Airplane design.

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?

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

Marc J. Zeitlin wrote:
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.

Yes, I needed to understand the parameters, and they were explained
better later in the book.. The book is kind of pricy for the number of
pages, but on the other hand, it gets to the core of what you need to
know without a lot of (unnecessary) theory and explaination, so you're
getting good bang for the buck. I can relate to that.

My problem came from the parameter "a". The book gives 1.19 for a single
engine, metal design - and this is the paramter I used. Way in the back
of the book, on page 135 (yeah, it's not a thick book) it shows how the
parameter a is calculated. You should find similar planes to the one you
want to build, and find the ratio of their empty weight to fully loaded
weight, and graph that on a chart against the fully loaded weight.

Among home built planes, there is a large cluster near 0.6-0.65 for
We/Wo, resulting in an a of over 1.2. However, there is another cluster
near 0.45 to 0.59 that result in an a closer to an a of 1.0 or below. I
think the airplane I want to build is in this grouping and not the
other. I suspect the difference is the smaller grouping is a high wing
with struts, while the other grouping is a low strutless wing.

It appears to be worthwhile to read the entire book. :-) Doh!