Free Flight Duration - 2004/03

146 MODEL AVIATION
SOMETIMES WE CAN learn a lesson from the birds; after all,
they have been flying much longer than we have. A while back I
was watching egrets at the local rookery. These are fairly goodsized
birds, with long legs and a long neck designed for feeding
in the coastal marshes. The egret’s neck shortens considerably in
flight, but on landing it extends its neck all the way out as the
angle of attack increases. In effect, the bird shifts its center of
gravity (CG) forward as it slows and flares for landing. I haven’t
seen an egret stall yet.
Many of us have experienced the problems of a model with a
CG that is too far back. The tail-heavy model, if it flies at all,
will be overly sensitive in pitch and prone to stall. The slightest
disturbance will upset the flight pattern.
(This is true of all models, not just Free Flight. A Control
Line or Radio Control model with the CG too far to the rear is
difficult to control. Full-scale aircraft are also adversely affected
by a too-rearward CG. The weight and distribution of cargo is an
important safety concern for all types and full-scale aircraft. As
shown by the egret, it’s true of birds as well.)
The idea of varying the CG during flight has been around for
years. Many of the early aviation experimenters controlled pitch
by shifting their body weight forward and aft. Free Flight
modelers often make slight adjustments in the CG to suit
changing atmospheric conditions. This typically involves adding
weight to the nose of a model trimmed for calm conditions when
the wind or turbulence picks up.
Another approach, which is popular with inexpensive, Readyto-
Fly models, is a sliding wing. As the instructions read, if the
model dives, move the wing forward; if the model stalls, move
the wing back. In effect, moving the wing changes the CG.
As far as I know, no one has developed an adjustable-CG Free
Flight model that moves the CG in flight. However, it wouldn’t
be too difficult to arrange something using a spring-loaded
weight that would release at a preset time. This might be useful
as a way to circumvent the rules against
auto surfaces in some events. (Dropping or
swinging weights have, of course, been
used for years to dethermalize small
models such as Hand-Launched Gliders.)
A too-far-forward CG will also cause
problems, although usually of a less severe
nature. For Control Line and Radio
Control, a model with a far-forward CG
will need more control input to get it to
maneuver. That’s why Control Line Speed
models and Radio Control trainers often
have a more forward CG than do Stunt or
Aerobatics models. For Free Flight
airplanes, a CG that is too far forward will,
of course, cause the model to glide steeply
or, in severe cases, to dive.
So where should the CG be? The best
answer is: it depends. The CG location is
closely tied to the stabilizer area, tail
moment arm, and decalage. The more
powerful the stabilizer, the more rearward
the CG will need to be. For any given
moment arm, the larger the stabilizer, the
farther back the CG will need to be. For
any given stabilizer area, the longer the
Louis Joyner, 6 Saturday Rd., Mt. Pleasant SC 29464
FREE FLIGHT DURATION
You must consider many factors when you are selecting the
correct center-of-gravity location for a Free Flight model.
Even twin fins, as on Tom Laird’s Nostalgia Wakefield, can affect optimum CG location.
Fins increase stabilizer aspect ratio, allowing more rearward CG.
moment arm, the farther back the CG will
need to be.
Decalage, also called longitudinal
dihedral, is the difference between wing
and stabilizer incidence. This is the
absolute difference independent of the
fuselage reference line. A model with the
wing set at 0° relative to the fuselage and
the stabilizer set at –3° will have the same
decalage as a model with the wing at +3°
and the stabilizer at 0°. As a rule, the
larger the decalage, the farther forward the
CG can be.
Many power models from the 1950s
used little decalage in an effort to reduce
the model’s tendency to loop under power.
(0-0 decalage and 100% CG were not
unheard of.) When everything worked,
these models would fly great, but often the
slightest upset would result in a
spectacular straight up and straight down
power pattern. The introduction of
downthrust allowed the CG to be moved
slightly forward and the decalage
increased, resulting in a safer, more
consistent climb, recovery, and glide.
The development of timer-controlled
auto surfaces allowed the climb and glide
segments of the flight to be isolated and
dealt with separately. This has allowed
the Power models’ CGs to move forward
considerably, into the 50-55% range.
Contest rules during the 1930s and
1940s often imposed a minimum fuselage
cross-section based on the model’s overall
length. The longer the model, the fatter the
fuselage. Since a fat fuselage caused extra
drag, designers kept the fuselage as short
as possible and used a large stabilizer.
With the elimination of the cross-section
rule based on fuselage length, models got
longer and stabilizers got smaller.
A typical Wakefield Rubber model of
the late 1930s measured approximately 36
inches in overall length with a 33%
stabilizer. Today’s Wakefields are well
more than a foot longer and have a
stabilizer area of 20% of the wing or less.
This small stabilizer, combined with an
auto stabilizer to control the power burst,
allows a more forward CG, typically near
55%.
March 2004 147
Mike Evatt’s Tandem P-30 uses 80% stabilizer to reduce wing loading and improve
glide. CG is at 210%—more than one chord length behind wing trailing edge.
The CG’s optimum location can be
calculated. The basic idea is to find the
neutral point (NP) and then locate the CG
forward of the NP. The amount that the
CG is located in front of the NP is called
the stability margin (SM). Increasing the
SM improves stall recovery but can
decrease the model’s still-air performance.
The NP’s location depends on the wing
area, stabilizer area, tail moment arm, and
average wing chord. Some other factors
enter in, including wing and stabilizer
airfoils, stabilizer aspect ratio, and location
of the wing relative to the fuselage
centerline.
Perhaps the most useful method of
calculating the CG for all types of Free
Flight models is a formula that French
modeler Rene Jossien developed some
years back. It takes into account the type
of model, weather conditions, airfoils, and
several other factors.
148 MODEL AVIATION
It’s too long to present here, but it has
been widely published through the years.
You can find the formula on the Internet at
www.acsol.net/~regiaero/centerofg.html.
Using his formula should get you within
1% or 2% of the optimum CG. That’s
much closer than the usual guess.
Tandem P-30: Some of today’s events
such as P-30 place a maximum overall
length and maximum wingspan on the
models. Since there is no restriction on the
wing area or the total area, modelers are
free to try a variety of approaches. This
has led to models with extremely wide
wing chords, to pack as much area as
possible into the allotted 30-inch span.
However, these low-aspect-ratio wings
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suffer from increased drag.
Mike Evatt in Great Britain is
exploring another interesting approach:
the tandem. Mike’s idea is to increase the
total area to reduce wing loading and
improve the glide without resorting to a
wide wing chord. His tandem designs use
a stabilizer that is only slightly less than
the wing area (80-85%) with a CG
between 180% and 210% of the wing
chord. Yes, the model balances one chord
length behind the trailing edge!
For comparison, his conventional P-
30 design uses a 35% stabilizer and a
60% CG. Based on glide tests, the
tandem configuration offers a significant
increase in performance, with a higher
climb and a longer glide. Mike’s
predictions are a total duration increase
of just more than 11% compared to his
conventional P-30 with geared motor.
For more about Mike’s P-30
experiments, order the 2003 Symposium
report of the National Free Flight Society
(NFFS). Copies are $25 for NFFS
members and $30 for nonmembers.
Postage is $4 extra. Order from NFFS
Publications, c/o Jim Zolbe, 4801
Bradock Ct., Lincoln NE 68516. Jim can
be reached via E-mail at jzolbe@
neb.rr.com.
For information about joining the
NFFS, visit the Web site—
www.freeflight.org—or contact the new
membership officer, J.P. Kish. Please
send membership applications to J.P. at
NFFS Membership Office, 22 Pine St.,
Homosassa FL 34446. The dues are $25
per year or $48 for two years. For more
information, contact J.P. at
[email protected]. MA