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.
