INTRODUCTION: In the late 1960s, the US Army
contracted with Lockheed for the construction of a
revolutionary attack helicopter called the AH-56
Cheyenne. In addition to the normal tail rotor, the
Cheyenne had a variable-pitch pusher propeller. For
hovering and low-speed flight, the propeller would spin
at flat pitch, but for high-speed flight the propeller
would gradually increase pitch, propelling the
Cheyenne to higher speeds until its stubby wings were
providing most of the lift.
Flight photos by Greg Gimlick
The Cheyenne made its flying debut at the 2011 IRCHA Jamboree. The sleek
machine looks great in the air, and the wings and color scheme help visual
orientation.
This static shot shows the scale four-bladed tail rotor and
three-bladed pusher propeller. Callie Graphics supplied the
CNC stencils.
30 MODEL AVIATION
Static photos by the author
With the smaller hatch option, the mechanics are a tight fit.
The hatch opening was reinforced with .030-inch G-10 fiberglass,
but 1/32 plywood would work just fine. The larger hatch option
makes installation easier.
The wings are carved from soft balsa, and the sharp TEs are
reinforced with ¼-inch strips of 1/64 plywood. The joiners are
snug-fitting carbon-fiber tubes, and music wire pins through the
joiners lock the wings in place.
The prototype plug was carved from foam
and balsa then glassed and primed. This
pattern was used to make the production
fiberglass molds. Molded fiberglass and
vacuum-formed parts are available from
the author.
The exhaust pipe is a rolled strip of paper, saturated with
finishing resin. Once cured, it’s trimmed flush with the opening.
To fit inside the body, the stock main gear
needs to be replaced with a 132-tooth gear.
The author machined out the hub on
the gear so it could be mounted in a
standard Align gear hub.
The retracts fold back into the sponsons. The prototype’s
actuation system was gear-driven, but a bellcrank will also work.
Install carbon fiber or 1/16-plywood doublers at the front of the
retract openings to provide a positive stop.
December 2011 31
The main frame was built using modified T-Rex 450
parts. The rear (pitch) cyclic servo needed to be
relocated to avoid touching the body. The aluminum
mounting flanges were cut from ½-inch aluminum
angle and installed with 16mm socket-head screws.
The removable canopy has a balsa cockpit floor,
finished with resin-saturated paper. The hatch has
a plywood tab at the rear and is secured with ¼-
inch rare-earth magnets at the front.
If you opt for the 132T main gear, you’ll need to
mill the slots on the motor mount. This allows
the motor longer travel to engage the smaller
main gear.
The compound tail unit,
featuring 2½-inch long
aluminum stand-offs, the offset
bellcrank, and the tail-rotor
pitch linkage, is the most
complex part of the build. The
thrust collar on the tail-rotor
shaft keeps the bevel gears
from driving together.
The Cheyenne placed first in the Helicopter Class at the 2011 Toledo Weak
Signals Expo. Note the attack heli’s compact size.
Specifications:
Rotor span: 28 inches (711mm)
Length: 30 inches (762mm)
Weight: 38 ounces (1077g)
Gear Used:
Transmitter: Multiplex Royal Evo 12 with Spektrum
conversion
Receiver: Spektrum AR9000
Servos: Cyclic Hitec HS-5055MG
Tail: JR DS287MG
Motor: Scorpion HK-2221-12
ESC: Castle Creations Phoenix 35
Battery: 2200-3S LiPo
Accessories: Align GP750
heading-hold gyro and Castle
CC-BEC unit
At this point the main rotor was running
nearly flat pitch, but it continued to provide
pitch and roll control. This ingenious design
gave the Cheyenne unprecedented speed and
range.
In addition to its unique propulsion
system, the AH-56 incorporated a number of
other novel features that would eventually
become standard on the next generation of
high-performance attack helis. These included
a rigid rotor head, terrain mapping navigation,
and helmet-mounted sights.
Progress on the project was slowed by the
ongoing war in Southeast Asia, and by the
early 1970s the Army was becoming
increasingly focused on Soviet armor. In the
end, the Pentagon changed its mind and
decided that instead of a high-speed gunship,
it really needed a tank killer that would use
terrain as its main defense. The AH-64
Apache was the result, and it has served
capably for more than 30 years.
As a lifelong aviation nut, I was fascinated
with the Cheyenne program when I was a kid.
I was disappointed when it was canceled, so
nearly 40 years later I decided to design my
own AH-56, based on mechanics from the
ubiquitous Align T-Rex 450.
The key to making this project feasible
was Align’s release of a torque-tube tail
retrofit kit, because this was the only practical
way to drive the complex tail gearbox. With
this important requirement checked off, I set
out to build my own Cheyenne.
Builder Notes and Considerations:
Replicating the compound tail of the original
Cheyenne was central to the project. The tail
unit adds a degree of complexity, and some
builders may opt to go with a normal tail
rotor.
I wanted the body to be as true to scale as
possible. Because the Cheyenne was an
unusually long and slender helicopter, this
imposed some challenges to fitting the
mechanics inside. A standard 150-tooth main
gear is too large to fit inside the fuselage. The
builder can cut clearance slots in the fuselage
sides, but I opted to modify the mechanics to
use a smaller 132T gear.
The low fuselage profile doesn’t provide
enough height for a standard 450-size motor. I
opted to cut a clearance hole, which also
provided cool air to the motor.
During the construction steps, I’ll outline
my solutions to the various challenges that
arose, but I’ll also offer alternatives that will
simplify the build. This advanced construction
project requires machining and fabrication
skills beyond those usually required for a
helicopter build.
(Note: A molded-parts kit that includes the
fiberglass fuselage, sponsons, the vacuumformed
canopy, chin turret, and tail rotor
housing is available from the author. Email
him for details.)
Tail Unit: The tail unit on the prototype was
fabricated using parts from the Align torquetube
conversion kit. Because an extra gearbox
side plate was needed, I purchased a spare tail
unit as well. The standoffs for the extended
tail-rotor mount were fabricated from 4mm
aluminum knitting needles, cut to 2½-inches
(63.5mm) in length, with the tips drilled and
tapped for M2 screws. The extended tail-rotor
shaft is a 120mm piece of 3mm stainless drill
rod, and the modified bevel gear was secured
on the shaft with J-B Weld.
Pitch control on the tail rotor was achieved
by fabricating an offset bellcrank, with a pivot
arm to keep the pitch-control slider from
flopping around. The pusher propeller can be
controlled with a simple straight pushrod.
The one component in the tail unit that
requires real precision is the rear bearing
block. Although not complex in shape, the
holes for the mounting screws and the center
hole for the propeller-shaft ball bearing must
be perfectly centered if the propeller is to spin
without runout.
The pusher propeller shaft is a spare tailrotor
shaft, press-fit into the bevel gear.
Again, drilling this hole precisely on center is
critical, so using a lathe is a necessity.
With the tail unit completed, I installed it
on an otherwise stock T-Rex 450 (with the
main rotor assembled for counterclockwise
rotation) and logged a series of test flights.
These tests revealed no major handling
problems, and the pusher propeller proved to
be effective.
Main Frame: The main frame is primarily
the upper side plates for a stock T-Rex 450-
V2. The mounting flanges were cut from ½-
inch aluminum angle stock from the hardware
store. Because of the thickness of the
aluminum, I increased the length of the lower
assembly screws to 16mm. I trimmed the
lower side frames flush with the bottom of the
mounting flanges.
To get the rear cyclic servo to fit inside of
the body I had to relocate it as shown on the
plans. This is a relatively simple modification
that I’ve used on previous scale helicopters.
As noted earlier, in order to have the
mechanics completely concealed, I opted to
replace the stock 150T main gear with a 132T
gear. This led to two or three other changes. I
had to modify the motor mount to allow the
motor to slide far enough to mesh with the
teeth of the smaller gear.
I also had to grind down the base of the
pinion so that it wouldn’t rub on the
autorotation gear. You can save some trouble
by using the stock gear train if you don’t mind
cutting clearance slots in the body.
Body:With the mechanics thoroughly tested,
the next step is installing them in the body.
The Cheyenne’s fuselage is slender by scale
helicopter standards, and careful planning is
needed to get everything to fit.
For my build, I elected to use a removable
canopy for battery access, and I made the
hatch between the cockpit and main shaft as
small as possible. This works, but easing the
mechanics through the opening is a very tight
squeeze. You could make your life easier if
you cut the hatch to the larger opening shown
on the plans. This works particularly well if
you opt to cut clearance slots for the stock
150T main gear.
After cutting the hatch opening, the 1/8-inch
plywood bearers are epoxied into place. These
have 4-40 blind nuts fitted for securing the
mechanics and 1/16-inch vertical doublers to
provide reinforcement for the wing joiners and
landing gear.
After drilling holes for these and test-fitting
the landing gear, I glued the molded sponsons
in place with thin CA. Note that the sponsons
stiffen the forward fuselage significantly.
Stub Wings: I elected to carve the small wings
from soft balsa, and I mounted them to the
body using carbon-fiber joiner tubes from The
Composites Store Inc. (CST). I molded their
roots using Bondo, reinforced the TEs with a
¼-inch strip of 1/64-inch plywood and finished
them with ½-ounce fiberglass cloth.
Landing Gear: The retractable landing gear
folds backward into the sponsons. There are
several ways to actuate the gear, but I opted for
a simple gear train built with RC car pinions. I
used a metal gear microservo to actuate the
gear.
Finishing: The preproduction Cheyenne wore
a number of color schemes during the life of
the test program. I opted for the standard Army
markings shown on the example at the Army
Aviation Museum at Fort Rucker, Alabama. I
generated CAD drawings for all of the
markings and ordered paint stencils online from
Callie Graphics. Callie has terrific service, and
the price was surprisingly inexpensive.
The overall color scheme was done with
Testors Model Master enamel paints, and
applied with an airbrush. The body was then
clear-coated with matte lacquer.
Test Flying:My first priority was to get the
mechanics flying, and I logged more than 50
flights in that configuration before I added the
body. With the propeller at flat pitch, the
Cheyenne flies similarly to a stock T-Rex
except that it’s slower because of the propeller
disk drag.
As propeller pitch is increased, the
helicopter accelerates forward, and top speed is
impressive. There’s very little pitch change and
no adverse handling issues with the pusher
propeller, and the heli carries more speed into
vertical maneuvers with the added thrust.
I logged all of my early flights with a
standard flybar head, reversed for
counterclockwise rotation. For a better scale
appearance (and greater lift) I switched to a
Black Angel four-bladed head from
eHirobo.com, with weighted scale blades from
SmartModel. You could certainly stick with the
stock flybar head and save some weight in the
process.
After your initial test flights, I recommend
adding ballast to the mechanics to approximate
the weight of the completed helicopter. This
gives you a chance to preview the handling
characteristics and to make any needed tweaks
to the head speed and pitch curve.
The completed Cheyenne made its flying
debut at the 2011 IRCHA Jamboree. After
working out some issues with controller
programming, the heli flew beautifully,
meeting all my hopes for the project. With the
added lift capacity of the four-bladed head, the
Cheyenne doesn’t handle at all like a heavy
Scale model.
In subsequent flights the Cheyenne
continued to improve. At flat pitch, forward
flight is slow and predictable, and the pusher
propeller can actually be a benefit because
there’s little chance of the heli getting away
from you. At full propeller pitch, the Cheyenne
accelerates briskly and really comes to life. It’s
truly a delight to fly.
Conclusion: Designing and building the
Cheyenne was one of the most challenging
projects of my RC career, but also one of the
most rewarding. Building the complex tail
went smoothly, and the real challenges didn’t
begin until I had to squeeze the mechanics into
the slender fuselage. The good news is that all
these puzzles have been solved, so your build
should go easily. Good luck! MA
Jim Ryan
[email protected]
Sources:
CST
(800) 338-1278
www.cstsales.com/index.html
Callie Graphics
(505) 228-2692
www.callie-graphics.com
Testors Model Master paint
(800) 837-8677
www.testors.com
eHirobo
[email protected]
www.ehirobo.com
SmartModel
[email protected]
www.smartmodel.com.hk/index.asp
International Radio Controlled Helicopter
Association
www.ircha.org
Edition: Model Aviation - 2011/12
Page Numbers: 28,29,30,31,32,33,34
Edition: Model Aviation - 2011/12
Page Numbers: 28,29,30,31,32,33,34
INTRODUCTION: In the late 1960s, the US Army
contracted with Lockheed for the construction of a
revolutionary attack helicopter called the AH-56
Cheyenne. In addition to the normal tail rotor, the
Cheyenne had a variable-pitch pusher propeller. For
hovering and low-speed flight, the propeller would spin
at flat pitch, but for high-speed flight the propeller
would gradually increase pitch, propelling the
Cheyenne to higher speeds until its stubby wings were
providing most of the lift.
Flight photos by Greg Gimlick
The Cheyenne made its flying debut at the 2011 IRCHA Jamboree. The sleek
machine looks great in the air, and the wings and color scheme help visual
orientation.
This static shot shows the scale four-bladed tail rotor and
three-bladed pusher propeller. Callie Graphics supplied the
CNC stencils.
30 MODEL AVIATION
Static photos by the author
With the smaller hatch option, the mechanics are a tight fit.
The hatch opening was reinforced with .030-inch G-10 fiberglass,
but 1/32 plywood would work just fine. The larger hatch option
makes installation easier.
The wings are carved from soft balsa, and the sharp TEs are
reinforced with ¼-inch strips of 1/64 plywood. The joiners are
snug-fitting carbon-fiber tubes, and music wire pins through the
joiners lock the wings in place.
The prototype plug was carved from foam
and balsa then glassed and primed. This
pattern was used to make the production
fiberglass molds. Molded fiberglass and
vacuum-formed parts are available from
the author.
The exhaust pipe is a rolled strip of paper, saturated with
finishing resin. Once cured, it’s trimmed flush with the opening.
To fit inside the body, the stock main gear
needs to be replaced with a 132-tooth gear.
The author machined out the hub on
the gear so it could be mounted in a
standard Align gear hub.
The retracts fold back into the sponsons. The prototype’s
actuation system was gear-driven, but a bellcrank will also work.
Install carbon fiber or 1/16-plywood doublers at the front of the
retract openings to provide a positive stop.
December 2011 31
The main frame was built using modified T-Rex 450
parts. The rear (pitch) cyclic servo needed to be
relocated to avoid touching the body. The aluminum
mounting flanges were cut from ½-inch aluminum
angle and installed with 16mm socket-head screws.
The removable canopy has a balsa cockpit floor,
finished with resin-saturated paper. The hatch has
a plywood tab at the rear and is secured with ¼-
inch rare-earth magnets at the front.
If you opt for the 132T main gear, you’ll need to
mill the slots on the motor mount. This allows
the motor longer travel to engage the smaller
main gear.
The compound tail unit,
featuring 2½-inch long
aluminum stand-offs, the offset
bellcrank, and the tail-rotor
pitch linkage, is the most
complex part of the build. The
thrust collar on the tail-rotor
shaft keeps the bevel gears
from driving together.
The Cheyenne placed first in the Helicopter Class at the 2011 Toledo Weak
Signals Expo. Note the attack heli’s compact size.
Specifications:
Rotor span: 28 inches (711mm)
Length: 30 inches (762mm)
Weight: 38 ounces (1077g)
Gear Used:
Transmitter: Multiplex Royal Evo 12 with Spektrum
conversion
Receiver: Spektrum AR9000
Servos: Cyclic Hitec HS-5055MG
Tail: JR DS287MG
Motor: Scorpion HK-2221-12
ESC: Castle Creations Phoenix 35
Battery: 2200-3S LiPo
Accessories: Align GP750
heading-hold gyro and Castle
CC-BEC unit
At this point the main rotor was running
nearly flat pitch, but it continued to provide
pitch and roll control. This ingenious design
gave the Cheyenne unprecedented speed and
range.
In addition to its unique propulsion
system, the AH-56 incorporated a number of
other novel features that would eventually
become standard on the next generation of
high-performance attack helis. These included
a rigid rotor head, terrain mapping navigation,
and helmet-mounted sights.
Progress on the project was slowed by the
ongoing war in Southeast Asia, and by the
early 1970s the Army was becoming
increasingly focused on Soviet armor. In the
end, the Pentagon changed its mind and
decided that instead of a high-speed gunship,
it really needed a tank killer that would use
terrain as its main defense. The AH-64
Apache was the result, and it has served
capably for more than 30 years.
As a lifelong aviation nut, I was fascinated
with the Cheyenne program when I was a kid.
I was disappointed when it was canceled, so
nearly 40 years later I decided to design my
own AH-56, based on mechanics from the
ubiquitous Align T-Rex 450.
The key to making this project feasible
was Align’s release of a torque-tube tail
retrofit kit, because this was the only practical
way to drive the complex tail gearbox. With
this important requirement checked off, I set
out to build my own Cheyenne.
Builder Notes and Considerations:
Replicating the compound tail of the original
Cheyenne was central to the project. The tail
unit adds a degree of complexity, and some
builders may opt to go with a normal tail
rotor.
I wanted the body to be as true to scale as
possible. Because the Cheyenne was an
unusually long and slender helicopter, this
imposed some challenges to fitting the
mechanics inside. A standard 150-tooth main
gear is too large to fit inside the fuselage. The
builder can cut clearance slots in the fuselage
sides, but I opted to modify the mechanics to
use a smaller 132T gear.
The low fuselage profile doesn’t provide
enough height for a standard 450-size motor. I
opted to cut a clearance hole, which also
provided cool air to the motor.
During the construction steps, I’ll outline
my solutions to the various challenges that
arose, but I’ll also offer alternatives that will
simplify the build. This advanced construction
project requires machining and fabrication
skills beyond those usually required for a
helicopter build.
(Note: A molded-parts kit that includes the
fiberglass fuselage, sponsons, the vacuumformed
canopy, chin turret, and tail rotor
housing is available from the author. Email
him for details.)
Tail Unit: The tail unit on the prototype was
fabricated using parts from the Align torquetube
conversion kit. Because an extra gearbox
side plate was needed, I purchased a spare tail
unit as well. The standoffs for the extended
tail-rotor mount were fabricated from 4mm
aluminum knitting needles, cut to 2½-inches
(63.5mm) in length, with the tips drilled and
tapped for M2 screws. The extended tail-rotor
shaft is a 120mm piece of 3mm stainless drill
rod, and the modified bevel gear was secured
on the shaft with J-B Weld.
Pitch control on the tail rotor was achieved
by fabricating an offset bellcrank, with a pivot
arm to keep the pitch-control slider from
flopping around. The pusher propeller can be
controlled with a simple straight pushrod.
The one component in the tail unit that
requires real precision is the rear bearing
block. Although not complex in shape, the
holes for the mounting screws and the center
hole for the propeller-shaft ball bearing must
be perfectly centered if the propeller is to spin
without runout.
The pusher propeller shaft is a spare tailrotor
shaft, press-fit into the bevel gear.
Again, drilling this hole precisely on center is
critical, so using a lathe is a necessity.
With the tail unit completed, I installed it
on an otherwise stock T-Rex 450 (with the
main rotor assembled for counterclockwise
rotation) and logged a series of test flights.
These tests revealed no major handling
problems, and the pusher propeller proved to
be effective.
Main Frame: The main frame is primarily
the upper side plates for a stock T-Rex 450-
V2. The mounting flanges were cut from ½-
inch aluminum angle stock from the hardware
store. Because of the thickness of the
aluminum, I increased the length of the lower
assembly screws to 16mm. I trimmed the
lower side frames flush with the bottom of the
mounting flanges.
To get the rear cyclic servo to fit inside of
the body I had to relocate it as shown on the
plans. This is a relatively simple modification
that I’ve used on previous scale helicopters.
As noted earlier, in order to have the
mechanics completely concealed, I opted to
replace the stock 150T main gear with a 132T
gear. This led to two or three other changes. I
had to modify the motor mount to allow the
motor to slide far enough to mesh with the
teeth of the smaller gear.
I also had to grind down the base of the
pinion so that it wouldn’t rub on the
autorotation gear. You can save some trouble
by using the stock gear train if you don’t mind
cutting clearance slots in the body.
Body:With the mechanics thoroughly tested,
the next step is installing them in the body.
The Cheyenne’s fuselage is slender by scale
helicopter standards, and careful planning is
needed to get everything to fit.
For my build, I elected to use a removable
canopy for battery access, and I made the
hatch between the cockpit and main shaft as
small as possible. This works, but easing the
mechanics through the opening is a very tight
squeeze. You could make your life easier if
you cut the hatch to the larger opening shown
on the plans. This works particularly well if
you opt to cut clearance slots for the stock
150T main gear.
After cutting the hatch opening, the 1/8-inch
plywood bearers are epoxied into place. These
have 4-40 blind nuts fitted for securing the
mechanics and 1/16-inch vertical doublers to
provide reinforcement for the wing joiners and
landing gear.
After drilling holes for these and test-fitting
the landing gear, I glued the molded sponsons
in place with thin CA. Note that the sponsons
stiffen the forward fuselage significantly.
Stub Wings: I elected to carve the small wings
from soft balsa, and I mounted them to the
body using carbon-fiber joiner tubes from The
Composites Store Inc. (CST). I molded their
roots using Bondo, reinforced the TEs with a
¼-inch strip of 1/64-inch plywood and finished
them with ½-ounce fiberglass cloth.
Landing Gear: The retractable landing gear
folds backward into the sponsons. There are
several ways to actuate the gear, but I opted for
a simple gear train built with RC car pinions. I
used a metal gear microservo to actuate the
gear.
Finishing: The preproduction Cheyenne wore
a number of color schemes during the life of
the test program. I opted for the standard Army
markings shown on the example at the Army
Aviation Museum at Fort Rucker, Alabama. I
generated CAD drawings for all of the
markings and ordered paint stencils online from
Callie Graphics. Callie has terrific service, and
the price was surprisingly inexpensive.
The overall color scheme was done with
Testors Model Master enamel paints, and
applied with an airbrush. The body was then
clear-coated with matte lacquer.
Test Flying:My first priority was to get the
mechanics flying, and I logged more than 50
flights in that configuration before I added the
body. With the propeller at flat pitch, the
Cheyenne flies similarly to a stock T-Rex
except that it’s slower because of the propeller
disk drag.
As propeller pitch is increased, the
helicopter accelerates forward, and top speed is
impressive. There’s very little pitch change and
no adverse handling issues with the pusher
propeller, and the heli carries more speed into
vertical maneuvers with the added thrust.
I logged all of my early flights with a
standard flybar head, reversed for
counterclockwise rotation. For a better scale
appearance (and greater lift) I switched to a
Black Angel four-bladed head from
eHirobo.com, with weighted scale blades from
SmartModel. You could certainly stick with the
stock flybar head and save some weight in the
process.
After your initial test flights, I recommend
adding ballast to the mechanics to approximate
the weight of the completed helicopter. This
gives you a chance to preview the handling
characteristics and to make any needed tweaks
to the head speed and pitch curve.
The completed Cheyenne made its flying
debut at the 2011 IRCHA Jamboree. After
working out some issues with controller
programming, the heli flew beautifully,
meeting all my hopes for the project. With the
added lift capacity of the four-bladed head, the
Cheyenne doesn’t handle at all like a heavy
Scale model.
In subsequent flights the Cheyenne
continued to improve. At flat pitch, forward
flight is slow and predictable, and the pusher
propeller can actually be a benefit because
there’s little chance of the heli getting away
from you. At full propeller pitch, the Cheyenne
accelerates briskly and really comes to life. It’s
truly a delight to fly.
Conclusion: Designing and building the
Cheyenne was one of the most challenging
projects of my RC career, but also one of the
most rewarding. Building the complex tail
went smoothly, and the real challenges didn’t
begin until I had to squeeze the mechanics into
the slender fuselage. The good news is that all
these puzzles have been solved, so your build
should go easily. Good luck! MA
Jim Ryan
[email protected]
Sources:
CST
(800) 338-1278
www.cstsales.com/index.html
Callie Graphics
(505) 228-2692
www.callie-graphics.com
Testors Model Master paint
(800) 837-8677
www.testors.com
eHirobo
[email protected]
www.ehirobo.com
SmartModel
[email protected]
www.smartmodel.com.hk/index.asp
International Radio Controlled Helicopter
Association
www.ircha.org
Edition: Model Aviation - 2011/12
Page Numbers: 28,29,30,31,32,33,34
INTRODUCTION: In the late 1960s, the US Army
contracted with Lockheed for the construction of a
revolutionary attack helicopter called the AH-56
Cheyenne. In addition to the normal tail rotor, the
Cheyenne had a variable-pitch pusher propeller. For
hovering and low-speed flight, the propeller would spin
at flat pitch, but for high-speed flight the propeller
would gradually increase pitch, propelling the
Cheyenne to higher speeds until its stubby wings were
providing most of the lift.
Flight photos by Greg Gimlick
The Cheyenne made its flying debut at the 2011 IRCHA Jamboree. The sleek
machine looks great in the air, and the wings and color scheme help visual
orientation.
This static shot shows the scale four-bladed tail rotor and
three-bladed pusher propeller. Callie Graphics supplied the
CNC stencils.
30 MODEL AVIATION
Static photos by the author
With the smaller hatch option, the mechanics are a tight fit.
The hatch opening was reinforced with .030-inch G-10 fiberglass,
but 1/32 plywood would work just fine. The larger hatch option
makes installation easier.
The wings are carved from soft balsa, and the sharp TEs are
reinforced with ¼-inch strips of 1/64 plywood. The joiners are
snug-fitting carbon-fiber tubes, and music wire pins through the
joiners lock the wings in place.
The prototype plug was carved from foam
and balsa then glassed and primed. This
pattern was used to make the production
fiberglass molds. Molded fiberglass and
vacuum-formed parts are available from
the author.
The exhaust pipe is a rolled strip of paper, saturated with
finishing resin. Once cured, it’s trimmed flush with the opening.
To fit inside the body, the stock main gear
needs to be replaced with a 132-tooth gear.
The author machined out the hub on
the gear so it could be mounted in a
standard Align gear hub.
The retracts fold back into the sponsons. The prototype’s
actuation system was gear-driven, but a bellcrank will also work.
Install carbon fiber or 1/16-plywood doublers at the front of the
retract openings to provide a positive stop.
December 2011 31
The main frame was built using modified T-Rex 450
parts. The rear (pitch) cyclic servo needed to be
relocated to avoid touching the body. The aluminum
mounting flanges were cut from ½-inch aluminum
angle and installed with 16mm socket-head screws.
The removable canopy has a balsa cockpit floor,
finished with resin-saturated paper. The hatch has
a plywood tab at the rear and is secured with ¼-
inch rare-earth magnets at the front.
If you opt for the 132T main gear, you’ll need to
mill the slots on the motor mount. This allows
the motor longer travel to engage the smaller
main gear.
The compound tail unit,
featuring 2½-inch long
aluminum stand-offs, the offset
bellcrank, and the tail-rotor
pitch linkage, is the most
complex part of the build. The
thrust collar on the tail-rotor
shaft keeps the bevel gears
from driving together.
The Cheyenne placed first in the Helicopter Class at the 2011 Toledo Weak
Signals Expo. Note the attack heli’s compact size.
Specifications:
Rotor span: 28 inches (711mm)
Length: 30 inches (762mm)
Weight: 38 ounces (1077g)
Gear Used:
Transmitter: Multiplex Royal Evo 12 with Spektrum
conversion
Receiver: Spektrum AR9000
Servos: Cyclic Hitec HS-5055MG
Tail: JR DS287MG
Motor: Scorpion HK-2221-12
ESC: Castle Creations Phoenix 35
Battery: 2200-3S LiPo
Accessories: Align GP750
heading-hold gyro and Castle
CC-BEC unit
At this point the main rotor was running
nearly flat pitch, but it continued to provide
pitch and roll control. This ingenious design
gave the Cheyenne unprecedented speed and
range.
In addition to its unique propulsion
system, the AH-56 incorporated a number of
other novel features that would eventually
become standard on the next generation of
high-performance attack helis. These included
a rigid rotor head, terrain mapping navigation,
and helmet-mounted sights.
Progress on the project was slowed by the
ongoing war in Southeast Asia, and by the
early 1970s the Army was becoming
increasingly focused on Soviet armor. In the
end, the Pentagon changed its mind and
decided that instead of a high-speed gunship,
it really needed a tank killer that would use
terrain as its main defense. The AH-64
Apache was the result, and it has served
capably for more than 30 years.
As a lifelong aviation nut, I was fascinated
with the Cheyenne program when I was a kid.
I was disappointed when it was canceled, so
nearly 40 years later I decided to design my
own AH-56, based on mechanics from the
ubiquitous Align T-Rex 450.
The key to making this project feasible
was Align’s release of a torque-tube tail
retrofit kit, because this was the only practical
way to drive the complex tail gearbox. With
this important requirement checked off, I set
out to build my own Cheyenne.
Builder Notes and Considerations:
Replicating the compound tail of the original
Cheyenne was central to the project. The tail
unit adds a degree of complexity, and some
builders may opt to go with a normal tail
rotor.
I wanted the body to be as true to scale as
possible. Because the Cheyenne was an
unusually long and slender helicopter, this
imposed some challenges to fitting the
mechanics inside. A standard 150-tooth main
gear is too large to fit inside the fuselage. The
builder can cut clearance slots in the fuselage
sides, but I opted to modify the mechanics to
use a smaller 132T gear.
The low fuselage profile doesn’t provide
enough height for a standard 450-size motor. I
opted to cut a clearance hole, which also
provided cool air to the motor.
During the construction steps, I’ll outline
my solutions to the various challenges that
arose, but I’ll also offer alternatives that will
simplify the build. This advanced construction
project requires machining and fabrication
skills beyond those usually required for a
helicopter build.
(Note: A molded-parts kit that includes the
fiberglass fuselage, sponsons, the vacuumformed
canopy, chin turret, and tail rotor
housing is available from the author. Email
him for details.)
Tail Unit: The tail unit on the prototype was
fabricated using parts from the Align torquetube
conversion kit. Because an extra gearbox
side plate was needed, I purchased a spare tail
unit as well. The standoffs for the extended
tail-rotor mount were fabricated from 4mm
aluminum knitting needles, cut to 2½-inches
(63.5mm) in length, with the tips drilled and
tapped for M2 screws. The extended tail-rotor
shaft is a 120mm piece of 3mm stainless drill
rod, and the modified bevel gear was secured
on the shaft with J-B Weld.
Pitch control on the tail rotor was achieved
by fabricating an offset bellcrank, with a pivot
arm to keep the pitch-control slider from
flopping around. The pusher propeller can be
controlled with a simple straight pushrod.
The one component in the tail unit that
requires real precision is the rear bearing
block. Although not complex in shape, the
holes for the mounting screws and the center
hole for the propeller-shaft ball bearing must
be perfectly centered if the propeller is to spin
without runout.
The pusher propeller shaft is a spare tailrotor
shaft, press-fit into the bevel gear.
Again, drilling this hole precisely on center is
critical, so using a lathe is a necessity.
With the tail unit completed, I installed it
on an otherwise stock T-Rex 450 (with the
main rotor assembled for counterclockwise
rotation) and logged a series of test flights.
These tests revealed no major handling
problems, and the pusher propeller proved to
be effective.
Main Frame: The main frame is primarily
the upper side plates for a stock T-Rex 450-
V2. The mounting flanges were cut from ½-
inch aluminum angle stock from the hardware
store. Because of the thickness of the
aluminum, I increased the length of the lower
assembly screws to 16mm. I trimmed the
lower side frames flush with the bottom of the
mounting flanges.
To get the rear cyclic servo to fit inside of
the body I had to relocate it as shown on the
plans. This is a relatively simple modification
that I’ve used on previous scale helicopters.
As noted earlier, in order to have the
mechanics completely concealed, I opted to
replace the stock 150T main gear with a 132T
gear. This led to two or three other changes. I
had to modify the motor mount to allow the
motor to slide far enough to mesh with the
teeth of the smaller gear.
I also had to grind down the base of the
pinion so that it wouldn’t rub on the
autorotation gear. You can save some trouble
by using the stock gear train if you don’t mind
cutting clearance slots in the body.
Body:With the mechanics thoroughly tested,
the next step is installing them in the body.
The Cheyenne’s fuselage is slender by scale
helicopter standards, and careful planning is
needed to get everything to fit.
For my build, I elected to use a removable
canopy for battery access, and I made the
hatch between the cockpit and main shaft as
small as possible. This works, but easing the
mechanics through the opening is a very tight
squeeze. You could make your life easier if
you cut the hatch to the larger opening shown
on the plans. This works particularly well if
you opt to cut clearance slots for the stock
150T main gear.
After cutting the hatch opening, the 1/8-inch
plywood bearers are epoxied into place. These
have 4-40 blind nuts fitted for securing the
mechanics and 1/16-inch vertical doublers to
provide reinforcement for the wing joiners and
landing gear.
After drilling holes for these and test-fitting
the landing gear, I glued the molded sponsons
in place with thin CA. Note that the sponsons
stiffen the forward fuselage significantly.
Stub Wings: I elected to carve the small wings
from soft balsa, and I mounted them to the
body using carbon-fiber joiner tubes from The
Composites Store Inc. (CST). I molded their
roots using Bondo, reinforced the TEs with a
¼-inch strip of 1/64-inch plywood and finished
them with ½-ounce fiberglass cloth.
Landing Gear: The retractable landing gear
folds backward into the sponsons. There are
several ways to actuate the gear, but I opted for
a simple gear train built with RC car pinions. I
used a metal gear microservo to actuate the
gear.
Finishing: The preproduction Cheyenne wore
a number of color schemes during the life of
the test program. I opted for the standard Army
markings shown on the example at the Army
Aviation Museum at Fort Rucker, Alabama. I
generated CAD drawings for all of the
markings and ordered paint stencils online from
Callie Graphics. Callie has terrific service, and
the price was surprisingly inexpensive.
The overall color scheme was done with
Testors Model Master enamel paints, and
applied with an airbrush. The body was then
clear-coated with matte lacquer.
Test Flying:My first priority was to get the
mechanics flying, and I logged more than 50
flights in that configuration before I added the
body. With the propeller at flat pitch, the
Cheyenne flies similarly to a stock T-Rex
except that it’s slower because of the propeller
disk drag.
As propeller pitch is increased, the
helicopter accelerates forward, and top speed is
impressive. There’s very little pitch change and
no adverse handling issues with the pusher
propeller, and the heli carries more speed into
vertical maneuvers with the added thrust.
I logged all of my early flights with a
standard flybar head, reversed for
counterclockwise rotation. For a better scale
appearance (and greater lift) I switched to a
Black Angel four-bladed head from
eHirobo.com, with weighted scale blades from
SmartModel. You could certainly stick with the
stock flybar head and save some weight in the
process.
After your initial test flights, I recommend
adding ballast to the mechanics to approximate
the weight of the completed helicopter. This
gives you a chance to preview the handling
characteristics and to make any needed tweaks
to the head speed and pitch curve.
The completed Cheyenne made its flying
debut at the 2011 IRCHA Jamboree. After
working out some issues with controller
programming, the heli flew beautifully,
meeting all my hopes for the project. With the
added lift capacity of the four-bladed head, the
Cheyenne doesn’t handle at all like a heavy
Scale model.
In subsequent flights the Cheyenne
continued to improve. At flat pitch, forward
flight is slow and predictable, and the pusher
propeller can actually be a benefit because
there’s little chance of the heli getting away
from you. At full propeller pitch, the Cheyenne
accelerates briskly and really comes to life. It’s
truly a delight to fly.
Conclusion: Designing and building the
Cheyenne was one of the most challenging
projects of my RC career, but also one of the
most rewarding. Building the complex tail
went smoothly, and the real challenges didn’t
begin until I had to squeeze the mechanics into
the slender fuselage. The good news is that all
these puzzles have been solved, so your build
should go easily. Good luck! MA
Jim Ryan
[email protected]
Sources:
CST
(800) 338-1278
www.cstsales.com/index.html
Callie Graphics
(505) 228-2692
www.callie-graphics.com
Testors Model Master paint
(800) 837-8677
www.testors.com
eHirobo
[email protected]
www.ehirobo.com
SmartModel
[email protected]
www.smartmodel.com.hk/index.asp
International Radio Controlled Helicopter
Association
www.ircha.org
Edition: Model Aviation - 2011/12
Page Numbers: 28,29,30,31,32,33,34
INTRODUCTION: In the late 1960s, the US Army
contracted with Lockheed for the construction of a
revolutionary attack helicopter called the AH-56
Cheyenne. In addition to the normal tail rotor, the
Cheyenne had a variable-pitch pusher propeller. For
hovering and low-speed flight, the propeller would spin
at flat pitch, but for high-speed flight the propeller
would gradually increase pitch, propelling the
Cheyenne to higher speeds until its stubby wings were
providing most of the lift.
Flight photos by Greg Gimlick
The Cheyenne made its flying debut at the 2011 IRCHA Jamboree. The sleek
machine looks great in the air, and the wings and color scheme help visual
orientation.
This static shot shows the scale four-bladed tail rotor and
three-bladed pusher propeller. Callie Graphics supplied the
CNC stencils.
30 MODEL AVIATION
Static photos by the author
With the smaller hatch option, the mechanics are a tight fit.
The hatch opening was reinforced with .030-inch G-10 fiberglass,
but 1/32 plywood would work just fine. The larger hatch option
makes installation easier.
The wings are carved from soft balsa, and the sharp TEs are
reinforced with ¼-inch strips of 1/64 plywood. The joiners are
snug-fitting carbon-fiber tubes, and music wire pins through the
joiners lock the wings in place.
The prototype plug was carved from foam
and balsa then glassed and primed. This
pattern was used to make the production
fiberglass molds. Molded fiberglass and
vacuum-formed parts are available from
the author.
The exhaust pipe is a rolled strip of paper, saturated with
finishing resin. Once cured, it’s trimmed flush with the opening.
To fit inside the body, the stock main gear
needs to be replaced with a 132-tooth gear.
The author machined out the hub on
the gear so it could be mounted in a
standard Align gear hub.
The retracts fold back into the sponsons. The prototype’s
actuation system was gear-driven, but a bellcrank will also work.
Install carbon fiber or 1/16-plywood doublers at the front of the
retract openings to provide a positive stop.
December 2011 31
The main frame was built using modified T-Rex 450
parts. The rear (pitch) cyclic servo needed to be
relocated to avoid touching the body. The aluminum
mounting flanges were cut from ½-inch aluminum
angle and installed with 16mm socket-head screws.
The removable canopy has a balsa cockpit floor,
finished with resin-saturated paper. The hatch has
a plywood tab at the rear and is secured with ¼-
inch rare-earth magnets at the front.
If you opt for the 132T main gear, you’ll need to
mill the slots on the motor mount. This allows
the motor longer travel to engage the smaller
main gear.
The compound tail unit,
featuring 2½-inch long
aluminum stand-offs, the offset
bellcrank, and the tail-rotor
pitch linkage, is the most
complex part of the build. The
thrust collar on the tail-rotor
shaft keeps the bevel gears
from driving together.
The Cheyenne placed first in the Helicopter Class at the 2011 Toledo Weak
Signals Expo. Note the attack heli’s compact size.
Specifications:
Rotor span: 28 inches (711mm)
Length: 30 inches (762mm)
Weight: 38 ounces (1077g)
Gear Used:
Transmitter: Multiplex Royal Evo 12 with Spektrum
conversion
Receiver: Spektrum AR9000
Servos: Cyclic Hitec HS-5055MG
Tail: JR DS287MG
Motor: Scorpion HK-2221-12
ESC: Castle Creations Phoenix 35
Battery: 2200-3S LiPo
Accessories: Align GP750
heading-hold gyro and Castle
CC-BEC unit
At this point the main rotor was running
nearly flat pitch, but it continued to provide
pitch and roll control. This ingenious design
gave the Cheyenne unprecedented speed and
range.
In addition to its unique propulsion
system, the AH-56 incorporated a number of
other novel features that would eventually
become standard on the next generation of
high-performance attack helis. These included
a rigid rotor head, terrain mapping navigation,
and helmet-mounted sights.
Progress on the project was slowed by the
ongoing war in Southeast Asia, and by the
early 1970s the Army was becoming
increasingly focused on Soviet armor. In the
end, the Pentagon changed its mind and
decided that instead of a high-speed gunship,
it really needed a tank killer that would use
terrain as its main defense. The AH-64
Apache was the result, and it has served
capably for more than 30 years.
As a lifelong aviation nut, I was fascinated
with the Cheyenne program when I was a kid.
I was disappointed when it was canceled, so
nearly 40 years later I decided to design my
own AH-56, based on mechanics from the
ubiquitous Align T-Rex 450.
The key to making this project feasible
was Align’s release of a torque-tube tail
retrofit kit, because this was the only practical
way to drive the complex tail gearbox. With
this important requirement checked off, I set
out to build my own Cheyenne.
Builder Notes and Considerations:
Replicating the compound tail of the original
Cheyenne was central to the project. The tail
unit adds a degree of complexity, and some
builders may opt to go with a normal tail
rotor.
I wanted the body to be as true to scale as
possible. Because the Cheyenne was an
unusually long and slender helicopter, this
imposed some challenges to fitting the
mechanics inside. A standard 150-tooth main
gear is too large to fit inside the fuselage. The
builder can cut clearance slots in the fuselage
sides, but I opted to modify the mechanics to
use a smaller 132T gear.
The low fuselage profile doesn’t provide
enough height for a standard 450-size motor. I
opted to cut a clearance hole, which also
provided cool air to the motor.
During the construction steps, I’ll outline
my solutions to the various challenges that
arose, but I’ll also offer alternatives that will
simplify the build. This advanced construction
project requires machining and fabrication
skills beyond those usually required for a
helicopter build.
(Note: A molded-parts kit that includes the
fiberglass fuselage, sponsons, the vacuumformed
canopy, chin turret, and tail rotor
housing is available from the author. Email
him for details.)
Tail Unit: The tail unit on the prototype was
fabricated using parts from the Align torquetube
conversion kit. Because an extra gearbox
side plate was needed, I purchased a spare tail
unit as well. The standoffs for the extended
tail-rotor mount were fabricated from 4mm
aluminum knitting needles, cut to 2½-inches
(63.5mm) in length, with the tips drilled and
tapped for M2 screws. The extended tail-rotor
shaft is a 120mm piece of 3mm stainless drill
rod, and the modified bevel gear was secured
on the shaft with J-B Weld.
Pitch control on the tail rotor was achieved
by fabricating an offset bellcrank, with a pivot
arm to keep the pitch-control slider from
flopping around. The pusher propeller can be
controlled with a simple straight pushrod.
The one component in the tail unit that
requires real precision is the rear bearing
block. Although not complex in shape, the
holes for the mounting screws and the center
hole for the propeller-shaft ball bearing must
be perfectly centered if the propeller is to spin
without runout.
The pusher propeller shaft is a spare tailrotor
shaft, press-fit into the bevel gear.
Again, drilling this hole precisely on center is
critical, so using a lathe is a necessity.
With the tail unit completed, I installed it
on an otherwise stock T-Rex 450 (with the
main rotor assembled for counterclockwise
rotation) and logged a series of test flights.
These tests revealed no major handling
problems, and the pusher propeller proved to
be effective.
Main Frame: The main frame is primarily
the upper side plates for a stock T-Rex 450-
V2. The mounting flanges were cut from ½-
inch aluminum angle stock from the hardware
store. Because of the thickness of the
aluminum, I increased the length of the lower
assembly screws to 16mm. I trimmed the
lower side frames flush with the bottom of the
mounting flanges.
To get the rear cyclic servo to fit inside of
the body I had to relocate it as shown on the
plans. This is a relatively simple modification
that I’ve used on previous scale helicopters.
As noted earlier, in order to have the
mechanics completely concealed, I opted to
replace the stock 150T main gear with a 132T
gear. This led to two or three other changes. I
had to modify the motor mount to allow the
motor to slide far enough to mesh with the
teeth of the smaller gear.
I also had to grind down the base of the
pinion so that it wouldn’t rub on the
autorotation gear. You can save some trouble
by using the stock gear train if you don’t mind
cutting clearance slots in the body.
Body:With the mechanics thoroughly tested,
the next step is installing them in the body.
The Cheyenne’s fuselage is slender by scale
helicopter standards, and careful planning is
needed to get everything to fit.
For my build, I elected to use a removable
canopy for battery access, and I made the
hatch between the cockpit and main shaft as
small as possible. This works, but easing the
mechanics through the opening is a very tight
squeeze. You could make your life easier if
you cut the hatch to the larger opening shown
on the plans. This works particularly well if
you opt to cut clearance slots for the stock
150T main gear.
After cutting the hatch opening, the 1/8-inch
plywood bearers are epoxied into place. These
have 4-40 blind nuts fitted for securing the
mechanics and 1/16-inch vertical doublers to
provide reinforcement for the wing joiners and
landing gear.
After drilling holes for these and test-fitting
the landing gear, I glued the molded sponsons
in place with thin CA. Note that the sponsons
stiffen the forward fuselage significantly.
Stub Wings: I elected to carve the small wings
from soft balsa, and I mounted them to the
body using carbon-fiber joiner tubes from The
Composites Store Inc. (CST). I molded their
roots using Bondo, reinforced the TEs with a
¼-inch strip of 1/64-inch plywood and finished
them with ½-ounce fiberglass cloth.
Landing Gear: The retractable landing gear
folds backward into the sponsons. There are
several ways to actuate the gear, but I opted for
a simple gear train built with RC car pinions. I
used a metal gear microservo to actuate the
gear.
Finishing: The preproduction Cheyenne wore
a number of color schemes during the life of
the test program. I opted for the standard Army
markings shown on the example at the Army
Aviation Museum at Fort Rucker, Alabama. I
generated CAD drawings for all of the
markings and ordered paint stencils online from
Callie Graphics. Callie has terrific service, and
the price was surprisingly inexpensive.
The overall color scheme was done with
Testors Model Master enamel paints, and
applied with an airbrush. The body was then
clear-coated with matte lacquer.
Test Flying:My first priority was to get the
mechanics flying, and I logged more than 50
flights in that configuration before I added the
body. With the propeller at flat pitch, the
Cheyenne flies similarly to a stock T-Rex
except that it’s slower because of the propeller
disk drag.
As propeller pitch is increased, the
helicopter accelerates forward, and top speed is
impressive. There’s very little pitch change and
no adverse handling issues with the pusher
propeller, and the heli carries more speed into
vertical maneuvers with the added thrust.
I logged all of my early flights with a
standard flybar head, reversed for
counterclockwise rotation. For a better scale
appearance (and greater lift) I switched to a
Black Angel four-bladed head from
eHirobo.com, with weighted scale blades from
SmartModel. You could certainly stick with the
stock flybar head and save some weight in the
process.
After your initial test flights, I recommend
adding ballast to the mechanics to approximate
the weight of the completed helicopter. This
gives you a chance to preview the handling
characteristics and to make any needed tweaks
to the head speed and pitch curve.
The completed Cheyenne made its flying
debut at the 2011 IRCHA Jamboree. After
working out some issues with controller
programming, the heli flew beautifully,
meeting all my hopes for the project. With the
added lift capacity of the four-bladed head, the
Cheyenne doesn’t handle at all like a heavy
Scale model.
In subsequent flights the Cheyenne
continued to improve. At flat pitch, forward
flight is slow and predictable, and the pusher
propeller can actually be a benefit because
there’s little chance of the heli getting away
from you. At full propeller pitch, the Cheyenne
accelerates briskly and really comes to life. It’s
truly a delight to fly.
Conclusion: Designing and building the
Cheyenne was one of the most challenging
projects of my RC career, but also one of the
most rewarding. Building the complex tail
went smoothly, and the real challenges didn’t
begin until I had to squeeze the mechanics into
the slender fuselage. The good news is that all
these puzzles have been solved, so your build
should go easily. Good luck! MA
Jim Ryan
[email protected]
Sources:
CST
(800) 338-1278
www.cstsales.com/index.html
Callie Graphics
(505) 228-2692
www.callie-graphics.com
Testors Model Master paint
(800) 837-8677
www.testors.com
eHirobo
[email protected]
www.ehirobo.com
SmartModel
[email protected]
www.smartmodel.com.hk/index.asp
International Radio Controlled Helicopter
Association
www.ircha.org
Edition: Model Aviation - 2011/12
Page Numbers: 28,29,30,31,32,33,34
INTRODUCTION: In the late 1960s, the US Army
contracted with Lockheed for the construction of a
revolutionary attack helicopter called the AH-56
Cheyenne. In addition to the normal tail rotor, the
Cheyenne had a variable-pitch pusher propeller. For
hovering and low-speed flight, the propeller would spin
at flat pitch, but for high-speed flight the propeller
would gradually increase pitch, propelling the
Cheyenne to higher speeds until its stubby wings were
providing most of the lift.
Flight photos by Greg Gimlick
The Cheyenne made its flying debut at the 2011 IRCHA Jamboree. The sleek
machine looks great in the air, and the wings and color scheme help visual
orientation.
This static shot shows the scale four-bladed tail rotor and
three-bladed pusher propeller. Callie Graphics supplied the
CNC stencils.
30 MODEL AVIATION
Static photos by the author
With the smaller hatch option, the mechanics are a tight fit.
The hatch opening was reinforced with .030-inch G-10 fiberglass,
but 1/32 plywood would work just fine. The larger hatch option
makes installation easier.
The wings are carved from soft balsa, and the sharp TEs are
reinforced with ¼-inch strips of 1/64 plywood. The joiners are
snug-fitting carbon-fiber tubes, and music wire pins through the
joiners lock the wings in place.
The prototype plug was carved from foam
and balsa then glassed and primed. This
pattern was used to make the production
fiberglass molds. Molded fiberglass and
vacuum-formed parts are available from
the author.
The exhaust pipe is a rolled strip of paper, saturated with
finishing resin. Once cured, it’s trimmed flush with the opening.
To fit inside the body, the stock main gear
needs to be replaced with a 132-tooth gear.
The author machined out the hub on
the gear so it could be mounted in a
standard Align gear hub.
The retracts fold back into the sponsons. The prototype’s
actuation system was gear-driven, but a bellcrank will also work.
Install carbon fiber or 1/16-plywood doublers at the front of the
retract openings to provide a positive stop.
December 2011 31
The main frame was built using modified T-Rex 450
parts. The rear (pitch) cyclic servo needed to be
relocated to avoid touching the body. The aluminum
mounting flanges were cut from ½-inch aluminum
angle and installed with 16mm socket-head screws.
The removable canopy has a balsa cockpit floor,
finished with resin-saturated paper. The hatch has
a plywood tab at the rear and is secured with ¼-
inch rare-earth magnets at the front.
If you opt for the 132T main gear, you’ll need to
mill the slots on the motor mount. This allows
the motor longer travel to engage the smaller
main gear.
The compound tail unit,
featuring 2½-inch long
aluminum stand-offs, the offset
bellcrank, and the tail-rotor
pitch linkage, is the most
complex part of the build. The
thrust collar on the tail-rotor
shaft keeps the bevel gears
from driving together.
The Cheyenne placed first in the Helicopter Class at the 2011 Toledo Weak
Signals Expo. Note the attack heli’s compact size.
Specifications:
Rotor span: 28 inches (711mm)
Length: 30 inches (762mm)
Weight: 38 ounces (1077g)
Gear Used:
Transmitter: Multiplex Royal Evo 12 with Spektrum
conversion
Receiver: Spektrum AR9000
Servos: Cyclic Hitec HS-5055MG
Tail: JR DS287MG
Motor: Scorpion HK-2221-12
ESC: Castle Creations Phoenix 35
Battery: 2200-3S LiPo
Accessories: Align GP750
heading-hold gyro and Castle
CC-BEC unit
At this point the main rotor was running
nearly flat pitch, but it continued to provide
pitch and roll control. This ingenious design
gave the Cheyenne unprecedented speed and
range.
In addition to its unique propulsion
system, the AH-56 incorporated a number of
other novel features that would eventually
become standard on the next generation of
high-performance attack helis. These included
a rigid rotor head, terrain mapping navigation,
and helmet-mounted sights.
Progress on the project was slowed by the
ongoing war in Southeast Asia, and by the
early 1970s the Army was becoming
increasingly focused on Soviet armor. In the
end, the Pentagon changed its mind and
decided that instead of a high-speed gunship,
it really needed a tank killer that would use
terrain as its main defense. The AH-64
Apache was the result, and it has served
capably for more than 30 years.
As a lifelong aviation nut, I was fascinated
with the Cheyenne program when I was a kid.
I was disappointed when it was canceled, so
nearly 40 years later I decided to design my
own AH-56, based on mechanics from the
ubiquitous Align T-Rex 450.
The key to making this project feasible
was Align’s release of a torque-tube tail
retrofit kit, because this was the only practical
way to drive the complex tail gearbox. With
this important requirement checked off, I set
out to build my own Cheyenne.
Builder Notes and Considerations:
Replicating the compound tail of the original
Cheyenne was central to the project. The tail
unit adds a degree of complexity, and some
builders may opt to go with a normal tail
rotor.
I wanted the body to be as true to scale as
possible. Because the Cheyenne was an
unusually long and slender helicopter, this
imposed some challenges to fitting the
mechanics inside. A standard 150-tooth main
gear is too large to fit inside the fuselage. The
builder can cut clearance slots in the fuselage
sides, but I opted to modify the mechanics to
use a smaller 132T gear.
The low fuselage profile doesn’t provide
enough height for a standard 450-size motor. I
opted to cut a clearance hole, which also
provided cool air to the motor.
During the construction steps, I’ll outline
my solutions to the various challenges that
arose, but I’ll also offer alternatives that will
simplify the build. This advanced construction
project requires machining and fabrication
skills beyond those usually required for a
helicopter build.
(Note: A molded-parts kit that includes the
fiberglass fuselage, sponsons, the vacuumformed
canopy, chin turret, and tail rotor
housing is available from the author. Email
him for details.)
Tail Unit: The tail unit on the prototype was
fabricated using parts from the Align torquetube
conversion kit. Because an extra gearbox
side plate was needed, I purchased a spare tail
unit as well. The standoffs for the extended
tail-rotor mount were fabricated from 4mm
aluminum knitting needles, cut to 2½-inches
(63.5mm) in length, with the tips drilled and
tapped for M2 screws. The extended tail-rotor
shaft is a 120mm piece of 3mm stainless drill
rod, and the modified bevel gear was secured
on the shaft with J-B Weld.
Pitch control on the tail rotor was achieved
by fabricating an offset bellcrank, with a pivot
arm to keep the pitch-control slider from
flopping around. The pusher propeller can be
controlled with a simple straight pushrod.
The one component in the tail unit that
requires real precision is the rear bearing
block. Although not complex in shape, the
holes for the mounting screws and the center
hole for the propeller-shaft ball bearing must
be perfectly centered if the propeller is to spin
without runout.
The pusher propeller shaft is a spare tailrotor
shaft, press-fit into the bevel gear.
Again, drilling this hole precisely on center is
critical, so using a lathe is a necessity.
With the tail unit completed, I installed it
on an otherwise stock T-Rex 450 (with the
main rotor assembled for counterclockwise
rotation) and logged a series of test flights.
These tests revealed no major handling
problems, and the pusher propeller proved to
be effective.
Main Frame: The main frame is primarily
the upper side plates for a stock T-Rex 450-
V2. The mounting flanges were cut from ½-
inch aluminum angle stock from the hardware
store. Because of the thickness of the
aluminum, I increased the length of the lower
assembly screws to 16mm. I trimmed the
lower side frames flush with the bottom of the
mounting flanges.
To get the rear cyclic servo to fit inside of
the body I had to relocate it as shown on the
plans. This is a relatively simple modification
that I’ve used on previous scale helicopters.
As noted earlier, in order to have the
mechanics completely concealed, I opted to
replace the stock 150T main gear with a 132T
gear. This led to two or three other changes. I
had to modify the motor mount to allow the
motor to slide far enough to mesh with the
teeth of the smaller gear.
I also had to grind down the base of the
pinion so that it wouldn’t rub on the
autorotation gear. You can save some trouble
by using the stock gear train if you don’t mind
cutting clearance slots in the body.
Body:With the mechanics thoroughly tested,
the next step is installing them in the body.
The Cheyenne’s fuselage is slender by scale
helicopter standards, and careful planning is
needed to get everything to fit.
For my build, I elected to use a removable
canopy for battery access, and I made the
hatch between the cockpit and main shaft as
small as possible. This works, but easing the
mechanics through the opening is a very tight
squeeze. You could make your life easier if
you cut the hatch to the larger opening shown
on the plans. This works particularly well if
you opt to cut clearance slots for the stock
150T main gear.
After cutting the hatch opening, the 1/8-inch
plywood bearers are epoxied into place. These
have 4-40 blind nuts fitted for securing the
mechanics and 1/16-inch vertical doublers to
provide reinforcement for the wing joiners and
landing gear.
After drilling holes for these and test-fitting
the landing gear, I glued the molded sponsons
in place with thin CA. Note that the sponsons
stiffen the forward fuselage significantly.
Stub Wings: I elected to carve the small wings
from soft balsa, and I mounted them to the
body using carbon-fiber joiner tubes from The
Composites Store Inc. (CST). I molded their
roots using Bondo, reinforced the TEs with a
¼-inch strip of 1/64-inch plywood and finished
them with ½-ounce fiberglass cloth.
Landing Gear: The retractable landing gear
folds backward into the sponsons. There are
several ways to actuate the gear, but I opted for
a simple gear train built with RC car pinions. I
used a metal gear microservo to actuate the
gear.
Finishing: The preproduction Cheyenne wore
a number of color schemes during the life of
the test program. I opted for the standard Army
markings shown on the example at the Army
Aviation Museum at Fort Rucker, Alabama. I
generated CAD drawings for all of the
markings and ordered paint stencils online from
Callie Graphics. Callie has terrific service, and
the price was surprisingly inexpensive.
The overall color scheme was done with
Testors Model Master enamel paints, and
applied with an airbrush. The body was then
clear-coated with matte lacquer.
Test Flying:My first priority was to get the
mechanics flying, and I logged more than 50
flights in that configuration before I added the
body. With the propeller at flat pitch, the
Cheyenne flies similarly to a stock T-Rex
except that it’s slower because of the propeller
disk drag.
As propeller pitch is increased, the
helicopter accelerates forward, and top speed is
impressive. There’s very little pitch change and
no adverse handling issues with the pusher
propeller, and the heli carries more speed into
vertical maneuvers with the added thrust.
I logged all of my early flights with a
standard flybar head, reversed for
counterclockwise rotation. For a better scale
appearance (and greater lift) I switched to a
Black Angel four-bladed head from
eHirobo.com, with weighted scale blades from
SmartModel. You could certainly stick with the
stock flybar head and save some weight in the
process.
After your initial test flights, I recommend
adding ballast to the mechanics to approximate
the weight of the completed helicopter. This
gives you a chance to preview the handling
characteristics and to make any needed tweaks
to the head speed and pitch curve.
The completed Cheyenne made its flying
debut at the 2011 IRCHA Jamboree. After
working out some issues with controller
programming, the heli flew beautifully,
meeting all my hopes for the project. With the
added lift capacity of the four-bladed head, the
Cheyenne doesn’t handle at all like a heavy
Scale model.
In subsequent flights the Cheyenne
continued to improve. At flat pitch, forward
flight is slow and predictable, and the pusher
propeller can actually be a benefit because
there’s little chance of the heli getting away
from you. At full propeller pitch, the Cheyenne
accelerates briskly and really comes to life. It’s
truly a delight to fly.
Conclusion: Designing and building the
Cheyenne was one of the most challenging
projects of my RC career, but also one of the
most rewarding. Building the complex tail
went smoothly, and the real challenges didn’t
begin until I had to squeeze the mechanics into
the slender fuselage. The good news is that all
these puzzles have been solved, so your build
should go easily. Good luck! MA
Jim Ryan
[email protected]
Sources:
CST
(800) 338-1278
www.cstsales.com/index.html
Callie Graphics
(505) 228-2692
www.callie-graphics.com
Testors Model Master paint
(800) 837-8677
www.testors.com
eHirobo
[email protected]
www.ehirobo.com
SmartModel
[email protected]
www.smartmodel.com.hk/index.asp
International Radio Controlled Helicopter
Association
www.ircha.org
Edition: Model Aviation - 2011/12
Page Numbers: 28,29,30,31,32,33,34
INTRODUCTION: In the late 1960s, the US Army
contracted with Lockheed for the construction of a
revolutionary attack helicopter called the AH-56
Cheyenne. In addition to the normal tail rotor, the
Cheyenne had a variable-pitch pusher propeller. For
hovering and low-speed flight, the propeller would spin
at flat pitch, but for high-speed flight the propeller
would gradually increase pitch, propelling the
Cheyenne to higher speeds until its stubby wings were
providing most of the lift.
Flight photos by Greg Gimlick
The Cheyenne made its flying debut at the 2011 IRCHA Jamboree. The sleek
machine looks great in the air, and the wings and color scheme help visual
orientation.
This static shot shows the scale four-bladed tail rotor and
three-bladed pusher propeller. Callie Graphics supplied the
CNC stencils.
30 MODEL AVIATION
Static photos by the author
With the smaller hatch option, the mechanics are a tight fit.
The hatch opening was reinforced with .030-inch G-10 fiberglass,
but 1/32 plywood would work just fine. The larger hatch option
makes installation easier.
The wings are carved from soft balsa, and the sharp TEs are
reinforced with ¼-inch strips of 1/64 plywood. The joiners are
snug-fitting carbon-fiber tubes, and music wire pins through the
joiners lock the wings in place.
The prototype plug was carved from foam
and balsa then glassed and primed. This
pattern was used to make the production
fiberglass molds. Molded fiberglass and
vacuum-formed parts are available from
the author.
The exhaust pipe is a rolled strip of paper, saturated with
finishing resin. Once cured, it’s trimmed flush with the opening.
To fit inside the body, the stock main gear
needs to be replaced with a 132-tooth gear.
The author machined out the hub on
the gear so it could be mounted in a
standard Align gear hub.
The retracts fold back into the sponsons. The prototype’s
actuation system was gear-driven, but a bellcrank will also work.
Install carbon fiber or 1/16-plywood doublers at the front of the
retract openings to provide a positive stop.
December 2011 31
The main frame was built using modified T-Rex 450
parts. The rear (pitch) cyclic servo needed to be
relocated to avoid touching the body. The aluminum
mounting flanges were cut from ½-inch aluminum
angle and installed with 16mm socket-head screws.
The removable canopy has a balsa cockpit floor,
finished with resin-saturated paper. The hatch has
a plywood tab at the rear and is secured with ¼-
inch rare-earth magnets at the front.
If you opt for the 132T main gear, you’ll need to
mill the slots on the motor mount. This allows
the motor longer travel to engage the smaller
main gear.
The compound tail unit,
featuring 2½-inch long
aluminum stand-offs, the offset
bellcrank, and the tail-rotor
pitch linkage, is the most
complex part of the build. The
thrust collar on the tail-rotor
shaft keeps the bevel gears
from driving together.
The Cheyenne placed first in the Helicopter Class at the 2011 Toledo Weak
Signals Expo. Note the attack heli’s compact size.
Specifications:
Rotor span: 28 inches (711mm)
Length: 30 inches (762mm)
Weight: 38 ounces (1077g)
Gear Used:
Transmitter: Multiplex Royal Evo 12 with Spektrum
conversion
Receiver: Spektrum AR9000
Servos: Cyclic Hitec HS-5055MG
Tail: JR DS287MG
Motor: Scorpion HK-2221-12
ESC: Castle Creations Phoenix 35
Battery: 2200-3S LiPo
Accessories: Align GP750
heading-hold gyro and Castle
CC-BEC unit
At this point the main rotor was running
nearly flat pitch, but it continued to provide
pitch and roll control. This ingenious design
gave the Cheyenne unprecedented speed and
range.
In addition to its unique propulsion
system, the AH-56 incorporated a number of
other novel features that would eventually
become standard on the next generation of
high-performance attack helis. These included
a rigid rotor head, terrain mapping navigation,
and helmet-mounted sights.
Progress on the project was slowed by the
ongoing war in Southeast Asia, and by the
early 1970s the Army was becoming
increasingly focused on Soviet armor. In the
end, the Pentagon changed its mind and
decided that instead of a high-speed gunship,
it really needed a tank killer that would use
terrain as its main defense. The AH-64
Apache was the result, and it has served
capably for more than 30 years.
As a lifelong aviation nut, I was fascinated
with the Cheyenne program when I was a kid.
I was disappointed when it was canceled, so
nearly 40 years later I decided to design my
own AH-56, based on mechanics from the
ubiquitous Align T-Rex 450.
The key to making this project feasible
was Align’s release of a torque-tube tail
retrofit kit, because this was the only practical
way to drive the complex tail gearbox. With
this important requirement checked off, I set
out to build my own Cheyenne.
Builder Notes and Considerations:
Replicating the compound tail of the original
Cheyenne was central to the project. The tail
unit adds a degree of complexity, and some
builders may opt to go with a normal tail
rotor.
I wanted the body to be as true to scale as
possible. Because the Cheyenne was an
unusually long and slender helicopter, this
imposed some challenges to fitting the
mechanics inside. A standard 150-tooth main
gear is too large to fit inside the fuselage. The
builder can cut clearance slots in the fuselage
sides, but I opted to modify the mechanics to
use a smaller 132T gear.
The low fuselage profile doesn’t provide
enough height for a standard 450-size motor. I
opted to cut a clearance hole, which also
provided cool air to the motor.
During the construction steps, I’ll outline
my solutions to the various challenges that
arose, but I’ll also offer alternatives that will
simplify the build. This advanced construction
project requires machining and fabrication
skills beyond those usually required for a
helicopter build.
(Note: A molded-parts kit that includes the
fiberglass fuselage, sponsons, the vacuumformed
canopy, chin turret, and tail rotor
housing is available from the author. Email
him for details.)
Tail Unit: The tail unit on the prototype was
fabricated using parts from the Align torquetube
conversion kit. Because an extra gearbox
side plate was needed, I purchased a spare tail
unit as well. The standoffs for the extended
tail-rotor mount were fabricated from 4mm
aluminum knitting needles, cut to 2½-inches
(63.5mm) in length, with the tips drilled and
tapped for M2 screws. The extended tail-rotor
shaft is a 120mm piece of 3mm stainless drill
rod, and the modified bevel gear was secured
on the shaft with J-B Weld.
Pitch control on the tail rotor was achieved
by fabricating an offset bellcrank, with a pivot
arm to keep the pitch-control slider from
flopping around. The pusher propeller can be
controlled with a simple straight pushrod.
The one component in the tail unit that
requires real precision is the rear bearing
block. Although not complex in shape, the
holes for the mounting screws and the center
hole for the propeller-shaft ball bearing must
be perfectly centered if the propeller is to spin
without runout.
The pusher propeller shaft is a spare tailrotor
shaft, press-fit into the bevel gear.
Again, drilling this hole precisely on center is
critical, so using a lathe is a necessity.
With the tail unit completed, I installed it
on an otherwise stock T-Rex 450 (with the
main rotor assembled for counterclockwise
rotation) and logged a series of test flights.
These tests revealed no major handling
problems, and the pusher propeller proved to
be effective.
Main Frame: The main frame is primarily
the upper side plates for a stock T-Rex 450-
V2. The mounting flanges were cut from ½-
inch aluminum angle stock from the hardware
store. Because of the thickness of the
aluminum, I increased the length of the lower
assembly screws to 16mm. I trimmed the
lower side frames flush with the bottom of the
mounting flanges.
To get the rear cyclic servo to fit inside of
the body I had to relocate it as shown on the
plans. This is a relatively simple modification
that I’ve used on previous scale helicopters.
As noted earlier, in order to have the
mechanics completely concealed, I opted to
replace the stock 150T main gear with a 132T
gear. This led to two or three other changes. I
had to modify the motor mount to allow the
motor to slide far enough to mesh with the
teeth of the smaller gear.
I also had to grind down the base of the
pinion so that it wouldn’t rub on the
autorotation gear. You can save some trouble
by using the stock gear train if you don’t mind
cutting clearance slots in the body.
Body:With the mechanics thoroughly tested,
the next step is installing them in the body.
The Cheyenne’s fuselage is slender by scale
helicopter standards, and careful planning is
needed to get everything to fit.
For my build, I elected to use a removable
canopy for battery access, and I made the
hatch between the cockpit and main shaft as
small as possible. This works, but easing the
mechanics through the opening is a very tight
squeeze. You could make your life easier if
you cut the hatch to the larger opening shown
on the plans. This works particularly well if
you opt to cut clearance slots for the stock
150T main gear.
After cutting the hatch opening, the 1/8-inch
plywood bearers are epoxied into place. These
have 4-40 blind nuts fitted for securing the
mechanics and 1/16-inch vertical doublers to
provide reinforcement for the wing joiners and
landing gear.
After drilling holes for these and test-fitting
the landing gear, I glued the molded sponsons
in place with thin CA. Note that the sponsons
stiffen the forward fuselage significantly.
Stub Wings: I elected to carve the small wings
from soft balsa, and I mounted them to the
body using carbon-fiber joiner tubes from The
Composites Store Inc. (CST). I molded their
roots using Bondo, reinforced the TEs with a
¼-inch strip of 1/64-inch plywood and finished
them with ½-ounce fiberglass cloth.
Landing Gear: The retractable landing gear
folds backward into the sponsons. There are
several ways to actuate the gear, but I opted for
a simple gear train built with RC car pinions. I
used a metal gear microservo to actuate the
gear.
Finishing: The preproduction Cheyenne wore
a number of color schemes during the life of
the test program. I opted for the standard Army
markings shown on the example at the Army
Aviation Museum at Fort Rucker, Alabama. I
generated CAD drawings for all of the
markings and ordered paint stencils online from
Callie Graphics. Callie has terrific service, and
the price was surprisingly inexpensive.
The overall color scheme was done with
Testors Model Master enamel paints, and
applied with an airbrush. The body was then
clear-coated with matte lacquer.
Test Flying:My first priority was to get the
mechanics flying, and I logged more than 50
flights in that configuration before I added the
body. With the propeller at flat pitch, the
Cheyenne flies similarly to a stock T-Rex
except that it’s slower because of the propeller
disk drag.
As propeller pitch is increased, the
helicopter accelerates forward, and top speed is
impressive. There’s very little pitch change and
no adverse handling issues with the pusher
propeller, and the heli carries more speed into
vertical maneuvers with the added thrust.
I logged all of my early flights with a
standard flybar head, reversed for
counterclockwise rotation. For a better scale
appearance (and greater lift) I switched to a
Black Angel four-bladed head from
eHirobo.com, with weighted scale blades from
SmartModel. You could certainly stick with the
stock flybar head and save some weight in the
process.
After your initial test flights, I recommend
adding ballast to the mechanics to approximate
the weight of the completed helicopter. This
gives you a chance to preview the handling
characteristics and to make any needed tweaks
to the head speed and pitch curve.
The completed Cheyenne made its flying
debut at the 2011 IRCHA Jamboree. After
working out some issues with controller
programming, the heli flew beautifully,
meeting all my hopes for the project. With the
added lift capacity of the four-bladed head, the
Cheyenne doesn’t handle at all like a heavy
Scale model.
In subsequent flights the Cheyenne
continued to improve. At flat pitch, forward
flight is slow and predictable, and the pusher
propeller can actually be a benefit because
there’s little chance of the heli getting away
from you. At full propeller pitch, the Cheyenne
accelerates briskly and really comes to life. It’s
truly a delight to fly.
Conclusion: Designing and building the
Cheyenne was one of the most challenging
projects of my RC career, but also one of the
most rewarding. Building the complex tail
went smoothly, and the real challenges didn’t
begin until I had to squeeze the mechanics into
the slender fuselage. The good news is that all
these puzzles have been solved, so your build
should go easily. Good luck! MA
Jim Ryan
[email protected]
Sources:
CST
(800) 338-1278
www.cstsales.com/index.html
Callie Graphics
(505) 228-2692
www.callie-graphics.com
Testors Model Master paint
(800) 837-8677
www.testors.com
eHirobo
[email protected]
www.ehirobo.com
SmartModel
[email protected]
www.smartmodel.com.hk/index.asp
International Radio Controlled Helicopter
Association
www.ircha.org
Edition: Model Aviation - 2011/12
Page Numbers: 28,29,30,31,32,33,34
INTRODUCTION: In the late 1960s, the US Army
contracted with Lockheed for the construction of a
revolutionary attack helicopter called the AH-56
Cheyenne. In addition to the normal tail rotor, the
Cheyenne had a variable-pitch pusher propeller. For
hovering and low-speed flight, the propeller would spin
at flat pitch, but for high-speed flight the propeller
would gradually increase pitch, propelling the
Cheyenne to higher speeds until its stubby wings were
providing most of the lift.
Flight photos by Greg Gimlick
The Cheyenne made its flying debut at the 2011 IRCHA Jamboree. The sleek
machine looks great in the air, and the wings and color scheme help visual
orientation.
This static shot shows the scale four-bladed tail rotor and
three-bladed pusher propeller. Callie Graphics supplied the
CNC stencils.
30 MODEL AVIATION
Static photos by the author
With the smaller hatch option, the mechanics are a tight fit.
The hatch opening was reinforced with .030-inch G-10 fiberglass,
but 1/32 plywood would work just fine. The larger hatch option
makes installation easier.
The wings are carved from soft balsa, and the sharp TEs are
reinforced with ¼-inch strips of 1/64 plywood. The joiners are
snug-fitting carbon-fiber tubes, and music wire pins through the
joiners lock the wings in place.
The prototype plug was carved from foam
and balsa then glassed and primed. This
pattern was used to make the production
fiberglass molds. Molded fiberglass and
vacuum-formed parts are available from
the author.
The exhaust pipe is a rolled strip of paper, saturated with
finishing resin. Once cured, it’s trimmed flush with the opening.
To fit inside the body, the stock main gear
needs to be replaced with a 132-tooth gear.
The author machined out the hub on
the gear so it could be mounted in a
standard Align gear hub.
The retracts fold back into the sponsons. The prototype’s
actuation system was gear-driven, but a bellcrank will also work.
Install carbon fiber or 1/16-plywood doublers at the front of the
retract openings to provide a positive stop.
December 2011 31
The main frame was built using modified T-Rex 450
parts. The rear (pitch) cyclic servo needed to be
relocated to avoid touching the body. The aluminum
mounting flanges were cut from ½-inch aluminum
angle and installed with 16mm socket-head screws.
The removable canopy has a balsa cockpit floor,
finished with resin-saturated paper. The hatch has
a plywood tab at the rear and is secured with ¼-
inch rare-earth magnets at the front.
If you opt for the 132T main gear, you’ll need to
mill the slots on the motor mount. This allows
the motor longer travel to engage the smaller
main gear.
The compound tail unit,
featuring 2½-inch long
aluminum stand-offs, the offset
bellcrank, and the tail-rotor
pitch linkage, is the most
complex part of the build. The
thrust collar on the tail-rotor
shaft keeps the bevel gears
from driving together.
The Cheyenne placed first in the Helicopter Class at the 2011 Toledo Weak
Signals Expo. Note the attack heli’s compact size.
Specifications:
Rotor span: 28 inches (711mm)
Length: 30 inches (762mm)
Weight: 38 ounces (1077g)
Gear Used:
Transmitter: Multiplex Royal Evo 12 with Spektrum
conversion
Receiver: Spektrum AR9000
Servos: Cyclic Hitec HS-5055MG
Tail: JR DS287MG
Motor: Scorpion HK-2221-12
ESC: Castle Creations Phoenix 35
Battery: 2200-3S LiPo
Accessories: Align GP750
heading-hold gyro and Castle
CC-BEC unit
At this point the main rotor was running
nearly flat pitch, but it continued to provide
pitch and roll control. This ingenious design
gave the Cheyenne unprecedented speed and
range.
In addition to its unique propulsion
system, the AH-56 incorporated a number of
other novel features that would eventually
become standard on the next generation of
high-performance attack helis. These included
a rigid rotor head, terrain mapping navigation,
and helmet-mounted sights.
Progress on the project was slowed by the
ongoing war in Southeast Asia, and by the
early 1970s the Army was becoming
increasingly focused on Soviet armor. In the
end, the Pentagon changed its mind and
decided that instead of a high-speed gunship,
it really needed a tank killer that would use
terrain as its main defense. The AH-64
Apache was the result, and it has served
capably for more than 30 years.
As a lifelong aviation nut, I was fascinated
with the Cheyenne program when I was a kid.
I was disappointed when it was canceled, so
nearly 40 years later I decided to design my
own AH-56, based on mechanics from the
ubiquitous Align T-Rex 450.
The key to making this project feasible
was Align’s release of a torque-tube tail
retrofit kit, because this was the only practical
way to drive the complex tail gearbox. With
this important requirement checked off, I set
out to build my own Cheyenne.
Builder Notes and Considerations:
Replicating the compound tail of the original
Cheyenne was central to the project. The tail
unit adds a degree of complexity, and some
builders may opt to go with a normal tail
rotor.
I wanted the body to be as true to scale as
possible. Because the Cheyenne was an
unusually long and slender helicopter, this
imposed some challenges to fitting the
mechanics inside. A standard 150-tooth main
gear is too large to fit inside the fuselage. The
builder can cut clearance slots in the fuselage
sides, but I opted to modify the mechanics to
use a smaller 132T gear.
The low fuselage profile doesn’t provide
enough height for a standard 450-size motor. I
opted to cut a clearance hole, which also
provided cool air to the motor.
During the construction steps, I’ll outline
my solutions to the various challenges that
arose, but I’ll also offer alternatives that will
simplify the build. This advanced construction
project requires machining and fabrication
skills beyond those usually required for a
helicopter build.
(Note: A molded-parts kit that includes the
fiberglass fuselage, sponsons, the vacuumformed
canopy, chin turret, and tail rotor
housing is available from the author. Email
him for details.)
Tail Unit: The tail unit on the prototype was
fabricated using parts from the Align torquetube
conversion kit. Because an extra gearbox
side plate was needed, I purchased a spare tail
unit as well. The standoffs for the extended
tail-rotor mount were fabricated from 4mm
aluminum knitting needles, cut to 2½-inches
(63.5mm) in length, with the tips drilled and
tapped for M2 screws. The extended tail-rotor
shaft is a 120mm piece of 3mm stainless drill
rod, and the modified bevel gear was secured
on the shaft with J-B Weld.
Pitch control on the tail rotor was achieved
by fabricating an offset bellcrank, with a pivot
arm to keep the pitch-control slider from
flopping around. The pusher propeller can be
controlled with a simple straight pushrod.
The one component in the tail unit that
requires real precision is the rear bearing
block. Although not complex in shape, the
holes for the mounting screws and the center
hole for the propeller-shaft ball bearing must
be perfectly centered if the propeller is to spin
without runout.
The pusher propeller shaft is a spare tailrotor
shaft, press-fit into the bevel gear.
Again, drilling this hole precisely on center is
critical, so using a lathe is a necessity.
With the tail unit completed, I installed it
on an otherwise stock T-Rex 450 (with the
main rotor assembled for counterclockwise
rotation) and logged a series of test flights.
These tests revealed no major handling
problems, and the pusher propeller proved to
be effective.
Main Frame: The main frame is primarily
the upper side plates for a stock T-Rex 450-
V2. The mounting flanges were cut from ½-
inch aluminum angle stock from the hardware
store. Because of the thickness of the
aluminum, I increased the length of the lower
assembly screws to 16mm. I trimmed the
lower side frames flush with the bottom of the
mounting flanges.
To get the rear cyclic servo to fit inside of
the body I had to relocate it as shown on the
plans. This is a relatively simple modification
that I’ve used on previous scale helicopters.
As noted earlier, in order to have the
mechanics completely concealed, I opted to
replace the stock 150T main gear with a 132T
gear. This led to two or three other changes. I
had to modify the motor mount to allow the
motor to slide far enough to mesh with the
teeth of the smaller gear.
I also had to grind down the base of the
pinion so that it wouldn’t rub on the
autorotation gear. You can save some trouble
by using the stock gear train if you don’t mind
cutting clearance slots in the body.
Body:With the mechanics thoroughly tested,
the next step is installing them in the body.
The Cheyenne’s fuselage is slender by scale
helicopter standards, and careful planning is
needed to get everything to fit.
For my build, I elected to use a removable
canopy for battery access, and I made the
hatch between the cockpit and main shaft as
small as possible. This works, but easing the
mechanics through the opening is a very tight
squeeze. You could make your life easier if
you cut the hatch to the larger opening shown
on the plans. This works particularly well if
you opt to cut clearance slots for the stock
150T main gear.
After cutting the hatch opening, the 1/8-inch
plywood bearers are epoxied into place. These
have 4-40 blind nuts fitted for securing the
mechanics and 1/16-inch vertical doublers to
provide reinforcement for the wing joiners and
landing gear.
After drilling holes for these and test-fitting
the landing gear, I glued the molded sponsons
in place with thin CA. Note that the sponsons
stiffen the forward fuselage significantly.
Stub Wings: I elected to carve the small wings
from soft balsa, and I mounted them to the
body using carbon-fiber joiner tubes from The
Composites Store Inc. (CST). I molded their
roots using Bondo, reinforced the TEs with a
¼-inch strip of 1/64-inch plywood and finished
them with ½-ounce fiberglass cloth.
Landing Gear: The retractable landing gear
folds backward into the sponsons. There are
several ways to actuate the gear, but I opted for
a simple gear train built with RC car pinions. I
used a metal gear microservo to actuate the
gear.
Finishing: The preproduction Cheyenne wore
a number of color schemes during the life of
the test program. I opted for the standard Army
markings shown on the example at the Army
Aviation Museum at Fort Rucker, Alabama. I
generated CAD drawings for all of the
markings and ordered paint stencils online from
Callie Graphics. Callie has terrific service, and
the price was surprisingly inexpensive.
The overall color scheme was done with
Testors Model Master enamel paints, and
applied with an airbrush. The body was then
clear-coated with matte lacquer.
Test Flying:My first priority was to get the
mechanics flying, and I logged more than 50
flights in that configuration before I added the
body. With the propeller at flat pitch, the
Cheyenne flies similarly to a stock T-Rex
except that it’s slower because of the propeller
disk drag.
As propeller pitch is increased, the
helicopter accelerates forward, and top speed is
impressive. There’s very little pitch change and
no adverse handling issues with the pusher
propeller, and the heli carries more speed into
vertical maneuvers with the added thrust.
I logged all of my early flights with a
standard flybar head, reversed for
counterclockwise rotation. For a better scale
appearance (and greater lift) I switched to a
Black Angel four-bladed head from
eHirobo.com, with weighted scale blades from
SmartModel. You could certainly stick with the
stock flybar head and save some weight in the
process.
After your initial test flights, I recommend
adding ballast to the mechanics to approximate
the weight of the completed helicopter. This
gives you a chance to preview the handling
characteristics and to make any needed tweaks
to the head speed and pitch curve.
The completed Cheyenne made its flying
debut at the 2011 IRCHA Jamboree. After
working out some issues with controller
programming, the heli flew beautifully,
meeting all my hopes for the project. With the
added lift capacity of the four-bladed head, the
Cheyenne doesn’t handle at all like a heavy
Scale model.
In subsequent flights the Cheyenne
continued to improve. At flat pitch, forward
flight is slow and predictable, and the pusher
propeller can actually be a benefit because
there’s little chance of the heli getting away
from you. At full propeller pitch, the Cheyenne
accelerates briskly and really comes to life. It’s
truly a delight to fly.
Conclusion: Designing and building the
Cheyenne was one of the most challenging
projects of my RC career, but also one of the
most rewarding. Building the complex tail
went smoothly, and the real challenges didn’t
begin until I had to squeeze the mechanics into
the slender fuselage. The good news is that all
these puzzles have been solved, so your build
should go easily. Good luck! MA
Jim Ryan
[email protected]
Sources:
CST
(800) 338-1278
www.cstsales.com/index.html
Callie Graphics
(505) 228-2692
www.callie-graphics.com
Testors Model Master paint
(800) 837-8677
www.testors.com
eHirobo
[email protected]
www.ehirobo.com
SmartModel
[email protected]
www.smartmodel.com.hk/index.asp
International Radio Controlled Helicopter
Association
www.ircha.org
