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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.


Use of glow motors in Vintage Combat 2000

By Joe Supercool

The rules we use in WA for Vintage Combat permit the use of glow motors made prior to 1970. The best of these are the Super Tigre G20/15 and the G15. I have rarely come across a contemporary diesel motor that can match them, although the G20/15 diesel of Mark Ellins was a match for my G15 at the first Busselton Nats. I have also seen Gary Turna’s diesel (type unknown) come close to matching the G15.

However, in both of these cases I was using the highly flexible Taipan 7X4 obtained via Kim Parks, which is somewhat down in performance over the older, rigid Taipan 7x4. As a genuine Vintage pilot (flying the same G15/Dominator combo in the 1960’s), my favourite prop was the Tornado 7X4, which could tow the Dominator at 90 MPH on 60’ lines (single strand .012”), which meant the model could also be used in Open Combat, a quite remarkable circumstance.

At that time, I was using Burford manufactured pistons diamond lapped by Andy Kerr. These pistons got better and better with more running, until the point was reached where the Tornado 7X4 threw blades and the conrods started to fail. As the replacement Super Tigre conrods were of poor quality, and my pistons were ruined by the passage of their disintegrated parts through the motor, I rather lost interest in continuing with the old FAI combat. I also tried the Kavan “fibreglass” 7x4, but its blades flew off before it even got in the air.

As a consequence, my return to Vintage Combat in the late 1990’s meant I had to find a source, not only of G15’s, but also of conrods that would not break. Fortunately a bucketful of cash sufficed to twist Steve Rothwell’s arm, and he made a bunch of G15 rods to my specifications, which I now offer for sale at $40 each. These rods have massive bushed big-ends, similar in size to the later Rossi 15 MkII. They may give rise to a slightly higher level of motor vibration, but this is acceptable when one gains such a leap in reliability. Also the tags on the gudgeon pin retaining circlips should be cut short, as they fatigue, snap off and wreck both the piston and head.

Re props, I am not allowed by the rules to use my own fibreglass props, and I am unaware of a suitable alternative to the rigid Taipan 7X4 of yesteryear. I cannot understand why production of this excellent prop was resumed using the wrong material.

G15’s in various states of repair appear from time to time on ebay; even new, unrun motors show up, so they can still be obtained fairly readily. There is a concern I have with new G15’s. The finish on the piston ex-factory was rather rough, which, while great for running in, did nothing for later performance, either in power output or ease of starting. It was my practise to polish the piston, and then accept an extended running-in period: the running in was done at 22000 RPM, the G15’s peak power RPM, but with a light load, such as a 7X4 cut down to 6X4. And of course, I would not even consider running the motor with the factory conrod: it is a disaster waiting to happen.

I notice my missive is rather dwelling on the G15, but my following comments apply to any glow motor used in Combat, so please persist with my ramblings.

The classic argument against using glow motors, and in favour of Diesel motors, is the supposed ease of starting and restarting hot of the latter. I cannot say that my observations support the former, or, with less rigour, the latter. My G15 starts first flick, on tune and ready to launch. The Diesel’s seem to need to warm up, and require some fiddling with tune to get going. That loses bouts.

So how come my G15 starts so easily, while others flick and flick to no avail?

Here are the secrets.

  1. The G15 is best run on pressure, which usually means crankcase pressure. It is very easy to flood the motor with this set-up, and a flooded G15 will not start, period. My procedure is designed to stop flooding, and have just the right amount of fuel in the motor for it to start first flick. After filling the tank (which needs to be 90cc for a 4-minute bout), I leave the pressure line disconnected. Then I flick it over to see if it is flooded, by some misadventure. If it seems wet, I turn the model so that transfer passage is on the lower side of the motor, with head pointing down also. This ensures that excess fuel in the crankcase runs down the transfer port into the head. This excess is drained off by raising the head and opening the exhaust port so that the flood can run out. If this is done correctly, the motor can be flicked over without locking up due to too much fuel in the head. By now the plug is probably full of fuel, so remove it and blow out the excess and check for a good strong glow. Return the plug, and start the motor to burn out any remaining fuel and leave the motor dry. Allow the motor to cool.
  2. The motor is now dry and cool. With the pressure line still disconnected, tilt the model so that the head is upright, and dribble about 8 drops of fuel into the venturi. Flick the motor over repeatedly, until you feel that the motor is nice and wet. This foreplay guarantees a quick start. Close the exhaust port.
  3. Await the combat Marshal’s order to start. Tilt the model so that the head is slightly upright, so that if there is, by some horrible error, too much fuel in the head, it will run out the exhaust port. Connect the pressure line and attach the glow leads.
  4. Give the prop a nice sharp bang backwards against compression and it will start immediately. Tip the model so that head is slightly down, to ensure the fuel pick-up is in the fuel, and if necessary do a last minute tune. It is best to go off slightly on the rich.
  5. If the motor does not start right off, immediately disconnect the pressure line. The motor is either too dry or too wet. Respond accordingly. With practise, you will get the starting mixture right and never have this problem.
  6. If the motor does not have a good piston/cylinder seal, it will not start when hot, or even warm. The heat from the sun is enough to stop the motor from starting: it must be cold. Put a rag over the motor to keep the sun off, and put a squirt of raw fuel over the cylinder to cool it further.
  7. Hot restarts cannot be guaranteed by any method I know. The best you can do is drench the motor with fuel to cool it, and ensure the motor has not flooded. A hot, flooded, G15 will not start.

Well that’s about it. In respect of the foregoing, I owe a debt of thanks to John Williams, Andy Kerr, Steve Rothwell, Mark Giggins and Trevor Letchford. If anyone out there wants to sell G15 bits in good condition, I am interested (but not in conrods!)

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