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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.


 Stunt, Squish and Singh.


For those who came in late, Supercool has been trying to improve the performance of his stunt engines by increasing compression, as opposed to the more usual process of decreasing compression. No doubt these efforts will be washed away by the tide of history, but them's the breaks. We last left Supercool on his way to the Whiteman Park tick farm, ready for further testing with the trusty Enya 45.

This morning, however, we find Supercool has departed once again from the straight and narrow, this time via the sensual path of the Internet. How about a nice Google search for 'Squish Band'? You never know your luck in the big city! And what a gold mine this turned out to be!

But first, quite at random, Supercool struck a familiar vein in a Model Airplane News article dated January, 1976, page 54. Reader Neil Harrington of Connecticut wanted to know what it meant for a 2-stroke engine to '4-stroke'. In reply, Peter Chinn suggested, on every other stroke, that the engine did not fire, due to a combination of rich mixture and charge contamination from imperfect exhaust gas scavenging from the previous firing cycle.

Interesting, I wonder if it is true. I guess what I call '4-stroking' is really just running rich. Since there is virtually no RPM change when I needle in from '4-stroke' to '2-stroke', there is very little power change and hence every other cycle cannot be 'missing'.

Whatever, we are back at the old question of what does happen when the charge burns in the cylinder head at TDC. There seems to be a consensus that most of the combustion occurs while the piston is still close to TDC, with all the later action resulting from expansion of the hot gases after burn completion. There is also believed to be a considerable variation from fire-to-fire in the combustion process.

This latter variability results from the rather unpredictable state of the mixture, which includes methanol vapour, methanol/castor droplets, un-scavenged burnt gases from the previous firing, plus oxygen and nitrogen. All this mess has to be compressed before it can produce a powerful and fast burn. Anything that can improve the quality of this mixture is going to improve the engine run, and that is where the 'squish' comes in.

Something like a hundred years ago, a gentleman by the name of Ricardo found that modifying the combustion chamber shape to include flat areas which closely oppose the piston could improve power and running characteristics. The improvement was due to increased turbulence at TDC, which improved mixing and hence burning of the charge.

The Supercool internet study found that the squish had the following properties:

  1. Timing: reducing the squish clearance, and using a wider squish band with a smaller, deeper combustion dome makes the motor 'act' as though the timing has been advanced.
  2. Vibration: To make maximum power, the squish band should be flat. If the resulting pressure rises by more than 35 PSI/degree, the engine will run roughly.
  3. Grunt: At lower RPM, 'grunt' is achieved by higher squish velocities, which means wider squish bands. For power at higher RPM, narrower squish bands and shallower hemispherical heads are required.
  4. Cooling: The fuel trapped in the squish does not burn readily, due to the cooling proximity of the piston crown and the metal in the squish band. This provides a small cooling effect.

If one is to apply this to F2B stunt, then a wider squish band is indicated for low down grunt. The ST G21/46 has this feature. Motors that tend to 'run away' may benefit from having a new head with a wider squish band. It is also suggested that reducing the squish clearance will improve low-down grunt as well.

The photo below shows the heads for Supercool's ST G21/46. Next to the original head, are 2 new heads, with identical wider-squishes and deeper bowls. One has grooves cut in as suggested in US patent 6,237,579 of May 29, 2001, as defined by Mr. Somender Singh of Mysore (www.somender-singh.com), India. You can view the patent details just using a Google search. The argument behind the patent, and the diagrams of head mods can be read on the US Patent Office website, no charge or other exploitation.

Click to enlarge

Mr Singh's idea is to create extra turbulence by adding grooves to the squish band. He has made some remarkable claims regarding low-end grunt, and this I found attractive as I am trying to use 11X8.5 props on my F2B ship. Anything that permits me to run rich with grunt is desirable.

Ground running of the new heads showed excellent behaviour while rich. I could needle down to 4000 RPM comfortably, lean out to 6500 at the break and peak out at 7400. This latter may be too high for my comfort, will need to test fly.

Sadly, I could not see any difference in the head with squish band grooves. Maybe the grooves are not the right shape: but there it is.

Test flying: During the desultory break as the morning Easterly gale switches to the afternoon South-Westerly gale, Supercool managed about 8 flights with the Singh head. Performance was fine, no nasty signs of SMS (sudden motor silence), and 5.2 laps on 60' lines and straight fuel with 30% Castrol M. Compared to the original ST 46 head, the level flight '4-stroke' was somewhat faster and sounded to have more 'bark'. This could be consistent with the more advanced timing expected with the wider squish. No effort was made to alter the compression with shims.

What an anti-climax. I seem to have achieved the state of getting the ST46 to run about the same as it does out-of-the-box!

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