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Aviation History
1939
1939 - 1180.PDF
April 20, 1939. Supplement to ^@CJff E AIRCRAFT ^ ENGINEER No. 159. (Volume XVII) 14th Year April 20, 1939 THREE NEW BALANCED FLAPS Results with a Modified Irving Design, a Slotted Flap and a Modified Zap Type By W. E. GRAY, D.F.O. ANUMBER of attempts have been made in the past few years to produce wing flaps with a reasonably-close degree of aerodynamic balance, and at the Editor's request the following brief account is given of three designs evolved by the writer, and of the tests so far carried out on them. Whether the tardiness in adopt ing the more closely balanced designs that have already been tested in flight is due to excessive caution on the part of "designers, or to their dislike of any mechanical complication, or (horrible thought!) to mere inertia and conservatism, is perhaps open to debate; there are no doubt some, however, to whom any fresh attempt is of interest—hence these notes. By reading between the lines the conservatives will doubtless be able to sum up just . why all three attempts are so much waste of time and effort; but it was not for them that these were expended. The main advantage in having flaps that can be applied and varied in setting easily is that one can use them as a gliding angle control while approaching to land, and so '.add to the safety of that operation. To feel that one has . a powerful control over the gliding angle is to feel that . one is more fully master of the situation, and can, in Professor G. T. R. Hill's apt words, impose one's will upon the aeroplane. Unfortunately none of the three flaps has yet been flown, but one has been thoroughly tested on a very large model on a car at high speed, and the results may be regarded as bevond doubt, while the other two have only been , tested in a tunnel at. low Reynolds Number ; similar tunnel tests on a plain split flap, however, are in close agreement with car tests on a like flap, and this gives a reasonably ' good assurance of the validity and accuracy of the tunnel • results. The Irving Flap The first flap is of the type invented by Mr. Irving of the National Physical Laboratory nearly four years ago, . and it is this one that has been tested by the car method ; : the Irving flap has been modified by cutting off the rear third of the lower half of the flap and re-attaching it with .a hinge, as shown in Fig. i. By giving the pilot control of this hinge the rear part of the flap may be rotated so as to upset the balance of air forces on the flap, and so control : the degree of flap opening in an indirect way. The control is arranged so as to be unaffected by the degree of flap opening, so that as the flap opens the airflow strikes the controlling part at an increasing angle until equilibrium is reached and the flap takes up a heavily damped position of opening corresponding to the control setting. The initial opening movement is caused by the airflow cling ing to the controlling part as it is turned slightly into the wing, and-the suction thus caused draws the flap out— assuming that its weight is balanced by springs. The possibility of the flap fluttering or fluctuating has been carefully investigated and there seems to be no possi bility of this happening. For the car tests the flap was made very light, so that it was a true dynamic model in the region of 30 m.p.h., and it was tested at all speeds up to 60 m.p.h. ; its moment of inertia was then trebled and the tests repeated right up the speed range. In this way a wide range was covered, representing flaps of a fraction of the weight of real flaps at the one extreme and flaps of many times their probable weight at the other extreme, and the degree of damping was very marked in all cases. " Stalling" the Servo Only one unforeseen feature was discovered, namely, that it was possible by a very rapid movement of the control at the beginning of the opening to '' stall'' the controlling part by turning it within the wing so far and so quickly that the air did not cling to it long enough to open the flap properly; the result was that the flap took up a stable position very slightly open. This peculiarity can be easily avoided by arranging a stop to prevent the pilot moving his control to big angles until the flap has opened some way, should this prove necessary. The tests were done on a wing of 30 in. chord by 80 in. span, mounted several feet above the car, and at speeds from 30 to 60 m.p.h., and of course they had to be done in calm air and on a good road surface; speeds were given by ordinary pitot, and hinge moments were recorded in graphical form with a simple spring arrangement. The flap span was 40 in. and its half-width was 3.3 in. A plain split flap was also tested to get'a direct com parison; unfortunately the only hinge moment test with the flap fixed as an Irving one was-made with the drag arms running to the 0.25 half-chord position; this was suitable for the modified flap but not for the Irving one,
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