A6M2 with overboost?

[1K3]

(very interesting...)

ZERO-SEN Model 21 Performance: Unraveling Conflicting Data
Richard L. Dunn © 2004  
INTRODUCTION

The Mitsubishi Type Zero Carrier Fighter Model 21 was the fighter that opened the war in the Pacific over Hawaii and the Philippines . It was the primary fighter used by the Japanese navy from the beginning of the war until early 1943 and remained in front line service until well into 1944. As important as this fighter was in the Pacific air war there is little agreement in published sources about some aspects of its performance. In particular, the aircraft's maximum speed is given by different post-war publications in a range from 316 mph to 345 mph. This disparity of nearly 30 m.p.h. is sufficiently broad that at the lower end the aircraft might be deemed relatively slow by 1942 fighter standards and at the upper end it might be considered relatively fast and definitely competitive. This article attempts to unravel conflicting data and provide a likely, if not definitive, answer to the question of the Zero's maximum speed[...]

http://www.j-aircraft.com/research/rdunn/zeroperformance/zero_performance.htm

[Karnak]

Interesting.  If true it would explain why the Zero was seen as such a threat in a way that AH's A6M2 cannot.

[BUG_EAF322]

It would also explain why the lightning succes should not be underestimated.

Hence zero's where too slow to compare to the uber luftwaffe aircraft.

As well the enemy faced in the PTO was at least as dangerous as in the ETO.

[HoHun]

Hi 1K3,

http://www.j-aircraft.com/research/rdunn/zeroperformance/zero_performance.htm [/B]

While this article is really nice to read, I think it is hard to reproduce Dunn's conclusion because he has fragmented the data sets in his article.

Unfortunately, some speeds are given without the corresponding altitude, and these don't help much :-(

The attempt to build A6M2 speed data based on combat reports is hardly reliable, though it could be used as a sanity check. One problem is that the F4F engines apparently were modified at some point, allowing them to run with the first supercharger stage in neutral gear, which boosted sea level speed considerably. The use of indicated air speeds is problematic since they are typically reading too high in the top speed range.

Let me try to re-build Dunn's data sets:

SET A

Intelligence Summary No. 85, December 1942:

(3) 321 m.p.h. at 20,000 ft. (Mikesh, p. 123);
(4) 326 m.p.h. at 16,000 ft. (Reardon, p.113);

Condition: Intelligence Brief #3 "It is probable that the airplane in original condition was somewhat faster than is indicated here, due to lack of flush fit at wheel well fairings and cabin enclosure in the overhauled plane, and the addition of non-specular paint."

Boost for the flight tests: +150 mm Hg (rated)

Overboost: +250 mm Hg

SET B

275 knots at 4,400m

(JICPOA Item No. 5981, dated October 1943, captured on Kwajalein February 1944).

SET C

289 m.p.h. at 15,000 feet @ 2050 rpm

(Holloway)

SET D

Zero rated 2500 r.p.m, maximum take-off rating of 2550 r.p.m.

(no source)

Regards,

Henning (HoHun)

[HoHun]

Hi again,

After checking it against a British report, it seems that the airspeed indicator of the P-39 indicated around 10 mph too high at sea level top speed, which casts a slightly different light on the combat reports quoted in Richard Dunn's article.

I've tried to find information on A6M performance, and it seems there were a few tests of captured aircraft of this type, many of which were problematic in one way or the other.

Here is my evaluation and a calculation for A6M performance, based on the drag of the airframe in the best condition and the TAIC data on the Japanese engines. (I'm not sure how trustworthy the latter is.)

http://hometown.aol.de/HoHunKhan/A6M_Speed.png

Note 1: Neumann's Zero was limited to 2050 rpm by a malfunction of the constant speed propeller. Only the 10000 ft data point was using full boost, and it coincedes with the Akutan Zero at the ca. 80% that were available.

Note 2: The Akutan Zero data is from the A6M2 that crashed on one of the Aleutian Islands. It was restored to a good, though maybe not perfect condition. My estimates are based on the assumption than all Zero models had the drag of this Zero.

Note 3: The Akutan Zero was found to be as fast as an F4F-4 in neutral gear at sea level. According to my calculations, it would be a bit short of that, and that's at WEP which the TAIC manual quotes as cleared for 60 s only. I assume in the comparative trials, it was used for longer than these 60 s or the F4F-4 comparison would make little sense. (Apparently, US combat pilots at Guadalcanal felt the A6M was superior to the F4F-4 at sea level, and from what I've read neutral gear operation was pioneered at Guadalcanal.)

Note 4: The Australian Hap was flown at lower rpm and higher boost than given by the TAIC WEP data. This might account for the low full throttle height. Estimating the power fraction it got from these settings, it confirms the drag assumption for the A6M2.

Note 5: The Patuxent River A6M5 was tested twice, once by the US and once by the UK (Royal Navy, I suppose, as they tested against a Seafire.) The US reports warns against airframe damage and high drag, but provides a sea level WEP speed that's only 12 km/h lower than my estimate. The UK report has data that's 10 - 20 km/h faster than the US report,  but a lower full throttle height and no indication of using the supercharger low gear.

Note 6: I don't know whether the TAIC engine data is realistic. Especially the A6M5 full throttle height appears to be rather high.  I get a top speed of more than 600 km/h for the A6M5, which tops all popular books. On the other hand, the popular books might be based on the sub-standard US tests, so who knows?

Note 7: I'm aware of one other test of an A6M5 that was captured on the Phillipines. It might be quoted in Mikesh's "Zero" book, but I don't have that at the moment. If someone could provide the data, that might help me with the data evaluation :-)

Henning (HoHun)

[gwshaw]

One problem is that the F4F engines apparently were modified at some point, allowing them to run with the first supercharger stage in neutral gear, which boosted sea level speed considerably. The use of indicated air speeds is problematic since they are typically reading too high in the top speed range.

Note 3: The Akutan Zero was found to be as fast as an F4F-4 in neutral gear at sea level. According to my calculations, it would be a bit short of that, and that's at WEP which the TAIC manual quotes as cleared for 60 s only. I assume in the comparative trials, it was used for longer than these 60 s or the F4F-4 comparison would make little sense. (Apparently, US combat pilots at Guadalcanal felt the A6M was superior to the F4F-4 at sea level, and from what I've read neutral gear operation was pioneered at Guadalcanal.)

I'm not following you, or you misunderstand how the P&W 2-stage/2-speed superchargers worked. There would be no need to modify the supercharger to run in neutral blower at SL.

The R-1820 in the F4F-4, as well as the R-2800-8 and -10 in the F4U and F6F all had an integral supercharger ALWAYS running at a single fixed gear ratio off of the crankshaft. They also had an aux stage blower with three settings; neutral, low and high blower. Neutral blower simply meant that the aux stage was clutched out, and only the integral blower was running.

So you had three settings for the blower;

neutral with the aux stage clutched out and only the integral blower operating

low with the aux stage in low gear and the integral blower operating

high with the aux stage in high gear and the integral blower operating

Neutral blower typically had the intake air coming directly into the carb, then from there to the integral blower and into the intake manifolds leading to the cylinders. High and low blower had the charge air go through the aux stage blower, then the intercooler and finally into the carb, integral blower, intake manifolds.

If you look at a power curve for the P&W figther engines there are three distinct steps in it, as opposed to two for a two-speed blower drive like RR used. Neutral, which was used from SL up to perhaps 5000 ft, low blower used from there up to around 15,000 ft or so, and hi used above that level.

Greg Shaw

[HoHun]

Hi Greg,

>I'm not following you, or you misunderstand how the P&W 2-stage/2-speed superchargers worked.

Most probably the latter :-)

>So you had three settings for the blower;

I was aware of that, I just mis-interpreted some read-and-half-remembered comment on the genesis of this system and thought it resulted from a field modification of a two-speed, two-stage system.

So the F4F-3 would have had the same neutral gear power curve as the F4F-4?

>Neutral blower typically had the intake air coming directly into the carb

Ah, thanks, that was new to me! :-)

Regards,

Henning (HoHun)

[helkos]

Would be interested to see an HTC response on this one myself

[HoHun]

Hi Widewing,

I think this thread is better suited for our analysis of the A6M than the great Luftwaffe thread where we originally started :-)

For reference:

http://www.hitechcreations.com/forums/showthread.php?s=&threadid=165257

Originally posted by Widewing
The Patuxent River A6M5 (yellow plot) data came from Naval Air Tactical Note 106, distributed to Royal Navy F6F squadrons deployed to the Pacific. The tests were performed by TAIC at Patuxent River, which the RN used to generate NATN 106. As far as I know, only the Patuxent River facility used independent air speed measuring equipment. I seem to recall that field tests performed by TAIC in the war zone relied on aircraft instrumentation for speed data. However, my memory is vague on this. Do you have any info on test methodology?

Unfortunately, I have few details on the tests. The Neumann Zero was tested in the field and made mention of relying on the original Japanese airspeed indicator, so at least for this test we know the instrumentation. I believe that is not an overly serious source of error as it would be possible to calibrate the airspeed indicator by either flying in formation with an aircraft with a sufficiently well-known indicator (standard operation procedure at test establishements at least) or by making a timed run over a known distance. I believe the runway length should normally be known accurately enough for the latter. Methodically, I imagine the latter might not yield perfect results for high altitude, but probably be quite satisfying for low altitude.

The Neumann A6M2, the Akutan A6M2 and the Australian A6M3 display good coincedence of their drag condition if you allow for the very different power settings they were run at.

I'd be confident that I could prepare a fairly good estimate of all A6M variants' speed curves using that data. It might be slightly on the low side because at least both A6M2s were (competently) patched up and not in optimum condition, but I believe that would be a minor factor.

However, such an estimate would have to be built on accurate engine power curves, which I haven't found yet.

The rest of the A6M tests paint a rather confusing picture, I haven't been able to learn anything useful from them so far :-(

I would like to ask you for your opinion on the following power curves, which I have derived from the TAIC intelligence data and which I regard with some suspicion:



(Now that I prepared such a nice diagram, I notice that I have probably underestimated power drop above full throttle height. My mistake, not the TAIC's.)

Here is the original TAIC data I used as a basis:

Sakae model 12, TAIC 1944 p. 851   
925 hp/2550 rpm/39.8" SL
925 hp/2550 rpm/39.8" SL
1010 hp/2550 rpm/39.8" 11000*
820 hp/2500 rpm/35.8" SL
935 hp/2500 rpm/35.8" 13800 ft
   
Sakae model 21, TAIC 1944 p. 852   
1115 hp/2750 rpm/41.7" SL
1115 hp/2750 rpm/41.7" SL
1180 hp/2750 rpm/41.7" 7500*
1040 hp/2750 rpm/41.7" 18000*
995 hp/2700 rpm/37.8" SL
1085 hp/2700 rpm/37.8" 9350 ft
965 hp/2700 rpm/37.8" 19700 ft
   
Sakae model 31A, TAIC 1944 p. 853   
1120hp/2800 rpm/41.7" SL
1120hp/2800 rpm/41.7" SL
1210 hp/2800 rpm/41.7" 8000*
1055 hp/2800 rpm/41.7" 20400*
995 hp/2700 rpm/37.8" SL
1085 hp/2700 rpm/37.8" 9300 ft
965 hp/2700 rpm/37.8" 21700 ft

The two main problems of the TAIC graphs are:

- 31A power gain over 21 in low gear (at just +50 rpm speed)
- 31A full throttle height gain in high gear without corresponding power loss

The only way I see for the 31A to achieve a higher full throttle height than the 21 is to use a longer high-gear drive ratio. That would however mean that effective power is reduced at the same time, resulting in a different power from the one I modelled from the TAIC data points.

In fact, since this data looks pretty generic, I'm actually having some doubts about the other data since it might be just as generic. TAIC data was wartime intelligence, which always is the best guess based on the currently available information at any moment.

I haven't been able to find anything substantial on the hardware of the Sakae engines either, such as supercharger dimensions, gear ratios etc., which could be used to double-check the TAIC data even in the absence of definite power curves.

A general statement TAIC makes on the engine data section:

"All ratings tabulated are taken from documents unless followed by asterisk (*). The latter are TAIC estimates.

Take-off - This is usually a one-minute rating.

W.E.P. - (War Emergency Power) - These ratings are in most cases estimated extrapolations based on take-off boost pressure. For most engines, documents indicate that take-off boost is permissible for one minute for either take-off or emergency. If one minute is permissible for take-off it is quite probable that under certain flight conditions, the period could be exceeded. Its exact duration is conjectural, but until better information is available, should, to be conservative from a tactical point of view, be considered as at least a five minute rating."

It would be interesting to hear more about the way the USN operated their radials and on what they considered advisable and possible. A one minute WEP duration doesn't seem to be of much combat value in my opinion.

Thanks for your help! :-)

Regards,

Henning (HoHun)

[Squire]

The part that needs explaining from the "the A6M2 did 345mph" group is how it is that a radial engined fighter with a 950 hp engine comes within 5mph of the following inline engined machines:

Ki-61 Hien @348mph 1175 hp
Spitfire IIA @350-355mph 1175 hp (100 octane) capable of overboost
109E-4/N @350-355mph 1200hp capable of overboost

330mph at FTH to me seems reasonable as an upper limit to me. I think 345mph is fancifull at best especially considering the A6M5 did @351mph tops? and the fuel grades the Japanese were using was not equivilant at all to RAF and LW fuels during the war.

I could see it getting 345mph on 100 octane grades, being run overboosted, maybe.

[Widewing]

Hiya Ho-Hun,

One problem I am encountering is finding any references to the Sakai 31a engine ever attaining genuine production status installed in any model Zero. As far as I know, all models of the A6M5 were powered by the Nakajima Sakae 21 engine w/two-stage supercharger rated at1,130hp at take-off, 1,100hp at 9,350ft, 980hp at 19,685ft.

So, I'm somewhat dismayed that you have TAIC data for the Nakajima Sakae 31a, when all of my references say that this engine was never installed in any A6M5s.

A handful of 31a powered A6M6c fighters were built in late 1944 and early 1945, but appeared far too late to be captured and shipped stateside for testing in October of 1944.

A6M8 and A6M8c prototypes were flown with the Mitsubishi Kinsei Model 62 engine, but the war was just about over when these made their first flights.

What document reports the 31a powering an A6M5?

My regards,

Widewing

[Squire]

Sakai 31A powered the A6M7. It had MW50.

The A6M7 was the last version to see service. The A6M6 was never put into production, neither was the A6M8. War ended before the order could be filled. The exact # of A6M7s is speculative at best.

[HoHun]

Hi Widewing,

>One problem I am encountering is finding any references to the Sakai 31a engine ever attaining genuine production status installed in any model Zero. As far as I know, all models of the A6M5 were powered by the Nakajima Sakae 21 engine w/two-stage supercharger rated at1,130hp at take-off, 1,100hp at 9,350ft, 980hp at 19,685ft.

Ah, thanks, that's a very important observation!

The TAIC summary states that the Sakae 21, Sakae 31A (without ADI) and possibly the Sakae 31 (with ADI) were used in the A6M5. I excluded the ADI-injected engine but failed to notice that not even the 31A was ever installed.

With the Sakae 21 in the A6M5, the only difference to the A6M3 in motive power would come from the additional exhaust thrust due to the individual exhaust stubs. I'll prepare a new analysis based on these assumptions :-)

Regards,

Henning (HoHun)

[F4UDOA]

I can remember reading a number of claims of Zero's being much faster in combat than during testing.

One of which is Ken Walsh (20+ kills) in a F4U-1 in mid 1943 running from a large number of A6M2's for miles starting at 5K and ending at sea level until his engine overheated and still having to do a hard break turn at high G to escape and head back toward home. This chase was at speads from 400MPH down to about 300MPH for an extended period of time and he could never get far beyond firing range of at least one Zeke.

Hohun,

Where did you get the speeds for the A6M5 at Pax River? I have the full report of the Army and Navy test of the A6M5 but I don't have the speed chart but your speeds for the TAIC test seem to slow especially at sealevel.

I have the Army flight test climb and speed chart of the A6M5 with HP curve and MAP but that was a broken A/C according to the AAF and was not running at full MAP as well as having drag problem because of structural defects.

I do not have the Navy test results except for specific altitudes.

The top speeds from the report are 413MPH at 20K for the F4U. 409MPH at 22,600FT for the F6F-5 and 335MPH at 18K for the Zero. 321MPH at 13K for the FM-2.

Note the F4U was listed as being 48MPH faster than the A6M5 at sealevel. It is hard to find a report that shows the F4U any slower than 350MPH at sealevel so the A6M5 in this test would have been unusually fast.

It must also be noted than the F6F was unusually fast in the test as well. I think it's results may have been skewed at lower altitiudes.

I will post the full test when I can.

[Grits]

Originally posted by F4UDOA
I can remember reading a number of claims of Zero's being much faster in combat than during testing.

One of which is Ken Walsh (20+ kills) in a F4U-1 in mid 1943 running from a large number of A6M2's for miles starting at 5K and ending at sea level until his engine overheated and still having to do a hard break turn at high G to escape and head back toward home. This chase was at speads from 400MPH down to about 300MPH for an extended period of time and he could never get far beyond firing range of at least one Zeke.

I have a VHS tape about VF-17 from 1990 and it has clips from the surviving members. In one clip Roger Hedrick talks about the F4U and says "The corsair had a slight speed advantage over the Zero". When he said "slight" he stressed that word to make clear the difference was not great.

I dont usually put much stock in anecdotal accounts of aircraft performance, but with the current accepted wisdom of the speeds of the F4U and A6M we have now, I cant see anyone calling it a "slight" speed advantage.