Cherokee getting fat

Does anyone know how they compute the maximum allowable take-off weight
(and, thus, the useful load) of a Spam Can like we fly? I assume it's
got something to do with minimum climb performance, but what are the
parameters?

I don't think there's a general answer. I think the answer has to be
specific to the model you're asking about. For one, it might be climb
performance, for another the wing spar strength, for another landing gear
strength, for another control authority, etc.

It's one of those bottleneck performance questions where you beef up one
area only to find out that just gets you to the next bottleneck.

I don't think there's anything in the public record that specifies what
the limiting parameter is for given type certificate.

Well, taking our Pathfinder as an example, somehow Piper managed to get a
1400+ pound useful load out of a 235, which is a huge difference from the
180.

True, that's a 30% horsepower increase, and Piper did have to beef up the
sheet metal in some places, but essentially they got the greatest useful
load of any GA 4-seat plane by simply slapping a 6-cylinder engine on the
front of an Archer.

But how did they determine what that useful load was? Did they keep adding
lead ingots until the plane couldn't climb more than "x" feet per minute?
Did they keep landing it at greater weights until the gear failed? You
always hear stories about the guy who "flew out with a moose in the back" --
which implies that useful load is set conservatively, with at least some
margin or fudge-factor built in.

I kick myself for not asking this at the Cherokee Pilots Association fly-in
last year. One of the original Cherokee designers was the keynote speaker,
and I'm sure he could have answered this question. (Chuck Suma, president
of New Piper, is the keynote speaker this year -- so maybe I'll ask him...)
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay Honeck wrote:

But how did they determine what that useful load was? Did they keep adding
lead ingots until the plane couldn't climb more than "x" feet per minute?

They start by calculating the performance. Then they do take it up with weight.
Maule uses 50lb sacks of lead shot. Frequently the limiting factors are stall
speed and the requirement that the plane be able to climb with full flaps on a
standard day. Maule had to reduce the flap setting on the MX-7-160 after tests
showed it wouldn't do that at max gross.

Did they keep landing it at greater weights until the gear failed? You
always hear stories about the guy who "flew out with a moose in the back" --
which implies that useful load is set conservatively, with at least some
margin or fudge-factor built in.

Any light plane will fly when overloaded. Usually they can get off the ground at
about 1.5 times max gross, and this is sometimes done for overseas ferry
flights. You know from your experience in that Cessna 150 how this affects the
performance -- long takeoff runs and poor climb rates. Sometimes the W&B
envelope gets narrower at high loadings also. IMO, this is not a margin or
fudge-factor.

George Patterson
Why do men's hearts beat faster, knees get weak, throats become dry,
and they think irrationally when a woman wears leather clothing?
Because she smells like a new truck.

Any light plane will fly when overloaded. Usually they can get off the
ground at about 1.5 times max gross, and this is sometimes done for
overseas ferry flights. You know from your experience in that Cessna 150
how this affects the performance -- long takeoff runs and poor climb
rates. Sometimes the W&B envelope gets narrower at high loadings also.
IMO, this is not a margin or fudge-factor.

Thanks, George.

I guess it all comes down to what you're comfortable with, and what your
plane will do. I've flown my 235 at gross many times, without difficulty,
which always makes me wonder what the REAL capability of the plane is...

With our plane, though, we run out of internal room long before we run out
of weight-carrying capacity. It's a nice change, after all those years of
sweating weight and balance.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"