Firewall Forward: Cowls, Baffles and Intake Air

The cowl gives form to the front (or sometimes the rear) of the fuselage and channels air around and into the engine. A well-designed cowl will reduce drag and provide important cooling and intake air to the engine with a minimum impact on the aerodynamic performance of the airplane. Baffles work in conjunction with the cowl to direct air where it needs to go inside the cowl to optimize engine performance. Getting air in and out of the engine compartment efficiently becomes a very important task to the amateur airplane builder, one that sometimes leads to a great deal of frustration. There are numerous ways to approach these issues, but experience has shown that some of them work better than others.

Intake Air

Clean air must get to the carburetor or fuel-injection servo with a minimum of fuss and muss. How we do that depends on the engine being used. Some engines have updraft or vertical intakes, and others have horizontal intakes (front or rear facing). The configuration of your engine will significantly affect the design of the cowl and baffling.

The RV series of airplanes uses an air box attached to the bottom of the carburetor to channel air into the engine. This air box incorporates an air filter and a carburetor-heat valve into a compact unit that then protrudes into the lower part of the cowl, that in turn has a dedicated air-intake “snout” on it. This snout points forward into the oncoming air and channels it into the air box. This is a distinctive visual feature of most RVs, and it works well. The simplicity and effectiveness of this configuration has led more than a few non-RV builders to attach RV snouts to the bottoms of their cowls. Other kit or plansbuilt airplanes use a similar air-box arrangement but dispense with the snout and simply face the air cleaner into the oncoming air in a manner similar to older Cessna 172s. This also works effectively, but lacks the distinctive visual character of the RV design.

The GlaStar uses an air cleaner mounted on the side of the cowl that is fed through an external NACA scoop. The Sportsman 2+2, the GlaStar’s younger brother, uses the same side-mounted air cleaner but employs an internal plenum to channel air into the air-filter housing. In both cases, a separate chamber is attached to the bottom of the carburetor to help the air make the turn into the engine. A carburetor-heat valve is attached to the base of the side-mounted air-filter housing. This arrangement is cleaner aerodynamically, but it sends the intake air through a bit of a torturous path. It nevertheless works fairly well. Updraft fuel injection can also easily be accommodated by this arrangement.

Horizontal or forward-facing fuel injection simplifies the intake-air problem by simply allowing an air cleaner to be attached to the front of the servo body, where it can draw air from the general stream of engine-cooling air. Since no carburetor-heat valve is required with fuel injection, this system is simple and compact. Because it lends itself so well to this easy and efficient method of installation, the forward-facing fuel-injection system has become popular with Experimental/Amateur-Built aircraft builders.

Air Filters

Air filters prevent dust, dirt and other foreign matter from getting into the air intake of the engine. In racing applications, air filters may be eliminated in the ultimate quest for that last ounce of power, but for everyone else, an air filter is a must. Air filters come in two types: paper or reusable. Paper air filters have the advantages of working pretty well and being fairly inexpensive. Many Experimental airplanes have intake systems that are designed around paper automotive air filters, making replacement elements readily available and reasonably priced. If you can make a common automotive air filter work for your Experimental airplane, then by all means do so.

Reusable air filters provide a popular alternative to replaceable paper air filters. K&N is the leading maker of reusable air filters for airplanes, cars, motorcycles and just about anything else with an engine that moves. They cost more initially, but they seldom, if ever, need to be replaced. They can easily be cleaned, oiled and reused many times. K&N also makes air filters with built-in features that allow some models to attach directly to the fuel-injection intake and thus eliminate the need for a separate air box. This saves weight and building time. The Glasair Sportsman was designed from the beginning to take advantage of this convenient feature, and there is no reason why many other Experimental airplanes could not do the same.

Cooling Air

Since most airplane engines are primarily air cooled, it is a really big deal to get sufficient airflow in, through and out of the cowling. There are some important details to consider. First, bring air into the cowl from the point at which the highest pressure is available. In a tractor-engine installation—in other words, when the engine is in the front—the front of the cowl immediately behind the propeller is the first-choice location for getting high-pressure air. In a pusher installation, a scoop at the top or bottom of the fuselage in undisturbed air will provide the best source of high-pressure air in most cases.

The configuration of the cowling is something that the designer of the airplane should have already worked out for you, but if you are designing your own airplane, you need to give this some thought. Look at similar airplanes with similar engines and see how they handled the issue. There is no need to start from scratch when so much work has been done by others before you.

Besides getting high-pressure air into the cowl, it is important to get as much of that air as possible to flow past the engine’s cylinders, and, of course, through the oil cooler. Well-made baffles force air to go where it is needed and prevent it from going where it will do no good. As an alternative, some builders use plenums to channel air over the cylinders. A slight weight penalty comes with this choice, but the benefit is better control of the airflow through the engine compartment. A number of RV builders have used plenums, and CubCrafters now uses a plenum to cool its Carbon Cub. A number of custom airplanes have also gone this way, dazzling AirVenture onlookers with their slick-looking engine bays. Just don’t forget that if you use plenums, you must provide some means of getting air to the carburetor or fuel injection. The hot air coming off the cylinders would be a poor choice for that.

Air can be pushed up from under the cylinders or down from above them—thus updraft or downdraft cooling. Most tractor installations use downdraft cooling, because it is more convenient to have air exit the cowl along the bottom of the fuselage (and get soot and oil on the belly of the airplane) than to have it exit the top of the cowl right in front of the windscreen. It is also generally more efficient to push cooling air directly onto the cylinders from above than to have it snake its way past exhaust and intake tubes on the bottom side of the engine. That doesn’t mean you have to do it that way, but most people do. With a pusher installation, it may be more convenient to collect air from the underside of the fuselage and have it exit above the cylinders, such as we see with the Long-EZ. The biggest negative to updraft cooling is that the exhaust system adds heat to the cooling air before it gets to the engine. There are ways to overcome this problem with plenums, but it makes things more complicated. In any event, both ways can work, but any one design may favor one choice over the other. It is worth noting that many Long-EZ builders have converted to downdraft cooling and gotten better results.

It is important to remember that air expands when heated, so the exit opening to your cowl will need to be much larger than the entrance. The same applies to the cooling air going through the oil cooler. There should be plenty of room for hot, expanded air to exit the oil cooler, in addition to ample air going into it. A restriction in the air going out is just as harmful as a restriction in the air coming in.

Cooling problems bedevil many amateur builders, including those who build from kits. Their problems often stem from insufficient airflow through the cowl. This raises two questions: How much airflow do you need to cool an engine, and how do you know if you are getting enough? Experience has shown that a pressure drop of about 8 inches of water is needed between the cowl inlet and the cowl outlet to move enough air to properly cool the engine. This pressure drop can be measured a couple of ways. A water manometer can be constructed with some plastic tubing and a yardstick. This is a bit awkward, but it has the advantages of being pretty accurate and really inexpensive. Another way is to borrow an airspeed indicator from someone and use it to measure the pressure differential.

In either case, two quarter-inch plastic tubes should be run from the cockpit into the engine bay and secured well. One tube should end just inside the cowl inlet, and the other should be secured near the exit to the cowl. The hose from the cowl inlet connects to the pitot port of the airspeed indicator, and the cowl exit line goes to the static port. If you are using a manometer, it doesn’t matter which hose goes to which end, because we just care about the difference in height of the water columns. With the manometer, the top of the water in one tube should be about 8 inches higher than the top of the water in the other tube. If you are using an airspeed indicator, it should read at least 127 miles per hour, or 110 knots. If you are seeing numbers less than that, you need to work on increasing airflow. If your pressure numbers are equal to, or greater than, 8 inches/110 knots, you probably need to look elsewhere for your cooling problem.

Cowl Flaps

Piper drivers are largely spared the annoyance of cowl flaps, but these flaps will be familiar to pilots who have spent time in the larger single-engine Cessnas. Cowl flaps allow for the adjustment of the airflow exiting the cowl by making the opening larger or smaller as required. When an airplane moves slowly through air, making larger cooling demands on the incoming air, such as during climb, the cowl flaps can be opened up. When the cooling demands decrease, as in cruise or descent, the cowl flaps can be closed to limit the air flowing over the engine, maintaining optimum cylinder-head temperatures and increasing speed. In Experimental airplanes, we see fixed cowl outlets quite often, but some planes have adjustable cowl flaps. The GlaStar came with a fixed cowl flap that could be adjusted by unbolting the flap and securing it in new holes. The new Carbon Cub EX also has ground-adjustable cowl flaps. Some builders have developed cabin controls for their cowl flaps to give them more control. Many amateur-built airplanes rely on a cowl opening that works fairly well for all conditions.

Most amateur builders are quite content to take their cowls the way they come, but others add special touches to optimize performance. Lower cooling drag can be achieved by closing down the cowl exit during cruise. Some other builders just want to be able to increase the airflow through the engine compartment during warm weather. In any case, adjustable cowl flaps are not too hard to make.

Baffling and Plenums

Once air has entered the cowling, it must be directed onto the cylinders and heads to cool them. Since the No. 1 and 2 cylinders face directly into the oncoming air, the flow must be diverted around the front faces of these cylinders to avoid over-cooling them. Thus air is diverted up and over the fronts of these cylinders and directed downward onto all four cylinders in roughly equal amounts. Of course, with an updraft system, the air would be forced up from underneath the cylinders instead. To effectively force all of the air to go where it is needed, a system of baffles and rubber seals closes off airflow to the rest of the cowl, except for whatever air is needed for the engine intake.

The standard system will have aluminum baffles made of .025 sheet aluminum that extends to within about half an inch of the cowl. The gap is sealed with silicone rubber or fiber-reinforced rubber material. Air is often tapped off the back of the baffles to provide air to the oil cooler. These baffles come in kits sold by Glasair, Van’s and other kit manufacturers, or you can make your own. They are not hard to make if you can get a pattern to fit your particular engine or simply copy someone else’s, but the kits make it easier and are highly recommended.

A special note on baffles: Lycoming installs and requires inner-cylinder baffles below the spaces between adjacent cylinders. These must be installed to ensure proper cooling of your engine. If you don’t have them on your engine, get some new ones before you run it.

There are two popular baffle configurations to consider. In one, ramps are formed that channel air directly to the front cylinders, sealing off the lower part of the engine from cold air flow. These baffles are easier to make, but they have the disadvantage of requiring a separate inlet for carburetor or fuel-injection air. Furthermore, they do not provide any air for cooling the alternator, which can be a big problem if electrical loads are high. The newer configuration places a vertical baffle behind the alternator and allows cooling air to flow into an air chamber in front of the engine, where it can flow into the engine and over the cylinders. These baffles are somewhat harder to make, especially the portion that goes behind the alternator, and they don’t channel air over the cylinders quite as effectively, but they minimize the number of air inlets in the cowl and better cool the alternator. Both ways work, so you can take your pick.

An alternative to baffling is the plenum. It takes air directly from the cowl intake and forces it over the cylinders, allowing no air to escape into the cowl until it has done its fair share of the work. They add slightly to the empty weight of the plane but work well to control the flow of cooling air. Aftermarket suppliers make kits for the RV series of kits. With some effort, these plenums can be adapted to fit other planes, or you can make your own. Just be sure to provide a source of air to the carburetor or fuel injection apart from the plenum. You do not really want heated air going into the engine intake.

Cooling Problems

If your engine does not cool well during the flight-test period, it is important to carefully look at the baffling and seals to be sure there are no gaps that allow air to escape. Close is not good enough when it comes to sealing the baffling; there must be no gaps. If you are convinced that your baffles are well made, next look to enlarging the exit from the cowl. By making the opening a bit larger, you can often greatly improve airflow through the cowl, but take it in small steps to avoid having to reconstruct your engine cowl. Sometimes a reshaping of the air inlet will help, but do not look there first, because this is seldom effort well spent.

If your cylinder-head temperatures are acceptable but your oil temperature is high, you may have an airflow problem through the oil cooler. Make sure the exit air path is as clear as possible first, then consider enlarging the SCAT duct to the oil cooler or getting outside air from a separate NACA scoop. Finally, you may want to go with a larger cooler, but start off with the cheapest, easiest fixes first and work your way up in cost and complexity. This is a good time to tap into the knowledge of your builder group. Many people before you have had to deal with cooling problems, so don’t waste their experience by going over the same ground yourself.

If your front cylinders are running much cooler than the rear cylinders, you may want to limit the air going to them by increasing the height of the air dams (baffles) directly in front of those cylinders. This will force the air up and over the top of the cylinders instead of hitting the cylinder fronts. This will warm up the front cylinders and divert more air to cool the rear cylinders. These front cylinder air dams will also help reduce shock cooling of the front cylinders in low-power descents.

An improperly timed magneto can elevate cylinder-head temperatures; too much advance can easily push CHTs into the red. It is also common to see this with electronic ignition systems that can have advance curves that run as high as 39°. Using a less aggressive advance curve can drop cylinder-head temperatures by 20˚ or more in some cases.

The major kit manufacturers usually have a system of cooling that works well, and that should serve as your starting point. However, if you use your plane in a non-standard way, you may need to alter your cooling arrangement to accommodate that use. A good example is putting a plane on floats or very large tires, such as Alaska Bush Wheels. Both of these installations produce a lot more drag than the standard gear configuration and will slow the airplane down. Slower flying speeds and more drag mean less airflow to cool the engine, even as more drag is increasing the workload on the engine. Operating in areas where temperatures and humidity are high will also lead to a greater demand on the cooling system. Expect to need a larger cowl exit and a larger oil cooler with a larger supply of air to it in such situations.

Next time we will take a look at ignition systems—the traditional but still popular technology of the magneto versus modern electronic ignitions.