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Aviation History
1944
1944 - 1811.PDF
AUGUST 3IST, I944 FLIGHT 239 moulded vanes, and the other pair angular moulded vanes. In both %ases 18 vanes were applied on each side of the tyre. Tyre speeds were obtained by an electric tachometer installation, and test flights were made at constant airspeeds to compare the efficiency of radial and angular vanes, it being found that, while the angular vanes produced quicker acceleration, the radial vaned tyres reached higher terminal speeds. Further tests were then run with the* airspeed held con stant at 150,. m.p.h. for one minute and then the airspeed was reduced, simulating a landing approach. At the end of the one-minute constant- speed run, the radial vanes again showed their superiority in speed by running 43 per cent, faster than the angular vanes, but on Vne decreasing I.A.S. part of the run the angular vaned tyres showed much less decrease in speed. It was noted that on both the radial and angular vanes, the vane at the lowest part of the wheel was completely closed. The vanes immediately ahead and behind this one were 100 per cent, open, and the next two vanes adjacent were partially open. This condition is no doubt caused by tur bulence set up by the front vane, thus causing the next vane behind it not to open. Consequently, it was believed that every alternate vane might be eliminated, and the same effectiveness obtained. Therefore, further constant I.A.S. tests were made with the angular vanes having one-half of the width of every alternate vane cut away, and also with these half-vanes removed completely, leaving nine full-size vanes. As a result of these preliminary tests, it was decided to run normal landing tests, using 18 radial vanes, 18 angular veins, and nine angular vanes respectively. No vibration trouble was experienced throughout the tests,- and it should be noted that these tyres were statically balanced within 30 oz.-in. as compared to the usual limit of 52. Cemented and Moulded Vanes Kegarding the very important matter of weight increase, it should be noted that the g-vane construction increases the tyre weight by approximately two pounds, which is less than the difference often found between two tyres of _^;he same make. Two sets of 19.00 x 23 tyres were also tested by Eastern Air Lines on a C-46 aircraft, one set being equipped with radial cemented vanes and the other with radial moulded vanes. Constant-speed flight tests were run for four dif ferent indicated airspeeds and a graph was prepared show ing the peripheral speeds developed by the tyres with moulded vanes. In comparing the terminal speeds reached, The present production tyre fitted with 9 angular vanes on each side. The increase in tyre weight is only 2 lb. TIME IN ko SECONDS Solution for wheel acceleration/time, n is assumed constant=.55. Tyre rolling radius assumed constant—44* tyre. it was interesting to note that the tyre speed differentials exceeded the I.A.S. differentials, indicating in creased vane efficiency for the higher airspeeds. For an airspeed of 100 m.p.h., there would apparently be no tyre rotation at all, which would seem to indicate the existence of appreciable frictional resistance, possibly due to brake drag. Landings were also made on both sets of tyres, and it was observed that in both cases, to achieve a minimum of 50 per cent, pre-rotation, it is necessary to lower the landing gear at an I.A.S. of not less than 140 m.p.h. For 150 m.p.h. the moulded vanes developed 58.7 per cent, pre-rotation and the cemented vanes 60.8 per cent. This is believed to be due to the greater flexibility of the cemented vanes, these tyres having hjrtl approximately 200 hours of service before the test. Jf^as decided to determine if any vibration would be noticed at an increased wheel speed. Therefore the landing gear was lowered in level flight at 150 m.p.h. and the airspeed in creased to 160. As the wheel reached a tyre peripheral speed of 87 m.p.h. slight vibration was noted; when the speed reached 98 m.p.h. the vibration was excessive, and brakes were applied to stop the wheel rotation. This vibra tion was no doubt caused by wheel assembly unbalance, as no precautions had been taken to balance them. However, these tyre speeds were in excess of pre-rotational speeds that would be developed during normal landings, and it was, therefore, the general opinion that no trouble should be encountered in normal service operations , Flight tests supplementing wind tunnel tests were also made on 17.00 x 16 tyres with vanes set at a reverse angle to that now generally used. For a landing made with wheels extended at 145 I.A.S., the pre-rotation was only 15 per cent. Comparing this with the 80 per cent, pre- rotation obtained with vanes set at the angle now adopted, it is interesting to speculate upon the cause of this wide difference. With the vanes set at the adopted angle they are normal to the direction of airflow in the trailing lower quadrant of the tyre, whereas the vanes set at the reverse angle are normal to the airflow in the leading quadrant. The explanation of the greater effectiveness of the air- stream at the trailing half of the tyre might be found from a study of the airflow pattern around the tyre. Possibly also the Magnus effect (the flow distortion caused by a rotatin^'body in an air stream) may be involved. It'lsobvious that there are many variables affecting the performance of vaned tyres, and for optimum performance, the vane design for any given size of tyre should be based on the airflow characteristics of the aircraft on which that tyre will be used. However, it is hoped that by deter mining the general principles of vane performance, we can establish a uniform design that will give generally satis factory results. Ultimately it will be necessary for the aircraft designer to be familiar with these various factors, so that he may provide the same advantageous flow of air around the tyres that he provides for the wings and control surfaces. Liberator Landing Tests The results of recent tests by Consolidated-Vultee Air craft Corp. on 56m. vaned tyres fitted to a B-24-D aircraft revealed that for steady flight condition, with landing gear extended at 120 m.p.h., the tyre pre-rotation was 38 per cent.; and at 187 m.p.h. it was^.35 per cent. With the landing gear retracted, due to ttfe fact that one side of the tyre was in the airstream on the lower surface of the wing, the pre-rotation was only 7 m.p.h. slower in each case. Quoting from the report, '' There was no vibra tion or buffeting due to tyre rotation during any of the test conditions, and the pilot stated that he could detect no gyroscopic effect during turns. During four landings
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