How to not lose your cool
Professional prognosticators and armchair future-seers alike have come to almost universally predict the decimation of the old-guard, air-cooled piston engine for aircraft any day now - as soon as the next generation of high-tech, liquid-cooled wonders appears. Problem is, few of them have materialized in the last decade or four. Sure, the 500-horsepower Orenda V-8 has been certified, and there's been lots of work on alternative powerplants, but the rank-and-file aircraft owner now peers through the cowling at an air-cooled, opposed-cylinder engine.
And that's not likely to change - notice the work being put into computer control of otherwise conventional piston engines. Instead, most of the rational oracles - particularly those who have been proved wrong once or twice - see the air-cooled engine's reign continuing.
Air-cooled engines can be simpler than their liquid-cooled (water or oil) brethren. Even though they don't lend themselves well to en block construction - where the cylinders are set close together in line, as is common in auto engines - and it's difficult to get the bore centers together for a more compact engine without sacrificing cooling capacity, they can be made remarkably light. Because the cooling fins take advantage of the air flowing over them to shed combustion heat, there are no moving parts.
Cooling by air does, however, put tremendous demands on cowling design, both inside and out. Over the years, as airframe manufacturers tried to reduce various airframes' overall drag, cooling inlets have grown ever smaller. Look at the gaping hole of an early Mooney compared to the Coke-can-diameter portals of the current M20R Ovation - and remember that the first M20s had just 150 hp next to the M20R's 280. Owners report that the Continental-IO-550-powered Ovation is still remarkably well cooled.
Along with fashions, the cowl flap has gone in and out of style, like bell-bottoms and long sideburns. The cowl flap's job is simple: to increase the flow of air through the cowling during low-airspeed conditions by increasing the size of the cowling outlet. Some designers disdain the cowl flap as an inelegant fix to a flawed cowling design. The original Piper Malibu was designed without a cowl flap - the Lycoming-powered version sold today still doesn't use one - but one of the most popular mods for the pressurized single is RAM Aircraft's nosegear-door removal kit. This mod opens the lower cowling to permit better cooling, at the expense of a small amount of cruise speed.
Design considerations are one thing, but you are stuck with whatever came on your airplane. Unless, of course, you are willing to ante up for a replacement cowl. LoPresti Speed Merchants builds a number of retrofit cowls that are said to improve overall speed while also aiding engine cooling. Results on last year's Ultimate Arrow sweepstakes airplane were decidedly mixed - the cowling looked great but didn't provide much in the way of a performance increase - some of LoPresti's other mods have proven to offer better results. That said, the various Mooney cowlings - many of which are patterned after the comprehensive cleanup given to the M20 to create the 201 under Roy Lo Presti's reign - appear to offer measurable and worthwhile improvements over the 1960s standard.
For most owners, the greatest benefits are to be gained with careful upkeep of the existing cowl and baffling system. Because the efficacy of the cooling system lies in its ability to take high-velocity air into the engine compartment, slow it down, and then distribute it ideally around the engine, it's crucial that all the bits and pieces are in place. Surprisingly small alterations in baffle position or condition can make for large changes in indicated cylinder-head temperature.
Most airframes use what's called downdraft cooling - that is, the cooling air enters the top of the engine either side of the prop, flows across the upper portion of the cylinder heads, and then is forced down through the fins surrounding the heads to the lower part of the cowling. The intent is to provide as much cooling air as possible to the heads first, and worry about the accessories and lower half of the engine second.
A few large Lycomings have used updraft cooling, where the air enters below the prop spinner, turbine-style, and is forced up and over the top of the engine. The theoretical benefits include somewhat better ram recovery with the main inlet further outboard of the prop hub, but this setup makes packaging a real head-scratching project. In airplanes like the Piper Turbo Saratoga, this meant a serpentine path for the cooling air, as it was pushed up through the engine and back down over the accessory case to exit on the belly. (The alternative is an outlet or cowl flap at the windshield base, a convenient means of ice protection but an almost certain eyesore.)
We polled a handful of maintenance facilities as well as the two major engine manufacturers to get a list of common cowl and baffling maladies.
Deteriorated or missing flexible baffle seals
By far the most common sight is poorly maintained flexible seals. Because the engine is free to move relative to the airframe, and some accommodation must be made for openings, flex baffling is a necessity. Usually, it's a black, blue, or orange strip or sheet made of fiber-reinforced, high-temperature silicone. Mechanics often see baffle strips folded over the wrong way - the fit of the flex seal is supposed to be such that the high pressure of the upper air plenum pushes the seal closed. If it's able to bend over backward, much useful pressure and cooling potential is wasted. Similarly, if the material is so old that it just stays bent over - even in the right direction, it may not seal properly. Amazed inspectors frequently come upon baffling that's badly in need of TLC, even on recently overhauled engines.Care must be taken with the installation of these flexible components. If the seal must go around a corner, make sure there are sufficient reliefs that the material makes continuous contact with the fixed baffle plates. If it's allowed to ride like a wave against the baffle close-outs, some of the air will seek the path of least resistance and be dumped toward the lower cowling or the rear accessory case.
You may have to use a shop light in a darkened hangar to find gaps in the flex seal - place the light in the engine compartment near the accessory case and look for light shining through to the upper deck. You will see some, but if it's like daylight on top of the cylinders, you'll need to do some baffle-seal work.
Replacement of baffle seals is generally done on condition, or when required, but it's good form to rework all of the flexible material at an engine overhaul or replacement. Look also for improved baffle systems - you can buy later-style baffle kits for earlier Beech Bonanzas, for example, that are a significant improvement over the original setup. The best time to make this kind of switch is when the engine has been removed.
Pickoffs that go nowhere
As airplanes age, the likelihood increases that modifications will be made and then reversed. Often, an alteration that involves the baffling is not put right - or even done correctly the first time. If you see holes on any of the baffle plates that appear to do nothing, close them off - holes as small as an inch in diameter can dramatically influence the cooling of nearby cylinders. And for those pickoffs that are mated to a length of high-temperature flexible tubing (often called SCAT tubing), make sure that the tube is free of obstructions and kinks, and that it's actually hooked to something on the other end. There's always a chance it's not, particularly if the hose must be taken down to remove a part of the cowling.Cowl flap condition
Check your service manual for proper cowl-flap rigging. Slop in the system can hinder the operation of the flaps, perhaps not allowing enough cooling air through on hot days. Also, be sure that the cowl flaps do not touch the exhaust pipes when they're fully closed, or one part or the other will wear through and you'll introduce unwanted noise and vibration to the cabin. Also, take a moment to look up into the cooling outlet to make sure there's nothing blocking the way; the engine-installation engineers tried to keep the outlet path as clear as possible, so you'll want to make certain that any aftermarket add-ons haven't undone their work.Many owners ask to have their cowl flaps tweaked within the allowable range according to season. Some turbo installations are critical enough that a half-inch difference at the trailing edge of the cowl flap can mean the difference between being able to run them closed in hot weather; pushing them out to the in-trail position costs speed and can provide too much cooling. You can see why pilots come to appreciate continuously adjustable cowl flaps.
Silicone madness
Some mechanics think that filling the engine compartment with silicone seal is the answer to cooling problems. And while it's true that judiciously applied sealant will help keep precious upper-deck cooling air from leaking away, you have to consider the maintenance implications. If your shop has to spend an hour or two scraping away the silicone just to pull a cylinder, you've paid dearly for a minor cooling improvement. The guideline here is that a little goes a long way, and it's most effectively used near the front of the cowling where the air pressure is highest.Good record keeping will help alert you to cooling-system problems. A simple log of cylinder-head temps at various altitudes and outside-air temperatures will provide a baseline for any sudden changes in your airplane's cooling-system capabilities. If you see a change, do something about it, because while the air-cooled engine in most installations has sufficient cooling margins, they are margins that you nonetheless don't want to waste. Cooler is better.
