On 9/4/2012 10:13 AM, JohnDeRosa wrote:
I had been told, anecdotally, that having maximum aft CG based on PIC
weight is "best" for glider performance. I don't hope to understand the
aerodynamics of why this is so (but it would make an interesting read in
"Soaring").

It has to do with the theoretical minimization of trim drag, a conceptually
simple idea with "the usual" aerodynamically and mathematically complex details!
- - - - - -

My point is that if a glider was at max aft CG, and then you add a tail
dolly, the glider could be out of CG and you could have a major problem on
your hands. If at max aft CG the problem would be, I assume, the glider
pitching violently upwards as soon as sufficient speed for lift was
attained. Yes/no?

"No (though with "the usual caveats)," regarding the sentence immeedjutly
ahead of the question. Not necessarily "violently", and maybe not at all,
depending (on Joe Pilot, the plane, etc.).

The aft CG limit is not definitionally defined (by the FAA, anyway, so far as
I'm aware), though something called the "neutral point" IS. The N.P. is
definitionally the point at which the aerodynamic center (the point through
which the plane's total lift forces effectively act) and the plane's CG
coincide on the pitch axis. AT that point, the plane is
definitionally/mathematically neutrally stable. If the CG is aft of the
aerodynamic center, the plane is definitionally/mathematically - *genuinely*
complex stuff! - unstable. But what it actually DOES in the real world isn't
subject to "easy definition"...

Textbooks could be - and have been - written about this concept. Whole college
courses, in fact. Shoot - some mathematically gifted folks make specialized
*careers* in it! :-)

What the preceding means to a glider's designer, is that a decision needs to
be made about where the aft CG will be declared in the POH. I'd bet Real Money
most designers choose to place the aftmost allowable CG "somewhat forward" of
the aerodynamic center, for "perceived handling" reasons. Over the years, the
FAA has sometimes insisted on the FAA's opinions being adhered to in "the
handling regard". (The 1-35 springs immediately to mind...)

Point being that handling doesn't "change stepwise" as the CG passes some
arbitrary point. Understand, the handling may not change *linearly*. It's
possible (for example) perceived larger (more difficult for the pilot to
easily/instinctively/intuitively deal with) handling differences will occur
per unit-of-movement of the CG position as the CG nears the aerodynamic
center. Though this isn't necessarily a given, it's yet another reason to pay
Real Test Pilots. :-)
- - - - - -


Would this event be compounded by having a CG hook?

Entirely possible, and arguably likely...
- - - - - -

Mitigated by a nose
hook (until release from tow)? Elevator authority? Trim setting?

Yup, yup, yup...and, of course, the devil is always in the details.
- - - - - -

Have we any St'd Cirrus drivers willing to share 0'beer thirty tales of how
their ship's all-flying-stabilator early models transition from
positively/neutrally stable in pitch to "something abbie-normal"? By that I
mean most of us are used to having to apply increasing back force in order to
generate increasing G-load, but I've been told (never flown one) by more than
one "reasonably technical" St'd Cirrus driver they've experienced having to
REDUCE aft stick force at some speeds/CG's in order to NOT continue to
generate increasing G-loads on higher-speed-of-entry pullups. None of these
pilots admitted to flying with an out-of-aft-range CG.

Certainly, over the decades, the FAA has changed their opinions on what
handling standards must be met in order to obtain an Approved Type
Certificate. (F'r'example think powerplane spin criteria...)

Bob W.