Optimum CG Range

Bob, thanks for figuring out the MAC of the LS8. I wasn’t clear on
how to “fudge” (DJ Aerotech’s term) the winglet and the wing root to
get the required area for use in their graphical calculation of MAC.

Your values of 736 mm for MAC, with a MAC LE of 41 mm aft of the LE of
the wing root, jibe quite well with my arithmetic chord of 700 mm and
my arithmetic chord LE of 50 mm aft of the LE of the wing root. I’m
not clear why you said it didn’t seem right.

But thanks to you I can now calculate the LS8 CG limits in terms of
MAC. The fwd CG limit is 280 mm aft of the LE of the wing root, and
the aft CG limit is 400 mm aft of the LE of the wing root (both values
from TCDS G14CE). So:

Fwd CG limit = 100 * (280 - 41) / 736 = 32% MAC
Aft CG limit = 100 * (400 - 41) / 736 = 49% MAC

This result (a) makes the LS8 odd from the perspective of SE light
aircraft (typical CG range from 15% to 35% of MAC) and the HP-18 (was
25% to 40%, now 25% to 35%), and (b) means that the 1981 Frank Irving
optimum CG guideline of 30% to 35% of MAC isn’t useful. So that
answers my original questions.

-John

On Dec 13, 10:30 pm, Bob Kuykendall wrote:
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.

jcarlyle wrote:
Fwd CG limit = 100 * (280 - 41) / 736 = 32% MAC
Aft CG limit = 100 * (400 - 41) / 736 = 49% MAC

John,
My experience and judgement tell me your figures are way too far aft.
I suspect the distance aft of the leading edge to the zero MAC is in
error. One sure way to check this is to mark the MAC on both wings
then assemble the wing without the fuselage on saw-horses. Then snap a
string from both zero MAC's and measure the distance from the string
to the leading edge at root rib.
Cheers,
JJ

JJ, I place great weight on your experience and judgement! I agree,
what I calculated is way far back. But I've looked over my figures
very carefully a number of times, and if there's an error I sure can't
find it.

As for the LE to zero MAC dimension causing the problem, we can get a
figure of merit by calculating the CG limits for the root chord:

Fwd CG limit = 100 * 280 / 900 = 31% root chord
Aft CG limit = 100 * 400 / 900 = 44% root chord

Of course it's different from the 32% to 49% MAC, or the 33% to 50%
for the arithmetic chord, but all three result are in the same ball
park. Measuring the actual wings as per your write-up is a good
suggestion, but due to available work area and the weather I won't be
able to try that for about 5 months at the earliest.

-John

On Dec 15, 10:49 am, JJ Sinclair wrote:
jcarlyle wrote:

Fwd CG limit = 100 * (280 - 41) / 736 = 32% MAC

Aft CG limit = 100 * (400 - 41) / 736 = 49% MAC

John,
My experience and judgement tell me your figures are way too far aft.
I suspect the distance aft of the leading edge to the zero MAC is in
error. One sure way to check this is to mark the MAC on both wings
then assemble the wing without the fuselage on saw-horses. Then snap a
string from both zero MAC's and measure the distance from the string
to the leading edge at root rib.
Cheers,
JJ