Plane & Pilot
Saturday, July 1, 2006

Compress Your Power

Turbocharging. Is it worth its weight and heat?

Turbocharging. Is it worth its weight and heat?

  Scenario 1  
  Nestled in the tall pines of the Rockies southwest of Denver, is Lakeport County Airport, one of America’s more scenic airports. The runway runs roughly north-south for 6,400 feet; its approaches are relatively unobstructed. It’s a typical airport with one caveat: it’s 9,927 feet above sea level.

As the highest municipal airport in North America, this airport in Leadville, Colo., deserves special respect. Combine its two-mile-high elevation with even a modest summer temperature of 78 degrees F and you’ve got a density altitude well up in the mid-teens, specifically 13,224 feet. Normally aspirated airplanes needn’t apply.
  Scenario 2          
  Let’s say you live in Wichita, Kan., where the highest terrain is about 1,600 feet. Normal aspiration should be adequate to cover virtually any situation, right?

That won’t necessarily be the case. Say you decide to fly up to Hutchinson in mid-July for its buffet lunch. The temperature reaches 100 degrees F, nowhere near a record for Hutchinson, but unquestionably warm. Run that through your E6B (does anyone still use those?), and you’ll come up with a density altitude of nearly 4,400 feet. Chances are that your airplane will be notably less enthusiastic than you might expect.
  Scenario 3  
  Your Skyhawk is bouncing along at 7,500 feet over central Florida. The swamps below are generating the usual afternoon buildups. You can see the clouds ramping up ahead, gradually tilting well above 10,000 feet. Flight watch confirms that the tops are at 11,000 feet.

Your plane, however, has a service ceiling of only 13,000 feet on a good day with a new engine, perfect rigging, standard temperature and mid-cruise weight. The conditions are nowhere near optimum. A climb to 12,000 feet (1,000 feet above the clouds, but only 1,000 below your service ceiling), might take the rest of the day.

In each of these cases, the problem is that aircraft engines lose power as they climb away from sea level. What’s required is a method for delivering sea level power to engines at high altitudes.

Supercharging, which is more than a century old, is the original technique for maintaining power in piston engines at increasing altitudes. Mechanically driven superchargers were first employed in ground-bound diesel power plants long before the Wrights launched their way into aviation history.

In 1918, General Electric became the first group to apply an exhaust-powered turbocharger to an airplane (a Liberty). Supercharging was installed on race planes in the 1930s; it was practically mandatory on all World War II planes that fought or bombed at high altitudes. One of the most famous of these, the P-51 Mustang, employed a two-stage blower on its Rolls-Royce Merlin engine. This allowed a pilot to select more boost for max power at high altitudes.

These days, turbochargers are the rule for compressed power on piston, general aviation engines. A turbocharger allows pilots to utilize exhaust energy, which would otherwise be wasted and blown into the atmosphere.

Though turbocharging is a relatively simple concept, it’s rather complex in execution. Intake air is compressed by a centrifugal pump, which is driven by an exhaust-energized turbine. The turbine and compressor wheels are comparatively small, usually only about three inches in diameter, but they operate in an incredibly hostile environment. Typically, piston engines drive their propellers at a maximum 2,700 rpm, but the turbocharger spins at an amazing 100,000 rpm, and the turbine is immersed in an exhaust stream at 1,500 degrees F.

The wastegate, essentially a pressure relief valve, is the component that regulates a turbocharger’s output to the engine. The simplest form of turbocharger employs its own, independent throttle, which is little more than a manual wastegate control.

A pilot can regulate manifold pressure (MP) by opening or closing the wastegate. With the wastegate fully open, the system transports exhaust gases overboard without routing them through the compressor, thereby “wasting” the exhaust. As a pilot commands more boost, he or she gradually closes the wastegate, forcing more exhaust gas through the turbine. This results in progressively more compression until the system is producing maximum boost. More sophisticated turbo systems employ automatic wastegates that regulate power to maintain whatever power the pilot sets.

The maximum height at which a turbocharger can deliver sea level MP is known as the turbo’s critical altitude. Most turbos deliver sea level power to 16,000 through 18,000 feet. Some of the stronger turbochargers extend that height to nearly 25,000 feet. Above critical altitude, engine power will fall off at about a standard lapse rate of one inch per 1,000 feet. This means a typical turbocharger installation can maintain 75% cruise power to about 25,000 feet; a few can still deliver 100% power at FL250. As it turns out, that’s a convenient limit, as most oxygen systems aren’t approved above 25,000 feet anyway.

Unfortunately, in keeping with the universal principal of TINSTAAFL (There Is No Such Thing As A Free Lunch), turbocharging is far from free. Employing a turbo has both direct and indirect operational and financial costs. An aftermarket turbo may not be the easiest thing to install, and it will always add weight up front. (On a Mooney, an aftermarket turbo will probably rule out a three-blade prop—too much weight and stress on the engine mounts.)

Turbocharged engines cost more to buy, operate, maintain and overhaul. Additionally, turbo TBOs are typically 20% to 30% lower than comparable, normally aspirated powerplants. In order to realize their performance benefits, turbocharged engines are often operated at higher power settings and higher altitudes where, despite the colder temperature, the air is a lot less dense and the result is less cooling. An engine asked to deliver 75% at 15,500 feet is almost guaranteed to run hotter than one generating the same power at 7,500 feet. Excessive heat is the enemy of all things mechanical, which means flying behind a turbo demands higher fuel flows to keep the engine cool, often an additional two to three gph.

Even with the higher fuel flow, a turbocharged engine will run hotter. Compressing air increases its temperature, making it slightly less efficient. Every six to 10 degrees F of increased temperature decreases engine power by about one percent. Introducing that compressed, heated air to the cylinders results in higher operating temperatures. You could recover some of that lost power by intercooling (introducing a heat exchanger between the turbocharger and the engine’s induction inlet), but this adds a further level of complexity and extra weight, and it moves the center of gravity farther forward (the latter isn’t always a bad thing on some airplanes).

Turbocharged engines are also susceptible to overboosting, a maintenance concern that’s not common on normally aspirated powerplants. Contrary to popular belief, it’s possible to overboost a normally aspirated engine on a cold day at low elevation. Alaskan and Canadian pilots who fly in extreme winter conditions sometimes see extremely high MPs if they simply push power to the wall for every takeoff. At minus-30 degrees F at a 1,000- foot MSL field elevation, for example, density altitude is –5,282 feet. In those conditions, you could reasonably expect most engines to develop at least 34 inches of MP at full throttle—but not for long.

Turbocharged engines are either turbo-normalized (limited to 30 inches of MP) or turbo-supercharged (capable of producing more than 30 inches MP). If the turbo employs a manual wastegate and the pilot pushes too hard, he or she may exceed the allowable limit and overstress the engine.

For that reason, most turbocharged engines are equipped with pop-off valves designed to automatically limit an overboost to one or two inches. If you push too hard on takeoff and exceed the maximum MP by more than two inches, the pop-off valve will open and bleed away the excess MP.

Another phenomenon associated with turbocharging, known as bootstrapping, can be a source of frustration for pilots. It occurs when a pilot adjusts power at or above critical altitude, and the turbocharger begins chasing itself. With the wastegate fully closed, any increase in power increases exhaust output. That, in turn, increases turbine speed, which increases MP, which increases power, which increases exhaust flow, which increases turbine speed…you get the idea. Power adjustments become a continuous tail chase.

Some turbos are so sensitive that throttle adjustment of any kind results in bootstrapping. Others respond with less variation. Smart pilots always adjust power slowly on any airplane, normally aspirated or turbocharged, but it’s especially important on engines with a blower out front.

The final operational procedure on a flight behind turbocharged engines is one of the most important. It’s absolutely critical that you cool down the turbo after landing. (For that matter, it’s not a bad idea to cool down all piston engines before shutdown.) As mentioned above, a turbocharger at full power spins at nearly 100,000 rpm. During approach and landing, you normally won’t use anything like full power (if you’re doing it right), but you may still develop turbo speeds of 50,000 rpm. If you land short and make the first turnoff and pull directly into the first parking space, you could be theoretically ready to shut down in less than a minute from airborne. That, however, is a bad idea. A turbocharger draws its oil supply from the engine, and as soon as you pull the mixture to idle cutoff, you’ll stop oil flow to the engine and turbo. That’s not a problem for the engine, because it idles at only 700 to 1,000 rpm. If the turbine is still spinning at 20,000 rpm, however, and you shut off all lubrication, the turbo will quickly grind to a halt, generating extremely high heat in the process and causing the remaining oil to “coke,” or turn to carbon. Do this consistently, and the bearings in your turbo will tighten up, the turbo will begin to smoke and lose efficiency and it may even seize.

For that reason, most engine manufacturers specify a minimum of three minutes at idle power after parking to allow the turbo to spool down to idle before the engine is shut down.

There’s nothing that difficult about flying with a turbocharger and the performance advantages at all altitudes should be obvious. Preventive maintenance may be more of a challenge—you may need a slightly fatter wallet to keep your airplane flying and you may experience slightly more down time—but turbochargers offer benefits that can more than offset their costs.

Labels: Turbocharging


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