MIKE BUSCH COMMENTARY / SAVVY AVIATOR A GPS-coupled fuel totalizer (like this Shadin Digifl o that I have in my airplane) is a great help in achieving Flying maximum effi ciency. Efficiently It’s more important than ever to get the best bang for the buck HOW CAN WE GET THE BEST FUEL economy from our airplanes? Given much energy as possible from each combus- the painful cost of avgas these days, this question is on lots of airplane tion event before the exhaust valve opens and owners’ minds. That goes at least double for unfortunate folks like me discards what’s left. and Mac McClellan who fl y twins. It’s a subject I’ve thought about— Operating at WOT and low rpm will make and researched—quite a bit. some pilots uncomfortable, because they had a It turns out that there are lots of pieces to this puzzle. There are all fl ight instructor who taught them never to sorts of things we can do to optimize effi ciency. Some are simple; some operate the engine oversquare—that is, with are a bit more complex. It takes considerable attention to detail to fl y manifold pressure (in inches) greater than the as effi ciently as possible. rpm (divided by 100). This is exactly what I was taught by my primary instructor in 1965, THROTTLE and it took me more than a decade to fi gure Let’s start with something simple: throttle position. out that my CFI didn’t know what the heck he Our piston aircraft engines are always most effi cient at wide-open was talking about. I guess I’m a slow learner. throttle (WOT). Retarding the throttle from its wide-open position This “oversquare is bad” concept is just closes the throttle butterfl y valve in the carburetor or throttle body one of those old wives’ tales that refuses to and chokes off the engine’s air supply, intentionally reducing its volu- die. It’s dead wrong. For optimum effi ciency, metric effi ciency. It doesn’t take rocket science to fi gure out that this is we want to operate the engine as far over- not something you want to do when striving for maximum engine effi - square as the book allows. Most normally ciency. Could you operate at maximum effi ciency if someone was aspirated engines are approved to operate at trying to strangle you? Neither can your engine. least several inches oversquare. The turbo- As a general rule, for best effi ciency, we always want to be operat- charged engines in my Cessna T310R operate ing at WOT, or as close to WOT as we can without exceeding the all day long at 32 inches of manifold pressure engine’s operating limitations. Normally aspirated, turbonormalized, (MP) and 2200 rpm (10 inches oversquare), and mildly boosted turbocharged engines can and should be operated and they’re both beyond 200 percent of TBO, continuously at WOT. so they obviously don’t mind a bit. Highly boosted turbocharged engines may need to be throttled Have a look at an interesting test fl ight that back to remain within operating limits, but should not be throttled any my friend Elliott Schiff man made in his 1988 more than absolutely necessary. Most of these engines are permitted Beechcraft F33A a few years ago (see Figure 1). to operate continuously at full redline manifold pressure, but some are The fi rst eight minutes were fl own at 27 inches limited to fi ve minutes. Check your POH. MP and 2100 rpm (6 inches oversquare, if you must), and the next 10 minutes were fl own at RPM 21 inches MP and 2500 rpm (4 inches under- If the airplane has a constant-speed prop, best engine and prop effi - square). During both segments, fuel fl ow was ciency are generally achieved by setting the rpm as low as the book precisely the same, about 12 gph, which means allows. Low rpm dramatically reduces frictional losses in the engine, that the exact same number of BTUs of latent which vary with the square of rpm. Low rpm also tends to increase avgas energy was being consumed. But during propeller effi ciency, because it increases the angle of attack of the pro- the second (undersquare) segment, the engine peller blades. And low rpm gives the engine more time to extract as monitor data made it obvious that much more 28 Sport Aviation October 2012 PHOTOGRAPHY COURTESY MIKE BUSCH 028-032_savvyOCT.indd 28 9/12/12 8:45 AM latent energy was wasted, and substantially Naturally, if your less was being converted into mechanical airplane has a fi xed- energy and airspeed. pitch prop, you can’t The undersquare segment shows EGTs control rpm from the that are more than 100°F higher, refl ecting the cockpit. The best you increased amount of energy being wasted out can do to obtain maxi- the exhaust. Why is this happening? Because mum effi ciency is to at the higher rpm, there’s about 16 percent less have a cruise prop time between when the spark plug fi res and installed rather than a when the exhaust valve starts to open. As a climb prop. The cruise result, the engine has less time to convert the prop has greater blade heat and pressure of the combustion event into pitch and will operate useful mechanical work before the exhaust at lower rpm than a Figure 1—This test fl ight of a Beech F33A has the fi rst eight minutes fl own oversquare and the valve opens and discards the remaining heat climb prop, all other next 10 minutes fl own undersquare at precisely the same fuel fl ow. Note how much higher the and pressure out the exhaust pipe. things being equal. EGTs and CHTs are during the undersquare segment, refl ecting signifi cantly lower effi ciency. The undersquare segment also shows CHTs that are about 50°F higher, mainly due MIXTURE specifi c fuel consumption (BSFC) and maxi- to increased frictional losses in the cylinder Staying with the simple stuff : If we want mum nautical miles per gallon of fuel— assemblies. Those losses vary with the square to optimize effi ciency, we need to lean aggres- invariably occurs well on the lean side of peak of rpm, so theoretically they should be sively. (Duh!) EGT, typically somewhere between 30ºF and roughly 65 percent greater at 2700 rpm than So-called “best economy mixture”—which 70ºF lean of peak. 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One is enough to make the roughness go away (and achieves minimum drag. It is variously slower, the other is faster. not any richer), you’ll be close to best economy referred to as maximum L/D speed or mini- If we divide max L/D speed by 1.316, we mixture. Pilots have been using this technique mum thrust required speed. You can generally get best endurance speed or minimum power for about 100 years, and it works pretty well. fi nd it in the emergency procedures section of required speed. This is the speed you’d want There are all sorts of other benefi ts that your POH as best range glide speed. For my to fl y if your goal was to remain aloft for the come with operating the engine aggressively Cessna 310, it’s 111 knots indicated.