A way with winglets

More payload and winglets equal more lift and less drag

October 1, 2010

In some circles, it’s become fashionable to refer to the Hawker Beechcraft Corporation (HBC) King Airs as dated and dowdy looking. True, King Airs have been around since 1964, and comparisons with sleeker, newer light business jets do invite judgment. But the market has had its say: King Airs remain the most popular turboprop twins.

Turbine Edition Table of Contents

That’s especially true of the 90-series, which has benefited from continual upgrades over the years, and has racked up more than 2,400 total sales in that time. In 2008, the King Air C90GTi was introduced. This gave the basic C90B design a new pair of Pratt & Whitney PT6A-135A engines of 750 thermodynamic horsepower, which are derated to 550 shaft horsepower in order to boost power at altitude and under hot-and-high conditions. By derating an engine, you basically design it with more power, and then limit that power. This translates into an engine that can tolerate higher interturbine temperatures and keep producing its derated power levels even though the ambient air may be hotter and/or thinner than those prevailing under standard conditions. In short, the engine is loafing, which means it runs cooler, experiences less wear, and can have longer life.

Winglet power

Now comes the C90GTx, which retains the GTi’s Pratts and Rockwell Collins Pro Line 21 avionics, but adds winglets and a gross weight increase. “The idea is to make the C90GTi into a true one-pilot, four-passenger, 1,000-nm airplane,” says Trevor Blackmer, Hawker Beechcraft’s senior manager of product marketing.

HBC raised the C90GTx’s maximum takeoff weight to 10,485 pounds—up from the previous model’s 10,100 pounds. The extra 385 pounds is what allows for an “extra-passenger” payload, or an equal amount of extra fuel. The increased-weight provision comes via a supplemental type certificate (STC) that’s licensed to HBC, and incorporated on the GTx’s production certificate.

A similar arrangement, this one with Boundary Layer Research (BLR) of Everett, Washington, lets the HBC factory install the winglets. To briefly review, winglets increase a wing’s area and aspect ratio, and greatly diminish wingtip vortices, which otherwise would generate more induced drag.

HBC says that the results in the C90GTx are slight reductions in stall speeds, increases in climb rates and reduced times to climb to altitude, and reduced fuel burns in cruise. BLR goes even further, saying that their winglets reduce yaw-roll coupling (Dutch roll) tendencies. Owners of earlier C90s, by the way, can have their King Airs retrofitted with BLR winglets. You can go online for more information.

HBC makes no claims of cruise-speed boosts on account of the winglets, but does say that the reduced drag saves 50 to 60 pounds of fuel burn in the first hour of flight. “Where usually we’d burn 600 pph in the first hour of flight, we now see 540 pph,” Blackmer said. “So there’s a definite drag reduction.” When installed on 200-series King Airs, BLR claims an annual fuel savings of more than $8,000, based on flying 450 hours per year.

Flight notes

You can add all the Blackhawk Modifications engines and all the Raisbeck Engineering mods you want—like their aft body strakes and wing lockers—but a King Air still looks like a King Air. However, when you step up to a C90GTx you’ll definitely take notice of those winglets. They somehow clash with the King Air’s traditional image and give it a fresher look. To see what winglet power offered, I climbed up the C90GTx’s aft airstair door, went through its roomy six-place cabin, and settled into the cockpit. This will be familiar territory to anyone who’s ever flown any type of King Air—save those not having experience with the Pro Line 21 avionics.

At our light (9,271 pounds) weight, the airplane accelerated quickly to its V1 (takeoff decision speed) of 91 knots, then launched into a 2,300-feet-per-minute initial climb at 150 knots. Unfortunately, a couple of climb restrictions prevented a direct climb to FL260—where the C90GTx flies most efficiently—so our time to climb was 22 minutes. That’s about seven minutes longer than book-advertised time to climb to that altitude, but the steps in our step-climb lasted at least that long. And passing through FL200, our climb rate was still a hefty 1,500 fpm.

With maximum cruise power at FL260, our torque settings were 1,170 feet/pounds, our props were adjusted to 1,900 rpm, and our ITTs were comfortably below their 805-degree Celsius redlines on our ISA-plus-7-degree C day. We trued out at 263 knots (right on book), and our fuel burn was 240 pph (about 36 gph) per side—which is 24 pph (about 3.6 gph) less than the book numbers. So, thanks to the winglets for the total savings of seven gph compared to the C90GTi’s performance under the same conditions. HBC says that the airplane’s max cruise speed is 270 knots; that’s at FL200, where it burns a total of 612 pph, or approximately 91 gph.

On approach and in the pattern, all C90s handle beautifully, with target airspeeds that will make a Bonanza or Baron pilot feel right at home. The HBC brochures say that the GTx’s winglets provide “superior” handling qualities during low-speed flight, but I never felt that low-speed control response was ever an issue in the C90. So landing a GTx—on an IFR or VFR approach—is pretty much the same as any other C90: 500 pounds of torque, approach flaps, and gear down yield 120 to 130 knots.

At about 500 feet agl, select full flaps and airspeed should work out to around 100 knots. Go to flight idle at about 10 feet above the runway, add a little aft-stick pressure, and you should make an acceptably ego-satisfying landing.

The C90GTx represents yet more incremental improvements to an airplane with an exemplary cabin, solid performance, and a ramp presence that makes a statement without suggesting conspicuous consumption. That’s a formula that’s served all King Airs well for years. In these times those attributes make the C90GTx even more relevant.

The market apparently agrees. In the first two months since they began rolling off the assembly line, 20 GTxs have been sold, and five have been delivered.

E-mail the author at tom.horne@aopa.org.

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.