FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1924
1924 - 0119.PDF
FEBRUARY 28, 1924 LOW-POWERED FLYING By W. O. MANNING As " The Wren " light aeroplane was the first of its type to fly in England, it might be as well to put on record the origin of this design. This machine was designed originally at the end of October, 1922, and work was started on it in the beginning of February, 1923, the machine being finally finished and flown early in April the same year. It was not designed for the Daily Mail Competition at Lympne, but it was obvious that a machine of this type which was known after its first trials to have a sufficiently good performance for the purpose would have a very good chance in such a competition, and it was therefore entered. The machines which were entered at Lympne followed the lines of the first "Wren" very closely, with the exception of one or two slight alterations which were the result of experience with the first machines. These alterations consisted principally in reducing the dihedral from 4° to 2°, and swinging the wing forward so that the front edge was nearly at right angles to the centre line of the machine instead of sloping backwards as previously. This latter alteration was due to the fact that in the original machine the centre of gravity was somewhat too far forward. In getting out the original design it was clear that in order to make the machine fly satisfactorily with the A.B.C. motor, it was necessary to reduce all the subsidiary resistance to the minimum possible, and it was with this object that the wheels were half buried in the fuselage, thereby suppressing the resistance of the chassis. The wheels had consequently a very narrow wheelbase compared to usual practice, but no trouble due to this has arisen in use. As the centre of gravity of the machine is only about 18 ins. above the axle, the angle from the centre of gravity to the point where the wheels rest on the ground is probably about normal. It was anticipated that in the case of a bad landing, or in the case of a good landing on a very rough surface, the fuselage would come in contact with the ground, but it was not expected that this would normally damage the fuselage and this expectation has been realised in practice as no structural damage of this nature has ever occurred. In connection with the chassis it may be pointed out that one of the functions of the rather peculiar shape of the nose of the fuselage is to prevent the machine turning over after landing. This nose was stiffened up with three-ply so as to form a broad flat skid, and it would be quite safe to use it as a brake by holding the nose down with the elevator. As great efficiency was aimed at, and as the machine was small, it appeared reasonable to make the wings of the monoplane type, and the question as to what aspect ratio could be used was given very careful consideration. It is impossible to calculate accurately what the best aspect is under any particular set of circumstances, and the particular aspect selected is really the result of collating all the known facts and then using one's judgment. It is clear that the higher the aspect ratio the better the L/D of the wings, but if too large an aspect ratio is used the weight of the wings would increase seriously and trouble due to lack of rigidity might be expected. The aspect ratio eventually selected was 9 -25 to 1. The performance of the " Wrens " at Lympne are: Top speed, 52'm.p.h. ; minimum speed, 24 m.p.h. ; climb about 180 ft. per min. at ground level. It must, of course, be remembered in the design of machines of this type the question of L/D pure and simple is not the only consideration, as it is in the case of a pure glider. In the glider the performance is independent of the weight, the only effect of pushing up the weight being to increase the size of the machine if loading is to be constant. In the case of the power-driven machine, the performance is dependent on the weight divided by the L/D, which is the resistance, so that in this case weight comes in. It therefore will not pay to put on too much weight by increasing the aspect beyond a certain amount, and, of course, a biplane structure or any other of less actual efficiency, becomes possible if sufficient weight can be saved by its use. The wings were arranged as pure cantilevers, as on such a design diagonal supporting struts to the wing spars cannot be very satisfactorily arranged for. If they reach to anywhere near the best point of the spar they are very long, and have a very bad angle. If, on the other hand, they are attached to the spars close in, loads are seriously increased owing to negative shears, and, apart from this, although their angle is better, they are not very effective in relieving stresses. There appears to be no particular difficulty in connection * Abstract of paper read before the Inst, of A.E. on February 22, 1924. with controls of such machines. A rudder of ample size should be arranged for, and if ordinary practice is followed in connection with elevators and rudders, no trouble need be expected from either of these directions. These small machines have, however, shown one rather important characteristic. Owing probably to the high degree of lateral damping, they do not put one wing down suddenly when stalled, and certain machines of this type, including the " Wren," have been stalled repeatedly without anything whatevep happening. Engines I think, personally, that it would be advisable to keep to the two-cylinder engine as far as possible. This engine has the advantage of extreme simplicity, and can be over hauled, the valves ground, any cylinders cleansed, etc., at very small cost. An objection one sometimes hears to the use of this type is, that if one plug fails a large proportion of the power vanishes, but this is equally true of any small engine which is likely to be fitted in such machines. Such engines are not likely in any case to have a greater number of cylinders than four, and as a counterpoise to the above, it is clear that as there are twice as many plugs in the latter engine as there are in the two-cylinder type, the risk of failure of one of them is twice as great. In a small engine of this type the irregularity of the torque does not seem to have an appreciabfy detrimental effect on the propeller, and I think that it would be quite possible to drive a propeller satisfactorily with a single-cylinder only. Gearing, or Direct Drive There is another point to which attention may be directed in connection with engines, and that is the question of gearing driving the propeller. The French custom in all their light machines is to run the propeller at not more than about 1,700 r.p.m., and they either use a larger engine than we do and drive direct, or else they use a high-speed engine and gear down, and they seldom use a propeller less than 5 ft. in diameter. At Lympne several machines were fitted with propellers turning up to over 3,000 r.p.m. and about 4 ft in diameter. I believe that at Lympne the only reason why certain engines were geared was owing to the designers desiring to get more power than could be obtained at a speed practicable or the propeller. Some of the engines used were stated to give their maximum power at 5,000 r.p.m. or so. There is a good deal to be said for the small diameter hi<n-speed propeller. Though it is obvious that, apart fro n increase of power, a geared propeller may be con siderably more efficient, this is not the only point to consider, though it is clearly a most important one. The high-speed direct-driven propeller is much lighter than the geared one (the " WTren " propeller weighed 1 lb. 11 oz), and the arrange ment is much simpler. Lightness is also assisted by the absence of a gear-box. The srriall diameter of the high speed propeller also may enable some resistance to be saved ; with its use the wheels may possibly be put in the fuseage, while with a slow-speed propeller a higher chassis which would be heavier and of greater resistance than the alternative may have to be used. It is not a bad general rule when considering two alternative methods of doing something, and if the arguments in favour of each are so balanced as to make it difficult to know which to adopt, to adopt the simplest. Instruments The question of instruments for these small machines deserves some consideration, and in this connection it may be stated that the speed indicator used on the "Wren" was slightly heavier than the engine mounting. Very much lighter'instruments could and should be provided, and full- size machines would also benefit from a reduction in the weight obtained. Instruments have not so far given much trouble, the principal trouble with speed indicators being the temptation which is felt by members of the general public to blow down pitot tubes. We had three air speed indicators damaged at Lympne from this cause. Standard engine revolution „ indicators work well, but are much too heavy, but it is difficult to save much weight on cross levels and certain types of altimeters which are obtainable. The latter, however, are somewhat small. Other instruments are not usually fitted, though when the time comes when it is necessary to fit compasses to small machines of this type, it is clear that a new pattern would be desirable. Something both lighter and smaller than those at present in existence is wanted. 149
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
