Indoor
Scene 3
Co2
Motors
Continuing from my last article referring to Co2 Duration models.
After working my way through the scores of the Co2 Duration event
from Indoor 1, a serious re-think seems necessary to come up with
something to compete with the winning flights of Steve Glass, just 6
seconds off 9 minutes. This
is likely to hold for some time me thinks!
Steve uses a model of his own design, with an airframe weight of just 6
grams, the motor weighing in at 7 grams complete with propeller giving a
ready to fly weight of 13 grams. The
Co2 charge (liquid) was estimated at about 2 grams plus. Other specs of the model, if anyone cares to challenge Steve
are: Gasparin 28BB motor,
home made single blade propeller with counter balance weight, 30 inch
wingspan, carbon stiffened fuselage and clear ultra film covering.
These flights were flown in adverse conditions, cold, which tends
to reduce gas pressure to the motor, thus requiring the power to be
increased to maintain flying revolutions at the propeller.
It remains to be seen how this combination works in conditions
that are more favourable.
 At just about all of the civi indoor events and
meetings that I have attended over the past few years, I have been asked
as to how the motor works using Co2.
I imagine that members of RAFMAA who may not have come across
these small motors might well pose the same question.
Therefore, hoping not to make too big a fool of myself I shall
try to describe the workings as I see it.
If anyone out there has other ideas or can put me right if I get
it wrong, please put your thoughts in writing to the indoor column. You never know, even us so called experienced users may learn
something.
The Co2 motor is a very simple device, which uses Co2 gas under pressure
as its motive force. The
Co2 is supplied from a bulk pressure cylinder, usually a Soda Stream
bottle, or more uncommonly these days by Sparklets bulbs, the latter
being a bit more expensive to use.
A charge of Co2 gas or liquid is transferred from the bulk
cylinder bottle under its own pressure into the motors flight tank via
an adaptor nozzle mating with the motors charging nozzle.
The liquid/gas Co2 lifts a small ball bearing or rubber ball off
a taper seat allowing the gas to pass.
On disconnection of the bulk cylinder, backpressure re-seats the
ball bearing in the charging nozzle giving a gas tight seal.
At the other end, the cylinder head also has a ball bearing, this
time seated in a plastic insert with a tapered hole opening into the
cylinder above the piston. Gas
pressure from the flight tank also holds this ball on its seat providing
a gas tight seal. The system is now charged and ready for use.
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Now we come to the working parts. The
Co2 motor works on a two-cycle principle, but unlike a petrol engine,
which relies on crankcase pressure to transfer the fuel/air mix via
ports in the cylinder wall for the piston to compress before ignition,
the Co2 motor needs to release the stored gas through the cylinder head
valve. All Co2 motors, as
far as I am aware, use the same method to do this.
The crankcase on Co2 motors is open to atmosphere to prevent
pressurisation and to allow access for lubrication of the lower moving
parts.
 On the piston crown is a vertical pin, dead
centre. The function of
this pin is to lift the ball bearing off its seat in the plastic head
valve seat when the piston rises to top dead centre, thus allowing gas
under pressure to pass into the cylinder and push the piston down.
As the piston descends, the piston pin exits the head valve seat
and the ball bearing again re-seats to give a gas seal. The expanding Co2 gas and the flywheel action of the
propeller and other moving parts move the piston and crankshaft through
a revolution cycle. As the
piston approaches bottom dead centre, exhaust ports are uncovered and
the expanding Co2 releases its pressure to atmosphere and the flywheel
action of the propeller etc continues the cycle to raise the piston and
pin through the head valve to start the process all over again.
We now have a running engine.
This is fine and the motor can now be installed in a model and
flown, but it would be nice to have some control over the revolutions of
the propeller and flying speed of the model.
This can be done in several ways, depending on the manufacturer of the
motor, but all end up by doing the same thing, which is to adjust the
time that the cylinder head ball valve is open and how much gas is
released into the cylinder.
Telco motors effect this by
varying the height of the piston top dead centre through having the
crankshaft supported in an eccentric bush in the front crankcase
extension. Rotation of the
eccentric bush is achieved by turning a hexagonal nut bonded to the bush
with a spanner and moving the whole crankshaft/piston assembly up or
down, the cylinder is fixed.
Gasparin and Brown motors both use the same
idea of a fixed crankshaft and adjust the time of gas injection by
screwing the whole cylinder and head assembly in or out of the
crankcase. Brown motors
have a friction fit screw thread to prevent the cylinder from unwanted
movement and Gasparin use a lock ring to achieve this after adjustment.
In recent years, Stefan Gasparin has introduced a gas regulator valve to
control the power output on some of his motors. I do not know if any other manufacturer has used this,
Gasparin is the only one I know off.
This valve eliminates the need to slack off the lock ring to make
speed adjustments and is a useful means of connecting a radio control
throttle function to light models.
The valve assembly is usually fitted to the cylinder head,
although I believe he can supply a remote valve separately.
Generally, all Co2 motors follow much the same construction. The most
significant difference to me is in the manufacture of the piston and
piston to cylinder seal. Telco
motors use a one piece nylon moulding for their piston, which
incorporates both the piston crown pin to open the head ball valve and
also the cylinder gas seal in the form of a flange ring cup, works much
like a brake cylinder cup.
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Gasparin motors also use a nylon moulding for their piston, but have an
aluminium piston crown with integral pushpin for opening the head valve.
Between these is a radial groove for the fitment of a
rubber-sealing ring, which provides the gas seal between the piston and
cylinder.
 Finally, the Brown method is a one-piece solid steel piston assembly
complete with piston crown pin. This
component is highly machined and, I believe, is a lapped set with the
steel cylinder. This item
is such a fine fit that it requires no further seal apart from
lubricant. An expensive
motor but really reliable and consistent
All of the Co2 motors described above all like to have a good drink of
oil to prevent the piston and seals from generating friction.
A dry Co2 motor does not last very long and gives poor
performance. The method
that I use to lubricate any of these motors is the same, I use a small
syringe with ground down needle (remove the sharp bit) and inject oil
through the exhaust ports with the piston at bottom dead centre to
lubricate the upper cylinder. Then
rotate the propeller through 180 deg and inject oil into the crankcase through the oil/breather hole on Gasparin motors, or through
the exhaust port but under the piston skirt on the Brown to lubricate
the lower cylinder, connecting rod and crankshaft.
Gasparin motors also have an additional oil hole on top of the
crankshaft extension for crankshaft and bearing lubrication.
Excess oil is not a problem; the motor just throws it out.
This can be a bit messy after some time, but it soaks up easily
with a tissue.
Operating Co2 motors can at times can be very frustrating, especially if
going for duration flights. The slightest gas leak from any part of the system will blow your chances,
as will a lack of lubrication. Trying
to find just the right power setting can often be very critical in
duration flights, there seems to be a fine line between climbing,
cruising or descending. These
motors do not like to operate in very cold or humid conditions, they
either need to have their power increased or they tend to ice up.
Over working an engine will also induce icing and lower
performance, this is often seen in models being powered by too small a
For scale models, Co2 motors are great.
Apart from the conditions mentioned above, it is nice to hear the
putt-putt of the exhaust to give that air of authenticity to your
subject. There are now many
configurations of Co2 motors available, from tiny 1cubic mm to ½ cc
capacities in single cylinder engines and then on through various twins,
inline fours, box fours, V 12s, 9 cylinder
radials and now even twin row
radials. If you are
prepared to pay, anything goes.
Phil Morgan
Indoor Comp Sec.
email:
rafmaa-indoor@rafmaa.co.uk
 
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