Some of us on the list are getting hung up on the CG range as a
percent of mean aerodynamic chord. To an aircraft designer,
expessing the CG range this way is used for preliminary design purposes
only --we want to make sure that we have a usable product that can
support real world loading conditions, and also to know where the
landing gear needs to be. (It's bad when the airplane falls on its
tail when the pilot throws a suitcase in the back...)
Once the airplane is built and in flight test, we determine the
real CG limits by ballasting the airplane to different CGs and doing the
following:
1) Determining the maximum forward CG by determining where
tail power becomes insufficient to flare on landing. This is
actually not hard to calculate and simply verify during flight
tests.
2) Determining the aft CG at which the handling qualities
become unacceptable. As I mentioned in an earlier posting, this
depends on the intended pilot population (is it a trainer or an advanced
fighter jet) and its mission (is it a trainer, a Bonanza-class airplane
or an airliner). This is very difficult to determine analytically,
hence the intial reliance on rules of thumb. (Since you can't
calculate it precisely, you might as well use a quick estimate that's
just as good.)
Again, rules of thumb (such as a range of 15% - 30% MAC -- this
depends on airfoil section and the aft-loaded NLF airfoils are different
from the NACA 65-series, for example) are used only to estimate those
actual CG limits which are verified in flight test.
Scott Krueger is abolutely right when he says that there are
multiple factors which influence pitch sensitivity, not all of which
were mentioned in the CAFE reports. First and foremost is
CG. Other factors are (in the 320 series) the elevator bellcrank
arm and the tail size. And then there are odd airplanes like Dom
Crain's, which has the pitch damping of a large tail but (I'm guessing)
the tail power of the small tail. He likes it.
Here's one suggestion to non-builder owners of 320 and 360
airplanes. If you think the pitch sensitivity is too high, measure
the distance between the elevator axis of rotation and the bolt hole to
which the elevator pushrod is attached. There were two versions of
this bellcrank. Original ones had a longer distance and the newer
ones were 1 inch shorter. I have a copy of the Lancair factory
newsletter in the hangar addressing this change and I'll post it in the
next week or so. The shorter distance gives higher stick forces
but less control stick throw. You will notice the stick force
change but probably not the change in stick throw. You can modify
your bellcrank without replacing it, if you want. You just drill
one hole, although you'll have to remove at least one elevator (maybe
both) in order to do it, and you should check the rigging afterwards to
make sure the neutral position is still where you want it.
Elevator range of motion will be unaffected but with the elevator in
trail the stick might be tilted forward or aft a little.
- Rob
Wolf