MONTY THE ANSWER MAN ARCHIVE... METAL
FATIGUE
METAL FATIGUE...
(5699)
From: Rodger Herrington
<rodger@mail.utexas.edu>
Subject: metal fatigue
I was just curious about a problem that I discovered in another aircraft
type and if it could relate to the Swift. I have never heard of this problem
in a Swift and I have tried to read all of the AD and accident reports
that I could find. Has there ever been a problem, to your knowledge, of
metal fatigue in a Swift, particularly the higher stressed areas like
a spar? Visual inspections will not reveal the type of fatigue I'm curious
about. Only an electron microscope, it seems, will discover it. The other
aircraft was used for aerobatics and, I'm sure the situation would not
apply to most Swifts, but I was still curious. I realize that a well maintained
plane will last a very long time with no problems (My family has maintained
the same plane for 18 years) and, obviously, many Swifts are still flying
:) Thanks for any information! -- Rodger Herrington
Rodger,
I don't believe that (metal fatigue) has ever been found in a Swift wing
spar. Any problems in my opinion are more related to corrosion and prior
damage. There have been several cracks found in the horizontal stabilizer
spar, usually at the inboard rivet of the upper front spar. I had a Swift
with a not-too-smooth Beech-Roby prop. It was kind of scary to look back
at the horizontal in flight. It shook. I mean constantly, so I removed
the prop and had it overhauled, several lag bolts were broken in one blade.
(which retain the wooden blade in its metal ferrule) Inspection revealed
no cracks in the stabilizer spar. For similar reasons I limit practicing
stalls, the stabilizer shakes then also. Incidently, the early (GC-1A)
spars are .050 aluminum, the later ones are .065. I must also mention
backing the airplane into the hangar by pushing on the stabilizer must
be done with discretion. I feel the horizontal stabilizer is the most
likely place where 50+ years of use and abuse will reveal itself first.
-- Jim
(Editor's note: Don Bartholomew's
opinion was also solicited on this question. His reply follows...)
From: Don Bartholomew <spectro@nanosecond.com>
Subject: metal fatigue
Here are some thoughts and ideas to your question. In my dealings with
the Swift, I have seen a few problems caused by metal fatigue. These were
isolated cases of skin cracking due to vibration, and one noteworthy one
is the front horizontal spar cracking through the rivets in the inboard
rib. Both of these conditions were enhanced by rivet holes which acted
to concentrate the stress.
Fatigue is caused by two
things: load reversals or cycles, a component being loaded or stressed
and then unloaded, and the load or stress applied during these cycles.
The more load applied, the fewer cycles are needed to cause fatigue. The
less load applied, the more cycles are needed. Anything that causes a
stress concentration (rivet holes, scratches, nicks, etc) will contribute
to the fatigue. The small skin cracks were probably fatigue or work hardening
due to vibration of an unsupported panel. The vibration causes a low load,
high cycle condition. This can be from engine/prop vibration or vibrations
from air blowing over a surface. As far as fatigue of a major structure
component, the only problem I have seen is the horizontal spar crack.
This would be a high load and fewer cycle condition. The loading here
would be due to normal flying loads, gust loads, and even improper ground
handling.
I am not familiar with any
other areas of problems with the main structure. This is probably due
to a number of things. First, the load applied during normal flight is
fairly low compared to the load the plane is designed for. Second, most
Swifts are fairly low time compared to other aircraft that do see fatigue
problems. Acrobatics does put a load on the structure but the cycles are
fairly low. For example, a loop will put about three G's and the two cycles
on the structure during the initial pull up and the pull out at the bottom.
Of more concern to me is flying fast in turbulence. You may see four or
five G's and five or six cycles in a couple of seconds. The loads and
cycles are cumulative. Each one adds to the total and there is a finite
number before something will break. -- Don Bartholomew |