There are a few practices that will help you to get the best service life out of your engine and enhance safety.
They have been broken up into two topics:
Operational and Maintenance
- Full Throttle Take-Off: This engine is equipped with a float type carburetor that supplies more fuel to the engine during take off and during very high power output than the engine actually needs for power production! This extra fuel is metered to the engine through an economizer valve (actually a fuel enrichment valve) with its own jet. This extra fuel serves to cool the cylinder heads during high power application. If a pilot doesn’t use full throttle for take off, he is actually leaning the engine and possibly over heating the cylinder heads and exhaust valves! This can lead to increased potential for head failure and sticking valves. If you just can’t stand application of full throttle please consider 34.5 to 35 inches as a bare minimum!
- 10 Spread for Cruise (or Spraying): Develop a practice of not exceeding a “10 Spread” between engine revolutions per minute (RPM) and manifold pressure as a maximum power setting for RPM used. Example: 2000 RPM – 30 inches manifold pressure maximum. 1900 RPM – 29 inches MP maximum. Lower manifold pressures can be utilized but beyond certain limits may not be practical. Piston ring flutter at manifold pressures below 23.5 inches can cause accelerated piston and ring wear. Piston ring flutter has been known to produce very high crank-case vent pressure and related heavy oil venting overboard!
- Approach Minimum MP: It is recommended procedure to maintain one inch of manifold pressure for every one hundred propeller R.P.M. while on approach for landing. Example: 1700 RPM – 17 inches manifold pressure minimum. If a pilot ‘chops’ the throttle during descent to land he will ‘super cool’ or ‘shock cool’ the cylinder heads. This sets up tremendous stress in the aluminum head, and if done habitually, can lead to cylinder cracking.
Pilot Tip: Check your manifold pressure gage against reported field barometric pressure while the engine is shut down, they should match!
- Fresh Carburetor Installed at Engine Overhaul: If the carburetor’s fuel enrichment valve (mentioned above) is defective or its linkages worn it can cause unusually low fuel consumption. The results will be similar to not using full power on take off. IE: excessively lean mixture, high Exhaust Gas Temperatures, overheated exhaust valves and, high cylinder head temps.
- Good Sparkplug Maintenance: Good maintenance on sparkplugs helps prevent ignition harness and magneto problems! Electricity still takes the path of least resistance to ground and that should be at the electrode tip of your sparkplugs! If it becomes easier to go to ground through the phenolic components of your magnetos, carbon-tracking will develop. You’ll be changing magnetos instead of just spark plugs!
- Use the Best Fuel: Contaminated fuel (diesel, jet-fuel or alcohol mixed with gasoline) or gasoline of too low octane rating can cause detonation or pre-ignition during high power output. Detonation or pre-ignition cause’s distress to pistons and rings, cylinder mounting studs as well as distress to cylinder heads possibly to the extent of cracking!
Note: There’s been more than one blower changed due to excessive white smoke emission from an exhaust stack accompanied by “oil” fouled spark plugs, all caused by fuel-oil in gasoline accumulating in a radial engine blower case!
- Keep Your Eyes Open: Cracks can often be found in an aircraft engine cylinder head prior to failure. Look for black staining between any two fins across the top of the cylinder, or around the exhaust side. The classic indication will look very black, not dark brown (burned oil). If a black stain shows between two fins and not their adjacent fins it’s most likely a crack. These cracks are the subject of AD Notes 78-08-07 (985) and 99-11-02 (1340)!
An operator once told me he had offered his ground crew a bonus of $500.00 for finding any cylinder head crack or broken cylinder attach stud on his engines! He hadn’t had an aircraft downed for cylinder failure since that offer had been made (several years)! His crew had spotted the cracks before the component failed! A good move on his part!
I hope this information is of some help to you in safely and economically operating your radial engines.