Early cowl changed to pressure cowl

[c170b53]

Saw thread about advantages of a B over an A over a C (Hang in there Eric). The advantages or disadvantages of cowl types made me wonder if a early cowl could be used on a later version by using the later style baffling. Any thoughts?

[zero.one.victor]

A friend of mine flies a 52 B model (early cowl) with an O-360 Lycoming. It is set up as a pressure cowl-- no airbox on the top of the engine. Far as I know it works fine. Plenty of other airplanes with this style cowl (Pacer, Champ, etc) use them as pressure cowls. It'd just be a matter of building baffling and/or adapting later baffling to fit the early top. Approval- who knows? ("That's the stock cowl, isn't it officer?")

Eric

[Dave Clark]

Hi Eric

The STC for 180 conversion includes the baffle change so that makes it legal. It seems I've seen some early airplanes with C-145s' converted to pressure cowl systems though. Of course the early/later cowl difference is in the upper cowl where, like mine, the early cowl hinges on the top for both sides to be lifted in there entirety vs. the later cowl with a door in the upper cowl for adding oil and a peek a boo inspection. I don't think the cowl differences would be a problem in converting to a pressure cowl.

[tshort]

I'm assuming mine has the "old" type ... from what I remember looking under the cowl, there is a plenum / box over the engine and then the rest of the compartment is open.

What is the downside to this? I see how it is different thant the cooling system in my 172, but a lot of the RV / experimental guys are building plenums within the fiberglass cowl for improved engine cooling. Isn't this a similar idea?

[GAHorn]

You'd think that such a conversion would be a simple matter, but you'd be surprised at the seemingly small variations that make huge differences in "hot" spots in a cooling system. Cessna spent many hours flying the various cowlings with multiple temperature probes installed before they finally settled on the designs they certified. (W.D. Thompson wrote a little bit about this in his book.)
An owner-derived system that looks good and functional might actually have undocumented serious problems....or it might not. The problem is one of availability of accurate and appropriate testing and legality of the finished product.
I once had a Cessna 206 that flew just fine except for the fact that the engine (IO-520-A) had gone thru 3 number 5 cylinders in less than 900 hours. I discovered this as I had pulled cyl #5 for low compression. It was cracked at the head. (Most folks would say that the engine had been "super cooled" or something I suppose, but we came to a different conclusion as you can see.)
While re-installing it the inspector handed me two small sheet-metal nothings and insisted I install them. (They were letting me do the work in their shop, but they would sign off on it.) The parts had new Cessna picking tags on them (along with a $150 price tags.) One was about the size of a silver dollar and merely fit onto the END of the oil cooler, and was an intercylinder baffle which no other cylinder on the engine had. The other was a U-shaped peice of channel that slipped behind/above the oil cooler in front of cyl. #5. It was clear the installation had never been on the airplane during my 5-year ownership.
The inspector said the two insignificant little pieces were illustrated in the IPC, and then showed me the illustration. I swear, you would never believe they were anything but useless appendiges (sort of like an appendix.) But the inspector said it was the reason cylinder No. 5 had such a miserable survival record in my airplane.
I sold the airplane about 300 hours/2 years later and the engine was still doing just fine. I've seen other 206's who had the small parts in place, but you'd have to know to look for them or you'd never notice.
The point I hope to have made is: There's more to this than it might seem, and ... where the baffles are placed sometimes makes no logical sense at first glance. It takes testing and instrumentation to know if you've got it right. I don't think sticking B-model type baffles in an A-model cowl automatically guarantees that the engine is being cooled correctly. I'm uncertain as to whether it's legal without also installing a later B-model cowl to go along with it (unless some other basis of approval is obtained.) Finally, even if paperwork says the end result is now legal...without the previously mentioned testing/instrumentation, it may be possible to have hot spots. IMHO. (In this last regard, I wonder if any DER/DARs would sign off on a flight test that used temperature sensitive wax or marker in strategic locations during a flight test? Seems to me that might be a good way to test a final fitment/design.)