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DESIGN AND BUILD AN RC BIRD MODEL
FLYING LIKE A BIRD
It was a calm Saturday morning, and as I walked outside for the
morning paper, I spotted a raven gliding toward me at telephone-pole
height. Although this is a common sight here in the California desert,
I stopped to watch. I had just retired from the Air Force Flight Test
Center as a stability and control flight-test engineer, and now I had time
to expand my interest in bird flight as a new hobby. As I watched, the
raven started a slow bank to the right with its wings fully outstretched.
The turn got steeper and steeper until, at last, the raven dived-head-
first—into the middle of the road, about 30 yards in front of me. I was
dumbfounded. I had never seen a bird crash before!
Span • 4.17 ft. Length • 1.92 ft.
Wing area = 2.78 sq. ft. Aspect ratio = 6.2S
Weight = 1.1 Ib.
BASE-LINE RAVEN
There was no evidence of life after the impact, and I suspected that
the bird had suffered a stroke or a heart attack. In any case, this little
fellow had unwittingly given his life to science. I seized the opportunity
to weigh him and draw an outline of his wings and tail on a sheet of
butcher paper. The dimensions were surprisingly similar to a ^A-size
radio-controlled model that I had been flying. I reasoned that I could
probably learn something about how birds fly by building a glider model
with the same shape, size and planform of a large soaring bird.
I hoped to establish a "base-line" configuration that would fly, even
if I had to cheat a little at the beginning. The starting point was a series
of small, balsa-wood, profile models with 6- to 8-inch spans that I
glided around the living room. I was a little surprised to find that these
models were stable in all axes, even without vertical tails. Lateral
control was an enigma, but I soon discovered that either spoilers or
drag flaps caused the model to turn toward the high-drag wing.
To provide room for the radio gear, I built the prototype Raven
model about 8 percent larger than a real raven. Pitch control was
provided by deflection of the rear portion of the tail, and lateral-
directional control was provided by the use of drag flaps (downward
only) on the bottom of each wing. I built it to be sturdy, since I expected
frequent crashes. The model was launched from the top of a small hill.
It appeared to be stable but glided with a gentle rolling oscillation that
could not be controlled with the drag flaps, so a very small vertical fin
was added. Stable and controllable glides were then possible.
It seems that each successful test produces additional, unanswered
questions, such as: how do birds twist their wingtip feathers to produce
proverse yaw? Does a bird's airfoil really have a reflexed trailing edge
in flight? Do birds adjust the wing sweep at the wingtips as part of the
control mechanism? Of course, there is also a wide variety of species
with various wing shapes to try. I have only begun to explore the
technical aspects of how birds really fly. I am constantly amazed at
the incredible complexity of bird flight, and I marvel at that little
"bird-brain" that's able to coordinate all the required actions.
For the full story, take the "Click Trip"!
94 MODEL AIRPLANE NEWS
The aileron servo for each wing
panel is attached to the root
end of the wing. This keeps the
tips light for better turning per-
formance. A Kimbrough rotary
coupler connects the torque
rod to the servo.
Left: without a vertical fin
and rudder, bird models
can be very difficult to
launch with a bungee
high-start or a winch line.
To improve the ride, you
can use a drop-away
ventral fin like this one
that fits into a slot in the
bottom of the fuselage.
hints spur some interest in building models that look and fly like birds.
The analysis methods mentioned are good starting points, but they do
not ensure that a new bird model will fly well on the first launch. There
is still much to learn about how birds fly.
BUILDING A TURKEY VULTURE
Early flights of my Turkey Vulture model oscillated continuously in roll.
I made three changes to help it fly better. First, I built a new, lighter
wing. Next, I replaced the drag flaps with wingtip-aileron feathers, and
third, I installed Kimbrough rotary-drive couplers to control all moving
surfaces. In its current configuration, the model oscillates lightly in
turbulence, much as a real bird does, but it's easily controlled and will
stabilize nicely in still air.
The model is based on photos of vultures flying in thermals. It is
approximately full size (66-inch span) but is roughly half the weight of a
real turkey vulture. The wing construction incorporates a full-depth spar
and a cambered-wing airfoil with reflex. The fuselage (body) is built with
balsa formers, sheeting and balsa blocks for the head and minor fairing
pieces. I cover my bird models with MonoKote film and paint the heads
to match. This results in a model that is a true floater but does not penetrate
the wind as well as a model
with less camber. The model handles
ballast well, and this can be used to
somewhat improve penetration. Real
soaring birds reduce their wingspan
and area and increase their wing
sweep when flying straight between
thermals (more options for
experimentation?).
Detailed instructions for building
the Turkey Vulture model
accompany the full-size plan.
You can also view the detailed article
and some related aerodynamic illustrations
via the "Click Trip" Web
address at the end of this article.
Please let me know, through Model
Airplane News, of your experiences
with bird-like flight. ±
MODELAIRPLANENEWS.COM
For more information and to
see the model fly, click on
the article and videos link.