WIX Archives
No practical difference!
Posted by bdk on Wed Sep 18, 2002 03:18:57 PM
In reply top One for the real teccie guys! posted by Christer on Tue Sep 17, 2002 05:59:36 PM
The life expectancy is morte related to storage environmental conditions (corrosion) and physical (crash or storage) damage.
"Metal fatigue" is cumulative and depends on the stress level and the number of cycles. Most steel alloys however will never fatigue if kept below a particular stress level.
A B-17/Privateer/C-130 doing firebombing is in the worst environment possible. It is flying in rough (gusty) air at high "Q" levels (dynamic pressure), so both the number of cycles and the stress level are high. In cruise at altitude the air is smoother and the "Q" is low due to the thinner air. Pressurized aircraft are also subjected to pressurization cycles which can accelerate fatigue damage and cracking, especially in the presence of poorly designed features like the windows of the early DeHavilland Comets.
The Douglas Invader ended up being used as a low level attack aircraft (and as a firebomber), so many had a very hard life for the reasons noted above. The invader also had an Airworthiness Directive to trim a wing root fairing that was cutting into the spar over time and causing a stress riser (which leads to cracking at lower stress levels).
The actual G-levels that the pilot pulls are usually not harmful unless pulled repetetively and near the G-limit as was the case on some T-34's discovered with cracked spars. These aircraft had been used for aerobatic training and later for civilian air combat. Both these applications are also at high "Q" levels. There is also the opportunity for students (and in the case of civilian air combat- completely unskilled pilots at the controls) to overstress the aircraft repeatedly. Added to the corrosion of years of outdoor starage (some in Florida) and the use of much larger engines than the aircraft were originally designed for (from 225 HP originally to 260-325 HP), and you can imagine the result (AD notes requiring spar replacements and/or revised operating procedures).
The bottom line is that if you operated your Spitfire or Mustang as an airshow demonstration aircraft, you could do so nearly indefinitely without concern for metal fatigue. That assumes you don't have other mitigating factors like corrosion, crashes, or frequent G-overloads. Don't forget that most warbirds are flown without guns or armor plate, so they are considerably below the weights they were designed to fly at. This leads to much lower stress levels operationally than they were designed to withstand.
My experience is that typically the airframe parts that wear out are the engine cowlings and other parts subjected to fretting corrosion from engine vibration.
Brandon
: Anyone who knows the life expectancy of an airframe, say a
: Spitfire or Mustang, which has been totally rebuilt to ze
: ro airframe hours?
: Which will the difference be if it?s flown gently to 2-3 G
: 's rather than to 5 G's?
:
: Thanks in advance,
: Christer