leading Soviet research center for aerodynamics) about the I-15/I-153 (gull-type) wing. Polikarpov proved that the gull-type scheme does not have the disadvantages attributed to it by TsAGI and provides similar parameters as the "straight" wing. The P.24 wing scheme has a clear similarity to the gull type, so the reasons for the high aerodynamic drag are not obvious to me. Airplanes with open canopy, non-metallic wing skin and worse wing profile fabrication (typical for Soviet airplanes) demonstrated a higher flight performance with less engine power. Even some biplanes with approximately equal engine power (with even larger diameter!) outperformed the P.24. Why? I have already spoken about the non-retractable landing gear. There remain struts - apparently, the drag created by them was higher than previously estimated. Or are there other factors? Cowling? It was just terrible on the I-16. What else?
The main reason was very draggy technology of stressed skin wing construction (Wibault). First on market, not the best in the late 30's.
Master thesis that can give some answers:
It wasn't cheaper to produce than low-plane, and it wasn't bad plane in the time when it was designed and produced. It is simply previous generation, that was being replaced in 1939. Polish Air Force intended to replace them by PZL.38, multirole aircraft that required engines specially designed engine that happened to be a failure. Unfortunately there was no backup project for airplane, and no replacement for engine.
This generation was outdated no later than 1937. Whether the Poles underestimated their opponents or were too tight on funds - I don't have enough information. The engine (14K) was adequate and could provide 540-560 kph for all-metal aircraft.
Please, do tell.
Re.2000 - 530 kph, I-180 M-87B - 540 kph. Both were powered with 14K derivatives. I suppose, the I-180 with a closed cockpit canopy and more careful wing fabrication (e.g., better landing gear doors fit) could reach 560 kph.
Neither of these was powered by the actual 14K.
I am not sure that you can obtain reliable coefficients by simulation in this case - were the simulations verified by the experimental data (e.g., wind tunnel blowing of the P.11 models)?
I doesn't matter. The difference by parameters (diameter, power) was negligible. M-87B had 950hp on 4000m, 14K on P.24 - 930, 14N on P.24F/G - 970.
