The " tractor " is the more convenient design, slightly better aerodynamically and reputed safer in a " crash."
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FIG. 9. Propeller Biplane of 1914-16.
Weight and Head Resistance. The aeroplane designer is continually interested in the relative importance of weight and head resistance. At the start attention was naturally concen- trated upon the production of a light structure. Knowledge of the resistance to motion of bodies of various shapes was meagre and was most probably gauged in the mind of the designer by the frontal area exposed, irrespective of shape. It was not real- ized that a strut of circular section offers twelve times the resistance of a strut of the best " streamline " or " fair " shape of the same frontal area. The light biplane structure of the Wrights and the Farmans contained a network of struts and wires offering a very high resistance. To reduce resistance, exposed parts may be " faired," which involves adding weight; and the number of external parts may be reduced, which again increases the weight of the structure. Wrights and Far- mans may be contrasted with the fast monoplanes and biplanes, the latter employing only a single bay of struts on either side, and finally with the unbraced monoplanes of Junker and Fokker.
" Streamline " wires were first designed for the British army dirigible "Beta" in 1912, and fairshaped wires were in 1914 fitted to aeroplanes designed at the Royal Aircraft Factory. They have since become the most usual bracing of British aero- planes. They offer approximately one-eighth of the resistance of cable of the same tensile strength. Their metallurgy required careful study, and hence in other countries cable has con- tinued to be used, frequently duplicated, the cables lying one behind the other with a wood " fairing " between them. Struts of streamline shape were in use at an earlier date. The bodies of aeroplanes have improved in form, the crew has been pro- tected from wind pressure, and the spokes of wheels have been covered in with fabric.
The drag of a biplane of moderate speed is made up roughly as follows i- Main planes 3%
Bracing of main planes 20%
Body
30%
Undercarriage 15%
Tail surfaces . . . . 5%
These figures show the importance of careful design of all parts Much of the resistance of the wing-bracing occurs at the joints of wires and struts to the planes, and the resistance of the body is largely due to the necessity of cooling the engine, either by water radiator or by flow of air over the cylinders.
The weight of the complete structure, excluding the power unit, fuel, crew and other load borne, is about one-third of the whole weight of the aeroplane, but varies with the total weight with the weight carried per unit of area of lifting surface, anc with the strength of the structure. The following figures are averages for a number of British aeroplanes:
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Total weight
Area of lifting surface
Load borne per unit area
Load factor
Structure weight of % of
total weight. . . . 28 % 35 % 27 % 34 %
2,000 Ib.
33 fl sq -
4 I 8
200 sq. ft.
10,000 Ib.
l,7OOsq. ft.
31% 40% 29%
l.OOOsq. ft.
The " load factor " is the number of times the weight of the craft which the wings will support; a measure of the strength.
Using one of the light engines now available, the power unit to give a speed of 100 m. an hour will weigh about one- quarter of the total, leaving 40 to 45 % for fuel, crew and cargo.
Wing Loading and Horse-Power. The lift of a wing is pro- jortional to its surface, the atmospheric density, the square of
- he speed and the angle at which it meets the air measured
rom the angle giving no lift and up to an angle near that known as the " critical angle." At this angle the lift is a maxi- mum (if the other factors be supposed constant) and above it the lift decreases. The wing in passing through this angle is said to be " stalled." Stalling occurs when flying as slowly as possible. After stalling it is no longer possible to increase the lift by de- pressing the tail of the aeroplane and it is necessary to dive in order to recover flying speed. This has been a frequent cause of accidents when flying too low to have room for a dive. More- over, the wings when stalled have lost their normal tendency to oppose rotation about the line of flight and now tend to
auto-rotate " or act as a windmill. The aeroplane may therefore drop one wing and pass into a steep spiral glide known as a " spinning nose-dive " from which it may be brought to normal flight by the same diving process reducing the angle of attack of the wings. There is no danger in the stall or the spin so long as there is space for the recovery and knowledge of the action required.
100 M.P.H.
so
6 10 '6 Ibs./sq. ft.
FIG. 10. Curve showing lowest speed of flight possible with given wing-loading and the usual thin wings.
Wing-loading, the weight borne per unit area of sustaining surface, determines the speed at which the wings become stalled and therefore the slowest alighting speed. With constant loading, as the speed of aeroplanes increases, wings attack the air at ever finer angles, very soon passing the angle of lowest resistance for a given lift. To increase speed it therefore becomes desirable to increase the loading, or in other words to reduce the area of the wings. This reduction has also the merit that it reduces the bulk of the craft, the resistance of external bracing and the weight of the wing structure. To attain the greatest height heavy wing-loading is not required, and the best loading for a high ceiling would to-day be considered a light loading. For fighting, power of rapid manoeuvre is essential. The aeroplane of light loading can be turned in a smaller circle. The total weight is, however, approximately fixed by military considerations, and light loading implies large wing area and consequent greater resistance to angular acceleration, so that the lightly loaded aeroplane cannot so quickly be " banked " to the correct angle for the turn. Given the wing area, the aeroplane having the lighter loading is the more manoeuvrable; given the weight, the heavier loaded aeroplane is at least the equal of the other. Aeroplanes carry a larger area of sustaining surface than they require, except for alighting, and it is the
