Cooling Fans for Radials

Flight tests of a cooling fan, introduced by the Wright Aeronautical Corp for use on air-cooled aircraft engines, have shown that it increases a plane's rate of climb as much as 20%, the pay load of some types of twin-engined airplanes as much as 10,000 pounds.

The cooling fan was developed in close cooperation with engineers of the armed services to solve two problems which have become particularly acute as the power of engines and the speed and load of planes have increased in recent years. There were the problems of speed-killing drag created by the cowling and flow of air through it, and the problem of cooling the engine under difficult operating conditions, such as a steep climb or long takeoff with a heavy load. Under these conditions, the plane moves slowly, with an inadequate flow of cooling air over the engine. Very little air is swept over the engine by the propeller, because as engine power has increased, the propellers have grown larger, with the broad part of the blades well out from the engine.

Just as a fish in breathing takes in water through its mouth and expels it through gills along its side, so air sweeps in through the front of an engine cowling, flows over the cooling fins of the engine and exits through an opening in the rear of the cowling which is fitted with flaps or gills to control the volume of air. However, when these flaps are opened to provide more cooling air for the engine, in a hard climb or long takeoff, they extend out into the air and drag down the speed of the plane. Horsepower that could be used for forward thrust is required to overcome this drag.

The cooling fan maintains a high pressure flow of air through the cowling, even with the flaps closed. Furthermore, the exhaust air is pushed out into the slip stream at high speed, instead of moving out as a slow, dragging mass. As a result of this work by the fan, the drag is reduced so much that far more of the engine's power goes into flying the plane.

Of equally great importance is the work of the fan in permitting the engine to be run at high power for long periods of time. Since temperature is the main limiting factor on how long full power can be maintained in an engine, the cooling fan plays a highly important role when the plane is moving at a slow speed, on takeoffs, steep climbs or long taxi runs. It is especially valuable for large flying boats with engines in the upper horsepower range. On calm days, a big flying boat loaded to its gross capacity may require a takeoff run of several miles before it can get up flying speed and lift into the air. During this run, with the engines at full power, the speed of the plane is so slow that there is insufficient air forced over the engine to cool it. With the fan in use, however, not only the full load but a heavy overload may be carried in the plane and the takeoff extended to the limits of the space available, without damage to the engines.

The fan is also important at high altitudes, due to the fact that the air at stratosphere levels is so thin that a far greater volume of it has to flow over the engine for cooling purposes. On planes without a fan, the cowl flaps must be opened at high altitudes to increase the flow of air, again creating a drag. The fan not only overcomes this drag but under some high speed conditions actually gives the plane additional forward thrust, due to the jet-like action of air being expelled from the cowling at high speed.

While a small amount of horsepower is necessary to turn the fan, the amount required is only a fraction of the total gain in useful power which the fan makes possible. Likewise, though it adds some weight to the engine installation, this is largely balanced by the elimination of moving parts on the ordinary type of cowling.

Wright engineers have pointed out that the Germans have had a similar type of project under way for some time. However, the art of casting deep cooling fins on cylinder heads and machining fins on cylinder barrels has reached a higher peak in America than any other nation in the world, with resulting better cooling even as engines moved up towards 2,000 and more horsepower. In contrast, the Germans encountered difficult cooling problems at a much lower level of power.

This news clip was originally published in the "Shop Talk" column of the July, 1944, issue of Air Tech magazine, vol 5, no 1, p 10.
The original clip includes 1 uncaptioned photo of an engine nacelle and four-bladed prop.
Photo credited to Glenn L Martin Co.