The Cardboard Condor - 2009/09

WHEN I WAS a much younger kid, I would
scrounge up anything I could to build
something that resembled an RC model. I’d
try to fly most of my creations as gliders,
throwing them off of hillsides with a cobbledtogether
radio system for rudder and elevator
control.
I made wings from foam insulation that
I’d cover in packaging tape. Four sheets of
balsa for a fuselage and a little more sheeting
to make a tail, and I had my next chance at
getting something substantial to stay in the air
for more than a few seconds. I loved it. The
fact that everything I tried to fly usually
crashed immediately after the hand launch
didn’t stop me from trying again and again.
Currently, a few decades later, building
and flying come much easier to me. But
every now and then, I find myself
recollecting what a great time of discovery
those years were. In hindsight, that’s where
the idea to build the Cardboard Condor was born.
Now in my 30s, people probably thought I sounded like a
9-year-old a couple years ago when I said, “I’m gonna build a
really big RC airplane outta cardboard and pine. And it’s
gonna have four engines. And it’ll be really awesome!”
They must have thought I was joking. But I was excited by
the prospect of trying something new that, as far as I knew,
hadn’t been done on such a scale.
The idea of using cardboard came from my school days,
when I built an airplane wing cross-section from poster board
for use as a visual aid. I was surprised by its rigidity and
strength. So the thought of employing some type of cardboard
in building a model had been playing in the back of my mind
for years.
Designing the Cardboard Condor was mainly a combination
of building and design experience, experimenting with the
materials to be used, and basic formulas for surface area. I also
designed it with its cardboard covering in mind, using flat
surfaces wherever I could.
It didn’t have to carry anything but itself. With the
wingspan exceeding 12.5 feet, I allowed the fuselage and
wings to enjoy a fairly stout build at the cost of a few extra
pounds. I never had a multiengine model, but throughout the
years I had become comfortable and confident in operating my
engines to the point where it didn’t seem to be an unwise leap.
The initial estimates of the Condor’s flying weight resulted
in a range of 43-53 pounds. That included four 7-inch main
wheels at 14 ounces each, four two-stroke O.S. 61FX engines
at 23.6 ounces each, and approximately 80 square feet of
cardboard, which weighed roughly 10 pounds.
Tossing on 6 pounds here and 10 pounds there was
unfamiliar territory for me in building RC airplanes. I found
myself going back and staring closer at
the numbers as I built. I estimated that the
design could absorb this extra weight and
still keep the wing loading within an
acceptable range.
Another design requirement was that
this 12.5-foot-wingspan airplane had to fit
into my Pontiac Vibe. No problem, right?
That’s where the removable empennage
and wingtips came in.
The 4-foot-span tail section is secured
to the fuselage with four 1/4-20 nylon
bolts. The servos are located in the tail;
thus one had only to attach the servo
connectors upon assembly. The center
wing section supported all four engines,
which helped keep assembly simple.
I like my marriage, so I wanted to
spend as little money on this thing as
possible. The Condor used one four-cell,
3600 mAh NiMH battery. A 1/4-scale
analog servo was employed on each
control surface, of which the elevator used
only one. The throttle and nose gear servos
were standard size.
All servos used nylon or Karbonite
gears, with the exception of the metal-geared elevator servo. I
decided that my servos and their applications would not require a
PowerBox—another big cost reducer.
The radio I used had only one channel available for the throttle.
So they were all ganged; throttle linkages were built and adjusted to
be as identical as possible, from one engine to the next.
I wasn’t keen on the idea of spending the money on the two areas
where I did have to deviate to costly aircraft-grade plywood: select
wing ribs and the engine nacelles. However, I kept the price down
by using 1/8 birch light plywood instead. I merely doubled the
thickness for use with the ribs and tripled it for use with the
firewalls. I did use some balsa, but only on certain LEs, TEs, and in
the aileron cores.
I created a hatch in the nose to access the Condor’s electronics.
Two power switches were installed for redundancy.
I built two unique features into the access hatch in the nose to
add convenience in operating the four engines, the first of which is a
Before building, I played with cardboard and pine; I tried different glues and cardboard thicknesses, tested joints, and estimated the
Condor’s ready-to-fly weight. I even experimented with mat board for a short time, until I determined that it had nearly twice the weight per
area of corrugated cardboard.
A quality of corrugated cardboard that I found to be essential is that it is sandable. This meant that I could apply the material and sand the
edges flush later. More good characteristics are the cardboard’s strength in both tension and compression and its ability to bond well to wood
and itself with inexpensive exterior wood glue.
A big advantage of cardboard compared with polyester covering is price. The former is much less expensive—free if you know where to
look. I could cover the whole model with new cardboard sheets shipped to my door for the price of a couple rolls of polyester covering. Plus,
there are no wrinkles to chase away later.
My investigation revealed that the most common and applicable cardboard for my project was the single-corrugation-layer type that was
5/32 inch thick and available in a variety of sheet sizes from various online sources. I learned that the sheets had one “good” side, free from
irregularities, and the other side usually had a couple of minor indentations or folds from the manufacturing process. I made sure that the good
side was always on the model’s exterior.
People had two common questions about the Cardboard Condor, the first of which was how I protected it from fuel and water damage. I
applied rings of hot glue onto cardboard test samples that were coated with various fuel-proofing products, and then I filled them with fuel and
water to see how well the cardboard held up.
I also attempted to paint the cardboard with fuelproof LustreKote. The resulting surface remained “fuzzy” and didn’t look as good as a few
brushed coats of the polyurethane did.
In the end, I learned that Titebond III wood glue worked great for general construction; a few coats of polyurethane brushed onto the
cardboard’s exterior surface made it ready for the flying field and easy to clean.
The big benefit here was that polyurethane cost roughly $9 a quart, which was enough to apply three coats to the entire model. Besides that,
I liked the idea of showing off the cardboard covering by applying no color to it.
The second question was, “How the heck did you bend the cardboard around curved surfaces?” I tried many things, but I ultimately found
that cutting only the “interior” surface of the material between the corrugated flutes allowed it to conform perfectly around a simple curve, as
in the nose.
Compound curves were more challenging and involved a combination of precutting the cardboard’s interior surface and allowance for
folding to occur on the exterior surface. The only area where compound curves existed was on the LE of the wingtip sections.
Kraft packaging tape with fiberglass reinforcement lent itself well to covering the cardboard’s edges and corner joints. The tape had an
adhesive on one side that needed to be wetted before application. I applied it before I coated the sections with polyurethane.
I sourced the pine I used for the construction from the local home-improvement center. A majority of the fuselage and empennage is made
from 8-foot lengths of 3/4 x 1/2-inch stock, which I typically found to be free of knots and have a straight grain. I drilled a 3/16-inch-diameter hole
through each joint in the fuselage frame, into which I inserted a piece of dowel with wood glue to achieve the joint integrity I desired. MA
—Ryan Livingston
master remote glow plug connection that
supplies power to all four glow plugs
through four toggle switches. It’s wonderful;
there’s no fiddling with a glow starter near
the adjacent engines’ spinning propeller.
If you try this, make sure to use lowresistance
wire and connections. The glow
plugs’ high current draw at such a low
voltage can easily be compromised by tiny
amounts of resistance—even fractions of an
ohm.
Second, I installed a switch to turn power
on and off to each throttle servo
individually. This allowed me, for example,
to prime engine 4 (for starting) with wideopen
throttle, while keeping the other three
running engines from screaming away.
Instead, I’d have them running at idle with
the power shut off to those throttle servos
until all engines were started.
However, this is a double-edged sword.
A potential safety hazard is created without
the instant ability to stop all engines using
the transmitter during starting.
I decided that it was realistically safer to
have this system in place based on
observations at the field through the years.
It’s certainly a subject for debate. As we say
where I work, “I reserve the right to be
smarter five minutes from now.”
All of the Condor’s wheels had some
form of spring suspension. The nose strut
was kept simple. It traveled through a
couple of frame members.
I had originally installed brass sleeve
inserts in the frame to receive the nose strut,
but the strut tended to bind in the sleeves
when its wheel experienced any side loads.
Holes in the wood frame used as guides
seemed to work best.
As I got closer to the reality of how 50
pounds felt as I built, I made more changes
to lighten things. Before I was finished, the
main gear went through a second revision
and the third undercarriage design did a
better job of distributing the weight between
the nose and main wheels, which was the
result of having located all four main wheels
to one axle.
In keeping with the theme of simplicity,
the engine nacelle-mounting and wingmounting
methods involved the use of
screw-and-nut fasteners through holes
drilled into the wing. It uses a twin-mainspar
design, which offers the nacelles good
support. The wing didn’t have much
fuselage weight to lift; the bending stresses
were spread somewhat evenly along the
spars.
Only four hardened-steel 8-32 screws
were needed to lock the wing onto the
fuselage frame. That’s relatively light
hardware, but the majority of the Condor’s
weight was in the wings.
There was a good amount of preparation to
determine what checklists I wanted to go
through one more time at this point.
Basically, the model needed to establish a
good, fast taxi of which I had complete
directional control, before opening up the
engines all the way.
I stood at a point along the runway where
I decided to abort my takeoff attempt if the
airplane wasn’t off of the ground with a
healthy head of flying speed. Not as much
throttle as I anticipated was needed for the
Condor to roll over the grass under its own
power. I was pleased with its groundhandling
stability.
I tried a couple of fast taxis and then
checked the model one last time for anything
that may have come loose that didn’t shake
out during engine break-in or other testing.
Everything looked good. It was time to see if
my four .61s could actually pull an excess of
50 pounds into the sky.
Every fiber of my being wanted to jam
that throttle forward to see if this thing
would fly. Luckily I resisted and went
through my checks, including visualizing the
climbout I wanted.
I’m happy that I had the presence of
mind to take a minute to realize that this was
the moment I had pictured in my mind with
every ounce of effort toward making this
idea a reality. Otherwise I would have blown
right by it with a flick of the throttle.
When I was ready, I slowly opened the
throttles and the Cardboard Condor started
rolling. Having watched the video, in reality
the Condor took less than five seconds to go
from a standstill to airborne. (At the time, it
seemed to take considerably longer.)
It responded to my inputs and continued
down the center of the runway while
09sig2.QXD 7/23/09 11:47 AM Page 44
advancing to full throttle. The aircraft
heaved itself off of the ground and, with
little complaint, answered my respectful
request for a shallow climbout.
I did my best to keep the Condor on a
nice climbing turn back over the field.
Flyingwise, it looked fairly strong. I throttled
the engines back and my nerves followed.
After minor trimming, I knew that I had a
good airplane. The elevator was sensitive
and I had to correct the steeper banks with
some opposite aileron, but I was in business.
Wondering if and how a giant cardboard
airplane would fly was over. It was
“corrugated overcast.”
After flying around a bit and quickly
gaining a feel for the model, I did one slow
pass over the runway and saw that I could
expect a fair amount of flying stability on the
landing approach. The next time around, the
Condor settled downward so nicely that I
thought, “What the heck; let’s try a landing
now.”
The weight and wing loading kept things
moving slow enough for me to even enjoy it.
The model flew directly down to the runway,
set its 7-inch wheels on the grass, and
lumbered to a stop with four running
engines. To say I felt good at that point is an
understatement.
Since then, I have run out a tank or two
of fuel on a couple occasions while flying.
However, the relatively low wing loading for
such a large airplane helped me get to an
approach and landing each time. Each of
those times an engine stopped, I was in a
position to immediately bring the running
engines back to idle. So I haven’t been put
to the full engine-out test yet.
On landing, the wingspan’s long reach
doesn’t threaten to drag a wingtip, even with
a healthy correctional bank while touching
down. Landings with or without power seem
easy. The relatively large vertical surface
might give the Condor additional relief from
the ill effects of uneven engine speeds.
I ended up making small adjustments on
a couple throttle linkages to fine-tune the
engine synchronization. However, I noticed
no adverse yaw in the air from uneven
engine thrust that might have existed. I
could hear a couple of engines running
slightly slower than others in their midrange
and wanted to improve the synchronization
as a matter of good practice.
With such a strong wing spar and joiner
system, I was comfortable trying something
with a couple of “Gs.” On the second flight I
performed two consecutive inside loops,
starting from the bottom.
Coming down on the backside the first
time was a little scary; I wasn’t sure what to
expect. But the aircraft’s rather blunt frontal
profile seemed to keep the speed under
control. The Condor doesn’t resist inverted
flight too much, either.
Could one engine taxi the Condor over
the grass field? If not, could two? One of the
inboard engines at full throttle would not
move the Condor, even if I helped start it
rolling. But both inboard engines would taxi
it at half throttle.
The four engines get a workout during
flight, requiring me to apply much more
throttle than I normally use to fly a singleengine,
60-size model. I like the concept
that one engine is unable to taxi a fourengine
model on grass, but two should be
capable. It seems like a good check on
engine selection to me, although there are
many other factors.
Building a giant model mainly from
cardboard and pine is possible. Only time
will tell how it holds up with age. The
Condor hasn’t experienced extreme highs in
humidity and heat. I haven’t noticed issues
with the cardboard’s stability in the days
I’ve had it out in temperatures in the mid-
80s with medium-high humidity.
I don’t recommend that everyone tries
something similar to this project. The
Condor definitely requires an increased
level of attention to safety because of its
experimental nature and relatively large
size. The amount of care it takes in handling
and flying to avoid overloading the structure
does call for a fair amount of experience.
I hope this project encourages you to
consider new directions in which you can
take your RC flying. After all, exploring and
experimenting is what made flying possible
in the first place. MA
Ryan Livingston
[email protected]

WHEN I WAS a much younger kid, I would
scrounge up anything I could to build
something that resembled an RC model. I’d
try to fly most of my creations as gliders,
throwing them off of hillsides with a cobbledtogether
radio system for rudder and elevator
control.
I made wings from foam insulation that
I’d cover in packaging tape. Four sheets of
balsa for a fuselage and a little more sheeting
to make a tail, and I had my next chance at
getting something substantial to stay in the air
for more than a few seconds. I loved it. The
fact that everything I tried to fly usually
crashed immediately after the hand launch
didn’t stop me from trying again and again.
Currently, a few decades later, building
and flying come much easier to me. But
every now and then, I find myself
recollecting what a great time of discovery
those years were. In hindsight, that’s where
the idea to build the Cardboard Condor was born.
Now in my 30s, people probably thought I sounded like a
9-year-old a couple years ago when I said, “I’m gonna build a
really big RC airplane outta cardboard and pine. And it’s
gonna have four engines. And it’ll be really awesome!”
They must have thought I was joking. But I was excited by
the prospect of trying something new that, as far as I knew,
hadn’t been done on such a scale.
The idea of using cardboard came from my school days,
when I built an airplane wing cross-section from poster board
for use as a visual aid. I was surprised by its rigidity and
strength. So the thought of employing some type of cardboard
in building a model had been playing in the back of my mind
for years.
Designing the Cardboard Condor was mainly a combination
of building and design experience, experimenting with the
materials to be used, and basic formulas for surface area. I also
designed it with its cardboard covering in mind, using flat
surfaces wherever I could.
It didn’t have to carry anything but itself. With the
wingspan exceeding 12.5 feet, I allowed the fuselage and
wings to enjoy a fairly stout build at the cost of a few extra
pounds. I never had a multiengine model, but throughout the
years I had become comfortable and confident in operating my
engines to the point where it didn’t seem to be an unwise leap.
The initial estimates of the Condor’s flying weight resulted
in a range of 43-53 pounds. That included four 7-inch main
wheels at 14 ounces each, four two-stroke O.S. 61FX engines
at 23.6 ounces each, and approximately 80 square feet of
cardboard, which weighed roughly 10 pounds.
Tossing on 6 pounds here and 10 pounds there was
unfamiliar territory for me in building RC airplanes. I found
myself going back and staring closer at
the numbers as I built. I estimated that the
design could absorb this extra weight and
still keep the wing loading within an
acceptable range.
Another design requirement was that
this 12.5-foot-wingspan airplane had to fit
into my Pontiac Vibe. No problem, right?
That’s where the removable empennage
and wingtips came in.
The 4-foot-span tail section is secured
to the fuselage with four 1/4-20 nylon
bolts. The servos are located in the tail;
thus one had only to attach the servo
connectors upon assembly. The center
wing section supported all four engines,
which helped keep assembly simple.
I like my marriage, so I wanted to
spend as little money on this thing as
possible. The Condor used one four-cell,
3600 mAh NiMH battery. A 1/4-scale
analog servo was employed on each
control surface, of which the elevator used
only one. The throttle and nose gear servos
were standard size.
All servos used nylon or Karbonite
gears, with the exception of the metal-geared elevator servo. I
decided that my servos and their applications would not require a
PowerBox—another big cost reducer.
The radio I used had only one channel available for the throttle.
So they were all ganged; throttle linkages were built and adjusted to
be as identical as possible, from one engine to the next.
I wasn’t keen on the idea of spending the money on the two areas
where I did have to deviate to costly aircraft-grade plywood: select
wing ribs and the engine nacelles. However, I kept the price down
by using 1/8 birch light plywood instead. I merely doubled the
thickness for use with the ribs and tripled it for use with the
firewalls. I did use some balsa, but only on certain LEs, TEs, and in
the aileron cores.
I created a hatch in the nose to access the Condor’s electronics.
Two power switches were installed for redundancy.
I built two unique features into the access hatch in the nose to
add convenience in operating the four engines, the first of which is a
Before building, I played with cardboard and pine; I tried different glues and cardboard thicknesses, tested joints, and estimated the
Condor’s ready-to-fly weight. I even experimented with mat board for a short time, until I determined that it had nearly twice the weight per
area of corrugated cardboard.
A quality of corrugated cardboard that I found to be essential is that it is sandable. This meant that I could apply the material and sand the
edges flush later. More good characteristics are the cardboard’s strength in both tension and compression and its ability to bond well to wood
and itself with inexpensive exterior wood glue.
A big advantage of cardboard compared with polyester covering is price. The former is much less expensive—free if you know where to
look. I could cover the whole model with new cardboard sheets shipped to my door for the price of a couple rolls of polyester covering. Plus,
there are no wrinkles to chase away later.
My investigation revealed that the most common and applicable cardboard for my project was the single-corrugation-layer type that was
5/32 inch thick and available in a variety of sheet sizes from various online sources. I learned that the sheets had one “good” side, free from
irregularities, and the other side usually had a couple of minor indentations or folds from the manufacturing process. I made sure that the good
side was always on the model’s exterior.
People had two common questions about the Cardboard Condor, the first of which was how I protected it from fuel and water damage. I
applied rings of hot glue onto cardboard test samples that were coated with various fuel-proofing products, and then I filled them with fuel and
water to see how well the cardboard held up.
I also attempted to paint the cardboard with fuelproof LustreKote. The resulting surface remained “fuzzy” and didn’t look as good as a few
brushed coats of the polyurethane did.
In the end, I learned that Titebond III wood glue worked great for general construction; a few coats of polyurethane brushed onto the
cardboard’s exterior surface made it ready for the flying field and easy to clean.
The big benefit here was that polyurethane cost roughly $9 a quart, which was enough to apply three coats to the entire model. Besides that,
I liked the idea of showing off the cardboard covering by applying no color to it.
The second question was, “How the heck did you bend the cardboard around curved surfaces?” I tried many things, but I ultimately found
that cutting only the “interior” surface of the material between the corrugated flutes allowed it to conform perfectly around a simple curve, as
in the nose.
Compound curves were more challenging and involved a combination of precutting the cardboard’s interior surface and allowance for
folding to occur on the exterior surface. The only area where compound curves existed was on the LE of the wingtip sections.
Kraft packaging tape with fiberglass reinforcement lent itself well to covering the cardboard’s edges and corner joints. The tape had an
adhesive on one side that needed to be wetted before application. I applied it before I coated the sections with polyurethane.
I sourced the pine I used for the construction from the local home-improvement center. A majority of the fuselage and empennage is made
from 8-foot lengths of 3/4 x 1/2-inch stock, which I typically found to be free of knots and have a straight grain. I drilled a 3/16-inch-diameter hole
through each joint in the fuselage frame, into which I inserted a piece of dowel with wood glue to achieve the joint integrity I desired. MA
—Ryan Livingston
master remote glow plug connection that
supplies power to all four glow plugs
through four toggle switches. It’s wonderful;
there’s no fiddling with a glow starter near
the adjacent engines’ spinning propeller.
If you try this, make sure to use lowresistance
wire and connections. The glow
plugs’ high current draw at such a low
voltage can easily be compromised by tiny
amounts of resistance—even fractions of an
ohm.
Second, I installed a switch to turn power
on and off to each throttle servo
individually. This allowed me, for example,
to prime engine 4 (for starting) with wideopen
throttle, while keeping the other three
running engines from screaming away.
Instead, I’d have them running at idle with
the power shut off to those throttle servos
until all engines were started.
However, this is a double-edged sword.
A potential safety hazard is created without
the instant ability to stop all engines using
the transmitter during starting.
I decided that it was realistically safer to
have this system in place based on
observations at the field through the years.
It’s certainly a subject for debate. As we say
where I work, “I reserve the right to be
smarter five minutes from now.”
All of the Condor’s wheels had some
form of spring suspension. The nose strut
was kept simple. It traveled through a
couple of frame members.
I had originally installed brass sleeve
inserts in the frame to receive the nose strut,
but the strut tended to bind in the sleeves
when its wheel experienced any side loads.
Holes in the wood frame used as guides
seemed to work best.
As I got closer to the reality of how 50
pounds felt as I built, I made more changes
to lighten things. Before I was finished, the
main gear went through a second revision
and the third undercarriage design did a
better job of distributing the weight between
the nose and main wheels, which was the
result of having located all four main wheels
to one axle.
In keeping with the theme of simplicity,
the engine nacelle-mounting and wingmounting
methods involved the use of
screw-and-nut fasteners through holes
drilled into the wing. It uses a twin-mainspar
design, which offers the nacelles good
support. The wing didn’t have much
fuselage weight to lift; the bending stresses
were spread somewhat evenly along the
spars.
Only four hardened-steel 8-32 screws
were needed to lock the wing onto the
fuselage frame. That’s relatively light
hardware, but the majority of the Condor’s
weight was in the wings.
There was a good amount of preparation to
determine what checklists I wanted to go
through one more time at this point.
Basically, the model needed to establish a
good, fast taxi of which I had complete
directional control, before opening up the
engines all the way.
I stood at a point along the runway where
I decided to abort my takeoff attempt if the
airplane wasn’t off of the ground with a
healthy head of flying speed. Not as much
throttle as I anticipated was needed for the
Condor to roll over the grass under its own
power. I was pleased with its groundhandling
stability.
I tried a couple of fast taxis and then
checked the model one last time for anything
that may have come loose that didn’t shake
out during engine break-in or other testing.
Everything looked good. It was time to see if
my four .61s could actually pull an excess of
50 pounds into the sky.
Every fiber of my being wanted to jam
that throttle forward to see if this thing
would fly. Luckily I resisted and went
through my checks, including visualizing the
climbout I wanted.
I’m happy that I had the presence of
mind to take a minute to realize that this was
the moment I had pictured in my mind with
every ounce of effort toward making this
idea a reality. Otherwise I would have blown
right by it with a flick of the throttle.
When I was ready, I slowly opened the
throttles and the Cardboard Condor started
rolling. Having watched the video, in reality
the Condor took less than five seconds to go
from a standstill to airborne. (At the time, it
seemed to take considerably longer.)
It responded to my inputs and continued
down the center of the runway while
09sig2.QXD 7/23/09 11:47 AM Page 44
advancing to full throttle. The aircraft
heaved itself off of the ground and, with
little complaint, answered my respectful
request for a shallow climbout.
I did my best to keep the Condor on a
nice climbing turn back over the field.
Flyingwise, it looked fairly strong. I throttled
the engines back and my nerves followed.
After minor trimming, I knew that I had a
good airplane. The elevator was sensitive
and I had to correct the steeper banks with
some opposite aileron, but I was in business.
Wondering if and how a giant cardboard
airplane would fly was over. It was
“corrugated overcast.”
After flying around a bit and quickly
gaining a feel for the model, I did one slow
pass over the runway and saw that I could
expect a fair amount of flying stability on the
landing approach. The next time around, the
Condor settled downward so nicely that I
thought, “What the heck; let’s try a landing
now.”
The weight and wing loading kept things
moving slow enough for me to even enjoy it.
The model flew directly down to the runway,
set its 7-inch wheels on the grass, and
lumbered to a stop with four running
engines. To say I felt good at that point is an
understatement.
Since then, I have run out a tank or two
of fuel on a couple occasions while flying.
However, the relatively low wing loading for
such a large airplane helped me get to an
approach and landing each time. Each of
those times an engine stopped, I was in a
position to immediately bring the running
engines back to idle. So I haven’t been put
to the full engine-out test yet.
On landing, the wingspan’s long reach
doesn’t threaten to drag a wingtip, even with
a healthy correctional bank while touching
down. Landings with or without power seem
easy. The relatively large vertical surface
might give the Condor additional relief from
the ill effects of uneven engine speeds.
I ended up making small adjustments on
a couple throttle linkages to fine-tune the
engine synchronization. However, I noticed
no adverse yaw in the air from uneven
engine thrust that might have existed. I
could hear a couple of engines running
slightly slower than others in their midrange
and wanted to improve the synchronization
as a matter of good practice.
With such a strong wing spar and joiner
system, I was comfortable trying something
with a couple of “Gs.” On the second flight I
performed two consecutive inside loops,
starting from the bottom.
Coming down on the backside the first
time was a little scary; I wasn’t sure what to
expect. But the aircraft’s rather blunt frontal
profile seemed to keep the speed under
control. The Condor doesn’t resist inverted
flight too much, either.
Could one engine taxi the Condor over
the grass field? If not, could two? One of the
inboard engines at full throttle would not
move the Condor, even if I helped start it
rolling. But both inboard engines would taxi
it at half throttle.
The four engines get a workout during
flight, requiring me to apply much more
throttle than I normally use to fly a singleengine,
60-size model. I like the concept
that one engine is unable to taxi a fourengine
model on grass, but two should be
capable. It seems like a good check on
engine selection to me, although there are
many other factors.
Building a giant model mainly from
cardboard and pine is possible. Only time
will tell how it holds up with age. The
Condor hasn’t experienced extreme highs in
humidity and heat. I haven’t noticed issues
with the cardboard’s stability in the days
I’ve had it out in temperatures in the mid-
80s with medium-high humidity.
I don’t recommend that everyone tries
something similar to this project. The
Condor definitely requires an increased
level of attention to safety because of its
experimental nature and relatively large
size. The amount of care it takes in handling
and flying to avoid overloading the structure
does call for a fair amount of experience.
I hope this project encourages you to
consider new directions in which you can
take your RC flying. After all, exploring and
experimenting is what made flying possible
in the first place. MA
Ryan Livingston
[email protected]

WHEN I WAS a much younger kid, I would
scrounge up anything I could to build
something that resembled an RC model. I’d
try to fly most of my creations as gliders,
throwing them off of hillsides with a cobbledtogether
radio system for rudder and elevator
control.
I made wings from foam insulation that
I’d cover in packaging tape. Four sheets of
balsa for a fuselage and a little more sheeting
to make a tail, and I had my next chance at
getting something substantial to stay in the air
for more than a few seconds. I loved it. The
fact that everything I tried to fly usually
crashed immediately after the hand launch
didn’t stop me from trying again and again.
Currently, a few decades later, building
and flying come much easier to me. But
every now and then, I find myself
recollecting what a great time of discovery
those years were. In hindsight, that’s where
the idea to build the Cardboard Condor was born.
Now in my 30s, people probably thought I sounded like a
9-year-old a couple years ago when I said, “I’m gonna build a
really big RC airplane outta cardboard and pine. And it’s
gonna have four engines. And it’ll be really awesome!”
They must have thought I was joking. But I was excited by
the prospect of trying something new that, as far as I knew,
hadn’t been done on such a scale.
The idea of using cardboard came from my school days,
when I built an airplane wing cross-section from poster board
for use as a visual aid. I was surprised by its rigidity and
strength. So the thought of employing some type of cardboard
in building a model had been playing in the back of my mind
for years.
Designing the Cardboard Condor was mainly a combination
of building and design experience, experimenting with the
materials to be used, and basic formulas for surface area. I also
designed it with its cardboard covering in mind, using flat
surfaces wherever I could.
It didn’t have to carry anything but itself. With the
wingspan exceeding 12.5 feet, I allowed the fuselage and
wings to enjoy a fairly stout build at the cost of a few extra
pounds. I never had a multiengine model, but throughout the
years I had become comfortable and confident in operating my
engines to the point where it didn’t seem to be an unwise leap.
The initial estimates of the Condor’s flying weight resulted
in a range of 43-53 pounds. That included four 7-inch main
wheels at 14 ounces each, four two-stroke O.S. 61FX engines
at 23.6 ounces each, and approximately 80 square feet of
cardboard, which weighed roughly 10 pounds.
Tossing on 6 pounds here and 10 pounds there was
unfamiliar territory for me in building RC airplanes. I found
myself going back and staring closer at
the numbers as I built. I estimated that the
design could absorb this extra weight and
still keep the wing loading within an
acceptable range.
Another design requirement was that
this 12.5-foot-wingspan airplane had to fit
into my Pontiac Vibe. No problem, right?
That’s where the removable empennage
and wingtips came in.
The 4-foot-span tail section is secured
to the fuselage with four 1/4-20 nylon
bolts. The servos are located in the tail;
thus one had only to attach the servo
connectors upon assembly. The center
wing section supported all four engines,
which helped keep assembly simple.
I like my marriage, so I wanted to
spend as little money on this thing as
possible. The Condor used one four-cell,
3600 mAh NiMH battery. A 1/4-scale
analog servo was employed on each
control surface, of which the elevator used
only one. The throttle and nose gear servos
were standard size.
All servos used nylon or Karbonite
gears, with the exception of the metal-geared elevator servo. I
decided that my servos and their applications would not require a
PowerBox—another big cost reducer.
The radio I used had only one channel available for the throttle.
So they were all ganged; throttle linkages were built and adjusted to
be as identical as possible, from one engine to the next.
I wasn’t keen on the idea of spending the money on the two areas
where I did have to deviate to costly aircraft-grade plywood: select
wing ribs and the engine nacelles. However, I kept the price down
by using 1/8 birch light plywood instead. I merely doubled the
thickness for use with the ribs and tripled it for use with the
firewalls. I did use some balsa, but only on certain LEs, TEs, and in
the aileron cores.
I created a hatch in the nose to access the Condor’s electronics.
Two power switches were installed for redundancy.
I built two unique features into the access hatch in the nose to
add convenience in operating the four engines, the first of which is a
Before building, I played with cardboard and pine; I tried different glues and cardboard thicknesses, tested joints, and estimated the
Condor’s ready-to-fly weight. I even experimented with mat board for a short time, until I determined that it had nearly twice the weight per
area of corrugated cardboard.
A quality of corrugated cardboard that I found to be essential is that it is sandable. This meant that I could apply the material and sand the
edges flush later. More good characteristics are the cardboard’s strength in both tension and compression and its ability to bond well to wood
and itself with inexpensive exterior wood glue.
A big advantage of cardboard compared with polyester covering is price. The former is much less expensive—free if you know where to
look. I could cover the whole model with new cardboard sheets shipped to my door for the price of a couple rolls of polyester covering. Plus,
there are no wrinkles to chase away later.
My investigation revealed that the most common and applicable cardboard for my project was the single-corrugation-layer type that was
5/32 inch thick and available in a variety of sheet sizes from various online sources. I learned that the sheets had one “good” side, free from
irregularities, and the other side usually had a couple of minor indentations or folds from the manufacturing process. I made sure that the good
side was always on the model’s exterior.
People had two common questions about the Cardboard Condor, the first of which was how I protected it from fuel and water damage. I
applied rings of hot glue onto cardboard test samples that were coated with various fuel-proofing products, and then I filled them with fuel and
water to see how well the cardboard held up.
I also attempted to paint the cardboard with fuelproof LustreKote. The resulting surface remained “fuzzy” and didn’t look as good as a few
brushed coats of the polyurethane did.
In the end, I learned that Titebond III wood glue worked great for general construction; a few coats of polyurethane brushed onto the
cardboard’s exterior surface made it ready for the flying field and easy to clean.
The big benefit here was that polyurethane cost roughly $9 a quart, which was enough to apply three coats to the entire model. Besides that,
I liked the idea of showing off the cardboard covering by applying no color to it.
The second question was, “How the heck did you bend the cardboard around curved surfaces?” I tried many things, but I ultimately found
that cutting only the “interior” surface of the material between the corrugated flutes allowed it to conform perfectly around a simple curve, as
in the nose.
Compound curves were more challenging and involved a combination of precutting the cardboard’s interior surface and allowance for
folding to occur on the exterior surface. The only area where compound curves existed was on the LE of the wingtip sections.
Kraft packaging tape with fiberglass reinforcement lent itself well to covering the cardboard’s edges and corner joints. The tape had an
adhesive on one side that needed to be wetted before application. I applied it before I coated the sections with polyurethane.
I sourced the pine I used for the construction from the local home-improvement center. A majority of the fuselage and empennage is made
from 8-foot lengths of 3/4 x 1/2-inch stock, which I typically found to be free of knots and have a straight grain. I drilled a 3/16-inch-diameter hole
through each joint in the fuselage frame, into which I inserted a piece of dowel with wood glue to achieve the joint integrity I desired. MA
—Ryan Livingston
master remote glow plug connection that
supplies power to all four glow plugs
through four toggle switches. It’s wonderful;
there’s no fiddling with a glow starter near
the adjacent engines’ spinning propeller.
If you try this, make sure to use lowresistance
wire and connections. The glow
plugs’ high current draw at such a low
voltage can easily be compromised by tiny
amounts of resistance—even fractions of an
ohm.
Second, I installed a switch to turn power
on and off to each throttle servo
individually. This allowed me, for example,
to prime engine 4 (for starting) with wideopen
throttle, while keeping the other three
running engines from screaming away.
Instead, I’d have them running at idle with
the power shut off to those throttle servos
until all engines were started.
However, this is a double-edged sword.
A potential safety hazard is created without
the instant ability to stop all engines using
the transmitter during starting.
I decided that it was realistically safer to
have this system in place based on
observations at the field through the years.
It’s certainly a subject for debate. As we say
where I work, “I reserve the right to be
smarter five minutes from now.”
All of the Condor’s wheels had some
form of spring suspension. The nose strut
was kept simple. It traveled through a
couple of frame members.
I had originally installed brass sleeve
inserts in the frame to receive the nose strut,
but the strut tended to bind in the sleeves
when its wheel experienced any side loads.
Holes in the wood frame used as guides
seemed to work best.
As I got closer to the reality of how 50
pounds felt as I built, I made more changes
to lighten things. Before I was finished, the
main gear went through a second revision
and the third undercarriage design did a
better job of distributing the weight between
the nose and main wheels, which was the
result of having located all four main wheels
to one axle.
In keeping with the theme of simplicity,
the engine nacelle-mounting and wingmounting
methods involved the use of
screw-and-nut fasteners through holes
drilled into the wing. It uses a twin-mainspar
design, which offers the nacelles good
support. The wing didn’t have much
fuselage weight to lift; the bending stresses
were spread somewhat evenly along the
spars.
Only four hardened-steel 8-32 screws
were needed to lock the wing onto the
fuselage frame. That’s relatively light
hardware, but the majority of the Condor’s
weight was in the wings.
There was a good amount of preparation to
determine what checklists I wanted to go
through one more time at this point.
Basically, the model needed to establish a
good, fast taxi of which I had complete
directional control, before opening up the
engines all the way.
I stood at a point along the runway where
I decided to abort my takeoff attempt if the
airplane wasn’t off of the ground with a
healthy head of flying speed. Not as much
throttle as I anticipated was needed for the
Condor to roll over the grass under its own
power. I was pleased with its groundhandling
stability.
I tried a couple of fast taxis and then
checked the model one last time for anything
that may have come loose that didn’t shake
out during engine break-in or other testing.
Everything looked good. It was time to see if
my four .61s could actually pull an excess of
50 pounds into the sky.
Every fiber of my being wanted to jam
that throttle forward to see if this thing
would fly. Luckily I resisted and went
through my checks, including visualizing the
climbout I wanted.
I’m happy that I had the presence of
mind to take a minute to realize that this was
the moment I had pictured in my mind with
every ounce of effort toward making this
idea a reality. Otherwise I would have blown
right by it with a flick of the throttle.
When I was ready, I slowly opened the
throttles and the Cardboard Condor started
rolling. Having watched the video, in reality
the Condor took less than five seconds to go
from a standstill to airborne. (At the time, it
seemed to take considerably longer.)
It responded to my inputs and continued
down the center of the runway while
09sig2.QXD 7/23/09 11:47 AM Page 44
advancing to full throttle. The aircraft
heaved itself off of the ground and, with
little complaint, answered my respectful
request for a shallow climbout.
I did my best to keep the Condor on a
nice climbing turn back over the field.
Flyingwise, it looked fairly strong. I throttled
the engines back and my nerves followed.
After minor trimming, I knew that I had a
good airplane. The elevator was sensitive
and I had to correct the steeper banks with
some opposite aileron, but I was in business.
Wondering if and how a giant cardboard
airplane would fly was over. It was
“corrugated overcast.”
After flying around a bit and quickly
gaining a feel for the model, I did one slow
pass over the runway and saw that I could
expect a fair amount of flying stability on the
landing approach. The next time around, the
Condor settled downward so nicely that I
thought, “What the heck; let’s try a landing
now.”
The weight and wing loading kept things
moving slow enough for me to even enjoy it.
The model flew directly down to the runway,
set its 7-inch wheels on the grass, and
lumbered to a stop with four running
engines. To say I felt good at that point is an
understatement.
Since then, I have run out a tank or two
of fuel on a couple occasions while flying.
However, the relatively low wing loading for
such a large airplane helped me get to an
approach and landing each time. Each of
those times an engine stopped, I was in a
position to immediately bring the running
engines back to idle. So I haven’t been put
to the full engine-out test yet.
On landing, the wingspan’s long reach
doesn’t threaten to drag a wingtip, even with
a healthy correctional bank while touching
down. Landings with or without power seem
easy. The relatively large vertical surface
might give the Condor additional relief from
the ill effects of uneven engine speeds.
I ended up making small adjustments on
a couple throttle linkages to fine-tune the
engine synchronization. However, I noticed
no adverse yaw in the air from uneven
engine thrust that might have existed. I
could hear a couple of engines running
slightly slower than others in their midrange
and wanted to improve the synchronization
as a matter of good practice.
With such a strong wing spar and joiner
system, I was comfortable trying something
with a couple of “Gs.” On the second flight I
performed two consecutive inside loops,
starting from the bottom.
Coming down on the backside the first
time was a little scary; I wasn’t sure what to
expect. But the aircraft’s rather blunt frontal
profile seemed to keep the speed under
control. The Condor doesn’t resist inverted
flight too much, either.
Could one engine taxi the Condor over
the grass field? If not, could two? One of the
inboard engines at full throttle would not
move the Condor, even if I helped start it
rolling. But both inboard engines would taxi
it at half throttle.
The four engines get a workout during
flight, requiring me to apply much more
throttle than I normally use to fly a singleengine,
60-size model. I like the concept
that one engine is unable to taxi a fourengine
model on grass, but two should be
capable. It seems like a good check on
engine selection to me, although there are
many other factors.
Building a giant model mainly from
cardboard and pine is possible. Only time
will tell how it holds up with age. The
Condor hasn’t experienced extreme highs in
humidity and heat. I haven’t noticed issues
with the cardboard’s stability in the days
I’ve had it out in temperatures in the mid-
80s with medium-high humidity.
I don’t recommend that everyone tries
something similar to this project. The
Condor definitely requires an increased
level of attention to safety because of its
experimental nature and relatively large
size. The amount of care it takes in handling
and flying to avoid overloading the structure
does call for a fair amount of experience.
I hope this project encourages you to
consider new directions in which you can
take your RC flying. After all, exploring and
experimenting is what made flying possible
in the first place. MA
Ryan Livingston
[email protected]