Structural strength of fighters up to and including WW2 in various countries

[Pasoleati]

Interestingly, it seems that Italian Fighters had highest load factors. Fiat CR 42 and G. 50 were definitely designed to 14 g ultimate and apparently Macchi C. 202 was no less than 15.8 g (as per Macchi V. 202 Profile).

Germans theoretically had the figure of 12.8 g (7 g x 1.8) and the USAAC 12 h (8 h x 1.5). I said in theory for Germans) as I would never dare to pull 12.8 g in a 109.

What were the French reqs, for example?

By the way, anyone interested in a Finnish test report on the C. R. 32? I could translate it.

 

[riggerrob]

The later North American P-51H was lighter because it was designed to reduce-strength British standards.

 

[Foo Fighter]

Considering the tech of the times, why have an aircraft designed to carry much higher stresses (G) than the squidgy organic component they have to carry.

Later autonomous airframes maybe but otherwise you are just carrying around excess weight.

 

[Pasoleati]

The later North American P-51H was lighter because it was designed to reduce-strength British standards.

Was it? According the official Standard Aircraft Characteristics sheet it was designed to comply with standard US requirements.

According to a document the Tempest was stressed to 14 g, higher than American contemporaries.

 

[Pasoleati]

Considering the tech of the times, why have an aircraft designed to carry much higher stresses (G) than the squidgy organic component they have to carry.

Later autonomous airframes maybe but otherwise you are just carrying around excess weight.

For many reasons. One is to cater for manufacturing/maintenance hiccups. Dozens of pilots died because the Bf 109 structure failed without enemy fire.

 

[riggerrob]

Was it? According the official Standard Aircraft Characteristics sheet it was designed to comply with standard US requirements.

According to a document the Tempest was stressed to 14 g, higher than American contemporaries.

Young, scared, wounded, exhausted pilots are going to pull too many Gs, over-speed, land too hard, etc.
Typical civilian standards require structuraly testing airframes to 1.5 times the published (pilots’ operating handbook) weights, airspeeds, etc.
Only “god’s gift to test pilots” can fly an airplane exactly to the edges of the enevelope. All other pilots are doing well if they don’t break the airplane.
One hard landing can eliminate the structural margin of error.

 

[Calum Douglas]

Young, scared, wounded, exhausted pilots are going to pull too many Gs, over-speed, land too hard, etc.
Typical civilian standards require structuraly testing airframes to 1.5 times the published (pilots’ operating handbook) weights, airspeeds, etc.
Only “god’s gift to test pilots” can fly an airplane exactly to the edges of the enevelope. All other pilots are doing well if they don’t break the airplane.
One hard landing can eliminate the structural margin of error.

These things must have happened, but you need to remember that for most of the war nobody was using g-suits, so you`ll pass out before most common manoeuvres occur which can overload the airframe. Going back to a previous point, I`m not sure that the Mustang weight increase was ONLY due to some weird thing where the US happened to need far higher load cases. I`ve read a very detailed NAA study on the weight compared to Spitfire IX, and whilst there are SOME loading requirements which are higher in the US, it is by no means all. A lot of the weight increase of the Mustang was due to the wings needing to be much sturdier and less wavy than usual to ensure the wing profile`s worked as hoped for which would never have happened had the skins been the same thickness as those of the Spitfire. NAA also said that much of the weight increase was due to their inexperience and thus needing to play safe. Much of that was able to be reversed for the 51-H when years of operational experience was available.

The British started discussing g-suits not that long after the start of the war, and many Air Ministry staff were very concerned that should they be introduced it would be self defeating as pilots would be able to get too close to airframe limits (which had been designed on the basis of the pilot passing out about 6g at the very most), which at the time were about 10g in Britain - and that therefore should they be used all of our fighters would need to be significantly structurally revised at immense cost and effort with corresponding weight increase (note that's the requirement, obviously certain aircraft were a bit above depending on the detail of the design).

British RAE tests of Spitfire and 109 wings are infuriating as they are not done with exactly the same test setup, but, broadly it appears that both failed in bending at roughly similar loadings, the Spitfire wing maybe being a tiny fraction stronger. There are anecotal reports of 109 wings enjoying falling off, either they are just stories, or possibly the aerodynamics of that wing and the 109 with the slats might have enabled the 109 to induce particuarly severe loadings in certain circumstances, I dont think the wing taken as a mechanical beam was itself notably weaker than a Spitfire wing looking at the tests (which are of course isolated ground tests in a rig of the wing itself)

 

[edwest4]

Calum,

A bit about G-suits.

https://media.defense.gov/2010/Sep/23/2001330103/-1/-1/0/AFD-100923-014.pdf

 

[Pasoleati]

Calum, at least one Finnish pilot list his life when the tail of the 109 failed in dive recovery. Dozens of Bf 109Fs lost their tails killing the pilots.

As far as I know, the highest stress classification in German aircraft was H5 and that required 7 g with a safety factor of 1.8 (ultimate 12.8 g). If I remember correctly, the 109G without strengthened wings was limited to 6.5 g.

According to the specification the Tempest was designed for, the maximum g had to be "available" at VNE. If the pilot manuals are of any indicator, both the Hellcat and the Corsair were limited to only 3.5 g at their VNE. Possibly the USAAC standard was different being Birch Mathews's book on Bell Cobras it is mentioned that the test pilots had to demonstrate an 8 g pullout at VNE.

 

[Calum Douglas]

Calum, at least one Finnish pilot list his life when the tail of the 109 failed in dive recovery. Dozens of Bf 109Fs lost their tails killing the pilots.

As far as I know, the highest stress classification in German aircraft was H5 and that required 7 g with a safety factor of 1.8 (ultimate 12.8 g). If I remember correctly, the 109G without strengthened wings was limited to 6.5 g.

According to the specification the Tempest was designed for, the maximum g had to be "available" at VNE. If the pilot manuals are of any indicator, both the Hellcat and the Corsair were limited to only 3.5 g at their VNE. Possibly the USAAC standard was different being Birch Mathews's book on Bell Cobras it is mentioned that the test pilots had to demonstrate an 8 g pullout at VNE.

Tails, or horizontal stabilisers ? ("F" being the first version without diagonally braced stabilisers)

 

[Pasoleati]

Horizontal stabilizers.

 

[sienar]

One thing to remember is fatigue limits. Fighters, at least german, were designed to much lower life limits than transports - something like 200hrs for fighters at the end of the war vs 10000 for transports.

British RAE tests of Spitfire and 109 wings are infuriating as they are not done with exactly the same test setup, but, broadly it appears that both failed in bending at roughly similar loadings, the Spitfire wing maybe being a tiny fraction stronger. There are anecotal reports of 109 wings enjoying falling off, either they are just stories, or possibly the aerodynamics of that wing and the 109 with the slats might have enabled the 109 to induce particuarly severe loadings in certain circumstances, I dont think the wing taken as a mechanical beam was itself notably weaker than a Spitfire wing looking at the tests (which are of course isolated ground tests in a rig of the wing itself)

I wonder if 109 wings falling off could be related to the wing bolts. Maybe some crews just messed up attaching the wings?

 

[Pasoleati]

For the airframe, with usage figures expressed in hundreds of hours, fatigue isn't a factor.

 

[publiusr]

And if it is a bit overdesigned…bullet holes matter a tad less.

 

[sferrin]

Interestingly, it seems that Italian Fighters had highest load factors. Fiat CR 42 and G. 50 were definitely designed to 14 g ultimate and apparently Macchi C. 202 was no less than 15.8 g (as per Macchi V. 202 Profile).

Germans theoretically had the figure of 12.8 g (7 g x 1.8) and the USAAC 12 h (8 h x 1.5). I said in theory for Germans) as I would never dare to pull 12.8 g in a 109.

What were the French reqs, for example?

By the way, anyone interested in a Finnish test report on the C. R. 32? I could translate it.

Do these figures include a safety factor? For example, an F-16 might be "designed for 9Gs" but because of the safety factor the actual number will be higher.

 

[Ravinoff]

Do these figures include a safety factor? For example, an F-16 might be "designed for 9Gs" but because of the safety factor the actual number will be higher.

I know modern aircraft are rated to survive well in excess of their stated G-limits, civilian and military. During the Air Astana Flight 1388 incident an Embraer ERJ-190 experiencing some insane control issues (due to the aileron cables being connected backwards) safely landed after pulling upwards of 150% rated load in a bunch of places, to the point it was visibly warped. And in one event I still have no sane explanation for, a MiG-25 (max rated g-load 4.5) in low-level dogfight training pulled an 11.5G turn without coming apart, presumably because the MiG-25 is a flying slab of nickel steel. Both of those were complete airframe writeoffs, but the aircraft in question landed intact.

 

[sferrin]

I know modern aircraft are rated to survive well in excess of their stated G-limits, civilian and military. During the Air Astana Flight 1388 incident an Embraer ERJ-190 experiencing some insane control issues (due to the aileron cables being connected backwards) safely landed after pulling upwards of 150% rated load in a bunch of places, to the point it was visibly warped. And in one event I still have no sane explanation for, a MiG-25 (max rated g-load 4.5) in low-level dogfight training pulled an 11.5G turn without coming apart, presumably because the MiG-25 is a flying slab of nickel steel. Both of those were complete airframe writeoffs, but the aircraft in question landed intact.

Heard similar about the Skyray and Tomcat.

 

[Foo Fighter]

I am just going to leave this here as a demo of exceeding design stress limits.

The Spitfires that nearly broke the sound barrier

A handful of British Spitfire pilots cheated death in high-speed dives that helped pave the way towards supersonic flight. Stephen Dowling investigates.

www.bbc.com

 

[Avimimus]

Do these figures include a safety factor? For example, an F-16 might be "designed for 9Gs" but because of the safety factor the actual number will be higher.

I assume they do include the safety factor.

It also isn't just sustained g-forces, there are also instantaneous forces and other structural forces that the plane needs to withstand... so making it quite a bit stronger than what would cause the pilot to temporarily black out!

By the way, structural load/maximum gee force data exists for some WWI aircraft!

 

[sferrin]

I am just going to leave this here as a demo of exceeding design stress limits.

The Spitfires that nearly broke the sound barrier

A handful of British Spitfire pilots cheated death in high-speed dives that helped pave the way towards supersonic flight. Stephen Dowling investigates.

www.bbc.com

"On an afternoon in 1955, Bob Rahn leapt off the Los Angeles International Airport’s runway in a production Skyray, heading out over the Pacific a hundred feet above the waves. The idea was to see whether enough pitch trim was available with the new engine to compensate for the airplane’s tuck-under at transonic speeds. He later wrote, “I had accelerated to Mach .98 (approximately 750 mph) in afterburner. This Mach speed created the maximum tuck-under. Full trimmer deflection was required to maintain trimmed flight. Therefore I concluded that the engineers had done a good job with respect to adequate trim for this low-altitude, high-speed flight environment. For all practical purposes, the test was completed. So I nonchalantly shut off the afterburner.”

The Skyray decelerated so rapidly that the trimmer became super-effective, flipping the nose suddenly skyward. “My Skyray and I were pitched up at a gut-wrenching 9.1 Gs,” Rahn wrote. “The airplane had a design limit of 7.0 Gs. Moreover, I wasn’t wearing a g-suit…. I immediately blacked out.” Rahn lost his vision but was aware of his situation. Reluctant to touch anything for fear of making a bad situation worse, he endured the ride. When his eyes cleared, his windscreen was all blue Pacific. “I was in a vertical dive after completing three-fourths of a loop.” Gingerly recovering at about 3,000 feet, he looked out at the wings. “They were wrinkled from wing tip to wing tip, resembling dried prunes.”

Back on the ground at LAX, Rahn found that the rest of the Skyray’s skin was wrinkled, the wings were incurably bent, and some of the vertical stabilizer’s stringers were protruding, like broken bones. The engine had torn off its mounts and was resting on the engine-compartment access door, pinching a fuel line. Later, Rahn reckoned that the event had been caused by the added thrust and the resulting increase in tuck-under. The corresponding increase in nose-up trim had made the Ford go nuts when it suddenly decelerated. Scratch one Skyray."

Beautiful Climber

In the summer of '58, nothing was faster to 50,000 feet

www.smithsonianmag.com

 

[Archibald]

“They were wrinkled from wing tip to wing tip, resembling dried prunes.”

ROTFLMAO

That Skyray had aged instantly...

 

[icyplanetnhc (Steve)]

The later North American P-51H was lighter because it was designed to reduce-strength British standards.

I think the P-51H also had a smaller fuselage fuel tank as well, and the resulting range and endurance was quite a bit reduced compared to the D model.

Speaking of over-g, the third F-22, 91-4003, had a flight departure that resulted in something like a negative 11 g overload. That jet never flew again after that incident and is now a museum. The F-22’s high usage of titanium and composites may have helped, in fact more titanium than any fighter other than the YF-12, I believe.