FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1912
1912 - 0680.PDF
MONOPLANE 'WING SPARS. THE spars used in monoplane wings are, apparently, about to pass through the same changes as motor car chassis members passed through several years ago. There were, in the beginning of motor cars, frames of wood, of armoured wood, of stout steel tubes, of lighter tubes with bracings, and eventually of pressed steel. Although one would not go so far as to say that the ultimate type of wing spar will be one of pressed steel, such as is predominant in cars to-day, it is fairly safe to say that we shall all eventually adopt one design, whatever is found best, and the sooner we find this the better. Investigation of the points, good and bad, of present-day wing spars may help us to F"i >• form our opinions as to which school of thought to follow. Probably the commonest form of wing spar is one cut or shaped from a solid piece of ash or other wood and made of " H " or girder section. Fig. I shows one of this type. At the point of attachment of wing wires, the spar is either left solid, or has pieces let in flush with the sides. The chief stresses in a spar of this sort are those of compression, and it is a most unfortunate fact that, though ash has an average ultimate tensile strength of 19,000 lbs. per square inch, its strength in compression is seldom over 9,000 lbs. per square inch. The serious stresses arise from compression due to wing wire pull and compression on the under side due to bending up - wards. CThe former compression stress is spread over the whole of the area of the spar, but the latter is in the lower half of the spar only and varies in intensity from no stress in the centre to maximum stress in the lower extreme fibres of the spar. These two compression stresses, when added together, give the maximum stress the spar is subjected to in compression from these two loads. Investigating the stresses in the upper half of the spar, we find a compression due to wing wire pull and a tension due to bending upwards, this latter reducing, of course, the compression stress in the wood due to wing wire pull would be too great and our one mm. bolt would tear its way along the fibres of the spar. It looks as though many of our wing spars are fitted with bolts that are only designed to withstand the load in themselves, and no notice is taken of the load that the wood in actual contact with the bolt has to carry. For instance, if the compression stress in the spar due to wing wire pull on any one bolt is 1,500 lbs., and we are allowed, say, a factor of safety of 6 ; assuming the thickness of the spar where the bolt is fitted to be 2 ins. then the diameter of the bolt to enable the wood to resist the compression must be ^ in. although it is quite possible that a J in. bolt, as far as stress in the bolt is concerned may be much too large. Whilst on the subject of bolts fitted into woodwork, there is a method commonly used in motor body manufacture of increasing the hardness of the wood round the bolt, which method the writer has never seen in aeroplane construction. It is to drill a consider ably smaller hole in the wood than is desired, and to " rub" this hole out exactly to size by means of a red-hot iron. It is sometimes done as a makeshift when proper tools are not at hand, but the makeshift in this case is often a sounder and stronger job than the proper way. After the hot iron has been used the hole can be reamered out to the exact size of the bolt, and this reamering should just remove the chaned black powder. There are various forms of built up spars, one of which is shown in Fig. 4. The method employed is to have a central piece of ash or three-ply wood with four strips of ash glued together and riveted with copper rivets and washers. If the centre piece is of three-ply there may be some slight advantage in this spar over one cut from the solid, as the resistance to shear in three-ply is somewhat greater than is the resistance to shear of ash along the grain. If ash is used it would be a good plan to have the centre board in sections with the slope of the grain at about 300, and alternate sections should have the grain reversed as shown in Fig. 5. Taking this spar all round, it is very doubtful if it has any important advantages over the spar cut from the solid. The machine may have to remain out on a warm, damp night with a reduction in the strength of the glue by 50 or 60 per cent., even though the spar may have been varnished carefully to prevent this. Rivets or screws weaken the spar by the amount of sectional area of the spar that is removed and according to their distance from the neutral axis. If the thickness of the upper flange is f in. and the rivets employed are -fw in- in diameter the strength of the spar is reduced by approximately 25 per cent. Another spar of somewhat similar construction is shown in Fig. 6, the method employed in this case being to glue two pieces of ash and two pieces of three-ply wood together, screwed up with wood screws into the ash and varnished to prevent the glue perishing from damp. the upper half of the spar. A sectional stress diagram shown in Fig. 2 will serve to illustrate how disproportionate are the stresses in a spar with equal sectional area above and below the centre line, the area shaded showing the relative amount of compression stress. There is still another stress which is seldom recognised when designing these spars and this is a stress on, or near, the centre line, or neutral axis of the spar. It is a stress due to shear, and is caused by the tendency of the fibres of the spar to slide on one another when the spar is deflecting under its load. That this stress, though very low in steel girders, is of more than casual importance is shown by the fact that one spar tested by the writer fractured by shearing along the centre line before failing from compression or tension. From this it appears that the correct section of a spar between wire fastenings should be some thing like that shown in Fig. 3. Another point one must not fail to consider when designing spars is the size of the bolts used to attach the wing wire plates. For instance, if we used a bolt of a diameter of one millimetre through the spars and attached our wing wires to the ends of the bolt (we know the bolt would break under the load, but let us assume for <nr purpose that it would not), the compression per square inch on A glued joint is about as strong as can be when well made, but there is always the remote possibility of it being badly made or of inferior glue being used or of deterioration from damp, and a designer's chief motive ought to be to remove these possibilities. A built up spar is difficult to shape to suit the varying loads a spar has to carry, and thus one sees spars of this class with a constant taper from the fuselage to the tip. Spar sections, of course, ought to alter with the load each portion of the spar has to carry. This is done very accurately in the Nieuport wing, but then the Nieuport is mathe matically designed from tip to tail, and is just one of those instances that prove the worth of design as against guesswork. Built up spars are troublesome at the point of attachment of the wing wires, and generally have to have pieces let in, which in itself is a source of weakness. Solid spars are better in this respect. The rear spar in a monoplane very often has greater compres- sional stresses to withstand than has the front spar, as the rear spar 680
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
