Nakjima Homare - what's the verdict?

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BUNRIN-DO No.20 1971




And BUNRIN-DO No. 148 1985


As you can see, basically the same as the research done in 1956.

From Model Art 1997, the entire volume on Ki-84, in Japanese, and includes models and modifications as much as I can tell. An excellent book and an ad for a J2M in the same series.

Two more from the Japanese volume of the "Airview" 1956






Although you may not read Japanese, we can still see numbers that tell us speed, altitude etc.

Nice posts special ed. Thanks for sharing.

While putting books away, I began "re-reading" the Model Art 1997 book on the Ki-84. That's why I don't like getting them out. Here is another chart with performance numbers as well as the instrument page and the cockpit panel locations.

special ed said:

View attachment 660725View attachment 660726

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Thank you special ed for posting excerpts from those period Japanese books. They cement for us the basic understanding that while the Ki-84 was the fastest Japanese fighter to see service during the war it was nevertheless a sub 400 mph fighter.

The numbers published by TAIC are only theoretical and could be off due to inaccuracies in propeller efficiency, airfoil calculations, weight, and thrust augmentation numbers. Unfortunately there are many over the years who accept them as fact, and this only got worse when the IL2 flight sim made an uber-fighter out of it using these same unsubstantiated calculations.

special ed said:

The three N1K1 were to be escorted by four late model F4U-4s armed, while the Japanese planes had guns with no ammo. The seven aircraft took off together but the Japanese arrived at the port 20 minutes before the Corsairs.

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Sorry special ed but that's not exactly how things occurred...

Excerpt from Genda's Blade middle of page 183:

On 16 October 1945 the time came for the ferry flight to Yokosuka. LCdr Shiga met Cdr Yamada and several Marine officers to discuss plans. One of the officers, a handsome man, appeared very familiar to Shiga. The plan was for the flight of three Shiden-Kais to be escorted by four Corsairs, with the Shiden-Kais in the middle, flanked by a pair of F4Us on the outside. The Americans ordered the Japanese pilots to fly straight and level, otherwise they would be shot down. The 20 mm cannon were reinstalled on the fighters, but they were obviously not loaded. Mechanics attached belly tanks for the long journey.
When all the aircraft were airborne, the three Japanese pilots decided to unleash the full power of the Shiden-Kai. With no ammunition, the aircraft was a little lighter. Using high octane American aviation fuel, the Homare engine exhibited it's maximum power. As if on cue, the three ferry pilots gave full throttle and virtually left their escorts behind.
"The Corsairs raced after us at full speed!" recounted CPO Toshio Tanaka. "It was very thrilling and delightful! I gestured to them to speed up and try to catch us."
The fight reached Yokosuka Airfield without any problems and the three Shiden-Kais were turned over to the Technical Air Intelligence Center.

The Marine squadron mentioned was VMF 113 which at this time were equipped with F4U-1Ds (not F4U-4s as previously stated). Secondly, It said the flight reached the airfield without any problems. I would think that if the Marine pilots arrived a whole 20 minutes AFTER their charge I'm quite sure there would be quite a few problems for the American pilots!

This was basically the case of the Japanese pilots having a little fun with the Marines as they obviously had no desire to ignore the orders of their captures. Quality fuel or not, the Corsairs were still faster and they would have easily caught up to the Japanese if it became an extended pursuit, which it did not.

DarrenW said:

Thank you special ed for posting excerpts from those period Japanese books. They cement for us the basic understanding that while the Ki-84 was the fastest Japanese fighter to see service during the war it was nevertheless a sub 400 mph fighter.

The numbers published by TAIC are only theoretical and could be off due to inaccuracies in propeller efficiency, airfoil calculations, weight, and thrust augmentation numbers. Unfortunately there are many over the years who accept them as fact, and this only got worse when the IL2 flight sim made an uber-fighter out of it using these same unsubstantiated calculations.

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Unfortunately, we still don't know the Japanese test methodology for determining maximum speed. On top of that, when mentioned, the Japanese documentation mentions that the Ki-84 aircraft have 1,800 horsepower engines. I'm not sure why this is but I've heard two explanations:

1. The Homare was designed for 100 octane fuel. (The 1/8 compression ratio of the Homare suggests that it was designed for a high octane fuel)

2. To prolong the lifespan of the engine. (The Homare was well known as a high-maintenance engine.)

By the way, I found the original TAIC documentation for Japanese aircraft and they mention in their methods section that all VMAX calculations are synthetic unless otherwise mentioned (they have a box in which comments are made regarding virtual vs. real performance). It's mentioned that they use a "very careful" methodology. I'm not sure whether they mean they do their best to assure accuracy or whether they do everything possible to make sure pilots don't get any nasty surprises from the speed calculations. But this was published in September of '45 after going hands on with a Ki-84 from the Philippines (according to the Japanese documentation). So it's clear that they had correct measurements of the aircraft.

It appears (without evidence) that TAIC must have removed the limiter on HP and they MUST have used a higher octane fuel (as others have argued) if they made the calculation that this thing was a 427MPH aircraft. Because it seems clear at this point that its operational performance was less than 400 MPH for the Homare 21 series because of output problems at altitude. The Homare 23 almost certainly had better performance, although no Homare-23 equipped Hayate made its way to TAIC.

So if these are derated engines running at 1,800 HP, then it would make sense that the maximum speed was 388MPH.

AerialTorpedoDude69 said:

1. The Homare was designed for 100 octane fuel. (The 1/8 compression ratio of the Homare suggests that it was designed for a high octane fuel)

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Japanese sources note 7:1 CR for the Homare line of engines. US report notes that CR of the Homare 21 was 7.17:1 as measured:

Gentlemen,
Perhaps today I am in the bubble bursting business.
Please review IN DETAIL what has been presented in those "Period Japanese Publications".
You will find that quite a bit of it is basically translated from the TAIC reports we already know about. Even the graphs are reproduced. Most of the other data is regurgitated from sources we have already seen.
The dimensions where stated are generally from the manuals for the aircraft but the images are cleaned up a bit. I have rounded up a fair number of the images and they do have good dimensional though not performance data.

As for performance estimates, I didn't want to get into this again, but here goes anyway:
First of all, one has to decide whether or not the claimed performance for the Homare is believable or not. I tend to believe that it is. I also believe that TAIC did a bit more testing on Homare than is obvious. Please note the horsepower figures differ slightly between the N1K2-J and the Ki 84 in at least one table. I don't believe this was a mistake. I would quote the data, but my laptop died a few months ago and my reference data is not currently accessible to me.
Even without that table, a reasonable HP versus altitude graph can be plotted using data from the Middletown report.

Note that the original TAIC speed estimate was CALCULATED based on what would happen with an engine of Homare's power installed in a Ki 43 Hayabusa airframe. This was before a Ki 84 was captured and tested. When aircraft were captured, they were not actually tested for straight line performance. A second report in the same thread as the Middletown report states this. Testing was for handling and relative combat performance.
There are actually other report suggesting some level of performance, notably one against Spitfire XIV if I remember correctly.
I believe in that report, one can conclude that the Ki 84 was a 400+ MPH aircraft at a minimum.
After the end of the war when the Middletown report was written, there was again some testing of the Ki 84. The prior performance reports listing 427 MPH were referenced again. One has to wonder that with the effort put into the Middletown report, if that number were not credible, it probably would have been corrected at that time.
Do I really believe the Ki 84 could achieve 427 MPH? Maybe and maybe not, but I believe reality was pretty close.

- Ivan.

Ivan1GFP said:

Note that the original TAIC speed estimate was CALCULATED based on what would happen with an engine of Homare's power installed in a Ki 43 Hayabusa airframe.

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Thanks for clearing up so many rough edges Ivan! One quick question:

TAIC lists the dimensions for the Hayate as being equal to the historical record (37' length, etc...). Are you saying that TAIC generated the calculated performance data prior to getting their hands on a Ki-84, and then left outdated information in the pilot briefing? So even after getting a real Hayate, they changed the dimensions but not the calculated speed?

tomo pauk by the way, I found the source: SAE MOBILUS

The Homare was designed for 100-octane fuel according to Nakajima engineers. My guess is that the 1/8 compression ratio is erroneous, although it's frequently cited all over the internet. The source you found (is it TAIC?) seems to be authoritative, since they actually measured it. But even so, 1:7 is a really high compression ratio.

I don't believe TAIC ever tested the Ki 84 for maximum level speed in a manner that would be comparable to testing of Allied aircraft. Please note that just about all the American flight tests comment that testing only for handling, acceleration, and relative combat performance. Even the T-3 Report that is a few posts down from the Middletown Report is a little weird. It lists performance and maximum speed data that is pretty consistent with other TAIC estimates but on the last page states that no performance data was obtained in testing....
Japanese Aircraft Performance

Check out this old thread.
Ki-84 - The Real Maximum Speed?

Please note the comment that the Middletown test engine gave better results than the specifications from the manuals that are quoted in this thread. This was not done with any special fuel; it was 92 Octane and ADI.

My belief is that TAIC didn't revise their earlier estimate because it was pretty much correct from evidence they later collected.
From having been in numerous discussions on the same subject, I tend to believe that a perfectly maintained Ki 84 was actually capable of achieving the speeds listed. Obviously there are many who disagree.

As for dimensions of the aircraft, I would go with information from the Japanese books over that from TAIC.
The information from the books is directly from the manual and is nominal specifications down to the millimeter.
TAIC dimensions are from running a tape measure over a captured aircraft. It is better than nothing but not nearly as good as something from the actual aircraft manual.

- Ivan.

I Ivan1GFP thanks for clarifying. I'm mainly interested in locating, and vetting, the sources for the Homare and the Frank's performance so I can update Wikipedia. As it stands, I can't find for certain whether the calculations for the Frank's top speed are accurate. I ask because it's been mentioned that TAIC used an incorrect drag co-efficient and HP rating for the Frank, although I read through the report on the T-2 Frank as well as all the documentation sourced here, and can't find any reference to it. I suspect that TAIC probably knew what they were doing though and is probably the most authoritative source in English.

AerialTorpedoDude69 said:

So if these are derated engines running at 1,800 HP, then it would make sense that the maximum speed was 388MPH.

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More horsepower doesn't always equate to a higher absolute maximum speed. Even the TAIC data shows basically the same speeds, 426 mph vs 427 mph, attained in Military power (+350 mm) and WEP (+500 mm) respectively, the only difference is at which altitude the speed was obtained (23,000 vs 20,000 feet). This is because the supercharger can only perform so well as altitude increases. Eventually it doesn't matter how much boost is applied, engine horsepower will max out and nothing more can be produced. This is known as critical altitude.

This is why a speed of 390 mph would be the absolute maximum speed in both military and emergency power settings, with only the altitude at which the speed could be obtained changing as a result. This can still be beneficial of course, as emergency power would improve speed and climb below the critical altitude when using military power.

DarrenW said:

More horsepower doesn't always equate to a higher absolute maximum speed. Even the TAIC data shows basically the same speeds, 426 mph vs 427 mph, attained in Military power (+350 mm) and WEP (+500 mm) respectively, the only difference is at which altitude the speed was obtained (23,000 vs 20,000 feet). This is because the supercharger can only perform so well as altitude increases. Eventually it doesn't matter how much boost is applied, engine horsepower will max out and nothing more can be produced. This is known as critical altitude.

This is why a speed of 390 mph would be the absolute maximum speed in both military and emergency power settings, with only the altitude at which the speed could be obtained changing as a result. This can still be beneficial of course, as emergency power would improve speed and climb below the critical altitude when using military power.

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Thanks again Darren. So you're saying that the supercharger's blower settings can't be changed to match engine output at the critical altitude? This is an interesting line of research as I know absolutely nothing about the Homare's supercharger and this is one of the most important components for performance.

EDIT: By the way, I just noticed your quote. My father had this great anecdote from his service years about having met a cab driver in (IIRC) Nagoya city Osaka in Japan who claimed to have been a high-scoring fighter ace. I'm 90% sure (if this wasn't a lie) that he had met Takeo Tanimizu in person!

AerialTorpedoDude69 said:

Thanks again Darren. So you're saying that the supercharger's blower settings can't be changed to match engine output at the critical altitude? This is an interesting line of research as I know absolutely nothing about the Homare's supercharger and this is one of the most important components for performance.

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There is minor tweaking that can be performed but without a major upgrade in supercharger design, using higher octane fuels and/or water methanol injection will increase power lower down but it will eventually have no affect as one gets closer to the critical altitude while utilizing a military power setting. So while you might get an extra 200 hp at 15,000 feet when using emergency power the extra drag at the lower altitude will negate any real increase in absolute maximum speed which was obtained at 20,000 feet while in military power (maybe a one or two mph gain is possible). Of course the speed you once achieved at 15,000 feet while using military power could increase by 10 or 15 mph in the higher boost setting. Does this make sense?

AerialTorpedoDude69 said:

EDIT: By the way, I just noticed your quote. My father had this great anecdote from his service years about having met a cab driver in (IIRC) Nagoya city Osaka in Japan who claimed to have been a high-scoring fighter ace. I'm 90% sure (if this wasn't a lie) that he had met Takeo Tanimizu in person!

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Yes, they were called "Kamikaze Taxi" in the 1950s. They were so speedy and agile.

AerialTorpedoDude69 said:

EDIT: By the way, I just noticed your quote. My father had this great anecdote from his service years about having met a cab driver in (IIRC) Nagoya city Osaka in Japan who claimed to have been a high-scoring fighter ace. I'm 90% sure (if this wasn't a lie) that he had met Takeo Tanimizu in person!

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That is a very interesting story and it could very well be true. During a cursory check I found this on the web:

Following the end of the war, Tanimizu became a taxi driver until a minor stroke forced him into retirement. He passed away in a retirement home in Osaka, Japan on March 12th 2008, at the age of 88.

Mitsubishi A6M5c Type 0 Model 52c Reisen '03-09' by kanyiko on DeviantArt

www.deviantart.com

Shinpachi said:

Yes, they were called "Kamikaze Taxi" in the 1950s. They were so speedy and agile.

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That was a fascinating look. I see why they were ticketed. They were clearly driving on the wrong side of the road.