![]() Once there is a larger radius or taper, however, the actual corner becomes much less critical. It’s difficult to accurately and cleanly make a truly sharp corner, and small variations can have a large effect on the flow. The cross-sectional area of the orifice is the main factor, but the shape is also important…an orifice with a sharp-corner leading edge will pass only 65% as much flow as an orifice with a rounded entrance. Engineering handbooks provide information on flow versus pressure drop for short (length less than diameter), square-cornered or rounded orifices, but here we’re talking about an orifice length more than six times the diameter-more like a combination of an orifice and a short tube. Twice the pressure drop means √2 or 1.41 times the flow.īut what is the actual flow (in cubic feet per minute or any other units)? Although it doesn’t matter from a practical standpoint when checking your engine, I was interested in the actual number so I would understand the effects of other parts of the system knowing the actual flow is important when sizing hoses and fittings. The math to calculate the actual flow (in gallons per hour or cubic feet per minute or whatever) is messy and beyond the scope of this article, but in general, as long as the flow through the orifice is subsonic (as it will be above about 35 psi on the downstream gauge), it is proportional to the square root of the pressure drop. ![]() This principle is used on many flowmeters. If you know the characteristics of the orifice and measure the pressure differential, you can calculate the flow. The amount of that pressure difference is a function of the amount of flow passing through the orifice and also of the size and shape of the orifice. When a fluid (liquid or gas) flows through a restriction or orifice, there will be a pressure difference across it, which is the pressure it takes to force the fluid through the orifice. If you’re being hauled around the sky by a big round engine, though, you may need the larger size. Most of us, particularly those who are inclined to save a few dollars building a tester instead of buying, are probably dealing with the smaller engines and so will need the 0.040-inch diameter orifice. For an engine cylinder with a 5.00-inch bore and over: 0.060-inch orifice diameter, 0.250 inch long and a 60° approach angle. For an engine cylinder having less than a 5.00-inch bore: 0.040-inch orifice diameter, 0.250 inch long and a 60° approach angle.Ģ. The AC specifies the size and shape of the restrictor:ġ. Basically, it consists of a pressure regulator, two pressure gauges with a restrictor orifice between them, and a connection to the spark plug hole in the cylinder with an air valve to shut the flow off. COMPRESSION TESTER HOW TOThe FAA’s maintenance bible, Advisory Circular AC 43.13, gives a good description of a differential compression tester and how to use it, along with a schematic (see illustration). Advisory Circular AC 43.13 includes this schematic of a differential compression tester. The orifice is the critical part of the tester. There are less expensive ones sold for automotive use (usually called leak-down testers), but they may not (probably won’t) use the same orifice size as an aircraft tester, and the downstream pressure gauge is often marked in “percent” (percent of what?) and not the actual pressure, as an aircraft tester is. ![]() ![]() I didn’t own a compression tester, other than the common single-gauge automotive type that measures cranking pressure, and while I could easily borrow one from a friend, it’s always nicer to use your own tools.ĭifferential compression testers for aircraft are readily available from many suppliers, with basic ones starting around $100. The A&P doing the inspection with me wasn’t too concerned, but he suggested that I recheck it after flying for a while. When checking the compression of the O-290-D engine in my Hatz biplane during the annual condition inspection, we found two cylinders lower than the other two. ![]()
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