Conflicting Engine Data: Ha-115-II

[Laurelix]

its not a secret that
Ki-43-II used the Ha-115 engine
Ki-43-III used the Ha-115-II engine

However the data for Ha-115-II is all over the place…

Here are the figures I'm getting for Ha-115-II
A. 1150hp take off power
B. 1190hp take off power
-
A. 1100hp at 2850m
B. 1230hp at 2800m
-
A. 980hp at 6000m
B. 950hp at 6800m


Source A: Engine History of Nakajima Aircraft
Source B: Technical Colonel Ankai

C. Wikipedia even claims the figure of 1210hp, boosted by water-methanol injection, I've also seen 1300hp elsewhere.

The Issue I have with Nakajima Table is the fact that Ha-115 is rated 1130HP take off power, 1100hp at 2850m, 980hp at 6000m and Ha-115-II is also 1100hp at 2850m, 980hp at 6000m but with 1150HP take off power. Barely any difference. It claims the only difference is methanol injection. However Ki-43-III Ko which is heavier achieves a lot better rate of climb than Ki-43-II at military power (no methanol injection used)

Issue I have with Colonel Ankai is that the Ki-43-III Ko Top Speed is rated 555-560km/h at 6000m, whilst it claims the 2nd stage peaks at 6800m. If this was the case…. Ki-43-III Ko top speed would be at higher altitude than 6000m, however at the same time the table shows that Ki-43-III has superior performance at 8000-9000m, could that also be the side effect of the "rocket effect", or is the power at that altitude just better?

My Speculation that they probably only differ in terms of maximum power (W-M Injection) whilst military power is identical.
Ha-115 has 1130hp take off power
Ha-115-II has 1230-1300hp take off power
-
Everything military power wise is the same and that the performance difference between Ki-43-II Ko and Ki-43-III Ko at military power is probably due to the fact that Ki-43-III has individual exghaust pipes for each cylinder which is aimed backwards giving it a "rocket effect" as it did for A6M5's and the fact that Ki-43-III Ko is probably also more aerodynamic than the Ki-43-II Ko.
More about rocket effect is here:

Ki-43-II 'Oscar 2' performance differences

Ki-43's had different Nose designs... As you may recall... A6M3 Clipped with 1130hp engine achieved 545km/h at 6000m A6M5 had same wing area and the same engine and intially achieved also 545km/h at 6000m max speed. The first 200 A6M5's didnt get the individual exghaust pipes for each...

ww2aircraft.net

——

Ki-43-II Ko vs Ki-43-III Ko

 

[wells]

I'm gonna throw another curveball at you, hehe
Both Maru Mechanic 45 and "Japanese Army Fighters, Design with Precision" both agree on Ki-43-IIIa, in that the Ha-35-32 engine is used and the latter source cites the maintenance and flight manuals as sources. Data for the model 32 engine is 1150 for takeoff at 2800 rpm and rated power at 2700 rpm of 1210 (low gear) @ 2400 m and high gear is 1055 at 6200 m.

The numbers you posted of 1190/1230/950 are listed for the model 31 engine in MM45 ( chart on pg 10 ). There were prop ratio and supercharger ratio differences, depending on the application ( bomber vs fighter or Army vs Navy ). That's all I got.

 

[qaz]

I have been looking into the strangeness around the Sakae 31 and Ha-115II recently. Based on my research I believe that:

• Sakae 31 and Ha-115II had the same settings as Sakae 21 and Ha-115. Their outputs at rated power were comparable to the previous engines.
• The main purpose of WM injection in Sakae 31 and Ha-115II was to allow an extended use of WEP (10 minute interval).
• Ha-115II almost certainly also used WM injection on rated power as its performance is slightly increased from Ha-115. This makes sense because the Ha-115 had to be de-rated by 100rpm in service.
• I'm not sure if Sakae 31 used WM on rated. Oddly its rated power is slightly less than Sakae 21 overall.
• The high numbers for Ha-115II (like 1200+ PS on rated) are actually from its WEP output. Sakae 31 and Ha-115II are basically the only Japanese aero engines where you can find original WEP numbers at altitude.
• The power numbers usually associated with Ha-115 are actually closer to those of Ha-115II on rated.

Because the main feature of Sakae 31 and Ha-115II is the extended WEP duration, the top speed performance numbers for Ki-43III and A6M6 were measured/calculated using WEP. As far as I am aware, this is the only case of Japanese aircraft officially using WEP for the top speed measurement. It's why the Ki-43III and A6M6 reach top speed at 6000 meters despite having an increase of ~1000 meters in 2nd-speed critical altitude. In fact, I suspect the main point of raising the supercharger's 2nd speed critical altitude was so that these planes could reach WEP manifold pressure at the same altitude where the prior A6M5 and Ki-43II previously reached rated manifold pressure.
According to regulations, Japanese aircraft in the past usually had a limit of only 1 minute for emergency WEP use in combat. However, in practice, pilots utilized it more liberally.

My post is a bit useless without sources, but I'm writing an article to explain that I will probably (*) finish within 1 week. I also think that the performance of Sakae 21 is slightly suspect.

 

[Laurelix]

I have been looking into the strangeness around the Sakae 31 and Ha-115II recently. Based on my research I believe that:

• Sakae 31 and Ha-115II had the same settings as Sakae 21 and Ha-115. Their outputs at rated power were comparable to the previous engines.
• The main purpose of WM injection in Sakae 31 and Ha-115II was to allow an extended use of WEP (10 minute interval).
• Ha-115II almost certainly also used WM injection on rated power as its performance is slightly increased from Ha-115. This makes sense because the Ha-115 had to be de-rated by 100rpm in service.
• I'm not sure if Sakae 31 used WM on rated. Oddly its rated power is slightly less than Sakae 21 overall.
• The high numbers for Ha-115II (like 1200+ PS on rated) are actually from its WEP output. Sakae 31 and Ha-115II are basically the only Japanese aero engines where you can find original WEP numbers at altitude.
• The power numbers usually associated with Ha-115 are actually closer to those of Ha-115II on rated.

Because the main feature of Sakae 31 and Ha-115II is the extended WEP duration, the top speed performance numbers for Ki-43III and A6M6 were measured/calculated using WEP. As far as I am aware, this is the only case of Japanese aircraft officially using WEP for the top speed measurement. It's why the Ki-43III and A6M6 reach top speed at 6000 meters despite having an increase of ~1000 meters in 2nd-speed critical altitude. In fact, I suspect the main point of raising the supercharger's 2nd speed critical altitude was so that these planes could reach WEP manifold pressure at the same altitude where the prior A6M5 and Ki-43II previously reached rated manifold pressure.
According to regulations, Japanese aircraft in the past usually had a limit of only 1 minute for emergency WEP use in combat. However, in practice, pilots utilized it more liberally.

My post is a bit useless without sources, but I'm writing an article to explain that I will probably (*) finish within 1 week. I also think that the performance of Sakae 21 is slightly suspect.

That actually makes a lot of sense...
You see...
A6M5 Mod 52 initially had 545kph at 6000m , 7:27 to 6000m rate of climb. It has the same engine as Ki-43-II
Then from around Late September - October 1943 it received individual exhaust pipes and were aimed backwards which created extra thrust.
New performance became: 565kph at 6000m, 7:01 to 6000m

Ki-43-II Ko had 515kph at 6000m but later came Ki-43-II Kai around September 1943 which also incorporated this feature of additional thrust.

The Speed for Ki-43-II Kai would of increased to roughly 535kph at 6000m at military power

Now... Ki-43-III Ko also has the individual exhaust pipe feature getting benefits from it as well.
Ki-43-III ko new engine has 1190hp at SL with WEP, roughly 1065hp at military power at SL (also 1230hp at 2800m)
1190 / 1065 = 1.1174 , engine power WEP multiplier
Cube Root of 1.1174 is 1.0377
The Source shows that at military power the engine optimal altitude is up to 6800m and we can be more or less sure that the WEP at 5850m is offering full 1.1174x engine power boost as well compared to military power at that altitude of 5850m. Both Ki-43-II and Ki-43-III have rather similar engine power at military power at 6000m.
The Source says 550-560kph at 5850m.
The Speed at military power without WEP at 5850m would be 530-540kph for the Ki-43-III Ko
550 / 1.0377 = 530
560 / 1.0377 = 540

Considering Ki-43-II Kai has roughly 535kph at 6000m with roughly 950hp at 6000m and Ki-43-III Ko has 950-1000hp at 6000m
It shouldnt be surprising

 

[taly01]

The Ha115 short term max. power rating (~1min) was +200mm, the Ha115-II short term max. power rating was (<10min) and at +300mm, but could continually use +200mm until its water-methanol ran out!

Water-meth injection also adds some 5% power even at same boost level due to water evaporation cooling increasing density of intake mixture, this effect works up to around 6000m where things begin to get very cold, this is another data point the Ha115-II seemed to be aiming at with the new supercharger second speed!

 

[Laurelix]

I also like how the TAIC can't agree on a single value for Sakai 21 / Ha-115

Power Curve for Ha-115 according to the Japanese source (Technical Colonel Ando)
1120hp at Sea Level (WEP)
1020hp at 2800m (Military Power)
980hp at 5600m (Military Power)

Nakajima figures
1130hp at Sea Level (WEP)
1100hp at 2850m at Military Power
980hp at 6000m at Military Power

 

[taly01]

I think sometimes US data on Japanese planes also converted metric HP to imperial HP, 0.98 factor. Also a coarse conversion of metres to ft for altitudes was sometimes done only to closest 1000ft if lucky. Then people reconvert that back to metric and you get slightly off results!