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ith carech apparen t con t rov ersynds the use of the ca nardin t od ay s a ir B. R. A. Burns technica lo f BA e s Exper i

anards , current ly f inding favour inall classes of aeroplane, requirecareful design if they are to

e e.g. to the pitch c ontro l surface. A

n a s table tai led aeroplane the forward

Th e tai l mu st also counte ract the nopi tching mo me nt generated by wingcam ber ed wing produce s lift at zero

to tai lp lane maximum download capabi l i ty . For th is reason some aeroplanesfeature devices to augme nt tai lp lane negative liftleading-edge blowing on theBuccaneer and s l a t s on the Phan tom areexamples .The aft e.g. l imit is determined bystabili ty, and in a stable aeroplane islocated a few per c ent of wing chord ah eadof the neutral stabili ty point, where e.g.and a.c. coincide. The aft e.g. l imit movesrearwards wi th increas ing tai lp lane s ize,but only at a comparatively shallow ratebecause its effectiveness as a stabiliser isdiminished by wing downwash, typical lyby 50 per cent.I t i s a misconcept ion that tai led aeroplanes always carry tai lp lane downloads .They usual ly do, wi th f laps down and atforward e.g. positions, but with flaps up atthe e.g. aft, tail loads at high lift are

AboveBert Rutan s VariEze set a new fashionin general-aviation design. Below pitchingmoment is reversed in unstable configur tions

Y k-Control Stabilitypower Weight limitlimit

frequent ly posi t ive (up), al though tai l 's maximum l i f t ing capabi l i ty is raapproached.As our tail sizing diagra m sh ows (Figthere is a steep increase in e.g. range wincreasing tail size as the limits mapart . In the example i l lus t rated, a 40 cent increase in tail size doubles the range from 10 per cent to 20 per centwing chord. Also the e.g. range is locwel l wi thin the wing chord, so that t r imming load on the tai l (and t r im dris small.Put t ing a foreplane the same dis taahead of the wing as the tai lp lane behind leaves the forward e.g. lunchanged, but the surface, now cleawing downwash, becomes an efficdestabi l iser . If the aeroplane is to remstable, then the aft e.g. l imit must mforward.Both e.g. l imits move forward wincreasing foreplane size, the forwcontrol-power l imit moving at a s l iggreater rate propo rt ional to foreplane ing capability. To achieve a usable range in a s table aeroplane, therefforeplane design must emphamaximum lift , for control power, wreducing its destabilising effect minimising lift curve slope (the ratechange of l ift with incidence).Our foreplane sizing diagram suggthat a much larger foreplane than tplane is required to achieve the same range in a stable aeroplane. A factor of is indicated. Also the practical e.g. rais ahead of the wing leading edge, so the foreplane carr ies a s ignif icant portion of total l ift . Wing flaps canbe t r immed without a large increaseforeplane sizea further 50 per cenindicated.An ingenious solut ion to f lap t r immis employed by Beech on i ts StarshiWith wing f laps extended, thei r Fowaction moves wing a.c. aft , allowinmore destabi l is ing canard to be used.lift-curve slope decreases with swT International, 23 February 1985

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Germany s TKF90 tested the unstable canardconfigurationBeech is able to sweep the canard forward(maxim um des tabi l is ing effect ) to t r im theflaps, and back (minimum destabi l is ingeffect) for cruise. At the same time, as thecanard sweeps back, i t s moment armshortens , reducing i ts control power insympathy with f lap ret ract ion.I t i s imp orta nt to be r ight f i rs t t imewith a canard configuration. If the c.g./a.c.relat ionship proves to be wrong,increasing the size of the foreplane willnot get the designer out of trouble, as alarger tail willthe wing has to move atthe same t ime .Foreplane s ta l l

The foreplane must be made to s tal lbefore the wing to avoid severe pitch-up.Th is can be achieved in a num ber of ways;by careful choice of wing and foreplaneaerofoil sections and decalage (relativeincidence), or by stall delaying devices onthe wing, such as s t rakes inboard andcamber outboard.A foreplane does have advantages forthe s table aeroplane, as i t contr ibutessubstantially to total l ift . Wing flaps maybe unnecessary, leading to a s impler ,l ighter wing. The increased lift ing arearesults in a smaller total wing plus foreplane area th an w ing plus tai lp lane, wi th asaving in profile drag and weight, and in amore compact layout .Induced drag (drag due to lift) is greater,however, because of the high spa n loadingof the foreplane and the fact that the wingbehind it has to climb in the foreplanedownwash, so that i ts l ift vector is inclinedrearwards . In pract ice, th is in terferencebetween foreplane and wing overturnsempirical calculations which suggest lowerinduced drag when lift is shared betweentwo surfaces . The s i tuat ion is made worseif the canard configuration is scaled byreducing wing area to maintain the sametotal lift.This penal ty is not as serious as i tsounds , however, because induced drag is

TAIL/FOREPLANE SIZING DIAGRAMSNATURAL STABILITY ARTIFICIAL STABILITY

'rw L,

e.g.Forward limits

Aft limit(high lift, flaps u

e.g.range

30 40 0 10Position on wing cord ( )

Forward limits 10 e.g. range r ^

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