Aerodynamics of Japanese fighters (1 Viewer)

[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.

 

[wells]

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.

Interesting...It makes sense that the airplane would be tested with lower grade fuel ( hence being de-rated ), since the good stuff would go to the front line combat units as a priority. Or, it just wasn't available at the time. The US did the same thing. For training, they used 91/96 grade and gave the engine limits in the handbook. As an example,

The figure of 321 knots at 5600 m is actually exactly the same as 330 knots at 6000m, as given in the handbook. The indicated speed is 240 knots in both cases. The airplane would have been tested at 6000m, since it was to meet the same specification as the J2M ( 325 knots at 6000 m ) and the Americans actually captured a document containing that figure. They thought it was low also, but maybe didn't realize it was the speed to which the airplane was to meet or exceed in the specification. Later, they found more information that gave the figure of 354 knots. This is where the figure of 407 mph comes from in the TAIC manual. Anyway, if the density altitude was 5600 m, it means the test flight took place on a cold day, where the outside air temperature was about 15C colder than "standard". Once corrected back to "standard" atmosphere, you get the figure of 330 knots at a density altitude of 6000 m.

I was always under the impression that the figure of 348 knots was just a calculated figure to which the airplane failed to meet. According to Jiro Horikoshi in "Eagles of Mitsubishi", they were using an "obsolete" method of calculating top speeds, which is why the Zero never met their expectations. He doesn't say what the issue was, but I suspect it was compressibility correction. This would have been an industry wide problem. In any case, it now makes more sense that it corresponds to the fully rated engine, versus the derated engine, especially given the climb times, which compressibility drag by itself cannot make up the difference. And, why would they publish such data in the manual? "Look at how bad our airplane performs, compared to what we thought it would do! " haha

So, I agree that 348 knots at 6000 m is the speed with rated power of 1700 HP ( 1675 hp ) @ +350 mm boost / 3000 rpm ). I will post more in the other thread. On a final note, my calcs give a Cd0 ( Mach 0 ) of 0.0174, if 330 knots at 6000 m is achieved with 88% rated power ( about 1500 HP, if power varies directly with manifold pressure and RPM ). Then, applying rated power, I get the figure of 345 knots, only 3 knots less than that published in the manual. So, that makes perfect sense.

 

[Simon Thomas]

Nice work on the video. That would have taken a while to put together.

In post #22, was your prop efficiency of 77.% found or calculated?

Power does not vary linearly with RPM, however for small changes in RPM it is likely close enough.
Power is meant to vary linearly with manifold pressure, but typically other things are changing at the same time so it never seems to work out correctly. Again small changes will likely be close enough to linear.

 

[wells]

Nice work on the video. That would have taken a while to put together.

In post #22, was your prop efficiency of 77.% found or calculated?

Purely calculated. Then, the drag is determined from the calculated thrust at some known speed/alt/power combination. It is an iterative process to determine inflow losses. The propeller sees a higher velocity than the freestream velocity. The results are quite accurate when compared to airplanes with well documented performance ( mostly British and American ). I will probably post a couple of sample calculations in the P-51 or Corsair thread at some point to illustrate. But then, the same method can be used to determine performance of any airplane, even with minimal data. All I need really, is one airspeed/altitude/power combination. For example, I've recently done calculations for the J7W1 ( not published anywhere yet ) and the results are within 1% at both sea level and 8400 m, compared to the Japanese calculations done 80 years ago. The performance of that airplane was never determined by flight test, since it only made 3 short flights of about 15 minutes each and had major trim issues, which were never sorted before the war ended. My goal is to expand on known performance figures and show things like glide ratios ( range and endurance ), airspeed for best climb rate or angle, turn performance, etc.