I thought it would be neat to take
some aerial photos of a home site I am interested in. Kites,
balloons or paying for professional aerial photos were all
options but I thought it would be fun to try to do it from a
model air-craft of some kind. I had never flown models before and
the saga of learning to fly a model is elsewhere on this site
along with some videos. I fly full sized gliders and it wasn't
too hard to fly fixed wings after the initial prangs. I
completely ruled out regular helicopters as being too expensive
and too hard to fly. I was impressed with the quad rotor
dragonfly but it was too small and the professional version too
I bought a dragonfly micro heli. It took a month
to arrive and had serious drift problems in the gryo so it took
quite a while to install “separates”, get them
working and then learn to hover.
Bought a Blade runner. This is an amazing kids
toy. Co-axial rotors with no cyclic. If floats like a balloon. At
50 grams and limited to totally still air make it is useless for
camera work but steered me away from quad rotor towards co-axial.
I started my co-axial project.
I wanted to use
off the shelf parts as much as possible. I was fairly familiar
with dragonfly parts and they are relatively cheap. Other types
are probably better for this but this was an experiment.
on I was thinking of using two dragonfly (sans tails) mounted
right way up. I was also thinking of modifying the rotors so the
shaft extended through the rotor head so it could also be held at
the top as well.
One problem with this arrangement is a lot
of wiring has to be run between the two helis. I was thinking of
controlling both cyclics at that point so there would have been a
lot of wire to route and some of them passing high currents.
the co-axial designs I've seen have both rotors on one shaft,
this means major modifications and probably requires a fairly
decent workshop plus a lot of hours.
the problem upside down.
By turning the bottom heli
over all the interconnection problems go away. The down side - so
to speak - is the C.G. is very high. The heli would not be
inherently self-righting. I thought (wrongly) that is would be
stable though. By stable I mean it stays in the attitude you put
The frame is a bucky ball made
from 2mm carbon fibre rods - 90 rods 13cm long - 60 vertexes.
The idea is in part to provide a undercarriage but mainly it
is to partly protect the rotors from bumps when flying through
There are a lot of possible shapes that
might be better, this was a starting point that I thought would
The ball is about 650mm across so it will fit through
a standard doorway.
Making the reverse pitch
blades for the bottom was problematic. You can't buy them as far
as I know so I bought symmetrical profile blades (like-90) and
tried to modify then to add pitch.
The first set I removed
material at the root to add 10 deg pitch but they were destroyed
on the first test run due to a stupidity error.
The next set I
used PCB material to make wedges. I later damaged one blade in a
heat gun accident.
I had serious vibration problems which I
blamed on the damage but later found other factors.
blades were a little long anyway, then I used some "CP
twister" blades fitted using 15 deg balsa wedges.
wedges didn't last long and I replace them with spruce.
top rotor is powered by a brushless motor - bottom is brushed for
now. It remains to be seen if a brushless motor will respond fast
enough for yaw control.
With a 3 cell poly battery fitted the
top rotor has enough power to lift off without the lower assembly
Ideally the blades should be the same top and
Yaw control is through
differential speed control of the two rotors. I don't have a
co-axial mixer so I've fudged it for the tests by using a heading
hold gryo to control the lower rotor speed. This is not ideal but
works well enough for basic hover testing. Ultimately I'll
probably have to build a custom mixer.
Only the top rotor has cyclic
control. Servos are fitted to both rotors but the bottom ones are
Initially I had fly bar
stabilization on the lower rotor but this doesn't work well,
particularly if there is a wind shadow from the innards.
Sept - First test.
First tests were
hopeless due to extreme vibration. I expected the frame to
vibrate but not this much. The thing was literally shaking apart
and I had some rotor strikes – not clear which happened
first, the strike or the frame failure. I've only broken one rod
so far due to rotor impact all the other failures were glued
joins. The first tests were with just the top assembly fitted. I
tamed the shake a little by adding a cross brace and balancing
the rotor better.
With a three cells lipo fitted the top
rotor has enough grunt to lift off (I was holding it to stop it
With the bottom assembly fitted (and a few mistakes
and accidents) I got the gyro to lock properly and it held a
(fairly) stable heading.
I have more vibration problems but
eventually got it good enough to increase power to what I think
was the point of lift off – it just fell over.
what I did with the controls it would always fall over to the
After much head scratching I figured the problem was
that the lower fly-bar paddles were passing through the wind
shadow of the battery at the back. If I understand fly-bar theory
correctly this would cause the symptoms I was seeing.
Off with the bottom fly-bar paddles.
removed the lower paddles and found a way to re-arrange the
linkages to lock the pitch neutral. I didn't get it quite correct
because the swash plate position is moved in the process. So now
it always fell over backwards.
I re-adjusted and still had
problems. Again bad vibration which I then but down the a warped
swash plate, it is more like I have a bent shaft. I was able to
compensate for the wobble but still had it falling over.
added “training wheels” (as it the top photo) to give
me a better chance of keeping it upright and finally managed to
briefly lift off.
Lift off and let down.
I could lift off
but rather than float like a helium balloon as I'd hoped it flew
more like a balloon being deflated..
This was one of the many
times I was about to give up and try something else – maybe
a blade-runner on steroids.