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Canards: design with care Much apparent controversy surrounds the use of the canard configuration in today's air craft. B. R. A. Burns, technical manager of BAe's Experi mental Aircraft Programme, sheds some light on the subject. Canards, currently finding favour in all classes of aeroplane, require careful design if they are to outperform conventional configurations. While there are aerodynamic advantages, the benefits for stable aeroplanes are largely layout related. Canards come into their own on unstable, high-performance combat aircraft. Any aeroplane may be made stable by locating the e.g. ahead of the aerodynamic centre (a.c). Lift therefore acts behind weight, generating a nose-down moment which must be trimmed by some means. Pitch control of a tailless aeroplane is effected by wing-mounted elevators, or elevons. This requires a swept or delta wing, to obtain a reasonable moment arm from the e.g. to the pitch control surface. A stable tailless aeroplane has several disad vantages, not least an inability to use wing flaps because there is no means of trimming the additional pitching moment that they generate. Also trim drag is high, particularly at forward e.g. positions, because large elevon downloads are required for trim. As a result, e.g. range is limited. In a tailed aeroplane the range of e.g. positions required for various loadings of fuel, passengers, or weapons determines the size of the tailplane and locates the e.g. range in relation to wing chord. Tailplane download In a stable tailed aeroplane the forward e.g. limit is determined by the ability of the tailplane to produce a download to balance wing lift, which acts behind the e.g. The tail must also counteract the "no lift" pitching moment generated by wing camber, which is greatest with flaps down. (A cambered wing produces lift at zero angle of attack (AoA). At zero lift, nega tive AoA, the wing carries a download at the front and an upload at the rear, producing a nose-down moment.) The e.g. limit moves forward with increasing tail size at a rate proportional to tailplane maximum download cap ability. For this reason some aeroplanes feature devices to augment tailplane nega tive lift—leading-edge blowing on the Buccaneer and slats on the Phantom are examples. The aft e.g. limit is determined by stability, and in a stable aeroplane is located a few per cent of wing chord ahead of the neutral stability point, where e.g. and a.c. coincide. The aft e.g. limit moves rearwards with increasing tailplane size, but only at a comparatively shallow rate because its effectiveness as a stabiliser is diminished by wing downwash, typically by 50 per cent. It is a misconception that tailed aero planes always carry tailplane downloads. They usually do, with flaps down and at forward e.g. positions, but with flaps up at the e.g. aft, tail loads at high lift are Above Bert Rutan's VariEze set a new fashion in general-aviation design. Below pitching moment is reversed in unstable configurations A Y k- Control ' ' Stability power Weight limit limit frequently positive (up), although the tail's maximum lifting capability is rarely approached. As our tail sizing diagram shows (Fig A), there is a steep increase in e.g. range with increasing tail size as the limits move apart. In the example illustrated, a 40 per cent increase in tail size doubles the e.g. range from 10 per cent to 20 per cent of wing chord. Also the e.g. range is located well within the wing chord, so that the trimming load on the tail (and trim drag) is small. Putting a foreplane the same distance ahead of the wing as the tailplane was behind leaves the forward e.g. limit unchanged, but the surface, now clear of wing downwash, becomes an efficient destabiliser. If the aeroplane is to remain stable, then the aft e.g. limit must move forward. Both e.g. limits move forward with increasing foreplane size, the forward, control-power limit moving at a slightly greater rate proportional to foreplane lift ing capability. To achieve a usable e.g. range in a stable aeroplane, therefore, foreplane design must emphasise maximum lift, for control power, while reducing its destabilising effect by minimising lift curve slope (the rate of change of lift with incidence). Our foreplane sizing diagram suggests that a much larger foreplane than tail- plane is required to achieve the same e.g. range in a stable aeroplane. A factor of two is indicated. Also the practical e.g. range is ahead of the wing leading edge, so that the foreplane carries a significant pro portion of total lift. Wing flaps cannot be trimmed without a large increase in foreplane size—a further 50 per cent is indicated. An ingenious solution to flap trimming is employed by Beech on its Starship I. With wing flaps extended, their Fowler action moves wing a.c. aft, allowing a more destabilising canard to be used. As lift-curve slope decreases with sweep, FLIGHT International, 23 February 1985
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