Rock was a pioneer in precision radio-controlled helicopter design.
His 5th generation SSP helicopter (SSP-5) is detailed in this article
form the March 1973 American Aircraft modeler. Fixed pitch on the
main rotors was still de rigueur in R/C copters, but Gene did yeoman's
work on dynamic tail rotor compensation, model helicopter flight
dynamics, system minimization (the KISS principle), balance, fuel
delivery, and much more. Still, just as the brilliance of IBM's
engineers who designed the Selectric typewriters have largely disappeared
from memory due to the invention of computers, with only the keyboard
as the surviving remnant, so too has a lot of this hard-won technology
been replaced with electronic wizardry. Modern R/C helicopter owe
a lot to people like Gene Rock; he is a giant upon whose shoulders
we stand*.* "If I have seen further
it is only by standing on the shoulders of giants." Sir Isaac Newton
Gene Rock presented in AAM's July and August issues of last year
the successful original version of this model. Since then, many
many SSP's were made and improvements developed. These improvements
supplement the original plans still available through the plans
service. No additional plans are offered.
With many SSP's
under construction and flying, here are the designers latest improvements
for easier building and better performance.
Brisighella's model (right) illustrates many of the latest
improvements discussed by Gene Rock in this article and shows some
alternate construction methods and shapes. It flies great too. A
unique idea is to cover the RC compartment with clear stiff plastic
for instant visual inspections.
Since the last documentation
of the S.S.P. (August and September AAM) there have been four basic
revisions. Three of these revisions existed by the time the article
first major change came about a year ago with the ultimate goal
of incorporating a semi-scale fiberglass fuselage. When flying in
cold weather, the tail rotor belts lost some of their elasticity
and were therefore slipping. When the original plans were drawn,
larger belts were incorporated to meet with the manufacturer's suggested
belt tension. These larger belts, however, were never added to my
model because they would have been in the way of a fiberglass fuselage.
To make a fiberglass fuselage feasible, the following changes
were made. A gear drive tail rotor was incorporated along with an
extra gearbox to change the direction of the tail rotor drive shaft.
The gears in the change of direction box were two pinions from a
2:1 bevel set. The side struts came off next because they were in
the way. After several experiments, a spring-loaded flap hub was
found to make flying easier, especially in a wind. The old hub was
discarded because of the difficulty in the addition of springs.
In a fit of ambition, aluminum paddles were shaped to replace the
wooden ones and weights. Not only did they look better, they worked
better and last a lifetime.
A sub fin and a strap-on large
rudder were added to enable better visibility in flight. Surprisingly
my hovering ability was also improved because of this addition.
The horizontal stabilizer was moved out of the downwash of the main
rotor in order to lighten the nose weight.
After all of
these changes, the fiberglass fuselage was found too heavy (it
cut into my reserve power), and therefore discarded. Well, I still
had a more scale-like model.
The extra gear box was discarded
next because of added complexities and weight penalty. It was replaced
with a speedometer cable about three in. long and the complete drive
shaft was installed inside of the tail boom with Rulon bushings
supporting the shaft every six to eight in. Two of the supports
were on each end of the cable. The model was then flying well ...
I thought. (Boy, was I in for a surprise.) The model was flown extensively,
which brought up some more problems. The model fell out of the sky
many times because of fuel starvation and an incorrect setting of
the air bleed on the carburetor. During some of the hard landings,
the sub fin would break its bond to the tail boom. There also seemed
to be an interaction from the cooling fan when the model went into
forward flight. An extreme amount of nose weight was required to
counteract the down wash of the fan. With this CG 1/2 to 1" forward,
the model would pitch nose down when applying power and nose up
when decreasing it.
Shroud controls air-flow from the fan to really cool the Enya
45. Murphy muffler also featured. Electric starter always used.
That sump prevents fuel starvation with a fore-and-aft tank
arrangement. Pilot experienced several power failures before
this was added.
Tall rotor drive is taken through miter gears from clutch shaft.
One piece shaft drive and support plate holds four ball bearings.
Study complete control linkage and swash plate system. Specially
made parts used throughout. Rotor head is as on original plans.
Brisighella's model again showing Rock's new rotor head system.
It is very much simpler and more trouble-free than the original.
It is based on a cut and shaped piece of hard 1/4 in. aluminum
rather than multiple pieces and parts of sheet aluminum.
The tail gyro, like all other parts of Dario's model is beautifully
made and works like a charm, but it takes careful adjustment
of the many forces involved in its operation.
Landing gear drawing.
SSP is still the only model helicopter with gyro on tail rotor.
Text and drawings show how it works, pix show its looks.
Tail rotor takeoff drawing.
Tail rotor gearbox drawing.
Rotor head drawing.
The next revision solved the problems discussed above. The sub fin
was attached with three sets of rubber bands rather than bonding.
A fuel reservoir was incorporated and the air bleed closed. (This
doesn't mean that all Enya .45s need their air bleed completely
closed.) Next, a centrifugal cooling fan 2 1/2" in dia. was tried
with a shroud. Because of the belt drive, the blower could be only
1/2" deep. The blower did not give adequate cooling, therefore by
sizing what I had, I decided that the blower needed to be as large
as four in. in dia. Since the original SSP had a 4 1/2" dia. axial
blower - too small for a 90° day and full power - I decided to use
it with a shroud. By choking the outlet, the 4 1/2" dia. fan was
sufficient. The problem of the interaction of the fan was over.
The CG was then moved back to 1/4" in front of the rotor shaft.
An integral rudder was incorporated and the next flying session
indicated that scale-like skids could be added. The model was then
flying like a dream. You could take your hands off the controls,
reach out and touch the model in flight. What could be better?
The SSP was next flown in the Boeing-Vertol wind tunnel
three weeks before the Nationals in Chicago. This test, among other
things, brought out the 0° collective setting to the bottom surface
of the paddles. The test came off with barely a scratch - was I
lucky, flying in a 20 by 20' room with a thirty mph wind!
These drawings illustrate. the most significant and unique aspect
of the SSP helicopter. All torque or wind-gust inputs to the yaw
axis are compensated for by the tail rotor's mechanical gyro. Drawings
tell how it works and illustrate the mechanical operation clearly.
At the Nationals, the model flew very well, but the change
of altitude and air temperature left me somewhat short on maneuvering
power. The tail rotor hit the ground in a couple of hard landings,
the sudden shock to the speedometer cable would double it over.
It was very hard to hover in a crosswind in the windy city.
After the Nationals, the cable was replaced, and power lost
tests were conducted. These tests concluded that because of the
small bend radius in the cable and the fact that the cable was supported
on each end by a bushing instead of a bearing, the tail rotor was
taking twice as much horsepower to drive as normal.
last revision took shape in the form of a straight out tail boom
(negligible power loss). The rudder was reduced to 31 sq. in., approximately
half its former size. The model then had more power and was easier
to hold in a crosswind. In the last couple of weeks, the horizontal
stabilizer was moved back to its position shown on the original
plans. The reason for this was because the model would not fly forward
naturally without holding forward stick. The horizontal stab would
come into the downwash of the main rotor when the model reached
five to ten mph. This would cause the model to pitch up slightly,
slowing it down. The original position requires more nose weight
because it is always in the main rotor downwash, but the model will
not pitch with sudden power changes. A slightly forward (1/4 to
1/2") CG position is more stable and should work out well.
The next revision is anyone's guess. Who knows what we will
be flying in 1973? As for me, I am starting a new model - scale
this time. Questions
The most frequently asked question is, "Where can the tail rotor
gyro bearing be purchased?" Well fellows, I goofed. I had the wrong
Part No. The correct No. is SR 1028, thanks to Syd Horne of Ontario
who is building an SSP. The bearing can also be purchased from PIC
Design Corp., P.O. Box 335, Benrus Center, Ridgefield, Conn. 06877
(Part No. E 5-3 at $10.60). The main swash plate bearing should
not be difficult to obtain. Bearings, Inc. will handle it for about
-33: Another question asked is, "Where is Part -33?"
Somehow it was omitted from the reduced plans in the magazine, but
it is on the full-size drawings.
Tail Rotor Gyro: I wish
I had a dollar for each time I have been asked how the tail rotor
gyro works. The following is a brief explanation.
located just behind the tail rotor blades, is a rotating mass. When
the model yaws, the gyro wants to remain in its former plane. Notice
from the illustrations that the gyro has the same heading (Figures
A through C). If a horizontal pivot is installed on the gyro, the
gyro is then forced to change its heading following the model (Figures
D through F). When the gyro is forced to follow the model with a
horizontal pivot, the gyro will precess 90° later in the direction
of the force (Figures G through I). If the horizontal pin is not
on the centerline of rotation but above or below it, the gyro will
also slide in or out when it precesses. The slider on which the
gimbal and then the gyro is mounted is connected to the pitch arms
of the tail rotor blades by means of pitch links. When the gyro
precesses, the slider moves and changes the tail rotor b lades collective
pitch (Figures J through L).
Tail rotor collective control
from the servo is achieved by pushing or pulling the gyro with a
light spring. The gyro can override the spring, therefore the servo
travel at the tail rotor must be about twice the normal input. The
gyro would almost cancel a normal collective input. The springs
also provide a centering for the gyro. The tail of the model will
wag if the gyro is not sufficiently damped. STP on the tail rotor
shaft best accomplishes this.
The kinematics and dynamics
of a tail rotor gyro are fairly sensitive. The only change from
the drawings so far is to locate the tail rotor on the left side,
change the pitch arms to 3/4", and add an extra 1/32" thick ring
to the gyro. These changes allow better maneuverability especially
for stall turns. (Figure J and drawings). Do not make kinematic
or dynamic changes other than the ones mentioned above until you
are thoroughly familiar with its operation. The weight perpendicular
to the blade cancels the blade's tendency to go to flat pitch; a
light blade will require less weight, a heavier blade more. Adjust
this weight by spinning the whole tail rotor assembly. When the
weight is proper, the gyro will remain vertical and will not compress
either spring. When adjusting the tail rotor collective to cancel
main rotor torque, adjust the angle of the pitch arm relative to
the tail rotor blade. This will allow the gyro to be vertical when
proper tail rotor collective is reached. The gyro will not work
properly if it is compressing one of the springs or if it is not
vertical when operating at normal tail rotor collective.
Engine: About the biggest misconception is that a 60 engine
will fly the SSP. Sure it will, but you will be replacing belts
every other flight; a 45 has plenty of power. A 60 can be used if
a 15-tooth and a 72-tooth pulley are incorporated for the second
stage reduction. Also use a 11- or 12-tooth pulley on the engine.
Check Stock Drive's catalog to determine belt sizes. A 60-sized
SSP will then outfly anything, even some fixed wings, and will be
easy to handle.
Clutch: When making the clutch, almost any
thickness cork will do. Just machine it to size after bonding it
to the clutch drum. Although neoprene cork is mentioned, plain cork
will do. The spring clips on the clutch should pull the shoes together
slightly. The tighter the shoes are pulled together, the higher
the engagement rpm. Remember the clutch is doing 1000 rpm when the
engine is doing 2800 rpm.
Springs-General: My springs are
wound in a drill press or a lathe. The size rod that the spring
is to be wound around is chucked; about 1/4" of the end of the music
wire is bent at 90°. A DuBro collar with a knotch on the inside
diameter is fitted over the rod next to the chuck. The music wire
is inserted through the notch and into the void between the chuck
jaws. The collar is then tightened. On large wire, such as 1/16"
music wire, a hole is drilled into the rod. Hold the end of the
music wire with pliers and have someone turn on the machine - use
low speed! Keep the wire taut and the coils next to each other by
pulling the wire away from the spring generation. A tension spring
with a preload is the result. Then make a compression spring, and
pull the tension spring open. Put the spring back on the mandrel
and compress until the coils bottom. This is not the way normal
springs are wound but it is satisfactory for models.
Stock Drive Products (55 S. Denton Ave., New Hyde Park, N.Y. 11040)
is not making any of the special machined parts, therefore both
the assembly and .detail drawings are required. The kits include
only stock parts. The kit for the SSP-5 is No. HK 103 and is priced
at $44.95, or, if you have previous parts package for SSP, order
in addition 4 each 47Y 55 FSS 3718 bearing, 2 each IC4 Y3216 gears,
2 each IC4 Y2012 gears and 2 each 7B4 F006 bushing.
The hub drawing is an attempt
to simplify a spring-loaded hub. The lack of collective adjustment
also makes it more reliable and lighter. The 1/4" OD x 3/16 ID flap
pivot rod is a drill rod, heat treated after final machining. The
feather pivot rod is a drill rod, heat treated after final machining.
The feather pivot trunnion should be a heat treatable steel such
as 1/2" dia. 4340 or drill rod. Ream the hole for the flap pivot
rod, .249 dia. There must be a good press fit of the flap pin inside
the feathering pin to insure proper spring operation.
final machining of the feathering pin, it should be heat treated
so that it will not lose its close tolerance flap pinhole through
several disassemblies. The No. 4-40 shoulder bolt is a special and
can be made from a No. 6-32 socket head cap screw. The shoulder
should be about 1/2" long. The rubber on the clevis is mainly a
The tail rotor gear box shown is very similar
to John Burkam's in the August 1972 issue of AAM. The gear box is
closed out in back which adds strength in holding the gears in mesh.
Although a 1/8" dia. shaft is used because of the tail rotor gyro,
as much as a 3/16" dia. tail rotor shaft could be used on a different
control configuration. Lubricate with moly grease.
of the tail rotor takeoff shown does not require a new belt/pulley
height adjustment. The flat on the 3/16" dia. clutch shaft for the
gear set screw provides the shoulder when the No. 10-32 stop nut
is lightly torqued down. The 1/8" dia. music wire comes undersized
and oversized at the hobby shops. Take your micrometer along and
choose an undersized one for this application. A slip clutch to
drive the tail rotor is not needed when a 3/32" music wire drive
shaft is used.
shroud is constructed from .020" 2024 T3 aluminum with a very close
fit around the engine. The baffles should not be fastened to the
1/16" thick mount. They should slide on this mount for ease of removing
the shroud. The training gear attachment will be difficult when
using a shroud because it is hard to tie the compression strut to
a solid base. The ability to throw on training landing gear at any
time is good for experimenting and to help teach future helicopter
nuts! An angle bracket mounted sideways to the front top of the
radio box projected beyond the engine shroud would be one solution.
The fuel reservoir shown is necessary when the tank installation
is longitudinal. The fuel is constantly sloshing back and forth
and cannot be picked up by a weighted fuel line. The ideal fuel
tank installation is a lateral one right under the rotor shaft.
A lateral installation would not need a reservoir and would feed
the engine even through a loop. The fuel tank installation is not
for CG purposes but a compromise between the engine and CG.
The scale-like landing gear should be self-explanatory except
for the nose wheels. The nose wheels' main function is to provide
nose weight. By taking a standard wheel and machining an aluminum
and a steel hub for it, the CG can be shifted just by changing hubs.
The secondary purpose is to allow the model to pitch forward
and taxi. My landing gear struts are not made from aluminum but
from unidirectional fiberglass 1002S with BP 907 resin system.
This is about the same glass that is used in archery bows. If you
are not familiar with this glass, do not use it. The normal glass
cloth and resin bought at a hardware store will not suffice. If
you are still interested in this high temperature cure material,
contact the 3M Co.
There was an error on my part in detailing
-12 and -13, longitudinal control to the swashplate. This was pointed
out to me by Dario Brisighella of Milwaukee, also building and now
flying a highly modified SSP. I would like to point out that quite
a few parts are not drawn like the original. I try to incorporate
the way I would construct the part if I were to rebuild it.
As of now, all of the
controls are spring-loaded to eliminate backlash. The gimbal in
the swashplate is also springloaded because of the wear on the
Although shown on the plans, Rocket City missing links
are no longer recommended. Find some .10 thick aluminum and cut
it to 3/16" sq. like the end fittings in -12. This fitting coupled
to a Du-Bro Kwik-Link will replace a missing link and will be much
After about 40 hours running time on the
Enya .45, the crankcase started throwing oil through the front engine
bearing. This does not hurt the engine's performance, but makes
for a messy model. A sealed bearing from PIC Design Corp. was installed
with one seal removed on the outside. Sealed bearings add friction
and only a seal on the inside is needed. The prop drive washer will
need rework. A very slight power loss was observed but the oil slinging
was almost completely cancelled.
Vendor supplied radio batteries
have enough capacity for flying fixed wing aircraft, but I find
that more time is logged on a helicopter during a day than a fixed
wing. My batteries are C-sized NiCads which give at least five
hours flying time. The main disadvantage is that they weigh eight
oz., but this is convenient if the model is tail heavy.
Originally, drill rod was used for the main rotor shaft. Drill rod,
this length, will often distort when heat treated, so the material
has been changed to 17-4PH which is a high-yield stainless steel
even in the annealed condition. It is strong enough without a heat
treatment and can still be machined. If you cannot find this material,
send $1.50 and I will supply a 10" length (501 Meadow Park La.,
Media, Pa. 19063).
The side struts on the original SSP have
long since been removed. Their main function was to shorten the
compression strut of the training landing gear, therefore lessening
the chances of breakage. They are not essential and detract from
whatever looks the SSP might have. The compression strut can go
to the top of the main transmission structure.
I have also
increased the height of the main transmission box by 1 1/4" to give
better support to the main rotor shaft-not necessary but it helps.
My radio box is still .032 6061 T6 aluminum but .020 2024
T3 is recommended. The .020 is strong enough and will lighten the
model by at least three oz. Several models have been built with
The tail boom should always be made easily
removable and the fittings to it clamped on. The tail boom is next
to the rotor blades as the most often replaced item.
closing may I add, fellow helicopter nuts, you now should have enough
information to go out and build a chopper capable of flying circles
around my SSP-5. I vow not to be defeated and shall go into exile
in my basement for the next three months to emerge with a bigger
and better machine.
The AMA Plans Service offers a full-size
version of plans at a very reasonable cost. They will scale the
plans any size for you. Try out my
Scale Calculator for Model Airplane Plans
Posted November 27, 2011