Aerodynamics of Japanese fighters

[GregP]

Here's a Seafang I drew awhile back.

 

[wells]

@ThomasP my understanding is that it was 7075 not Alu 7076 but those are almost the same material anyway. Minor quibble on my part but totally agree with everything you wrote. EDIT: I misread and you did write that 7076 is essentially the same thing as ESD whereas 7075 was introduced by Japan in 1943. Mind blowing info. Thanks!

Regarding the primary discussion topic, aerodynamics, I believe the main things that we're all talking about are the zero-lift drag coefficient and wing area of an aircraft. I only have a low-level, mechanical understanding of these principles of flight, but a quick summary for WW2 aircraft is as follows:

In general, the major aerodynamic variables for drag are the zero-lift drag coefficient and the wing area. If you have both, then you can calculate how aerodynamically efficient an air frame is.

Japanese aircraft tended to have particularly good streamlining, but then they also had big wings relative to horsepower and weight, which watered down their excellent streamlining.

But this is all speculation as I have never seen a ZLDC calculated for any Japanese aircraft, not even the Zero. Although some aviation buffs have used estimated ZLDCs which were reverse engineered from TAIC records, these are of unknown pedigree as well. This video does a really great job of reverse engineering TAIC's calculations, and some Japanese sources, to get the ZLDC for the Ki-84. But as the video pointed out, TAIC appears to have just transplanted the Ki-43's ZLDC to the Ki-84's wing area. Additionally, Nakajima burned all of their records after the war, so the final drag estimate is also just an estimate with no valid sourcing. This is definitely not the right number. And we don't actually know how the Frank's aerodynamic efficiency was.

Just a side note: I'd love to see the zero lift drag coefficient calculated for the N1K2-J and the N1K1-J but my guess is that it doesn't exist. But because it has laminar airfoils and a large propeller, it's a good indicator of Japanese aerodynamic technology.

The Japanese were early adopters of "laminar" flow airfoils. Kawanishi was the first company to fly such an aircraft outside of the US, but the J2M1 Raiden also had a semi-laminar airfoil (according to Japanese Wikipedia, which sources Maru Mechanic IIRC for this). However, except for the Suisei dive bomber and the Ki-61, all Japanese aircraft were relatively high drag radial aircraft, with extra emphasis spent on streamlining to compensate for their lower horsepower output as well as extensive hand finishing using machinists.

The Mustang (D model), as far as I'm aware, has the lowest Zero-Lift Drag Coefficient out of all aircraft produced in large quantities during the war. I think much of that is because it uses a liquid-cooled engine. The Ki-64 probably had an overall lower zero-lift drag coefficient though but that was a prototype.

That was my ki-84 video and I'm working on calculations for other Japanese fighters as well. Regarding the N1K2-J, it is quite comparable to an F4F-4 in terms of wing span, area, weight, pudginess? So, if you put a 2000 hp engine on the Wildcat, it should be close to "George". Using the Standard Aircraft Characteristics data for the F4F-4, its drag works out to 0.0198 for 260 sq ft ( A base value, corresponding to Mach 0. Compressibility correction is applied based on Mach number ), or an "equivalent flat plate area ( f )" of 0.0198 x 260 = 5.15 sq ft. This was arrived at from knowing the speed of 284 mph at sea level with 1200 hp and a 9.75 ft prop with 77.5% propeller efficiency.

The next step was to put a 3.3 m prop on it and give it 2000 hp. The speed goes up to 337 mph at sea level. Taking it up to 6000 m, with 1625 hp, the speed is 375 mph TAS. So, looking at the published figures for "George" of 369 mph, you can see it is right in the ballpark and very comparable to a Wildcat in drag terms. I suspect the fatter fuselage counters the laminar wing to a large extent.

 

[wells]

I wish there was performance data available for the M6A1-K, which was the land-based version of Aichi's M6A1 Seiran.

Would probably be similar to the difference between A6m2 and A6m2-N

 

[AerialTorpedoDude69]

That was my ki-84 video and I'm working on calculations for other Japanese fighters as well. Regarding the N1K2-J, it is quite comparable to an F4F-4 in terms of wing span, area, weight, pudginess? So, if you put a 2000 hp engine on the Wildcat, it should be close to "George". Using the Standard Aircraft Characteristics data for the F4F-4, its drag works out to 0.0198 for 260 sq ft ( A base value, corresponding to Mach 0. Compressibility correction is applied based on Mach number ), or an "equivalent flat plate area ( f )" of 0.0198 x 260 = 5.15 sq ft. This was arrived at from knowing the speed of 284 mph at sea level with 1200 hp and a 9.75 ft prop with 77.5% propeller efficiency.

The next step was to put a 3.3 m prop on it and give it 2000 hp. The speed goes up to 337 mph at sea level. Taking it up to 6000 m, with 1625 hp, the speed is 375 mph TAS. So, looking at the published figures for "George" of 369 mph, you can see it is right in the ballpark and very comparable to a Wildcat in drag terms. I suspect the fatter fuselage counters the laminar wing to a large extent.

Hey thanks for providing that information. Your video is fantastic.

Regading the George 21, The George 21 came with three primary engine variants, two of which were derated from its maximum horsepower. The same is true for the Ki.84.

The derated engine that was listed in the George 21 Prototype's Handbook mentioned that the top military speed was 644 KPH. Laurelix calculated its WEP at around 658 KPH. Japanese Wikipedia also has the same numbers, which are derived from the Shiden Kai manual. I also applied the equation for determining the aircraft's top speed in that thread and the math appears to be correctly done (if you use the only authoritative source available).

Also, the top speed in the manual aligns with some IJN pilots' recollections for the top speed of the Shiden. As far as I'm concerned, this is the truest value we're going to get given the available data.

The 369 MPH number is of unknown pedigree. It was derived from a 1946 Japanese source hastily compiled from memory. I believe this is the military rating of the George 11b (or N1K1-J-Otsu) which had streamlined its wing. Japanese records specifically state that removing the gun pods added around 7 MPH.

The report originated from a US government request for official performance values from the Japanese. Unfortunately, many of the official records had been destroyed. The only authoritative text remaining is the Shiden Kai prototype handbook. The best secondary sources after that are pilot biographies which mention and corroborate the manual.

 

[GrauGeist]

Would probably be similar to the difference between A6m2 and A6m2-N

Well, I'm sure there would be a differwnce in performance, but it would certainly be quite a difference.

The A6M2 Model 21 had speeds of 317mph plus, while the A6M2-N had speeds of roughly 270mph due to the main float and outriders.

The M6A1 had a top speed of 295mph with it's twin floats where the M6A1-K had retractable landing gear (it was land-based) and the upper folding portion of the vertical stabilizer omitted.