Michael wrote in
:

On May 30, 6:25*pm, Dudley Henriques wrote:
I would respectfully disagree. An Extra 300, as do most high
performance aerobatic airplanes has neutral static stability, but
your vanilla Cessna or Piper with dihedral is designed with positive
static stability in mind.

The positive static stability, when present, is slight. More to the
point, it is only slightly positive around the zero point. Once an
excursion in roll gets past a few degrees, the stability is negative
(for a while) and the new stability point is in a significant bank (or
non-existent).

As for dynamic stability, it really isn't much of a factor in lateral
stability. The ailerons if mass balanced around the hinge line by
weight, and if comparatively free in movement, usually assure that
the pure lateral movement is heavily damped.

And that's the point. Dynamic stability is generally mildly negative,
and once a slight displacement from zero occurs, the restoring force
from the mild positive static stability starts a very mild oscillation
that will eventually take you outside the area of positive static
stability. Once that occurs, dynamic stability is indeed no longer
important.

Even a plane with comparatively good lateral stability will eventually
wind up in a spiral once something like turbulence disturbs it. In
some cases it won't happen - combine a very stable plane with very
smooth air, and it might fly wings level for a long time. It has
happened. Also note that the stability improves with lower weight
(this is true of the longitudinal axis as well - that's why the
allowable cg range on most light airplanes is wider at lower weights)
so we do have these stories of planes without pilots flying just fine
for hours.

I recall one particular sad incident where a Pilatus Porter dropping
jumpers lost the yoke - it literally came off in the pilot's hands.
The pilot elected to bail out (which was, IMO, the wrong call - a
plane can be landed with rudder, throttle, and trim). His parachute
malfunctioned, and the pilot died. The plane eventually ran out of
fuel and 'landed' in a field. After a few minor repairs, it was flown
out.

These things do happen, but they are the exception. Under most normal
conditions, the combination of neutral or weakly positive static
stability and negative dynamic stability in the lateral axis will
eventually put the plane into a spiral.

Remember, we're dealing here with a dead engine. I'm still going to
stick with the spiel :-)) that says with a light GA airplane with
positive lateral stability built in with the normal dihedral found on
such airplanes and the engine dead, we're going to need a source for
an outside the system force strong enough to offset the countering
dihedral to any roll input to initiate the roll or yaw (or coupling
if you wish) that would end up with the aircraft banked enough to
counter the dihedral correcting it back into the normal phugoid I'm
expecting.

Sure - but that source will be found in the normal turbulence found on
most days. There are also additional factors.

First off, any side-by-side airplane with only one person on board is
going to be slightly out of balance laterally. Any plane with fuel
feed from a single wing tank, or a less-than-perfect 'both' feed
(which is most of them) will develop an imbalance. So what we need to
postulate is a plane that is tandem seating with a header tank (like
an old-style Champ or maybe a Cub) and then, on a really smooth day,
it might actually stay wings level.

If the aircraft has dihedral, it has positive static lateral
stability.

That's not necessarily the case. There can be other factors that
affect stability that would overcome a small amount of dihedral.
Remember that most airplanes have strong yaw stability, and that
weakens roll stability because yaw and roll are strongly coupled.

That's interesting. I never knew that. A pretty good primer on
stabilit
y issues can be found he
http://selair.selkirk.bc.ca/aerodyna...ity/Page5.html
I know the site. His stuff is generally very good. I do have some
very minor issues with his presentation on a few things.

I also like the site. The particular link includes much of what I
posted, including the assertion that most light planes left to
themselves will wind up in a spiral.

Of course as you mentioned elsewhere, this isn't the sort of
discussion you would have with the average FAA ops inspector. I knew
a couple who would have appreciated it in Houston. Neither one is
still with the FAA.


Yeah, sadly this is true. I'm just reading up to renew my instructor
cert and have been reading some of the circulars on the FAAs website.
Most of them are excellent, but I was just reading one about stalling
cross controlled, for instance and it's crap.. Having said that, I
suppose it's all good enough for basic private knowledge..
Fortunately, the examiner falls into the pragmatic category.

Bertie