Penetrating the preconceptions of power
Engines are often the most misunderstood part of our aircraft. Perhaps no aspect of the powerplant is party to more misinformation than power management — opinions from pilots, mechanics, manufacturers, and other self-anointed masters of internal combustion proliferate like golf courses in Palm Springs.
Instrumental issues
Make an effort to determine the accuracy of the power gauges. Mechanical tachometers are notoriously inaccurate. Worse, they tend to indicate low, resulting in power settings higher than you think they are and, in some installations, continuous operation at forbidden speeds. Have the tach checked at each annual, or, better yet, install a more accurate tach. Horizon Instruments sells an electronic tach that has supplemental type certificate approval for use as a primary instrument; both Electronics International and J.P. Instruments have supplementary electronic tachs. There's also the Cardinal Electronics Proptach — a portable device that optically measures and displays prop speed.
In constant-speed prop installations, the manifold pressure (mp) gauge acts as an important partner to the tach. It is essentially a barometer scaled in inches of mercury. At sea level on a standard day (59 degrees Fahrenheit), the mp gauge should read about 30 inches with the engine off. Errors of more than about an inch can lead to significantly erroneous power settings.
Fuel-injected engines will have fuel flow gauges as well. Most are a simple pressure gauge with markings on the face in gallons per hour; this instrument is determining fuel flow indirectly. Fuel-flow computers (and flow gauges in airplanes made in the last 10 years or so) employ actual flow transducers and so, once calibrated, are quite accurate. Poorly calibrated or clogged fuel injectors can cause the pressure gauge's flow markings to be inaccurate.
Simple is as simple does
Pilots flying simple single-engine airplanes have an easy task of setting power. Pick your altitude, read the rpm needed to get your desired percent of power and you're done. Consider the outside-air temperature when referring to book speeds, though — at a given engine speed, a difference of 20 degrees Celsius (34 degrees F) above or below standard will influence power by 3 to 5 percent with corresponding changes in fuel flow. True airspeeds vary only slightly, however — faster on a colder day — primarily because the extra power available on the below-standard-temperature day is nearly consumed by the additional aerodynamic drag of the denser atmosphere.
Three-lever dilemma
Upon transitioning to complex airplanes, you were probably hit over the head with the proper procedures for using the constant-speed propeller. When increasing power, always adjust mixture first, propeller second, and throttle last; reverse this order for power reductions.
Don't believe that if you don't strictly adhere to the order, you'll blow up the engine. The truth is, a normally aspirated (nonturbo) engine will tolerate idle to full throttle settings at any governable propeller setting without damage. Moreover, as long as you have the right grade of fuel and the engine is properly maintained, it is impossible to induce detonation or other short-term engine damage with the mixture control. Even getting a turbo powerplant to detonate is difficult. According to a retired Continental engineer, it's not possible to get a certified turbo powerplant into destructive detonation except at full power and with fuel flows more than 10 percent below spec. At cruise settings, he says, you can't get the engine to detonate — period.
Climb settings
Although the airframe manuals are usually quite specific on how much power to use in cruise, not all are that clear-cut on climb settings. The most common blanket recommendation is to use 75-percent power or the highest allowed cruise number. You are not bound by these recommendations — and more than a few manuals recommend full-power climbs even for turbocharged models.
How much you use depends greatly on the performance of the airplane and the effectiveness of the engine cooling system. For acceptable performance, a modestly powered airplane should use all of the available horsepower, full throttle and maximum rpm. But noise and vibration at maximum rpm often create a strong disincentive for this tactic. Try using a lower rpm with full throttle to altitude, if the noise is bothersome.
Airplanes with fuel-injected, normally aspirated Continentals should be using full throttle throughout the climb — even though the airframe manual may call out a cruise-climb setting of 25 inches/2,500 rpm. Why? Inherent in the Continental fuel-injection system is a biasing circuit that increases fuel flow substantially with the last 15 percent of throttle travel. By pulling the throttle back to 25 inches during the initial climb, both cylinder-head (CHT) and exhaust-gas (EGT) temperatures skyrocket as a result of greatly curtailed fuel flow. Usually, a full-throttle climb is carried out at 2,500 rpm, for noise and vibration concerns.
The American Bonanza Society's proficiency clinics teach this method, and, in fact, Beech recommends the tactic in the IO-550-powered Bonanzas and Barons. Ironically, the manuals for the earlier airplanes, including the IO-520-powered Bonanzas, call for 25 inches/2,500 in cruise climb. There's no compelling reason to treat the 520 differently than the 550.
Lycoming engines with the Bendix injection system do not have the same degree of full-throttle enrichment, so unless engine cooling or fuel consumption are concerns, there's no strong reason to pull the throttle back during climb on these engines, either. As always, manage airspeed and cowl flap position to maintain minimum CHT and oil temperature. Current thinking says to hold CHTs below 410 degrees F in the climb.
Cruise criteria
Picking a cruise power setting involves juggling several variables, including noise, desired cruise speed (always faster than available, it seems), overall economy, and heat control. Manifold pressure indirectly measures the amount of fuel and air entering the cylinder. The higher the number, the stronger the combustion event and therefore the more power being produced. Engine rpm acts as a multiplier — the faster the engine spins, the more combustion events take place, resulting in more power.
Die, oversquare myth, die!
Theories abound as to why we should avoid oversquare operation — 27 inches of manifold pressure and 2,400 rpm, for example — none of them rooted in any apparent science. Both the airframe and engine manuals for modern engines specifically call out oversquare power settings for cruise. According to the manual, a Lycoming IO-360 in a Mooney 201 can be set for 75-percent power at 4,000 feet by using 23 inches/ 2,700 rpm, 24 inches/2,600 rpm, or 26 inches/2,400 rpm. Other powerplant manuals spell out approved cruise settings as much as 4 inches oversquare — and turbo applications tend to run even more oversquare.
You won't hurt the engine by cruising oversquare; indeed, there are some good reasons to do so. Noise and vibration tend to fall at lower rpm, and internal friction rises rapidly with engine speed. By using equivalent power at a lower rpm, you will likely see reduced cylinder-head and oil temperatures. Try different settings in your airplane to see which ones result in the best compromise of speed, vibration, and noise.
Managing the mixture
Leaning is always a controversial topic. Many an engine malady has been blamed on the pilot's operating the engine too lean. So it's popular now to recommend very conservative cruise-mixture settings, usually centering on best-power mixture. That occurs near 100 degrees F rich of peak EGT. (For simple airplanes with a fixed-pitch prop, best power can be obtained by leaning to peak rpm.) If you have a single-probe EGT, finding peak means peaking that needle. For multiprobe installations, use the first cylinder to reach peak as a reference. Remember that with one exception, the absolute EGT values are unimportant. It's the relationship to peak that matters most.
Keep this concept in mind: Except at high power settings — in excess of 75 percent or the highest allowable cruise setting — ultra-rich mixtures are unnecessary. And because you can't induce detonation at cruise settings, mixture control becomes an issue of temperature and engine smoothness.
Cylinder-head temperature tracks EGT very closely, peaking in the region of 10 to 25 degrees rich of peak EGT. For well-cooled and modestly powered engines, just about any mixture between best power (100 degrees F rich of peak) and peak will work fine as long as the engine runs smoothly. If your engine is prone to exhaust-valve and -guide wear, you may want to avoid peak EGT at high power settings. Richer than best power and you'll load up the combustion chamber with deposits and simply waste fuel.
High-powered engines need a bit more mixture care. For example, Beech and Cessna both specifically recommend remaining either richer than 20 degrees C (37 degrees F) rich of peak or leaner than 20 degrees C lean of peak in the IO-520 and IO-550 engines. High power in this case is above 65 percent, and the reasoning is simply to curtail CHTs. Running on the lean side of peak provides a substantial CHT reduction, as well as a marked improvement in economy — the bottom of the efficiency curve is on the lean side. Unfortunately, not all fuel-injected engines and relatively few carbureted models will run smoothly at or even a bit lean of peak.
Descent tactics
One of the best things you can do for maintaining a reasonable cooling gradient during descent is to keep the mixture leaned. Because CHT and EGT track so closely, you can use the mixture to help keep the CHTs from falling. In the end, let common sense be your guide. Treat the engine with care, but do what you need to do for flight safety. Know at all times whether you are at a specific power setting that allows you to lean the mixture — and do so. Be watchful for unusual indications — abnormally hot or cold temperature indications, and unduly high or low cruise speeds or fuel consumption. Listen to your engine — it'll tell you what it wants.
Is Lean Mean?
The dictum comes down with an iron fist: Run your engine lean and you'll burn it up. We've been handed this warning of mixture management with such force and self-righteousness that we never stop to consider whether it's true.
Extensive flight testing by the folks at General Aviation Modifications, Inc., makers of the GAMIjectors for Continental engines, has caused them to object to the orthodoxy rather strenuously. GAMI has been fitting Continental engines with matched and modified fuel-injector nozzles for more than 18 months. The company was recently awarded supplemental type certificate approval for turbocharged applications, bringing the total number of engine models covered to 75. Among GAMI's findings is this little nugget: Not only can the big-bore Continental be operated significantly lean of peak, it likes it there.
It would be one thing if some good old boys from Oklahoma had pulled this newfangled idea out of thin air, but GAMI's assertions are backed by reference material from as unassailable a source as Curtiss-Wright Corporation, Wright Aeronautical Division. In the days of the large piston-engine airliners, operation lean of peak exhaust-gas temperature (EGT) — to the tune of 40 to 70 degrees F lean of peak — was absolutely common. According to a manual printed by Wright on the operation of the R-3350, this was the norm not just because the engines consumed less fuel, but also because they lasted longer that way. (Typical TBOs on the later R-3350s in the DC-7s and Constellations are said to be between 2,000 and 3,000 hours — amazing, considering that the 2,800-pound, turbo-compound monsters were putting out 3,700 hp from 18 cylinders.)
These brutes like lean-of-peak operation for one simple reason — the engines run cooler. Curtiss-Wright includes a graph in the manual — one that, incidentally, presents substantially the same information as one printed by Continental for the IO-550 engine — showing the relationship of temperature to mixture strength. On the rich side of peak, the power and temperature traces are fairly flat from about 100 degrees F rich to 25 degrees F rich. From 25 degrees F to peak, the temps rise fairly rapidly and then fall even more dramatically on the lean side. In addition, if you could plot the specific fuel consumption — usually rendered in pounds per hour of fuel per horsepower — the curve reached the lowest (best) figure on the lean side of peak.
One interesting tactic used with these engines was to lean the mixture to a setting lean of peak — actually to a 10-percent drop in cylinder brake mean effective pressure (BMEP) — resulting in a 10-percent drop in power. Then the engineer would advance the throttles until the torquemeters indicated a return to desired power.
Why not just overshoot the power setting and lean through it, to arrive at the desired percent of power? A graph in the same manual shows the critical areas for detonation at high power settings to be between 40 degrees F rich of peak and 15 degrees F lean of peak. By allowing the leaning process to reduce the power output below the critical high-cruise settings and only increasing power with the mixture on the lean side of peak, the areas of potential detonation are avoided. If the big radials could do it, why isn't lean-of-peak operation recommended for most flat engines? A prerequisite for running lean of peak is excellent mixture distribution. The radials have good distribution because fuel arrives through the carburetor or injection system at the inlet to the supercharger, which, like a giant Cuisinart, helps atomize the fuel/air mixture. Some early high-powered opposed engines came with superchargers that did the same thing. In fact, an operator's manual for the Lycoming IGSO-540 engine specifically recommends cruising lean of peak EGT.
Continental has fitted tuned induction systems to several engines with the aim of improving mixture distribution. But the rank-and-file injected engines — to say nothing of the carbureted models with endemically poor mixture distribution — won't normally operate smoothly on the lean side of peak. So the recommendations from the early engines disappeared from POHs, replaced with the suggestion to run at or rich of peak.
GAMI is putting its lean-is-better philosophy to the test. One of the firm's test airplanes, a V35 Bonanza with a Flightcraft turbo-normalized IO-550 and Superior Air Parts Millennium cylinders, is being flown at high cruise settings exclusively lean of peak; the operating range has been between 40 degrees F lean and 70 degrees F lean of peak TIT. The company's pilots report that the engine runs significantly cooler and more efficiently lean of peak, even at power settings that would create some real heat problems on the rich side of peak. They are watching the cylinders and the exhaust system closely for any signs of premature deterioration.
After all, the GAMI crew believes that if the lean-of-peak tactic worked on the big radials, there's no reason — once the mixture distribution is corrected so that the engine runs smoothly — it shouldn't also work on the opposed powerplants we have today. — MEC
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