Nakjima Homare - what's the verdict?

[AerialTorpedoDude69]

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

Part of me wants to believe the guy wasn't lying, but there were so many frauds out there. We've all met our share. I even met a sushi chef in the early nineties in Sacramento who claimed to have been a kamikaze pilot who rammed a B-29, got stuck, and was captured. I know for certain that this is a pure fiction so someone claiming to have been an ace seems just as unlikely since they were so rare.

Oh and your explanation makes perfect sense, they teach those principles in ground school, although it's been ages since I went through ground school. It's good to review that information though, thank you. Your explanation was outstanding.

 

[special ed]

At one CAF airshow, an individual at a table selling Japanese headbands claimed to be the one who shot down Boyington. Although he appeared way too young no one challenged him. Fortunately, Greg Boyington was also a vendor at the show, and when I mentioned the claims made by the other vendor, Greg replied with a description of him in the language for which Boyington was legend.

 

[BiffF15]

At one CAF airshow, an individual at a table selling Japanese headbands claimed to be the one who shot down Boyington. Although he appeared way too young no one challenged him. Fortunately, Greg Boyington was also a vendor at the show, and when I mentioned the claims made by the other vendor, Greg replied with a description of him in the language for which Boyington was legend.

Knowing fighter pilots, and knowing Boyingtons' temper, he probably told you and any number of folks the same thing. Meanwhile he knew everyone would mention it to Greg, which in turn would keep him pissed off all day. Typical fighter pilot modus operandi. I would have done the same thing. If a man doesn't have an achilles heel, make one up and give it to him.

 

[Ivan1GFP]

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.

Hello AerialTorpedoDude69,
If you are REALLY interested in how the supercharger on the Homare-21 performs, you can get a VERY good plot based on the T-2 / T-3 Report and the Middletown Report.
Perhaps I am telling you things you already know..... I know this is a rehash for some folks here.

(Incidentally, there was a question earlier about the compression ratio of the Homare engine.
Please read page 84 of the Middletown Report. It states compression is 8:1.
I would tend to believe a group doing a tear down of an engine would get that characteristic correct.)

Here is how to go about it. Now keep in mind that the numbers that you will get here are the experimental numbers from US testing and not necessarily the specification numbers that one normally would get from design specifications. In other words, if the "official" critical altitude is 6500 meters and this particular engine happens to test at a slightly different critical altitude....

We know T-O power is achieved at +500 mm boost (49.6 inches Hg according to Middletown)
For this discussion, this is also considered War Emergency Power.
We know that Military power is achieved at +350 mm boost (43.7 inches Hg according to Middletown)

Here is where life may get a little confusing: Below "Critical Altitude", a supercharger has excess boost capacity. It has the capacity to deliver more boost than at critical altitude. It may have the capacity to deliver more boost than the engine can tolerate.
As for what "Critical Altitude" means, it is the maximum altitude at which the supercharger can deliver the boost pressure specified.
The Altitude changes DEPENDING ON THE BOOST PRESSURE SPECIFIED.
So, reading Page 3 of the T-2 Report:
In Low Blower -
For War Emergency Power +500 mm boost
2050 HP @ 2,500 feet
Above that, the supercharger can no longer maintain +500 mm boost in low blower.
For Military Power +350 mm boost,
1875 HP @ 5,900 feet
For Normal Power which is probably around +180 mm boost
1150 HP @ 8,000 feet

For High Blower, more power is consumed by the supercharger to compress the thinner air at high altitudes to maintain the same boost, so even though the boost is the same, the engine output is lower.
In High Blower -
For War Emergency Power +500 mm boost
1850 HP @ 17,900 feet (427 MPH @ 20,000 Feet)
For Military Power +350 mm boost,
1695 HP @ 20,000 feet (426 MPH @ 23,000 Feet)
For Normal Power which is probably around +180 mm boost
1000 HP @ 22,000 feet

As you go higher, engine power is dropping, but air density and air resistance are also decreasing, so it is a cool little balancing act.
Note that the maximum speeds are achieved a couple thousand feet above the engine's critical altitude.
This is probably because of ram effects from having the intake facing into the airstream on a fast moving aircraft.

There was a comment earlier about this aircraft and water injection and emergency power.
Note the "Normal Power" engine outputs. This was probably all this engine was capable of producing WITHOUT Water Methanol injection. I don't believe the aircraft could exceed 300 MPH at any altitude without Water Methanol injection. Such are the drawbacks of 91/92 Octane fuel standards so late in the war.

- Ivan.

 

[AerialTorpedoDude69]

At one CAF airshow, an individual at a table selling Japanese headbands claimed to be the one who shot down Boyington. Although he appeared way too young no one challenged him. Fortunately, Greg Boyington was also a vendor at the show, and when I mentioned the claims made by the other vendor, Greg replied with a description of him in the language for which Boyington was legend.

That's Mike Kawato! I have a signed book of his called "Bye Bye Blacksheep" (or "Flight Into Victory", he renamed the book to piss off Pappy) which is a partial work of fiction . The guy was only a partial fraud. He claimed to have been an ace, but Sakaida's post-war analysis of his victories showed he maybe had a couple, at most. And was shot down at least five times. So he was a kind of an ace

Despite his faults, Kawato was an accomplished pilot who was at least present when Pappy was shot down. So he might not be lying (but we're pretty sure he is).

By the way, I didn't mean to imply that Tanimizu could have been a fraud. His victory claims were disturbingly accurate in some cases.

I Ivan1GFP thanks again for helping me with sources. I'm unable to find the report on the T-3 aircraft, although I have read the TAIC report on the T-2 aircraft and must have missed the part on the supercharger. (Also, I gather you mean 400 MPH, not 300 MPH.)

 

[special ed]

That CAF airshow also had George Gay, former Ensign, with a vendor's table where I bought a signed copy of his book.

 

[Shortround6]

Am I missing something on the Homare engine?

Engine that is 0.78 the size of an R-2800 spins 1.11 times faster for about 0.866 the airflow at "normal" air pressure.

The R-2800 runs at 52in of manifold pressure while the Homare runs 49.6in manifold pressure (0.954 the manifold pressure).
The Homare is running at 2500ft (?) while the two speed single stage R-2800 is running at 1000-1500ft of altitude. Maybe a 2% difference?

The Homare "offers" a lot more in high gear.

The R-2800 is making 1650hp at 13,500ft at military power. That would be at 52in (or darn close) and we could "assume" the engine would lose about 2% per 1000ft for about 1320hp at 23,500ft compared to

"For Military Power +350 mm boost,
1695 HP @ 20,000 feet (426 MPH @ 23,000 Feet)"

This would seem to imply the R-2800 was not very good (and in may not have been.)

We could argue about the efficiency of the superchargers or about the size of the valves manifolds but the only really big difference seems to be the water injection system.

Water injection systems don't "make power" on their own.
They "allow" for higher power in two ways.
1. They allow for higher pressure to be used before detonation sets in. The Water injection on the F4U allowed for about 6in more manifold pressure. But the Homare was already at max pressure.
2. At the same pressure/s the engine using water injection has a denser mixture (more pounds of air per cubic ft of volume) which allows more fuel to be burned for the same volume of air.

For the Homare we are being asked to believe that cooling the intake charge (air) effect is enough to make up for the roughly 13% less air flow through the basic engine and/or really cool off the charge when the supercharger is operating in high gear.

Maybe the Homare engine's supercharger was that good?

 

[tomo pauk]

Maybe the Homare engine's supercharger was that good?

It was good (if not great) and big enough - 12.6 in diameter of the impeller.
S/C on the wartime 1-stage S/Ced R-2800 was neither good (at least not until the C series) nor it was big for the displacement it was supplying the air with: 11 in impeller on the A and B series, 11.5 in on the C series.

For comparison, both Hercules ('normal' versions) and BMW 801 have had the impellers of diameter of 13 in. Late-war versions (Hercues 100, BMW 801S) were making 1600+ HP at 18500+ ft, vs. the R-2800-22 making 1600 HP at 16000 ft. (all 'dry' power, no ram). Both of these 14-cyl engines have had much improved superchargers than the previous types, like the Hercules XVI or BMW 801D.

Wrt. mid-war engines, the fully-rated BMW 801D was making better altitude power than the R-2800s used on wartime A-26s or late B-26 versions.

 

[Shortround6]

You have two/three dimensions on impellers. The diameter of the impeller and depth and number of the blades.
The diameter of impeller can tell you something about the veleocity of the air leaving the impeller and that is strongly related to the power required to drive it.

But we often don't know the size and shape of the inlet and how well the inlet was designed (Early Merlin inlet anyone? and it was one of the better ones even before Hooker came along) or anything about guide vanes or anything about the impeller blades (straight or curved or box vanes) or the diffuser and the volute.

The P & W Pre "C" series supercharger may have been on the poor side but P & W was also one of the two most successful designers of two stage superchargers. Maybe they needed two stages to get around how poor the singe stage was?

But you are depending on a really good supercharger to make up for that difference in displacement the Homare had. You need an awfully good supercharger to make up the difference. Remember that the R-2800 making 1600hp at 13,500ft was making well over 2000hp in the cylinders. It was taking almost 400 hp more to drive the supercharger in high gear than in was in low gear (that does include heating the intake air more and the resulting less density and some other losses). Maybe the P & W Supercharger was really operating on the backside of it's efficiency curve when in high gear.
But the Homare wasn't exactly an late engine either, it took a while to get it into production.

Even with Hooker at RR, very few superchargers got over the mid 70s in efficiency.

The Homare may well have been better, but was it efficiency enough to free up several hundred HP to drive the propeller?

 

[tomo pauk]

You have two/three dimensions on impellers. The diameter of the impeller and depth and number of the blades.
The diameter of impeller can tell you something about the veleocity of the air leaving the impeller and that is strongly related to the power required to drive it.

Granted, "big impeller and/or greater impeller's RPM" will usually require more power, than it will be the case with a "smaller impeller and/or low RPM" combination. Return of the investment is usually in form of better altitude power.

But we often don't know the size and shape of the inlet and how well the inlet was designed (Early Merlin inlet anyone? and it was one of the better ones even before Hooker came along) or anything about guide vanes or anything about the impeller blades (straight or curved or box vanes) or the diffuser and the volute.

Hercules pre-100 series have had bad inlet (as it was the vogue for second half of 1930's engines, including the pre-Mk.XX Merlins). Impeller lacked the curved rotating vanes, like in the many of the other radials, and unlike the impellers on the V-1710 C series, DB 601A or Merlin I (and later).
BMW 801A/C and D have had the same problems.
For the 100 series and later, Bristol introduced the curved vanes for the impeller (similar to what Centaurus gotten), and 'straight' intake (similar to what Hooker did to Merlin 4 years prior). This might be very well worth a look: link
BMW 801E and S introduced the similar changes for the 801 line, with similar gains in altitude power because the losses on the S/C side were much reduced (or - efficiency was much improved).
(granted, the non-introduction of the BMW 801E was a boon to the Allies in 1943-44; the 801S was basically the power section of the 801D and S/C section of the 801E, for late 1944).

The P & W Pre "C" series supercharger may have been on the poor side but P & W was also one of the two most successful designers of two stage superchargers. Maybe they needed two stages to get around how poor the singe stage was?

Kinda

But you are depending on a really good supercharger to make up for that difference in displacement the Homare had. You need an awfully good supercharger to make up the difference. Remember that the R-2800 making 1600hp at 13,500ft was making well over 2000hp in the cylinders. It was taking almost 400 hp more to drive the supercharger in high gear than in was in low gear (that does include heating the intake air more and the resulting less density and some other losses). Maybe the P & W Supercharger was really operating on the backside of it's efficiency curve when in high gear.
But the Homare wasn't exactly an late engine either, it took a while to get it into production.

Even with Hooker at RR, very few superchargers got over the mid 70s in efficiency.

The Homare may well have been better, but was it efficiency enough to free up several hundred HP to drive the propeller?

We can take a look at Hercules again. The Mk.VI and XVI were making just under 1100 HP at 20000 ft (2900 rpm; ~30 HP less on 2800 rpm). Mk.100 was making 1600 at 20000 ft at 2800 rpm.
(all per Bristol data)
So I'd say that big and efficient superchargers (complete units, not just impellers) pay off handsomely.

The R-2800-34 was supposed to do 1700 HP at ~17600 ft, while the -30 (a later engine than the -34) was rated for 1660 HP at 20000 ft per the chart, or 1600 HP at 22000 ft per F8F-2 docs. All on 115/145 fuel, military power, no ram.

 

[Ivan1GFP]

I Ivan1GFP thanks again for helping me with sources. I'm unable to find the report on the T-3 aircraft, although I have read the TAIC report on the T-2 aircraft and must have missed the part on the supercharger. (Also, I gather you mean 400 MPH, not 300 MPH.)

I was goofing up the title of the report. It is the T-2 Report - Interim Report 3.... Just one report.
The data from the T-2 Report and Middletown Report are the same as far as engine performance though the Middletown Report gives a lot more detail on the ADI system and other aspects of the engine. As mentioned earlier, what we get from Middletown is experimentally derived settings. Nominal settings are in the manuals.

I really meant not able to exceed 300 MPH without Water Methanol. The engine had ADI come on at anything over +180 mm boost. That is not a lot of power at that boost level and whatever the corresponding RPM was. I know I have some notes on my dead computer but not accessible at the moment. There are several thousand books and manuals on that computer.
Figuring out the supercharger performance is just a matter of understanding what that engine performance table in the T-2 Report was telling you and putting it together in a more readable form.

- Ivan.

 

[K5083]

A while ago, maybe twenty years but who's counting, I did some work comparing the important fighter engines of the second half of WW2. When you compare turbines it's important to know the air mass flow rate, so I though I'd do that for the big piston engines.

I started with the easily obtained data. Capacity, RPM and boost for each engine. Put them all in the same units, in this case litres, RPM and atmospheres. The air mass flow in pounds per minute is capacity * RPM * boost * 0.001223 (air density at sea level in kg/litre) and divide by 2 because they are fourstroke engnes and only have one induction stroke in two revolutions.

The following are the air mass flow figures in pounds per minute for each engine with the data I used.

RR Merlin 27/3000/2.22 = 243 pounds/min = 1750hp

DB605 35.7/2800/1.98 = 267 = 2000hp

BMW801 44.5/2700/1.42 = 230 = 1700hp

R2800 45.8/2800/2.34 = 405 = 2800hp

Sabre 36.6/3750/1.61 = 298 = 2340hp

Jumo 213 35/2750/2 = 306 = 2240hp


Now, in each case divide the power by the air mass flow to get a figure to use as an index of efficiency. ( I understand that each engine has a different supercharger setup, different mechanical losses, different valve trains and so on, but they are all state-of-the-art, so it is fair to compare them.

Merlin, 7.2 HP per lb/min

DB605, 7.49

BMW801, 7.39

R2800, 6.91

Sabre, 7.83

Jumo213, 7.3

All in a fairly small range, IMHO, considering the difference between a big lazy radial at 6.9 and a frantic 24-cyl two-crank sleeve valve and a small highly boosted V12.

Now we come to the Homare. I used 35.9 litres, 3000 rpm and 500mmhg = 1.66 ata. The air mass flow turns out at 240 lb/min but divide that into 1990 horsepower and the comparison figure is 8.3 horsepower per lb of air mass flow. That is such a remarkable result that I can't explain it. This is a fairly pedestrian engine, no unusual boost figures, no H24 sleeve valve. The same mass flow as a Merlin running +18 boost, but 240 hp more.

Would anybody, Shortround6, Snowygrouch, anyone, like to say where I'm going wrong?

(If you don't like the parameters I've chosen try different figures, different boost/rpm/power combination. You'll find the result similar for each engine or others I didn't include. Except the Homare.)

 

[Shortround6]

I would cut the R-2800 back to 2700rpm and the power back to either 2300 or 2500.

The 2800 hp version only showed up in late 1944 was a completely different engine to the early R-2800.
It also used the improved turbo.
But it just makes the Homare even more of an outcast.

I agree that you reasoning seems sound.
Engines are air pumps, the fuel only takes up around one part out of around 9000 parts of air (depending on mixture). So looking at the engine air flow is a good starting point.

 

[Ivan1GFP]

Hello K5083,
If I am understanding you correctly, you are using claimed HP figures. Some of these figures may not be entirely correct. As Shortround6 pointed out, the figures for the R-2800 were not for the common version available for most of the war. Another correction is the RPM figure for the JuMo 213 is a bit off. I believe the RPM for maximum power should be about 3250. This engine was a screamer.
Another thing to consider is that the German ATA manifold pressure reading isn't exactly in "Atmospheres" pressure as we understand it. It is close but not exact. I do not have my customary spreadsheet available to do a precise calculation at the moment. The actual BMW 801D HP was actually a bit higher than you are listing.
If you already have all these numbers, you have enough to calculate Indicated HP as well. How do your numbers compare to indicated HP?

- Ivan.

 

[cherry blossom]

I like the idea that we can get an idea of how powerful an engine should be by simply treating it as an air pump. The actually power will also depend on the exhaust temperature that the engine can accept (which is why we need fuel!). This may give an advantage to Anglo-American metallurgy or sleeve valves. It may also give an advantage to liquid cooled engines, especially for WEP.

Of course, the power to the propeller requires that we subtract the power used by the supercharger (which is why turbocharged R2800s are so powerful) and the loss due to friction, which should give an advantage to engines with short strokes and low rpm.

Finally, I am not sure if we need to reduce the estimated power of the DB605 to account for the valve overlap.

Added as edit: What I was dimply remembering is discussed at http://www.enginehistory.org/members/articles/ACEnginePerfAnalysisR-R.pdf

 

[AerialTorpedoDude69]

Thanks all!

I am not so interested in the Hayate as I am in the Shiden, mainly because the story of the Shiden is more dramatic in its twists and turns. But just like the Hayate (as most of you know), the Shiden also uses a Homare 21.

A user of a Warbirds forum translated five Japanese documents on the N1K Kyofu, Shiden, and Shiden Kai. One of the five books is authoritative and a primary source. The other books are from Japanese aviation magazines. The first book is the "Shiden Kai Instruction Manual", which I am dying to get my hands on. According to the user, the Shiden's (he says the N1K1-Jb and N1K1-Jc) has a maximum speed of 670 KPH/416 MPH.

The pilot's manual almost certainly lists the maximum speed. The aviation magazines, on the other hand, are less authoritative. It's not clear to me what information the writer sourced from the aviation manuals or from the manual. But I tend to trust the data in that post a little more, since the writer can apparently speak and read Japanese and has a broad list of sources.

EDIT: I also found a blog post where a Japanese author discusses why the secondary sources are bad: they pull together data from a variety of sources, some of which use design estimates or design targets, and confuse these data points as being actual performance values. So if even people with access to primary sources are saying the data is bad, I do not think we are going to get any definitive answers as to the Homare's actual performance (other than the quantitative testing done by the US on the engine).

Here's the list of sources:

Primary Source

『紫電改取扱説明書』



Second Source

･『世界の傑作機 No.124 強風、紫電、紫電改 (世界の傑作機 NO. 124)』(文林堂･2007.11.1)

･『最強戦闘機紫電改』(光人社･2010.1.1)

･『局地戦闘機紫電改―海軍航空の終焉を飾った傑作機の生涯 (〈歴史群像〉太平洋戦史シリーズ (24))』(学研･2000.1.1)

･『世界の傑作機 No.53 強風,紫電,紫電改 (世界の傑作機 NO. 53)』文林道･1995.7.1)

 

[Shortround6]

I like the idea that we can get an idea of how powerful an engine should be by simply treating it as an air pump. The actually power will also depend on the exhaust temperature that the engine can accept (which is why we need fuel!). This may give an advantage to Anglo-American metallurgy or sleeve valves. It may also give an advantage to liquid cooled engines, especially for WEP.

Of course, the power to the propeller requires that we subtract the power used by the supercharger (which is why turbocharged R2800s are so powerful) and the loss due to friction, which should give an advantage to engines with short strokes and low rpm.

Finally, I am not sure if we need to reduce the estimated power of the DB605 to account for the valve overlap.

Added as edit: What I was dimply remembering is discussed at http://www.enginehistory.org/members/articles/ACEnginePerfAnalysisR-R.pdf

I like looking at the displacment X RPM X Boost as a rough guide.

But there was so much other stuff going on that trying to go out very far, like 1st decimal place let alone 2nd decimal place gets very tricky.
Just to pick on engine outside the list above.

We have a small supercharger and might not have been very good. Some acounts say the supercharger would get hot enough to blister the paint off the housing. That is going alter the density ratio of the intake charge
The six carburators might affect perfromance and each carb feeding two cylinders may not be the best idea either. You have only two valves in each cylinder and they are in line under the camshaft

So we know what the theoretical airflow is and we can guess it doesn't make very good use of the air it does get, but trying to figure out which features are robbing the potential power certainly gives you a range of options.
36 liters/ 2500rpm/1.35= .........XXX..............=1100hp at best.

I have no idea where they actually measured the boost from/actual location of the gauge sensor in the manifold.

 

[DarrenW]

I would cut the R-2800 back to 2700rpm and the power back to either 2300 or 2500.

The 2800 hp version only showed up in late 1944 was a completely different engine to the early R-2800.
It also used the improved turbo.
But it just makes the Homare even more of an outcast.

I agree that you reasoning seems sound.
Engines are air pumps, the fuel only takes up around one part out of around 9000 parts of air (depending on mixture). So looking at the engine air flow is a good starting point.

The P-47D was cleared for 70" Hg or 2,800 hp in late June 1944 while using 100/150 fuel. Of course this was only in the ETO where the fuel was available. With the more common 100/130 fuel only 64" Hg or 2,600 hp was authorized. In both these instances water injection was required.

http://www.wwiiaircraftperformance.org/p-47/24june44-progress-report.pdf