Letters: February 1999

Six Place Step-Up
I continue to be amazed by all I have read concerning the difficulty of flying T-tailed Pipers, particularly the Turbo Lance.

I bought a 1979 Turbo Lance the day after my private checkride with fewer than 60 hours in 172s. By the third landing, I was proficient. The statement that on the T-tailed aircraft: Nothing happens until 80 knots when, whoop, the stabilator comes alive is patently false. Anyone who cannot fly these airplanes after a little practice is simply sloppy.

Mike Rapoport
via e-mail

Not all T-tail owners agree with that point of view. We don’t either, having experienced the T-tail Lances occasionally odd runway habits. On the other hand, we do agree that once youre accustomed to it, the T-tails traits may be hardly noticeable.

RAM Filters
Re: Douglas Ritters article on the RAM oil filter. The RAM filter clearly appears to be a better filter. Being made from fiberglass, it will not take on water and swell shut like cellulose/paper does.

This is probably a more common cause of filter bypass than dirt overload. However, I would like to clarify and take exception to some of your points so people are not lead astray. Filters generally are rated for two different particle sizes, nominal and absolute. The absolute rating is the size of the largest particle that can get through the filter. Generally, the nominal size is about 2.5 times smaller than the absolute.

Simple physics tells us that a hole which has a diameter less than three diameters of whatever particle size we are interested in is likely to bridge and plug up. Therefore, a filter with an absolute rating of 25 microns in a re-circulating system will take out essentially all of the particles down to 10 microns. If the first pass through the filter doesnt get that 15-micron particle, then it is highly likely that the next one will.

Lycomings have connecting rod bearing clearances of 0.0015 to 0.0045 inches or 38 to 113 microns. This gives an annular space of 19 to 56 microns when centered, but on the power stroke, the oil film gets down to half of that in the 10- to 25-micron range. To me, its clear that you don’t want particles larger than 10 microns in the oil and probably don’t want many 3- to 5-micron particles.

I have not seen the test report you refer to for the two filters, but I believe that you have gravely misstated the performance of both filters. Typical modern filters tests use a Beta rating. A filter might be rated at Beta 10=75. The 10 means that it was tested with uniform glass beads, all of which have a 10-micron diameter. The 75 is the ratio of the particles up stream of the filter to those downstream on a single pass.

If the Champ filter mentioned in the article really has a Beta 20=36 rating, the true removal rate is actually 97 percent (35 out of every 36 particles removed and only one allowed to pass) of the 20-micron and larger particles, not the 36 percent stated in the article. The RAM numbers would also be erroneous; however, the RAM filter still removes particles that are three times smaller than the Champ filter, so your conclusion is not flawed.

The purpose of the bypass valve in the filter is to keep the filter media from rupturing and sending shredded cellulose, stainless steel wire and fiberglass throughout the engine if the filter pressure drop gets too high. The small pores of the RAM filter media cause a much higher pressure drop. This is why they need a higher bypass valve setting. I cant envision why Continental specifies a minimum pressure drop, whereas I see many reasons to limit the differential pressure and the total pressure required on the oil pump.

don’t change the filter too often. Dirty filters remove much finer particles and more particles than clean filters. The only hitch is that one doesnt know what the pressure drop is. Industrial filters have alarm switches or tattle tales to alert one to the need to change the filter. If your manual advises you to change the filter every 50 or 100 hours, changing it every 25 hours in normal use will actually increase the amount of dirt in your oil and reduce engine life.

As to whether better filtration actually helps, I am surprised you didnt quote an article from Aviation Consumer some years ago. The actual real-life TBOs for Cessna C-180s and C-182s with the same model O-470 engine were compared. Planes built before a certain model year had oil screens and a TBO that was roughly two thirds that of the newer airplanes with oil filters. Thats a 50 percent life increase.

Arden V. Tarum
Portland, Oregon

Actually, as the article states, we accept RAMs lab data at face value but absent any convincing data-lab or otherwise-were still skeptical of any claims that this level of improved filtration will decrease engine wear in a meaningful way. (RAM makes no such claims, by the way.) If a $48 filter seems like a good deal, we say buy it.

As for our previous pronouncements on filters, our search of the archives came up blank. You sure it was us?

More Oil Argument
With regard to comments on multi-weight versus straight-weight oil in your January issue, the point everyone seems to be missing is that what we are concerned about here is not the amount of oil that drains off the parts while the oil is warm.

In fact, I will even concede that my friend Ben Visser at Shell is correct in saying that multi-weight, when warm, adheres to parts better than straight-weight oil does. Our concern is maintaining a protective film on these parts between uses by the average owner, which could be a week or more between flights.

In this situation, what we are concerned about is how the cold oil film maintains on the parts. Clearly, from our experience, we have seen multi-weight oil, when cooled to ambient and allowed to sit for days at a time, will more readily strip off internal parts than will straight-weight oil. For this reason, we recommend straight-weight for aircraft likely to sit a week or more, when climatic conditions permit. This brings up another point: Everyone seems to be looking for the best oil. There is no such thing. There is an appropriate oil for a specific type of usage, but that oil might not necessarily be the proper oil to use for another type of usage.

For a flight school, where the aircraft fly every day, multi-weight may be the way to go. For the owner whose airplane might sit for a week or so and who is based where there are serious climatic changes through the year, it might be wise to use straight-weight oil for nine months of the year and shift to multi-weight during the winter. For the owner in Florida, where it rarely gets cold, it would be appropriate to use straight-weight the year around.

John Frank
Cessna Pilots Association
Santa Maria, California