From: highflyer <highflyer@alt.net>
Newsgroups: rec.aviation.homebuilt
Subject: Re: Aft CG?
Date: Fri, 11 Dec 1998 08:33:28 -0600

David Lednicer wrote:

> The concept that the CG must be forward of 25% MAC is just a rule of
> thumb.  What really matters is where the aircraft's CG position for
> neutral stability lies.  Many aircraft can be quite safely flown with
> the CG aft of 25% MAC.  The only way to find out if this applies to your
> aircraft is to get the recommended CG range from the designer.


David's advice is excellent.  Unfortunately there are many homebuilt
designs that were not designed by aircraft designers.  The old maxim
says that if it looks like an airplane it will fly like an airplane.
Sometimes that is even true.

Over the years I have consulted with a number of folks who designed
more or less successful homebuilts.  Some were one of a kind, and
some became popular and sold thousands of sets of plans.  Some of the
designers understood quite well how the CG range is determined.
Some did not.  In every case the 25% MAC rule did put them into a
location where the aircraft would be flyable, even if the CG range
was less than we would have hoped it would be.

With some of the designs, determining the MAC ( mean aerodynamic
chord ) was not a trivial undertaking.  For example, can anyone
quickly tell me where the 25% of MAC point is on a VariEze or a
Cozy.

HF