Relay-Version: B 2.11 6/12/87; site scolex Path: uunet!elroy.jpl.nasa.gov!ames!apple!geoff From: geoff@Apple.COM (Geoff Peck) Newsgroups: rec.aviation Subject: Re: Engine wear question Message-ID: <50732@apple.Apple.COM> Date: Sun, 24 Mar 91 23:00:36 PST References: <1991Mar20.132028.1464@nlm.nih.gov> <17410239@col.hp.com> Organization: Apple Computer Inc., Cupertino, CA Lines: 41 In article mferrare@physics.adelaide.edu.au (Mark Ferraretto) writes: > I was taught always to keep MP below RPM. Now here comes Dave [Allen] > saying he runs at 19, 23!!! Looks like this rule aint gospel. Let me try to make a long story short. The "square rule" says that it is safe to operate an engine with a constant-speed propeller such that the manifold pressure (MAP) is less than or equal to the RPM expressed in hundreds -- i.e., 24"/2400rpm is "safe", and is referred to as "twenty four squared". Setting the MAP below the "square" value is also "safe". However, setting the MAP _above_ this value is permitted only if the pilot's operating handbook and/or the engine handbook so permit. This is called "oversquare" operation. In the Arrow III in which Mark learned, "oversquare" operation is permitted in a number of cases. At takeoff, for example, one uses full throttle, which, at sea level, will be about 30" MAP, and full RPM, which will be about 2700. At, say, 4,000 feet, 45 degrees Fahrenheit, the following power settings are published in the pilot's operating handbook: 110HP 130HP 150HP 55% power 65% power 75% power 9.1 GPH 10.4 GPH 11.6 GPH ------------ ------------ ------------ RPM 2100 2400 2100 2400 2400 MP 21.9 19.5 24.8 22.0 24.4 Notice that _all_ of these settings except for the 22"/2400rpm setting are "oversquare"! In the case of most turbocharged engines, almost all of the recommended power settings will be "oversquare". The power settings which Dave Allen mentioned for a Bonanza, 1900RPM and 23" MAP are oversquare! So what's this "square rule" all about? It's a _very_ conservative approach to power setting. It's there so when you don't have time to look at a table, you can set the power in some non-destructive manner, fly the airplane, and get back to set the power "for real" later. One final note. Some airplanes have specific limitations on RPM/MAP combinations, or RPM restrictions. Know your airplane! Geoff