Originally Posted by
Shaft109
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?
I'll give this a shot, as I used to teach aspects of this stuff.
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.
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