What is a three –surface aircraft and do these aircraft offer any real benefits?
A three-surface aircraft (three-lifting-surface aircraft) has a foreplane (canard), a central main wing and an aft tailplane. The central wing surface always provides lift and is usually the largest, while the functions of the fore and aft planes may vary between types and may include lift, control and/or stability.
A three surface configuration may be used to give lower stalling speeds and offer short takeoff and landing (STOL) performance. The total wing surface area may also be reduced, lowering skin drag.
Due to insufficient knowledge at the time, pioneers were keen to try various configurations to overcome the instability problems of their aircraft.
Pitch control was on the front surface with the rear surface also acting as a lifting surface.
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.
More successful designs included the Voisin-Farman I (1907) and Curtiss No.1 (1909). However by the outbreak of the First World War, the rear surface became the conventional configuration of two surface aircraft. Tri-planes and bi-planes were not considered as three-surface aircraft, but rather as conventional designs.
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.
Robertson’s system was commercialised as the Wren 460, a modified Cessna 180/182 aircraft. Later this aircraft was licensed and produced during the 1980s as the Peterson 260SE. In 2006 the Peterson Katmai entered production.
It is possible to achieve a soft stall with a pure canard design, but it is difficult to control the pitching, as oscillations can develop as the foreplane repeatedly lifts the nose, stalls and recovers over and over again.
A three-surface aircraft overcomes this problem which is not usually present in conventional designs, but 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.
Examples are the designs of George Fernic of the 1920s. He developed the idea of two lifting surfaces in tandem, together with a conventional tailplane.
The foreplane was highly loaded and as the angle of attack increased so that it stalled first, causing the nose to drop and allowing the aircraft to recover safely without stalling the main wing. The Fernic T-9 (1929) had a rather large foreplane bigger than the tailplane. Today this sizing configuration has been dropped, with the tailplane larger than the foreplane.
Unfortunately Fernic was killed in his latter design, the Fernic FT-10.
Maneuverability Beyond the Stall
A few combat aircraft of today use three-surface configurations to enhance manoeuvrability both before and beyond the stall, often in conjunction with vectored thrust. This is especially helpful at low speeds and high angles of attack such as during takeoff and combat.
In 1984 in the United States flew a modified McDonnell Douglas F-15 and in 1988 the F-15 STOL/MTD, but unfortunately these aircraft never went into production.
However in the Soviet Union a Sukhoi Su-27 modified with canard foreplanes flew in 1985 and derivatives of this design became the one of the few military three-surface types to enter production.
Reducing Drag – Minimum Wing Surface
The three-surface configuration is claimed to reduce total aerodynamic surface area compared to the conventional and canard configurations, thus enabling drag and weight reductions.
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 takeoff 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 takeoff 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.
Minimum area in cruise can also 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
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.
Under the lead of Burt Rutan, two experimental aircraft adopting this configuration were subsequently built by Scaled Composites and flown in 1988.
The Triumph was a twin-turbofan very light jet aircraft designed for Beechcraft.
The Catbird was a single-engine 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.
In 1979, Piaggo in collaboration with Learjet, began design studies on a three-surface civil twin turboprop which would emerge as the Piaggo P.180 Avanti. The 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.
Piaggio attributes improved performance to a 34% reduction in total wing area compared to a conventional layout.
The 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.
Electric and hybrid aircraft and drones are making great strides in technology and are being billed as future inner city and intercity transport modes.
However a great problem is the weight of the batteries and the hybrid systems needed to make these forms of transport viable.
Recent designs have been employing three- lifting surface designs to make them lighter and more efficient.
No doubt that in the future we will see more and more of these aircraft and drones in our skies.