Ok, me again,
I will try and answer some of the (very valid) questions asked.
The question was asked "Why would XYZ build a plane with a shorter life than ABC ?"
Ok, without letting it sound like aerodynamics 101, here are SOME of the reasons this happens:
With airframe design, like most things in life, there is always a tradeoff, here we have a case of a plane that was designed to be one of the fastest (if not THE fastest ?) ML around, that along with having a good range.
So, staying within some limitations like the engines that are available and the 450kg limit of ML's, how do you make your plane faster than the opposition ?
There are a few way of making a plane faster, you can decrease the wing area (span usually), you can make the wing thinner (and use a airfoil, like a laminar flow one, that is more suitable for high speed flight but has other disadvantages like the need to keep it clean, accurate and smooth or else it does not function correctly), and you can make the plane lighter.
Making the wing area smaller helps with the top speed, the tradeoff is a higher landing/stall speed.
Making the wing thinner helps with speed as well, the tradeoff again being a higher stall speed AND with a thinner wing it is more difficult to make the wing strong, a thin wing has a lot more chance of flexing and is thus more prone to fatigue. This is ONE of the reasons a faster plane MIGHT have a shorter life.
Making the plane lighter helps a lot, remember plane needs less lift to fly and generating lift creates drag, thus, the lighter plane flies faster all being equal. How do you make a plane lighter ? Well, clever structural design helps, using "exotic" materials like Carbon or Boron fibers helps a lot, and making all components thinner/smaller helps as well. The tradeoff here is again that you might end up with a more flexible structure, remember a flexible structure will often be stronger than a rigid one (we have all seen a big jet's wings flex
) BUT, remember a more flexible airframe is more prone to fatigue, and thus a shorter expected life.
Combine the above with a plane that now flies faster than the opposition (as planned) and you have an airframe that has to handle higher loads due to the speed, think of the stress involved flying through turbulence at 70k compared to 110k. This is by the way why you will often see faster planes having a "max maneuvering or max turbulent air penetration speed" that is often less then the cruising speed the plane is capable of. This is a way of trying to manage and lower the stress on an airframe and extend the life. So, again, all this adds up to an airframe that might have a shorter life.
All the above choices are made when the plane is designed and are accepted tradeoffs that the engineers accept. We see this in other areas as well, we know how well the Robinson R22/R44's fly, but have you looked at the life expectancy of those helicopters before a total (expensive) rebuild ?
Sure, you can build fast planes, you can build strong planes and you can build long lasting planes but combining the 3 ? NOT that simple.
Other problem areas with composites are quality control (Maybe why the factory wants to inspect an airframe after 1000h ?) and factors like UV degradation of epoxies, It is very difficult for the designer or factory to predict how much UV exposure your plane will have during its life. Also important here is the heat it will be exposed to, almost all epoxies go soft in heat and the constant cycle of heating/cooling causes chemical breakdown and changing properies in epoxies, again the designer cannot predict the exposure and to play it safe has to build in safety factors and cannot commit to long airframe life.
A composite plane flying in Europe will almost certainly have a longer airframe life than one flying here in South Africa, or parts of the USA for that matter.
Ok, of the soap box again :D
Safe flying,
Rudi
