Outright aircraft strength
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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.
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.
As for the A10, it's "strength" comes from it's simplicity and that it's designed to fly with significant bits missing here and there...
As for the wingless F-15 who would have believed that?
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?
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?
Firstly, let's be clear about "strong". There are several ways you can define this where an aeroplane is concerned.
The simplest is the V-N diagram; the speed-g envelope which applies to any aeroplane, albeit with different values depending upon aeroplane role. A light aeroplane will have something like +3.8/-1.5g and 170kn. From memory the Hawk was around +8/-4 (a bit less in a training role) and 350kn/M=1.05, a modern fighter will be around +9/-4, 650kn/M=2.2, and a modern transport around +2.5/-1.25/350kn/M=0.85. Typically you design to that plus an appropriate safety margin, which is usually about 1.5 (i.e. add 50%) for metal structures, with another 20-50% over that for composite structures.
Additional to that will be a requirement for damage tolerance. For most civil or training aeroplanes, that's fairly trivial - requirements for a certain amount of hail / corrosion damage, or manufacturing defects. So, the V-N diagram requirements have to be met with predetermined levels of damage. Where you are dealing with aircraft that have any kind of combat role, you add in additional requirements there - in simplistic terms, it's still got to meet the requirements when it's been shot at a bit. That means inevitably a fair bit of redundant structure - Barnes Wallis' geodetic constructed Wellington did that really well - but any aeroplane can be designed with redundant structure. This is, incidentally, one reason why even the aerobatic variants of the various Spitfire replicas can afford ot be massively lighter than the original - they might be thrown around a great deal, but they're unlikely to ever get shot at. On the other hand, anything design for a low level surface attack role, such as the A10, will have a huge amount of structural redundancy built in.
Undercarriages are pretty straightforward for the vast majority of flying machines, as the task of landing doesn't vary much between a civil or military trainer, ditto transport, or even a fighter. There are known formulae for determining worst case impact loads, and they pretty much design the undercarriage for you.
The big exception is landing on a ship, whether that's a fixed or rotary winged arrival - where the requirement for a distinct "arrival" has to be designed in, combined with the obvious fact that ships keep moving up and down, and if that is mis-timed, the loads go up a lot. However, it is also important to realise that this isn't really about the forces, it's really all about the energy of impact. So, the best way to deal with lots of energy, is to make an undercarriage with a lot of travel, and within that plenty of damped energy absorption. That in itself brings the peak loads in the undercarriage down, and makes an undercarriage which will take a huge amount of abuse. Note, that this makes the way to build a maritime undercarriage long wth lots of travel, not just beef it up, as making it stiffer will actually just make things worse.
Control surfaces, linkages and actuators are a whole additional game, as you need to consider simple (only in one direction or centralising), control reversal, and then the speed at which they'll be operated. You pretty much want a fighter to be "carefree" as in full deflection up to Vne, which is a big ask structurally, and would mean than an aircraft in that class will be designed to a much tougher set of requirements. Its an interesting point there with civil transports apapted for military use where a certain amount of high speed severe manoeuvring may be required, so you're likely to need to beef the control runs of such an aeroplane up before you paint it green.
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.
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.
WW2 of-course the design methods were much cruder than we have now, so there was an inevitable need to overdesign somewhat. Also however there was extensive use of frame + lightweight covering structures (Wellington, Hurricane...) That fabric or light alloy covering was by and large not loadbearing, so you could burn or blow it away to your hearts content, and whilst it didn't do the aerodynamics many favours, it didn't worry the structure much.
Hope that helps a bit - a few books off the shelf that cover this a bit...
is the book I learned most of the basic theory from, although I see from the reviews that some of the mistakes are apparently still in there. (Megson was taught by a chap called Professor Dennis Meade, who taught me when I was an undergraduate at Southampton).
is the only aircraft design book I know which covers the military side well. John Fielding used to run Cranfield's MSc in aircraft design, as well as a lot of industrial experience.
has just come out, and is probably the only textbook out there covering the practice of airworthiness assessment. Not too much maths in it either and written by somebody who has a fairly substantial background in both military and civil certification. Stupidly expensive, presumably the publisher are profiteering off the fact that there's no real competition.
G
Last edited by Genghis the Engineer; 15th Feb 2015 at 18:57.
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.
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Thanks all - will be doing a little further reading with that Genghis.
Certainly clearing up a few things I've wanted to know.
One thing India touches on is the possibility of unseen or latent damage caused earlier in the a/c life - mulled this once after whizzing around at 3+g during hour building after getting bored with S + L practice. I was always honest about what I'd been up to but who knew what had really happened to it in the past.
Good to know it had probably been accounted for.
Certainly clearing up a few things I've wanted to know.
One thing India touches on is the possibility of unseen or latent damage caused earlier in the a/c life - mulled this once after whizzing around at 3+g during hour building after getting bored with S + L practice. I was always honest about what I'd been up to but who knew what had really happened to it in the past.
Good to know it had probably been accounted for.
Educated Hillbilly
Some very good replies so far, however the one thing that hasn't been mentioned so far is that for the static analysis there is both ultimate and proof / limit assessment.
A lot of you have mentioned the 1.5 Ultimate factor, that is only for the ultimate static load case (limit case x 1.5), in this instance the component is assessed against the material ultimate strength (that is the strength at which rupture will begin to occur).
It is also normal to consider the limit load (ie: no safety factor applied) or a reduced safety factor (proof) against the materials yield point (the point at which a material starts to deform and the deformation remains permanent).
Typically this is where the civil and military regs are different, in the civil world the proof factor is 1.0 (therefore the same as the limit load), the military have typically used a proof factor of 1.125.
So on the proof load check the aim is to show that no permanent deformation of the structure occurs.
So for aluminium structure, deformation is expected well before an out right failure in most circumstances.
A lot of you have mentioned the 1.5 Ultimate factor, that is only for the ultimate static load case (limit case x 1.5), in this instance the component is assessed against the material ultimate strength (that is the strength at which rupture will begin to occur).
It is also normal to consider the limit load (ie: no safety factor applied) or a reduced safety factor (proof) against the materials yield point (the point at which a material starts to deform and the deformation remains permanent).
Typically this is where the civil and military regs are different, in the civil world the proof factor is 1.0 (therefore the same as the limit load), the military have typically used a proof factor of 1.125.
So on the proof load check the aim is to show that no permanent deformation of the structure occurs.
So for aluminium structure, deformation is expected well before an out right failure in most circumstances.
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Smuj & KnC
The investigation, into the Ventus, concluded that: the failure of the glider wing was due to it being flown with the trailing edge flaps down, and possibly the airbrakes extended, beyond the flap limiting speed. Flap failure and subsequent damage to the rear spar caused a loss of torsional strength of the wings; these departed the fuselage following several violent oscillations.
There was no comment in the report regarding spinning or experienced G loads.
The investigation, into the Ventus, concluded that: the failure of the glider wing was due to it being flown with the trailing edge flaps down, and possibly the airbrakes extended, beyond the flap limiting speed. Flap failure and subsequent damage to the rear spar caused a loss of torsional strength of the wings; these departed the fuselage following several violent oscillations.
There was no comment in the report regarding spinning or experienced G loads.
Thought police antagonist
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RR Nemesis,
Given the content of this thread, and the tragedy involved, it would be inappropriate for it to descend into a slanging match.
I note the 4CGC annotation and I can say with all honesty, that the information I posted came from two very well respected and reliable sources within 4 C's, and I also suspect we know each other from that era.
Would you care to remove thesmilie please because I am not given to making comments about accidents in a frivolous or dismissive manner
There is a possibility, we could be talking about 2 separate accidents here however.
Check you PM's please
.
Given the content of this thread, and the tragedy involved, it would be inappropriate for it to descend into a slanging match.
I note the 4CGC annotation and I can say with all honesty, that the information I posted came from two very well respected and reliable sources within 4 C's, and I also suspect we know each other from that era.
Would you care to remove the
smilie please because I am not given to making comments about accidents in a frivolous or dismissive mannerThere is a possibility, we could be talking about 2 separate accidents here however.
Check you PM's please
.
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What a great post Genghis, may I please ask a supplementary question about the use of composites in the modern era. I do understand that higher reserve factors were applied in the early days of composites because not much was known about failures. Also the inability to inspect for defects. (I am guessing during manufacture or damage/ overstress in use)
With 787 and A350 are the RFs still increased. I would have thought that with all the experience gained and modern NDT 1.1 would have been sufficient.
I am with you all the way about deflection being used to absorb energy. Local stiffening just moved the problem. When I was a student I recall a book entitled the New Science of Strong Materials. It was littered with cartoons and phrases such as "why wings bend" and the likes.
Had I still been a student I would have asked my Dad to buy me the Guy Gratton book for Christmas or my Birthday.
With 787 and A350 are the RFs still increased. I would have thought that with all the experience gained and modern NDT 1.1 would have been sufficient.
I am with you all the way about deflection being used to absorb energy. Local stiffening just moved the problem. When I was a student I recall a book entitled the New Science of Strong Materials. It was littered with cartoons and phrases such as "why wings bend" and the likes.
Had I still been a student I would have asked my Dad to buy me the Guy Gratton book for Christmas or my Birthday.
What a great post Genghis, may I please ask a supplementary question about the use of composites in the modern era. I do understand that higher reserve factors were applied in the early days of composites because not much was known about failures. Also the inability to inspect for defects. (I am guessing during manufacture or damage/ overstress in use)
With 787 and A350 are the RFs still increased. I would have thought that with all the experience gained and modern NDT 1.1 would have been sufficient.
With 787 and A350 are the RFs still increased. I would have thought that with all the experience gained and modern NDT 1.1 would have been sufficient.
Yes, SFs are still increased for modern composite aeroplanes, but not by as much as they used to be because they're now using high temperature / pre-fatigued structural data.
There's more variability in composites: with temperature, moisture absorption, invisible stress / impact damage, manufacturing quality and ageing, than there is with metals. So, the larger safety factors are being applied, but they depend very much upon how you do it. The more work to demonstrate exactly what the safety factors are due to all of these factors, the more you can bring them down.
(SF = margin between limit and ultimate, whilst RF = margin between ultimate and failure)
I am with you all the way about deflection being used to absorb energy. Local stiffening just moved the problem. When I was a student I recall a book entitled the New Science of Strong Materials. It was littered with cartoons and phrases such as "why wings bend" and the likes.
Had I still been a student I would have asked my Dad to buy me the Guy Gratton book for Christmas or my Birthday.
G
I think Genghis has it pretty well covered but to add some colour, here are a few details from my current job certifying a composite CS-23 / FAR 23 aeroplane:
The regulatory loadings are minima. We have chosen (as have others in the same class) to certify ours to +/-10g which means we will do the certification ultimate load tests, after the fatigue test, to the loads for15g. we will do that at 72 centigrade. The airframe used for all that testing will also incorporate some tens of points of deliberate manufacturing error or deliberate impact damage. In addition, the material properties values we use in calculations for the structure are based on low expectations derived from statistics from multiple sample tests (that's not a very scientific sentence, I am trying to convey the ideas for general readers). Those sample tests are on test pieces that have been conditioned in a hot and humid environment for several weeks. In summary, for composite aircraft, there are multiple layers of pessimism in the calculating and testing of the airframe.
One last thing, for about the last twenty years, seats, which for small aeroplanes tends to mean a big chunk of the front end of the aeroplane as well, have been dynamically tested for occupant injury protection (similar to the crash tests used in car advertising). There are probably videos on the internet if you look for them. Discovery Canada did a good piece on it in the airliner context in 2013 (but I'm only saying that because I was the technical talking head
).
Oh yes, books. I found Cutler and Liber's Understanding Aircraft Structures to be a good introduction.
The regulatory loadings are minima. We have chosen (as have others in the same class) to certify ours to +/-10g which means we will do the certification ultimate load tests, after the fatigue test, to the loads for15g. we will do that at 72 centigrade. The airframe used for all that testing will also incorporate some tens of points of deliberate manufacturing error or deliberate impact damage. In addition, the material properties values we use in calculations for the structure are based on low expectations derived from statistics from multiple sample tests (that's not a very scientific sentence, I am trying to convey the ideas for general readers). Those sample tests are on test pieces that have been conditioned in a hot and humid environment for several weeks. In summary, for composite aircraft, there are multiple layers of pessimism in the calculating and testing of the airframe.
One last thing, for about the last twenty years, seats, which for small aeroplanes tends to mean a big chunk of the front end of the aeroplane as well, have been dynamically tested for occupant injury protection (similar to the crash tests used in car advertising). There are probably videos on the internet if you look for them. Discovery Canada did a good piece on it in the airliner context in 2013 (but I'm only saying that because I was the technical talking head
).Oh yes, books. I found Cutler and Liber's Understanding Aircraft Structures to be a good introduction.
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Thanks Genghis, and Joe, Don't wish to steal the OPs thread but more on composites. I am currently working in crash testing. We undertook a project recently with some EU funding looking into embedding sensors in composite wind turbine blades for early failure detection. I also had a bit of a dabble into acoustic emissions with a group up in Aberdeen (Composite pressure vessels) our longer term aspirations was to embed the transducers into the windings. I don't think this happened on this project before I moved on. I do wonder if this has been applied to composite aircraft structures. We were imposing significant damage to these vessels before testing to ultimate to ensure the sensors were able to detect early fibre failure. The Design Authority had applied factors way over the top but I did understand the caution. A fine example of multiple load paths on a different level working in your favour.
My experience in airframes is only at the small (part 23) end. Sensors would be painful, they fail, their wires fail, their displays fail. On very simple low cost (relative to airliners and biz jets) aeroplanes, we would rather avoid them.
On our programme, the only structural sensors are for temperature. It is theoretically possible to get the airframe temperature above the safe limit for the material (it is pretty improbable, one would probably have to have an all black aeroplane parked in Death Valley in the summer then pull straight to 10g on take-off) so we are obliged to do that. Otherwise we are avoiding electrical content wherever we can.
On our programme, the only structural sensors are for temperature. It is theoretically possible to get the airframe temperature above the safe limit for the material (it is pretty improbable, one would probably have to have an all black aeroplane parked in Death Valley in the summer then pull straight to 10g on take-off) so we are obliged to do that. Otherwise we are avoiding electrical content wherever we can.
