Plane & Pilot
Tuesday, April 21, 2009

Looking For 200 Knots

Forty years ago, the goal was 200 mph. Today, it’s 200 knots.

knottsFast feels good. For those of us obsessed with clocking along at the velocity of a Lamborghini, speed is the kinesthetic equivalent of beauty.
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Piper Malibu Mirage
Eventually, Lyle’s Bearcat became the world’s fastest propeller-driven airplane, setting the three-kilometer prop/piston speed record at 528.33 mph, winning Reno’s Unlimited Championship 10 times and winning or placing well in practically every other race he finished.

In a more moderate sense, the airplanes in the 200+ knot category are veritable masters of their own brand of aerodynamic art. They utilize engines as strong as 350 hp and employ airframes and wings as optimized for aerodynamic efficiency as production requirements will allow.

The manufacturers also know that weight is the final ingredient that constrains speed. Any airplane will fly faster if it flies lighter. Some aerodynamicists suggest the improvement may be as much as one knot for every 100 pounds reduced on aircraft lighter than 6,000 pounds. Here’s a quick rundown of the specs on the five airplanes in the 200+ knot class.

The Mooney Acclaim S leaves virtually nothing hanging in the wind. It’s a truly slick design that sports a drag coefficient of about 0.0190, nearly as good as the famous P-51 Mustang, which logged an amazing 0.017.

I flew an Acclaim S last year that couldn’t have been much cleaner and scored 240 knots at FL250, only two knots off the book promise. That’s 276 mph, far out in front of the goal and easily the equal of some turboprops’ best efforts. Even the step was missing, not necessarily a major inconvenience since the aft inboard section of the wing is close to the ground.

A development of the original (2006) Acclaim, the new “S” variation has a new prop, flap gap seals, improved gear doors, cleaner hinges and a low-drag cowling.

For motive force, the Acclaim S employs a severely derated, 280 hp Continental IO-550-G engine, breathing compressed sky through a pair of Kelly Aerospace turbochargers that maintain full power all the way to 25,000 feet. The Continental is approved for as much as 350 hp in other applications, so 280 hp is a bare 80% of the max rating. Accordingly, the big IO-550 carries a TBO of 2,000 hours.

Cessna’s Corvalis 400TT, formerly known as the Columbia 400, is all the more amazing considering that the quick Cessna is a fixed-gear design. The Cessna Corvalis 400TT is the logical extension of the original turbocharged Columbia 400.

As a fixed-gear airplane, the 400 couldn’t be much cleaner. A composite design with no rivet heads, section lines or any of the other inconsistencies of aluminum airplanes, the Corvalis 400TT is about as slick a machine as you’ll find, probably scoring a drag coefficient close to the Mooney.


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