Three-surface aircraft

In civil aircraft the three surface configuration may be used to give safe stalling characteristics and short takeoff and landing (STOL) performance.

It is also claimed to allow minimizing the total wing surface area, reducing the accompanying skin drag.

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.

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.

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.

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.

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 three-surface types to enter production.

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.

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.

This is typically intended to enhance control and manoeuvrability, especially at very high angles of attack beyond the stall point of the main wing.

Following the successful addition of canard foreplanes to a development aircraft, these were incorporated into a number of subsequent production variants including the naval Su-33 (Su-27K), some Su-30s, the Su-35 and the Su-37.

The McDonnell Douglas F-15 STOL/MTD was an F-15 airframe modified with canard foreplanes and thrust vectoring, designed to demonstrate these technologies for both STOL performance and high manoeuvrability.

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.

[9] On a canard aircraft, to allow natural static pitch stability in normal flight, the foreplane must provide lift.

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.

[9] Examples of reduced-area three-surface aircraft include the Piaggio P.180 Avanti, and the Scaled Composites Triumph and Catbird.