Plenums

[Zzz]

I've been doing some research on 540s and came across someone who swore by the James Aircraft plenum for their RV-10 over the traditional baffle design. Anyone ever put one of these in a BH?

Supposedly very effective, but... those inlets.

[jaredyates]

What is the goal of the plenum, to cool more efficiently? To reduce drag? The major cost would be the ongoing effort required for accessing the engine during maintenance. The Bearhawk cowling design is wonderful for access. On one of our family trips away from home with everyone loaded and the babies being heavy-eyed, we found a fouled spark plug during the pre-takeoff mag check. I taxied back to the parking area and swapped the plug with my travel tool set, and we took off less than 10 minutes later. At least a few minutes of this was devoted to removing the front right seat to gain access to the tool bag, from which I removed a total of four tools: 7/16 and 7/8 wrenches, a spark plug socket, and a ratchet. I was just visiting an RV-8 builder friend who expressed his reluctance to balance his fuel injectors because of the hassle of removing the cowl and plenum several times to get to them.

I think you'd have less performance return than the RV guys in part because of your lower cruise speed. The RV guys are going much faster, with an airframe that has much less overall drag in the first place. Since parasite drag is related to the square of airspeed, the overall savings of a particular drag improvement at 180 mph is going to be many more total pounds than at 120 mph, and much more likely to justify the costs. With a 540, you'll readily be able to get your Bearhawk into level-flight speeds where the rest of the airframe drag is going to be huge, so that's another reason why I doubt saving a little bit of drag in cooling will make as noticeable difference cruise as it would in an RV.

The cooling efficiency will likely be much better with a non-cowl plenum, but at Bearhawk speeds you'll have much less air coming into the inlets than the RV plenum designer intended. This makes me wonder if the benefits would be diminished to a point where they would be that much less likely to offset the access problem. I'd be interested to hear what the plenum designer would say about how well it should work in our type of plane. If he says it would be great, then he should be able to tell you why. If he says it wouldn't be worth it, then you'd know that even the person with a financial interest in saying yes has said no.

[Zzz]

The Bearhawk cowling design is wonderful for access.

This alone is a good enough argument against I think. I was just curious. It does look like a major pain in the ass to access/view the engine.

[dedgemon]

EVERY Rv owner that I know with the James cowl/plenum setup fights high temps. When you ask them if its faster they say, its supposed to be. Of course there is no data to support it. Reducing cooling drag is certainly possible, but its hard. I'd rather have a simple cowl that cools well. For reference the James inlets are around 60% of the area of the stock Vans inlets.

[Battson]

I think there is a case for them, but it's marginal. I often though about the benefits since finishing the build. The cooling air losses through my baffle must be large, from what I can see, and my engine still runs too cool when running LOP. The cooling air flow could definitely be reduced significantly with a proper plenum. I would want to control that though, rather than fix it and take a guess.

The only job you really have is changing a plug, and some of the plugs you need access to change anyway. Look at current aircraft with a plenum, they have access plugs you can remove to access the plugs Most other jobs, well you can either reach in through the inlet, or you probably need to pull the top cowl anyway.

So far I have only used my large access doors to get at the electrical, oil, fuel and other systems which live around the bottom and back of the engine.
2c

[N942VT]

I open my cowl before every flight, both sides to take a look at things, put my hands on them, wiggle and shake them. It's good peace of mind and I have caught minor issues by doing so. Seems like a plenum would limit that ability, they look cool though.

[Battson]

I do the same preflight check, I think it's a good practice to get into. I would have said 95% of the stuff I touch and fiddle with is after the baffles.
Really the only thing I check on top of the engine is the injector lines, plug leads and fuel spider, and I can see and reach the last two from the cowl inlets.