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Aviation History
1951
1951 - 0835.PDF
524 FLIGHT SPEED IN GLIDING Some Suggested Methods for use in Planning Cross-country Flights By J. C. NEILAN IN Fig. i is a pictorial representation of the method bywhich the majority of glider cross-country flights aremades using convection currents (hereafter referred to as "thermals") for regaining height between periods of cruising descent. In the old days, a glider was thought to have two main speeds, a soaring speed and a gliding speed. The soaring speed was that at which its sinking speed was least, and the gliding speed, usually about 3 m.p.h. faster, was that at which the flattest gliding angle in still air was achieved. Fig. 2 shows a typical form of glider performance curve, with sinking speed plotted vertically against forward speed horizontally. The sinking speed can be seen to be a minimum at X, and the speed for the flattest glide, shown at Y, was found by drawing a tangent to the curve through the origin of the diagram. The best speed at which to glide against a head-wind could similarly be found by drawing a tangent to the curve from a point along the horizontal axis equal to the wind strength. This is seen at Z, at which speed the maximum possible distance from any given height would be flown against the wind. In the days when flights were all made down-wind, so as to get the most help from it, speeds X and Y were all that mattered. Now, however, the performance of sailplanes has been improved to give a flatter curve, with smaller rates of descent at higher speeds than hitherto; and it is how best to use these higher cruising speeds that matters most to a pilot making competitive cross-country flights. With higher cruising speeds longer flights can be made in the limited time during which convective lift is available, or, alternatively, they make possible flights against or across the wind which were impossible with the slower machines. Referring to Fig. I, if section BC is flown at a speed higher than Y, the flattest gliding angle, then (i) the time taken will be less, but (ii) the point C will be at a lower altitude, and the subse- quent climb CD to regain height will take longer. If, however, the thermal CD is a strong one, there will still have been an overall saving of time between points B and D, and a higher average air-speed will have been achieved. What it amounts to is that for any particular sailplane there is a certain optimum cruising speed for any particular thermal strength. The thermals are flown at the minimum sinking speed, as usual, but the cruising descents between them are flown at a speed chosen in relation to the average rate of climb, thereby achieving the maximum average air-speed, and therefore the maximum ground-speed, while iaining the same average cruising height. Fig. 3 Sis simply Fig. 2 with the vertical scale extended upwards for rate of climb. By drawing a tangent to the curve from any particular rate of climb, the optimum cruising speed for that rate can be found, and the point where that tangent cuts the horizontal axis of the diagram gives the average air-speed which will be obtained under those conditions. If a higher cruising speed is used, the extra height lost in the cruise will not be regained within the time saved. If a lower cruising speed is chosen, the other fellow will be up at D while you are still somewhere between C and D. In other words, you have not made the best average air-speed or ground-speed. . It might be thought that where there is a head-wind a higher air-speed should be used, and as far as headway over the ground in a straight glide is concerned that may be so; but if the object -»- FORWARD SPEED Fig. 2. Performancejcurve, sinking speed against forward speed. is to continue the flight by the use of more thermals, maintaining an average cruising altitude, it will not be so. The figure for average air-speed can also be regarded as the maximum wind- speed against which any ultimate progress can be made while maintaining an average cruising height. If the wind is stronger than that figure, a positive average ground-speed in an up-wind direction can be maintained only at a higher cruising air-speed, which will either have to be balanced by stronger thermals or by a loss of average cruising height. Normally the only occasions when the optimum cruising speed can be exceeded with impunity are when there is an immediate geographical goal which can be reached in a straight glide from the current cruising height, or if the next lot of lift is capable of being used without having to circle in it. For instance, it might be a cloud street, along which one could cruise in the direction of the flight without loss of height or even with a gain of height; or there might be hill lift or standing-wave lift in which one doe& not drift down-wind while keeping in the lift area. For easy use in the air, it is better to tab- ulate the data obtained from Fig. 3 as follows : Average rate of climb Cruise Average air-speed (ft/nun)IOO 150 200 250 300 350 400 450 500 (m.p.h.)43 4547 4951 5355 5862 (m.p.h.)18 22 2528 30 3234 35i37 Fig. I. Normal method of using thermal currents for cross-country flights. If cross-wind nights are contemplated, it may be as well to get a rough idea beforehand what course will have to be flown
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