Keep cylinder head temperatures in the green
Take a look at the new single-engine airplanes from Columbia Aircraft Manufacturing, Mooney Airplane, and The New Piper Aircraft, among others. One thing you'll notice is that the cowl cooling-air openings are markedly smaller than the same openings in the cowlings of high-performance designs produced in the 1960s, 1970s, and 1980s. Yet the certification standards for engine cooling haven't lessened; if anything they're tougher. So what gives?
The smaller inlets are evidence that airframe and engine manufacturers have learned how to lessen drag — cooling drag in this instance — by working together to create well-engineered installations.
The evolution of engine air-cooling systems is because of a couple of factors. The first air-cooled aircraft engines were configured in a radial layout. The cylinders radiated out like spokes of a wheel from the crankshaft. Each cylinder barrel and head was exposed directly to ram air. The cylinders got plenty of cooling air but the cost was high levels of cooling drag. Find a picture of Charles A. Lindbergh's Spirit of St. Louis and you'll see what I mean. The Wright J-5 was the 223-horsepower engine that carried Lindbergh on his record flight. Fred Weick of Langley Memorial Aeronautical Laboratory published a technical note for the National Advisory Committee for Aeronautics (NACA) titled "The Drag of a J-5 Radial Air-Cooled Engine." Weick concluded that there were 85 pounds of drag at 100 mph.
The first step taken to reduce the drag was the introduction of a Town-end ring, often called a speed ring. This circular airfoil-shape sheet-metal ring rested on the top of each cylinder and smoothed and accelerated airflow past the heads. This reduced drag by approximately 11 percent. This concept was expanded with the NACA cowling — also designed by Weick — that fully enclosed the engine. A series of adjustable openings (cowl flaps) at the trailing edge of the NACA cowl permitted the pilot to control cylinder head temperatures (CHTs) by controlling the cooling-air exit size.
Radial engines had their day, but didn't cross over into the light-airplane fleet. The most common piston-airplane engine configuration is the flat, or horizontally opposed, engine. Cooling is provided by a pressure-cooling system. In this system ram air enters the fully enclosed cowling through openings on each side of the propeller, is turned 90 degrees by a series of sheet-metal pieces called baffles, and is then forced past the engine cylinder cooling fins into the low-pressure area below the cylinders before exiting the cowling.
The paragraph above describes the details of a downdraft pressure-cooling system, but the same principles also apply to updraft configurations where the high- and low-pressure areas are reversed, and pusher configurations where the cooling air enters the high-pressure zone through a forward-facing scoop.
These systems have three restrictions — one at the cooling-air inlet(s), one as the air flows from the high-pressure inlet side of the engine past the fins that radiate out from the cylinder barrels and cylinder heads to the outlet side of the system, and one at the outlet. For the system to work well the cross-section of the exit area must be larger than that of the inlet.
High and low pressures
Lycoming engine installation manuals require a pressure differential of 5 and a half inches of water — a pressure measurement determined by manometer gauges installed in the cowling — across the ram-air side and the exit-air side of the engine for its 150-horsepower O-320, and 6 and a half inches of water for its 180-horsepower O-360 engines to ensure good cooling under all air-temperature conditions. A measurement of 1.23 inches H2O is required for an airspeed gauge to measure 50 miles per hour. Five inches of water is equivalent to 105 mph, or 91.2 knots; six inches of water is equivalent to 112 mph. It's important to realize that these pressure increases are logarithmic in nature; e.g., once enough speed is obtained to result in 4 inches of water pressure, it takes only a little more speed to get 5 and 6 inches of water pressure. For comparison to a more common value, 112 mph is equivalent to 0.220 pound per square inch, or less than one-quarter psi.
Now that the numbers mean something, it's obvious that keeping airspeeds up is critical to engine cooling during climbs. Fortunately cooling is not an all-or-nothing affair, and reduced levels of pressure still provide reasonably good cooling during most environmental and flight conditions.
Air cooling works best when there's a large temperature differential between the cooling air and airplane engines. As the differential lessens — during hot summer days, for instance — the cooling system's efficiency decreases. It's easy to see why steep-climb angles coupled with high outside air temperatures (OATs) result in high cylinder head temperatures. Air cooling also works best when the cooling air is dense. As airplanes climb into thinner air, air cooling becomes less efficient. Pilots of high-flying turbocharged airplanes need to realize this fact and manage their engines with an eye toward maintaining moderate CHTs.
Is the CHT gauge truthful?
The typical cylinder head temperature is obtained by screwing a temperature pickup — called a bayonet probe — into a threaded hole in the cylinder head. Almost every cylinder assembly comes from the manufacturer with threaded holes for these probes. There is also a type of CHT pickup that looks like a copper washer. This type, called a spark-plug gasket type, is installed under a spark plug in place of the copper spark-plug sealing washer. The correct installation of these probes is on the low-air-pressure side of the cylinder.
CHTs do vary between the two pickup locations. Technicians and owners must account for the position and type of the probes on an engine before troubleshooting temperature deviations. It's not unusual to have both types installed on one engine.
Studies by General Aviation Modifications Inc. of Ada, Oklahoma, have shown that the hottest spot on a cylinder is always near the exhaust port of the cylinder. This can present cooling problems, especially if engine cooling-air baffles don't provide adequate cooling to the exhaust port area.
Inlets
Take a look at the size of the cooling-air inlets in the cowling of the 2006 AOPA sweepstakes Cherokee Six online. Each opening is big enough to insert a six-pack. Now go to the New Piper Aircraft Web site and look at the cooling-air inlets in the (redesigned) cowling on the 6X. The two airplanes are almost identical except that the 6X has a fuel-injected engine that puts out 40 more horsepower and has a maximum takeoff weight that's 200 pounds greater. The inlets in the newer 6X are quite a bit smaller. What changed?
An airflow expert named Roy LoPresti was hired by Piper to get more speed out of the Piper fleet. The least expensive method for increasing speed on any airplane is to reduce drag. LoPresti's first goal when he was hired as president of Mooney Airplane was to reduce drag. Guess where he started. Yep, he streamlined the cowling. That, plus reducing the angle of the windshield, changed Mooney's look and the company's future.
By the book
According to the book Tony Bingelis on Engines, a dependable rule of thumb for determining the size of cowling air inlets consists of multiplying the engine's maximum horsepower by 0.35. The product is the opening in square inches. According to this rule, the size of the cowling air inlets for a 150-horsepower Lycoming O-320-equipped Cessna 172 should be 52.5 square inches. Divide this number by two since the 172 has an opening on each side of the propeller, and the result is two openings that measure 26.25 square inches each. Two openings of 3.25 by 8 inches would do the trick. Air that enters the cooling-air inlet in excess of what's required for cooling merely increases cooling drag.
How big a factor is cooling drag? According to Bob Minnis, a consulting engineer specializing in engine installations who has worked for many airplane manufacturers, cooling drag makes up about 7 percent of an airplane's total drag. So reworking a cowling to reduce cooling drag, even by a large factor such as 20 percent, will not yield big performance gains. The biggest advantage gained with the increased knowledge about modern air-cooling cowlings is the realization that cowl flaps — which add weight and maintenance costs as well as increase pilot workloads — can be eliminated without compromising cooling efficiency.
Minnis says the biggest reason for the reduction in the size of cowling air inlets is an improved understanding of the effects of the propeller hub and propeller spinner on airflow. It's now known that the airflow in this area is very turbulent and greatly disturbs the orderly flow of cooling air. Current designs position the cooling-air inlets away from the propeller hub area.
Improving engine cooling
Wouldn't it be great if we could all run down to the local airplane parts store and buy an easily installed, FAA-approved modern cowling that incorporates improved cooling for our 30-, 40-, or 50-year-old air chariot?
Unfortunately, there are only a few manufacturers that have invested the money to pull this off. LoPresti Speed Merchants is the best known of the companies that have developed new cowls with reduced-area cooling inlets (see " Win A Six in '06 Sweepstakes: Airframe Soup-Up," page 127). LoPresti sells these for a wide range of Pipers, Grumman American singles, and Mooneys. AOPA chose LoPresti cowlings for the 2004 and 2006 sweepstakes airplanes, and both the Twin Comanche and the Cherokee Six sweepstakes airplanes have gained some speed with the retrofits.
Aviation Performance Products, of Melbourne, Florida, produces a new lower-drag cowling for single-engine Piper Comanches, and Atlantic Aero of Greensboro, North Carolina, includes a modern-design cowling for Bonanzas with its IO-550 engine installation.
GAMI has developed its Liquidair baffle system for Bonanzas. This product improves the cooling airflow to cylinder numbers two and six. Beryl D'Shannon, a well-known Beechcraft modification and parts seller located in Minneapolis, also markets improved Bonanza, Baron, and Debonair baffle kits. The kit makers claim their kits lower CHTs on number-two and number-six cylinders by 25 to 35 degrees Fahrenheit.
Tornado Alley Turbos, of Ada — a division of GAMI — has recently received FAA approval to install Superior Air Parts' Millennium tapered cooling-fin cylinders on its turbo-normalized IO-550 installations on Bonanzas. Studies have shown that it is more efficient to redirect the available high-pressure cooling air from the coolest part of the cylinder assemblies — the portion of the cylinder barrel nearest the engine case that doesn't require maximum cooling airflow — to the aluminum cylinder head which does. To reduce weight, Superior tapered the cylinder-barrel cooling fins from the head down to the cylinder-mounting flange. The reduced cooling-fin area is blocked off with larger intercylinder baffles. This simple change forces more cooling air past the hotter cylinder heads.
The cooling solution
The sheet-metal baffles and the silicone-rubber baffle seals — the flexible strips that bridge the gap between the sheet-metal baffles and the cowling — are critical components in engine cooling. A few hours spent on these oft-ignored parts of the engine-cooling system will pay big dividends in cylinder life.
One easy way to check the fit of the baffles and baffle seals is to put a mechanic's drop-light up inside the lower cowl from underneath the engine, and look through the cooling-air inlet for light leaks where the baffle seals contact the cowling. If there are big leaks, or holes in the baffles, seal them. Small holes in the baffles can be sealed with high-temperature room-temperature-vulcanize (RTV) material, which can be bought at auto parts stores. Baffle seals that don't provide a light-tight seal should be replaced. If your airplane still has the original black-coated asbestos seals, the odds are 10 to 1 that these seals have reached the end of their useful life.
GeeBee Aero Products, of Palm Springs, California, uses die-cutting machines to produce pre-cut silicone baffle seals kits for most GA airplanes. Guy Ginbey, owner of GeeBee (800/556-3160), promises special pricing for AOPA members.
The few hours spent installing a new set of baffle seals and plugging leaks will pay off during those hot summer days by helping keep cylinder head temperatures in the green. Not only will the engine run cooler, but also you'll feel better knowing you've taken a vital step toward maintaining your engine's air-cooling system.
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