FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1913
1913 - 0086.PDF
JUGHT] SOME THOUGHTS III. Neutral Tips aod the Negative Warp. Work of Dunne, Weiss and others. Dr. Hankin's Obser vations on Steering in the Soaring Flight of Birds THE subject as thus far discussed has been mainly related to steering, which aspect of the situation I regard as fundamentally the most important, partly for the reason that aeroplanes must be built to be steered, and partly because the assumption of a machine being canted necessarily supposes also that a steering movement has, whether intentionally or otherwise, been initiated thereby. Practical considerations of the use to which aeroplanes might be put renders it out of the question to suppose that they should never be canted accidentally. An internal displacement of the load by a movement of one of the occupants would, of course, tend to upset the balance of a machine that in itself might be stable against veering winds. Thus, the situation, it seems to me, resolves itself far more into a question of safe control than of permanent inherent stability on straight-line flight. Permanent negative down pressure wing tips potentially afford such stability. The warping of such tips so as to differentiate their negative angles as in the Dunne aeroplane tends to give rise to reactions that are in the proper sense for initiating a bank and the general tendency of the negative tips themselves is towards stability. The problem, in detail, however, is complicated by the difference in mean speeds on the part of the positive portions of the two wings, which also, it would seem, ought properly to be warped. But, as was pointed out in the first part of this article, there is also the centrifugal couple tending to destroy the bank. From general considerations in this connection, especial interest would appear to attach to the angle of 45', which also has particular importance from the point of view of power required, as has already been explained. When the subject at large is regarded from a broad standpoint, and due consideration is given to the fact that the pilot must always be an essential part of the machine, it would appear that an aero plane not permanently stable, but nevertheless fitted with a safe control, might well be the best combination for practical purposes. Thus, if a machine with normally neutral wing tips were fitted with a control that would enable the pilot to warp them negatively at will, it seems to me that the combination of safety with efficiency tends to reach its highest limit in such a system. In any case, adequate power must be provided on the machine which, if necessary, must be capable of supporting itself with both tips fully negative to the extent required for stability in straight-line flight. Having presented to the best of my ability my own argument in respect to lateral and directional stability, I desire, before passing on to discuss longitudinal stability, to make reference to the work of others. Whether what I have put forward proves to be true in practice or not, the present discussion thereof affords an excellent opportunity of drawing attention to various matters that seem to be significant on the light of it. First and foremost there is, of course, the work of Lieut. J. W. Dunne who spent years experimenting with all sorts of models until he at length evolved his present machine with negative wing tips and retreated wings. I had always supposed, however, that the significance of the negative tips was related to their position on the retreated wings and not something that might equally well be applied to straight wings such as are used on most modern machines. The possible shielding effect of the down-turned weather tip, coupled with the tendency to a positive angle on the part of the lee- tip of a retreated wing in an oblique wind, suggested that the general tendency of the system was towards conferring what I then had occasion to describe as " stiff " stability in order to distinguish it from the rolling stability of the dihedral, which implies the power to recover balance. It is naturally very interesting to read at the present time what Mr. Dunne had to say on the subject of stability so long ago as 1909 in his patent No. 8,118 of that year. Thus : " In order to render the machine free from oscillation, it is advisable that as the wing extends outwards the angle of incidence should decrease gradually so that there may be no abrupt alteration of angle at any part of the wing. With a wing so formed, alterations of the mean angle of incidence bring into play gradual alterations in the pressures on the wings gently to correct the deviation from normal conditions. It is of the greatest importance that this correction should be gentle at the commencement of an oscillation, and it is equally important that the tendency should persist through wide ranges of variation in the mean angle of incidence and increase as the departure from the normal condition increases. This latter condition requires that the difference between the angles of incidence of the inner and outer portions of the wing should be considerable, a condition JANUARY 25, 1913. ON STABILITY AND CONTROL. By A. E. BERRIMAN. (Continuedfrom page 63.) The Pioneer which in turn requires in general a negative angle of incidence at the tips under normal flight conditions. 86 I have found that twisting the wings or rendering them flexible so that they twist under air pressure, to cause the tips to present the requisite small angle, involves the disadvantage that sections taken fore and aft across the tips of the wings or from the inner portion to the tip gives curves more or less concave on their upper side, thus greatly detracting from the lifting powers of the wing. A twisted wing, therefore, is unable to give large pressure reaction when driven at an angle against the air ... . " In order to obtain the correct form, therefore, it is necessary to consider : " Firstly, how the angle of the fore and aft section can be made gradually to decrease as the wing is built outwards without producing points of inflection in the surfaces ; and "Secondly, how considerable differences in the angles of the inner and outer portions can be maintained without too much loss of pressure under the outer portions and, therefore, loss of lifting power .... " For the guidance of all machines constructed in accordance with this specification, I consider that the control described in the patent specification No. 1,469 of May 21st, 1870, which comprise horizontally pivoted flaps at the rear tips of the wings, is particularly suitable. ... If with such flaps it is desired to incline the front of the machine upward, the flaps would b^ inclined upwards so as to receive air pressure upon their upper surfaces and so force the rear of the machine down. To incline the front of the machine downward the flaps would be lowered so as to receive air pressures on their under sides and so lift the rear of the machine. In order to turn to the right, the right-hand flap would be raised and the left- hand flap lowered, and in order to turn to the left the left-hand flap would be raised, and the right-hand flap lowered .... " I have found by practical experiment that machines, as described, supported mainly by wings of which the whole or the outer portion are inclined backward and constitute portions of the surfaces of cones or cylinders, fully satisfy the conditions of stable and steady flight." A later patent, No. 11,021, of 1909, relates more particularly to the features necessary in biplane as distinct from monoplane design, thus:— "In order to obtain the same degree of stability when the principles embodied in the monoplanes above indicated are applied to biplanes, I have found that the tips of the upper wings should be inclined downwards and forwards in relation to the tips of the lower wings. " While the angle of incidence of the upper wing should decrease towards the tip and preferably become negative as before, the lower wings may have a uniform angle of incidence or even one increasing slightly towards the tips; if such increase exceed a comparatively moderate extent, however, the stability of the aeroplane will suffer. But, I consider it preferable, and the best results are obtained, when both pairs of wings are so constructed that while their inner portions have a positive angle of incidence, their tips have a much smaller or actually negative angle of incidence, the variation being greater in the upper than in the lower pair of wings, and the super posed wings being so set relatively to each other that the tips of the upper pair are inclined forward and downward in relation to those of the lower pair. " With the machine described, steering in either a horizontal or vertical plane may be effected by the ailerons or flaps arranged, as described, behind the tips of the upper wings. . . . " The action of these ailerons may be best understood by the consideration that raising one of them, besides decreasing the angle of incidence of the tip of that side, also increases the existing difference the angles of incidence of the tips of the upper and lower planes, and therefore for a double reason increases the resistance of that side of the machine. On the other hand, when one of the flaps is lowered, the angle of incidence of the tip on that side is rendered more positive, while the difference between the angles of incidence of the tips of the two wings is decreased, thereby for both reasons decreasing the resistance of that side of the machine." Although from the patent descriptions it might not seem perhaps as if the actual down pressure on the wing tips was sufficiently emphasised as an essential condition of stability, nevertheless, from the known behaviour of the machine in flight and the details of its con struction, it seems to me that in fact the wing tips of the Dunne aero plane must have been definitely negative (down pressure) from the first. Moreover, it is quite conceivable that the negative tip as such may give trouble, unless obtained by the gradual change of angle that is
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
