Earlier, (Shin Gou) wrote:

Bob, could you tell me more about the constrains of the RV-9A wing on
its current fuselage?

An RV builder who is familiar with both RV-6 and RV-9 could tell you
more about that than I ever could.

In specific, I can only repeat some of what I've heard that seems to
make sense. That is, that the longitudinal location of the RV-9 spar
is constrained lightly by the original design of the RV-6 fuselage
(elements of which of course Vans is at liberty to change) and
constrained more tightly by the size and shape of the pilot and their
relationship with the seat and controls. A similar fuselage design
will have similar general constraints - but the devil is in the
details.

I like Rotax 914/912s and Jabiru 3300 for their light weight and fuel
efficiency. If a lighter and better engine can do the same job as an
O-235, why not?

Having spent well into the five digits on composite aircraft tooling
and with the end barely on the horizon, here's the one bit of specific
advice I feel qualified to give:

If the RV-9 is anywhere near what meets your needs, buy the kit and
build it as shown in the plans. Take the extra money you would have
spent developing a new fuselage, and put it in the bank. It will way
more than cover the extra fuel burned by the O-235. The O-235 might
seem old-tech and sound like a tractor. But its extra oomph below
10000 feet will make more difference than you might expect in how the
airplane feels like it performs.

You will have the good feeling that comes from building airplane
parts, and you will not have the many sleepless nights that come from
wondering whether something you are developing is going to work as
planned. Be guided by the hand; your building experience will be
happier, quicker, and more peaceful if your mind is at ease and your
fingers are busy.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24