FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1910
1910 - 0370.PDF
I/HI MAY 14, 1910. The changes in position of the centre of pressure are eif graphically illustrated in an accompanying diagram, where the line AX represents the length of the aerofoil and 40* the abscissae of the curves show the distances in per cent, from the front of the « aerofoil. The ordinates *° show the corresponding degree of inclination. The data represented by jp the diagram are also con tained in the accompanying table. It will be seen from the curves that centre of pressure corresponding to zero angle of incidence is about two-thirds of the chord from the leading edge. It will further be noticed that the position of the centre of pressure moves forward with an increase in the angle of incidence, till it reaches a point one-third of the chord from the leading edge. This position is obtained with an angle of incidence of from 160 to 200. From this point the centre of pressure begins to move slowly backward and is near the centre of the chord at an angle of incidence of 450. This movement of the centre of pressure is least in a parabolic curve and greatest in a circular arc. If we decrease the camber, the centre of pressure for all angles moves forward, and the angle at which reversal takes place is also lessened. It has also been found that changing the relative dimensions of an aerofoil, changes the relation of the angle of inclination to the centre of pressure. It has been stated repeatedly that, for the same angle, Diagram illustrating the shifting of the centre of pressure asl deduced from the results of Mr. Sellers' experiments. the centre of pressure moves forward with an increase in air velocity ; this may be so, but I have no positive proof of it and see no reason why it should be so. The air velocity used was 1,400 feet per minute. Centre of Pressure. Distance from Nearest front edge. per cent. Angles C C 1/3 P 1 in 12 1 in 24 I in 12 I in 12 75 A 00 57 54 51 49 67 62 5* 54 S° 46 42 40 67 62 57 S^ 48 46 44 43 Distance from Nearest front edge. per cent. Angles C C 1/3 P 1 in 12 1 in 24 iini2 I in 12 59 55 51 49 47 46 44 45 25 40 47 46 44 39 38 42 45 40 39 38 37 35 38 42 45 46 42 41 40 y> 36 37 41 4? 42 41 40 35 33 30 40 44 EDITORIAL NOTE ON MR. SELLERS' EXPERIMENTS. THERE are two entirely separate reasons for regarding the above paper by Matthew B. Sellers as a contribution of great interest and importance. In the first place it describes an apparatus that commends itself to us as a simple and straightforward method of obtaining the desired result with a fairly high degree of accuracy. It will be observed that th e important feature of the apparatus lies in the use of springs for holding the aerofoil in a position of •equilibrium upon its pivot. It will further be observed that the method of obtaining a reading is to adjust the apparatus to a position of equilibrium of the aerofoil; a principle of working that has been adopted as the basis of many of the. most accurate instruments known to science. It is very easy for an observer to say whether or no a pointer coincides with a mark, and it is equally obvious that when, in the apparatus in question, the pointer does coincide with the mark, that the centre of pressure on the aerofoil must be very close indeed to the pivot. It is of course true that the springs only completely neutralise one another in one position, and that any shifting of the centre of pressure can only make itself observed by •a stretching of one spring or the other. If the stretch of the springs was used as a means of measuring the movement of the cp. there would no doubt be room for considerable inaccuracy, but in Mr. Sellers' apparatus they play no part in the actual reading at all, their action being entirely confined to a condition in which they are, or at any rate could be made, extremely sensitive. The second consideration that makes Mr. Sellers' paper a con tribution of extreme interest and importance is the nature of the results obtained. These are of a somewhat startling character, inasmuch as the data that he has collected go to show that the reversal of the centre of pressure occurs when the angle of incidence of the chord to the real wind is as great as 20°. It has always been known that this reversal of the centre of pressure takes place in cambered aeroplanes, but there has been no reliable data on the subject, and it has, we believe, been tacitly assumed by most people that the phenomenon occurs at a much smaller angle than is indicated by the Sellers figures. Wilbur Wright has referred to the subject as follows ("Flight Manual," Note 22) :—" In deeply curved surfaces the centre of pressure at 90 degrees is near the centre of the surface, but moves forward as the angle becomes less till a certain point is reached varying with the depth of the curvature. After this point is passed the centre of pressure, instead of continuing to move forward with the decreasing angle, turns and moves rapidly towards the rear. These phenomena are due to the fact that at small angles the wind strikes the forward part of the surface on the upper side instead of the lower, and thus this part altogether ceases to lift, instead of being the most effective part of all as in the case of the plane." It will be observed that the results of the Sellers experiments show the same rapidity of retrogression on the part of the centre of pressure after the reversal has once taken place. We are regarding these results in the reverse sequence to that in which Mr. Sellers deals with them. Mr. Sellers starts with a horizontal chord and speaks of the centre of pressure advancing as the angle increases, whereas the more common terms of reference hitherto have been to start with a vertical chord and to speak of the centre of pressure advancing as the angle of inclination decreases. The value of the critical angle, which is given by Mr. Sellers as lying between 16° and 20°, is of immense importance to aviators, because it is perfectly obvious that this shifting of the centre of pressure might quite well be of a character to destroy the equilibrium of an aeroplane in flight. The fact that the critical angle has a high value according to the Sellers experiments (the normal angle of incidence in flight is much below 200 in all modern machines) is of course also of great importance, but it must be borne in mind that the position of the centre of pressure is not alone affected by a change of attitude on the part of the machine ; it is, in fact, much more likely to be brought about by a change of direction on the 368
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
