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Amen, Brent.
Strength is only half of it; stiffness
i.e. torsional or bending frequency tuning is probably more important...
especially balance of the control surfaces. Sympathetic vibration at some
(unmeasured) frequency, which one does not want to match some energy
input from engine torsional pulses, or airlow pulses from prop blade,
etc.
I had an interesting conversation
with the survivor of a flutter (unbalanced but irreversible horizontal
tail) failure at about 500 mph and going up through 100 ft. (one hundred
feet) altitude. Like, hummmmmm-BAM. His black XP-89's tail came off at the
star, his ejection seat failed to fire, Gs locking him down, then one wing
snapped off and spun the fuselage throwing him out. His guardian angel smiled a
second time causing the 600 ft/sec airflow to rip open his chute before he could
pull the ripcord, blowing out two panels... so he could land in a lone peach
tree in Hawthorne, CA, thence to the hospital for dislocated shoulder, leg,
broken pelvis, etc.
The pitch control was irrevesible, but
the hydraulic lines back to the actuators could flex a little. Flex, as
in vibrate....
Terrence
N211AL L235/320
----- Original Message -----
Sent: Thursday, April 24, 2008 09:52
AM
Subject: [LML] Re: Window Strength
"I am sure 8' deep girders will be
sufficient." Leon Moisseiff, Chief Engineer, Tacoma Narrows Bridge
"If
you cant find 20' lengths of All-Thread then substitute two 10' lengths"
Jack D. Gillum, Hayatt Regency hotel "Tea Dance" walkway
designer.
"Sure, it's a little stronger with the window bonded
in, but that small amount of extra strength is not required. The
fuselage is strong enough even with the big hole in it." Rob Wolf
Rob
is correct that the static strength of the fuselage is determined without the
windows BUT the windows do have significant "in plane" (npi) shear strength
which will have an effect on the torsional stiffness of the fuselage. This, in
turn, effects the natural frequency of the empennage. Making the fuselage less
stiff by decoupling the window reduces the empennage flutter margin. Only
analysis and testing will tell us by how much. Pushing the aircraft faster
with bigger engines and making the fuselage less stiff will eventually
result in structural failure. I know of at least one IV -P that disintegrated
in a 0.63 Mach dive.
Rob may be correct that the structural effects are
negligible but his confidence is unsupported by analysis or testing. I doubt
he has the factory's agreement on this matter.
Prudence, and
experience, dictates a less cavalier approach to structural
modifications.
Regards
Brent Regan
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