Plane & Pilot
Tuesday, February 11, 2014

Turbocharging


The benefits of getting high


As we all learned in flight school, most engines are rated for max cruise at 75% power. A normally aspirated aircraft engine will develop 75% to about 7,000 to 8,500 feet, depending upon the efficiency of the induction system. Above that height, full throttle power gradually drops off at the rate of roughly five percent per thousand feet. That means you can reasonably expect a properly leaned engine to develop 65% power at 9,500 to 10,000 feet and 55% at 11,000 to 12,000 feet. (Your mileage may vary.)

Turbocharging adds another dimension to flying, a spectrum of sky well above the bottom two-and-a-half miles of airspace that clings to the Earth. A turbo typically contributes at least another 10,000 feet of vertical altitude to an airplane's flight envelope, expanding available cruise levels from roughly 13,000 to 14,000 to 23,000 to 25,000 feet. A turbocharger, not so simply, compresses intake air and delivers it to the engine at a graduated rate.

Most modern turbos employ automatic waste gates that limit boost depending upon altitude and provide sea-level power until the waste gate is fully open. (Some even utilize intercoolers to cool the intake air and provide additional prospective power, but that's another story.)

This allows the engine to develop full- rated horsepower to the critical altitude, the height at which the waste gate is wide open and the engine can still deliver sea level power. On most turbos, that's usually at least 17,000 feet, but often as much as 25,000 feet. Logically, if the engine can still deliver 100% power at 17,000 feet, it should now deliver the recommended 75% maximum allowable cruise power at 24,000-25,000 feet.

Flying high has a number of advantages, and while operation in the mid-teens to mid-20s may offer obvious benefits, a turbo can provide better performance at lower levels, as well. Cruise at 10,000 to 13,000 feet offers 10 to 12 knots better speed, because the engine can still deliver 75% long after the standard breather is down to 55% or less.

The big benefit comes in the aforementioned block of airspace between 13,000 and 25,000 feet. If your airplane is turbocharged but unpressurized, oxygen is mandatory for the flight crew above 14,000 feet and for everyone on board for flights above 15,000 feet. Today's oxygen systems have become so simple and transparent, with the gas dispensed from either a cannula or the microphone stalk on a headset, that you may hardly know you're using O2. Those systems generally are limited to operation at 18,000 feet and below. Full face masks are required above 18,000 feet.

Still, the oxygen requirement automatically excludes most of the normally aspirated general aviation fleet, so there's little traffic to contend within the middle altitudes. File IFR, and direct routings are likely to be the rule, since controllers have sparse conflicting traffic in the middle altitudes.

If you do happen to be using VOR navigation rather than GPS, range is usually excellent, even over high, mountainous terrain. In the southern 48 states, there's nothing higher than 15,000 feet, so even a modest turbocharger will let you fly high above the peaks.



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