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Aviation History
1964
1964 - 1687.PDF
926 FUGHT International, 4 June (964 AIR COMMERCE... V, V2 AND ALL THAT Part 4 of C.C.J's "V" Classifications HERE we publish Part 4* in C.C.J.'s series of articles on speed terminology. The series should prove valuable in filling a gap in the field of current aviation literature; we know of no single reference which deals with this subject authoritatively and comprehensively and which at the same time could be described as a textbook suitable for introducing the jet pilot to the performance concepts behind the techniques recommended to him. C.C.J. asks us to say that for this reason—i.e., the lack of references—there may be shortcomings; he will accordingly welcome readers' criticisms and incorporate any necessary amendments in a concise "summary of definitions" with which he intends to conclude the series, and which it is hoped will be useful to airline pilots for some years to come. (4) The Decision Speed (V^ continuedI N the March 5 issue I dealt with the case of a fixed Vj and balanced field lengths, and the article ended in anticipation that the next of the series would be on the variable case. However, I had a lot of trouble with the variable Vx (as the reader might also by the end of this article!) and lost time doing a rewrite or two. To maintain the continuity of the series, part 5 (Vr, Vmu, Viot, V.>min) was therefore printed ahead of part 4. This had the disadvantage of disturbing the upward climb of the speed values but perhaps it has yielded an advantage in that the relationship between V\ and the above group of speeds has been illustrated in its simplest form—i.e., with only one value of Vi to consider. It is important to remember that this relationship is exactly the same in the case of the variable Vl and the unbalanced field length. But, before dealing with the "unbalanced" case, there are one or two other considerations which apply generally to Vi. Since, on losing an engine just after \\, it is obviously undesirable to have to use brakes or to throttle back an engine so as to maintain a straight take- off path, Vj must never be less than the minimum Control speed oh the ground (Vmcc). And since, having rotated the aircraft, it would be undesirable on losing an engine to slap the nosewheel down again and apply brakes and reverse thrust, Vt must not exceed the rotation speed (Vr). The complete picture of the fixed V, together with the speeds and distances related to it is given at A in the diagram and this should be taken as basic to the remaining figures and descriptions. In these figures the relative movements of V,, Vr and Viof are given in the correct sense; the amount of the movement is, however, purely arbitrary. Throughout all the cases described the governing influence on these movements is that pulling back V, speed means pulling back V! distance, which in turn means that, in the continued take-off case, a greater three-engine time will be spent accelerating on the runway from Vj to Vr and Viof. So, as Vt moves back, Vr and Viof move forward. Proceeding now to unbalanced field-lengths and variable Va, I must confess that in applying either the UK or the US method of calculation I find the procedures difficult and should very much have liked to skip all references to them—just as most airlines apparently do for the purposes of present-day operations! Unfor- tunately from this point of view, the unbalancing process can be made to add appreciably to payload or to stopping margin and these gains tend to get bigger as speeds become higher; probably the SST will be operated throughout the world on unbalanced field lengths and variable Vj. This being so, I will try to deal at least with the principles involved; if anybody wants to go more authoritatively into the matter, I can only suggest that he wades through the ARB publication Specimen Performance Charts and the April 1963 issue of The Boeing Airliner. I would say quite frankly that I do not know a pilot who has completed this exercise; indeed, a friend who until recently was chief training pilot of one of the largest jet fleets remarked to me: "The procedures do not seem to be practical for the pilot in the field"—and I agree with him. However, a man at a desk at a fixed airport, dealing with only one type of aircraft which can operate from only one or two of the runways and which will always take-off within a comparatively narrow weight range, can use many short cuts through the labyrinthine curves and this is the picture I have of him as he "unbalances the field-length" and resorts to a variable V!. As a means of illustration, the simplest way in which the field- length could be unbalanced would be to remove weight from the aircraft. The effect on climb would not normally correspond exactly to the effect on stopping distance and the result is therefore an unbalanced effect as shown in case B. A further result of remov- ing weight is, of course, that (assuming the same runway length as in A) the take-off" is no longer field-length limited. When the runway length is not critical there are two possibilities with regard to Vj. By the first method Vt could be left as in case A but would, by reason of the weight reduction, be attained a little earlier in the take-off run, making the emergency distance available a little longer; on the other hand, for the continued take-off, the aircraft (as shown in case B) would be appreciably higher than the prescribed 35ftf by the end of the "take-off distance available" (which is here the same as the runway). This represents the sort of solution which is useful if noise abatement take-off procedures are dominant. By the second method the reduction in weight could be used to pull back V, and this would pull back the V! point very much more, yielding appreciably more stopping distance but providing only the minimum screen height of 35ft at the end of the runway. This is case C and represents the sort of solution which is useful if the runway is slippery and it is desired to provide maximum stopping distance. Between case B and case C is a range of Vx speeds and, on the basis of traditional performance codes, the choice of any one of them is legal. An alternative method of unbalancing the field-length—and one usually more popular with the man on the ground in that he can add weight rather than take it away—is to stick on a length of sea (or other obstruction-free area) at the far end of the concrete and call it a "clearway." This immediately "unbalances" the field-length in that, from the point of view of climb-out, the aircraft can afford to take up a much longer distance before attaining the required 35ft. In the technical jargon of case D in the diagram, the take-off distance available goes up and with it the possibility of increasing the weight. It is, however, also necessary to ensure that the weight does not go up so much that the wheels are still on the ground at the end of the concrete—hence the seemingly naive directives: "the take-off run should not exceed the length of the runway" (ICAO Doc. 8283, pg. 46, PAMC 2.3.1.) or a point halfway between Viof and the 35ft height point, as shown at A in case E (ICAO Doc. 8283 pg, 35, PAMC 6.5.2(a)). Within these limitations, weight may be increased with increasing clearway so long as the accelerate-stop distance can be increased to absorb the additional momentum associated with the greater weight. This is effected by pulling back Vj and with it, the Vt point. In case D, where the clearway '•> assumed to be long compared with the runway, the pulling back of Vj so that no surplus height margin above 35ft is available for the continued take-off case has meant that the stopping requirement is in fact more than satisfied. But, if the clearway is limited, ?* >n case E, Viof and Vr must take place earlier along the runway an<< we concluded on page 931 * Part 5 was intentionally published before Part 4—see national" April 16, 1964. t Referred to in the figures as the "screen height." t For new concepts, see case F. "Flight Inter-
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