Most of us "know" that conventional aircraft engines are normally cooled using downdraft air flow. Looking at the design of the intercylinder baffling supplied with the engine and the shape of the head fins confirms that the ideal cooling scenario would be a vertical duct fitted right on top of the engine in place of the cowl, and a large supply of forced air from the top down. Obviously this is impractical for an airplane, particularity a tractor configuration. Complicating “ideal” cooling is the fact that the airplane’s direction of flight and propeller slipstream are essentially perpendicular to the cooling flow. So the challenge is to capture the slipstream at the front of the cowl, slow the velocity and raise working pressure in the upper plenum, then get it to change direction 90 degrees and flow past as much fin area as possible on its way out the bottom of the cowl.
So if you’re trying to “fill a bucket” (the upper plenum area), why are the inlets almost always centered on the crankshaft centerline where the front cylinders present a major obstacle to airflow? Wouldn’t it make more sense to raise the inlets enough to provide a well designed inlet duct with enough length to actually get some pressure recovery? And if these ducts went right past the first two cylinders and discharged this low velocity, high pressure air in the “middle” of the plenum, wouldn’t you have a better chance at uniform, effective downdraft cooling?
You see a lot of baffle modifications and it’s clear that people are treating the front two cylinders differently than the rest. You see blocker dams, ramps, etc – all in an effort to manage the inrushing air, but in reality, each cylinder is designed to use airflow the same way… Doesn’t matter if it’s the front two, rear two, or in between, cooling goes down.
Maybe it’s time to rethink “conventional wisdom”
So if you’re trying to “fill a bucket” (the upper plenum area), why are the inlets almost always centered on the crankshaft centerline where the front cylinders present a major obstacle to airflow? Wouldn’t it make more sense to raise the inlets enough to provide a well designed inlet duct with enough length to actually get some pressure recovery? And if these ducts went right past the first two cylinders and discharged this low velocity, high pressure air in the “middle” of the plenum, wouldn’t you have a better chance at uniform, effective downdraft cooling?
You see a lot of baffle modifications and it’s clear that people are treating the front two cylinders differently than the rest. You see blocker dams, ramps, etc – all in an effort to manage the inrushing air, but in reality, each cylinder is designed to use airflow the same way… Doesn’t matter if it’s the front two, rear two, or in between, cooling goes down.
Maybe it’s time to rethink “conventional wisdom”
