About Propellers Other Products Articles Bookshop Gallery Links Contact Supercool

Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


F2B: the mighty Enya 45 1 6001
(AKA tuning F2B stunt engines)

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.

 Back to Top