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Aviation History
1969
1969 - 2478.PDF
122 AIR TRANSPORT FLIGHT International. 24 July 1969 CCJ Undershoots and heavy landings PART 2 IN THE LAST ARTICLE for this column (Flight for July 10, page 44) I argued that angle-of-attack should be used to identify excursions from desired glidepath and to minimise those excursions by initiating throttle movement for recovery of airspeed. Now all that is old hat, but what was new was the suggestion at the end of picking off the direct-lift control (DLC) increments from the angle-of-attack signal. DLC, as at present conceived, takes its signal from the position of the control column. But where should the control column be for any given recovery demand? Even, assuming that each pilot can work this out according to his unique column-to-girth gap, how fast should the stick movement be made? These become important considerations when one is talking in the special case of the performance margin being at the lowest point in the flight envelope (in fact, of course, it has to be reduced to near zero at touchdown and so is necessarily small at the flare). What I am saying is that, having put in all that work to get a good usable signal from angle-of- attack, why not take the philosophy one stage further and couple the a signal not only to the throttles but also to the direct-lift device? Not only should this provide its near- instantaneous direct-lift effect, but it should do so without substantial change in angle-of-attack—which is a serious problem especially with the second-generation jet and will be more so for the aircraft of the future. Not so simple The central thought here is that throttle opening is an indirect and therefore a slow way of getting lift but, when you've got it, it stays; the DLC, on the other hand, is (as its name implies) a direct way to get lift, but it is necessarily tied to a wash-out signal and so some of the increment soon fades away. The obvious move is therefore to combine the strong points of automatic throttle (slow but lasting lift) and DLC (quick but fading) so that one compensates the other. That no doubt sounds simple enough, but that is about as far as CCJ's first principles will take him. In fact, I know just enought to be aware that it is not all that simple. It has taken BLEU and BEA years to get auto-throttle programmed with the required lead and lag; it took a major NASA study (with Ames, Boeing, Douglas and Lockheed participating) to get the DLC project off the ground; and the basic signal on which I would hang so much, namely angle-of-attack, itself needs balancing out against an inertial device. But what has swung me at least a little way from the crude demand for more concrete as a means of reducing the under shoot problem is not only that there has been a near-revolution in instrument accuracy at all points, but that the method of testing the various signal relationships ha's been made eminently practicable by the introduction of airborne analogue computers such as the TR 48 of Electronic Associates—this now being a main tool at BLEU and elsewhere. So, with good signals and a good method of analysis, I have no doubts that the boffins can carry on from these first principles. But before they rush too far ahead I hasten to add a caution, since not all signals in all aircraft are in fact good. No system can make something out of nothing and whatever combination of angle-of-attack, DLC and inertia emerges, it will break down if one of the inputs is false and this applies especially to the lift margin associated with the DLC. When the device is activated, it raises the spoilers (sym metrically) and so, initially, it adds to drag and subtracts from lift. For the DC-10 the effect on the margin above stall is about 4kt. It retains the power to make up for this later (by lowering the spoilers), but, as emphasised throughout, one is dealing here with comparatively small margins and if, for example, the certification of the aircraft had permitted 8kt between the speed Vs and the 1-g stall (which is much greater than is in fact the case for the DC-10 but is not unknown elsewhere), one would not be able to afford to give away that 4kt of stall margin: to do so might mean, especially with the so-called "slotted" approach where little more than threshold speed is carried all the way down, that the aircraft was exposed for a comparatively long time to a narrower lift margin than it otherwise would be, and so become an easy prey to wind-shear. But, if you don't like the above CCJ divinations, all this matter of the g available at or near the flare (which is, of course, the most critical area for the undershoot), can be checked. Both the ARB and the FA A* (and probably others) have for some years been carrying out extensive automatic recordings, by radar and/or photometry, of day-to-day take- offs and landings and from these programmes precise flight- paths, airspeeds and g-loadings should be ascertainable. The ARB has in fact published some of these results in Civil Aircraft Data Recording Programme: Stall Margins during the Take-off Manoeuvre (Tech. Memo 94), but it has not published the corresponding data on the landing. Now until this type of basic information is available (and I haven't seen the FAA results either), I would not trust any of the performance margins resulting from theoretical signal combinations of angle-of-attack, DLC, inertia, or anything else. It is a case of the old mensonge aeronautique which I dealt with in April 1963. It takes a long time for some signals to be effective but, until that little matter is cleared up, I would aim at improving the undershoot record by reverting (if I could not have more concrete, of course) to the primitive but nevertheless effective advice: "If at any time below 1,000ft your sink-rate exceeds 1,000ft/min, reject the approach imme diately"; that has so far probably done more to prevent undershoots than has any playing around with sophisticated instruments. NOISE THE BIG PROBLEM THE noise generated by supersonic transports is one of the most serious problems facing the air transport industry, according to Mr A. Baltensweiler, executive vice-president of Swissair. He stressed in a recent speech that the degree to which it could be accepted would make or break the SST. Sideline noise in areas adjacent to runways used by SSTs would be very much greater than levels experienced with the jet operations of today, he said. There were three possible ways of alleviating the nuisance in populated areas: the building of special SST airports in less populous areas, the evacuation of villages and zoning of building near to existing airports, and the placing of night-time restrictions on SST operations. The first and last of these measures, he pointed out, would have an adverse effect on the economics of SST operation, while the second might prove difficult to achieve for political, legal, or financial reasons. *FAA Aircraft Development Service tests at Philadelphia and O'Hare in April and May 1968.
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