We squeeze engines into tight, streamlined cowls and allow the least amount of (drag-producing) cooling air through there because what we really want is speed. But even tiny leaks in baffles can have huge effects on engine cooling and, therefore, engine longevity as Marc Cook discusses in this month's Motor Head column.
December 4, 2005
This will be the column that allows devotees of liquid-cooled engines to rock back in their Aerons, place palms on paunch and proclaim, "Aren't we the smart ones?!" With some design care, a liquid-cooling system for aircraft can be simple to execute and remarkably insensitive to variations brought by series production or less-than-surgical ongoing maintenance. My rationale for this statement is this: Once the position and size of the main radiator is established and the inlet/outlet ramps crafted to maximize flow (and reduce drag), you end up with fewer components that are absolutely critical to cooling efficiency. Not to say designing such a system is anywhere near trivial, mind, but that's all upfront work, something the owner (or builder of an Experimental) need not worry about. To take the insensitivity idea one step further: If you can keep coolant in the system and can manage to place the ducts the way the designer intended, much of your work is done.
From what I understand talking to pilots flying behind Subaru engines in Experimentals, the cooling system is often the least touched part of the installations. The same is true from what I hear from the Rotax ranks. The sturdy Rotax 912 is flying in a lot of airplanes and, based on what I've read and the feedback I get from maintenance personnel, has offered up no recurring issues beyond the engine just not being a low-revving Lycontiental. Expect to see more of these little four-bangers if the Light Sport Aircraft segment finds its feet; the 912 appears to be the engine of choice for the category.
Why Is It So Hard?
Let's contrast a well-designed liquid-cooled setup with the typical air-cooled engine's baffling system, which is, as the name implies, often truly baffling to owners and maintenance personnel. I'll give you a firsthand example, too. A stalwart Beech A36 I fly often -- now that my friend Troy Foster has moved from the city to a veritable paradise in central California and taken his Viking with him -- recently came out of the shop after an engine overhaul. Proclaimed ready to go, the baffling was a disaster. In fact, months after the fact, the owner is still trying to get all the little bits and pieces in the right place. The JPI tells the tale: Without careful airspeed and fuel management, this engine runs hot, hotter than it did before the overhaul and hotter than other IO-520-powered A-36s I've flown.
Backing up a bit, then: It was a trying experience for the owner, as overhauls often are. He had arranged for a local shop -- a generalist, not a Beech specialist -- to remove the old engine, send it out for field overhaul, and then reinstall it. I owned a P-model Bonanza with the IO-470-N -- an airplane and engine I truly miss, but that's another story -- and so am fairly familiar with the Beech baffling system. There are some important differences between the 470 and the IO-520 in this A-36, it's true; but the underlying concepts are the same. Even without the parts manual in front of me, I noticed several major baffling transgressions right off. More on them later.
Nevertheless, the baffling (literally) idea is simple on the face of it: You must direct as much of the air entering the cowling through the cylinder fins as you can, do it efficiently, and as evenly from cylinder-to-cylinder as you can. It's this evening-out of the flow -- an ungainly way of saying that each cylinder should receive the amount of air it needs, not necessarily the most cooling air -- that is the hardest to obtain. (See Mike Busch's excellent discussion of a similar set of problems facing a Cessna T210 pilot in Savvy Aviator #14.)
We know this because of the proliferation of engine monitoring equipment in the last decade and a half. Before, we'd stare at the single CHT indicator, by decree placed on the critical cylinder during the climb phase, to help determine power, speed and even cowl-flap management. Oh, how we were uninformed. No doubt the airframe and engine manufacturers began to hate the engine monitor, as now pilots call in listing a 40-degree CHT spread as a problem when it had been there all along.
Well Educated Is Well Armed
Engine monitors are a great tool, as I've said before, but require some interpretation, especially where data acquisition is concerned. Fortunately, the subject Bonanza has a JPI system with fuel flow, so it wasn't too hard to see where the baffling was affecting CHTs during the break in. The CHTs were a bit high, but were they high because the baffling was botched or because, in the normal course of things, a breaking-in engine runs hot? We wouldn't know conclusively for a few flights, especially as we were running the engine at high power at low altitude, climbing at a higher-than-normal airspeed and pumping a great deal of fuel through it to reduce ICP (internal cylinder pressure).
But after the break-in flights were completed and other pilots began to fly the airplane, it became obvious that something still wasn't right. We decided to take another shot at it.
I say "another" because after the first flight we had to make some remedial repairs to the flexible baffling. As you Bonanza owners know, there are two basic types of baffling designed for these airplanes. The original is called the hanging baffle because there is a strip of flexible material hanging from the hinged, opening cowling that fits down across the faces of the rocker boxes to seal the top half of the engine. The entire top area of the cowling is open. Small close-out plates seal the area between the cylinders at the valve covers, and there are myriad fixed baffles between the cylinders, between the front of the engine and the inside, forward edge of the cowling, and up around the back of the engine to seal the upper plenum from the area just behind the engine.
The hanging, flexible baffling was about three times too long, so that instead of just nestling against the rocker covers, it wrapped over and curled back on itself. Not only was the cowling hard to close, the exposed lip of the flexible material was now pointing into the airflow, where it could easily be lifted off the mating surface. At that point, you might as well not bother putting it in there at all. Moreover, the flexible seal around the vertical baffle behind the engine had not been relieved, so that it gapped noticeably where it met the cowling doors. Again, more leaks and air left to stream toward the accessory case without being sent around the cylinders.
We did our best to trim the flexible seal and close up the gaps, but the more we looked the more leaks we saw: The closeout piece around the front of the engine near the prop was not fitted with the correct length of flexible material, so that the area was open to the lower part of the cowling. More air lost.
Finally, we starting making progress, but the standard Bonanza bugaboo of cylinder two running hot in the climb along with number six running warm in cruise spoke up. At least we knew we were back at a baseline setting, where what we expected of a Bonanza was what we got. Between our progress and getting the takeoff fuel flow turned up where it should be -- for the IO-520, think of the redline fuel-flow value as an absolute minimum -- this airplane is getting there.
What's the Point?
It occurs to me that there are several factors at work here: 1. Any shop planning to remove and install an engine at overhaul time needs to have (and actually use) the parts manual. It became clear right away that the shop committing the work on this Bonanza lacked one or the other. I know that shops are supposed to have the documents, but you can bet that some do not. As an aircraft owner, it's worth the investment in the manuals to have a copy for yourself. 2. Baffling condition is far more critical than is generally appreciated, especially in high-performance airplanes. 3. From what I've seen in the field, we aren't always paying the closest attention to baffling condition.
On any stroll down the ramp, you can find chafed or missing flexible baffling, cracked rigid baffles, holes drilled in the sheetmetal that lead nowhere ... all wasting precious cooling air. Think of baffling systems as a pair of pants, and think of your airplane investment as a pocket full of heavy coins. Any gaps will let the change fall out.
Making It Better
I'm encouraging the owner of this Bonanza to take it a step further and consider aftermarket baffling kits, which are available from Beryl D'Shannon and General Aviation Manufacturing Incorporated (GAMI) to fit this engine/airframe combination. Among the mods, these systems switch over to a different kind of outer-cylinder baffle that places the flexible strip on the engine for a better seal against the cowl door. There are other less obvious tweaks to improve airflow (and airflow attachment) around the cylinder heads and barrels. I've heard much good about both systems and have had the engineering process described as tedious at best. "You make one, small, seemingly insignificant change and it makes a world of difference in the CHTs," said one engineer. "And then you can make what seems like a big change and it makes no difference. The devil is in the details." Indeed, GAMI's subsidiary company, Tornado Alley Turbo, recently received approval to use tapered-fin cylinder assemblies on its TNIO-550 installations; these fins are tapered on the barrel portion only and allow modified baffling to push more air through the hotter heads and less through the heat-uncritical barrels. Every little bit ...
Beyond ensuring that all the available cooling air is being forced through the cylinder deck and the oil cooler, it's a matter of smoothing the flow and equalizing the air distribution where you can. The rear oil cooler for the Permold-case Bonanzas creates a challenge or two in getting the air to flow smoothly and efficiently around that rearmost, number-two cylinder. Just as getting the flow around the front oil cooler of the sandcast-case Cessna T210s requires a bit more effort and finesse than the original manufacturers cared to deliver. (In the T210 I also fly, the number-five cylinder, just behind the oil cooler, is the problem child. Every other cylinder runs cool, so we end up managing power to keep that one cylinder happy, wasting fuel and airspeed in the process.)
But the advantages to improving your engine's baffling are huge, in my view. More efficient cooling reduces CHTs, and that's a good thing -- no, make that a great thing -- just on the face of it. I'm among those who believe there's almost no such thing as a too-cool cylinder based on the metallurgy of the things. At the nominal redlines of 450 or 460 degrees F, the head alloy has lost a huge amount of strength. Current thinking is that 380 is a good working maximum, with up to 400 allowed for short periods. Remember that when most of our airplanes were certified, it was permissible to let the CHTs get to the redline value during the worst-case climb-cooling test. Permissible, maybe, but not smart of you're paying the bills.
One Case Study
On my old Bonanza, I spent a fair amount of time during the first annual working on the baffling, getting the flex material up to scratch and sealing leaks. There were a couple of places on the rigid, vertical baffle that had holes drilled to nowhere, leftovers from the original generator, perhaps, or bits and pieces from the Rajay turbo that had been on there. There were glaring gaps between the corner edges of the hanging baffle. After a bit of work, some of which the technicians who were watching over me said was utterly without merit, my IO-470 ran 10-20 degrees cooler in CHT, permitting me to use more power (higher rpm) in the climb if I needed it and even climbing with the cowl flaps partially closed in cooler weather, which itself improved climb rate.
And when I had to overhaul that engine -- I bought the airplane with the engine near TBO and had to overhaul due to case cracks -- we took the time to replace all that hard work and make sure every bit of the baffling was true and correct. It added a day or more to my personal time spent on the changeout, but I think the effort probably added many more hours to the life of that engine.