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Aviation History
1935
1935 - 0672.PDF
33o FLIGHT. MARCH 28, 1935. Fig. 6.—The Focke-Wulf From the foregoing it will be seen that with a tandem aerofoil arrangement it is only part of the wing area which stalls at any given time, i.e., the front one. Any tendency which the front wing might have to go into a spin is resisted by the rear wing which is still carrying its full load, and the combination is not, therefore, likely to spin unless the relative angles of the two aerofoils are too close together or unless the wing loadings of the two are too nearly equal. If that is the case there is always a possibility that unpleasant characteristics might develop, and in order to make quite sure of avoiding these, designers who have produced tandem aerofoil machines have always provided for a considerably greater load per square foot of the front wing than of the rear. From an aerodynamic point of view the tandem arrangement is not as efficient as that of a monoplane wing because the rear wing works in disturbed air and is not carrying as much load as the front one, so that full use is not being made of both wings at any given angle. Shortly after Mr. Handley Page introduced the leading-edge slot for the purpose of preventing the air from breaking away from the wing at large angles, the Aeronautical Research Com mittee had certain experiments carried out on tandem arrange ments of aerofoils. For the best efficiency it was found that the front aerofoil should be at a negative angle relative to the rear wing, as shown in Fig, 5, This combination of aerofoils was very nearly stable over a large range of angles of incidence, but at very small angles the centre of pressure of the com bination was found to move back suddenly, and in order to make the combination stable it would be necessary to use a tail. The main advantage of the arrangement was that a very considerable increase in maximum lift coefficient could be obtained. According to the published results (R. and M., No. 804, March, 1922) the maximum lift coefficient of the combination was in excess of 0.8 for certain arrangements of gap and stagger, while with the two wings placed immedi ately behind one another the maximum was 0.5. The object of these experiments was not at the time so much that of getting fore and aft stability as that of getting a high maximum lift coefficient so as to retain a low landing speed combined with a high wing loading. A "Tailfirst" Type A practical arrangement making use of tandem wings was used in the German Focke-Wulf " Ente," produced some years ago. This machine paid a visit to this country, during which the present writer was privileged to make a flight in it, and there was certainly no doubt whatever about the longitudinal stability. When a certain angle was reached the front wing stalled mildly, the machine began to gather speed, and soon stability was restored. From Fig. 6 it will be seen that the front wing was quite a small one, while the main rear wing was large. Actually, the difference was greater than shown in the sketch, because, in addition to having a very small chord, the front wing also had quite a small span. To get the relative loadings of the front and rear wing right, the centre of gravity of the machine had to be fairly far back. For purposes of directional stability this fact necessitated the use of a very large fin and rudder behind the main wing, and, in addition, fins were placed under the wing. The machine was not of very high aerodynamic efficiency, but this was not a necessary consequence of the tandem arrangement. In the Pou-du-Ciel, shown in Fig. 7, M. Henri Mignet has made use of a combination of the various principles outlined in the foregoing notes. Not only has he arranged his wings in tandem, but by placing the front one slightly above the rear one he has, he claims, obtained a slot effect. When the trailing edge of the front wing is pulled down it approaches closely to the leading edge of the rear wing, and the claim is that the high velocity caused by the slot thus formed prevents the air from breaking away on both front and rear wings. Whether or not this claim can be substantiated it is impos sible to say, but from the fact that M. Mignet has carried out model tests, in a wind tunnel which he himself has constructed one is inclined to think that he may conceivably get the effect which he claims. By placing his main weights well forward, the centre of gravity of the combination is somewhere below and behind the line of the windscreen, so that the front wing, which is a good deal larger than the rear, is more heavily loaded per square foot of wing surface. The reason for using a reflex curvature in the wing section is that a wing with this profile has a stationary centre of pressure. If a wing of the ordinary curvative were used the centre of pressure would move back as the angle of incidence decreased, and the load on the Fig. 7.—The latest tandem-wing development —M. Mignet's Pou-du-Ciel. The dihedral angle of the wings is not shown. control stick would be very much increased and might possibly become too great for the pilot. Having solved the problem of longitudinal stability by using a tandem wing arrangement, and that of fore and aft control by pivoting the front wing, M. Mignet attacked the subject of lateral stability. Experience with a number of orthodox aeroplanes had convinced him that the usual types of control— elevator, rudder and ailerons—were too complicated for the average man to master easily. To make a correctly banked turn it was necessary on some machines to increase the bank by using the ailerons, while on others there was a tendency to over-bank and "opposite ailerons" had to be used. The whole process of turning was complicated, and misuse of the controls might easily lead to stalling and spinning. After pondering the subject, M. Mignet said to himself that, as the ailerons were the main cause of the trouble, the solution might lie in suppressing them altogether. But how to provide lateral control? He argued that if the centre of gravity were placed low enough in relation to the centre of side area, the machine would swing its fuselage out on a turn owing to the pendulum action, and if he could hit upon a suitable proportioning of his side areas in relation to the e.g. the machine ought to bank correctly for any radius of turn. Actually this problem took longer to solve than the fore-and-aft stability. Originally he had a fixed fin placed ahead of his rudder and the machine refused to turn. Then one day he took the fin off and with the material made a horn balance for the rudder. On the next flight the machine turned perfectly ! M. Mignet has now got his fuselage and rudder side areas and his dihedral angles on front and rear wing so pro portioned that he claims that it is impossible to make an in correct turn on the Pou-du-Ciel. Up to a bank of some 45 deg. the bank follows automatically, on the use of the rudder. If an even steeper bank is wanted it is only necessary to pull a little on the stick as in an orthodox aeroplane. The rudder, it will be remembered, is connected to the stick, and the feet are not used at all. On a gusty day the Pou-du-Ciel rolls slightly from side to side, but after the pilot becomes accus tomed to this it does not worry him, and he merely concerns himself with keeping the machine on its course, and in so doing he corrects the more pronounced rolling movements. "Pott" Developments One of the difficulties which builders of the " Pou-du-Cwk" will have to face in this country is the selection of a suitable engine. Mr. S. V. Appleby, whose " Pou" is progressing fast at Heston, intends to use a new engine which Sir John Carden is designing and building. This is a four-cvl inner water-cooled upright unit, very much like a small car engine, and as simple as possible. Weight has been kept down an- is expected to be loolb. for a horse-power of thirty. T£e drive will be direct. The price, it is understood, will oe about £50 Mr. Appleby is a close and personal friend of M. Mignet. the designer of the "Pou." and has built and flown one: 01 the earlier models; he is being kept au fait with the lates development and is quite willing to assist, with advice, a""" one else who is building.
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