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Aviation History
1914
1914 - 1028.PDF
QFySHTj OCTOBER 9, 1914. Edited by V. E Mr. H. Gilbert's Model Orthopter. "I CONSIDER," writes our correspondent, "the problem of the orthopter a fascinating one, and it is certainly a pity that model aeroplane makers do not tackle the problem with more z:st than appears to have b:en the case. This model orthopter is the first I have built, and is the result of observations of seagulls and rooks. It will be seen that it has a fairly larg; span and wing area, and is consequently a rather slow ' flapper." The wings are built up in rather a novel way, and by this method an unusual amount of flexibility U obtained where it is needed most. The angle of incidence near the Ixxly is rather large, and it gradually decreases to a wash out at the tips, which possess a negative angle of incidence. A good idea of the wing formation is obtained by looking at the photo., which shows the spars and ribs very clearly. The reason for this flexibility is that it acts as a propeller blade when it is flexed by the air-pressure while flapping. Indeed, quite a considerable draught is noticeable at the back of the planes while they are flapping, which serves to show that they must propel. The model is constructed in the usual manner, and I consider that the reason for its flying at its first attempt is that nothing freakish is incorporated in its design and no attempt was made to build any fancy and useless parts. The adjust ment of the wing< and motor was just right, and its gliding angle was quite correct when assembled. Th: disposition of weights had to be carefully con sidered, as when they are once fixed they cannot be moved without spoiling the whole model. The very small tail will be noticed, and this was decided on because all birds seem to seldom use their tails except for alighting purposes. The gull is an example. The reason for providing a rather large elevator was that it is designed to carry an appreci able amount of weight and to effect slight adjustments for the gliding angle. The clockwork—[? —V.E.J.] —motor is of the same type that was illustrated some time ago in FLIGHT, so there is no need to describe it. As regards the flying of the model, it was very interesting and provided food for much thought. It rises at rather a steep angle while the full power is on, and as the power gradually gels less it proceeds in a rather jerky manner. The following illustration describes it. " This is no doubt due to the up-and-down motion of the wings. As the wing is on the down (impulse) stroke it shoots upwards, and as it is on the upward stroke (which is slower than the down stroke owing to the rubber which provides the impulse being against it), it glides down till the impulse stroke sends it up again, and so on. The best flight up to date is 12 seconds, which, to the average duration exponent, may seem very small, but as the motor is a geared DOWN one, and the length of the rubber motor is rather small JOHNSON. M.A. (the length of the rubber motors is 12 ins. each), it will be seen how the duration is curtailed. In my opinion a direct-driven motor would be impissib'e, as it would have to have an enormous amount ofrubber, and the frame would consequently have to be of abnormal weight .0 withstand the strain. This applies chiefly to large span models, although for small spin models it may be feasible. I have found it practically impossible to get a flight unless I provide a greater impulse to the down stroke than to the upward one. The rubber for these impulses is not shown on the photo., as it was more convenient to photograph without it on. The angle of the wings from the bottom ol the down stroke to the top of the upward stroke is 75° " The following measurements and description of model may be of interest:— . . . , " The span of the wings from tip to tip is 38 ins. ; the front spar Bearing frSlxt S/Ar s£ilk-&ound. Silk-bound to X_ \ H.Gilbert I Orthopter. UpSrrzke ft-Z Doun Stroke- Mr. H. Gilbert's orthopter to scale, with some detailed parts. is i>e in. at the beginning, and tapers to in. at the tips; the space between the wings is 6 ins. ; the back spar is i in. by g- in., the same all the way; the average chord is 5! ins. ; the wings slope slightly backwards ; the ribs are of yellow bamboo ; the curved tip is of bamboo ; the elevator has a span of 10 ins., a chord of 3 ins., and is also made from bamboo; the tail is a semi-circle, with a diameter of 5J ins., and is very flexible; this is also made of bamboo ; the weight of the complete model is 3f ozs. ; the rubber motors consist of 9 strands of iVin- square rubber each ; all joints are bound with silk steeped in glue; the main planes, elevator, and tail are covered with Clarke's flight silk, which makes a splendid Mr. H. Gilbert and two views of his model orthopter. 1028 Photo, by Mr. A. Fineberg.
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