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!