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Aviation History
1964
1964 - 0120.PDF
;L.:,HT International, 16 January 1964 83 AIR COMMERCE . . . V,, V2 AND ALL THAT SOMB time ago the Editor confided to me that he had always been somewhat vague in his understanding of V2 and Vs but that now Vr, Vrcf, Vnc, Mno, etc, were coming into daily operational use, he was completely confused; no glossary contained the lot and, even if one tracked down an official definition here and there, it did not necessarily supply an answer in terms which the layman, or even the average pilot for that matter, could readily understand. Could I not write out a few definitions in popular form, in the hope that they would catch on and last for at least the next decade or so? I have jibbed at the job for a month or two: I recognize the desirability of some catchy definitions but how can one make an attractive article from V!, VmCg, Vno? Another deterrent is the sheer proliferation and complexity of the terms. I find that over a score are in common use and that many of these have variants according to country or manufacturer; some of the variants are exact equivalents but some contain small though significant differ- ences. Thus the speed between the point where the aircraft leaves the ground and the point of initial power reduction will be referred to, with very nearly unanimity, as take-off safety speed, initial climb-out speed, or as V2, V2min, Vref- The slight nuances between these terms tend to vary according to the individual national code or to the date of the publication in question—the different connota- tions of each emerging only as new specifications evolve to match the new concepts of the designer. No wonder the Editor gets confused. I am. However, with the tacit (though as yet unratified) acceptance of the ICAO PAMC (Provisional Acceptable Means of Compliance) on performance, the industry as a whole seems to be settling for the standard terms of the ICAO Airworthiness Committee. Only when shese definitions seem too drastic (as in the definition of the stall*) does a manufacturer tend to introduce his own version and attempts to talk a certificating authority into accepting it. With this en- couraging trend and with an obvious and growing need for a practical set of definitions, I propose, in the next few articles to have a go at the job. However, before one can even make a start it is necessary, in the interests of arriving at fairly brief definitions, to lay down quite a string of ground rules. First of all, I propose wherever possible to take the ICAO PAMC concepts and definitions as the norm and to treat the others as variants. Secondly I propose, with one or two exceptions (e.g., where it is desirable to keep related speeds close together), to start it the lowest speed and work up. On this basis I will first comment a little on each speed as we go along, then try to reduce it to a practical definition. No Room For Feelings I say "try to" reduce it because I am not sure that one can, in he true academic sense, reduce most of these definitions; many of aem have been born out of fifteen years of controversy and in the ourse of this have acquired numerous subtle but important ouches. For example, the minimum unstick speed (Vmu) used to ^ such that lift-off (Viof) occurred at 1.12 Vmu or above. Another •xeption (5 per cent less margin) was made for aircraft which were difficult to stall on the ground because the tail bumper touched, and - modified definition was therefore introduced by which "geometry 'smited" aeroplanes need demonstrate only that VW is 1.07 times Vmu; finally this proved rather too much of a concession and air- craft taking advantage of this criterion had to conform to another, namely to demonstrate, just after lift-off at Vmu, a minimum hori- zontal acceleration of (\.\ ^EDY By now someone will probably -•<•• trying to introduce a further qualification to that! These are subtle but important touches which cannot be ignored in certifica- tion. This is how the aerodynamicists and the certification authori- ses work. Good I uck to them, but they do not have to fly the aero- P'anes and, at the expense of hurting their feelings, they have to be * See "Flight International," March 7, April 4, April 18, June 13. told that one cannot arrive at a "practical definition" for use by pilots and operators on the basis of all these exceptions and quali- fications. For our present purposes therefore, I propose to ignore all but a few of the exceptional cases. It does mean, however, that a true academic definition is out as far as I am concerned, and if anybody wants all the refinements, he must go to the official manuals. Not that they will always help him; for example, if he goes to our appointed bible, the ICAO PAMC on Performance, and looks for the definition of Vr, he will find that it is "The rotation speed (see 6.2.5)"; and if he goes to 6.2.5, he finds, among other requirements, that Vr must produce a climb-out speed at 35ft of not less than V2min; and if he goes to V2mjn, he finds, among other things, that it has to be at least 1.10 Vmc; and, if he goes to Vmc, he finds, among other things.... In fact, I am not sure that anyone has yet got all the rounds and there seems to me to be more than a chance that the studious researcher who tries to do so could find himself with a 3d Underground ticket, going round the Inner Circle indefinitely: everything is fine so long as he does not want to get anywhere. However, let it not be said that we have no thought for the studious researcher, and for this reason we will supply him with the 3d ticket to start him off—in the form of leading references. A Definition of Style - As a means of reducing the verbiage, I propose to apply what I call a "commonsense" set of conditions in all the definitions. For example, for all speeds applicable to the take-off and accelerate-stop, the power, flap setting, gear position, ground effect, etc, will be those normally associated with the take-off run—it would hardly be commonsense in the take-off case to assume a landing flap setting or a gear up configuration and I don't propose to spell out the obvi- ous. But I mean a little more than simple commonsense: I mean also that all weightings will be reasonably on the adverse side. For example, the CG will always be adverse, the engine failure will apply to the most critical engine and the trim will not be moved during a critical manoeuvre; on the other hand, automatic feather- ing and anti-skid braking may be assumed where they are fitted and proven and, additionally, no actual abuse of settings or controls will be assumed. As certification definitions are on critical values, the concept of "the expected configuration but the moderately adverse (though within range) setting" is, I believe, a "common- sense" line to take and it will simplify the definitions a great deal if throughout we can take this as read. Where, however, the assump- tion might lead to serious error, we will include the "conditions" in more detail. Again, to keep the wording down, I will relate all definitions to the case of a four-engine aircraft; general principles, however, should not be different for threes and twos. A useful convention in BCARs is that all the speeds which the certifying authority normally requires to be displayed in the cockpit (i.e., the "placarded" speeds) areannotated by an asterisk; I propose to keep to this and also, for added clarity, to mark the speeds which are used in design and in certification, but which are rarely needed for operations, with the symbol t- One further editorial convention: I will use only small type for the suffixes; thus Vno, not VNO- In a few cases this may look unusual. For example, I have never seen the lift-off speed written Viof; it is always VLOF ; but there seems no special reason for these peculiari- ties and it would seem sensible to go in for uniformity while we are about it. Finally, all speeds are, unless otherwise stated, in Calibrated Airspeed, that is the airspeed shown by the pilot's ASI, corrected for known position error but not for density or compressibility error. With this clearing of the decks for a cut-and-thrust action the definitions of the principal operating speeds, I propose next week (?) to get down to business. At the end of the series, I will try to satisfy Roger Bacon and give all the recommended definitions, shorn of frills, explanations or qualifications, "on one sheet of paper."
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