Page:EB1922 - Volume 30.djvu/49

Rh is surely without parallel in the annals of engineering. And in this story may be found the hint of a tremendous future.

See also: The Royal Aero Club Year Books (1911-9); Flight (Jan. 1909 to Dec. 1920, the Official Organ of the Royal Aero Club) ; Captain McCudden, Five Years in the Royal Flying Corps (1918).

(R. M. H.)

Design of Lifting Surfaces. The determination of the forces acting upon a body moving through a viscous fluid, such as the atmosphere, is a problem so far not amenable to mathematical solution, and design must therefore be based upon experiment. A vast mass of experimental data has been obtained by testing models in wind tunnels (by Eiffel in Paris, by Prandtl at Gottingen, at the National Physical and other laboratories) and by experiments upon aeroplanes in flight, principally in England at the Royal Aircraft Establishment, Farnborough. A very use- ful amount of information had been acquired before the war, but this has been greatly extended during the war period.

Lifting-surfaces of various shapes have been used in the design of aeroplanes, disposed in a variety of ways. It was immediately evident that the span or spread of the wing across the line of flight should be large in comparison with the " chord " or dimension along the flight path. The ratio of the span to the chord has been termed the " aspect ratio." Aerodynamic efficiency increases with increasing aspect ratio; but it is desirable to limit the aspect ratio for constructional reasons and in order to reduce the room required for housing. The greater aerody- namic efficiency, moreover, becomes neutralized after a point by the head resistance due to the additional external bracing re- quired. A compromise must be made, and the average figure used is in the region of six to one. It was also evident that the wings should be cambered along the line of flight. The early aeroplane wings had approximately the same curvature of upper and lower surfaces. Wind-tunnel experiments, however, showed that the curvature of the under surface had but small influence compared with that of the upper surface, a result which enabled the designer to increase the thickness and in- ternal strength of the wings and reduce external bracing.

Extensive wind-tunnel research has been carried out to find the best cross-section shape of wings. Greater lift can be ob- tained from highly cambered wings, but thinner wings offer less resistance to motion at small angles. An aeroplane should have as large a speed range as possible. While a wing of high lifting-capacity is required to fly slow, small resistance is re- quired for fast flying, that is at fine angles of attack. A greater speed range is obtained by the use of wings of small curvature (about i in 15), the same lower limit being attained by the use of a larger area to carry a given weight. Wind-tunnel experi- ments further determined the extent to which the curvature should be greater towards the leading edge of the wing.

Early writers sometimes stated the requirements of a wing as consisting purely of a high ratio of lift to resistance at some angle of attack. The requirements are in reality more complex. To secure a wide range of speed a high ratio of lift to resistance is required at fine angles (fine in comparison with the angle at which the wing attains its greatest lift at a given speed) and in addition a high value of this ratio is required at the inter- mediate angle at which the aeroplane climbs. This is not all. For longitudinal stability the travel of the centre of pressure when the angle of attack varies should be small, as this travel on a curved surface produces instability. The wing section best meeting all these requirements is probably the British Royal Aircraft Factory's No. 15, designed early in 1916.

-Length of Chord >

Rear Spar

Front Spar

Leading ; Edge.

Trailing Edge

FlG. 6. Wing Section R.A.F. 15.

The resistance of a wing must, however, be considered in rela- tion to the resistance of the external bracing attendant upon

its use. It has bee'n suggested that the thick wing, in spite of greater head resistance due to the wing, might prove superior by making possible the suppression of all external bracing, and

o s 10 is 20

FIG. 6b. Variation of the ratio of lift to Resistance for the wing alone as the Angle varies.

10

15

20

FRONT EDGE

5 10 15 2f

FIG. 6a. Variation of the Lift FIG. 6c. Travel of Centre of and Resistance of awing with Pressure as Angle of Attack Angle of Attack. varies.

the German Junker and others have designed aeroplanes on these lines.

The term " wing " is commonly used of the half of a lifting- surface on one side of the aeroplane, the whole surface con- stituting a " plane." Thus a monoplane has one pair of wings. A tandem aeroplane has two or more pairs of wings arranged as the name implies. The terms " biplane," " triplane," " quad- ruplane " denote that two, three, or four planes are superposed. Langley's " aerodrome " is an early example of the tandem aeroplane. This type is inconvenient structurally and aerody- namically very inefficient. The rear plane acts upon air to which a downward trend has been imparted by the plane in front. The reaction upon the rear plane is therefore inclined backward by the angle through which the air has been " downwashed " by the leading plane. In multiplane systems in which the planes are placed one above the other, each plane operates in air whose motion is influenced by the others, and the ratio of resistance to lift is less than the ratio which each would expe- rience if acting alone. If, however, the planes are placed at a sufficient distance apart, so that the gap between is roughly equal to the chord of the planes, the mutual interference pro- duces an effect comparable with that due to a reduction in aspect ratio such as is found necessary in the design of a mono- plane. Using the same aspect ratio a given area is disposed in a biplane in half the span required in a monoplane. The biplane forms a good structure, the planes forming the flanges of a box girder. In the monoplane the bracing wires make small angles with the planes, with consequent high tension in the wires and high compression in the spars of the wing. In the biplane the wires make obtuser angles with the planes. In reviewing the examples of the two types, it is found that the monoplanes are relatively of heavy wing loading and low aspect ratio. In the triplane the upper and lower planes may form the flanges of the girder, or the structure may consist of two girders superposed. This does not possess the same structural superiority over the biplane, as does the latter over the monoplane. The triplane arrangement provides a means of reducing span by increasing height. An early example of the triplane is that designed and flown by A. V. Roe in 1909. A Sopwith triplane was used by the British army during the war. The type may be suitable to large aeroplanes, in which reduction of the weight of the structure and of bulk is especially needed.

The great majority of aeroplanes have been of the monoplane and the biplane types, the latter predominating since 1912. The first aeroplanes to fly were biplanes and by far the larger number of aeroplanes in use to-day are of this type. The monoplane appeared about the opening date of the period under