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Fuel Miser

The Piaggio P.180 Avanti II is one of the most unusual, even exotic, looking airplanes in production today. When you look at the airplane and wonder "why did they do that," the answer is always the same - to fly the biggest cabin the fastest for the least fuel. Efficiency drove every decision Piaggio engineers made when the airplane was created in the late 1980s.

Facing the huge runup in fuel prices of the past year, it is sometimes hard to remember that the worry in the previous oil crisis was of availability more than price. In the 1970s oil experts -- at least the ones who got our attention -- warned that the world was running out of oil at any price, and that the regions that had the oil were so politically unstable that supply disruptions were a certainty. It was in that climate of very little oil left that Piaggio set out to make the best use of the fuel available with the Avanti.

The decision to power the airplane with a turboprop instead of a turbofan jet engine was easy because the turboprop, at least up to cruise speeds in the mid-300-knot range, is more efficient than the jet. Piaggio wanted to match the speed of many of the light and even some midsize jets without the high fuel burn. But after that choice was made, Piaggio set off on a unique design path, with each decision to improve efficiency leading to another unusual feature.

Low drag, and thus fuel efficiency, begins with the wing. In general the longer the wingspan, and the smaller its area, the lower the drag created by the wing as it does its work of generating the lift needed to carry the airplane. A glance at any glider shows how well understood the efficiency of a high aspect ratio -- long and slender -- wing is. Piaggio selected a wing with a span greater than 45 feet, but total area of 172.2 square feet, about the same as a Cessna 172 or other popular light singles.

The Avanti wing is also very thin with an airfoil shape that achieves laminar flow over about 50 percent of the chord. Laminar flow means that air molecules are moving in an orderly and smooth stream, creating the least drag possible. Most wings have only a small amount of laminar flow before the air becomes turbulent and the molecules tumble and jumble in a layer flowing over the wing, and that adds drag. To maintain laminar flow over significant sections of the wing demands a careful airfoil shape, but also a very smooth surface free from ripples and ridges, rivet heads or other fasteners.

In addition to low drag, the other Avanti design requirement was the biggest possible cabin, and a large fuselage creates drag. The Piaggio solution was to design a fuselage of constantly changing cross section that is free from the distinct nose and canopy sections of a conventional fuselage. By blending the windshields into the fuselage as it expands gradually from the pointed nose, the company hoped to achieve some laminar flow over the fuselage as well as the wing. Such flow is difficult to measure, but it appears that airflow remains attached to the fuselage in a laminar flow for at least some distance aft of the nose.

With such a low-drag fuselage shape the benefits would be squandered if it were to sit atop the wing as in a conventional design. The lowest drag way to join a wing and fuselage is to place the wing near the center of the body. That´s great for aerodynamic textbooks, or maybe for fighters or Reno racers, but passengers are not going to think highly of a cabin that has a wing spar penetrating the middle. That´s why all recently designed jets put the wing entirely under the fuselage and then use large and elaborate fairings to make the air believe the wing is mounted higher and more toward the center of the fuselage.

The Piaggio solution to this problem was to locate the wing aft of the passenger cabin where it could pass through the center of the fuselage without robbing room from people. But this wing location creates its own problems because the tail arm -- the distance from the aerodynamic center of the wing to the horizontal tail -- is short. An airplane pitches around the center of its wing and the horizontal tail balances those pitching forces. When the tail arm is short there is less leverage available to counteract these pitching forces and the size of the horizontal would need to be huge to generate sufficient force.

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.

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.

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.

The Avanti does, however, have a number of features typically found only in jets, including single-point pressure refueling and heated wing leading edges for ice protection. Fuel is carried in the wings and in tanks in the fuselage. Fuel management for the crew requires nothing more than to have enough onboard, or to occasionally transfer fuel from side to side if an imbalance occurs. All of the feeding from various tanks is automatic, as is the fuel distribution to the tanks when the line crew hooks up to the single point refueling port under the right wing root.

Read the full version of this article at Flyingmag.com

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