N1K2-J Shiden-Kai Performance

[Laurelix]

Thank you, that makes perfect sense. My best attempt to calculate how an 175 HP increase impacts the Shiden Kai's speed matches your calculation as well. As far as I can tell you are correct about the continuous military speed of the N1K2-J and I have to compliment your math skills.

But just so others can follow, I've done my best to show my work below:

1. Military speed of Shiden Kai with full-rated Ha-45-21 = Speed of derated Ha-45-21 *(1625 HP/1475 HP)(to the 1/3rd power)

2. Military speed of Shiden Kai with full-rated Ha-45-21 = 610 KPH * (1.11 to the 1/3rd power = 1.03228011)

3. Military speed of Shiden Kai with Full-rated Ha-45-21 = 630 KPH

I rounded off a lot so my numbers are not as precise as yours. Overall, your estimates are the best calculated top speed for the fully rated Shiden Kai available, although I think there are some assumptions made in your WEP calculation that might need some additional research in order to validate. For example, the WEP setting for the Ha-45-21 is 500mm of mercury at 3,000 RPMs. But given that the Homare was already using MW50 injection at military power, then I don't know if it could get another 200 HP because MW50 already added 200 HP according to Japanese records.

But that calculation itself is probably better than anything else out there. However, we could make a better estimate if we knew the Sumitomo-VDM prop efficiency used in the Shiden-Kai. It's also worth noting that their props got better throughout the war. Although US bombing eventually destroyed the Sumitomo Water factories, they did manage to relocate to a basement beneath a department store and continued production.

By the way, I went ahead and uploaded the Shiden-Kai manual to a free filehosting solution, which anyone can download here. You already have it but in case someone else wants to download it, I hope it helps them.

At nearly 400 pages it will take forever to translate, and the scan quality is bad enough that even AI OCR cannot do an adequate job at translating it automatically. There are a few options available to me but none are good enough to do a comprehensive translation. This AI seems to be one of the better free options out there.

I have a few questions:

First, are we sure the Shiden Kai in the manual has a derated Ha-45-21? I could not find what engine they are referring to in the engine section. The kanji is almost unreadable.

Second, are we sure that the Homare 21 overboost setting was capable of adding 200 horsepower? It was probably weaker than that given that it used MW50 injection at its military power setting.

Third, if would be helpful if we knew the four-blade VDM propeller efficiency in order to determine how an increase in horsepower would impact top speed. Although this isn't necessary, the fact that their props got better over time means there could have been a speed bump later in the war that's not accounted for.

Since the U.S. knew the engine power curve of Ha-45-21

They calculated the performance of Japanese fighters based on that

Let's look at the Ki-84 TAIC - Ha-45-21
663kph (412mph) at 6000m

With WEP
687kph (427mph) at 6000m
687 / 663 = 1.0362

635 x 1.0362 = 658

 

[MIflyer]

My favorite N1K2 artwork, but then I am a bit biased...

 

[AerialTorpedoDude69]

Since the U.S. knew the engine power curve of Ha-45-21

They calculated the performance of Japanese fighters based on that

Let's look at the Ki-84 TAIC - Ha-45-21
663kph (412mph) at 6000m

With WEP
687kph (427mph) at 6000m
687 / 663 = 1.0362

635 x 1.0362 = 658

Thanks, that makes sense. But am I understanding correctly that you are using the TAIC speed calculation to derive the Ha-45-21's fully rated power output? We're both familiar with the numerous reasons (such as the Pe-32 prop and TAIC's miscalculation of drag) that number can't be fully trusted. But I am interested in why you consider it to be valid as you have been using valid methodology.

Also, thank you for entertaining my questions. Regarding the Shiden Kai, I'm trying to run an AI translation on the manual but so far it hasn't been successful. But does it mention in the manual that the N1K2-J used the derated Ha-45-21 special? I vaguely remember reading somewhere that it did use the derated engine but I can't find the source.

 

[Laurelix]

Thanks, that makes sense. But am I understanding correctly that you are using the TAIC speed calculation to derive the Ha-45-21's fully rated power output? We're both familiar with the numerous reasons (such as the Pe-32 prop and TAIC's miscalculation of drag) that number can't be fully trusted. But I am interested in why you consider it to be valid as you have been using valid methodology.

Also, thank you for entertaining my questions. Regarding the Shiden Kai, I'm trying to run an AI translation on the manual but so far it hasn't been successful. But does it mention in the manual that the N1K2-J used the derated Ha-45-21 special? I vaguely remember reading somewhere that it did use the derated engine but I can't find the source.

In the opening post you can see the lower manifold pressure compared to what fully rated Ha-45-21 has

Furthermore we see Japanese prototype flight test results and beside it there's calculated figures

It wouldn't make sense to have 2 different performance columns if there was no difference

 

[Laurelix]

Thanks, that makes sense. But am I understanding correctly that you are using the TAIC speed calculation to derive the Ha-45-21's fully rated power output? We're both familiar with the numerous reasons (such as the Pe-32 prop and TAIC's miscalculation of drag) that number can't be fully trusted. But I am interested in why you consider it to be valid as you have been using valid methodology.

Also, thank you for entertaining my questions. Regarding the Shiden Kai, I'm trying to run an AI translation on the manual but so far it hasn't been successful. But does it mention in the manual that the N1K2-J used the derated Ha-45-21 special? I vaguely remember reading somewhere that it did use the derated engine but I can't find the source.

Ki-84 TAIC says that the engine has
1765hp at SL at Military power
1970hp at WEP
1970 / 1765 = 1.1161
Cube Rooted = 1.0373
It also says
350mph at SL at military power
363mph at SL at WEP
350 x 1.0373 = 363

Now according to the taic, Ki-84 has 662-663kph at 6000m and we know the WEP multiplier is 1.0373
662 x 1.0373 = 687

The TAIC also gives the Ki-84 a speed of 687kph at 6000m. This means the WEP multiplier hasn't still lost power due to being over the maximum optimal altitude yet. If x1.0373 is still applicable at 6000m

It means that if 1625hp is the power at 6000m at military power , then it has ~1815hp at 6000m with WEP (1970 / 1765 = 1.1161, 1625 x 1.1161 = 1814hp)

If then the N1K2-J achieved 611km/h at 6000m with 1450 horsepower…Then with 1815hp (Ha-45-21 full power with WEP)
It would have;
1815 / 1450 = 1.2517
Cube Rooted = 1.0777
611 x 1.0777 = 658kph

 

[AerialTorpedoDude69]

Ki-84 TAIC says that the engine has
1765hp at SL at Military power
1970hp at WEP
1970 / 1765 = 1.1161
Cube Rooted = 1.0373
It also says
350mph at SL at military power
363mph at SL at WEP
350 x 1.0373 = 363

Now according to the taic, Ki-84 has 662-663kph at 6000m and we know the WEP multiplier is 1.0373
662 x 1.0373 = 687

The TAIC also gives the Ki-84 a speed of 687kph at 6000m. This means the WEP multiplier hasn't still lost power due to being over the maximum optimal altitude yet. If x1.0373 is still applicable at 6000m

It means that if 1625hp is the power at 6000m at military power, then it has ~1815hp at 6000m with WEP (1970 / 1765 = 1.1161, 1625 x 1.1161 = 1814hp)

If then the N1K2-J achieved 611km/h at 6000m with 1450 horsepower…Then with 1815hp (Ha-45-21 full power with WEP)
It would have;
1815 / 1450 = 1.2517
Cube Rooted = 1.0777
611 x 1.0777 = 658kph

Sorry for the late reply.

I've scrutinized your calculation on my own and using AI and believe that you are the first person to figure out the top WEP speed for a factory-fresh, fully-rated Ha-45-21 N1K2-J Shiden-Kai. When I hand calculated the top speed, I got the same performance as you arrived at, although my math skills are terrible.

The only issue is that the manual is almost impossible to read. The Kanji is almost unreadable even to a machine using optical character recognition. And it's not clear where the tables came from which have the N1K2-J's with a derated Ha-45-21's top speed. If we could find where in the Japanese manual this information is located, it would be possible to update Wikipedia to reflect the actual top speed.

But also, if someone publishes an article somewhere it's possible to edit Wikipedia with the correct data, including the WEP value. All we need is adequate citations.