Aerodynamics of high-winged fighters.

peceed said:

When you analyze structural differences you should compare the masses of empty aircrafts.

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I do not analyze structural differences. I try to evaluate the aerodynamic perfection. Empty weights according to Shavrov were 1348 (I-153) and 1428 kg (I-16 type 18)

peceed said:

Leading edge seems to be gulf shaped

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It never seemed that way to me.

bf109xxl said:

Please in numbers. What is "good enough"? Why even small difference by diameter was so important for designers that they prefer to install less powerful engines with a smaller diameter than more powerful, but larger engines?

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They didn't have wind tunnels or wind tunnels that worked at the speeds they were trying to fly at.
Designers did a lot of "dumb" stuff in the late 20s and early 30s but that is how they learned.

A lot of American racers used 450-550hp P&W Wasps. (51-52in dia) instead of 525-700hp Hornets (54.4-55.5in dia)
However there may have been other things going on. Weight, suitability of propellers, At this point in time a ground adjustable prop was about as good as it got.
A lot of the racers pushed their engines so book power should be taken with salt. Some of the racers simply bolted the different engines on sometimes used the same cowl so both streamlining and cooling might be a bit sketchy. Nobody was using adjustable cowl flaps at this point and in fact even baffles between the cylinders was something of a rarity.

The planes I showed in post #16 had about the lowest zero lift drag coefficient of their time and in fact were better than most single engine fighters of the 1930s. Because of their size the total drag may have been more. But getting 228mph from a 450hp engine with a 5400lb 294 sq ft wing airplane that carried one pilot and 5 passengers and 105 US gallons of fuel was pretty darn good.
Especially considering that the plane was "wood".

Shortround6 said:

They didn't have wind tunnels or wind tunnels that worked at the speeds they were trying to fly at.
Designers did a lot of "dumb" stuff in the late 20s and early 30s but that is how they learned.

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Polikarpov preferred the 1700hp M-90 with a diameter of 1296 mm over the much more powerful, but larger M-71 (2000hp, 1375 mm). According to his calculations, the maximum speed of the I-185 with a less powerful but more compact engine should be higher (680 vs. 655kph). And by this time wind tunnels with desirable speed were available. Unfortunately, the M-90 was too far from perfect and had no chance to achieve the required performance.

Apparently the M-71 wasn't quite perfect either.
Frankly both engines seem to be on the sketchy side. A lot of displacement for not enough weight.

Shortround6 said:

Apparently the M-71 wasn't quite perfect either.
Frankly both engines seem to be on the sketchy side. A lot of displacement for not enough weight.

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M-71 _was_ accepted and might have been accepted much earlier if Shvetsov had been properly ordered to concentrate his efforts on perfecting this engine rather than the M-82. Both engines suffered from numerous defects, but the design bureau had enough strength only to perfect one of them.
Maximum speed is determined by the ratio of power to aerodynamic drag, the latter depends quadratically on engine diameter, and weight has no direct impact on maximum speed.

bf109xxl said:

weight has no direct impact on maximum speed.

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No but.
It may have an effect on wing size indirectly to meet landing/take-off requirements.
More directly it has an impact on engine life/durability.
M-90 engine was about the same weight (or less) than Wright R-2600 of 14 cylinders. The M-90 was an 18 cylinder engine of 3034 (?) cu in.
The M-71 was an 18 cylinder of 3643 cu in. Literarily two R-1820s placed one behind the other. But the engine was lighter than an R-2800 let alone the Wright R-3350.

Now another reason that Polikarpov may have wanted the M-90 engine was that it had a 2 speed supercharger and the M-71 and a single speed supercharger so that the power difference may not have as great at higher altitudes?

Shortround6 said:

No but.
It may have an effect on wing size indirectly to meet landing/take-off requirements.
More directly it has an impact on engine life/durability.
M-90 engine was about the same weight (or less) than Wright R-2600 of 14 cylinders. The M-90 was an 18 cylinder engine of 3034 (?) cu in.
The M-71 was an 18 cylinder of 3643 cu in. Literarily two R-1820s placed one behind the other. But the engine was lighter than an R-2800 let alone the Wright R-3350.

Now another reason that Polikarpov may have wanted the M-90 engine was that it had a 2 speed supercharger and the M-71 and a single speed supercharger so that the power difference may not have as great at higher altitudes?

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Engine diameter was the only reason. And only after proving the impossibility to finalize the M-90 in time (design bureau engineers were sent to Zaporozhye to clarify the situation) Polikarpov decided to install the M-71. No other reasons were mentioned, and I am not familiar with any sources stating otherwise.

bf109xxl said:

Engine diameter was the only reason.

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It doesn't make any sense. M-71 with power 2000 hp (version used on I-185) has better power to frontal area ratio than M-90 (1750 hp).
14% higher power, 12% higher frontal area (assuming that in the case of M-90 diameter was identical as in base 9K engine).
Total drag totally favored M-71, if you ignore double speed supercharger.

The main factor was projected availability of these engines.

peceed said:

It doesn't make any sense. M-71 with power 2000 hp (version used on I-185) has better power to frontal area ratio than M-90 (1750 hp).

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The designers of the M-90 already at that time promised (toooo optimistic) _up to_ 2000 hp of TO power. Perhaps Polikarpov based his estimates on this figure. Or more likely he expected to use a new close-fitting cowling (for a smaller engine) with a ducted spinner and fan providing extra thrust by the air after cooling the cylinders (similar to Fw 190V-1).

peceed said:

14% higher power, 12% higher frontal area (assuming that in the case of M-90 diameter was identical than in base 9K engine).

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The achieved M-71 power was 1940 hp - i.e., only 11% higher. In addition to the increased cross-sectional area, the larger diameter results in an increase in the total fuselage volume and surface area. In any case, there is a document (project explanatory note) containing estimates of maximum speed for variants with different engines. Supercharger was not mentioned in this document.

peceed said:

Total drag totally favored M-71, if you ignore double speed supercharger.

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Both the engines had a two-speed supercharger. The difference in altitude performance was not essential.

peceed said:

The main factor was projected availability of these engines.

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Polikarpov actually wanted the BMW801, but quickly realized that the Soviet government was not going to buy this engine.
I wrote above about availability - both of these engines were at the early perfectioning stage. It was impossible even to estimate availability at the time of the project submission - it could change dramatically during the engine development.