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REMOTE CONTROL
BY BOB YOUNG
Internal combustion engines
for model aircraft
The subject of this month's column was prompted
by a reader's letter enquiring about the possibility
of converting a glow plug model aircraft engine to
operate on petrol. Inevitably, as part of the
conversion, the reader wants a design for an
ignition system to complete the conversion.
Why would you want to do such a
conversion? The reader, Leon Burkamshaw, of Coffs Harbour, NSW, is
quite succinct when he states that the
cost for methanol-based fuel is becoming prohibitive. He is right.
The short answer is to his query is
"yes but with some difficulty and great
care". However, I am not renowned
for my short answers so sit back while
I endeavour to fully explain this subject. First though, I will give a brief
This is particularly true of 4-stroke
motors which can run too hot when
fuelled by petrol. This excess heat
can cause severe damage to the valves
and head. And, as a secondary effect,
the spark plug element can begin to
glow under the heat and compression
and cause severe pre-ignition, with
attendant problems to the conrod and
bearings.
While fuel prices are important, so
is safety and petrol is a very explosive
"Anyone attempting to convert a modern motor
to petrol ignition is advised to proceed with
caution as there are vast differences between
motors designed for methanol and petrol fuels".
explanation and a word of warning
on operating petrol motors.
Anyone attempting to convert a
modern motor to petrol ignition is
advised to proceed with caution, as
there are vast differences between motors designed for methanol and petrol
fuels. The main areas of difference are
in compression ratios, timing and carburation; in other words, in virtually
every area of the motor design.
58
SILICON CHIP
chemical when compared to methanol. Handling methanol is a fairly laid
back business and in over 40 years of
hanging around methanol powered
models, I have rarely seen a model
catch fire and I have never seen anybody seriously burnt in an accident.
Some of the fuel additives are, however, · very toxic when inhaled and
great care is called for when using all
chemicals. Nitro-benzene was a very .
popular additive in my early years
and was particularly dangerous. Fortunately, it is rarely used these days. I
must add here that one should never
run motors or fly indoors and for those
who think I am mad for even suggesting it, I once flew a helicopter inside
my factory and nearly gassed myself.
To go one better, a friend of mine
once flew a helicopter in his hotel
room and suddenly found himself and
his helicopter on the footpath outside
the hotel; along with all of his luggage!
Fuel dangers
On a more series note, there was an
article in an American modelling
magazine some time ago, written by
an American modeller who was also a
trained military aviator. He had a
methanol fuel-can fitted with an electric pump explode and catch fire under his face. He was immediatelv engulfed in flame from the chest up. His
survival training had taught him never
to breathe in until the flames went
out if caught in this situation, as otherwise one's lungs and throat can be
badly seared. He survived to write the
story.
An untrained person may not have
been so lucky so please be careful,
even when using methanol based fuels. As a personal idiosyncrasy, I will
not use an electric fuel pump. I realize that such accidents are rare but
they do happen.
The situation with petrol is potentially much more dangerous. Again, I
have not heard of any accidents in the
modelling field but stories abound in
general society of the dangers of petrol. Carrying petrol in a. model box
Speed Control Still Coming
This month, I must apologise to
those who have been eagerly awaiting the speed control construction
article. Judging by the response to
the speed control series, there are
many more interested in this subject than I first anticipated. Unfortunately, pressure of work has prevented me from devoti ng the amount
of time necessary to complete such
a complex project.
And here I must point out that I
am quite surprised at just how complex this project has become. When
Leo Simpson first asked me to do a
construction article on a simple
speed controller, I had exactly the
type of circuit I wanted in mind. Leo
applied only one constraint - that
the design was to be based on commonly available components. The
NE544 servo amplifi er ch ip is no
longer current and other chips can
suffer the same fate at any moment,
so I considered this a reasonable
request. It did, however, introduce
containing batteries is just not on and
it should always be contained in a
well sealed can, painted red and
clearly marked PETROL - INFLAMMABLE. Tanks should be fi lled in such a
way as to prevent spillages an d the
motor sh ould not be started over any
area where a spillage has occurred.
Even a small spark from a glow
battery or starter battery can start a
petrol fire and the really wise will
carry a small powder fire extinguisher
in their flight box as a matter of course.
I might add that electric fuel pumps
and petrol sounds a really dangerous
combination to me. Thus endeth today's sermon .
Motor types
The motors most commonly employed in models fall largely into two
categories: single cylinder 2-stroke
and single cylinder 4-stroke. But, in
keeping with all fie lds of modern human endeavour, the recent technological revolution has resulted in a
proliferation of very expensive multicylinder motors. These days , it ap pears that the sky is the limit in regards to cost, with radios selling for
some unexpected complications.
This constraint also had a second
and less obvious consequence in
that it 'also precludes a microprocessor based system on the grounds
that the software also becomes a
single source component and therefore difficult to obtain . But because
the speed control ler lends itself so
well to a micro-based system, I have
finall y managed to twist Leo's arm
into a painful enough position for
him to agree to let me do a second
construction article at a later date
on a more specialised unit.
The good news is that as a result
of the enormous amount of work put
into analysing the prevailing speed
contro l design philosophy, we have
come up with a very interesting new
wrin kle to the design approach,
again using readily available corn:
ponents. Prototypes are currently
under construction for testi ng and
more will follow shortly, so stay
tuned .
up to $2000 and multi-cylinder engines and the n ew gas turbines exceeding even that figure.
Prices of this order were unheard of
in th_e late 1960s but th ey do give the
manufacturers a chan ce to recover
costs so that they can develop .newer
and even more specialised units.
The two categories may be broken
down again into three sub-categories:
Diesel, Glow Plug and Spark Ignition.
Each type has its own advantages and
disadvantages and each has its own
particular set of design rules, all of
which are too complex for a full analysis in a column such as this. There
are , however, many books covering
engine theory and operation for those
interested in this fi eld.
The names "4 -s tro ke" and " 2stroke" derive from the number of
basic operations required to complete
one cycle of operation. In a 4~stroke
motor, there are inlet and exhaust
valves to enable the fuel mixture to
get into the cylinder and the exhaust
to get out. In a 2-stroke motor, by
contrast, there are no valves and the
crankcase is used to transfer the mixture into the cylinder and to scavenge
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CATALOGUE APOLOGY
Our sincere apologies to all our customers awaiting
copies of our catalogue We are having to endure a
systems error at our typesetter He 1s 1nstalhng a new
system and production should be finished shortly
MAJOR CREDIT CARDS/CHEQUES WELCOME
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Telephone: (03) 662 3506
Fax: (03) 663 3822
A UGU ST 1992
59
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MODEL AIRCRAFT ENGINES - CTD
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60
SILICON CHIP
The invention of the modern glow
plug model motor is attributed to an
American engine enthusiast, Ray
Arden, and the idea swept the world,
pushing the diesel and spark ignition
motors into the background. Here it
seems was a great new idea, yet the
Wright Brothers flew at Kittyhawk
with a glow motor of their own design. The old adage of "there is nothing new under the Sun" still holds it
appears.
The method of generating the ignition is the next most important factor.
Diesel motors use a self-ignition process which relies on the heat generated
in the compression process to raise
the fuel/air mixture to flash point.
Diesel motors used in model work
have a sub-piston fitted above the piston and a screw on top of the cylinder
head to adjl.).st the compression ratio
whilst the motor is running. The compression is reduced for starting and
then increased for maximum revs.
They are very difficult to throttle effectively and do not run well at low
speeds. Fuel is commonly a mixture
of ether, kerosene and castor oil. The
piston/cylinder seal must be good.
Glow plug ignition is again a selfignition process, in which a glow plug
which consists of a wire element, usually made of platinum, supplies the
ignition. To start this type of motor,
the plug is raised to bright red heat by
connecting a battery across the element. Once the motor begins to run,
the platinum wire element will continue to glow under the heat from
compression and the chemical effects
of the methanol fuel. At this point,
the battery is disconnected and the
motor will continue to run. As in the
diesel motor, the piston/cylinder seal
must be good which these days means
piston rings.
·
If there is a noticeable drop in en-
gine revs when the battery is removed,
the plug is "too cold" and a hotter
plug must be fitted. What this means
is that a more active element must be
fitted for that compression ratio. The
art of motor tuning is very subtle and
such things as plugs, porting and fuels make a very big difference to engine performance. These motors throttle extremely well and are very popular in R/C work as a result.
The fuel is usually methanol and
castor oil with extra oxygen supplied
sometimes in the form of nitro-methane. Glow fuels make excellent paint
strippers and many paint jobs have
been destroyed completely because
the model was not fuel proofed or the
incorrect paint was used in the first
place.
Spark ignition
And now to get to the question
which started this article. Spark ignition is the oldest form of model engine, the first engines being petrol
powered "gas jobs". And very quaint
they were.
The compression ratio in these motors is usually much lower and the
spark is supplied by a true spark plug
as used in cars. This plug is connected to a coil, condenser and a set
of points and the spark ignites the
fuel -air mixture. Because of the lower
compression ratio, the piston/cylinder seal does not need to be as good as
for glow plug motors. A good bright
spark cures a lot of the ills encountered in model motors, such as worn
pistons and cylinders. The early motors did not throttle well, however,
and a 2-speed set of points was often
fitted, to provide spark advancing and
retarding.
Modern spark ignition motors are
now fitted with electronic engine management systems and many of the old
ills are now cured. The fuels used are
most commonly petrol and mineral
oil but methanol-based fuels can be
used.
Once again though, the dreaded
space restriction brings us to the end
of this month's column and we must
await next month's issue for a more
detailed analysis of the ins and outs
of modern spark ignition and 4-stroke
engines.
SC
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