ELECTRIC STARTER MOTOR
Applied to
a four-stroke engines:
OS FS-120
SURPASS II
When I started the contruction of Stinson
L5 Sentinel it didn’t come to my mind an electric starter, but I thought about
an old 3000ST, which could give me the necessary strength to draw streamers and,
if I had been able to, to carry out a “container” which helped me to toss
parachutes, sweets, confetti, shooting-stars…Well, I wanted to turn back a bit
when I was a child and have a model to play with and enjoy myself. Stinson was
what I was looking for: it had a huge cargo bay where I could put a big basket
with several partitions containing the items to throw. While I kept on
contructing, that 3000 ST seemed more and more unsuitable: the idea of having to
cut the cowl, to whom I had devoted so many hours of my
sleep, in order to let the head of the motor get out made me to look for a
different choice.
One day, while I was surfing on the web markets, this choice was handy: using a
four-stroke engine my reproduction would have had an doubtlessly more realistic
noise. Once settled the contract, the price and the delivery, just few days
later I was able to put my OS 120 Surpass into the cowl : it perfectly suited,
without removing anything, on the other hand, I still had room for the silencer,
the fuel tank and maybe for a small electric starter; well, I had to find a way
to fill the gap in weight between the 3000 and the four-strokes!
Since that day I spent each moment of my spare time on designs, plans, projects,
schemes and ideas which led nowhere. I remember I tried to involve some friend
of the group, but without results. According to them, the solution to my problem
was easy: motor, geared belt and free wheel as they already exist on the market.
Why make it more complicated?
Remembering that the similarity with the full scale plane is important, the
pinion coupled with the gear on the engine shaft, is quite similar to the full
scale engine starter mounted on the real plane! So, I put aside projects and
plans and one night I clamped a piece of aluminium in the mandrel of my small
lathe began to cut it. The further the work went on, the surer I felt to be on
the right direction, to have had a good idea and to have real chances on
succeeding on it. At last I really got it: it works very fine . During the
tests I carelessly switched on the starter while the motor was working without
any damage to the gear. Even though I haven’t got a particular equipment I can
confirm that I realized all the items on my own; I asked for help just once:
the slot on the slotted shaft part.12 where the pinion and the tap run.
Now let me add few more notes to what already explained and to the pictures. The
starter motor ( Speed 480 Race) is powered by 7,2V-1800 mAh battery pack, while
a 1,2V-1800 mAh battery feeds the sparking-plug just when the motor turns on.
The servos used are 2.
The first one does three things
at the same time:
-
it powers the starter
-
it moves forward the pinion and the
-
it turns on of the plug
These operations are achieved through the 2 micro-switches controlled by some
leverages mounted on the proper shaft part 28.
The second one is used both as gas controller ( when the motor is already
running ) and to supply and enrich the mixture/fuel ratio when a cold start is
needed.On the bauden which connects the servo-motor to the gas lever, I put a
nylon U hook ( part. 36). During the starting the square part. 34 is dragged
through a spring by square no.1 (part. 26) causing the hooking of part. 37 to
the nylon U hook. Now, opening the throttle with the stick the plug part. 38 of
the carburettor completely closes. The hooking of part. 37 of the nylon U hook
doesn’t take place when the starter motor is turned off. I don’t think the free
wheel it is essential, but it is useful. The starter works even with a geared
wheel directly mounted on the driving shaft, thanks to the loading spring part.
21, which let the pinion to be coupled to the geared wheel in whatever position
it is. However, I suggest to use the free wheel to avoid damages in case of the
starting procedure begins when the motor is still turning from a previous
starting trial. Moreover, according to my experience, I have to admit that the
free wheel , thanks to its weight and to the way it works on the drivinf shaft
( the wheel works during the burst stroke and it acts as fly-wheel during the
other strokes), improves the idling performances of the motor as much as that,
while flying, even lowering the gas trim, the engine doesn’t stop immediately.
One more thing.
The gear ratio is 1 to 23. In order
to have good performances, when starting, even when the engine is cold, lower
gear ratio do not have to be used; whereas if you want to increase it to have
more torque I suggest to substitute gear part. 13 with one with a wider diameter
in order to increase the distance between the motor and the slotted shaft part.
12.
Enjoy your work!