Airplane design.

I was looking at the design of the Murphy Moose:

http://www.murphyair.com/

They put the cost at about 100k$, cheap compared to the two Cessna's and
the Bush Hawk to which Murphy compares the Moose:

http://www.murphyair.com/Product_Info/Super/compare.htm

From their engine page, half of that cost is probably engine.

Empty weight is 1450 pounds.

At Boeing surplus, a sheet of aluminum runs about $1.64/pound (Not
including the Boeing discount). At 1450 pounds, that's only about $2400
worth of aluminum. The rest is labor (they half build the airplane),
mark up, insurance, support and so on. A really cheap guy who doesn't
have a lot of money, such as myself (who some say don't deserve to fly,
and maybe so...) might be tempted to get Daniel P. Raymer's book,
“Simplified Aircraft Design for Homebuilders” and take a stab at
designing something similar. (okay, not just this book but a whole bunch
of books, along with long visits to the FAA website.)

http://www.aircraftdesign.com/sadfh.html

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!)

Stuart Grey wrote:
I was looking at the design of the Murphy Moose:

http://www.murphyair.com/

They put the cost at about 100k$, cheap compared to the two Cessna's and
the Bush Hawk to which Murphy compares the Moose:

http://www.murphyair.com/Product_Info/Super/compare.htm

From their engine page, half of that cost is probably engine.

Empty weight is 1450 pounds.

At Boeing surplus, a sheet of aluminum runs about $1.64/pound (Not
including the Boeing discount). At 1450 pounds, that's only about $2400
worth of aluminum. The rest is labor (they half build the airplane),
mark up, insurance, support and so on. A really cheap guy who doesn't
have a lot of money, such as myself (who some say don't deserve to fly,
and maybe so...) might be tempted to get Daniel P. Raymer's book,
"Simplified Aircraft Design for Homebuilders" and take a stab at
designing something similar. (okay, not just this book but a whole bunch
of books, along with long visits to the FAA website.)

I would buy a set of plans and use an existing design. I would
consider wood and fabric and steel tubing as well as aluminum. I would
shun certificated engines or else buy a used certified aircraft and
restore it.

Because most homebuilders today are affluent retirees and yuppies the
market has become cost-insensitive. Most designs popular today are
designed around the overpriced museum piece Lycoming engine because it
is faster and less hassle to use one.

I would leave airplane design alone until I had completed a couple of
other people's designs and done some college level coursework in
mechanical engineering or allied subjects. I have my own ideas on what
would be a good airplane but I know I'm not ready to do them yet.

Bret Ludwig wrote:
Stuart Grey wrote:

I was looking at the design of the Murphy Moose:

http://www.murphyair.com/

They put the cost at about 100k$, cheap compared to the two Cessna's and
the Bush Hawk to which Murphy compares the Moose:

http://www.murphyair.com/Product_Info/Super/compare.htm

From their engine page, half of that cost is probably engine.

Empty weight is 1450 pounds.

At Boeing surplus, a sheet of aluminum runs about $1.64/pound (Not
including the Boeing discount). At 1450 pounds, that's only about $2400
worth of aluminum. The rest is labor (they half build the airplane),
mark up, insurance, support and so on. A really cheap guy who doesn't
have a lot of money, such as myself (who some say don't deserve to fly,
and maybe so...) might be tempted to get Daniel P. Raymer's book,
"Simplified Aircraft Design for Homebuilders" and take a stab at
designing something similar. (okay, not just this book but a whole bunch
of books, along with long visits to the FAA website.)


I would buy a set of plans and use an existing design. I would
consider wood and fabric and steel tubing as well as aluminum. I would
shun certificated engines or else buy a used certified aircraft and
restore it.

Because most homebuilders today are affluent retirees and yuppies the
market has become cost-insensitive. Most designs popular today are
designed around the overpriced museum piece Lycoming engine because it
is faster and less hassle to use one.

I would leave airplane design alone until I had completed a couple of
other people's designs and done some college level coursework in
mechanical engineering or allied subjects. I have my own ideas on what
would be a good airplane but I know I'm not ready to do them yet.

I think half the fun is doing the design. It is why I became an
engineer. :-) True, I'm just an electrical engineer, but I am familiar
with airplane design and certification to a small degree. I thought
about going back and getting an MS in Aero engineering, but you know,
desgrees are just for people who want to show to someone else who
doesn't know squat about the subject, so they'll give you a job. Anyone
with half a wit and the will can learn just about anything on their own.

So, what's the deal with Raymer's book? Does it have an excessive margin
of safty, or was I doing something wrong?

"Bret Ludwig" wrote in message
ps.com...

Because most homebuilders today are affluent retirees and yuppies the
market has become cost-insensitive. Most designs popular today are
designed around the overpriced museum piece Lycoming engine because it
is faster and less hassle to use one.

or designed around over-rare VW engines that 20 years ago were in abundance

(till homebuilders got to them!)

Stuart Grey wrote:

So, what's the deal with Raymer's book? Does it have an excessive
margin of safty, or was I doing something wrong?

After playing with the spreadsheet for about 15 minutes, it looks as
though the weight calcs are VERY sensitive to a few parameters that
aren't well explained in the spreadsheet. Unless you know exactly what
those parameters are, I don't think you should trust the weight #'s that
you get. Maybe the book has in-depth explanations of what the
parameters are and how to set them.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2006

Tater Schuld wrote:
"Bret Ludwig" wrote in message
ps.com...

Because most homebuilders today are affluent retirees and yuppies the
market has become cost-insensitive. Most designs popular today are
designed around the overpriced museum piece Lycoming engine because it
is faster and less hassle to use one.

or designed around over-rare VW engines that 20 years ago were in abundance

They're still in abundance. Get a copy of the VW magazine and you can
see that there is a worldwide VW parts industry based around importing
VW parts from every country that makes them. All the hot rod stuff is
California.


The problem with the VW is it was never designed to directly turn a
prop and a direct drive VW combines every disadvantage of LyCon direct
drive engines with all those of the VW in the car, and more. A blower
cooled VW with redrive would work, but the general perception is, why
bother?


Homebuilt aircraft have no effect on non-aircraft parts prices they
use. Even if 100% of homebuilts used VW power there would still be 10
times as many aircooled VW cars licensed in Los Angeles County as
homebuilt airplanes nationwide. I have mentioned the 3.8 Javelin Ford
to junkyard operators and Ford mechanics over the last 20 years
probably 100 times and 99 of those they'd never heard of such a thing,
were amazed anyone would fly _that_ engine, or flat out didn't believe
me.

"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

-----------much informative stuff snipped---------
(till homebuilders got to them!)

Very true! Plus the ones that found their way into and onto dune buggies,
small airboats, old BMW motorcycles, and heaven knows what else. The same
problem is true of Corvair engines, of which I've even seem one mounted in
the bed of a pick-up truck and driving an electric generator. The BMW 600
and 700 automobiles were never popular enough to have single seat aircraft
designed around them, at least in the US, and the little air cooled Subaru
engines were probably gone from the stock-piles before the Part 103
ultralight rule allowed 254 pounds empty. I suspect that the BMW and Soob
engines would have been chosen over the 1/2VW if availability permitted.

Peter Dohm wrote:
-----------much informative stuff snipped---------
(till homebuilders got to them!)

Very true! Plus the ones that found their way into and onto dune buggies,
small airboats, old BMW motorcycles, and heaven knows what else. The same
problem is true of Corvair engines, of which I've even seem one mounted in
the bed of a pick-up truck and driving an electric generator. The BMW 600
and 700 automobiles were never popular enough to have single seat aircraft
designed around them, at least in the US, and the little air cooled Subaru
engines were probably gone from the stock-piles before the Part 103
ultralight rule allowed 254 pounds empty. I suspect that the BMW and Soob
engines would have been chosen over the 1/2VW if availability permitted.



Nope.

The nonautomotive uses of VWs-the most creative was probably the 2
cylinder integral air compressor-and dune buggy building made only the
smallest dent in VW supplies. They sold probably fifteen million
aircooled VWs in this country and probably fifty thousand dune buggies
were the high point.

The BMW 600 and 700 used BMW motorcycle engine cores and the bikes
were far more popular.

The Citroen 2CV twins and GS fours were used in homebuilt airplanes
and kit bikes in Europe. Same deal.

Corvairs have probably had the highest percentage of predation along
with the 215 aluminum GM V8 but in both cases more have been summarilyy
scrapped than all hobby uses combined. Corvairs are still not
scarce-any old smallblock brings more money most of the time.

Just as a friendly challenge to anyone foolish enough...

Design an airplane within the 254 pound weight limit.

Clean sheet of paper.
254 pounds empty weight.
Your choice of engines, design, materials.

Where would you start?

Richard