G load design limits for P-38, F7F, B-25, A-20 and A-26

[warbird51]

Are there any books out there that have the design G limits for the aircraft listed? I'm trying to find out what the G load limits they were designed to.

[bdk]

From the Type Certificate Data Sheet (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/3b5192366ab5f6f4862578140063d192/$FILE/AL-2%20Rev%202.pdf):

"This airplane must be operated at all times within the limitations set forth in T.O. AN01-60GA-1 for (RB-25),
T.O. AN01-60GB-1 (for B-25C), T.O. AN-01-60GC-1 (for B-25G), T.O. AN01-60GD-1 (for B-25H), T.O.
AN01-60GE-1 (for B-25J), T.O. 1B-25 (T)K-1, T.O. 1B-25(T)L-1, T.O. 1B-25(T)M-1 or T.O. 1B-25(T)N-1 (for
B-25N and TB-25N) except for limitations specifically called out in Limited Type Certificate Data Sheet No. 2, in
which case the values given in the specification must be observed. A copy of the pertinent Technical Order must
be carried in the aircraft during flight."

Those are the operating limits. Typically those are considered "limit loads." "Ultimate loads" are generally 1.5 times the limit loads. That is the factor of safety in the design. Safety factors and other design requirements are generally called out in the contract specifications. In current USAF parlance, the "Air Vehicle Specification." Where you might find a copy of that, I have no idea.

[Bill Greenwood]

I am not up on max g limits for the bombers, however I expect they would be somewhere between what a civilian plane in the utility category is such as for the T-34, some Bonanzas, which I think is 6 positive and 3 negative; and that of the fighters.

I think P-51s are 8 positive and negative. Spitfires are 9 positive and 8 negative.

[bdk]

I wouldn't want to be in a B-25 pulling 6Gs, but at typical civilian operating weights you might get by once or twice.

How many Gs did Bob Hoover routinely pull in his Aero Commander?

[Bill Greenwood]

It's hard for me to believe that a B-25 would not be designed for 6gs stress. That is only what a T-34, is and that is not even a combat plane. That 6 gs would be at gross weight and should be not be a problem at all at lighter civilian weights.

I am almost certain that civilains limits for a T-34 were 6 and 3, (before the ad modifications).
My Bonanza is in the utlilty category , Beech likes to have a little more margin than normal category. I can't recall for sure if utility is 4.5 gs or6 gs.; I think it might be 4.5.

Bob likely did his show at about 3 or 4 gs, but they limit was probably higher and he was also very smooth and would be below gross weight.

In normal civilian flying even an airshow in the Spitfire there really is no need to go over 5. It will turn very tightly at 5gs. I was flying a Canadian air force jet instructor (T-33s) once and making some tight turns about 4 gs and he said , "This thing will turn up his own tailpipe" at least that is the cleaned up version of what he said.

[cwmc]

T.O. 1B-25(T)N-1U 20-Jun-56 SOF, This supplement is issued to impose flight restrictions and to apprise the pilot of flight conditions during which the maximum recommended flight limit load factors can be exceeded.

I cannot find the TO and my manuals are at the hangar, but I recall during check out that a 60 degree bank would be a great maximum (2G). I think the max G limit was restricted to less than 3 G's and that is why the bombers are not allowed to race at Reno. The airliner I fly has a max G limit of 2.5 clean and 2.0 with flaps more than 11. This is typical of a transport type, which is what a bomber is essentially.

No way on the B-25 and 6G. Less than 3.5G was probably war time, reduced depending on weight and it was summarily reduced during the trainer era of the 50's.

Chris...

[cwmc]

Hi Bill,
Normal category G limit is 3.8, utility is 4.4 (weight dependent, over a certain weight it is a normal category airplane) and aerobatic +6/-3 (sometimes weight dependent, like an Aerobatic Bonanza).
Chris...

[Bill Greenwood]

Thanks, guys.
Chris, only 2.5 seems low even for an airliner. I guess that that is the operating limit that you fly under, but not the design max which would be a little higher.
I think the original question in this thread was about the design limit. The title says "Design Limits"

CWMC, thanks , I think your figures are exactly right , and that jogs my memory and agrees with what I was thinking.
One can do a lot of flying, even most acro and never exceed 3 gs. I have done it like the last flight I did in my T-34 before delivering it to the new owner. Mine had no spar or web cracks upon inspection,and while I had never been to 6 gs I had been to 4 in it.

It is a nice feeling in the Spitfire to know that its limits are far in excess of my one if I am awake.

This is a real guess, but I think I might have read that a P-38 design limit was 8 positive. You would be in even bigger trouble if you had a Zero on your tail and had to stay down around 4 gs.

[davidbray]

Thanks, guys.
Chris, only 2.5 seems low even for an airliner. I guess that that is the operating limit that you fly under, but not the design max which would be a little higher.
I think the original question in this thread was about the design limit. The title says "Design Limits"

CWMC, thanks , I think your figures are exactly right , and that jogs my memory and agrees with what I was thinking.
One can do a lot of flying, even most acro and never exceed 3 gs. I have done it like the last flight I did in my T-34 before delivering it to the new owner. Mine had no spar or web cracks upon inspection,and while I had never been to 6 gs I had been to 4 in it.

It is a nice feeling in the Spitfire to know that its limits are far in excess of my one if I am awake.

This is a real guess, but I think I might have read that a P-38 design limit was 8 positive. You would be in even bigger trouble if you had a Zero on your tail and had to stay down around 4 gs.

I believe the required design factor is 150%. At least Boeing, not sure about Airbus, aims for a bit higher than that. There is one story that I read about that had a 747SP exceed 5gs (probably just a momentary force) and survive. Flight was China Airlines 006.

[mike furline]

For what it's worth, http://www.airportjournals.com/Display. ... tPage=True

"The key was to see whether I could perform the P-38 maneuvers within the structural design limits of the Aero Commander," he said. "The P-38 was a fighter capable of pulling 7.33 g's, whereas the Shrike Commander, a twin-engine business aircraft, was limited to 4.4 as a utility design."

[bdk]

Some terms are being tossed around here without full understanding of what they mean.

Operating limits are set in the flight manuals for various flight conditions and configurations.

Design limit load is the maximum load expected during operation. There should be no permanent deformation up to limit load.

Ultimate load is generally set at 1.5 times limit load. There can be permanent structural deformation between limit and ultimate load, but no failure.

If you fly above limit load in a metal airplane, you may inflict permanent damage to the airframe (i.e. popped rivets, wrinkled skin, etc.).

All of this ignores durability & damage tolerance (D&DT). Metal fatigue is accumulated over time and degrades the structure. Metal fatigue can cause a failure well below limit load. The accumulation of fatigue damage occurs more quickly at higher stress levels (like air tanker operations in turbulent low level environments and ACM). Metal fatigue was not well understood or seriously considered in aircraft applications until it became a focus after the Comet airliner disasters.

[davidbray]

Ultimate load is generally set at 1.5 times limit load. There can be permanent structural deformation between limit and ultimate load, but no failure.

I thought it was somewhere around that, couldn't remember for sure though. I remember watching the video about the making of the 777, the wing ultimate load test was amazing.

[cwmc]

Makes sense, BDK.
The MD-80 certification ended with two hull losses, one on a landing at too high of a descent rate and the other a 4G pull from a botched stall. The 4G pull put a permanent set in the spar box as it was explained to me. 1.5 x 2.5 = 3.75, so 4 is definitely more than it was designed to take.
I saw the title and since ultimate load factors are so esoteric I figured a few normal operating figures might be appropriate, with all of the opinion being bandied about.
Chris...

[warbird51]

Thanks for all the feedback. I am looking for design load limits. Not ultimate load limits.
I figure that when these aircraft were designed, there were certain limits specified by the military that the aircraft was to meet. There are 2 fighters listed, a medium bomber and 2 attack aircraft listed that I wanted to find out about. That way I can compare how the different mission requirements for each type are affected by the design limits required by the military. I would expect that the fighters (P-38/F7F) would have higher G ratings than the attack aircraft A-20/A-26 and the B-25 would be the lowest. I know that the Germans made some of their twin medium bomber's be able to dive bomb and that really added to their weight to be able to meet the requirements of their military.

[bdk]

Fighters are also expected to have higher maneuvering speeds (yanking and banking simultaneously), so that has an effect. The G-loads may not be that much higher, but the "load condition" of the combined forces will require a stronger airframe. There is more to the equation than just the positive and negative G-loadings.