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Outright aircraft strength

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Outright aircraft strength

Old 12th Feb 2015, 16:11
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Outright aircraft strength

Mods please feel free to move this to a more relevant area if appropriate.

All airframes are designed and built to be strong where necessary but as light as possible with a safety factor built in - but given certain job descriptions some are 'more equal' than others i.e. the A10 is specifically built to be 'tough' and I expect all carrier aircraft are that bit overbuilt to take the pounding of deck landings.


So my question is are some air frames simply tougher than others far in excess of the normal limits or are they made from stronger materials or what?

Sorry if it's a bit vague I'm struggling to be specific in what I mean - on the Bucc thread running Beagle refers to the Buccaneer as 'immensely strong' which i can understand given it's original role but would a Tornado or Hawk et al. survive the same maneuver in the same circumstances?

Or in the Second World War were certain bombers more likely to get you home missing a few bits and bobs?

The analogy I have in mind is the Toyota pick up on top gear - beat to sh!t but still intact where some other cars would be toast in minutes.
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Old 12th Feb 2015, 16:30
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Hence the T-45 Goshawk vs the original. Among other naval-specific mods, it has a strengthened airframe, I believe.

I don't suppose you want to incur the cost of implementing those extra features if they won't be required in the aircraft's intended role. You don't get cargo doors on most airliners. You can have them, but you pay for them, and floor strengthening etc.

There's a vid out there of an old Airbus (A310, I think) being stripped of passenger fit and converted to a freighter.
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Old 12th Feb 2015, 17:02
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The F4 was a naval aircraft of the 50s and no doubt stronger than needed as a land fighter.

The F4K was procured as a carrier aircraft whereas its stablemate, the F4M, was only a land plane. I doubt it was made less strong though it lacked hydraulic wing fold and several other mods need for RN carriers.
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Old 12th Feb 2015, 19:11
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SU25 has a titanium cabin that can take direct 30mm hits at close range. You just wouldn't expect it on an aircraft - but that's design for you.
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Old 12th Feb 2015, 19:46
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The A-10 has a titanium cockpit tub for pilot protection.

One of my friends flew the A-10 and he said pilots always asked where is the titanium canopy? Rolling in on a target and taking a round or two through there might mean you're in a blender while the rounds try to 'escape'....
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Old 12th Feb 2015, 19:54
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Or in the Second World War were certain bombers more likely to get you home missing a few bits and bobs?

see Reg Levy's NIGHT FLAK AND HIJACK PP 42-43. His description of the punishment his Mosquito took might put your hair on end.
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Old 12th Feb 2015, 19:55
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Shaft, there are so may variables that this is tough to answer in a quick post.

Yes some airframes are stronger than others, and there are a whole range of compromises and tradeoffs that go into the design of an aircraft. Military specifications calling for ballistic protection and crash survivability will generally result in beefier, heavier airframes. Aircraft designed to operate aboard aircraft carriers are more robust to cope with the stresses of catapult launches and arrested landings.

Most carrier aircraft have a good reputation for being strong and include the F-4, A-4, Hellcat, etc. Besides the fatigue issues in later life (land based), the Bucc was indeed known as a robust aircraft.

Some aircraft later served in roles that they were not inteded to serve, such as most of the cold war fleet going to low altitude penetration and later suffering fatigue issues or limitations (Valiant, B-52, etc).

I have some knowledge of the H-60 airframe, and can atest it is a robust airframe, mostly because of the initial design requirements for a battlefield helicopter. Other civil helicopters of similar size may have less stringent requirements. I know which one I would want to be in a rollover or crash.

A common rule of thumb is many modern aircraft are designed for a certain load limit, and then a 150% failure limit is built into the design. That is the structure can survive 150% of the normally anticipated highest load limit it will see in normal service. Structures, or even whole aircraft, can then be tested, and if they fail before that point, it may be time to go back to the drawing board. Some more weel-knowm examples include the the C-17 and C-5 that suffered wing issues and needed to be re-worked, and the C-133 and Comet suffered fuselage cracking.

WWII provided some great examples of different design philiopshies. The Japanese Zero was designed for optimum range an manueverabilty, but did so by sacrificing armor protection and had a less robust structure. The Hellcat was designed to take a beating, and did so by having a beefier structure- which had a weight penalty.

Manufactures use a a variety of materials to meet weight, cost and strength requirements. Through history we have seen different materials introduced to meet ever increrasing requirements: wood and fabic was replaced/augmented by aluminum, then alloys, tintaium, composites etc. Titanium is a great example as it was lighter and stonger than earlier material, and had better heat properties, but was quite expensive- it is all a trade off.

Some manufactures had or have a reputation for strong aircraft. Grumman was known as the "iron works" for good reason. They built strong aircraft, and needed to as most were intened for naval use.

The B-17 had a good reputation for taking damage. It was well built. Many bush planes are quite robust.
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Old 12th Feb 2015, 20:15
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In the 30s and 40s one didn't have computer calculated stress patterns or ultrasound inspection procedures, the aircraft had to stand up by itself. The designers and the people who were going to build them used to look at something like the main spar, suck their teeth and then add on a few layers.
This attitude gave birth to aircraft like the Douglas DC3 which has never suffered from fatique problems
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Old 12th Feb 2015, 23:05
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Carrier based aircraft would primarily be excessively strong in the fuselage barrel area - right?
I seem to remember some discussion of this when there were cracks found in Hornet fuses.
I assume you essentially build a very strong `centre bit' of the fuse where the gear attaches... to absorb the loads... and then similarly the spine of the aircraft from the barrel to the tail is excessively strong to hang the engines off, and attach arrestor hook?
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Old 13th Feb 2015, 08:32
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In WW 2 the Halifax was reputed to be more survivable than the Lancaster.

At one point, I think on the Lancaster, the engineers examined badly damaged aircraft with a view to strengthening. It was pointed out that these aircraft were fine. What needed beefing up were the undamaged areas as damage in those areas may have resulted in the missing one's loss.
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Old 13th Feb 2015, 08:44
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The Valiant losses were not due to an operational change from their original requirement but through metallurgy and the premature failure of the advanced alloys in the spars, spars never installed were found to exhibit the same stress signs. Aircraft assigned to RRE were very low hours and similarly afflicted.

The RAF knew in at least 1956 that there was a problem with DTD 683, the alloy the Valiant centre plane spar was made out of, but the materials low fatigue resistance was known from the very begining of the Valiant project in 1947, it was hoped that new techniques in manufacture would solve this inherent weakness. The other problem for the Valiant was that it was built to a 'safe-life' strategy, the 'safe-life' strategy of A/c design was abandoned in around 1956 as it could not ensure safety in a catestrophic failure. Also in 1956 this article:-
1790 Structural Changes Caused by Plastic Strain and by fatigue in Aluminium-Zinc-Magnesium-Copper Alloys Corresponding to DTD.683 (Broom and Mezza)
appeared in
The Journal of the Institute of Metals (JIM) Vol 86, 1957-1958, (written in November 1956)
The President of the Institute of Metals was Lord Tedder, Marshal of the RAF .

Lord Tedder

Last edited by Pontius Navigator; 13th Feb 2015 at 08:55.
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Old 13th Feb 2015, 14:08
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Tough jet

I was shocked when the 1998 jet hits cable car story was reported. I had no idea that such an aircraft could conceivably sever the steel cables like those supporting a cable car and fly on. I guess perhaps that the cables struck the landing gear or a similar tough bit.

Wiki says:
"The cable was severed and 20 people in the cabin descending from Cermis plunged over 80 metres (260 ft) to their deaths. The plane had wing and tail damage but was able to return to its base, "

The cables supporting cable cars are not flimsy.

A couple of poor images of the damage to the aircraft at starboard wing root.

Aircraft was EA-6B Prowler.

google for images [Damage to prowler jet cable car "archive.militarytimes.com"]
Site is subscription only, and I don't have a subscription, so any link I post may be no use.
The returned image may be a reduced resolution preview, not sure.
If anyone has a link to the US official report which has apparently been released to the Italian newspaper La Stampa it might be interesting.

Exclusive: Classified Documents Show U.S. Full Responsibility For 1998 Italy Ski Gondola Disaster
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Old 13th Feb 2015, 17:13
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JJ,http://en.m.wikipedia.org/wiki/Caval...disaster_(1998)
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Old 13th Feb 2015, 17:17
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A Group Captain at Laarbruch (cant remember his name) once told us on our arrivals course "...the Buccaneer is made of Ships Plating and Girders while the Jaguar is made of resin-impregnated paper, tin-foil and locking wire".


...if that's any help?
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Old 13th Feb 2015, 19:55
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Perhaps drifting away from the OPs intent, if it related solely to Military aircraft. I had some 500 hours solo experience of flying gliders over my time in the RAF. By basic trade I'm an Airframe tradesman, and was somewhat surprised when our CFI told me that modern gliders, spar wise, are stressed to take more than 20G loading, on account of spending most of our time in rough air and turbulence. His assertions were put to the test not long after when a fellow club member managed to fly into the blackest cu nim she could find that day. Her exit from the bottom of the cloud, minus wings, cost her her life. It appears she had found a way of exceeding whatever the true limits were on that aircraft, assisted by the nature of a violent piece of weather. As a C130 Ground Eng I never doubted the integrity of the airframe, yet could never forget seeing the wings fold on the firefighting Herk lost a few years after I left the service. i suspect though, that as is mentioned with the A10, designers "build in" strength based on the intended use of the aircraft. Who knows what happens when that aircraft is taken out of its designed environment. As a starter, the"Australia" patch on the underwing of the Vulcan, a direct result of its new low level role. I'm sure many others can offer input on the subject, it's certainly a "meaty" subject.

Smudge
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Old 13th Feb 2015, 21:11
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Smuj, not heard it called that before but it certainly contributed to the weight growth. IIRC we had zfw iro 96-98k when I started and near 110k 10 years later. Obviously some related to extra kit such as RS 2, HRS, TFR, Swivel Seats although the PTR175 and Collins may have been lighter. And of course polyurethane paint.
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Old 14th Feb 2015, 02:05
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jimjim1 - the landing gear was retracted, and to get to it the cable would have had to cut through the radar and radar-mounting bulkhead for the front landing gear, and through all that and the bottom of the cockpit and front of the engines to reach the mains.
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Old 14th Feb 2015, 05:35
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when our CFI told me that modern gliders, spar wise, are stressed to take more than 20G loading,
smuj,

Not quite 20 G, but still very impressive. Here's a video of a DG-1000 wing, being tested to destruction. The initial test was to the maximum certified flight load (6.4 G, J = 1, force applied by the crane 3240 lbs), with a cold wing. The wingtip bent upwards by 2.3 m!

They then heated the wing to 54 C (to simulate central Australia) and bent it until it failed at J=1.95. I calculate that is 12.5 G and a force of 6318 lbs! The certification requirement is J=1.725 for 3 seconds.





I was surprised by the lack of safety equipment and the close proximity of the testers to the wing.

There is a very good description of the test here:
DG Flugzeugbau: Destruction of the Wing

although when I went to the site today, the pictures were missing.

Last edited by India Four Two; 14th Feb 2015 at 06:29.
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Old 15th Feb 2015, 15:09
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Happily, the "strength" of an aircraft structure is complex and multi faceted. There are many design opportunities to optimize the design. Strong is nice, but it's usually either very heavy, or very expensive. But too strong may eventually break, either by overload, or fatigue. As India Two Four shows, "strong" is better when it is carefully balanced with flexible, as the flexing will relieve the need to carry the load somewhat.

Changing an airframe by "strengthening" requires a lot of applied knowledge. Just adding a gusset or doubler here and there can have the effect of shifting the load to be carried to another part of the airframe, which is perhaps even less able to sustain those loads, and possibly eve concentrating the load at that more vulnerable place.

The change to the structure can also cause the intended flexing to happen differently, and disastrously. I recall reading of an early German monoplane, which suffered wing failure during test flight. The designer strengthened the wing spars, and failed at even a lesser load. By strengthening the spar, the designer unwittingly changed changed how the wing flexed under G load, and now, as G was pulled, the wings flexed so as to increase their angle of incidence near the tips, and ripped them off. The spar had been strengthened behind the flexing axis of the wing. So if you're going to stiffen a wing structure, the leading edge is probably the place to do it.

The "get you home" properties of an airframe are often as much about multiple load paths, as a "strong" or "tough" structure. It's nice to have a wing with many spars (Lear Jet, for example), so if a spar fails, the rest will get you home. COnsidering the durability of the dH Mosquito. it was wood, with so many load paths that if you shot away some, the rest would carry the load, hoping of course that the pilot then recognized the need to handle compromised airframe with a gentle touch.

This nice characteristic of a well designed aircraft is found in many good designs as layered structure, so one layer can fail, and the others carry the load. "Built up" wing spars are generally an example of this, where multiple pieces make up the spar caps, so if one is broken the others carry the load until you get home.

The back side of this is the possibility of a "latent failure" such that an element of the structure has been damaged (fatigue/corrosion/overload) but the associated structure is still carrying the load, so the failure is undetected. Good inspectability is vital.

Aircraft structure is a fascinating subject to learn, and there is seemingly endless material.
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Old 15th Feb 2015, 15:42
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" Her exit from the bottom of the cloud, minus wings, cost her her life. It appears she had found a way of exceeding whatever the true limits were on that aircraft, assisted by the nature of a violent piece of weather

As I understood it, she had entered cloud and the accident report surmised she had entered a spin with the wings parting company at about 11g.

I knew the lady concerned.
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