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

[drgondog]

BDK has covered it pretty well - I can only speak for Lockheed and Bell, but a little more detail might be useful. First Limit Loads are based on yield stress level, and 1.5 x Yield is often failure mode for aluminum and steel.

Second, the P38 and P-51 were designed to 8g limit and 12g ultimate but in each case that was linked to a specific gross weight. For the Mustang, limit load of 8G applied to 8,000 pound gross weight which was middle range for an XP-51 and P-51A. By the time the P-51D was operational a limit load for 10,000 pounds Gross weight was 6.4 G (8 x 8000/10,000).

The detail stress analysis is for every component in the airframe and for many flight conditions, one area that is under severe scrutiny may have little reason for concern in another flight profile. Areas you look for immediately are fuselage/eppenage carry through structure when the airframe is is rolling and combination roll/yaw where the aerodynamic loads are asymmetrical, or dive pull out on the main wing spar/carry through structure, or Corner Speed turns, or carrier landings, etc, etc.

The harder part of airframe structures analysis was getting the aerodynamic load profiles for such manuevers so you could take it to assy level (wing, horizontal stab, etc) to main spar/torque box, longeron, beam caps, webs, shear panels, stiffener, rivet level detail stress analysis.

The P51H was a 99% redesign and the structures guys were designing to 7.5 G/11.3 for 8,000 pounds and that worked out to about the same for an equivalent amount of fuel and ammo for the H versus the D (7.5x8000/9500=6.3G limit load) as the H was about 500 pounds lighter.

As to fatigue, for Bell Helicopter a design life for a high reversible load like the pylon area took the allowable stress for 2024-T4 down from ~ 50K psi to 25K psi as the limit stress for that particular design feature. For 7075-T6 (more brittle) the percentage redution in allowable stress went from 75K psi to about 35K psi (can't remember exact level -CRS) -

again - BDK touched on this fatigue/aeroelastic consideration in airframe structures.

[warbird51]

quote
Second, the P38 and P-51 were designed to 8g limit and 12g ultimate but in each case that was linked to a specific gross weight. For the Mustang, limit load of 8G applied to 8,000 pound gross weight which was middle range for an XP-51 and P-51A. By the time the P-51D was operational a limit load for 10,000 pounds Gross weight was 6.4 G (8 x 8000/10,000).

That is what I am looking for. That answers the P-38. How about the other aircraft?

[JDK]

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.

That's correct in that the Ju 88 was designed to act as a dive bomber. What's more amazing is that the four engine He 177 'Grief' was also required to be able to act as a dive bomber, and even a journalist like me realises that Mass x acceleration = problems if it's big and heavy (and been going downhill fast just before!).

This was a hangover from a (slightly earlier justified) Luftwaffe love affair with the Ju 87 and its efficacy as a weapon - literally more (accurate) bang for your buck (Reichmark/metal).

Interestingly the brief heyday of the dive bomber as a critical battle-winning weapon (1939-1942) has everything to do with (in time order) poor bombsights; developments in aircraft structural engineering to take 'proper' dive bomber loads, and it was eclipsed not by better bombsights but by fighter aircraft's design strengths growing to enable near-full dive bombing (coupled with better self-defensive capability) as fighter bombers.

Off the answer, but probably on topic!

Regards,