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Aviation History
1964
1964 - 0018.PDF
RIGHT International, 2 January 1964 13 The High-speed Shape PITCH-UP —AND PALLIATIVES ADOPTED ON SWEPT-WING AIRCRAFT By K. G. Hecks (Hawker Siddeley Aviation Ltd) THE widespread adoption of the swept-back wing for high-speed aircraft during the past 20 years achieved a partialsolution to the problems of drag and buffet due to air compressibility at high subsonic speeds. At the same time it brought with it a formidable array of problems of its own, both aerodynamic and structural. Some of these have carried over into the supersonic age, in cases where wing sweep has shown itself to have substantial advantages beyond Mach 1. One such problem is a form of longi- tudinal instability, known as pitch-up, which frequently limits flight performance. An aircraft is stable longitudinally (i.e., in the pitching plane) if the net effect of the aerodynamic forces and moments produced on it by an inadvertent small change of incidence is such that the air- craft tends to return to its original incidence. Fig 1 shows a typical pitching-moment : incidence curve for an aircraft which is stable over the range of incidences shown. If the aircraft is trimmed to fly at the CL corresponding to point A, then a small increase in incidence (to B, say) will give an untrimmed nose-down moment (CM negative) which will make the aircraft revert to its original incidence. For stability the slope of the line in Fig 1 must always be nega- tive, and if the required stability level is to be maintained through- 'PITCHING MOMENT ,COEFFICIENT] CHOSEN FLIGHTCONDITION INCIDENCE ORLIFT COEFFICIENT PITCHING MOMENT INCREMENT WHICH MUST BE PROVIDED BY THE TAILPLANE FOR TRIM STABILITY REDUCED BUT STILL POSITIVE NEUTRAL POINT (COMMENCEMENT OF PITCH-UP) Fig / Generalized curve of pitching moment against wing angle of attack (incidence) or lift coefficient . out the usable incidence range the pitching-moment variation with incidence must be approximately linear. The latter requirement involves keeping the position of the overall centre of lift of the wing approximately constant under all flight conditions (except perhaps those with very low incidences when wing lift is small). Further, to reduce trim drag, and to keep trimming and control require- ments within the capabilities of a tailplane of reasonable size, this centre of lift position must be close to the aircraft e.g. As far as a swept wing is concerned this means that the wing position along the fuselage must be such that lift achieved in the wing-root region (well forward of the e.g.) must be balanced by lift at the wing-tip region (well aft of the e.g.), and for stability this balance must be closely maintained at all times. Unfortunately an inherent characteristic of a swept wing is that under certain condi- tions, which may well fall within the desired flight envelope of the aircraft, the airflow separates from the upper surface at or near the tip. The resulting loss of lift, appearing as a reduction in lift-curve slope, is of little consequence in itself since it can readily be restored by a slight increase in incidence. But the position of the lift decre- ment is of great importance, for the longitudinal lift balance is up- set by the lift aft of the e.g. being considerably less than that for- ward of the e.g. At incidences where this first occurs stability may be reduced but not lost; the load increment at the tailplane may still be sufficient to restore the balance and maintain stable flight. As the incidence is raised, however, the separated-flow region increases in extent by spreading inboard and eventually the nose-up moment due to this tip loss becomes too great for the tailplane. The aircraft then undergoes a rapid, uncontrolled nose-up movement which constitutes the phenomenon of pitch-up. The divergence feeds upon itself, and the separation swiftly spreads right in to the wing root—at which time stability may return. If stability does not return of its accord considerable difficulty may be met in unstalling the wing. In the case of the prototype BAC One-Eleven, a stall with aft e.g. and flap slightly deflected led to pitch-up developing into a stable "super-stall" with very high angle of attack. In this condition the servo-tab elevator had insufficient power to get the nose down and unstall the wing. Several other undesirable phenomena may take place as a result of separation at the tip, ambng them buffeting, loss of aileron effi- ciency, and even aileron reversal. The latter effect is an aerodyna- mic one, and is essentially different from the familiar case of aileron reversal due to aeroelastic distortion of the wing. Also, if the tip loss is asymmetric (occurring on one wing before the other) the air- craft will experience wing dropping or "roll off," possibly in com- bination with an uncontrolled yawing manoeuvre. This was a characteristic of the F-86 Sabre, one of the first swept aircraft. The degree to which these effects limit the performance of a wing de- pends largely on the planform. Increasing the sweep distorts the spanwise load distribution in that more and more lift is carried by the outboard region (Fig 2). Therefore, since the onset of flow-separation is strongly dependent Fig 2 Outward distor- tion of the lift distribution on a swept wing: broken line, curve for unswept wing; full line, curve for 45° sweep, no twist, aspect ratio 3.0 on the local aerodynamic loading, a highly swept wing will tend to be prone to early tip stalling. Moreover, the higher the sweep- angle, the more critical will be the effects of such tip-stalling. This is partly because the lift loss will be greater, and partly because the region of lift loss will be further behind the e.g. The effects of taper are rather more difficult to pin down. The local CL at the tip will tend to increase with taper (as with sweep), but because the chord (c) is reduced the span loading (CL X c) will probably not alter much. For the particular case of leading-edge separation, the reduced area at the tip of a highly tapered wing may substantially improve the pitch-up characteristics. A different planform effect arises from the fact that the local chordwise centre of lift, normally at about 25 per cent chord in subsonic flow, shifts rearwards to near mid-chord when the airflow separates. If the aspect ratio is sufficiently low, this rearward shift of the centre of lift in the stalled region may, as far as pitching moment is concerned, compensate for the reduction in magnitude of the tip lift. However, the low lift-curve slope and high induced drag associated with such a low aspect ratio makes the elimination of pitch-up in this way generally impractical. After planform geometry the most important factor to be con- sidered is tailplane position, since the stability contribution of the latter must be suitably matched to the wing/fuselage characteristics to give the required overall stability level. The present trend to- wards low-set tailplanes or, in some cases, high "T-tails" (Fig 3), is an attempt to get the tailplane as far as possible clear of the intense downwash field which exists behind the wing at high incidence. This increases the incremental load produced at the tailplane by a
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