Optimum CG Range

On Dec 13, 1:34 pm, jcarlyle wrote:

...I then calculated the arithmetic mean chord of the LS8 wing by
dividing span by aspect ratio, and got 700mm. This looks about right,
since the root chord is 900mm, and yes, I know it's not the MAC. Then,
I found a scale drawing of the LS8 at: http://www.dg-flugzeugbau.de/Data/3s-ls8-s.pdf
If I'm reading it right, the 25% MAC will be located 225mm behind the
wing LE...

Hmm... That doesn't seem right. Using the DJ Aerotech graphical MAC
method, the same LS8 drawing, and information on the LS8 from Thomas'
Fundamentals of Sailplane Design (thanks again, Judah!) that places
the planform break at 0.6 semispan, I got:

* MAC length of ~736mm
* MAC LE location of ~41mm aft of the wing LE at side of body _or_
* MAC LE location of ~45mm aft of the intersection of the projected
leading edge and the plane of symmetry (yeah, who uses that?)

Given that the LS8's double-trapezoid planform gives it more MAC per
unit area than the HP-18's eminently buildable single trapezoid
planform, and that the LS8's wing is unswept along the 25% chord line
as opposed to the HP-18 being unswept along the 41.25% chord, the MAC
and MAC LE numbers I got sound about right to me. But, hey, I'm a
college dropout with no engineering training, what do I know?

As regards the suggested CG location for the HP-series, Dick Schreder
typically suggested 25% to 40% MAC as the allowable range. Based on an
analysis of the margin of static stability of the HP-18 done by Steve
Smith (that's Dr. Smith to you Mythbusters fans), and based on my own
experience flying an HP-18 with CG back around 40%, I currently
recommend that HP operators limit their operation to 25% to 35% MAC.

For an extra 245 Europes, I will be glad to translate that into a
dimensional range aft of the wing leading edge at side-of-body. For
245 Australias, I will do all the above and throw in a wisecracking
reality-show cameraman. We are not accepting any other continents at
this time.

Thanks, and best regards to all

Bob K.
www.hpaircraft.com ---- now with 245% less ondulation!