Three-surface aircraft

1908 Voisin-Farman I

During the pioneer years of aviation a number of aircraft were flown with both fore and aft auxiliary surfaces. The issue of horizontal stability was poorly understood and typically pitch control was on the front surface with the rear surface also lifting, leading to instability problems. The Kress Drachenflieger of 1901 and Dufaux triplane of 1908 had insufficient power to take off. More successful types included the Voisin-Farman I (1907) and Curtiss No. 1 (1909). The Wright Brothers too experimented on the basic Flyer design in an effort to obtain both controllability and stability, flying it at various times in first canard, then three surface and finally conventional configurations.[2][3] By the outbreak of the First World War in 1914, the rear surface had become the conventional configuration and few three surface types would be flown for many years. The Fokker V.8 of 1917 and Caproni Ca.60 Noviplano of 1921 were both failures.

Molniya-1

In 1920s George Fernic developed the idea of two lifting surfaces in tandem, together with a conventional tailplane. The small foreplane was highly loaded and as the angle of attack increased it was designed to stall first, causing the nose to drop and allowing the aircraft to recover safely without stalling the main wing. This "soft" stall provides a level of safety in the stall which is not usually present in conventional designs. The Fernic T-9, a three-surface monoplane, flew in 1929. Fernic was killed in an accident while flying its successor the FT-10 Cruisaire.[4]

It is possible to achieve such a soft stall with a pure canard design, but it is then difficult to control the pitching and oscillations can develop as the foreplane repeatedly lifts the nose, stalls and recovers. Also, care must be taken in the design that the turbulent wake from the stalled foreplane does not in itself disturb the airflow over the main wing sufficiently to cause significant loss of lift and cancel out the nose-down pitching moment. In the three-surface design the third, tail surface does not stall and provides better controllability.

In the 1950s James Robertson developed his experimental Skyshark. This was a broadly conventional design but with a variety of features, including a small canard foreplane, intended to give not only a safe stall but good Short takeoff and landing (STOL) performance. The foreplane allowed STOL performance to be achieved without the high angles of attack and accompanying dangers of stalling required by conventional STOL designs. The aircraft was evaluated by the US Army.[5] Robertson's system was commercialised as the Wren 460, a modified Cessna light aircraft. This in turn was later licensed and produced during the 1980s as the Peterson 260SE and with the foreplane modification only as the 230SE. In 2006 a ruggedised variant, the Peterson Katmai, entered production. A broadly similar approach is taken by the 1988 Eagle-XTS[6] and its derivatives, the Eagle 150 series.

Grumman X-29, rear strake flaps deflected

Around 1979, military jet designers began studying three-surface configurations as a way to provide enhanced manoeuvrability and control, especially at low speeds and high angles of attack such as during takeoff and combat.[7] In the United States the experimental Grumman X-29 flew in 1984 and a modified McDonnell Douglas F-15, the F-15 STOL/MTD, in 1988 but these designs were not followed up. In the Soviet Union a Sukhoi Su-27 modified with canard foreplanes flew in 1985[8] and derivatives of this design became the only military types to enter production.

Scaled Composites Triumph

Also in 1979, Piaggio began design studies on a three-surface civil twin turboprop which, in collaboration with Learjet, would emerge as the Piaggio P.180 Avanti. This type first flew in 1986 and entered service in 1990, with production continuing today. In the Avanti, the three-surface configuration is claimed to significantly reduce wing size, weight and drag compared to the conventional equivalent.[9]

Two experimental aircraft adopting this configuration were subsequently built by Scaled Composites under the lead of Burt Rutan and flown in 1988. The Triumph was a twin-turbofan very light jet aircraft designed for Beechcraft. Flight testing validated the targeted performance range.[10][11] The Catbird was a single-engined propeller-driven aircraft, envisioned by Rutan as a replacement for the Beechcraft Bonanza. It holds the world record for speed over a closed circuit of 5,000 km (3,100 mi) without payload of 334.44 km/h (207.81 mph) set in 2014.[12]

On most aircraft, the wing centre of pressure moves forward and backward according to flight conditions. If it does not align with the centre of gravity, a corrective or trim force must be applied to prevent the aircraft pitching and thus to maintain equilibrium.[16]

On a conventional aircraft this pitch trim force is applied by a tailplane. On many modern designs, the wing centre of pressure is normally aft of the centre of gravity, so the tailplane must exert a downward force.[17] Any such negative lift generated by the tail must be compensated by additional lift from the main wing, thus increasing wing area, drag, and weight requirements.

On a three-surface aircraft, the pitch trim forces can be shared, as needed in flight, between the foreplane and tailplane. Equilibrium can be achieved with lift from the foreplane rather than downforce from the tailplane. Both effects, the reduced downforce and the extra lifting force, reduce the load on the main wing.

The Piaggio P.180 Avanti has flaps on both its forward wing and main wing. Both flaps deploy in concert to maintain pitch neutrality for take-off and landing.[9]

On a canard aircraft, to allow natural static pitch stability in normal flight, the foreplane must provide lift. Also, in order for the aircraft to have safe stall characteristics the foreplane must stall before the main wing, pitching the aircraft down and allowing the aircraft to recover. This means that a safety margin must be used on the main wing area so that its maximum lift coefficient and wing loading are never attained in practice. This in turn means that the main wing must be increased in size.

On a three-surface aircraft, the tailplane acts as a conventional horizontal stabiliser. In the stall condition, even if the main wing is stalled the tailplane can provide a pitch-down moment and allow recovery. The wing may thus be used up to its maximum lift coefficient, an advantage that may translate into a reduction of its area and weight.

A lifting foreplane is positioned ahead of the centre of gravity, so its lift moment acts in the same direction as any movement in pitch. If the aircraft is to be naturally stable, the foreplane’s size, lift slope and moment arm must be chosen so that it does not overpower the stabilizing moment provided by the wing and tailplane. Stability constraints thus limit the foreplane’s volume ratio (a measure of its effectiveness in trim and stability terms), which may in turn limit its ability to share pitch trim forces as described above.

The minimum size of the lifting wings of an aircraft is determined by: the weight of the aircraft, the force required to oppose the negative lift produced by the horizontal stabilizer, the targeted take off and landing speeds, and the coefficient of lift of the wings.

Most modern aircraft use trailing edge flaps on the main wing to increase the wings lift coefficient during take off and landing; thus allowing the wing to be smaller than it would otherwise need to be. This may reduce the weight of the wing, and it always reduces the surface area of the wing. The reduction of surface area proportionately reduces skin drag at all speeds.

A drawback of the use of trailing edge flaps is that they produce significant negative pitching moment when in use. In order to balance this pitching moment the horizontal stabilizer must be somewhat larger than it would otherwise be, so that it can produce enough force to balance the negative pitching moment created by the trailing edge flaps. This, in turn, means that the main wing must be somewhat larger than it would otherwise have to be to balance the larger negative lift produced by the larger horizontal stabilizer.

On a canard aircraft the foreplane can provide positive lift at takeoff, reducing some of the down force the rear stabilizer would otherwise have to create. However, the main wing must be large enough to not only lift the aircraft's remaining weight at takeoff but also to provide adequate safety margin to prevent stalling. On a three-surface aircraft, neither of these handicaps is present and the main wing can be reduced in size, so also reducing weight and drag. It is claimed that the total area of all wing surfaces of a three-surface aircraft can be less than that of the equivalent two-surface aircraft, so reducing both weight and drag.

Minimum area in cruise can be further reduced through the use of conventional high-lift devices such as flaps, allowing a three-surface design to have minimum surface area at all points in the flight envelope.[9]

Examples of reduced-area three-surface aircraft include the Piaggio P.180 Avanti, and the Scaled Composites Triumph and Catbird. These aircraft were designed to expose a minimum of total surface area to the slipstream; thus reducing surface drag for speed and fuel efficiency. Several reviews compare the Avanti's top speed and service ceiling to that of lower-end jet aircraft, and report significantly better fuel efficiency at cruise speed.[18][19] Piaggio attributes this performance in part to the layout of the aircraft, claiming a 34% reduction in total wing area compared to a conventional layout.[9][15]

• Garrison, P; TECHNICALITIES: Three's Company; Flying, December 2002, pp85–86