Plenums for Engine Cooling, trend or hi temp solution?
From several homebuilt aircraft featured in recent magazines (HOMEBUILT, KITPLANES, EAA Sport Aviation) I'm seeing a frequent use of the "plenum" or pressure chamber used to force air onto the engine to increase airflow pressure for cooling. Doe anyone know if this is just a trend , for those wanting to invest in the fiberglass cap over the engine, or genuinely the accepted method of solving high engine temperatures. I dont see most F1's, or One Design high performance aerobatic aircraft using it, but see it used mostly on the RV's. I would prefer not to have the engine top hidden from my pre-flight inspection, as it would be with use of the plenum. Welcome any insight anyone can share on the use of the plenum.
I'm not sure of the prevalence or lack there of in homebuilts, but they've been used common use in commercial aircraft for over seventy years. You can either build an internal plenum or seal up the cowling such that you use the cowling as the plenum itself.
Cooling an air cooled horizontal opposed aircraft engine is all about maximizing the pressure difference, or delta-P, between the top and bottom of the engine. Streamlining the cooling airflow usually provides the required delta-P with less cooling drag than sealing off a big boxy cowl. Mooney capitalized off this for many years. I remember the first time I removed the cowl on a Mooney still couldn't see the engine because there was another cowl, or plenum.
I think most builders or cross-country airplanes want to squeeze out as much performance as possible. Cooling drag accounts for a lot of overall drag so attention in that area gives good returns.
The Rockets should have a plenum and the appropriate inlets to feed it. Adding that will make the airplane go faster. I can report from experience that you want to balance the airflow over the top of the engine and as much as possible you want all of the air forced through the cylinder fins. An example of how to bias the airflow across the top of your engine and cause uneven cylinder head temps is mounting the oil cooler on a rear baffle. Care to guess which cylinder runs hotter because you are stealing air from it?
As for preflight inspection of the engine compartment, I will note that the builders of newer certificated aircraft (Cessna, Piper, Mooney....) seem to think that we can see all that we need to by looking through a small door over the oil filler and in the forward air inlets. For extra credit you can look up from the bottom of the cowling.
A plenum is used to increase static pressure of incoming cooling air. Assuming perfect flow, the plenum is where we trade dynamic pressure (q, airspeed pressure, ram air pressure, etc.) for static pressure. Ptotal = Pstatic + Pdynamic. It also makes the flow more uniform for more even cooling. Higher Pstatic (differential across the cylinders) cools the engine better. In addition, external aerodynamics around the cowling inlets can be made better if what happens inside the cowling (the plenum) is known, fixed or doesn't change a lot. In this case, we know the air is coming to rest (yes, not quite, but ...) and that the inlets will be spilling or diverting air around them as aircraft speed increases. Simple answer: Plenums have been around for decades. They increase cooling efficiency and can reduce cooling drag ... a major drag factor on piston powered airplanes.