Flexi-Flier Article & Plans
April 1974 American Aircraft Modeler
the request of website visitor Guillermo A ., here is the article
for Dick Eipper's Flexi-Flier Rollago wing for R/C. I electronically
scanned the article, including images, from my purchased copy of
the April 1974 American Aircraft Modeler magazine, beginning on
page 21. It is a 1/6-scale version of the original that uses a GI
Joe as the functional pilot. You might be able to scale up the image
below if you cannot find suitable plans for sale. The article was
written and plans drawn by Mr. Frank G. Kelly. All copyrights (if
any) are hereby acknowledged.
FRANK G. KELLY
Hang gliding is a new fad that is getting
a great response on both coasts.
This distinctive scale
model of a Rogallo wing glider is not only fun, but somehow mysterious...
first became aware of hang gliders in their present form when I
met Bill Moyes. While experimenting with a Rogallo wing as a potential
sidelight to his Australian water ski exhibitions, Bill discovered
that he had broken the altitude records for such things. Perhaps
more significant, he found that the rogallo wing ski-kite was a
stable aircraft with the line slack or disconnected. The conventional
water ski-kite is in no way stable with a slack line. When Bill
showed the movie of himself jumping off the south rim of the Grand
Canyon for a nine-min. flight to the river, I knew I was hooked!
On another occasion, he was towed to 10,000 ft. by an airplane,
then he released himself to glide back.
During the past
year or so, hundreds of young people in the Los Angeles area have
assembled hang gliders, mostly the products of available hardware.
Some are frightening creations of bamboo, clothesline and garbage
bags; others are very professional, made of the proper kind of aluminum
tubing, stainless steel cable, and rip-stop Dacron.
one modeled here is a nearly exact 1/6 scale "Flexi-Flier," a popular
contemporary design by Dick Eipper. (If you want to build a full-size
glider, get a set of plans from him. DO NOT scale the model plans
up. Parts and materials must be carefully selected to avoid disaster!)
Dave Cosgrove recently flew one (full-size) continuously for over
two hours in unusually favorable conditions near Palmdale!
The model, like the prototype, has a primitive but interesting
and effective control system. Pitching and turning is done by shifting
the center of gravity by means of body english. The pilot is suspended
from a harness or swing seat, and the control bar provides something
to push against. The model is equipped with only two muscles, so
the body is suspended from a string leading under the arms and around
the back. This worked out much better than any of the more scale
versions, and does not detract from the appearance. It is interesting
to note, if only for academic interest, that this control system
is effective at zero velocity. Ponder that during your next hammerhead
Now ponder this: Two rotary servos operate the two arms. BOTH must
move for each direction. To pitch up, move BOTH arms forward to
cause the body to move back. To pitch down, move BOTH arms the other
way. To turn right, move RIGHT arm back and LEFT arm forward (try
this while holding onto an imaginary towel bar).
The mannequin doll looks like a robot
from science fiction as he admires his electronic innards
in a mirror. Even with KPS-10 size servos, there is adequate
room inside the chest for a complete flight package.
The doll in position on his hang bar, amid a maze of rigging.
Tow hook release bridle is shown here. GI Joe is also available
with a beard.
GI Joe takes a hop aboard his Flexi-Flier. Note that the
author holds the transmitter at a 45°
angle, a clever way to get elevon control. Watch that you
don't detune the signal by accidentally grounding the antenna
with your hand.
(Photo by Jerry Trager)
this complex set of motions, simply rotate the transmitter 450.
This way the aileron/elevator stick operates in a normal manner:
"forward" moves both servos in such a way that the model responds
appropriately. Right stick turns right, etc., and after just a few
min., the controls feel quite natural. The same system could be
used for a V-tail airplane with NO extra linkage! The completed
glider weighs just eight oz.; after his extensive transplant, GI
Joe weighs one lb.
The model didn't take long to assemble,
but working with 0-80 screws involves some fumbling, at best. I
bought a 12-ft. length of 1/4" 2024-T3 aluminum tubing from a nearby
industrial supply house and settled for .028 wall thickness, rather
than the more scale .008. This was a good thing because it has to
take some lumps. All the rest of the materials are available at
hobby shops or hardware stores, although finding long steel 0-80
screws is something of a challenge. I made a drill jig out of a
block of wood to help get the little holes lined up properly.
The rest of the assembly is pretty much straightforward,
with just a few exceptions. The thimbles for the cable loops were
made by threading bare copper wire through a piece of 1/16 aluminum
tubing. This was wrapped tightly around a long 4-40 screw and chucked
up in a drill motor. After filing half way through, the copper wire
was removed and the aluminum tubing sawed lengthwise with a modeling
saw. This provided a handful of split rings which resembled thimbles.
The crimped ferrules were made of short bits of brass tubing and
crimped carefully with wire cutters. They're too small to crimp
any other way.
The control bar was made of 1/8" soft aluminum
hobby tubing but, since strength was necessary here, I selected
a coat hanger which fit inside snugly. Music wire won't do because
it has to be drilled. Bending is easy with the wire inside.
Now that the hang glider is finished, you'll need a pilot. You've
survived the jibes about playing with little toy airplanes, so don't
worry about playing with dolls again. GI Joe is really a remarkable
doll, beautifully articulated to move in all the proper pl-aces.
Pull an arm and a leg out of their sockets and snip the elastic
holding them on. Now saw through the torso at shoulder height. I
did this on my table saw and found it to be a curiously satisfying
experience! My radio is a six-channel Kraft with KPS-10 servos and
I found it necessary to discard GI Joe's hip sockets to make room
for the radio. With a two- or four-channel rig and KPS-12 servos,
you might be able to retain them, but it's not terribly important.
The two servos are taped together back-to-hack with the
rotary shafts available for attaching to the arms at the shoulders.
The receiver can now be slipped between the bottom mounting ears
of the servos. This will locate the "brains" appropriately in the
seat of his pants. The flat battery pack will fit in the chest area.
There's even room for a switch under the battery in the general
vicinity of the belly button. An S- shaped wire hook can attach
the head to the neck, and a 4-40 screw holds the legs together in
the upper thigh. A big rubber band holds the head and legs on like
Drill out the wrist and elbow rivets and tap
to accept 4-40 headless set screws. Carve these joints until they
move freely. Remove the rivet from the shoulder area of the upper
arm, and with a small soldering iron, fill in the area where the
hook had been. The plastic melts and fuses easily. When cool, cut
a square hole to accept the servo output shaft and screw on the
arms. The flight suit can now be slipped over the pilot and when
zipped, he can't be distinguished from the original G I Joe. At
this point, he is capable of doing pushups to get in training. With
the doll suspended from his armpits, and his hands attached to the
control bar with little rubber bands, the effect is absolutely captivating!
He swings around in a very realistic way.
of the completed model were very realistic. it has about a five-to-one
glide path (remember, this is a glider, not a sailplane), and will
flare out very nicely for landing. Upon landing, the legs swing
back gently and everything is protected by the hang bar and rigging.
Control capability is about the same as the prototype, but I found
that control could be overpowered by a strong updraft. A half-oz.
fishing sinker in the nose corrected this .
For the first few test flights, follow the technique used by the
big ones. Find a gentle slope which the prevailing wind blows toward
and which has a loose sand surface. start at the BOTTOM and, holding
the model loosely at the top of the control bar, get the feel of
wind filling the sail and lifting the craft. Avoid touching the
transmitter antenna while holding it sideways because it will detune
somewhat. Now make sure that the sail is filled. Ease it gently
straight into the wind and slightly downward. The shove will cause
the body to swing aft, giving it a tendency to pitch up a little.
|The mini-man does his thing. Control
inputs should find the doll in each of the positions shown:
It's a real marvel just to watch the
mechanical man do tricks.
After a few preliminary flights like this, you can start
working your way up the h ill. Notice that you haven't gotten into
a dangerous situation yet. I f nothing works, the worst thing that
can happen is a mild jolt. The jump suit helps to keep sand out
of everything but doesn't prevent it. Finally, standing on the edge
of the cliff, point the hang glider directly into the wind and launch.
It will gain altitude as, it moves through the updraft, and the
model can compete with a full-size manned hang glider on absolutely
equal terms. Sink rate, glide path, and directional control are
all quite comparable. The only problem is that everyone stops flying
and crowds around to watch when the model is there!
become inured to the thrills of jumping off cliffs in relative safety,
you can add the tow hook release mechanism. Full "up" control pulls
the body back, and a string from the chest releases the catch. Full-size
ones use a motorcycle clutch cable. Towing is much more dangerous
than cliff jumping, because the rigging can be overstressed at high
angles of attack. Higher speeds are attained, and it is possible
to get into a condition which cannot be corrected before impacting.
Longer flights can be sustained with the model though, and it makes
for another interesting novelty. With the tow hook, the model can
be flown like a kite, towed aloft like a tow line glider, launched
with a long rubber band, towed from another RC craft (I'd like to
try water skis behind an RC boat, but GI Joe hasn't learned to swim
yet), or dropped from a weather balloon. My longest flight so far
was from a manned hot air balloon at 5000 ft.
line is hollow like polypropylene water ski tow rope, and a loop
can be formed by threading an end back into the core. This bridle
can be attached to the control bar with a clove hitch. The towline
has a drapery ring on the end, through which the bridle is threaded.
The ring can ride up and down the bridle freely and the pitching
moment induced by towing can be overcome. (If the towline is attached
directly to the control bar, the pitching moment will cause an immediate
problem.) Upon release, the bridle pulls through the ring and comes
free. For the balloon launch, I melted a small hole in the sail
with a soldering iron and allowed the bridle loop to pull through
Under tow, the model has much more lateral control
than in a free glide, due to the way the different forces act and
the moments associated with them. For this reason, the kite technique
is suggested for training purposes before attempting a tow. The
ground speed is initially zero. It is still possible for undampened
oscillations to build up, with hard dirt at both ends; so be prepared
to release the instant a dangerous situation develops. I f you can
find another RC modeler willing to tow you up with his airplane,
the following precautions are important: attach the airplane tow
hook , just behind the cockpit, not at the tail.
for either end of the line to be released in an emergency. The tow
plane can use a spare channel or release with full left rudder.
The glider can get airborne in the first few t., then after a brief
period of uncertainty, both can fly together. Use a 50-ft. towline
of four-lb. test nylon monofilament fishing line. It will stretch
a little, maintaining tension under load, and will break if overstressed.
Teamwork is required to pull this off. Gentle turns and a steady
pull are important. The tow pilot should throttle back a bit at
release to avoid overstressing the glider rigging, but also I the
release will pitch the glider up, slowing down the tow plane. If
the tow plane turns down and left at release while the glider turns
right and down, there will be less of a tendency to interfere with
If you like something different, try a hang
glider. It is in no way an improvement over more conventional aircraft,
but it's fun and very easy to fly.
<click for larger version>
The AMA Plans Service offers a full-size
version of many of the plans show here at a very reasonable cost. They will scale the plans any size for you. It is always
best to buy printed plans because my scanner versions often have distortions that can cause parts to fit poorly. Purchasing
plans also help to support the operation of the Academy of Model
Aeronautics - the #1 advocate for model aviation throughout the world. If the AMA no longer has this plan on file, I
will be glad to send you my higher resolution version.
Try my Scale Calculator for Model Airplane Plans.
Posted June 10, 2010
Even during the busiest times of my life I have endeavored to maintain
some form of model building activity. This site has been created to help me chronicle my journey
through a lifelong involvement in model aviation, which
all began in Mayo, MD. There
is a lot of good information and there are lot
of pictures throughout the website that you will probably find useful, and might
even bring back some old memories from your own days of yore. The website began life around
1996 as an EarthLink screen name of ModelAirplanes, and quickly grew to where more server
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