Nerf Japanese planes

[Irregular23]

Yes, according to the US translated version of the ki-61 flight manual 「キ６１操縦法」, the official dive limit was 700 kph IAS. Unfortunately, it seems both ki-61’s original flight manual and operation manual in Japanese may have been lost, since it does not appear in the market.

You might have seen some Japanese aerospace magazines list ki-61’s dive speed limit to be 850kph, that number is definitely too high, presumably derived from static load data that does not reflect aeroelastic and compressibility that airframe will meet at critical Mach number. As there are even few US aircraft like P-51 and P-47 were hardly allowed to perform a dive at that high speed. The reason was compressibility, the airflow will separate at high speed, causing the aircraft to experience buffet/flutter/stall/control reversal. Some pilot may claim to perform a dive higher than this speed, either they had enough experience to recover from a compressibility dive, or the pitot static tube shows incorrect data at critical mach number ( sometimes off by 100 knots). N1K2 was designed to perform 450 knots IAS dive, but in reality the number was limited back to 400 knots due to unsolved compressibility shock wave issue.

Furthermore, Ki-61 airframe does not show any property to support it to dive at 850kph, yes it has tri-spar design. But don’t forget that ki-43 too used tri-spar and proved to be structurally weak. Ki-61’s airfoil was quite conventional with a quite convex mean chamber line at leading edge (NACA 24000), a large aspect ratio of 7.2 , and with a large aileron span( about 50% of wing span). All of those properties means it will have a quite low critical mach number (airfoil cause early separation), a larger AR ratio results in earlier separation on the wing tip further reducing its critical mach number, and its large ailerons would likely encounter heavy flutter due to aeroelastic effect at this compressibility dive.

The policy of War Thunder on dive speed limit is usually said to be +30kph on the number displayed in official flight manual, thus we see quite low dive speed limit on soviet planes.

[Josephs_Piano]

Official limits are not absolute limits - they include some safety margin “just in case”

[Irregular23]

That was why “+30kph” policy was incorporated for dive speed in War Thunder. All aircraft in other nations follows the policy of manual number + safety margin, clearly the late Japanese planes were not using the same standard, except for N1K1 and K2 that corrected implemented such limit ( 740kph limit + 30kph margin).

[PercussionCap]

@Irregular23

Some pilot may claim to perform a dive higher than this speed, either they had enough experience to recover from a compressibility dive, or the pitot static tube shows incorrect data at critical mach number ( sometimes off by 100 knots).

My claim comes from the Bunrindo book on the Ki-61, which I’m still slowly going through as translating every single page is tedious. But there are pilot testimonies of them maxing out the airspeedometer (well past 700kph) and obviously living to tell the tale.

As for the various features you mentioned, they can all be found on other piston engined fighters with similar IAS limits. The mach limit for the Ki-61s isn’t impressive.

@Irregular23

As there are even few US aircraft like P-51 and P-47 were hardly allowed to perform a dive at that high speed

Unless the rivet counters haven’t gotten to the P-47 and called them unbalanced, the Ki-61 is quite a ways behind it in mach limit. In fact, you’d have to go all the way down to the Wildcat to find a M0.75 limit, with all the other single engine fighters that come after it being able to go at least past M0.8.

Then we can compare it to the Bf 109s:

Here’s an F-4. Single spar wing, yet boasts a higher mach limit and only slightly lower IAS limit. It’s a similar story even with the earlier E-4.

So I doubt that a plane that took very obvious inspiration from a certain european fighter and was remarkably rugged (with its wings tested to 15G equivalent, surviving that with little damage, at which point they stopped testing and moved on) would somehow have both a lower IAS and mach limits.
The Ki-43s were all very lightly built and not as fast, so they’re more exceptions than a rule.

[Irregular23]

So I doubt that a plane that took very obvious inspiration from a certain european fighter and was remarkably rugged (with its wings tested to 15G equivalent, surviving that with little damage, at which point they stopped testing and moved on) would somehow have both a lower IAS and mach limits.

Static load test does not reflect aeroelastic nor compressibility effect, which is a common since, also “14G” was an optimistic number for ultimate load factor ( real structure failure beyond this limit), without considering real air load and buffet that will encounter in actual flight. That is a consensus in aircraft design. The compressibility effect itself has little to do with the structure, its more of an aerodynamic stuff, and a flutter induced by compressibility has to do with the natural frequency of the structure, it was never to be that you build a plane more rugged and it will dive faster, the aircraft design wasn’t that simple. A rugged aircraft could still meet an early separation of the airflow and been thrown into an uncontrolled compressibility dive.

Here you mentioned the BF-109 dive limit in game to be 790kph, that is a very good example of such “manual + margin” policy. The bf109 has similar airfoil as ki-61, but shorter wing span, smaller ailerons, thus has a faster dive limit than the ki-61. The manual of bf109 limits its dive at 750kph IAS, thus Gaijin gives it 790 with 40kph margin. Although there were a lot of testimonies claimed bf109 experienced dive as fast as 900kph. Again that don’t count, since the pitot static system error in compressibility dive.

Unless the rivet counters haven’t gotten to the P-47 and called them unbalanced, the Ki-61 is quite a ways behind it in mach limit. In fact, you’d have to go all the way down to the Wildcat to find a M0.75 limit, with all the other single engine fighters that come after it being able to go at least past M0.8.

The P-47 was designed to have large dive speed with its Seversky airfoil, and incorporated many compressibility dive tests, later on equipped with recovery flap to obtain a such hight dive speed. Another example would be the F6F-5 in game ,which was limited only with 803kph in a dive, which was slower than ki-61. While in reality its dive limit was 440 knots IAS and some testimonies said it dove at 870kph, the Ki-61 pilots in Kanto air battle found their dive speed couldn’t match the Hellcat.

三式戦
イ旋回性能我と概ね同等若しくは敌やや优る。我が高度差１５００ｍ付近より突进せば、敌の旋回回避により射距离内に入りたるときは追随困难なること多し。
ロ急降下速度は、我よりやや大なり。

The 803kph limit in-game came from the F6F-3’s limit with 415kts due to tail buffeting coupled with natural frequency of the tail. That was another example about dive speed of an aircraft has a very complex nature.

Again, I would say other than this dive limit, ki-61 family was quite accurate in the game. This dive speed issue can be accepted to make Japanese plane in game competitive, but I have to say the reality would be less optimistic.

[skultew1234]

@Irregular23

Yes, according to the US translated version of the ki-61 flight manual 「キ６１操縦法」, the official dive limit was 700 kph IAS. Unfortunately, it seems both ki-61’s original flight manual and operation manual in Japanese may have been lost, since it does not appear in the market

It looks like this limitations based a just on airspeed indicator which was limited to 700km/h

The same with J2M airspeed limit

And mostly for Ki-84 airspeed limit

So limitations at manuals might be caused by indicators what maximim numbers was less than actual dive speed

[Irregular23]

Ki-44II was using the same airspeed indicator with Ki-61, while the manual showed 650kph as the limiting airspeed.

The airspeed indicator somewhat reflects the lack of confidence for allowing higher airspeed to be achieved. It could explain why ki-84 switched to indicator with larger range. The same can be explained by the lack of experiments on performing compressibility dive.

The 400knots limit on J2M should be the same, since the N1K2 manual explained that 400 knots limit was set due to compressibility shock wave, thus the targeted 450 knot limit was not achieved.

[skultew1234]

@Irregular23

Ki-44II was using the same airspeed indicator with Ki-61, while the manual showed 650kph as the limiting airspeed.

Ki-44 is also good example at this questions.
It is similar to Bf109 va Ki-61 dive speed. Ki-44 has smaller wing spar than Ki-61 but has lower dive speed what is strange.

@Irregular23

The airspeed indicator somewhat reflects the lack of confidence for allowing higher airspeed to be achieved. It could explain why ki-84 switched to indicator with larger range. The same can be explained by the lack of experiments on performing compressibility dive.

The exact coincidence of the dive speed, as well as the speed indicator, may indicate that the dive speed limit was set due to the fact that the pilot could not know higher speeds than the indicator showed.

@Irregular23

The 400knots limit on J2M should be the same, since the N1K2 manual explained that 400 knots limit was set due to compressibility shock wave, thus the targeted 450 knot limit was not achieved

The problem is that it is similar story to the Bf109 and Ki-61. J2M had much lower wing spar so I really doubt it should has the same dive limit

[Irregular23]

It was common to see pilot push dive speed beyond the manual limits during the war, but again, I was talking about the game policy that simply set the rip speed to be around 30-40 kph higher than the manual value, most notably the soviet props suffer the most from this policy.

Ki-44 is also good example at this questions.
It is similar to Bf109 va Ki-61 dive speed. Ki-44 has smaller wing spar than Ki-61 but has lower dive speed what is strange.

Its more related with the airfoil, since Ki-61 used similar airfoil as Bf109, but different wing planform, so that comparison make sense. The Ki-44 used different airfoil. Another theory was that IJA really got freak out from the early Ki-43 accidents, and thus went conservative for later aircrafts.

Another feature about IJ fighter planes was the unusually large portion of wing span was occupied by ailerons, which can be bad when high speed flutter happens. One exception was the A7M which featured a twin-aileron design, just like Me-262 and many later-on jet designs that help to overcome the aileron flutter issue,

[Teh0]

Years ago I read that some Japanese aircraft were manufactured with wider safety margin. Minor structural bending was sustained past the safety speed, but the destruction speed was far away from that. This might have been a thing practiced only by certain manufacturers and I would suggest Nakajima, but impossible to say. It’s also not known what the “dive speed” really stood for in IJA. It translates into just that. It could include recovery for an average pilot in mind.

Ki-43 is specially an interesting case. Model 1 was well known from it’s tendence to break at high speeds and model 2 is said to have been a major upgrade. However in manual the dive speed is the same or very close to it from what I can remember. At any case Ki-43s and A6M had trouble keeping up with an enemy accelerating into dive. Extreme speeds weren’t often needed. This might have been due to old float type carburetor not keeping up at angles, resulting into significant power loss.

Ki-61’s 850km/h is from a test flight. The main designer stated there was no fluttering and no visible damage. After that they even wanted to make a lighter wing as they saw no need for that exessive structural strength. Eventually the new wing ended up quite different. It was larger and failed in tests with a Ki-61-II prototype. Later 1000km/h was claimed in combat with Ki-100s, which had the same old wing and new speedometer. Of course margin of doubt is far wider since these weren’t recorded in controlled tests. At the very least they say no Ki-61 or Ki-100 were lost due to structucal failure.

One peculiar thing about Ki-61s is the lack of fancy combat flaps, which became practically a standard thing in other Japanese single engine fighters. Also Ki-61 pilot had to keep the engine within certain RPM and boost settings during high speed dives, which apparently wasn’t done automatically. From other Japanese fighters I haven’t tried looking for these engine limits, though they weren’t that uncommon. Anyway the engine or new tail might have made Ki-100 more practical for higher speeds. It is also very clearly stated in many books how it was the only Japanese fighter capable of keeping up with Americans.

In game “rip speed” is supposed to be IRL dive speed limit +5%. I have no idea where Ki-44 and Ki-84 get their numbers from. J2M and A7M limits I haven’t seen either, so I suppose they are vaguely modelled to fit 400knot requirement.

[skultew1234]

@Teh0

This might have been due to old float type carburetor not keeping up at angles, resulting into significant power loss.

I remember Tester188 report where was reported that A5M and newer engines (A6M and Ki-43 and etc) was available to work with even -G load for some time so not sure it is correct

[Irregular23]

Years ago I read that some Japanese aircraft were manufactured with wider safety margin. Minor structural bending was sustained past the safety speed, but the destruction speed was far away from that.

I know the IJN had 1.8 as safety factor regulated, and some static test showed failure happens over 2.0. Considering the light weight and the use of super duraluminium, I would see A6M is structurally strong in static, but with the airload, things get different.

Ki-61’s 850km/h is from a test flight. The main designer stated there was no fluttering and no visible damage. After that they even wanted to make a lighter wing as they saw no need for that exessive structural strength.

There’s little information on whether the airspeed was correctly calibrated. Ki-61 with 7.2 AR and utilizes high camber convexity airfoil (NACA24000) should meet compressibility well before 800kph. Even the P-51 Mustang, utilizing an laminar flow airfoil, meets compressibility starting at 813kph IAS below 5000ft.

Later 1000km/h was claimed in combat with Ki-100s, which had the same old wing and new speedometer.

Seems like the pitot static system failed in compressibility, which is common at that time. There were pilots of bf109 made similar claims, and Me262 pilot claims on breaking the sound barrier, which wasn’t possible.

Also Ki-61 pilot had to keep the engine within certain RPM and boost settings during high speed dives, which apparently wasn’t done automatically.

This is quite common, diving RPM limit widely exist at that time.

One peculiar thing about Ki-61s is the lack of fancy combat flaps, which became practically a standard thing in other Japanese single engine fighters.

Neither did combat flap saw in Ki-84 manual, nor in the American/British test of Ki-84. The operation seems to only provides 15deg and 30deg. However, pilots with experience could use flap switch, or hydraulic valve to make flap stop at any intermediate position, thus providing a combat flap setting, but less convenient. It looks like IJA tends to remove the use of combat flap in late war, due to unknown reason. Perhaps to make pilots more used to energy fight? I heard that the Ki-84’s high stick force was also made in purpose.

In game “rip speed” is supposed to be IRL dive speed limit +5%. I have no idea where Ki-44 and Ki-84 get their numbers from. J2M and A7M limits I haven’t seen either, so I suppose they are vaguely modelled to fit 400knot requirement.

If 400knots was incorporated, we should expect a rip speed around 790kph.

[RayLeight]

learn to play

[Teh0]

@Irregular23

It looks like IJA tends to remove the use of combat flap in late war, due to unknown reason.

Nakajima’s butterfly flaps are said to have been slow and difficult to use and I don’t think navy’s automatic system ever found it’s way into army aircraft. It might have been impressive in tests, yet in practice it wasn’t used that much either. Unknown how reliable the hydraulic parts were.

[PercussionCap]

@Irregular23

also “14G” was an optimistic number for ultimate load factor

It’s not optimistic, they went beyond that. I have also found zero mentions of Ki-61s breaking up in flight due to high G forces, or even issues relating to this.

@Irregular23

it was never to be that you build a plane more rugged and it will dive faster, the aircraft design wasn’t that simple. A rugged aircraft could still meet an early separation of the airflow and been thrown into an uncontrolled compressibility dive.

These effects were already very well known by the time the Ki-61 was made.

@Irregular23

The bf109 has similar airfoil as ki-61, but shorter wing span, smaller ailerons, thus has a faster dive limit than the ki-61

And an inherently more fragile wing spar design, on top of easily removable wings that introduced an obvious point of failure. Still, the 109 was quite a tough little plane.

@Irregular23

Although there were a lot of testimonies claimed bf109 experienced dive as fast as 900kph. Again that don’t count, since the pitot static system error in compressibility dive.

If it was reading accurately at 750kph IAS (the manual limit) then anywhere above that would be faster, proving that the 109 could dive well beyond 750kph.

@Irregular23

Another example would be the F6F-5 in game ,which was limited only with 803kph in a dive, which was slower than ki-61

Higher mach limit, which is what really counts here if we’re talking about compressibility. And one of the P-38’s minor weaknesses.

@Teh0

Ki-61’s 850km/h is from a test flight. The main designer stated there was no fluttering and no visible damage.

Sounds like I was right.

[Irregular23]

I found this quote:

なお開発時に、土井技師の不適切な対応もあり、急降下時に補助翼がフラッター(異常振動)で千切れ飛ぶと言う事故が発生しているが、無事着陸に成功し事なきを得ている。

There were flutters happen on the Ki-61 during the development phase, and caused its aileron been blown off. This coincides what I have said, that over-spanned aileron causes flutter issue on almost all japanese fighter design, and it seems ki-61 was not an exception.

If it was reading accurately at 750kph IAS (the manual limit) then anywhere above that would be faster , proving that the 109 could dive well beyond 750kph.

In fact different nations did have different standards on dive limits. The German and Japanese manual tends to give ONE specific IAS limit, just like the in-game mechanics, instead of a piece wise limit corrected to Reynolds effect as British and Americans did.

For example, Spitfire was known to have extremely good dive limit and high critical mach number of 0.89, but in-game figure used its manual IAS limit at 20000ft (450mph @ 6000m) + 5% margin, causing them to rip at airspeed significantly slower(only 774kph) compared to Ki-61’s 850. Clearly the Spitfire could dive to a much faster IAS at 10000ft (presumably 970kph IAS could be allowed), and even more near the ground.

This had backed to the main conclusion: Japanese aircrafts in-game do not follow the same policy that applied to every other nation.

As for your claim about 850kph, that was really doubtful, was that a calibrated reading? Did they used the correct the pitot static reading? Were they giving IAS,CAS or TAS? Almost every other aircraft had dirty claims on achieving much faster dive than on the manual, said once P47-C achieved 725mph in a dive, yet the game still complied with its manual number of 522mph IAS at 5000ft + 5%margin as the rip speed.

The manual gives 700kph, which may be somewhat conservative, but so for almost every other aircraft. There were no other first-hand source that can support Ki-61 to do a higher speed dive. If following the same policy, Ki-61 should be given 735kph as its IAS limit, instead of 850kph.

[PercussionCap]

@Irregular23

I found this quote:

なお開発時に、土井技師の不適切な対応もあり、急降下時に補助翼がフラッター(異常振動)で千切れ飛ぶと言う事故が発生しているが、無事着陸に成功し事なきを得ている。

“During development, due to inappropriate handling by Engineer Doi, an accident occurred in which an aileron fluttered (abnormal vibration) during a steep dive and was torn off, but the aircraft was able to land safely without incident.”

Pilot error, according to your own quote.

@Irregular23

The manual gives 700kph, which may be somewhat conservative, but so for almost every other aircraft.

It is not impossible, but extremely unlikely that several aircraft, with completely different design philosophies, manufacturers, and powerplants have the same exact rip speed.
The manual numbers are more likely than not a placeholder, using those makes no sense when we do have primary sources of the Ki-61’s flight testing.

[skultew1234]

@PercussionCap

It is not impossible, but extremely unlikely that several aircraft, with completely different design philosophies, manufacturers, and powerplants have the same exact rip speed.
The manual numbers are more likely than not a placeholder, using those makes no sense when we do have primary sources of the Ki-61’s flight testing.

As it was said earlier Ki-61 airspeed indicator even couldn’t show speed over 700km/h so I really think that limit was done according to this fact

[Irregular23]

Dr.Doi is the chief designer of the ki-61, not the test pilot.

[Irregular23]

Test flight aircraft often uses different airspeed tube and indicator to acquire correct value. At least in other nations they did.