NAVIGATIONAL ASPECTS by J. L. Mitchell Assistant Air Attache, British Embassy Washington, D. C. The advance of the helicopter to its present-day technical, achievements has interesting navigational implications, because of the widening of the speed range, particularly at its lower limits. Most modern airspeed indicators are able to read in terms of indicated, i.e., "uncorrected," speeds above fifty miles or so an hour. Owing to the flat nature of the logarithmic curve of the pitot pressure, speeds below this figure are invariably portrayed inaccurately and erratically. With the ability of a helicopter to hover, and of course, to drift sideways and backwards, it is necessary to produce an airspeed instrument reading to zero, and perhaps negatively or backwards, with a high degree of accuracy. It is suggested that some form of anemometer cups, as employed in a wind gauge, might be feasible, as these could record the slightest movement in any lateral direction. Forward speed would then be measured as a true vector of any movement on the positive scale, instead of as the product of the sine of the drift angle; and any backward motion could be measured in the same way on a negative scale. Another disadvantage of the pitot head airspeed system would be the uneven airflow caused by the rotor blades. The movement of these blades is likely to produce a vertical airflow, which, in combination with varying horizontal speeds of the helicopter, is likely to give most erratic results to both the pressure and the static lines. Probably above fifty miles an hour the anemometer cups would suffer damage from high rotational speeds and would have to be clamped or locked at an arbitrary speed from which the more conventional piror head would take over. By our present ideas, the radio altimeter should be sufficiently sensitive to provide accurate readings of true height above the terrain, to the nearest five feet or so. Such accuracy would be particularly desirable in descents through fog or mist on to a requisite spot. The conventional aneroid altimeter would be too insensitive to small changes of altitude and its lag might even prove dangerous. In conjunction with an accurate altimeter, we should surely like to have a climb and dive indicator, or rate of climb indicator, as it is known. Once again the aneroid principle introduces insensitivity and time lag, both dangerous in an aircraft designed to move as freely in a vertical direction as it is in the lateral. It is possible that an electronic device might be perfected, working off the radio altimeter, to show rate of descent or ascent, instead of actual distances descended or ascended. So much for our aptitude to record the speed of our movements in the vertical and horizontal planes. What of direction? The directional gyro will remain the mainstay of our blind flying instrument panel, along with its cousin the artificial gyro horizon. The magnetic compass, however, still remains the datum for our directional settings. Large aircraft with overworked crews can defy the old turnIng errors with impunity, as in other than extremely light aircraft the weight justifies the installation of a distance reading or remote reading compass. Both the gyrosyn and the fluxgate types of compass have no turning errors, but it is likely that until the large helicopter makes its successful appearance, the driver will have to endure the antics of a simple unadulterated magnetic compass. We know we can cope with the perambulations on northerly and ,southerly courses, and with acceleration errors, too, no doubt. But now we can have reversed acceleration errors by the backward movement of our flying machine. At this point our critics will allege that if we have weight to spare for a radio altimeter, surely we can fit a gyromagnetic compass. We must, of course, concede this point. But the antics of the pure magnetic compass when our aircraft is going astern will, to say the least, be most interesting! What about a drift sight? The gyro stabilized graticule represents the latest advance, so we must surely fit one of these; but a ninety degree field to left and right will be needed, for drift angles at slow speeds are likely to be large. This may be sufficient for the recording instruments. As for the determination of actual position, radar has its part to play here. The aircraft could be fitted no doubt with GEE, LORAN and H2S, to name but three electronic devices which are capable of giving precise results. The necessity for accuracy need hardly be stressed, for in whatever role the helicopter is to be used, its variable flight characteristics clearly requires the pilot to define exactly the spot on which he is to alight. Owing to the comparatively short range of flights likely to be undertaken, it is doubtful whether celestial navigation will have more than academic interest, The sextant operator would be able to instruct the pilot to hover while shots of the stars were being taken, in order to eliminate acceleration errors of the bubble. With the slow speed of the helicopter he could spend adequate leisure-time in solving the celestial triangle, rather like the nautic who can spend from dawn to dusk computing his line of position. We omit to add here that the sai!or probably ignores it and decides that his dead reckoning is more likely to be correct. Finally, we might include a weathervane, a Swiss cottage, and a piece of seaweed for meteorological observations ; and a calendar for the use of the pilot. No doubt the helicopter experts will debunk much of this article as academic theory; but even if the author is proved wrong on every count, as he expects to be, he will have achieved one important objective: that of calling attention to undiscussed navigational aspects of the helicopter.