The A6M could not do negative G maneuvers?

Jenisch · May 6, 2018

So, here's what the Americans had to say about the Akutan Zero (an excerpt from a Wik article about the aircraft):

On September 20, 1942, two months after the Zero's capture, Lieutenant Commander Eddie R. Sanders took the Akutan Zero up for its first test flight. He would make 24 test flights between September 20 and October 15. According to Sanders' report:

These flights covered performance tests such as we do on planes undergoing Navy tests. The very first flight exposed weaknesses of the Zero which our pilots could exploit with proper tactics ... immediately apparent was the fact that the ailerons froze up at speeds above 200 knots so that rolling maneuvers at those speeds were slow and required much force on the control stick. It rolled to the left much easier than to the right. Also, its engine cut out under negative acceleration due to its float-type carburetor [emphasis mine]. We now had the answer for our pilots who were being outmaneuvered and unable to escape a pursuing Zero: Go into a vertical power dive, using negative acceleration if possible to open the range while the Zero's engine was stopped by the acceleration. At about 200 knots, roll hard right before the Zero pilot could get his sights lined up.[29]

Akutan Zero - Wikipedia

I have already read somewhere on the internet that the Americans installed the carburetor incorrectly during the evaluation of the Akutan Zero, so it did not behave as it should, as according to what I have read the plane could do 0 and negative G maneuvers without losing power (have read this in a forum years ago, don't have the link anymore).

Since I did not find any answer with a reliable source, I have decided to send a message to Shinpachi, our very helpful member from Japan, who kindly reply and give me an explanation of how the carburetor from Nakajima works. The answer follows:

By 1937, Nakajima was successful to cope with not only the negative G but zero G and excessive +G by adopting a slide valve. The slide valve is balanced with a coil spring. In +1G, it slightly downs by its own weight to allow the fuel stream. In 0G, it rises to close the hole to shut the fuel flow but a small bypass which is drilled inside the valve allows minimum fuel flow to run the engine. In minus G, it totally shut the fuel flow like a ball valve but a small bypass again allows minimum fuel flow like in the 0G. In the excessive +G like air-combat, another acceleration valve is activated by the increased fuel weight itself to run the engine.

Point is Nakajima did not rely on the ball valve like Americans.

I have asked to Shinpachi what he thinks about why the Americans arrived at the conclusion the Zero (the A6M2 specifically) could not do negative G maneuvers, but Shinpachi was not aware they arrived at this conclusion (it was news for him). He suggested that "Americans were unable to understand the Nakajima's mechanism" and "they should have cleaned the bypass hole." So I have asked and obtained his consent to copy his replies and create a thread about the subject, so we can understand the situation better. Basically: what exactly happened during the evaluation of the Akutan Zero that resulted in the conclusion it could not do negative G maneuvers? It was indeed an incorrect installation of the carburetor by the Americans as I have read in a forum years ago?

XBe02Drvr · May 10, 2018

GregP, where are you?

GregP · May 10, 2018

I'll ask Steve Hinton on Saturday, assuming he is present.

We have a Zero, the drawings, and it flies regularly. I do not know if he flies it inverted, but I can certainly ask.

Cheers!

BiffF15 · May 10, 2018

Greg,

It's negative G flight they are looking for.

Cheers,
Biff

XBe02Drvr · May 10, 2018

BiffF15 said:
Greg,

It's negative G flight they are looking for.

Cheers,
Biff

Like serious negative.

Mike Williams · May 10, 2018

General Technical Data and Flight Characteristics of the Japanese Zero Fighter Plane. 6 February 1943.

Tests carried out at Kunming China. Page 8: 9. c. "The engine will not run under any conditions of negative acceleration, inverted, or in a steep skid. A Zero is unable to follow any airplane which does a sharp pushover unless it rolls and it cannot roll at high speed."

GregP · May 10, 2018

I was going to ask about negative g, not just inverted flight. But I get why you replied as such.

And thanks, Mike Williams!

Cheers.

ODonovan · May 10, 2018

Jenisch said:
I have already read somewhere on the internet that the Americans installed the carburetor incorrectly during the evaluation of the Akutan Zero, so it did not behave as it should, as according to what I have read the plane could do 0 and negative G maneuvers without losing power (have read this in a forum years ago, don't have the link anymore).

Since I did not find any answer with a reliable source, I have decided to send a message to Shinpachi, our very helpful member from Japan, who kindly reply and give me an explanation of how the carburetor from Nakajima works. The answer follows:

In 0G, it rises to close the hole to shut the fuel flow but a small bypass which is drilled inside the valve allows minimum fuel flow to run the engine. In minus G, it totally shut the fuel flow like a ball valve but a small bypass again allows minimum fuel flow like in the 0G.

Click to expand...

From that, it's clear the only thing the bypass would do is allow minimum fuel, which would keep the engine running at idle but in no way be enough for zero G or negative G maneuvers. The engine would certainly lose power, just as claimed by the American testers. It just wouldn't stall out entirely. It clearly doesn't allow anything near a full fuel flow, which would be necessary for combat maneuvers. Or am I reading this incorrectly?

-Irish

XBe02Drvr · May 10, 2018

ODonovan said:
Or am I reading this incorrectly?

No, you got it right. Best explanation yet.

Shinpachi · May 11, 2018

Mike Williams said:
General Technical Data and Flight Characteristics of the Japanese Zero Fighter Plane. 6 February 1943.

Tests carried out at Kunming China. Page 8: 9. c. "The engine will not run under any conditions of negative acceleration, inverted, or in a steep skid. A Zero is unable to follow any airplane which does a sharp pushover unless it rolls and it cannot roll at high speed."

Thanks for sharing an interesting report, Mike.
Being afraid of destroying the sample, they did not necessarily carried out tests thoroughly and the report had involved not a few estimations but there left two basic questions.

Page 2
f. Engine Accessories
(1) Boost control. An automatic boost control is provided. A selector lever is provided in order that this control may be thrown in or out of operation as desired. Details of construction and operation of the boost control are unknown.

Probably, this would be an idling controller. According to manual, when it's ON, idling can be controlled automatically and idling stops when landed.

Page 5
(20) Mixture control - Use of this control is not fully understood, as in the rear position of this lever, mixture control is automatic. Movement of the lever to any forward position at any altitude causes no perceptible change in engine operation.

This could be a cylinder temperature compensator which supplies extra fuel to cool it down.

The army Ki-43 also mounted the same engine as the navy A6M.
Nakajima's footage introduces how it flew.

GregP · May 19, 2018

I spoke with Steve Hinton.

He says the stock wartime Zeros had float-type carburetors and would cut out under negative g.

As it happens, our A6M5 Model 52 Zero is not quite stock in that regard, despite having a Sakae 31 engine, because we are running a PD24 carburetor that can run inverted. I didn't ask him if there was a time limit on inverted flight, because , in airshow work with it, any negative-g is momentary.

For discussion, the 1,350 HP R-1820 in a Navy T-28 has a 5-second negative g limit, and I'd guess the Sakae 31 with a negative-g carb is similar.

Just FYI.

XBe02Drvr · May 20, 2018

GregP said:
For discussion, the 1,350 HP R-1820 in a Navy T-28 has a 5-second negative g limit

The Navy-version T-34B we had in the flying club had a 90 second inverted limitation based on engine oil scavenging, not fuel feed. Its Bendix pressure carb would run inverted as long as the fuel system could feed it, and through an ingenious network of header tanks, boost pumps, and gravity pickups, that was more or less indefinite.
The plane's inverted endurance was much greater than mine.
Cheers.
Wes

GregP · May 20, 2018

I didn't know it had a 90-second inverted rating. I also have seen several sporting bigger Continentals and Lycomings. Several around here have 285 HP Lyc 540s in them.

XBe02Drvr · May 20, 2018

GregP said:
I didn't know it had a 90-second inverted rating. I also have seen several sporting bigger Continentals and Lycomings. Several around here have 285 HP Lyc 540s in them.

Ours had the stock Continental 0-470-4. It had a fairly large oil header tank with a gravity pickup and scavenge pumps in both the bottom and the top of the crankcase. The inverted rating was based on how long the oil pressure pump took to empty the header tank, assuming both scavenge pumps would become starved for oil and not replenish the header.
Cheers,
Wes

XBe02Drvr · May 21, 2018

GregP said:
I also have seen several sporting bigger Continentals and Lycomings. Several around here have 285 HP Lyc 540s in them.

Are you sure those 285 HP T-34s have Lycomings in them? I know the 285 Continental O-520 is a popular conversion, and Lyc O-540s usually come in 260 and 300 HP flavors. I think the O-540's oil pan/induction manifold arrangement has some sort of conflict with the T-34's nose gear support structure. Some tidbit from the mists of memory.
I believe most all civilian privately owned T-34s are the Air Force "A" model, since the Navy's contract with Beech supposedly called for all aircraft to be returned to manufacturer for deconstruction rather than sold surplus. I know our club T-34B was still on the BuAer active list and we wrote a check for $1 to the Navy June 30 every year. The T-34A had a different fuel system without all the inverted fuel feed features our "B" had.
Cheers,
Wes

GregP · May 21, 2018

No, I'm sure you are right. They are probably Continentals. I have not had much to do with them, not from any lack of desire, but rather from lack of opportunity. I have flown a straight-tail Bonanza, but have never ridden or flown a T-34 of any variety. I like the T-34's looks, and it is a quality aircraft.

I would probably opt for a CJ-6 with an M-14 engine and save a LOT of money if I were getting a similar type aircraft. I have flown a CJ-6 and had a friend with a Yak-52. The value of the CJ/Yak is very high compared with the money you spend. Of course, it DOES burn more fuel ... and that's a trade off every potential buyer must consider. The CJs are a LOT easier to work on than the Yak-52 series, especially around the aft fuselage.

The A6M could not do negative G maneuvers?

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Jenisch

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XBe02Drvr

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