The Aeroplane Speaks Part 17

"VETTING" BY EYE.--This should be practised at every opportunity, and, if persevered in, it is possible to become quite expert in diagnosing by eye faults in flight efficiency, stability, and control.

The aeroplane should be standing upon level ground, or, better than that, packed up into its "flying position."

Now stand in front of it and line up the leading edge with the main spar, rear spar, and trailing edge. Their shadows can usually be seen through the fabric. Allowance must, of course, be made for wash-in and wash-out; otherwise, the parts I have specified should be parallel with each other.

Now line up the centre part of the main-plane with the tail-plane. The latter should be symmetrical with it. Next, sight each interplane front strut with its rear strut. They should be parallel.

Then, standing on one side of the aeroplane, sight all the front struts.

The one nearest to you should cover all the others. This applies to the rear struts also.

Look for distortion of leading edges, main and rear spars, trailing edges, tail-plane, and controlling surfaces.

This sort of thing, if practised constantly, will not only develop an expert eye for diagnosis of faults, but will also greatly a.s.sist in impressing upon the memory the characteristics and possible troubles of the various types of aeroplanes.

MISHANDLING ON THE GROUND.--This is the cause of a lot of unnecessary damage. The golden rule to observe is, PRODUCE NO BENDING STRESSES.

Nearly all the wood in an aeroplane is designed to take merely the stress of direct compression, and it cannot be bent safely. Therefore, in packing an aeroplane up from the ground, or in pulling or pus.h.i.+ng it about, be careful to stress it in such a way as to produce, as far as possible, only direct compression stresses. For instance, if it is necessary to support the lifting surface, then the packing should be arranged to come directly under the struts so that they may take the stress in the form of compression for which they are designed. Such supports should be covered with soft packing in order to prevent the fabric from becoming damaged.

When pulling an aeroplane along, if possible, pull from the top of the undercarriage struts. If necessary to pull from elsewhere, then do so by grasping the interplane struts as low down as possible. Never pull by means of wires.

Never lay fabric-covered parts upon a concrete floor. Any slight movement will cause the fabric to sc.r.a.pe over the floor with resultant damage.

Struts, spars, etc., should never be left about the floor, as in such position they are likely to become scored. I have already explained the importance of protecting the outside fibres of the wood. Remember also that wood becomes distorted easily. This particularly applies to interplane struts. If there are no proper racks to stand them in, then the best plan is to lean them up against the wall in as near a vertical position as possible.

TIME.--Learn to know the time necessary to complete any of the various rigging jobs. This is really important. Ignorance of this will lead to bitter disappointments in civil life; and, where Service flying is concerned, it will, to say the least of it, earn unpopularity with senior officers, and fail to develop respect and good work where men are concerned.

THE AEROPLANE SHED.--This should be kept as clean and orderly as possible. A clean, smart shed produces briskness, energy, and pride of work. A dirty, disorderly shed nearly always produces slackness and poor quality of work, lost tools, and mislaid material.

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[Ill.u.s.tration]

GLOSSARY

_The numbers at the right-hand side of the page indicate the parts numbered in the preceding diagrams._

=Aeronautics=--The science of aerial navigation.

=Aerofoil=--A rigid structure, of large superficial area relative to its thickness, designed to obtain, when driven through the air at an angle inclined to the direction of motion, a reaction from the air approximately at right angles to its surface. Always cambered when intended to secure a reaction in one direction only. As the term "aerofoil" is hardly ever used in practical aeronautics, I have, throughout this book, used the term SURFACE, which, while academically incorrect, since it does not indicate thickness, is the term usually used to describe the cambered lifting surfaces, _i.e._, the "planes"

or "wings," and the stabilizers and the controlling aerofoils.

=Aerodrome=--The name usually applied to a ground used for the practice of aviation. It really means "flying machine," but is never used in that sense nowadays.

=Aeroplane=--A power-driven aerofoil fitted with stabilizing and controlling surfaces.

=Acceleration=--The rate of change of velocity.

=Angle of Incidence=--The angle at which the "neutral lift line" of a surface attacks the air.

=Angle of Incidence, Rigger's=--The angle the chord of a surface makes with a line parallel to the axis of the propeller.

=Angle of Incidence, Maximum=--The greatest angle of incidence at which, for a given power, surface (including detrimental surface), and weight, horizontal flight can be maintained.

=Angle of Incidence, Minimum=--The smallest angle of incidence at which, for a given power, surface (including detrimental surface), and weight, horizontal flight can be maintained.

=Angle of Incidence, Best Climbing=--That angle of incidence at which an aeroplane ascends quickest. An angle approximately halfway between the maximum and optimum angles.

=Angle of Incidence, Optimum=--The angle of incidence at which the lift-drift ratio is the highest.

=Angle, Gliding=--The angle between the horizontal and the path along which an aeroplane, at normal flying speed, but not under engine power, descends in still air.

=Angle, Dihedral=--The angle between two planes.

=Angle, Lateral Dihedral=--The lifting surface of an aeroplane is said to be at a lateral dihedral angle when it is inclined upward towards its wing-tips.

=Angle, Longitudinal Dihedral=--The main surface and tail surface are said to be at a longitudinal dihedral angle when the projections of their neutral lift lines meet and produce an angle above them.

=Angle, Rigger's Longitudinal Dihedral=--Ditto, but subst.i.tuting "chords"

for "neutral lift lines."

=Angle, Pitch=--The angle at any given point of a propeller, at which the blade is inclined to the direction of motion when the propeller is revolving but the aeroplane stationary.

=Altimeter=--An instrument used for measuring height.

=Air-Speed Indicator=--An instrument used for measuring air pressures or velocities. It consequently indicates whether the surface is securing the requisite reaction for flight. Usually calibrated in miles per hour, in which case it indicates the correct number of miles per hour at only one alt.i.tude. This is owing to the density of the air decreasing with increase of alt.i.tude and necessitating a greater speed through s.p.a.ce to secure the same air pressure as would be secured by less speed at a lower alt.i.tude. It would be more correct to calibrate it in units of air pressure. [1]

=Air Pocket=--A local movement or condition of the air causing an aeroplane to drop or lose its correct att.i.tude.

=Aspect-Ratio=--The proportion of span to chord of a surface.

=Air-Screw (Propeller)=--A surface so shaped that its rotation about an axis produces a force (thrust) in the direction of its axis. [2]

=Aileron=--A controlling surface, usually situated at the wing-tip, the operation of which turns an aeroplane about its longitudinal axis; causes an aeroplane to tilt sideways. [3]

=Aviation=--The art of driving an aeroplane.

=Aviator=--The driver of an aeroplane.

=Barograph=--A recording barometer, the charts of which can be calibrated for showing air density or height.

=Barometer=--An instrument used for indicating the density of air.

=Bank, to=--To turn an aeroplane about its longitudinal axis (to tilt sideways) when turning to left or right.