Fatigue life in Aircraft (particularly Fins)

[Paul K]

I remember seeing some kind of 'G' counter inside the BBMF Lancaster, mounted on an overhead bracket.

[Motormouse]

just as an aside, "100FI" is not necessarily the end of the line,but does need the design authority apporoval to continue thereafter; we had Phantoms that had exceeded 100FI, and after being modified by BAe,were good for up to 140FI

Modification to post 100FI standard depended on the results of a structural sampling survey,(basically we drilled little holes all over the structure and pointed a borescope inside to check certain areas).

ttfn

Pete

[jonesey2k]

What about aircraft like DC-3's? A few of them have been going for quite a while!

[SkippyBing]

Not 100% sure about DC-3s however I know some aircraft due to their design have no lifed parts as such. I think the Hiller HT.2 springs to mind, and I'm sure the wing structure of the DC-3 is built like the Forth Bridge so that combined with not being pressurised means it's probably pretty much bullet proof.

[forthbridge]

Generally speaking, do smaller aircraft suffer less?

Would a 75% size Trident (for example) - built 'as strongly' as the full sized version last longer?

[Garry Russell]

Surely a 75 scale Trident built like a full size one would be overweight

It would be easy to build an aeroplane so strong as to never fall apart but may not be able to fly. The problem is trying to build it with minimal structure because it has to be light.

Garry

[TSR2]

if you think about it, the size is not all relivent, the 25% smaller would mean that the compents would be 25% weaker etc. Its not strictly true, but if you were to build a acurate scale model (which they used to do when testing some of these aircraft) stress would happen in the same areas at a rate proportional to the finished full size product. There are plenty of engineers here who can elaborate further.

[DaveB]

I think the inner wing sections on the DC3 are built from solid The main areas of stress seem to be the outer wings and these are swapped at regular intervals to preserve/slow fatigue life

ATB

DaveB :tab:

[forthbridge]

Interesting....

How about wings? Aside from the detail design, would a wing of the sazie of, say that on a VC10 get more fatigue if it held podded engines than having them mounted at the tail?

Similarly - I think the Nimrod is an example here.. (does the MRA4 have a larger wing??) - if an aircraft is fitted with a largr wing, will that extra area 'create' more stress/fatigue?

[TSR2]

These are all very subjective Jim. A wing which has engines mounted on it will be supject to different stress that one without. The comet / Nimrod / Victor / Vulcan have the engines actually mounted in the main wing box, so are a very different beast to say a tristar.

Engineers have to take all of these things into consideration when designing an aircraft. So if the design spec was to requite a wing which would have underslung engines and a give fatigue index, it could be done, but it would offset something else somewhere else in the design... ying and yang. Its all about balancing the engineering so that the aircraft meets or exceeds its design specs.