Fuel Miser
(continued) And that's why the Avanti has a forward wing. Many confuse the Avanti wing with a canard, but a canard is a pitch control surface that replaces the standard horizontal tail. The forward wing in the Avanti only lifts and has a conventional wing flap, but no pitch control capability. The lift contribution of the forward wing lowers the necessary balancing force from the horizontal tail so that it can be smaller and operate with the short tail arm between the tail and wing. In other words, it is the forward wing, or third surface as Piaggio calls it, that allows the main wing to be mounted aft of the cabin where it is most efficient while still having enough pitch control authority from the horizontal tail. The lift contribution of the forward wing also allows the main wing to be as small as it is in area.
The unique configuration of the Avanti also dictates the location of the main landing gear in the aft fuselage where it retracts aft to be fully enclosed by tight- fitting doors. Landing gear and wing loads are concentrated in a single location, which also helps save weight because those loads do not spread to the rest of the airframe. Large ventral fins under the tail act as feathers on an arrow to naturally damp yaw and Dutch roll, and when the airplane is at high angles of attack they produce nose-down pitch force that helps recovery from an impending stall.
In the photo above you can see clearly how the Avanti fuselage swoops from a pointy
nose to pointy tail.
Another visible drag fighting measure is the shape of the engine nacelles with their pinched-in "area rule" wasp-waist shape. The Pratt & Whitney PT6 engines are mounted-in the pusher position which gives the propellers a theoretical efficiency edge over a standard tractor arrangement. Keeping the disturbed air of the propeller slipstream behind the wing also helps maintain laminar flow. The pusher location makes sense of the PT6 airflow where air enters the rear of the engine and exits the front. In a normal PT6 installation inlet air must be turned 90 degrees within the nacelle to be pushed into the rear of the engine. In the Avanti nacelle inlet air is fed directly to the engine air intake, though it still must pass over an inertial separator that will spit out any significant bits of ice or FOD before they can reach the engine.
What is not visible from the outside is the construction techniques necessary to achieve the shape the aerodynamics dictate. Many at first assume the Avanti is made from composite materials because the surface is so smooth and the shapes so unusual, but more than 90 percent of the airplane is made from metal with composites used for fairings, nacelles and the horizontal stabilizer.
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The Avanti cabin door is a hybrid arrangement with a clamshell lower portion that drops down to provide an airstair while the top half swings on a forward hinge.
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The wings are made from thick slabs of aluminum that have been machined to create integral stringers and stiffeners in the skins. This is standard in larger jets, but not turboprops. To produce the critical fuselage shape, Piaggio and its initial development partner Learjet pioneered an "outside in" method of construction. Very precise tools were built to hold the fuselage skins in perfect conformity to the design while ribs and stringers were fastened from the inside. All adjustments during the construction process were made on the inside so that the external shape remained true to the design. Other manufacturers now use a version of this process but Piaggio and Learjet were there early.
The Avanti has succeeded in meeting its objectives of speed, large cabin and fuel efficiency, but the airplane is unique in many ways so it is difficult, actually impossible, to generalize. For example, the new Avanti II with its Dash 66B engines can hit 400 knots true airspeed at 31,000 feet, and that puts it into the jet arena of performance. But slowed down to, say, 350 knots at a higher altitude, the fuel efficiency soars as fuel flow drops down to around 600 pounds per hour, or less. The cabin is also difficult to compare because of its constantly changing shape. When measured at its maximum the Avanti cabin is actually a little larger than the Hawker 125 series that has been a perennial midsize jet leader. To say the Avanti is bigger than a Hawker is not accurate, but to say that the cabin is roomy and comfortable is exactly correct. And the Avanti is a turboprop, and like other turboprops weighing 12,500 pounds or less it doesn't meet the engine failure on takeoff requirement of the jets, so runway performance comparisons are not equal.
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