On Feb 3, 9:05 pm, "Marc J. Zeitlin"
wrote:

Any aircraft in which the CG can be ahead of the AIRCRAFT AC, but
behind the main wing AC. There are many aircraft that when flown at
rear CG positions and low speeds will have the tail producing lift.
Gliders, in particular. Off the top of my head, I don't know of any
in particular, although I remember being told that a C-172 will have
an upforce on the tail at low speeds and rear CG. Can't cite it, though.

Even with the CG at its most aft position, a 172's AC is still
behind the CG. CG range is typically 25-33% of MAC, while AC is around
40%.


The web site I indicated:

http://www.av8n.com/how/htm/aoastab.html

explains all this.

That's a good site that I've used for more than four years. I
think we're talking about two different things here and there's a
misunderstanding that leads to argument.
I find a fairly widely-held opinion that the aft CG can be
(legally) at or behind AC. This isn't true for any "modern"
lightplane. FAR 23.173 requires that the airplane return to trimmed
speed after being slowed or accelerated using pitch inputs only and
releasing them; this won't happen in a CG-behind-AC situation. As the
wing slows its AC moves forward due to the breakup of the boundary
layer toward the trailing edge, the CG therefore ends up even farther
behind the AC, the situation gets worse as the nose rises with the
forward-moving AC, and it eventually stalls. This is what I meant by
the illegality of a lifting tail.
Here's an example of some common miperceptions:
http://answers.yahoo.com/question/in...9225927AAfYZDU
Even the first answer, the one the voters liked, says that the CG is
12" ahead of the AC even in the most-aft position, This is extreme;
it's a lot less than that, but it's still forward. Other posters think
that with the CG at the aft position the tail must produce lift.

Dan

Dan