P-51D Center of Gravity

[Ken Browning]

I purchased an already built, but unflown P-51D Mustang. Supposedly it is
a Royal model, possibly their "junior" version P-51. Wingspan is 51" and
length is 48", weight about 7 lbs. It's named Miss Miami with fuselage
marking "LC F" and a white "F" in a black circle on the fin. I installed
a K&B .61 with a 3" spinner. This should be at least an average to
heavy engine for this size plane.

The center of gravity is about on the main spar measured with the
plane inverted. This is about right for any other models I have flown.
Getting ready for the maiden flight a club member with lots of P-51
experience indicated that every P-51 he has had experience with balanced
just behind the wingtip. This would make my plane very tailheavy. I
measured on the spar near the fuselage. He has built 2 Reno/Madera
unlimited racers (one 1st and one 2nd in silver class) and has built many
warbirds so I decided not to take the risk and fly that day. After all, it
took me 3 months to get around to installing the engine and radio so a few
more days won't matter.

Now the questions:

Has anyone any knowledge of the Royal P-51D and its correct C of G?

Does the P-51D usually balance further ahead than other wing planforms?

Does anyone know the Fax number for Royal Products. I would like to Fax
them a picture of the plane to see if it really is a Royal kit.

Ken Browning
brow...@nicad3.nic.bc.ca

[JP1]

In article <1994Oct12.2...@nicad3.nic.bc.ca>,
brow...@nicad3.nic.bc.ca (Ken Browning) writes:

>>The center of gravity is about on the main spar measured with the
>>plane inverted. This is about right for any other models I have flown.

Ken, I urge you _not_ to relate the CG to the location of a structural
member, such as a wing spar. The spar might..._might_ be located in such a
way as to allow coincidental relation to the proper CG, but that's pure
chance. The stability and control principles involved don't give a hoot
where the spar is located.

A fairly conservative CG location for more-or-less conventional designs is
at 25% of the mean aerodynamic chord (MAC). There are algebraic methods
for finding the MAC , but for a straight taper wing, it can be done
geometrically.

Sketch the planform of one wing panel--ignore the 'crank' in the leading
edge, if the wing has one, and just continue the leading edge straight
from the tip to the centerline of the wing. Extend the root chord forward
one tip chord length 9ignore the crank), and aft one tip chord length. At
the tip, extend the chord one root chord length forward (ignore the crank)
and one root chord length aft. This will yield a trapezoid. Draw a
straight line between the forward-most point at the tip (that is, at the
extended tip that came from drawing in a root chord) to the aft-most point
at the root (the 'extended' root). Similarly, connect the forward-most
root point to the aft-most tip point.

These two diagonal lines will intersect at some station along the panel
span, and that location defines the mean aerodynamic chord. Measure about
1/4 of that chord from the leading edge _at that chord location_, and
that's a reasonable starting point for CG location. To effect balancing,
of course, you'll need to transfer the 1/4 MAC location to the
tips--remember to extend the location straight out along the span, _not_
to measure back from the tip leading edge.

Using the 1/4 MAC as the (tank-empty) CG location is a pretty safe bet if
the horizontal stab isn't on the small side. It would be wise to
calculate the horizontal stab's area and its percentage of the wing's
area. If it's any less than 20-22%, I'd move the CG forward a little...to
maybe 22% of the MAC.

If you get the CG too far forward, about the worst result is lessened
control surface authority--the aircraft will react less to aileron,
elevator, and rudder deflections. If you get it too far aft...well,
you'll remember that flight for a _long_ time.

Hope this helps.

John P.
.

[Ken Browning]

Further to my previous post about my P-51 C of G, there was a typo. The
wingspan is 55" not 51". Also, for what it is worth, it has mechanical
retracts and flaps if that helps someone to identify the kit.

Ken Browning
brow...@nicad3.nic.bc.ca

[Dave Wright]

I have a Royal P-51 (.60), but the kit brand doesn't matter- I wouldn't
dream of trying to fly it with the balance behind the wingtip. As for your
friend, I would have to see him fly a P51 (or any warbird) balanced like that,
to believe it. Seven pounds with a 51" wing is going to be two handfulls to
begin with. Stick with the conventional rules for balancing, and your plane
may get more than one short flight.

Dave Wright