Update – Piaggio Avanti

Piaggio of course were no mere neophytes to aeronautical engineering. The company had been in existence since 1884 and had built its first aircraft in 1915, making it one of the oldest aircraft manufacturers in the world. Piaggio also were highly respected for their engine expertise and in addition to the 49 different aircraft types they have designed, they also lay claim to having been associated in 16 separate engine programmes. The company presently is involved in a number of major programmes including the AMX tactical fighter, Tornado, 767, DC-10/KC-10 and the Aeritalia G. 222 tactical transport. Thus when in 1979 the Piaggio board decided to actively pursue the development of a revolutionary high speed turboprop, they had a considerable fount of aeronautical knowledge on which to call.

What the engineers came up with was truly radical, in many ways far more radical, yet practical, than what had been achieved with the eventual Learfan design or even Bert Rutan’s efforts with the Starship.

Piaggio opted for a unique three-surface lifting body design that effectively reduced aerodynamic drag by 20% over conventional designs.

The concept is simplicity itself and works roughly like this – On takeoff, lift created by the canard-like foreplane lifts the aircraft on rotation thus effectively reducing the download required on the horizontal tailplane. As a result less main wing lift is required. Consequently, the aircraft can operate with a much smaller main wing with an aft-loaded Centre of Gravity (CofG).

Typically an aircraft generates considerable downloads on its tailplane in all flight regimes which ‘consumes’ between 15 to 20% of the lift produced by the main wing area. By eliminating this download Piaggio claim that their unique three surface design thus allows for a main wing typically a third smaller than any conventional design. To highlight this point it is worth noting that the Avanti’s total wing area is roughly 45% smaller than a Citation SIi and 35% smaller than the Starship which literally replaces its tail with a canard.

The other positive aspect of this unique design is that the very high 11. 7/1 main wing aspect ratio allows a higher degree of Laminar Flow than would normally be viable. Basically speaking, Laminar Flow is where the boundary layer airflow flows smoothly over the airframe with the degree of aerodynamic separation graduating progressively and smoothly through what we term the boundary layer. In other words, Laminar Flow, if achievable, greatly reduces drag, increases lift and even reduces noise, inside and outside the aircraft. Piaggio engineers are claiming their ratio of Laminar Flow to be at least double that of any competing design, a factor which they justifiably claim is worth at least another 20kts at maximum speed.

Returning to the unusual wing configuration. The forward wing of the Avanti actually helps to trim the aircraft over a wide range of flight conditions. Unlike the Starship, it is set at a fixed incidence on the fuselage with slight anhedral. However, a movable surface is attached to the trailing edge of the forward wing (which Piaggo do not refer to as a canard). This is in turn connected to the main wing flap control so that when the main wing flaps are lowered, the trailing edge of the forward wing is deflected downward. This acts to keep the aircraft in trim and produces additional lift at the same time in all flight regimes.

The benefits of this are that the aircraft’s weight acts ahead of the main wing. This effectively makes the Avanti nose-heavy and requires a nose-up trimming force for balance. On the Avanti this trimming force is provided by positive lift on the forward surface, reducing the negative lift required on the horizontal tail. Effectively this results in the forward surface helping to support the aircraft’s weight while it also serves to trim the aircraft.

The benefits of over 5,000 hours of wind tunnel testing, most of which was undertaken in the US with NASA and General Dynamics’ Convair Division, have refined the Avanti’s Laminar Flow characteristics to the best yet produced in a small civil aircraft. Ideally, the goal is to achieve a maximum airspeed of over 400kts with a specific range of 1 nautical mile for 1 pound of fuel burn at optimum cruise speed. Piaggio are already very close to these figures with around 300 of the 500 test flight hours completed between the three prototypes as we closed for press. Recent test data revealed a fuel flow of 460lb/hr total at long-range cruise at 310KIAS, very frugal indeed.

Power for the Avanti is supplied by two Pratt & Whitney Canada PT6A-66s flat. Rated at 850shp at 26,000ft. Piaggio chose the venerablePT6 instead of more exotic options due to its commonality, excellent track record and the fact that it was the best current design optimised for a pusher layout.

The ironic thing about the exotic-looking Avanti though is that – apart from the radical aerodynamics – it is a rather conventional aircraft. For instance, the hydraulic system is used only for the landing gear, flaps and brakes. The flight controls are cable-operated and the flap motors are electrical. Composites comprise only 20% of the total structure weight and have been used only where they have offered a real weight saving. – Piaggio are not fully sold on the large scale practicality of composites and are wary of their long-term longevity, particularly when exposed to jet fuel and routine damage tolerance, etc. Thus the wing is milled as a single alloy extrusion, as in an airliner, indeed a process that achieves better end quality and a far smoother finish that in turn assists in achieving Laminar Flow.

While the aircraft is now being primarily built in Italy, owing to Gates Learjet’s withdrawal from the programme in January of 1986 following their impending insolvency, Piaggio are still hoping to team up with an American partner to assist North American marketing efforts and to split the development costs. Ironically, quite a – lot of the aircraft is actually sourced from the US as Sikorsky builds the entire composite empennage and supplies a number of other components, Pratt & Whitney Canada supply the powerplants and much of the avionics is from Collins and other US sources.

The Avanti cabin is larger than any competing aircraft and can comfortably· accommodate between seven and nine passengers. Internal cabin height and width is significantly greater than in a Citation 11, a King Air 200/300, Beechjet or Cheyenne, in fact the internal dimensions are similar (excepting length) to a Falcon 200 or BAe 125. Piaggio are thus hoping that the docile handling and spirited performance of the Avanti, coupled with a fuel burn around half that of the competing light jets, will make it a sales winner.

Performance figures (please refer to data table) won’t be totally clarified till FAA FAR Part 23 certification is gained in 1988, however preliminary Piaggio estimates are firming as the test programme matures. Targets of a 320kts long-range cruise at 4t,000ft and 400kts high speed cruise at lower altitudes (rather like the Cheyenne 400LS) look attainable, as is a four passenger maximum range of 2200nm with NBAA VFR reserves. Stall speed on approach is a mere 90kts which also highlights the Avanti’s excellent field performance.

Aviation through this century has produced many exciting new and truly revolutionary designs that have served to set a trend in their respective areas of application. The Piaggio Avanti could well be just such an aircraft that is certainly long overdue in corporate aviation.