Brian

AAC/USAF did the same thing. WWII and prior birds were built hell for
stout (over built)

After War they started building light as they could and test flew
until they broke and just fixed that part. They ended up with the best
performing A/C at the lowest gross weight.

Big John



On 29 Sep 2004 11:49:20 -0700, (Brian Case) wrote:

"John Harlow" wrote in message ...
Chris wrote:
How do aircraft manufactures determine the maximum gross weight for a
given plane? I know I've come across articles that mention on a cool
day you can go over the max. gross weight by a percentage, I think
20% was thrown out there but not sure. Then again, on a hot day you
might not get any where near that. Is it determined by a standard
temp?

Which articles suggest it is OK to fly a plane over gross?

AC 21-4b special flight permits for operation of overwieght aircraft

for one example.

I do believe there is are waivers for operations in Alaska and for
operations with floats as well but I haven't found the documentation
to prove it.

That being said and having spent 10 years involved in aircraft design
I can easily say that Manufactuers determine Gross weight based on a
number of factors. Primarly the limiting factors are minimum aircraft
performance, Cost and Reasonable aircraft Structures.

Many times the Gross Wieght is determined by the Maximum allowable
stall speed for the aircraft.

Say I were to design a 1000 lb airplane stressed for acrobatics
(6G's). Lets say the wings weigh 200 lbs so the non lifting weight is
800lbs. To redesign this same airplane into a Less than Normal
Category airplane(just for easy numbers) (3G's) all I have to do is
change the manual to limit it to 3g's and I can double the non-lifting
weight of the aircraft so the new Gross wieght becomes 1800lbs. Likely
the only part of the aircraft that has not been designed to handle
1800 lbs at 3G's is the landing Gear which was designed for only
1000lbs on the ground and for landing.
Of course the take off and landing distances will increase with the
new gross weight as will nearly all performance speed except Vne.

Common Myths:

Aircraft are required to be designed to 1 -1/2 times the rated load,
so there is lots of safety factor built in.
Actually: All that is required is that aircraft be designed to
repeatly handle the rated load without damaging the structure.
Additionally the structure must not fail if subject to less then 1-1/2
times the rated load, However the strucure may be damaged if the rated
load is exceeded. Wo if I take my aluminum airplane rated at 6G's and
do an 8G pull out the wings will probably not fall off. However it
will probably bend the wings and leave them permantly bent. I have
seen a C-182 that this happened to. Aluminum makes a great material
for aircraft because it complies almost exactly to the regulation. You
can load it up to the limit load with no permanent damage. And you can
load it up to the ultimate load without it breaking however it will
permanently bend it.

Myth#2. Manufactures overbuild the aircraft.
Actually: a former Cessna Engineer told me that the way they
designed a structure was they would have the engineer design the
structure they would then load test it. If the structure did not fail
on the 1st test they fired the engineer for overbuilding it. After it
failed the reeinfored the area that failed and tested it again. They
would repeat this process until the structure just barely passed.

Every lb the manufacturer could take off of the airplane is 1 more lb
of performance they could sell and 1 lb less of material they had to
buy. Cessna didn't paint their airplanes for years in order to save
the weight of the paint. The only reason they changed is they could no
longer get consistantly unscratched aluminum to work with.

Brian