Cooling Plenums

By definition a twin-engine airplane requires installing two engines. But most twins can use the same install in each nacelle, making for less work. With the Defiant, we have a push and a pull engine. Ideally we will try to make them as much alike as possible, but to get the most performance out of each engine there will be some differences.

The front engine came with the project and is a parallel valve Lycoming O-360 A2A. The rear engine I acquired separately and is a parallel valve Lycoming O-360 A4A. As I’ve already mentioned, I am trying to follow the Dave Anders methodology and science behind the engine installations. Brian already set me well along this path with the cowl and cooling inlets already done, and done well. The cowl is relatively tight to the engine for minimum drag, so the installation and packaging will be more a la Lancair 360 than anything. There isn’t quite as much in-cowl room as other Defiant’s I’ve seen. If I can channel my inner Dave Anders, Tom McNerney, and Chris Zavatson, I think we will have a real nice front engine installation.

One thing all three have in common is a cooling plenum. I have done a lot of reading, including NACA technical document 3405 on cooling drag. There were two very shocking numbers in that document. The first is that in the test aircraft with traditional silicone baffle seals, up to 55% of the cooling air was leaking. Fixing this by doing what we now call a plenum (referred to as a Doghouse in NACA 3405) translated to being able to attain the same cooling performance with 38% less flow. Less cooling flow is less drag. Good cooling also means being nicer to your engine. A two-fer. Since both engines are going to be downdraft cooled, we can use the plenum advantage for both engines.

Like any good homebuilder does, I first looked to see if I can R&D the solution. (Rip off and duplicate.) There are a lot of critical dimensions and shapes when interfacing the cooling apparatus to the engine. Therefore, for the front engine, I stated with the Van’s Baffle Kit. It was a simple matter of trimming them down to fit the front cowl. From there, I trimmed further to make them height appropriate for the plenum.

Then it was a simple matter of making a lid for the plenum that matched up to the Van’s baffles. Nothing a little foam core board and tape can’t take care of. Once the lid was made, a secondary layup around the perimeter making a 90 degree flange to allow for fasteners completes the structure of the plenum. A little crafty sealing with either weather stripping or RTV and we will have a virtually leak free plenum.

For the rear engine, I am going to actually be able to cheat even more. I was able to find a plenum designed for a pusher airplane that I could purchase. So the plenum was a “plug and play” solution. The inlets for the Defiant are in a different location than what the plenum was designed for, so I have to devise my own way of getting air from the inlet to the engine. But that is actually the easy part. So in short order we have two plenums, of different design; one for a pusher engine, one for a puller engine.

There are still more details to finish fabricating and checking. The two big items for both front and rear engine fabrication include some further inter-cylinder baffles and diffusers that connect the inlet to the plenum. Then there is a long list of components to install to ensure there aren’t conflicts (there will most certainly be) between the plenum and other accessories that are engine mounted. But we will get to that in due time.

4 thoughts on “Cooling Plenums

    1. There are a lot of theories about updraft vs downdraft. The only pro in my opinion for updraft is the concept that heat rises. But at the end of the day, that is such a minuscule consideration in a cooling system. Anecdotally, most airplanes and airplane manufacturers do downdraft. The majority of the heat that is carried away is transferred to the cooling air which will flow in the direction of high to low pressure, so the effect of heat rising, if you do a good job of selecting inlet location, outlet location, and sealing the plenum won’t matter. Downdraft also makes for a much easier installation and easier plenum fabrication because there are less things in the way.

  1. Great “scientific” take on push pull, up and downdraft engine cooling in one aircraft. Lots of food for thought on unravelling this black art. Thanks James.

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