"Stuart Grey" wrote in message
. ..

I put into Raymer's spreadsheet a few things like 619 mile range, max
speed 160 mph, payload weight of 1190 lbs, wing taper ratio 1.0 and I get
a whopping gross weight of 4322 pounds, not the 3000 pounds of the Murphy
Moose. The engine suggested by the spreadsheet was also huge.

Well, that's a bit of a disconnect. I got similar disconnects when I put
in sizing data from other airplanes. The Raymer spreadsheet gives much
heavier designs. What's with that?

Q1) Is this because the Raymer book recommends overbuilding so that the
typical home designer doesn't have the engineering skill to make it
lighter?

Q2) Are there any better books out there?

Please be kind. I'm a very sensitive fellow. (HA!)


I've only given Raymer's book a cursory look, but I wouldn't say he's overly
conservative. I do believe he assumes an effort to meet the intent, if not
the letter, of Part 23. That could introduce significantly more
"conservatism" than some kit makers have put into their designs. Would that
make up the difference you cite? Maybe, but probably not.

I expect a big factor in the difference is the basic assumptions made
regarding manufacturing materials and design. Aircraft design is a lesson
in compromise. Change one thing and it ripples thru affecting a dozen other
things. The fact is most a/c designs start out too heavy, too slow, and not
enough payload or performance.

Then the real work starts. Find a few little things to reduce drag. That
can give you a few extra knots or let you cut a few HP. Cut that HP and you
save structure and engine weight. With less to haul around, you can reduce
your fuel tanks and save some more weight or trade it for payload, and so
on...

I'd consider Raymer's numbers a starting point. The Murphy is one possible
end point. Give the same starting point to a Van or a Rutan, and you'll
have a different end point.

There are other books out there. They're different. Better is a very
subjective term.

Gerry