Du-Bro's Whirlybird 505
November 1972 American Aircraft Modeler
Whirlybird 505 was the first successful commercial helicopter kit (although
successful is a relative term as applied here). I was fortunate as a kid
Holly Hill Harbor, Mayo, Maryland, because there was a man down the
street from me who was an avid radio control modeler and seemed to buy just
about every new type of radio, engine, and kit available. I would anxiously
await the sound of an engine running, and instantly jump on my bicycle to
ride down and see what he was doing. The strange thing about it was that
he had three step-sons who were notoriously bad actors who counted it as
sport to harass and occasionally beat up guys like me, so I always approached
the yard with a bit of trepidation. The gentleman himself was very nice,
and a few times even gave me some of his beat-up models and an engine or
two. I thought it was a magnanimous act of kindness, even if the stuff never
worked. Just having in my possession some of the great stuff that appeared
in American Aircraft Modeler and Flying Models was appreciated.
I certainly could not have afforded such treasures on my paltry newspaper
delivery route earnings, and my father's salary barely covered food and
clothing for our family of seven. Anyway, he had a Whirlybird, and I remember
seeing him learning to fly it while being tethered. That method never was
a very good one; use of training gear like the one shown in this article
works a lot better better because the model is not abruptly halted when
it reaches an extremity. He later advanced to one of the Kalt Huey Cobras,
but I never witnessed any more than a brief hover by him. It sure was cool
Du-Bro's Whirlybird 505
Author with our bird at the time of early testing with tripod gear.
Note ideal shape and fit of the O.S. Wankel engine. Spinner for starting
only but cooling fan needed.
Helicopter training gear plans.
Complete frame with Ross engine installed. Short bar of lead on this
side of engine balances carburetor and exhaust slacks. All parts are
Canopy removed to view inside. Snug body keeps the exhaust mess out
fairly well. Incidentally, weight is not critical if you have plenty
of available power.
Tail rotor operates at nearly zero pitch being used only for rudder
function, not anti torque. Since photo was taken, we learned stabilizer
should be omitted.
Six piece plastic body is easily assembled and long lasting.
Tail boom is tapering fiberglass rod and main frame is spruce. Sturdy
Silvertone servo linearity plot.
PART II THAT "GOOFY THING" DAVE GREY DESIGNED, DU-BRO'S WHIRLYBIRD 505.
BUILDING AND FLYING THIS MODEL WAS PURE FUN AND CHALLENGE
AND ED SWEENEY
Building the Du-Bro Whirlybird was sheer delight,
for two reasons: First, it was a new experience in modeling. Helicopters
are about the last "frontier" left in RC modeling, and it is fascinating
to dig into the details of their construction and operation. Second, it's
difficult to see how one could improve on the kit itself. It's not an ARF,
and several enjoyable hours can be spent building it. The quality of the
many metal, wood and plastic parts is excellent; the instructions are complete
and easy to follow.
One note of caution: If you don't know how to
solder, learn before you start constructing the Whirlybird. Most of the
mechanical assemblies depend on solder joints; neat, effective soldering
is a must. And keep in mind that this is soldering for mechanical strength,
not electrical connection. Sta-Brite low temperature silver solder is recommended.
Construction of the chopper is so well covered in the instruction
book that only two comments are needed here. The holes for the mounting
screws at the lower end of the cyclic pitch servos should be drilled before
installing the mounts for the throttle and tail rotor servos. And it is
recommended that the horizontal stabilizer be omitted. Experience, confirmed
by Du-Bro, indicates that the bird is more stable in windy conditions without
Early in the construction of the Whirlybird the decision
was made to try the OS Wankel rotary piston engine. This engine seemed to
be a natural for this application, even though its nominal displacement
is less than the 0.40 called for.
Mounting the engine requires fabrication
of a new mounting plate. A piece of 1/8" aluminum sheet is turned to the
outside diameter of the Wankel's mounting ring, the same diameter as the
fuel tank on the Whirlybird. The new mounting plate is then attached to
the rotor head assembly with counter-sunk 4-40 machine screws. Three 8-32
machine screws hold the Wankel mounting ring to the plate. The engine is
located on the plate with the throttle in line with the rotor blades. Final
balancing of the rotor assembly is accomplished by placing small pieces
of solder in the end of the rotor blade.
Several tanks of fuel were
run through the engine before attempting tethered flight. With the chopper
firmly anchored (bricks on the landing skid), this engine break in time
also afforded an opportunity to check and observe the action of the controls.
Final trimming of the flybar linkage was made by observing the disc of the
flybar in motion and adjusting for a level condition with all controls in
Initial flight attempts were made with the two-line tether
system described in the instructions. The model weighed just under four
lb. dry, and ten oz. of additional weight was attached to the landing gear.
The first "flights" were made in Bob Beckman's backyard heliport, using
a sheet of plywood as a landing pad. Two things became immediately obvious:
There was enough lift from the Wankel; there wasn't enough tail rotor control.
A quick trip to the shop to adjust the tail rotor linkage, and we were back
to the now greasy plywood.
Several days and many penguin-type flights
later we were beginning to wonder. The model obviously got light as power
was applied, but it also started jittering and swinging around as soon as
it started to lift. Even the most charitable of spectators couldn't say
that the model was flying. At times it did seem 'that we had some control
over the bird's gyrations, but for the most part it was just banging back
and forth between tethers. We were beginning to question whether the engine
had the power to turn the rotor fast enough to provide the necessary control.
At this point we got a break. Dave Gray was in town to help with
the AMA demonstrations at TRANSPO '72, and he gave our bird a test flight.
After removing the weights and checking over the engine installation, Dave
was ready to go. As he didn't feel that Bob's backyard heliport was big
enough, we all trooped out into the street in front of the house. The tank
was filled, the engine started, and Dave took the bird up. Just like that!
All doubts about the capability of the model were removed. Our difficulty
had really been the capability of the pilot. Dave made several flights that
evening (we wound up under the street light at the corner) with both the
Wankel-powered version and one of his own 40-powered birds. We reached the
conclusions that: 1) the Wankel will fly the Whirlybird nicely when it is
putting out peak power, but none of the margin of power is available that
is so important to helicopter flying; 2) the marginal power situation further
complicates the already difficult task of learning to fly a new way. That
the power is marginal is not surprising, since the Wankel is a nominal 30.
What is pleasantly surprising is that this rotary piston engine produces
enough power to fly the Whirlybird smoothly, once you know what you're doing.
Dave had another valuable hint regarding the landing gear. Up to
this point we had been using the skid gear that came with the kit. This
is quite adequate and looks very scale-like, but as Dave pointed out, its
narrow tread is what allows the chopper to jitter and bounce around just
before it becomes airborne. One of Dave's models was fitted with wide stance,
tricycle, training gear. The advantages are: 1) as the throttle is advanced,
the model remains stable until adequate control forces have been developed
by the main and tail rotors; 2) the wide stance of the gear will help to
avoid tipping over and breaking rotor blades; 3) the tether system is not
needed. Once the pilot's capability has been developed, the skid-type gear
can be used.
Several versions of the training gear have been made,
and the final version is shown in the drawings. Construction is simple,
but a few words on evolution are in order. The first gear made was fabricated
from 1/4" dowels with rubber balls on the ends. Looked great, but the rubber
balls wouldn't slide so it was hard to move the model around, the dowels
were both springy and weak so on a hard landing they either broke or bounced
the model over and banged up the rotor. Ping-pong balls were tried in place
of the rubber balls, but they were too fragile. Small rubber-tired wheels
looked good for a while, but going sideways they would dig in and flip the
bird just like the original balls. The final combination of fiberglass rods
and plastic practice golf balls seems ideal.
By this time we really
had the bit in our teeth on this business of helicopter flying and we wanted
to master it. The Wankel was temporarily retired, and we looked around for
something with enough power to make our tyro attempts a little easier. The
Ross twin looked like the answer. Here we had a setup where the counter-balance
has a piston in it to contribute to the power available. Mounting the Ross
was simply a matter of making another mounting plate and rigging a balance
weight opposite the exhausts.
Flying the Whirlybird with the Ross
has been a pleasure. The engine is easy to start, smooth and quiet running,
with more than enough power to do the job. The first takeoff resulted in
the bird climbing to a six ft. altitude before we realized what was happening.
That we got it back down undamaged proved that we were making some progress
in learning to handle this new type of RC flight. Since then we have progressed
to short duration hovering flights and, more important, learned that we
can fly the Whirlybird in moderate wind.
The Du-Bro Whirlybird has
proven itself to be a fascinating and successful introduction to RC helicopters.
Its greatest value may be as a trainer in preparation for flying other types
of choppers. But we have to repeat the often-stated fact that it takes a
lot of time to learn to fly a model helicopter. After about four gallons
of fuel we're still a long way from being accomplished RC helicopter pilots.
But we'll get there!
Additional comments by the Editor:
modelers have given up on their Du-Bro 505's and claimed that they are not
flyable. This is just not so. They fly well and are a true helicopter even
with the torque reaction drive. Think of this as a coaxial helicopter. The
Du-Bro is the easiest helicopter to fly, too. I have become a pretty good
fixed-wing RC pilot, but I felt like a rank beginner in RC with my first
three gallons of fuel through the Whirlybird. It takes about four gallons
to achieve smooth hovering. With a helicopter, four gallons are used surprisingly
Comments by other would-be Du-Bro 505 pilots indicate much
misunderstanding of helicopter characteristics. For example, the Du-Bro
is not top-heavy just because the engine is on top. Consider where the lift
is coming from and realize that all the weight is below the lift. The chopper
may seem top-heavy because of marginal control and stability when the rotors
slow down or are unpowered when chopping the throttle. Because this model
has a low rotor speed, that speed is critical.
within one rotor diameter distance from the ground are flying on a very
slippery bubble of air. All control inputs are concentrated on keeping on
top of that bubble. When the model is up to five ft., it is riding on a
more stable column of air, not just a bubble. Also, the Du-Bro is a rigid
rotor design and must be in full free flight off the ground (by at least
an inch) before any control inputs are meaningful. Inputs on the ground
have no relation to the direction of flight or movement of the model. So,
bring the rotors up to speed, then lift off quickly but slightly before
expecting controls to take effect.
As mentioned by Bob Beckman in
the above text, we switched from the Wankel engine to the Ross to try the
model with additional power. I made this conversion after achieving significant
power increases. Unfortunately, Bob did not get to fly it with the improved
The engine loves lots of nitro and K&B 500
seems to be its best fuel. A Fox short idle-bar plug improves rpm by nearly
800 rpm over the original O.S. Wankel plug. The engine overheats and then
sags fairly soon after running up to full speed on the helicopter, as there
is almost no cooling in this application. Simple aluminum cooling fan of
eight blades, 4-1/2 in. dia. mounted just under the flying prop, provides
the necessary engine cooling air. Use an 11x4 prop to fly the helicopter
You just can't beat having extra power and the Ross is the way
to over-power the Du-Bro 505. However, by using a high pitch prop (11x7-1/2),
more energy is put into the main rotor via torque at liftoff speed. More
torque means more rotor rpm and thus both more control and more stability.
The tripod landing gear is quite easy to make either with fiberglass
rods into a base block or with bent wires into the original landing gear
mounts. We'll sketch the elaborate tripod gear, you can simplify it to suit
yourself. Please, build one for your model, it makes learning to fly more
successful and possible.
Posted October 1, 2011
Even during the busiest times of my life I have endeavored to maintain
some form of model building activity. This site has been created to help me chronicle my journey
through a lifelong involvement in model aviation, which
all began in Mayo, MD. There
is a lot of good information and there are lot
of pictures throughout the website that you will probably find useful, and might
even bring back some old memories from your own days of yore. The website began life around
1996 as an EarthLink screen name of ModelAirplanes, and quickly grew to where more server
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