A6M3 safe loading factor

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Oct 29, 2012
This screenshot from this.
I believe 7.0G is safe loading factor and 1.8 is safety factor. The P-51D structural load limit is +8G and -4G (plus a standard safety factor of 1.5).
Is it possible that Zero was more durable than the Mustang aircraft (7*1.8=12.6>12=8*1.5)?


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From "Eagles of Mitsubishi" by Jiro Horikoshi:

(....and I am paraphrasing here.)
While the maximum load factor of the Zero was the same as any other fighter at 6G, the safety factor was much reduced.
There did not seem to be a point in designing a part that was 1.5 to 2 X as strong as it needed to be.

This was for the initial design, so perhaps the general strengthening of later models for greater aerodynamic loads (diving speed) also increased the G Load factor.
I am inclined to believe this was not done.

- Ivan.
 
Loading factors for most WW2 fighter aircraft were in the 12g range. Paper is patient, though, since weight of the aircraft usually increased progressively, while structural reinforcements rarely came, as they required a redesign of major components and later versions of the same aircraft had gradually reduced sustainable load factors. An extra gun, protection and so on worked into the A6M would quickly reduce the permissible load factors, because of its generally low weight. The difference between case A and B in the table is a mere ~50 kg extra weight.

1.8 might be design requirements to take multi directional loads into account, for instance high g at high speed will stress the structure both through dynamic pressure and the load factor - so why the airframe can take 7 g's on a test bench without any additional air pressure and be good for a 700 km/h dive without any g, it might still fail at 6g in a 500 km/h flight because the sum of the two loads is too much.

So regarding the low safety factor, I suppose the column with the 1.0 - 1.21 figures are the safety figures, with which the various requirements were met. German load requirements followed a similar approach, defining safety factors for overload conditions of nominal loads. I suppose the table is not contradicting the statement of Jiro Horikoshi, but rather supporting it.

From what I know, the A6M could take quite a lot of g's, not because the airframe was so tough, but rather because it was so light. And as far as I know, at least on paper, it could take more g than most contemporary US designs, which were structurally stronger, but also 2 to 3 times as heavy.
 
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The original requirement called for the 7 G with safety factor of 1.8

That is precisely written by Jiro Horikoshi in his book. I can quote this even :

By 1932, a new standard called "Summary of Airplane Planning" had been established which had to be adhered to in airplane design. It contained a rule that called for a safety factor prescribing the strength of structural members. This safety factor was defined as the ratio of an airplane's destructive load to the maximum load it was expected to be subjected to during operations. According to the legal regulation, this factor must be 1.8 regardless of the type of airplane, the application of the force or the material characteristics. In other words, the rule required the airplane not to be destroyed by loads less than 1.8 times the maximum load experienced in flight. For fighters, the maximum load had been established as seven times the force of gravity, or 7 Gs. Thus, when the 1.8 factor was applied to this, all airplane parts must be able to withstand a load of 12.6 Gs. Even when the airplane went through a maneuver of 7 Gs, which was the maximum load to which it would be subjected, the strength of all parts must have additional capability equal to 5.6 Gs
Click to expand...

Source : Page 37 of "Eagles of Mitsubishi. The Story of the Zero Fighter" by Jiro Horikoshi, translated by Shojiro Shindo and Harold N. Wantiez. University of Washington Press.

It also has a fancy drawing showing precisely the stages of the material under the G loads, when up to 7 G's material returns to original shape and is fully reusable, above that it may not return to original shape and eventually is not reusable, and the breakage at 12.6 Gs.

Going through the book "The Zero fighter" which contains a full reprints of factory documentation and maintenance manuals for Zeros model 11 to model 63:
- for A6M2 there is table same as presented in the first post in this thread which contains the 7.0 G value with safety factor of 1.8 - this is point 3.2.1 or page 220 in my book.
- for A6M3 models such table is also available, with exactly the same value of 7.0 G and safety factor of 1.8 - page 315 of my book.
- etc.

What is more I also checked the Ki-61-I (Tony I) flight manual - it has written :
" Section VI. Flight Limitations
a. Pull Out ..... 7 G "

Same thing is also in Ki-84 (Frank I) flight manual, where its in table 7 G normal and 12.6 maximum. And same values I found in Ki-44 manual.

Basing on what Jiro wrote and what all the manuals and documents I have indicate it seems that all Japanese fighters built throughout the WWII had same safety values for maximum G overloads, which seems to be logical considering there was a legal obligation to design the aircraft with such measurements.

Regards,
Hiro
 
Hi Hiromachi!

Jiro Horikoshi helped restore our A6M5 Model 52. He visited the museum and stayed long enough to assist in all the little things we needed to get it flying back in 1976 - 1977. That's where we got the 6 / 12 g ratings from.

Do you think it is possible the specifications changed between the intial A6M, the A6M3, and the A6M5? I believe the A6M5 has slightly shorter wings and that could account for a small bit, but am somewhat baffled at the change from 6 - to 7 g. As I don't read Japanese I am taking the word of the guys who restored the Zero along with Mr. Horikoshi way back in the day.

It is also possible that Mr. Horikoshi deliberately slightly down-rated it since it was to be a museum aircraft and not a military fighter. I'll ask Steve Hinton. He worked on the original restoration, knew Mr. Horikoshi and worked alongside him on the aircraft.

I believe I told you we have a volunteer who lives near Nagoya and visits once to twice a year. Next time he comes maybe we can look together at the documents we have.

Incidentally, we are coming along well with minor repairs to the center section and I'll send you more pics when anything significant changes. At present they are reinforcing the forward wing fillets that managed to accumulate a few cracks in the 37 years we flew it since the last restoration. Also, when the aircraft was transported to Japan the first time back in 1977, we managed to get a small bend in the trailing edge of one aileron. Steve made a temporary repair so it could be flown duing the visit, and we just finished replacing that trailing edge. So the original temporary repair only lasted 36 years!

I am fairly sure it will fly again for the airshow this year in early May, assuming no big-ticket items rear up and bite us. So far, the only things we have needed to repair are things we already knew about, and there haven't been any surprises.
 
I doubt so. I dont want to take any assumptions but all the sources I have indicate same 7.0 G limit.

The lowering of the limit would make no sense considering increased limits of dive speeds (up to 660+ km/h Indicated) and higher speeds plane was supposed to fight in. The airframe should become stronger, not weaker.

I will later ask a question about the A6M you have, as I'm curious about the button on the left side of control panel - its a Boost button supposed to allow to use emergency power for a short times. I'm curious how exactly that mechanism work. But to ask details, its almost 2 AM and I got work to do tomorrow.

Regards,
Hiro
 
Hi Again,

You may have the nail on the head. If the dive speed was higher and there was no structural change, then the g limit would drop a bit. I can't say for sure since we only have documentation for the A6M5 Model 52, but if the A6M3 had the same structure, it would have been slightly stronger at a lower speed.

I'll ask Steve Hinton. He knows about the A6M5, but may or may not know about the A6M3. We have worked on one while I was there as a volunteer, but it was a replica built in Russia, not an original A6M3, so it well may not have been 100% to original specification. The one we worked on is now in Paul Allen's collection in Seattle, WA, U.S.A. .
 
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There were strength improvements among the A6M5s depending on the ordinance but between the M3 and the earlier type of M5.
The PoF Zero looks belonging to the earlier one.
Just my impression.
 
Hey Shinpachi,

I spoke with Steve Hinton about the Museum Zero. He acknowledged that some people around our museum have siad ours A6M5 Model 52 was stressed to +6 / +12 g and said the design document we have says different. He said that due to the decreased wingspan relative to an A6M3, our Zero is actually considerably stronger than an A6M3 and he recalls it was considerably stronger, with a max of some 16 g before structural failure. He has the documents but will not let them out of museum hands as these are originals.

Due to the fact that this a museum aircraft, we operate it as though it were in the aerobatic category here in the U.S.A. and limit it ti +6 / -3 in normal operation, and further stated that we really don't pull more than +4.0 to +4.5 g in this aircraft as 1) here is no need and 2) we are not at war and 3) if we limit it to aerobatic category, we are not overstressing the airframe and it will last longer. He said that naturally, he'd pull 8 g or more to avoid disaster, but under normal conditions, no.

I found out it will likely not make our airshow this year and is likely to fly around December of this year or maybe a month or so later.

I didn't press for more since he was rather obviously busy with airshow prep and was busy trying to get some of our ground equipment working again. We have 2 or 3 ground units that are used to start our jets and one had gone down. He needed to get it working again as we will need it for the airshow as we sometime need to start 2 or 3 jets simulneously. That will be the case when the Horsemen F-86 act flies with three planes.
 
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The Japanese airframe Engineer was every bit as good as US/Euro counterparts.

The bottom figure is the important one as that is Normally the Limit load in which the designed part under the applied load condition reaches elastic Yield, and hence returns to original state after the load is removed. The top figure is Normally the Ultimate load. at which point the material has exceeded the elastic limit, has proceeded into the plastic range and reached Failure

For 2024 and other similar (Clad 24S-T3, etc) the Tensile strength is 64KSI, and for purposes of design is that stress 'point' at which the permanent set is .002 strain from original length. MOST airframe structural materials then were Aluminum alloys and high carbon/alloy steels and the Maximum Apparent Stress is the design Ultimate, which is short of actual fracture point - but is the point where continued deformation occurs absent any increase in Load. That Stress is usually about 50% higher than the Specification Tensile Stress in the 'Book'

So, the Point:

If 6g was the Design Limit Load (usually angle of attack related but also Q)) then the part/structure analyzed for say 24-T3 would approach 64,000 psi BUT that same part with an applied load of approximately 9g (a 1.5 factor) would reach the Proportional limit of 96,000 psi and flirt with a break.

Simply stated - "I don't get 6 and 1.8x6 G for the defined Limit and Ultimate G) load" UNLESS the Japanese engineer was extremely conservative on the analysis of individual parts because of the approach to stripping weight from critical airframe components.

If 6G translated to 64,000 psi Tensile (book) for 24S-T3 or Japanese equivalent, that aluminum material (whatever) would NEVER get past 96,000 psi safely, and at 115,000 psi stress (1.8 x 64) that part would be two or more parts. What it implies to me is that perhaps the A6M engineers increased dimensions between rivets on shear panels (perhaps), risking buckling at a lower stress than Yield to save on rivet weight or took aluminum panel thickness to lower threshold and decided on setting the design stress for some critical parts below yield to say 54,000 psi, well below spec Yield.

Is there any Mitsubishi explanation for the design approach?
 
My mistake, Bill. Steve said the load limit for the A6M5 Model 52 was "considerably higher than for the A6M3, due to shorter span." He didn't specifically state the limit normal g limit and said he recalled 16 g untimate at filuire, but would have to check the documents to be sure. He hadn't seen the document since about 1977 when the original restoration was completed. The A6M3, according to Shinpachi, was stressed to 7 g with a 1.8 safety factor, making it 12.6 g failure limit.

Since the A6M5 srtucture is essentially the same with shorter span, I infer 8.89 g and 16 g, using the same 1.8 safety factor. Either way, it is stronger than 7 g, which makes semse if nothing changes except to shorten the span.

They do that for some kitplanes, too. I have a friend who built a Harmon Rocket (which I occasionally get to fly) and they removed one rib bay from then RV-4 wing to shorten the span and stay in the aerobatic category while adding a Lycoming AIO-540 instead of an O-360 up front. I have not calculated iot, but they call it a +6 / -3 aircraft. Could be stronger actually.
 
My mistake, Bill. Steve said the load limit for the A6M5 Model 52 was "considerably higher than for the A6M3, due to shorter span." He didn't specifically state the limit normal g limit and said he recalled 16 g ultimate at failure, but would have to check the documents to be sure. He hadn't seen the document since about 1977 when the original restoration was completed. The A6M3, according to Shinpachi, was stressed to 7 g with a 1.8 safety factor, making it 12.6 g failure limit.

Since the A6M5 srtucture is essentially the same with shorter span, I infer 8.89 g and 16 g, using the same 1.8 safety factor. Either way, it is stronger than 7 g, which makes sense if nothing changes except to shorten the span.

They do that for some kitplanes, too. I have a friend who built a Harmon Rocket (which I occasionally get to fly) and they removed one rib bay from then RV-4 wing to shorten the span and stay in the aerobatic category while adding a Lycoming AIO-540 instead of an O-360 up front. I have not calculated it, but they call it a +6 / -3 aircraft. Could be stronger actually.
 
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