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. To make things even sim- at 2100 rpm. is the mixture that achieves minimum brake pler, if you lean the engine until the onset of

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30 Sport Aviation October 2012 PHOTOGRAPHY COURTESY MIKE BUSCH

028-032_savvyOCT.indd 30 9/12/12 8:46 AM perceptible roughness, then richen just One such contender is the airspeed that L/D speed (aka best range speed). 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. For an A36 maximum time or if you want to use the mini- LOP. This method is both cleaner and cooler. Bonanza, it’s 110 knots indicated. Those speeds mum amount of fuel per hour. This speed is It reduces stresses on critical engine compo- are at max gross weight; at lighter weights, the useful for loiter-mode operations like aerial nents and minimizes the buildup of harmful speed is a bit lower. (It varies with the square surveillance, fi sh spotting, and pipeline exhaust deposits. It’s the kindest thing you root of weight.) patrol. This is a very slow airspeed, and defi - can do for your engine and will provide you When used as a cruise speed, this maxi- nitely will not satisfy your need for speed. It maximum longevity. (My own geriatric mum L/D speed provides best range. This is also puts maximum stress on your bladder. engines are a perfect case study of this.) the speed you’d want to fl y if you want to go A faster and more satisfying most effi cient Maximum longevity plus maximum effi - as far as possible on a given amount of fuel, or speed is obtained by multiplying max L/D ciency—what’s not to like? if you want to get to your destination with the speed by 1.316. This gives us what is com- maximum amount of fuel reserve. This speed monly referred to as the Carson speed after AIRSPEED is sometimes useful when fl ying maximum- renowned Naval Academy professor Bernard If you ask which airspeed is most effi cient, range missions, but it’s slower than most of us H. “Bud” Carson, Ph.D., who wrote the semi- things start to get a bit more complicated. That’s are willing to fl y. nal paper on this subject in 1980 and passed because there are several diff erent airspeeds Two other contenders for the title of most away in 2009. This speed attempts to opti- that compete for the title of most effi cient. effi cient speed can be derived from this max mize the two things we most care about:

www.eaa.org 31

028-032_savvyOCT.indd 31 9/12/12 8:46 AM MIKE BUSCH

airspeed and fuel economy. It’s the speed at exceptionally low fuel prices and fi ll all the payload. The weight-related performance which the product of speed and fuel economy tanks in order to avoid having to buy high- penalty often off sets any savings involved in is maximized. For most of us, the Carson priced fuel at your planned destination. But tankering cheap fuel. So while it’s painful to speed is probably as good a tradeoff between succumbing to such temptation can be buy high-priced fuel, it often costs more to buy speed and economy as we’re likely to fi nd. counterproductive. low-priced fuel if it involves an extra landing Reducing this theory to some actual num- For one thing, it almost never makes and/or tankering extra fuel weight. bers, my Cessna 310 POH lists best range sense to make an extra fuel stop to load up glide speed (max L/D) as 111 KIAS at max on cheap fuel. That’s because the additional Mike Busch, EAA 740170, was the 2008 National Aviation gross weight. (It decreases to 102 knots at 800 fuel it takes to make the descent, landing, Maintenance Technician of the Year and has been a pilot pounds under gross.) This means that my takeoff , and climb usually costs more than for 44 years, logging more than 7,000 hours. He’s a CFI Carson speed would be 111 times 1.316, or the cost savings from the lower-priced fuel. and A&P/IA. E-mail him at mike.busch@savvyaviator. about 146 KIAS. For a Cessna 210, max L/D is For another, it’s important not to underesti- com. Mike also hosts free monthly online presentations listed at 88 KIAS at max gross, so Carson mate the reduced effi ciency that occurs when as part of EAA’s webinar series on the fi rst Wednesday of speed would be about 116 KIAS. we carry unnecessary weight, either fuel or each month. For a schedule visit www.EAA.org/webinars.

ALTITUDE So far, we’ve determined that to achieve opti- mum effi ciency, we want to cruise at 250 wide-open throttle, minimum allowable rpm, BEST and an IAS of approximately Carson speed 200 BEST RANGE (which is relatively low as indicated airspeeds ENDURANCE go). Let’s see if you’ve been paying attention. Here’s a pop quiz. 150 POWER Question: How can we do all this stuff at REQUIRED the same time? (HP) 100 P Answer: By fl ying at a relatively high alti- V tude that allows us to fl y WOT without exceeding Carson speed. 50 In a high-performance normally aspirated airplane, this generally means cruising up 0 around 12,000 feet, give or take. In a turbo- 0 20 40 60 80 100 120 140 160 180 charged airplane, it usually requires climbing IAS up to the low fl ight levels. These are the alti- tudes at which these airplanes are happiest and most effi cient. BEST 250 RANGE (MAX L/D) RANDOM TIPS ON EFFICIENCY A GPS-coupled fuel totalizer, like the Shadin 200 Digifl o I have in my Cessna 310, can be invalu- able in achieving maximum effi ciency. The V 150 BEST instrument has a mode that continuously dis- TOTAL ENDURANCE plays actual fuel effi ciency (nm/gallon or nm/ DRAG OPTIMUM pound) and lets you try diff erent power set- (lbs.) 100 CRUISE tings, airspeeds, and altitudes and see (CARSON SPEED) immediately what eff ect such changes have 50 on fuel effi ciency. Like many pilots, I frequently use web- sites like AirNav.com and 100LL.com to 0 check fuel prices along my route of fl ight. 0 20 40 60 80 100 120 140 160 180 It’s amazing how wide the variation in fuel IAS prices can be from one airport to another. It’s often tempting to land at an airport with Three key indicated airspeeds: best endurance, best range, and optimum cruise (Carson speed).

32 Sport Aviation October 2012 ILLUSTRATION BY CHRIS LIVIERI

028-032_savvyOCT.indd 32 9/12/12 8:46 AM