FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1918
1918 - 1164.PDF
*"'-:%: .".~ .•?•-,• OCTOBER 17, 1918, LATERAL STABILITY IN AEROPLANES. By C. THE stability of an aeroplane as built to-day is almost in the hands of the pilot. By governing the controls he alters the aerofoils, rudder and elevators, in such a manner as to cause forces to come into play which neutralise those created by the wind and the varying density of the air. Apart from special machines designed to be auto- matically stable, such as the Dunne biplane, or the original Fokker monoplane, there are remarkably few devices invented which tend to give stability. One of the best known and most used methods of gaining lateral stability is the practice of setting the aerofoils at an angle—called the dihedral angle—to the hori- zontal. This angle is as a rule very small, varying from 2° to 5°, but in spite of this has a great effect on the behaviour of the machine. It has stood the test of time well, having been employed constantly since the early days of aviation, when it was one of the distinctive points of both the Antoinette and Black- burn monoplanes. LEVICK. been displaced, is being brought back to , a level position by a force which is automatically getting less and less, but is zero only when the board is horizontal. It is difficult to find the dimensions of the couple that may be relied upon, but the result worked out below is probably a normal example. Viz.:— Let S = span of each wing in feet. C — chord of each wing in feet. A = area of each wing in square feet. Then A = SC square feet. If the velocity of the machine is such as to cause a lift of 5 lbs. per square foot and r is the total lift or thrust in lbs. normal to wing surface. Then r - 5 SC lbs. If S - 18 feet and C - 6 feet. r =5 xi8 x6 = 540 lbs. " If the dihedral angle <p = 30. Then vertical lift for each wing in Fig. 1— r — rCos<p — 540 x .9986 •» 539 lbs. Now in Fig. 2, suppose 8 •» 300, R=,2r F10. 1. Fie.2. FIG. 3.' 385 >«t 2 Ok It is interesting to note the effect of a roll on a machine with wings constructed as explained above, the dihedral angle being </> degrees. Fig. I shows how a part of the normal reaction of the air on the wings is lost, so that one of the dis- advantages of the dihedral angle is that it decreases the efficiency of the aerofoil. This effect, however, is very small, amounting to something less than 1 per cent, for an angle of 3 degrees, and is therefore almost negligible. Fig. 2 shows a machine having been canted through an angle 8 degrees by some force, and indicates the vertical thrusts on the wings It will be seen that the lower wing has moved through a smaller angle than the higher one in relation to the horizontal plane, and is therefore giving a greater vertical lift. The thrust on the lower wing is rCos (9 — </>) and on the high rCos (S + ^), or on the left and right respectively, presuming the machine to be moving forward perpendicularly to the plane of the paper. One of the effects of this unequal lift on the wings is to set up a couple, which tends to bring the planes back to their original position. An analogy is the action of the see-saw, the loaded end of which, having Then vertical lifts for left wing— r — rCos (9 + <f>). = 540 Cos 330 - 540 x .8387. = 452 lbs. And for right wing— r = rCos (8 - <f>) = rCos 270. •= 540 x .8910. = 481 lbs. Showing a difference of 28 lbs. From the above results it is an easy matter to find the resultant of these two unequal-like parallel forces, and thus to find the couple. The best method is to draw a diagram, such as Fig. 3, but to a very large scale, and thus measure the distances AC and AB. Those given, however, were calculated and are more -exact, although of no more value than an approximate result found by taking AB equal to 18 ft., owing to the very small variation for small and practical value of 0. The couple is 934 x .2385 lbs. ft. or 223 lbs. ft. It should be noticed that the total vertical thrust when the machine is not level is less than when in the normal position, and also that in the example given the centre of gravity of the machine was supposed to be at the intersection of the wings, whereas in practice it is rather lower down. Feeding Refugees by Aeroplane. IT now appears that when during the Turkish advance in North-West Persia some thousands of refugees were isolated in the Urumia region, the British succeeded in establishing communication' with them by aeroplane and endeavoured, though unsuccessfully, to send a relief convoy. 1165
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
