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Aviation History
1964
1964 - 0019.PDF
14 FLIGHT International, 2 January 1964 Fig 3 One of the largest examples of the "T-tail" yet built is that of the Lockheed C-I4IA StarLifter military transport, which rises almost 40ft from the ground THE HIGH-SPEED SHAPE... change of incidence; i.e., the tailplane contribution to stability is increased, and the aircraft can go to a higher incidence before over- all stability is lost. However, for the case of a tail position above the wing wake, an increase in incidence will give a reduction in the stability contribu- tion of the tailplane, because the latter will be moving down into the intense downwash field. This will aggravate the pitch-up problem unless the tailplane is either (a) still above the wing wake or (b) well through it and out the other side, when wing flow- separation occurs. This state of affairs does not apply to a low-set tailplane, which can be expected to achieve a greater stability con- tribution as incidence increases by virtue of its movement down- ward and away from the severe downwash region. Thus, making due allowance for the possibility of a reduced moment arm, a low- set tail is generally preferable to a high- or mid-set one as far as pitch-up is concerned. The chosen aircraft layout may not allow the use of a low tail, but in many cases where this is so (e.g., transport aircraft) the prob- lem is in any case less severe because of the reduced downwash field obtained with a wing of relatively high aspect ratio. Further, judicious use of dihedral or anhedral can give a substantial im- provement in the stability contribution of a tailplane where the attachment point has already been fixed by layout requirements. Thus, for a mid-set tail position, dihedral will help keep the tail- plane above most of the downwash from a high-aspect-ratio wing, while anhedral will take it through the intense downwash from a low-aspect-ratio wing at a lower incidence (Fig 4). It is important to note that the way the downwash over the tail varies with incidence determines not only the increased effectiveness which can be obtained by a change of position, but also the extent to which this increased effectiveness can be utilized. An increased stability margin under less severe flight conditions may be undesirable from a control standpoint, because of the ex- change rate between stability and manoeuvrability. It is also true that buffeting from the separated-flow region at the tip may prevent high incidences from being used, even though the tailplane position is such that pitch-up has not yet occurred; in this case the increased range of incidences for stable flight provides a very desirable safety margin but does not extend the flight envelope of the aircraft. Bearing in mind these various qualifications to the magnitude of the instability, it can be said that in the general case the onset of pitch-up on a swept wing sets an upper limit to the incidence (and therefore the CL) which can be used at a given speed. This limit is less than the lift theoretically obtainable—and also usually less than that required for reasonable operation, particularly at low speeds. In the latter case the net effect is that the take-off and approach speeds used by a swept-wing aircraft are appreciably higher than they would be otherwise. At high speed, the onset of pitch-up severely restricts manoeuvrability and altitude. In view of these effects, there is often a strong case for modifying a basic swept-wing design so that its pitch-up characteristics are improved—even if its other characteristics are compromised in the process. Exceptions to this general rule result mainly from the presence of other limitations on usable incidence and CL. Apart from the related case of severe buffeting preceding the actual pitch- up, such limitations can arise from (a) pilot's view, (b) ground clearance of rear fuselage, (c) the lateral instability of swept wings at high incidence, and (d) speed instability when approaches are made at less than VMD. In most cases pitch-up is the dominant problem, and hence much of the research work on swept wings carried out since 1945—millions of man-hours in all—has been devoted to the alleviation of pitch-up and the related phenomena described previously. Early attempts to improve actual aircraft were necessarily of a rather hit-or-miss nature, because so little was known of the mechanisms of tip flow breakdown. Extensive wind-tunnel and flight testing over the years has gradually resulted in a fairly com- prehensive picture of the way the flow pattern over a swept wing varies with Mach number, incidence and section. Consequently, the suppression or control of tip flow separation has in recent years become amenable to treatment in a more logical (if still largely empirical) 'manner; in this connection the work of Newby of RAE Farnborough is particularly noteworthy. A great advance was made when it was realized that, at subsonic speeds, flow separation from the surface of a wing of low t/c ratio is fundamentally different from that of a thicker wing. In the case of a thicker wing used for high-subsonic flight (e.g., the 11 per cent t/c wing of the F-86 Sabre), boundary-layer separation is turbulent, and occurs first at the trailing edge near the tip. Since a swept wing tends to carry a substantially increased load over the outboard part of the span, the low-energy boundary layer has to travel against a much steeper chordwise pressure-gradient in this region than it would do on a corresponding straight wing. Moreover, the boundary-layer thickness is aggravated by the span- wise flow induced within it by the spanwise pressure-gradient which exists on a swept wing (Fig 5). Unlike the relatively fast free-stream air passing over the wing, the slow-moving boundary layer acquires a substantial spanwise velocity component which increases with distance outboard (Fig 6). The adverse effects of spanwise flow were a factor in the choice of transverse-tip ailerons for the Lightning. The spanwise flow and boundary-layer thickness increase with incidence, until the flow separates from the wing surface. Since the onset of this separation is largely a function of inci- dence, a beneficial effect can be obtained by giving the outer wing a built-in downward twist relative to the rest of the wing (wash-out). In this way the effective tip incidence is reduced, and separation is delayed to a higher overall CL. AS will be described later in con- nection with shock-induced separation, a high-subsonic wing may Fig 4 One of the most remarkable combinations of exceptionally high flight performance with good handling qualities has been achie- ved by the McDonnell F-4 Phantom series. These Mach 2.6 multi- mission aircraft have sweep, kinks, dog teeth, droop, blown flaps, dihedral on the outer wings and anhedral on the tailplane
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