THE TWO MAJOR REQUIREMENTS for good cooling of an air-cooled cylinder-head are (a) adequate conductivity from the zones of maximum heat-flow, that is, the spark-plug bosses and the exhaust-valve seats, elbows and guides, to a sufficient area of finning, and (b) the maintenance of a high-velocity air-flow over the entire length and depth of all fins. Solution of the problem of (b) depends upon many items in the engine installation outside of the cylinder-head.
A limit to possible power output of the cylinder is set by detonation, which, with a given fuel, depends upon the cylinder-head temperatures. As these temperatures are the basic index of operating conditions of air-cooled engines, the author states that a head thermocouple instrument should be standard equipment on every airplane, and pilots should be trained to respect head temperatures as much as they now respect oil pressures and temperatures. Much higher ratings and cruising speeds could then be used with assurance under all normal conditions, because the head-temperature indication will give warning before harmful conditions develop.
Much interesting and valuable supplemental information is brought out in the discussion*. Rate of impact of the air on the fins is believed to be an important factor in cooling. Apparently small design changes to reduce air resistance can have as much effect in increasing speed as an increase of many horsepower in the engine. Fuels of 80-octane antiknock rating, if generally distributed, would enable the supercharged-engine rating to be increased one-third without changing anything but the blower ratio and would lower the piston temperature, reduce some of the stresses in the engine parts and be less severe on the lubricating oil. It is said to be better to boost the power output by increasing the supercharger pressure a moderate amount and increasing the piston compression slightly than to accomplish the result by either the piston or the supercharger alone.
Emphasis is placed on cylinder-head temperatures as a basic indication of safe engine-operation. A thermocouple attached to the rear spark-plug of the cylinder that becomes hottest warns the pilot when to reduce the throttle opening. Questions regarding the spark-plug cooler and radio shield are raised and answered. The question of cylinder-head design to get intimate thermal contact of cooling fins and the best air-flow around the parts is debated. One discusser points out that the use of longer pistons would permit of more piston-rings which would aid heat-flow to the cylinder-walls, but the reason for using short pistons is that the engine diameter must be reduced to the minimum. Poor mixture distribution rather than detonation is said by one discusser to be responsible for most of the piston trouble.