Airframe and Powerplant

Slick Sifters

July 1, 1999

Unlocking the door to future filters

Volumes have been written about oil selection, replacement intervals, the value of spectrographic analysis, and a host of other subtopics that are enough to keep the Society of Automotive Engineers' printing presses running around the clock. Consumers have had choices in brands and types of oil, including full synthetics, semisynthetics, and conventional dinosaur oils. But aircraft owners have not really had many choices when it came to oil filters. Today, many engines are fitted with spin-on filters made by Federal Mogul subsidiary Champion Aviation Products. Waco, Texas-based RAM Aircraft threw a punch at the status quo when it introduced its own high-performance oil filter last summer. Priced at a whopping $60 — the average Champion is around $20 — the RAM filter seeks to address perceived shortcomings in the design and execution of the most common filters.

That engines have full-flow, replaceable oil filters at all is not, even today, a given. Many engines still rely upon a comparatively primitive screen to "filter" the oil supply. Mechanics have been known to comment that the coarseness of the screen might help if you accidentally dropped some buckshot into the filler neck, but that it doesn't do much in the way of scouring the oil.

Over the years, engine manufacturers have attempted different filtering methods — but the purpose is the same: to hold solid contaminants such as carbon, dirt, and metal debris so that they don't continue to circulate around inside the engine. In the early days of aviation, there was little in the way of filtration, but by World War II, when engines had become incredibly complex and powerful in a short design lifetime, the need for good filtration became apparent. Most wartime powerplants used something called a Cuno filter, somewhat misleadingly labeled as self-cleaning. A stack of permeable disks trapped oil-borne contaminants, but a blade-powered by the oil pressure itself — continuously scraped the contaminants from the disks into an integral sump. Of course, you still had to drain the sump. Other engines used small, gear-driven centrifuges placed in the oil stream; sludge and contaminants were forced to the walls of the device, from which they were periodically removed.

Down in the ranks of GA powerplants, the need for advanced filtration followed better oils and more sophisticated engines. Even so, many low-horsepower engines are flying even today with nothing more than screens, despite the availability of kits to install spin-on filters. For many, the thinking is: These engines have gone to TBO more than once, so what's the problem?

Here it is: Every engine, no matter the hours, experience, or specification, makes — for lack of a better term — junk. Small bits of wear metals, flecks of carbon, dirt, dust, and other contaminants are, by design, picked up by the oil and held in suspension. That is why your oil changes color; the dark hue indicates that the oil is doing its job in suspending these particles. In a perfect world, this debris would be vacuumed out of the oil as you fly, allowing for a constant stream of uncontaminated oil to go about its business of lubricating, cooling, and preserving the engine.

Conventional oil filters are not perfect, but they do an admirable job in plucking the waste from the oil. A test conducted by the Milwaukee School of Engineering's Fluid Power Institute showed that a conventional Champion spin-on filter was successful in removing 100 percent of the particles down to 60 microns. (One thousandth of an inch equals 25.4 microns.) By 20 microns, the Champion was able to stop only 30 percent of the particles. By comparison, a standard Lycoming screen stopped just 49.2 percent of the 60-micron particles and 11.5 percent of the 20-micron pieces. A typical Continental screen posted similar results, although it was worse with larger pieces but better with smaller ones.

So what's this mean? Consider that in, say, your average Lycoming, the main-bearing and big-end clearances are down to 0.0008 inches (about 20 microns) so you might think that any particle larger than that could lead to bearing damage. Maybe yes and maybe no. Understand that these bearing areas are highly dynamic — that is, oil is constantly moving in and out of the bearings, and the actual clearances under load will vary from one side of the bearing to the other . According to the engine manufacturers, there usually isn't enough time or opportunity for these smaller particles to get in the way and get squished under the bearing load, and thereby get mashed into the bearing surfaces. Naturally, different engines offer different environments, and the high-horsepower models present the greatest challenges to oil-film integrity.

If the bearings aren't critical, why have an oil filter at all? Because you really want to get the accumulated junk out of the oil stream as quickly and efficiently as you can. Otherwise, it'll end up in places that act like natural centrifuges, such as the undersides of the pistons, in the dome of a constant-speed propeller, and in the hollow nose of the crankshaft. One more voice: The late Carl Goulet, once head of engineering at Continental, maintained that "cleaner is better, cooler is better," without regard for power levels or displacement.

What makes a good filter? Obviously its ability to filter and hold garbage is high on the list. RAM's filter uses a micro-fiberglass element, while the Champion filters until recently used a treated-paper media. There's a trade-off here. Better filtering comes from "tighter" media, but it is more restrictive and creates a greater pressure drop across the element. As a result, the so-called beta ratio goes up, resulting in a greater oil-pressure drop across the element. Some elements have internal relief valves that open in the event the pressure differential across the element exceeds a preset figure. For the short Champion filters, this is about 8 pounds per square inch differential (psid) and it's about 10 psid on the long ones. RAM has bumped the bypass pressure up to a whopping 20 psid.

Ideally, you never want the bypass open during normal operation. This is because the location of the valve — in the top of the filter opposite the mounting flange — tends to allow the sediment on the surface of the filter to wash into the oil stream, in essence cleaning the filter of some of the accumulated junk and putting it back into the oil stream. Unfortunately, it's difficult to know from the cockpit if the bypass is open. The RAM filter's higher bypass will show a greater pressure indication change than a Champion. If the idle oil pressure drops by 10 psi without any other changes, it's time to check the filter. RAM recommends changing the filter at 50-hour intervals.

Another trait of the filter to consider is how much of any contaminant it can retain. Typically, the larger the filter — and, subsequently, the greater area of the media — the more it'll hold.

Aircraft owners who wish to improve their engine's chances of making TBO and save a bit of money by stretching oil-change intervals have found filter options compelling. In general, the manufacturers recommend 25-hour oil-change intervals for engines with screens, compared to 50-hour intervals for those with filters. Airwolf Filter Corp and Aviation Development Corporation (ADC) both make remote filter kits for a wide variety of engine models; ADC also makes its own, disk-style wire-mesh-media filter.

Airwolf has gone the simpler route, producing handsome aluminum kits that include a puck-shaped filter carrier with an anodized bracket for firewall mounting and a choice of adapters that thread into or bolt onto the engine where the standard screen used to be. The pieces are joined by hoses. You can position the filter in the engine compartment with an eye toward convenience. In addition, because the filter is remotely mounted, you can choose to use a larger element on some engines. For example, the large Continentals with the cantilevered oil filter adapter are sensitive to filter weight; the long arm of the adapter combines with engine vibration to make life hard on the adapter. Long, heavy filters exacerbate this; so most owners fit the short-element Champions with this adapter. Airwolf lets you fit the taller cans, which have more filter media and bypass valves with slightly higher opening pressures. These kits appear to be well made and are priced at $495 complete with hoses.

ADC's filter adapter kits have some features not available on the Airwolf products, including a port for an optional chip detector that can be hooked to a cockpit lamp. In addition, the ADC adapter includes a hex machined into the top to help steady the bracket in case you have to free a stuck filter. (In fairness to Airwolf, if you use Dow DC-4 lubricant on the seal and stick to the recommended filter torque specs, you're not likely to have the filter stick to the adapter.) Finally, the ADC bracket has a much larger footprint, which helps prevent firewall distortion, the company claims. It also makes mounting the filter in confined spaces more difficult, so check your engine compartment to see if you have an unobstructed 4 inches by 6 inches of room. ADC undercuts Airwolf's prices slightly; most configurations run $415 including hoses and engine-specific adapter. The chip detector is $228.

In addition to its filter adapters using conventional spin-on elements, ADC sells a unique disk-style filter made up of woven stainless-steel cloth. It's said to provide 25-micron absolute filtration. The filter is held in a clamshell enclosure and protected by a ball-and-spring bypass valve. And here's the key element of the ADC setup — when the bypass valve opens, a lamp installed on the instrument panel lights. This way, the pilot knows when the filter goes into bypass and can make arrangements to look at the element. ADC acknowledges that its filter has much less media area than conventional filters, but says this is central to the design. You don't want the filter to be full of a lot of metal before you find out about it; with the ADC element, an abnormal amount of engine detritus will clog the element and open the bypass sooner than normal. ADC also says that its filter can benefit oil cooling thanks to the heat-sink-like fin arrangement. This is likely to be true, but you'll need to make sure the filter is in an area of good air flow to maximize this effect. Finally, in testing against conventional filters, the ADC disk fared poorly in the particle-entrapment tests. ADC counters that the tests measure the performance of the filters in new condition and that the ADC disk actually increases its filtration as it accumulates muck. True of paper-media filters as well, this characteristic is more pronounced in the ADC arrangement.

Leaving well enough alone isn't the RAM Aircraft way. So when it introduced a $60 filter last summer, only those who don't really understand the company were surprised. RAM has refined the filter to work better with its own engines — high-horsepower turbo-charged models that live a hard life in high-flying (and, particularly, pressurized) airplanes.

As part of its ongoing engine development, RAM elected to increase the oil flow on its modified Continental TSIO-520s to 14 gallons per minute, up from a nominal 10 gpm for most of the big-bore Continentals. The company's reasoning is simple: provide more oil flow to help improve lubrication and to assist in cooling these high-power engines. (Understand also that a turbocharged engine loses some of its oil capacity to running the wastegate system and to lubricating the turbocharger itself; to ensure the engine gets all it needs, an increase in total flow may be necessary.)

At these higher flow rates, the average Champion would, according to RAM's data, go into bypass even when completely clean. RAM's solution is its own filter.

Increasing both the flow rate in the RAM-modified engine and the bypass pressure in the filter called for other changes. For example, the base plate of the RAM filter has more and larger supply holes than a comparable Champion's, as does the internal media-support spindle. The media itself is supported on both sides to help prevent its collapse; a Champion filter is supported by the top and bottom edges and the perforated spindle alone.

RAM also cites better filtration, based on tests performed at its behest by Fluid Technologies in Stillwater, Oklahoma. According to these tests, the RAM filter is 99 percent effective in stopping particles down to 20 microns, while a similarly sized Champion posted 36 percent efficiency in this range. (These tests were performed with MIL-H-5606 fluid, lighter than conventional oil, and at 100 degrees Fahrenheit; Champion says flatly that its filters all meet the 20-micron requirement set by the engine manufacturers for filter efficiency.)

Would this better filtering result in less metal in the engine? In theory yes, but don't expect much difference in oil-analysis figures. Pilot Associate Editor Pete Bedell ran a RAM filter on one engine of a Beech Baron (an IO-520-C) and the oil analysis that came back from Engine Oil Analysis in Tulsa, Oklahoma, showed a very slight decrease in most wear metals. However, the numbers in this engine were trending down before the filter was installed, so these are hardly conclusive findings.

Bottom line, then: Do you really need a $60 oil filter? The answer is easy if you have a RAM-modified engine with the higher oil-flow rates — absolutely. For the rest of us, the answer is a bit less clear. Typical engines, particularly low-power, normally aspirated models, probably don't need this level of filtration or bypass resistance. Certainly, the engine may stay cleaner longer with the RAM filter, but there's not likely to be a correlation to better engine life. For that, you'd be better off changing the oil more often and using a standard filter.

Airwolf Filter Corp
15369 Madison Road
Middlefield, Ohio 44062
Telephone 800/326-1534 or 440/632-5136
Fax 440/632-1685

Aviation Development Corporation
1305 NW 200th Street
Seattle, Washington 98177
Telephone 800/944-3011 or 206/546-3011
Fax 206/546-8035

RAM Aircraft
7505 Airport Drive
Waco Regional Airport
Waco, Texas 76708
Telephone 254/752-8381
Fax 254/752-3307