Aerodynamic properties of a hemispherical cup with application to the hemispherical cup, windmill and anemometer

Windmills of the hemispherical cup anemometer type have been used on aeroplanes for driving auxiliary apparatus, and it therefore appeared desirable to be able to calculate their performance. To do this it was necessary to know the forces on a cup, and as this data was not available, the present work was set in hand. The lift, drag, and yawing moments of a hemispherical cup have been measured at several values of lv. Hence the characteristic curves for a windmill of this type when used as a means of obtaining power have been deduced. Two fans were tested in the wind channels for comparison with the calculated results. The effect of shielding the half revolution of the cups during which they return against the wind was ascertained, both with the anemometer half shielded by sinking it in the side of a large body, and with a windguard exposed to the wind. For the unshielded windmill the agreement obtained between the experimental torque and thrust and the calculated curves is very close. With a guard an approximate curve has been calculated, which gives good general agreement with the experimental results for the windmill as sunk in the side of a large body. The case with the exposed guard gives considerably larger values of torque and thrust, which effect is shown to be explained by the disturbance in the flow due to the guard. The aerodynamic properties of the cup, though investigated in this connection, are of more general interest and are therefore given in some detail. A note on the Robinson Anemometer is appended

Experiments on balanced control surfaces for rigid airships

The experiments were conducted at the request of the Airship Design Department of the Admiralty in order to obtain data to assist in designing balanced control surfaces for airships of the R.38 class. Pitching moments (about C.G.) were measured on a model of R.33 (see R & M 361) with stabilising surfaces of the same overall dimensions. The balancing area did not, however, extend along the whole length of the control surfaces, but was confined to the outer end of the elevators, loss of length being partially balanced by increased width. Two different balancing areas were used. The effect of cutting off a piece of the fin immediately in front of the balancing area was investigated

Second report on the twisting of propeller blades. Supplementary to R. & M. 454

The method of investigating the twist of propeller blades, which was developed in R. & M. 454, is interpreted mathematically by making a certain assumption as to the shape of the cross-sections. A general equation expressing the twist as a function of the radius is obtained, and an experimental method of solving it is evolved. It is shown that blades of certain shapes may be peculiarly liable to torsional vibration, and that a plan form.common in current practice possesses this property to an appreciable degree. It is further shown that the maximum stress due to torsion may determine fracture in this case. A method of calculating the shape of plan form in any given case, in order that the blade shall not twist, is deduced, and it is shown that this leads to a nearly symmetrical form in one instance. The effect of the large torsional hysteresis of timber in damping out vibrations is discussed, and it is suggested that herein may lie the reason for the comparative failure o5 metal propellers up to the present. Finally, suggestions are made for the modification of current practice in accordance with the indications of the present theory

The design of a sensitive yawmeter

The investigation was undertaken in response to a request from the Technical Department of the Air Board for information as to the most sensitive form of yawmeter, and for a calibration curve for such an instrument. The original form of yawmeter suggested by Mr. (now Sir) Horace Darwin was used by Mr. E. T. Busk in his experiments in 1912 (see Rpt. 1912-13, p. 254) and a yawmeter on the same principle has been used in several wind channel investigations at the N.P.L. and has been described in R. and M. 156 and 371. A direct reading instrument of this type was described by Sir Horace Darwin in his Wilbur Wright Lecture of 1913. The variation of pressure with angle of inclination to the wind was determined on several sizes of pitot tubes, and from this curve it was predicted that the best angle between the axes of the two tubes of the yawmeter would be 120 deg. The sensitivity was found experimentally to be about 1.7 times as great as for the original form in which the angle was 90 deg. Various forms of yawmeter were tested until one was found which gave a result which could have been predicted from the experiment with the single pitot tube giving greatest sensitivity. The experiments indicate that, in plan view, the arms of the yawmeter should be straight and bevelled to a sharp edge at the end. The embraced angle should be 120 deg and the tube should not be of very small diameter. A tube of 0".30 internal diameter was found to be satisfactory, and a calibration curve for this instrument is given in the report. The instrument is capable of measuring angles with considerable accuracy, and can be used on aircraft or in the wind channel. If measurements are required in one plane only, they can be made very simply by turning the yawmeter till the pressure difference is zero, and reacting off the angle from a degree scale