Tuesday, June 19, 2012
Turbines vs. Pistons
A turbine may be more logical than you imagine
TURBINES. A turbine offers amazing reliability and impressive longevity. It's inherently smoother than a piston and is remarkably compact and lightweight.
Of course, you do pay for the turbine's added reliability. It operates at much higher temperatures, and the turbine spins as fast as 45,000 rpm, so a turboprop engine must be constructed of extremely heat-resistant metals and be very well balanced to handle rotational forces.
Turbine engines are remarkably compact and lightweight compared to piston mills. The Meridian's 500 shp PT6A is comparatively small and weighs a little over 300 pounds, whereas the big-bore, turbocharged, 350 hp Lycoming on the Mirage weighs more like 450 pounds. This imposes considerations of both design and weight and balance. The Meridian's fuselage is extended an extra foot over that of the Mirage, and that's all in the longer nose to control the airplane's balance point.
Turbines aren't as efficient as piston mills, but the difference isn't as much as you might think if you consider the improved performance. At optimum altitude, the Meridian burns about 31 gph compared to 20 gph on the Mirage, roughly 50% more. That's because piston engines are more efficient and offer a lower specific fuel consumption (.43 lbs./hp/hr) compared to turbines (.58 lbs./shp/hr).
As partial compensation, the Meridian climbs to its maximum altitude, 28,000 feet, in about half the time and cruises roughly 40 knots faster than the Mirage. (In fact, the Meridian is approved for flight at 30,000 feet, but few of the type are RVSM equipped for operation above 28,000 feet.)
As partial counterbalance to the efficiency of avgas, it's not efficient if you can't find any. Jet fuel is often less expensive than avgas, and it's more universally available around the world. (A few years ago, I delivered a Cessna 421 to a company in Subic Bay, Philippines, only to go back a year later and ferry the airplane back to the states. The company executives had grown tired of limiting their destinations to airports with avgas and decided to buy a King Air C90.)
The extra fuel burn of a turbine also has an effect on payload. Jet fuel is 13% heavier than avgas, so the Meridian must carry more fuel weight to manage reasonable range, specifically 150 gallons vs. 120 gallons on the Mirage. The Meridian's bigger power allows it to be certified at 5,092 pounds, almost 800 pounds heavier than the Mirage, but the top piston Piper still scores about 150 pounds more payload than the Meridian.
The logic of insurance is always difficult to define, and you'll pay a disproportionately higher premium to fly a turbine. Apparently, only pilots recognize the ease and simplicity of flying behind a jetprop. Both types demand high-performance/high-altitude checkouts, but for reasons that have nothing to do with accident history, insurance companies seem reluctant to acknowledge the benefits of a turbine.
Finally, there's resale value to consider. If you compare a seven-year-old Mirage to a similar-age Meridian, the Mirage has lost about 37% of its value, whereas the Meridian has declined about 39%, not much difference.
I've been lucky to fly both types quite a bit, including a dozen or so international deliveries. While that doesn't make me an expert, I'll take turbine every time. (Easy for me to say. I don't have to pay for it.)
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