Beginner's Guide to Free Flight 2012/07

F ree Flight is the original form of heavierthan-
air aviation, dating back to Alfonse
Penaud’s 1871 rubber-powered Planophore.
Much has changed since that fi rst 11-second
fl ight in Paris, but the essence of FF remains
the same. It is about the purity of fl ight, and
confi dence to make an aircraft fl y stably and
effi ciently, with no piloting after the launch.
Probably the easiest way to get involved in
FF—or model aviation, for that matter—is with
a simple, handheld catapult glider. Plenty of kits
exist from various sources, and RTF models are
legal for AMA competition.
One of the best is Stan Buddenbohm’s Scout.
It is a simple, 16-inch wingspan design that is
mostly balsa, easy to build, and fl ies superbly.
Others include the Cata-Piglet from Campbell’s
Custom Kits and the Sting series by A2Z.
The idea behind the Catapult Glider event
is straightforward: the models are adjusted to
launch vertically from a 9-inch, handheld, rubber
band-powered catapult. In less than two seconds,
they reach speeds in excess of 100 mph and
heights of more than 100 feet.
That’s exciting, but then the magic happens as
the gliders slow down at the top of the launch, their
noses drop, and they transition into slow, circling,
floating glides of roughly 5 mph. From a good launch,
a well-trimmed catapult glider can remain aloft for
approximately 90 seconds without thermal help.
Catapult gliders aren’t difficult to adjust for flights
provided one understands the dynamics involved.
Rudder offset controls the roll/transition and is effective
mainly at launch speeds. Stabilizer tilt and center of
gravity (CG) are generally only effective during the glide.
And incidence changes affect both launch and glide.
Begin your initial flight trimming by setting the CG
at the plans location and hand gliding the model in calm
conditions at a local park. Look for a gradual left glide
turn with no tendency to spin or dive.
If the model dives, add incidence (stabilizer trailing
edge [TE] up) until the model is at the edge of a stall. If
the model spins or banks drastically, you probably have a
crooked fin or wing.
A proper launch should be pitched up roughly 45° to
60° and banked right at approximately 45°. Reverse this
scenario for a left-handed flier; bank left at launch and
the trim should be reversed for transition to a right glide
circle.
For more than 30 years, the best starting point for
powered FF has been the P-30 model. True to its name, this
is a simple-to-build-and-fly competition class that provides
loads of fun at a low cost. General specifications are a 30-
inch wingspan and length, 40-gram minimum weight, and a
commercially available 91/2-inch diameter plastic propeller.
An excellent and competitive P-30 kit is the PAL
Model Products Square Eagle, thousands of which have
been built in the past three decades.
The Square Eagle can be built in a
week of evenings by even the most
inexperienced builder. Basic familiarity
with stick-and-tissue construction
techniques is helpful but not required.
Probably the most important
thing about building FF models is
recognizing the importance of precision.
Sloppiness, at even the earliest stages
of construction, will show up later with
warped fl ight surfaces, and a model
that is diffi cult to adjust for fl ight.
Work on a completely fl at tabletop
surface. A hollow door from a home
store makes a good fl at surface. Some
builders go a step further and work
on 3/8-inch thick (or thicker) glass
tabletops.
For most traditionally constructed FF
models (balsa wood, open structure),
you’ll need a surface you can stick pins
into as you frame up structures over
full-size plans covered with plastic
kitchen wrap.
The basics of Free Flight stability
In order to fl y autonomously, Free
Flight models must be suffi ciently stable
in all three dimensional axes: pitch,
roll, and yaw. This is opposite of most
forms of active-control fl ight where
maneuverability is desirable.
For conventional (wing in front,
horizontal stabilizer in back) FF aircraft,
there is a narrow longitudinal (fore/
aft) CG range. The CG position is the
bedrock of any FF model; it determines
the critical angular settings of the wing
and horizontal stabilizer, which enable
effi cient fl ight.
Because FF models are optimized
for maximum lift and minimum drag,
airfoils are much different from most
RC and CL airfoils. We almost always
use undercambered or fl at-bottomed
airfoils in the range of 6% to 9% wing
chord thickness.
Thinner is generally better, but it
is usually only attainable with strong,
high-tech construction materials.
Stabilizer airfoils aren’t nearly as critical.
They are usually fl at bottomed with 5%
to 8% maximum thickness, but can also
be simple fl at plates on smaller models
such as gliders.
The wings of most FF models are
set at 0° to +3° positive incidence. The
horizontal stabilizers are set at a range
of 0° to -3°. This difference of angles—
usually 2° to 3° total—in concert with a
safe CG location is what yields adequate
longitudinal (or pitch) stability.
The tendency of a FF model to
diverge laterally (“fall off” on a wing)
is largely controlled by the amount of
wing dihedral used. Roughly 10° (or the
equivalent) on each wing half is needed
for optimum performance. This is more
than a typical RC model. The idea is
to have a model that resists upsets and
returns to level, stable fl ight without
dangerous, spiraling dives.
FF models must also have horizontal
stabilizers that are adequately effective
in order to resist longitudinal instability
(unrecoverable dives). Most horizontal
stabilizers for FF are in the range of 20%
to 40% of the wing area.
Large stabilizers were common
in the older, slower designs, until
approximately 1970. Since then, the
trend has been toward smaller ones.
Generally, the faster the model is, the
smaller the stabilizer can be. Long tail
moments make the stabilizer more
effective, so these models can have small
stabilizer areas—even less than 20% of
the wing area.
The vertical stabilizer area is a fi nal
issue of importance on a FF model. It
should only be large enough to prevent
the Dutch roll or tail wagging. RC
models tolerate much larger vertical
stabilizers because they are under the
pilot’s control.
A too-large vertical stabilizer on a FF
model can cause spiral instability. This is
manifested when the model is resistant
to recovering from a spiral dive; extreme
cases of spiral instability can cause a
crash. Spiral instability is also evident in
a FF model’s inability to climb steeply,
which can be a major detriment to
performance.
A good pin board is a 2 x 4-foot
acoustic ceiling tile. Even better is
the 1/2-inch thick sound-proofing
fiberboard available at home supply
stores in 4 x 8-foot sheets. Both options
are inexpensive; the sound proofing is
my favorite because it’s slightly denser
and holds pins more firmly.
Small rubber-powered models,
such as the P-30, are almost always
open-structured balsa frames covered
with an ancient but superb material:
Japanese tissue. This fine tissue is still
made by the Esaki company in Japan,
as it has been for generations.
What makes Esaki tissue so desirable
for FF is its low density (roughly
3.5 grams per 100 square inches)
combined with amazing skin strength
when it is water-shrunk. This skin
strength translates to finished flying
surfaces that are much stiffer than the
uncovered structures.
The downside of tissue covering
is it is time-consuming and more
difficult than iron-on films. It requires
the builder to brush on some kind of
adhesive. White glue (thinned 50%
with water) or unthinned nitrate dope
work well. The latter is mildly toxic, so
open a window or wear a respirator.
Tissue-covered structures are then
dampened with a light mist of water
and brushed with two or three light
coats of non-tauntening nitrate dope
(thinned 50/50 with dope thinner)
roughly 5 minutes apart.
This seals the pores of the tissue,
makes it reasonably glossy and
considerably stronger. In lieu of thinned
dope, some modelers use Krylon
Crystal Clear #1303 out of a spray can;
it works well and is actually slightly
lighter.
As previously stated, the importance
of precisely aligned, warp-free
structures cannot be overstated. Most
critical is the vertical stabilizer; glue it
on absolutely straight unless the plans
say otherwise.
The horizontal stabilizer is also
critical. It must be adjustable
longitudinally, preferably via a small
2-56 nylon screw on the TE. Small
1/64-inch plywood shims are a passable
substitute, although a screw is much
better.
No warps should be present in the
horizontal stabilizer. Remove any
you see with a heat gun or hair dryer.
Be careful not to get the structure
too hot; balsa and doped tissue are
excellent fi re starters!
The wing is a different matter. It
should have roughly 1/16 to 1/8-inch
washout (TE higher than the leading
edge [LE]) in the tips. Unless your
plans say otherwise, the washout
should be equal in both tips. Again,
use your heat gun and get the warps
right before attempting that fi rst
fl ight.
Your fi rst fl ight with a P-30 should
be an unpowered glide with the 10-
gram rubber motor installed and the
CG located as shown on the plans. Find
a grassy spot and gently toss the model
forward with the nose slightly down.
Shim or screw up the stabilizer’s TE
until you see a slight stall. This means
you’ve slightly exceeded the upper
incidence limit for that CG position.
Lower the stabilizer slightly and toss
again; the stall should be gone. You’re
now ready for powered fl ights.
Your fi rst powered fl ight should only
be attempted in a fairly large fi eld and
in light breeze. Wind roughly 50 turns
into the motor and release the aircraft,
carefully observing it. Chances are that
the model will pitch up slightly and
power stall or “mush” forward slowly;
this indicates a need for downthrust.
Most FF models need approximately
2° to 4° of downthrust for optimum
fl ying. You only need enough
downthrust to prevent a power stall
at full power; any more than this will
limit your climb height.
Keep increasing turns in increments
of 50 until you see the model turn in
the climb. The desired climb is a right
spiral (left is the direction of torque
and is unsafe under high power) using
slight right thrust. Most rubber models
use roughly 1° to 3° of right thrust to
affect a right-spiraling climb.
Keep tweaking the thrustline and
increasing turns until you’ve reached
maximum power, which is roughly
1,100 to 1,200 turns on a typical sixstrand
x 1/8-inch P-30 motor.
For this you’ll need a mechanical
winder and a larger fi eld—200 acres
minimum—more if you live in a windy
area. Set the DT on every fl ight; I’ve
seen models fl y away in thermals from
modest heights.
Welcome to Free Flight!

F ree Flight is the original form of heavierthan-
air aviation, dating back to Alfonse
Penaud’s 1871 rubber-powered Planophore.
Much has changed since that fi rst 11-second
fl ight in Paris, but the essence of FF remains
the same. It is about the purity of fl ight, and
confi dence to make an aircraft fl y stably and
effi ciently, with no piloting after the launch.
Probably the easiest way to get involved in
FF—or model aviation, for that matter—is with
a simple, handheld catapult glider. Plenty of kits
exist from various sources, and RTF models are
legal for AMA competition.
One of the best is Stan Buddenbohm’s Scout.
It is a simple, 16-inch wingspan design that is
mostly balsa, easy to build, and fl ies superbly.
Others include the Cata-Piglet from Campbell’s
Custom Kits and the Sting series by A2Z.
The idea behind the Catapult Glider event
is straightforward: the models are adjusted to
launch vertically from a 9-inch, handheld, rubber
band-powered catapult. In less than two seconds,
they reach speeds in excess of 100 mph and
heights of more than 100 feet.
That’s exciting, but then the magic happens as
the gliders slow down at the top of the launch, their
noses drop, and they transition into slow, circling,
floating glides of roughly 5 mph. From a good launch,
a well-trimmed catapult glider can remain aloft for
approximately 90 seconds without thermal help.
Catapult gliders aren’t difficult to adjust for flights
provided one understands the dynamics involved.
Rudder offset controls the roll/transition and is effective
mainly at launch speeds. Stabilizer tilt and center of
gravity (CG) are generally only effective during the glide.
And incidence changes affect both launch and glide.
Begin your initial flight trimming by setting the CG
at the plans location and hand gliding the model in calm
conditions at a local park. Look for a gradual left glide
turn with no tendency to spin or dive.
If the model dives, add incidence (stabilizer trailing
edge [TE] up) until the model is at the edge of a stall. If
the model spins or banks drastically, you probably have a
crooked fin or wing.
A proper launch should be pitched up roughly 45° to
60° and banked right at approximately 45°. Reverse this
scenario for a left-handed flier; bank left at launch and
the trim should be reversed for transition to a right glide
circle.
For more than 30 years, the best starting point for
powered FF has been the P-30 model. True to its name, this
is a simple-to-build-and-fly competition class that provides
loads of fun at a low cost. General specifications are a 30-
inch wingspan and length, 40-gram minimum weight, and a
commercially available 91/2-inch diameter plastic propeller.
An excellent and competitive P-30 kit is the PAL
Model Products Square Eagle, thousands of which have
been built in the past three decades.
The Square Eagle can be built in a
week of evenings by even the most
inexperienced builder. Basic familiarity
with stick-and-tissue construction
techniques is helpful but not required.
Probably the most important
thing about building FF models is
recognizing the importance of precision.
Sloppiness, at even the earliest stages
of construction, will show up later with
warped fl ight surfaces, and a model
that is diffi cult to adjust for fl ight.
Work on a completely fl at tabletop
surface. A hollow door from a home
store makes a good fl at surface. Some
builders go a step further and work
on 3/8-inch thick (or thicker) glass
tabletops.
For most traditionally constructed FF
models (balsa wood, open structure),
you’ll need a surface you can stick pins
into as you frame up structures over
full-size plans covered with plastic
kitchen wrap.
The basics of Free Flight stability
In order to fl y autonomously, Free
Flight models must be suffi ciently stable
in all three dimensional axes: pitch,
roll, and yaw. This is opposite of most
forms of active-control fl ight where
maneuverability is desirable.
For conventional (wing in front,
horizontal stabilizer in back) FF aircraft,
there is a narrow longitudinal (fore/
aft) CG range. The CG position is the
bedrock of any FF model; it determines
the critical angular settings of the wing
and horizontal stabilizer, which enable
effi cient fl ight.
Because FF models are optimized
for maximum lift and minimum drag,
airfoils are much different from most
RC and CL airfoils. We almost always
use undercambered or fl at-bottomed
airfoils in the range of 6% to 9% wing
chord thickness.
Thinner is generally better, but it
is usually only attainable with strong,
high-tech construction materials.
Stabilizer airfoils aren’t nearly as critical.
They are usually fl at bottomed with 5%
to 8% maximum thickness, but can also
be simple fl at plates on smaller models
such as gliders.
The wings of most FF models are
set at 0° to +3° positive incidence. The
horizontal stabilizers are set at a range
of 0° to -3°. This difference of angles—
usually 2° to 3° total—in concert with a
safe CG location is what yields adequate
longitudinal (or pitch) stability.
The tendency of a FF model to
diverge laterally (“fall off” on a wing)
is largely controlled by the amount of
wing dihedral used. Roughly 10° (or the
equivalent) on each wing half is needed
for optimum performance. This is more
than a typical RC model. The idea is
to have a model that resists upsets and
returns to level, stable fl ight without
dangerous, spiraling dives.
FF models must also have horizontal
stabilizers that are adequately effective
in order to resist longitudinal instability
(unrecoverable dives). Most horizontal
stabilizers for FF are in the range of 20%
to 40% of the wing area.
Large stabilizers were common
in the older, slower designs, until
approximately 1970. Since then, the
trend has been toward smaller ones.
Generally, the faster the model is, the
smaller the stabilizer can be. Long tail
moments make the stabilizer more
effective, so these models can have small
stabilizer areas—even less than 20% of
the wing area.
The vertical stabilizer area is a fi nal
issue of importance on a FF model. It
should only be large enough to prevent
the Dutch roll or tail wagging. RC
models tolerate much larger vertical
stabilizers because they are under the
pilot’s control.
A too-large vertical stabilizer on a FF
model can cause spiral instability. This is
manifested when the model is resistant
to recovering from a spiral dive; extreme
cases of spiral instability can cause a
crash. Spiral instability is also evident in
a FF model’s inability to climb steeply,
which can be a major detriment to
performance.
A good pin board is a 2 x 4-foot
acoustic ceiling tile. Even better is
the 1/2-inch thick sound-proofing
fiberboard available at home supply
stores in 4 x 8-foot sheets. Both options
are inexpensive; the sound proofing is
my favorite because it’s slightly denser
and holds pins more firmly.
Small rubber-powered models,
such as the P-30, are almost always
open-structured balsa frames covered
with an ancient but superb material:
Japanese tissue. This fine tissue is still
made by the Esaki company in Japan,
as it has been for generations.
What makes Esaki tissue so desirable
for FF is its low density (roughly
3.5 grams per 100 square inches)
combined with amazing skin strength
when it is water-shrunk. This skin
strength translates to finished flying
surfaces that are much stiffer than the
uncovered structures.
The downside of tissue covering
is it is time-consuming and more
difficult than iron-on films. It requires
the builder to brush on some kind of
adhesive. White glue (thinned 50%
with water) or unthinned nitrate dope
work well. The latter is mildly toxic, so
open a window or wear a respirator.
Tissue-covered structures are then
dampened with a light mist of water
and brushed with two or three light
coats of non-tauntening nitrate dope
(thinned 50/50 with dope thinner)
roughly 5 minutes apart.
This seals the pores of the tissue,
makes it reasonably glossy and
considerably stronger. In lieu of thinned
dope, some modelers use Krylon
Crystal Clear #1303 out of a spray can;
it works well and is actually slightly
lighter.
As previously stated, the importance
of precisely aligned, warp-free
structures cannot be overstated. Most
critical is the vertical stabilizer; glue it
on absolutely straight unless the plans
say otherwise.
The horizontal stabilizer is also
critical. It must be adjustable
longitudinally, preferably via a small
2-56 nylon screw on the TE. Small
1/64-inch plywood shims are a passable
substitute, although a screw is much
better.
No warps should be present in the
horizontal stabilizer. Remove any
you see with a heat gun or hair dryer.
Be careful not to get the structure
too hot; balsa and doped tissue are
excellent fi re starters!
The wing is a different matter. It
should have roughly 1/16 to 1/8-inch
washout (TE higher than the leading
edge [LE]) in the tips. Unless your
plans say otherwise, the washout
should be equal in both tips. Again,
use your heat gun and get the warps
right before attempting that fi rst
fl ight.
Your fi rst fl ight with a P-30 should
be an unpowered glide with the 10-
gram rubber motor installed and the
CG located as shown on the plans. Find
a grassy spot and gently toss the model
forward with the nose slightly down.
Shim or screw up the stabilizer’s TE
until you see a slight stall. This means
you’ve slightly exceeded the upper
incidence limit for that CG position.
Lower the stabilizer slightly and toss
again; the stall should be gone. You’re
now ready for powered fl ights.
Your fi rst powered fl ight should only
be attempted in a fairly large fi eld and
in light breeze. Wind roughly 50 turns
into the motor and release the aircraft,
carefully observing it. Chances are that
the model will pitch up slightly and
power stall or “mush” forward slowly;
this indicates a need for downthrust.
Most FF models need approximately
2° to 4° of downthrust for optimum
fl ying. You only need enough
downthrust to prevent a power stall
at full power; any more than this will
limit your climb height.
Keep increasing turns in increments
of 50 until you see the model turn in
the climb. The desired climb is a right
spiral (left is the direction of torque
and is unsafe under high power) using
slight right thrust. Most rubber models
use roughly 1° to 3° of right thrust to
affect a right-spiraling climb.
Keep tweaking the thrustline and
increasing turns until you’ve reached
maximum power, which is roughly
1,100 to 1,200 turns on a typical sixstrand
x 1/8-inch P-30 motor.
For this you’ll need a mechanical
winder and a larger fi eld—200 acres
minimum—more if you live in a windy
area. Set the DT on every fl ight; I’ve
seen models fl y away in thermals from
modest heights.
Welcome to Free Flight!