several airframes at a contest. The speed, turning
ability and being able to fly in a confined space all
contribute to the carnage. Given that this is how
combat is flown, can a plane be designed that lasts an
entire contest even if hit? That is the purpose of this
project, to design and promote a "survivable" combat
plane.
design process I wanted to have a soft Leading Edge, from tip to tip so that a collision with another airplane would be
blunted. With a conventional airplane the hardest point of the plane, the engine usually strikes the other plane causing
damage. I also wanted to get the whirling prop away the front of the plane. After engine first collisions, prop cuts are
the next most frequent cause of airframe damage. I selected a flying wing platform for my initial tests. A commercial kit
with EPP wing called the "Bat" was acquired from Atlanta Hobby for $49. It is designed as a slope soarer.
horns, pushrods, two rolls colored tape,
one roll strapping tape plus hatch covers
carbon fiber spar. Wing panels
have pre-molded tunnel for rod.
showing pre-moulded pockets for
servos, receiver, and battery
reasons. First it is all EPP foam, second it has a span of 53" making it large enough to easily see, third it is a
very complete kit with a true flying wing airfoil that is reflexed at the trailing edge. Also it has a "nose" section that
is soft EPP and this would be the first thing to hit an opponents plane. The nose is there to provide cushion in
front of the battery compartment. The wing has less taper on the leading edge making it "straighter" than the Zagi
type wing. With a straight wing it is easier to get "wing" streamer cuts.
The first step was to select a motor/battery/prop combination to test on the plane. I would like to than all the folks
who thoroughly confused me their logic, and thank those who were sympathetic, but finally than most of all Cliff
Whitney of Atlanta Hobby for putting up with all my questions, running a couple of hundred motocalc scenarios
and finally after we had selected the motor put up with bargaining to get it at the best possible price (which I
didn't, because Cliff deserved to profit from all the time he spent with me.
The prop had a large part in the motor decision. The room between the ailerons, which would ultimately be
where the rudder fins were located was a little less than 8 inches. This meant I had to use a 7" inch prop. As a
starting point I chose an APC 7X4. We wanted enough rpms to generate at least 35mph, and that would be
about 10,000 rpm. We knew a Mega 16/14-X or Astro 020 would easily get us more than enough speed but I felt
at 4 ounces they were too heavy, and would require a bigger battery (or shorter flight times) and speed
controller. I was hoping to fly two 7 minute combat matches on one charge. I did not want a gear box so the
decision criteria came down to what could swing a 7X4 at 10K rpm, it would be a brushless outrunner, and it had
to weigh less than four ounces.
We ended up with a HiMax 2815-1080 brushless outrunner, Castle Creations Phoenix 25A speed control, and
ThunderPower 3S 2,100 Mah Lithium Poylmer battery. The next step in the process was to acquire or design an
extension shaft for a pusher propeller that would let the motor be mounted right on the CG.
tube 1/8" diamter with two 5/32
wheel collars. The tubing is drilled
so that the set screw of the collar
can contact the prop shaft on one
end and the motor shaft on the
other.
cut to 10". Shown with Himaxx
propeller mount and APC 8X4 test
stand prop. The Himaxx mount
slides onto the graphite rod easily.
A very nice set up.
is unsupported. The 8X4 test prop
read a consistent 9,100 rpms but
had a bad wobble. You will be
surprised at what caused it. Read
the text below for an explanation.
Initially I planned to mount the motor on the CG and have the shaft supported near the prop with a
bearing mount. To save development time I researched the internet to find out how others had solved
similar problems. I thought I would find the answer in model boat prop drive systems but the answer was
really even simpler.
It turns out that quite a few people have had a similar idea of mounting the engine on the CG and using
an extension shaft to the prop. One of the illustrated articles told of how the builder had aligned the rear
support bearing carefully and eliminated all but a little vibration. Upon flying the plane for a while he
noticed a change in its sound, it became much quieter. On landing he found that the rear shaft support
had broken away leaving the shaft to wobble or not on its own. He then found that as he powered up the
prop would wobble for the first 15%-20% of its rpms but then smooth out and find its center of rotation.
The test rig shown above was designed to test the same thing. It worked perfectly with a 7X4 but would
wobble with an 8X4 after about 10 seconds of run time. I first thought the problem was balance so I put
the entire assembly on a prop balancer and brought it into static balance. The same thing happened and
may have been a little worse. What was really odd was that the shaft was bowing in the middle. It was
then I realized that the 1/8" graphite rod was not stiff enough. The push of the prop through the shaft
towards the motor was deflectiing the shaft. The spinning effect caused centrifugal force to magnify the
slight deflection making it worse. So everything is fine with a 7X4 and that is the prop I will stay with for
now.