| Now that I am serving
out my 61st year on this blood-drenched ball we call planet
Earth, Fate has offered me the opportunity to correct some
of my failures from the earlier years. The most frustrating
episodes concern F2B stunt engines. Many a time and oft have
I watched other competitors stunt engines running magnificently
while mine ran rich, cut, or took off with a howl at 80 MPH.
Click to enlarge
Now if you are one of those whose motor runs
well every time, and can concentrate on performing a flawless
pattern, as opposed to avoiding a crash, read no further.
This is not for you. I insist. Stop reading now. But if you
are a dunderhead like me, persist and maybe we’ll come out
of this for the better
Early Enya 45, wedge combustion chamber. The wedge is intended
to drive the charge into the combustion chamber near the source
of ignition, the glo-plug
In recent times (the last 25 years), my Albatri
have been the ST G21/46 and the Enya 45 I 6001. Tell me, how
many people do you know can’t get a ST 46 to run right? None,
right? Well, let me tell you.
I bought my ST 46 from Laurie Cantwell at his
Kingsford model shop in Sydney, in the early 80’s. With it
I purchased a spare P/C set and ring. Not being familiar with
ringed motors, I figured to give it a very careful, rich-running
period of one hour. This I conscientiously performed, only
to find that in the model it was gutless. No power.
In desperation I pulled it down and the ring
fell out in 2 pieces. Broken from new. I went to replace it
with the spare ring, but it didn’t fit: too big. Also the
spare sleeve didn’t fit, it had a thinner wall and couldn’t
be used. I appealed to Brian Eather, who fitted a new ring
for me, and so off flying. Now the motor would lean out and
go like a rocket, but not slow down!! Unflyable.
Twenty years later (count ‘em!), I tried again,
this time with first-pressings virgin castor oil from the
hobby shop. Now the motor would run nicely for 5 or 6 laps,
then slowly lean out until it was thoroughly fried at the
end of the run. Hopeless.
I switched to Shell Racing M and made a 2:1
mix. Eureka! Superb run, just what I always dreamed of. So
off to the first Albury-Wodonga Nats (was that 2000?), where
the motor ran wonderfully. I was too sloppy in the first round
(no practise) but tightened up for the second and scored 2800
with a motor cut half-way thru the clover. Not bad, 200 ahead
of the pack and 200 behind the leader. Came back for the 3rd
round and found my “Firecracker” had broken its back from
the 6000km ride in the back of my Laser from Perth , so I
withdrew. Second last in Expert.
Early Enya 45, with no cooling fins on the
head and wedge combustion chamber
Several years later, back in Perth, I dragged
it out again. Hopeless, another broken ring. With a lot of
help from Daryl Mills, Brian Gardner and Ranjit Phelan, I
got it going again! Hoorah! The motor ran very sweet, with
a wonderful strong, rich burble in level flight and leaning
out nicely for the hard work.
But the Devil was riding again. There were 2
problems. Narrow needle margin and sagging power in the overheads.
When I say narrow, I mean one click of an Enya needle on the
Enya spraybar assembly. Not good, too critical. Nitro did
not help, which was a worry, since nitro is supposed to make
needle setting less critical. Not this time, Bro!
Having twiddled the needle on Norm Kirton’s
Brodak 40 a few weeks earlier, maybe a finer needle was the
answer. The Brodak seemed to have a huge needle margin, possibly
due to the very fine-threaded needle. Brian shot me over an
ST assembly, but by now the old Brain started working involuntarily.
You know, the 3am stuff when you can’t sleep. Never works
when I want it too, so better then than never.
Just where did the ST46 get that lovely rich
burble from? I checked out my Enya 45’s (6001 and 6002). They
had the same shaft, transfer and exhaust timings as the Tigre,
well near enough, but they didn’t run like that. Why not?
OK. Lets work thru this. Once the piston is
on its way up, and the exhaust port is closed, one motor is
much like another. The timing’s are forgotten, the size of
the transfers is forgotten: everything is forgotten except
for burning the fuel! So there is something about burning
the fuel that separates the Tigre from the Enya’s. Lets look
at this more closely.
Later, kid, later. Several years ago I did a
prop for a design contest. Never heard the result, so I guess
I didn’t win (see Travelling Dynamometer on this site) It
was an 11X8.5, rather an odd size and no use for anything
. What would happen if I bolted this onto the Tigre?? So off
to the Whiteman Park Kangaroo-tick factory for a test fly.
Amazing. The Tigre ran up to only 6500 RPM (no
nitro), but it sounded good! Into the air and to my astonishment
it burbled around beautifully for 5.3’s, pulled strongly in
the overheads and turned as tight as an FAI combat model!
As I say, astonishing! Previously I had good results with
the Bolly 11.5X7, but this was something else. Brian reckons
the tight turns were the result of my using a lighter prop.
That was true, the Supercool was half the weight of the Bolly,
and lighter again than the APC 11X8 that I also tried. Gyroscopic
forces cut in half.
This is revolutionary stuff. You ask any expert.
You want to turn tight? Use less pitch.
You want needle margin? Use nitro. You want
four-stroke? Raise the head. Baloney, all wrong.
Well things were going so well, how could I
lose by trying 10% nitro? Disaster. The motor would not tune
rich. With the plug leads on, yes: but it would go gobble-gobble
and stop rich with the leads off. Could the new batch of Castrol
M be the problem? Now that its cut with synthetic, could it
be poisoning the platinum plug element? Thus making the plug
go cold. Tried 3 new plugs, still no good. Switched to Shell
Racing M, which I know works well. Still no good, so its not
the Castrol M (Mystery?) oil.
Back to zero nitro: problem gone. After some
more testing, I decided I needed more power in the overheads,
The Tigre was not quite there. So I switched to my Enya I
6001. Ranjit had kissed the bore with the hone and checked
the rings. Good as new.
With zero nitro, and Bolly 11.5X6, it ran OK,
but not with the controlled authority of the Tigre. Flakey
on the level, too much power in manoeuvres, not nice at all.
Tried the Supercool 11X8.5 on straight fuel. Ran at 6700 on
the break, but cut when the plug leads were pulled off: even
on straight fuel, not like the ST.
So where are we now. The piston has closed,
the exhaust port and is on its way up. That only leaves the
combustion chamber as the source of differences between the
two motors. Pulled the heads off: quite a lot difference.
(see photos).
The Tigre has a wide squish band with a hemispherical
combustion chamber, while the Enya had an insignificant squish
and a large dish for the combustion chamber.
The ST 46 head and the Enya 45 II 6002 have
the same configuration, and they both drop straight onto the
6001! So I ran the Enya I 6001 with both of these heads, and
was rewarded with excellent performance.
With .012” of shims under the 6002 head, tuning
rich was poor. With the shims out, tuning was good with 6500
at the lean/rich break, and 7300 leaned out.
Now you know, F3D pylon racing guys are very
fussy with the compression on their racing forties. I mean,
if you’re getting 4HP at 33000 RPM and 200 MPH from a 40,
you better be doing things right. They have a test flight,
go blank for a while, wave a wet finger in the air, and loudly
say “take out a thou”. Take out a thou!? For Gods sake, do
you know how thin that is?
By now I was starting to recall some of Ranjits
words of wisdom. I’m not a very good listener. I much prefer
to talk than listen, but something had sunk in. Methanol has
an explosive limit. A range of fuel/air mixture over which
it will burn rapidly and produce power. The trick is to get
the combustion conditions inside that range, so that the fuel
burns strongly and smoothly. Hey, that’s what I want in my
stunt engine.
Not only that, he said “ If you want to run
cool so you don’t burn plugs, run it rich and up the compression”.
Got that? Increase the compression if you want to run rich!!!
Hell, that’s the direct opposite of what the stunt guru’s
say. Can they be wrong? Well of course, otherwise would I
be wasting my time writing this crap for you swine out there?
Talk about Pearls before Swine. I bet your eyes have all glazed
over and you are thinking of the Britney Spears video! Well
stop it, pay attention!!
So how does combustion work in our engines?
Well we seem to have a number of variables. The squish band,
the deck height (squish to piston at TDC distance), the combustion
chamber shape and volume, and the fuel burning characteristics.
Let’s do the last first. The fuel/air mixture
is ignited by the constantly glowing platinum plug: a flame
front then spreads out over the combustion chamber. The speed
of this front is important, as the piston is only in the right
position to accept pressure from the burning gas for a very
short time. We also want the right ratio of fuel to air, which
is ultimately determined by the needle valve setting.
But having the right ratio is not enough. The
fuel and air must be intimately mixed for controlled burning
to occur. This is not so easy, such mixtures do not form spontaneously:
they must be forced to mix. This is the job of the squish
band.
“Squish is the name given to the radially inward
gas motion that occurs toward the end of the compression stroke
when a portion of the piston face and cylinder head approach
each other closely”. Thus spake John B. Heywood in “Internal
Combustion engine Fundamentals, page 353. 500 pages later
he tells us some more. “The impact of radially inward squish
motion (see page 353) on in-cylinder turbulence, and hence
combustion, is unclear” !!!!! Thank you John, well Hello!!
“ Chambers with significant squish are also more compact:
for this reason alone they would be faster burning”
Ye Gods, what a Turkey. Lucky I got this book
at 1/10 the new price. Now for the Supercool version.
As the piston face approaches closely to the
squish area, the trapped mixture is blasted toward the combustion
chamber at high speed, where it mixes violently with the charge
there. The turbulence created by this event mixes the fuel
and air intimately (how nice!), permitting the flame front
to advance in a controlled way.
So, if you are trying to run a rich mixture
(and we are), then if you want it to burn without spluttering
and popping, you might be advised to use a squish band head.
Thus the distance between the piston at Top Dead Centre (TDC)
and the squish band (deck height) is very important for smooth
running when rich. It is necessary to be able to adjust this
parameter using head shims, say .003” thick for best effect.
As examples, I run my G15’s at .004”, Rossi 15’s at .012”,
F3D motors at .016”, Brodak stunt engine at .016”, and my
Enya 6001 at .015” with my own head design.
Now what happened to our problem with the nitro
fuel? Recall, the motor would not run rich with the plug leads
off? Well, the fuel mixture must be too cold; it’s cooling
off the plug. So much for nitro keeping the fires lit: it’s
actually putting out the wick in the engine! Well, there is
a certain way to fix this: increase the compression ratio.
Compressing gases makes them hot, just what we want here.
Unfortunately, none of the heads available to
me could be set to give increased compression. Less yes, more,
no. So I made my own head.
The deck height on the new head was set to zero.
The piston just touched the squish. This meant that the squish
adjustment was essentially infinite, so I popped in a .015”
shim to give me a known deck height. The squish band was made
wide, and a small combustion chamber was adopted to give heaps
of compression and a small distance of travel for the flame
front.
So it was off to the TARMAC field at the end
of the runway of Perth domestic airport. This is an amazing
field. The wind coming off the trees produces a wonderful
rotor effect, so you never know from which direction the wind
is coming. It is actually different at the model to what it
is at the pilot, and can be by 180 degrees. You have to be
insane to fly F2B there, as you can be crashed just doing
a loop. It’s no place for test flying a radical new engine
mod, so I rolled out my lines and test flew.
Amazing. The Enya ran like a Mack-Truck diesel.
It growled around the circle on the 11X8.5 at 6500 RPM, leaning
out a little to power over the top of the circle and perform
a neat Lomcevak as the rotor hit it. Let the FAI add that
to the pattern!!
I was stoked. There was heaps of needle margin,
5.3 second laps, a real dream. So tomorrow it’s off to the
tick factory for some blood poisoning and a field with very
little turbulence. |
