An aircraft with three lifting surfaces (3LSC) is an aircraft with a canard wing mounted in the forward section of the fuselage in front of the main wing, a main wing and an horizontal tail installed in a classical way, behind the main wing.
One of the major advantages of the 3LSC derives from the added flexibility in selecting the aircraft geometry for what concerns the payload/wing/fuselage relative position, due to the possibility of complying to control & stability requirements for a larger range of c.g. positions. This allows a unobstructed passenger cabin to be positioned in front of the wing, mid fuselage-wing intersection, landing gears retracted in the aft fuselage body without external protrusions, engines in pusher configuration far back from passengers. All this leads to a significant reduction of aircraft cross section area and consequent reduction of aerodynamic drag, especially in small aircraft like general aviation and commuter ones.
The other significant advantage is the dramatic reduction of the total lifting area required to fly the aircraft allowed by the 3LSC, with consequent reduction of total wetted area and aerodynamic drag. It is well known that the canard + wing configuration is potentially capable of offering better aerodynamic efficiency than the wing + tail configuration. This derives from the fact that the rotation of the aircraft at take-off, or approaching the landing speed, is obtained applying upward aerodynamic forces on the front surface instead of down loads on the conventional tail, so a lesser lifting surface is required to balance the aircraft weight. Small and commercial aircraft have the lifting area determined by the minimum speed requirements for those two flight conditions. As a consequence, when they fly at cruise speed, they have a total lifting surface exceeding the one required for just cruising. Considering that those aircraft spend the most part of flight time at cruise, the negative impact of this additional wing areas on the aerodynamic efficiency is very significant.
Theorically the canard configuration shows the possibility of reducing the total wing area of the aircraft, but, in the real word, this is spoiled by a number of problems typical of pure canard aircraft. Differently from an horizontal tail located behind a wing, a surface in front of a wing has a destabilizing effect, so it cannot always be as large as it would be required to balance all of the pitching moments generated by the c.g. excursion and, at the same time, be small enough to comply with the stability requirements. The pure canard configuration cannot always comply to the control and stability requirements for all practical c.g. range and for every flight conditions.
Another adverse effect for the canard aircraft is determined by the flap extension, because of the large variation of pitching moment produced by this operation. All the actual flying pure canard aircraft have a very limited c.g. range and no flaps, or flaps with minimum effect. The flap reduction effect requires larger lifting surface to maintain the same minimum speed, reducing the advantage of the canard configuration, unless artificial stability augmenter system is installed, but this is not appropriate for small aircraft. The other challenging possibility is to add a small horizontal surface behind the wing to compensate the reduction of the aircraft stability due to the presence of a canard surface large enough to ensure the required controllability of the aircraft in every flight conditions, and c.g. position.
Analysis and tests demonstrate that a well optimized Three Lifting Surface Configuration may offer exceptional efficiencies, also due to several other aerodynamics benefits, not mentioned in this brief description. The Piaggio P180 AVANTI, a twin engine, executive aircraft, designed to exploit all benefits of the 3LSC, is showing, with its actual every day flight operations, dramatic flight performances, not achieved by the contemporary classic aircraft. All flight parameters, predicted for that aircraft during the design definition, were achieved and demonstrated during the flight test and certification program.
Page prepared by Dott. Ing. Alessandro Mazzoni
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