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Whichever method is used, be sure that after the job is done the spars are perfectly straight.

STAGGER.–The stagger is the distance the top surface is in advance of the bottom surface when the aeroplane is in flying position. The set measurement is obtained as follows:

Plumb-lines must be dropped over the leading edge of the top surface wherever struts occur, and also near the fuselage. The set measurement is taken from the front of the lower leading edge to the plumb-lines. It makes a difference whether the measurement is taken along a horizontal line (which can be found by using a straight-edge and a spirit- level) or along a projection of the chord. The line along which the measurement should be taken is laid down in the aeroplane’s specifications.

If a mistake is made and the measurement taken along the wrong line, it may result in a difference of perhaps 1/4 will, in flight, be nose-heavy or tail-heavy.

After the adjustments of the angles of incidence, dihedral, and stagger have been secured, it is as well to confirm all of them, as, in making the last adjustment, the first one may have been spoiled.

OVER-ALL ADJUSTMENTS.–The following over-all check measurements should now be taken.

The straight lines AC and BC should be equal to within 1/8 inch. The point C is the centre of the propeller, or, in the case of a “pusher” aeroplane, the centre of the nacelle. The points A and B are marked on the main spar, and must in each case be the same distance from the butt of the spar. The rigger should not attempt to make A and B merely the sockets of the outer struts, as they may not have been placed quite accurately by the manufacturer. The lines AC and BC must be taken from both top and bottom spars–two measurements on each side of the aeroplane.

The two measurements FD and FE should be equal to within 1/8 inch. F is the centre of the fuselage or rudder- post. D and E are points marked on both top and bottom rear spars, and each must be the same fixed distance from the butt of the spar. Two measurements on each side of the aeroplane.

If these over-all measurements are not correct, then it is probably due to some of the drift or anti-drift wires being too tight or too slack. It may possibly be due to the fuselage being out of truth, but of course the rigger should have made quite sure that the fuselage was true before rigging the rest of the machine. Again, it may be due to the internal bracing wires within the lifting surfaces not being accurately adjusted, but of course this should have been seen to before covering the surfaces with fabric.

FUSELAGE.–The method of truing the fuselage is laid down in the aeroplane’s specifications. After it has been adjusted according to the specified directions, it should then be arranged on trestles in such a way as to make about three- quarters of it towards the tail stick out unsupported. In this way it will assume a condition as near as possible to flying conditions, and when it is in this position the set measurements should be confirmed. If this is not done it may be out of truth, but perhaps appear all right when supported by trestles at both ends, as, in such case, its weight may keep it true as long as it is resting upon the trestles.

THE TAIL-PLANE (EMPENNAGE).–The exact angle of incidence of the tail-plane is laid down in the aeroplane’s specifications. It is necessary to make sure that the spars are horizontal when the aeroplane is in flying position and the tail unsupported as explained above under the heading of Fuselage. If the spars are tapered, then make sure that their centre lines are horizontal.

UNDERCARRIAGE.–The undercarriage must be very carefully aligned as laid down in the specifications.

1. The aeroplane must be placed in its flying position and sufficiently high to ensure the wheels being off the ground when rigged. When in this position the axle must be hori-

nontal and the bracing wires adjusted to secure the various set measurements stated in the specifications.

2. Make sure that the struts bed well down into their sockets.

3. Make sure that the shock absorbers are of equal tension. In the case of rubber shock absorbers, both the number of turns and the lengths must be equal.

HOW TO DIAGNOSE FAULTS IN FLIGHT, STABILITY, AND CONTROL.

DIRECTIONAL STABILITY will be badly affected if there is more drift (i.e., resistance) on one side of the aeroplane than there is on the other side. The aeroplane will tend to turn towards the side having the most drift. This may be caused as follows:

1. The angle of incidence of the main surface or the tail surface may be wrong. The greater the angle of incidence, the greater the drift. The less the angle, the less the drift.

2. If the alignment of the fuselage, fin in front of the rudder, the struts or stream-line wires, or, in the case of the Maurice Farman, the front outriggers, are not absolutely correct–that is to say, if they are turned a little to the left or to the right instead of being in line with the direction of flight–then they will act as a rudder and cause the aeroplane to turn off its course.

3. If any part of the surface is distorted, it will cause the aeroplane to turn off its course. The surface is cambered, i.e., curved, to pass through the air with the least possible drift. If, owing perhaps to the leading edge, spars, or trailing edge becoming bent, the curvature is spoiled, that will result in changing the amount of drift on one side of the aeroplane, which will then have a tendency to turn off its course.

LATERAL INSTABILITY (FLYING ONE WING DOWN).–The only possible reason for such a condition is a difference in the lifts of right and left wings. That may be caused as follows:

1. The angle of incidence may be wrong. If it is too great, it will produce more lift than on the other side of the aeroplane; and if too small, it will produce less lift than on the other side–the result being that, in either case, the aeroplane will try to fly one wing down.

2. Distorted Surfaces.–If some part of the surface is distorted, then its camber is spoiled, and the lift will not be the same on both sides of the aeroplane, and that, of course, will cause it to fly one wing down.

LONGITUDINAL INSTABILITY may be due to the following reasons:

1. The stagger may be wrong. The top surface may have drifted back a little owing to some of the wires, probably the incidence wires, having elongated their loops or having pulled the fittings into the wood. If the top surface is not staggered forward to the correct degree, then consequently the whole of its lift is too far back, and it will then have a tendency to lift up the tail of the machine too much. The aeroplane would then be said to be “nose-heavy.”

A 1/4-inch area in the stagger will make a very considerable difference to the longitudinal stability.

2. If the angle of incidence of the main surface is not right, it will have a bad effect, especially in the case of an aeroplane with a lifting tail-plane.

If the angle is too great, it will produce an excess of lift, and that may lift up the nose of the aeroplane and result in a tendency to fly “tail-down.” If the angle is too small, it will produce a decreased lift, and the aeroplane may have a tendency to fly “nose-down.”

3. The fuselage may have become warped upward or downward, thus giving the tail-plane an incorrect angle of incidence. If it has too much angle, it will lift too much, and the aeroplane will be “nose-heavy.” If it has too little angle, then it will not lift enough, and the aeroplane will be “tail-heavy.”

4. (The least likely reason.) The tail-plane may be mounted upon the fuselage at a wrong angle of incidence, in which case it must be corrected. If nose-heavy, it should be given a smaller angle of incidence. If tail-heavy, it should be given a larger angle; but care should be taken not to give it too great an angle, because the longitudinal stability entirely depends upon the tail-plane being set at a much smaller angle of incidence than is the main surface, and if that difference is decreased too much, the aeroplane will become uncontrollable longitudinally. Sometimes the tail- plane is mounted on the aeroplane at the same angle as the main surface, but it actually engages the air at a lesser angle, owing to the air being deflected downwards by the main surface. There is then, in effect, a longitudinal dihedral as explained and illustrated in Chapter I.

CLIMBS BADLY.–Such a condition is, apart from engine or propeller trouble, probably due to (1) distorted surfaces, or (2) too small an angle of incidence.

FLIGHT SPEED POOR.–Such a condition is, apart from engine or propeller trouble, probably due to (1) distorted surfaces, (2) too great an angle of incidence, or (3) dirt or mud, and consequently excessive skin-friction.

INEFFICIENT CONTROL is probably due to (1) wrong setting of control surfaces, (2) distortion of control surfaces, or (3) control cables being badly tensioned.

WILL NOT TAXI STRAIGHT.–If the aeroplane is uncontrollable on the ground, it is probably due to (1) alignment of undercarriage being wrong, or (2) unequal tension of shock absorbers.

CHAPTER IV

THE PROPELLER, OR “AIR-SCREW”

The sole object of the propeller is to translate the power of the engine into thrust.

The propeller screws through the air, and its blades, being set at an angle inclined to the direction of motion, secure a reaction, as in the case of the aeroplane’s lifting surface.

This reaction may be conveniently divided into two component parts or values, namely, Thrust and Drift.

The Thrust is opposed to the Drift of the aeroplane, and must be equal and opposite to it at flying speed. If it falls off in power, then the flying speed must decrease to a velocity, at which the aeroplane drift equals the decreased thrust.

The Drift of the propeller may be conveniently divided into the following component values:

Active Drift, produced by the useful thrusting part of the propeller.

Passive Drift, produced by all the rest of the propeller, i.e., by its detrimental surface.

Skin Friction, produced by the friction of the air with roughnesses of surface.

Eddies attending the movement of the air caused by the action of the propeller.

Cavitation (very marked at excessive speed of revolution). A tendency of the propeller to produce a cavity or semi-vacuum in which it revolves, the thrust decreasing with increase of speed and cavitation.

THRUST-DRIFT RATIO.–The proportion of thrust to drift is of paramount importance, for it expresses the efficiency of the propeller. It is affected by the following factors: Speed of Revolution.–The greater the speed, the greater the proportion of drift to thrust. This is due to the increase with speed of the passive drift, which carries with it no increase in thrust. For this reason propellers are often geared down to revolve at a lower speed than that of the engine.

Angle of Incidence.–The same reasons as in the case of the aeroplane surface.

Surface Area.–Ditto.

Aspect Ratio.–Ditto.

Camber.–Ditto.

In addition to the above factors there are, when it comes to actually designing a propeller, mechanical difficulties to consider. For instance, the blades must be of a certain strength and consequent thickness. That, in itself, limits the aspect ratio, for it will necessitate a chord long enough in proportion to the thickness to make a good camber possible. Again, the diameter of the propeller must be limited, having regard to the fact that greater diameters than those used to-day would not only result in excessive weight of construction, but would also necessitate a very high undercarriage to keep the propeller off the ground, and such undercarriage would not only produce excessive drift, but would also tend to make the aeroplane stand on its nose when alighting. The latter difficulty cannot be overcome by mounting the propeller higher, as the centre of its thrust must be approximately coincident with the centre of aeroplane drift.

MAINTENANCE OF EFFICIENCY.

The following conditions must be observed:

1. PITCH ANGLE.–The angle, at any given point on the propeller, at which the blade is set is known as the pitch angle, and it must be correct to half a degree if reasonable efficiency is to be maintained.

This angle secures the “pitch,” which is the distance the propeller advances during one revolution, supposing the air to be solid. The air, as a matter of fact, gives back to the thrust of the blades just as the pebbles slip back as one ascends a shingle beach. Such “give-back” is known as Slip. If a propeller has a pitch of, say, 10 feet, but actually advances, say, only 8 feet owing to slip, then it will be said to possess 20 per cent. slip.

Thus, the pitch must equal the flying speed of the aeroplane plus the slip of the propeller. For example, let us find the pitch of a propeller, given the following conditions:
Flying speed ………….. 70 miles per hour. Propeller revolutions ….. 1,200 per minute. Slip …………………. 15 per cent.

First find the distance in feet the aeroplane will travel forward in one minute. That is–

369,600 feet (70 miles)
———————— = 6,160 feet per minute. 60 “ (minutes)

Now divide the feet per minute by the propeller revolutions per minute, add 15 per cent. for the slip, and the result will be the propeller pitch:

6,160
—– + 15 per cent. = 5 feet 1 3/5 inches. 1,200

In order to secure a constant pitch from root to tip of blade, the pitch angle decreases towards the tip. This is necessary, since the end of the blade travels faster than its root, and yet must advance forward at the same speed as the rest of the propeller. For example, two men ascending a hill. One prefers to walk fast and the other slowly, but they wish to arrive at the top of the hill simultaneously. Then the fast walker must travel a farther distance than the slow one, and his angle of path (pitch angle) must be smaller than the angle of path taken by the slow walker. Their pitch angles are different, but their pitch (in this case altitude reached in a given time) is the same.

In order to test the pitch angle, the propeller must be mounted upon a shaft at right angles to a beam the face of which must be perfectly level, thus:

First select a point on the blade at some distance (say about 2 feet) from the centre of the propeller. At that point find, by means of a protractor, the angle a projection of the chord makes with the face of the beam. That angle is the pitch angle of the blade at that point.

Now lay out the angle on paper, thus:

The line above and parallel to the circumference line must be placed in a position making the distance between the two lines equal to the specified pitch, which is, or should be, marked upon the boss of the propeller.

Now find the circumference of the propeller where the pitch angle is being tested. For example, if that place is 2 feet radius from the centre, then the circumference will be 2 feet X 2 = 4 feet diameter, which, if multiplied by 3.1416 = 15.56 feet circumference.

Now mark off the circumference distance, which is represented above by A-B, and reduce it in scale for convenience.

The distance a vertical line makes between B and the chord dine is the pitch at the point where the angle is being tested, and it should coincide with the specified pitch. You will note, from the above illustration, that the actual pitch line should meet the junction of the chord line and top line.

The propeller should be tested at several points, about a foot apart, on each blade; and the diagram, provided the propeller is not faulty, will then look like this:

At each point tested the actual pitch coincides with the specified pitch: a satisfactory condition.

A faulty propeller will produce a diagram something like this:

At every point tested the pitch angle is wrong, for nowhere does the actual pitch coincide with the specified pitch. Angles A, C, and D, are too large, and B is too small. The angle should be correct to half a degree if reasonable efficiency is to be maintained.

A fault in the pitch angle may be due to (1) faulty manufacture, (2) distortion, or (3) the shaft hole through the boss being out of position.

2. STRAIGHTNESS.–To test for straightness the propeller must be mounted upon a shaft. Now bring the tip of one blade round to graze some fixed object. Mark the point it grazes. Now bring the other tip round, and it should come within 1/8 inch of the mark. If it does not do so, it is due to (1) faulty manufacture, (2) distortion, or (3) to the hole through the boss being out of position.

3. LENGTH.–The blades should be of equal length to inch.

4. BALANCE.–The usual method of testing a propeller for balance is as follows: Mount it upon a shaft, which must be on ball-bearings. Place the propeller in a horizontal position, and it should remain in that position. If a weight of a trifle over an ounce placed in a bolt-hole on one side of the boss fails to disturb the balance, then the propeller is usually regarded as unfit for use.

The above method is rather futile, as it does not test for the balance of centrifugal force, which comes into play as soon as the propeller revolves. It can be tested as follows:

The propeller must be in a horizontal position, and then weighed at fixed points, such as A, B, C, D, E, and F, and the weights noted. The points A, B, and C must, of course, be at the same fixed distances from the centre of the propeller as the points D, E, and F. Now reverse the propeller and weigh at each point again. Note the results. The first series of weights should correspond to the second series, thus:

Weight A should equal weight F.
“ B “ “ “ E.
“ C “ “ “ D.

There is no standard practice as to the degree of error permissible, but if there are any appreciable differences the propeller is unfit for use.

5. SURFACE AREA.–The surface area of the blades should be equal. Test with callipers thus:

The points between which the distances are taken must, of course, be at the same distance from the centre in the case of each blade.

There is no standard practice as to the degree of error permissible. If, however, there is an error of over 1/8 inch, the propeller is really unfit for use.

6. CAMBER.–The camber (curvature) of the blades should be (1) equal, (2) decrease evenly towards the tips of the blades, and (3) the greatest depth of the curve should, at any point of the blade, be approximately at the same percentage of the chord from the leading edge as at other points.

It is difficult to test the top camber without a set of templates, but a fairly accurate idea of the concave camber can be secured by slowly passing a straight-edge along the blade, thus:

The camber can now be easily seen, and as the straight- edge is passed along the blade, the observer should look for any irregularities of the curvature, which should gradually and evenly decrease towards the tip of the blade.

7. THE JOINTS.–The usual method for testing the glued joints is by revolving the propeller at greater speed than it will be called upon to make during flight, and then carefully examining the joints to see if they have opened. It is not likely, however, that the reader will have the opportunity of making this test. He should, however, examine all the joints very carefully, trying by hand to see if they are quite sound. Suspect a propeller of which the joints appear to hold any thickness of glue. Sometimes the joints in the boss open a little, but this is not dangerous unless they extend to the blades, as the bolts will hold the laminations together.

8. CONDITION OF SURFACE.–The surface should be very smooth, especially towards the tips of the blades. Some propeller tips have a speed of over 30,000 feet a minute, and any roughness will produce a bad drift or resistance and lower the efficiency.

9. MOUNTING.–Great care should be taken to see that the propeller is mounted quite straight on its shaft. Test in the same way as for straightness. If it is not straight, it is possibly due to some of the propeller bolts being too slack or to others having been pulled up too tightly.

FLUTTER.–Propeller “flutter,” or vibration, may be due to faulty pitch angle, balance, camber, or surface area. It causes a condition sometimes mistaken for engine trouble, and one which may easily lead to the collapse of the propeller.

CARE OF PROPELLERS.–The care of propellers is of the greatest importance, as they become distorted very easily.

1. Do not store them in a very damp or a very dry place.

2. Do not store them where the sun will shine upon them.

3. Never leave them long in a horizontal position or leaning up against a wall.

4. They should be hung on horizontal pegs, and the position of the propellers should be vertical.

If the points I have impressed upon you in these notes are not attended to, you may be sure of the following results:

1. Lack of efficiency, resulting in less aeroplane speed and climb than would otherwise be the case.

2. Propeller “flutter” and possible collapse.

3. A bad stress upon the propeller shaft and its bearings.

TRACTOR.–A propeller mounted in front of the main surface.

PUSHER.–A propeller mounted behind the main surface.

FOUR-BLADED PROPELLERS.–Four- bladed propellers are suitable only when the pitch is comparatively large.

For a given pitch, and having regard to “interference,” they are not so efficient as two-bladed propellers.

The smaller the pitch, the less the “gap,” i.e., the distance, measured in the direction of the thrust, between the spiral courses of the blades.

If the gap is too small, then the following blade will engage air which the preceding blade has put into motion, with the result that the following blade will not secure as good a reaction as would otherwise be the case. It is very much the same as in the case of the aeroplane gap.

For a given pitch, the gap of a four-bladed propeller is only half that of a two-bladed one. Therefore the four- bladed propeller is only suitable for large pitch, as such pitch produces spirals with a large gap, thus offsetting the decrease in gap caused by the numerous blades.

The greater the speed of rotation, the less the pitch for a given aeroplane speed. Then, in order to secure a large pitch and consequently a good gap, the four-bladed propeller is usually geared to rotate at a lower speed than would be the case if directly attached to the engine crank-shaft.

CHAPTER V

MAINTENANCE

CLEANLINESS.–The fabric must be kept clean and free from oil, as that will rot it. To take out dirt or oily patches, try acetone. If that will not remedy matters, then try petrol, but use it sparingly, as otherwise it will take off an unnecessary amount of dope. If that will not remove the dirt, then hot water and soap will do so, but, in that case, be sure to use soap having no alkali in it, as otherwise it may injure the fabric. Use the water sparingly, or it may get inside the planes and rust the internal bracing wires, or cause some of the wooden framework to swell.

The wheels of the undercarriage have a way of throwing up mud on to the lower surface. This should, if possible, be taken off while wet. It should never be scraped off when dry, as that may injure the fabric. If dry, then it should be moistened before being removed.

Measures should be taken to prevent dirt from collecting upon any part of the aeroplane, as, otherwise, excessive skin- friction will be produced with resultant loss of flight speed. The wires, being greasy, collect dirt very easily.

CONTROL CABLES.–After every flight the rigger should pass his hand over the control cables and carefully examine them near pulleys. Removal of grease may be necessary to make a close inspection possible. If only one strand is broken the wire should be replaced. Do not forget the aileron balance wire on the top surface.

Once a day try the tension of the control cables by smartly moving the control levers about as explained elsewhere.

WIRES.–All the wires should be kept well greased or oiled, and in the correct tension. When examining the wires, it is necessary to place the aeroplane on level ground, as otherwise it may be twisted, thus throwing some wires into undue tension and slackening others. The best way, if there is time, is to pack the machine up into its “flying position.”

If you see a slack wire, do not jump to the conclusion that it must be tensioned. Perhaps its opposition wire is too tight, in which case slacken it, and possibly you will find that will tighten the slack wire.

Carefully examine all wires and their connections near the propeller, and be sure that they are snaked round with safety wire, so that the latter may keep them out of the way of the propeller if they come adrift.

The wires inside the fuselage should be cleaned and regreased about once a fortnight.

STRUTS AND SOCKETS.–These should be carefully examined to see if any splitting has occurred.

DISTORTION.–Carefully examine all surfaces, including the controlling surfaces, to see whether any distortion has occurred. If distortion can be corrected by the adjustment of wires, well and good; but if not, then some of the internal framework probably requires replacement.

ADJUSTMENTS.–Verify the angles of incidence; dihedral, and stagger, and the rigging position of the controlling- surfaces, as often as possible.

UNDERCARRIAGE.–Constantly examine the alignment and fittings of the undercarriage, and the condition of tyres and shock absorbers. The latter, when made of rubber, wear quickest underneath. Inspect axles and skids to see if there are any signs of them becoming bent. The wheels should be taken off occasionally and greased.

LOCKING ARRANGEMENTS.–Constantly inspect the locking arrangements of turnbuckles, bolts, etc. Pay particular attention to the control cable connections, and to all moving parts in respect of the controls.

LUBRICATION.–Keep all moving parts, such as pulleys, control levers, and hinges of controlling surfaces, well greased.

SPECIAL INSPECTION.–Apart from constantly examining the aeroplane with reference to the above points I have made, I think that, in the case of an aeroplane in constant use it is an excellent thing to make a special inspection of every part, say once a week. This will take from two to three hours, according to the type of aeroplane. In order to carry it out methodically, the rigger should have a list of every part down to the smallest split-pin. He can then check the parts as he examines them, and nothing will be passed over. This, I know from experience, greatly increases the confidence of the pilot, and tends to produce good work in the air.

WINDY WEATHER.–The aeroplane, when on the ground, should face the wind; and it is advisable to lash the control lever fast, so that the controlling surfaces may not be blown about and possibly damaged.

“VETTING” BY EYE.–This should be practiced 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 horizontal.

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 practiced constantly, will not only develop an expert eye for diagnosis of faults, but will also greatly assist in impressing upon the memory the characteristics and possible troubles of the various types of aeroplanes.

MISHANDLING OF 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 pushing 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 lay fabric-covered parts upon a concrete floor. Any slight movement will cause the fabric to scrape 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.

GLOSSARY

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 a 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 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 substituting “chords” for “neutral life 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 altitude. This is owing to the density of the air decreasing with increase of altitude and necessitating a greater speed through space to secure the same air pressure as would be secured by less speed at a lower altitude. It would be more correct to calibrate it in units of air pressure.

Air Pocket–A local movement or condition of the air causing an aeroplane to drop or lose its correct attitude.

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.

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.

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.

Biplane–An aeroplane of which the main lifting surface consists of a surface or pair of wings mounted above another surface or pair of wings.

Bay–The space enclosed by two struts and whatever they are fixed to.

Boom–A term usually applied to the long spars joining the tail of a “pusher” aeroplane to its main lifting surface.

Bracing–A system of struts and tie wires to transfer a force from one point to another.

Canard–Literally “duck.” The name which was given to a type of aeroplane of which the longitudinal stabilizing surface (empennage) was mounted in front of the main lifting surface. Sometimes termed “tail-first” aeroplanes, but such term is erroneous, as in such a design the main lifting surface acts as, and is, the empennage.

Cabre–To fly or glide at an excessive angle of incidence; tail down.

Camber–Curvature.

Chord–Usually taken to be a straight line between the trailing and leading edges of a surface.

Cell–The whole of the lower surface, that part of the upper surface directly over it, together with the struts and wires holding them together.

Centre (Line) of Pressure–A line running from wing-tip to wing-tip, and through which all the air forces acting upon the surface may be said to act, or about which they may be said to balance.

Centre (Line) of Pressure, Resultant–A line transverse to the longitudinal axis, and the position of which is the resultant of the centres of pressure of two or more surfaces.

Centre of Gravity–The centre of weight.

Cabane–A combination of two pylons, situated over the fuselage, and from which anti-lift wires are suspended.

Cloche–Literally “bell.” Is applied to the bell-shaped construction which forms the lower part of the pilot’s control lever in a Bleriot monoplane, and to which the control cables are attached.

Centrifugal Force–Every body which moves in a curved path is urged outwards from the centre of the curve by a force termed “centrifugal.”

Control Lever–A lever by means of which the controlling surfaces are operated. It usually operates the ailerons and elevator. The “joy-stick”.

Cavitation, Propeller–The tendency to produce a cavity in the air.

Distance Piece–A long, thin piece of wood (sometimes tape) passing through and attached to all the ribs in order to prevent them from rolling over sideways.

Displacement–Change of position.

Drift (of an aeroplane as distinct from the propeller)–The horizontal component of the reaction produced by the action of driving through the air a surface inclined upwards and towards its direction of motion PLUS the horizontal component of the reaction produced by the “detrimental” surface PLUS resistance due to “skin-friction.” Sometimes termed “head-resistance.”

Drift, Active–Drift produced by the lifting surface.

Drift, Passive–Drift produced by the detrimental surface.

Drift (of a propeller)–Analogous to the drift of an aeroplane. It is convenient to include “cavitation” within this term.

Drift, to–To be carried by a current of air; to make leeway.

Dive, to–To descend so steeply as to produce a speed greater than the normal flying speed.

Dope, to–To paint a fabric with a special fluid for the purpose of tightening and protecting it.

Density–Mass of unit volume, for instance, pounds per cubic foot.

Efficiency–Output
Input

Efficiency (of an aeroplane as distinct from engine and propeller)– Lift and Velocity
Thrust (= aeroplane drift)

Efficiency, Engine–Brake horse-power Indicated horse-power

Efficiency, Propeller– Thrust horse-power Horse-power received from engine (= propeller drift)

NOTE.–The above terms can, of course, be expressed in foot- pounds. It is then only necessary to divide the upper term by the lower one to find the measure of efficiency.

Elevator–A controlling surface, usually hinged to the rear of the tail- plane, the operation of which turns an aeroplane about an axis which is transverse to the direction of normal horizontal flight.

Empennage–See “Tail-plane.”

Energy–Stored work. For instance, a given weight of coal or petroleum stores a given quantity of energy which may be expressed in foot-pounds.

Extension–That part of the upper surface extending beyond the span of the lower surface.

Edge, Leading–The front edge of a surface relative to its normal direction of motion.

Edge, Trailing–The rear edge of a surface relative to its normal direction of motion.

Factor of Safety–Usually taken to mean the result found by dividing the stress at which a body will collapse by the maximum stress it will be called upon to bear.

Fineness (of stream-line)–The proportion of length to maximum width.

Flying Position–A special position in which an aeroplane must be placed when rigging it or making adjustments. It varies with different types of aeroplanes. Would be more correctly described as “rigging position.”

Fuselage–That part of an aeroplane containing the pilot, and to which is fixed the tail-plane.

Fin–Additional keel-surface, usually mounted at the rear of an aeroplane.

Flange (of a rib)–That horizontal part of a rib which prevents it from bending sideways.

Flight–The sustenance of a body heavier than air by means of its action upon the air.

Foot-pound–A measure of work representing the weight of 1 lb. raised 1 foot.

Fairing–Usually made of thin sheet aluminum, wood, or a light construction of wood and fabric; and bent round detrimental surface in order to give it a “fair” or “stream-like” shape.

Gravity–Is the force of the Earth’s attraction upon a body. It decreases with increase of distance from the Earth. See “Weight.”

Gravity, Specific–Density of substance Density of water.
Thus, if the density of water is 10 lb. per unit volume, the same unit volume of petrol, if weighing 7 lb., would be said to have a specific gravity of 7/10, i.e., 0.7.

Gap (of an aeroplane)–The distance between the upper and lower surfaces of a biplane. In a triplane or multiplane, the distance between a surface and the one first above it.

Gap, Propeller–The distance, measured in the direction of the thrust, between the spiral courses of the blades.

Girder–A structure designed to resist bending, and to combine lightness and strength.

Gyroscope–A heavy circular wheel revolving at high speed, the effect of which is a tendency to maintain its plane of rotation against disturbing forces.

Hangar–An aeroplane shed.

Head-Resistance–Drift. The resistance of the air to the passage of a body.

Helicopter–An air-screw revolving about a vertical axis, the direction of its thrust being opposed to gravity.

Horizontal Equivalent–The plan view of a body whatever its attitude may be.

Impulse–A force causing a body to gain or lose momentum.

Inclinometer–A curved form of spirit-level used for indicating the attitude of a body relative to the horizontal.

Instability–An inherent tendency of a body, which, if the body is disturbed, causes it to move into a position as far as possible away from its first position.

Instability, Neutral–An inherent tendency of a body to remain in the position given it by the force of a disturbance, with no tendency to move farther or to return to its first position.

Inertia–The inherent resistance to displacement of a body as distinct from resistance the result of an external force.

Joy-Stick–See “Control Lever.”

Keel-Surface–Everything to be seen when viewing an aeroplane from the side of it.

King-Post–A bracing strut; in an aeroplane, usually passing through a surface and attached to the main spar, and from the end or ends of which wires are taken to spar, surface, or other part of the construction in order to prevent distortion. When used in connection with a controlling surface, it usually performs the additional function of a lever, control cables connecting its ends with the pilot’s control lever.

Lift–The vertical component of the reaction produced by the action of driving through the air a surface inclined upwards and towards its direction of motion.

Lift, Margin of–The height an aeroplane can gain in a given time and starting from a given altitude.

Lift-Drift Ratio–The proportion of lift to drift.

Loading–The weight carried by an aerofoil. Usually expressed in pounds per square foot of superficial area.

Longeron–The term usually applied to any long spar running length- ways of a fuselage.

Mass–The mass of a body is a measure of the quantity of material in it.

Momentum–The product of the mass and velocity of a body is known as “momentum.”

Monoplane–An aeroplane of which the main lifting surface consists of one surface or one pair of wings.

Multiplane–An aeroplane of which the main lifting surface consists of numerous surfaces or pairs of wings mounted one above the other.

Montant–Fuselage strut.

Nacelle–That part of an aeroplane containing the engine and pilot and passenger, and to which the tail plane is not fixed.

Neutral Lift Line–A line taken through a surface in a forward direction relative to its direction of motion, and starting from its trailing edge. If the attitude of the surface is such as to make the said line coincident with the direction of motion, it results in no lift, the reaction then consisting solely of drift. The position of the neutral lift line, i.e., the angle it makes with the chord, varies with differences of camber, and it is found by means of wind-tunnel research.

Newton’s Laws of Motion–1. If a body be at rest, it will remain at rest; or, if in motion, it will move uniformly in a straight line until acted upon by some force.

2. The rate of change of the quantity of motion (momentum) is proportional to the force which causes it, and takes place in the direction of the straight line in which the force acts. If a body be acted upon by several forces, it will obey each as though the others did not exist, and this whether the body be at rest or in motion.

3. To every action there is opposed an equal and opposite reaction.

Ornithopter (or Orthopter)–A flapping wing design of aircraft intended to imitate the flight of a bird.

Outrigger–This term is usually applied to the framework connecting the main surface with an elevator placed in advance of it. Sometimes applied to the “tail-boom” framework connecting the tail-plane with the main lifting surface.

Pancake, to–To “stall ”

Plane–This term is often applied to a lifting surface. Such application is not quite correct, since “plane” indicates a flat surface, and the lifting surfaces are always cambered.

Propeller–See “Air-Screw.”

Propeller, Tractor–An air-screw mounted in front of the main lifting surface.

Propeller, Pusher–An air-screw mounted behind the main lifting surface.

Pusher–An aeroplane of which the propeller is mounted behind the main lifting surface.

Pylon–Any V-shaped construction from the point of which wires are taken.

Power–Rate of working.

Power, Horse–One horse-power represents a force sufficient to raise 33,000 lbs. 1 foot in a minute.

Power, Indicated Horse–The I.H.P. of an engine is a measure of the rate at which work is done by the pressure upon the piston or pistons, as distinct from the rate at which the engine does work. The latter is usually termed “brake horse-power,” since it may be measured by an absorption brake.

Power, Margin of–The available quantity of power above that necessary to maintain horizontal flight at the optimum angle.

Pitot Tube–A form of air-speed indicator consisting of a tube with open end facing the wind, which, combined with a static pressure or suction tube, is used in conjunction with a gauge for measuring air pressures or velocities. (No. 1 in diagram.)

Pitch, Propeller–The distance a propeller advances during one revolution supposing the air to be solid.

Pitch, to–To plunge nose-down.

Reaction–A force, equal and opposite to the force of the action producing it.

Rudder–A controlling surface, usually hinged to the tail, the operation of which turns an aeroplane about an axis which is vertical in normal horizontal flight; causes an aeroplane to turn to left or right of the pilot.

Roll, to–To turn about the longitudinal axis.

Rib, Ordinary–A light curved wooden part mounted in a fore and aft direction within a surface. The ordinary ribs give the surface its camber, carry the fabric, and transfer the lift from the fabric to the spars.

Rib, Compression–Acts as an ordinary rib, besides bearing the stress of compression produced by the tension of the internal bracing wires.

Rib, False–A subsidiary rib, usually used to improve the camber of the front part of the surface.

Right and Left Hand–Always used relative to the position of the pilot. When observing an aeroplane from the front of it, the right hand side of it is then on the left hand of the observer.

Remou–A local movement or condition of the air which may cause displacement of an aeroplane.

Rudder-Bar–A control lever moved by the pilot’s feet, and operating the rudder.

Surface–See “Aerofoil.”

Surface, Detrimental–All exterior parts of an aeroplane including the propeller, but excluding the (aeroplane) lifting and (propeller) thrusting surfaces.

Surface, Controlling–A surface the operation of which turns an aeroplane about one of its axes.

Skin-Friction–The friction of the air with roughness of surface. A form of drift.

Span—The distance from wing-tip to wing-tip.

Stagger–The distance the upper surface is forward of the lower surface when the axis of the propeller is horizontal.

Stability–The inherent tendency of a body, when disturbed, to return to its normal position.

Stability, Directional–The stability about an axis which is vertical during normal horizontal flight, and without which an aeroplane has no natural tendency to remain upon its course.

Stability, Longitudinal–The stability of an aeroplane about an axis transverse to the direction of normal horizontal flight, and without which it has no tendency to oppose pitching and tossing.

Stability, Lateral–The stability of an aeroplane about its longitudinal axis, and without which it has no tendency to oppose sideways rolling.

Stabilizer–A surface, such as fin or tail-plane, designed to give an aeroplane inherent stability.

Stall, to–To give or allow an aeroplane an angle of incidence greater than the “maximum” angle, the result being a fall in the lift- drift ratio, the lift consequently becoming less than the weight of the aeroplane, which must then fall, i.e., “stall” or “pancake.”

Stress–Burden or load.

Strain–Deformation produced by stress.

Side-Slip, to–To fall as a result of an excessive “bank” or “roll.”

Skid, to–To be carried sideways by centrifugal force when turning to left or right.

Skid, Undercarriage–A spar, mounted in a fore and aft direction, and to which the wheels of the undercarriage are sometimes attached. Should a wheel give way the skid is then supposed to act like the runner of a sleigh and to support the aeroplane.

Skid, Tail–A piece of wood or other material, orientable, and fitted with shock absorbers, situated under the tail of an aeroplane in order to support it upon the ground and to absorb the shock of alighting.

Section–Any separate part of the top surface, that part of the bottom surface immediately underneath it, with their struts and wires.

Spar–Any long piece of wood or other material.

Spar, Main–A spar within a surface and to which all the ribs are attached, such spar being the one situated nearest to the centre of pressure. It transfers more than half the lift from the ribs to the bracing.

Spar, Rear–A spar within a surface, and to which all the ribs are attached, such spar being situated at the rear of the centre of pressure and at a greater distance from it than is the main spar. It transfers less than half of the lift from the ribs to the bracing.

Strut–Any wooden member intended to take merely the stress of direct compression.

Strut, Interplane–A strut holding the top and bottom surfaces apart.

Strut, Fuselage–A strut holding the fuselage longerons apart. It should be stated whether top, bottom, or side. If side, then it should be stated whether right or left hand. Montant.

Strut, Extension–A strut supporting an “extension” when not in flight. It may also prevent the extension from collapsing upwards during flight.

Strut, Undercarriage–

Strut, Dope–A strut within a surface, so placed as to prevent the tension of the doped fabric from distorting the framework.

Serving–To bind round with wire, cord, or similar material. Usually used in connection with wood joints and wire cable splices.

Slip, Propeller–The pitch less the distance the propeller advances during one revolution.

Stream-Line–A form or shape of detrimental surface designed to produce minimum drift.

Toss, to–To plunge tail-down.

Torque, Propeller–The tendency of a propeller to turn an aeroplane about its longitudinal axis in a direction opposite to that in which the propeller revolves.

Tail-Slide–A fall whereby the tail of an aeroplane leads.

Tractor–An aeroplane of which the propeller is mounted in front of the main lifting surface.

Triplane–An aeroplane of which the main lifting surface consists of three surfaces or pairs of wings mounted one above the other.

Tail-Plane–A horizontal stabilizing surface mounted at some distance behind the main lifting surface. Empennage.

Turnbuckle–A form of wire-tightener, consisting of a barrel into each end of which is screwed an eyebolt. Wires are attached to the eyebolts and the required degree of tension is secured by means of rotating the barrel.

Thrust, Propeller–See “Air-Screw.”

Undercarriage–That part of an aeroplane beneath the fuselage or nacelle, and intended to support the aeroplane when at rest, and to absorb the shock of alighting.

Velocity–Rate of displacement; speed.

Volplane–A gliding descent.

Weight–Is a measure of the force of the Earth’s attraction (gravity) upon a body. The standard unit of weight in this country is 1 lb., and is the force of the Earth’s attraction on a piece of platinum called the standard pound, deposited with the Board of Trade in London. At the centre of the Earth a body will be attracted with equal force in every direction. It will therefore have no weight, though its mass is unchanged. Gravity, of which weight is a measure, decreases with increase of altitude.

Web (of a rib)–That vertical part of a rib which prevents it from bending upwards.

Warp, to–To distort a surface in order to vary its angle of incidence. To vary the angle of incidence of a controlling surface.

Wash–The disturbance of air produced by the flight of an aeroplane.

Wash-in–An increasing angle of incidence of a surface towards its wing-tip.

Wash-out–A decreasing angle of incidence of a surface towards its wing-tip.

Wing-tip–The right- or left-hand extremity of a surface.

Wire–A wire is, in Aeronautics, always known by the name of its function.

Wire, Lift or Flying–A wire opposed to the direction of lift, and used to prevent a surface from collapsing upward during flight.

Wire, Anti-lift or Landing–A wire opposed to the direction of gravity, and used to sustain a surface when it is at rest.

Wire, Drift–A wire opposed to the direction of drift, and used to prevent a surface from collapsing backwards during flight.

Wire, Anti-drift–A wire opposed to the tension of a drift wire, and used to prevent such tension from distorting the framework.

Wire, Incidence–A wire running from the top of an interplane strut to the bottom of the interplane strut in front of or behind it. It maintains the “stagger” and assists in maintaining the angle of incidence. Sometimes termed “stagger wire.”

Wire, Bracing–Any wire holding together the framework of any part of an aeroplane. It is not, however, usually applied to the wires described above unless the function performed includes a function additional to those described above. Thus, a lift wire, while strictly speaking a bracing wire, is not usually described as one unless it performs the additional function of bracing some well- defined part such as the undercarriage. It will then be said to be an “undercarriage bracing lift wire.” It might, perhaps, be acting as a drift wire also, in which case it will then be de- scribed as an “undercarriage bracing lift-drift wire.” It should always be stated whether a bracing wire is (1) top, (2) bottom, (3) cross, or (4) side. If a “side bracing wire,” then it should be stated whether right- or left-hand.

Wire, Internal Bracing–A bracing wire (usually drift or anti-drift) within a surface.

Wire, Top Bracing–A bracing wire, approximately horizontal and situated between the top longerons of fuselate, between top tail booms, or at the top of similar construction.

Wire, Bottom Bracing–Ditto, substituting “bottom” for “top.”

Wire, Side Bracing–A bracing wire crossing diagonally a side bay of fuselage, tail boom bay, undercarriage side bay or centre-section side bay. This term is not usually used with reference to incidence wires, although they cross diagonally the side bays of the cell. It should be stated whether right- or left-hand.

Wire, Cross Bracing–A bracing wire, the position of which is diagonal from right to left when viewing it from the front of an aeroplane.

Wire, Control Bracing–A wire preventing distortion of a controlling surface.

Wire, Control–A wire connecting a controlling surface with the pilot’s control lever, wheel, or rudder-bar.

Wire, Aileron Gap–A wire connecting top and bottom ailerons.

Wire, Aileron Balance–A wire connecting the right- and left-hand top ailerons. Sometimes termed the “aileron compensating wire.”

Wire, Snaking–A wire, usually of soft metal, wound spirally or tied round another wire, and attached at each end to the framework. Used to prevent the wire round which it is “snaked” from becoming, in the event of its displacement, entangled with the propeller.

Wire, Locking–A wire used to prevent a turnbuckle barrel or other fitting from losing its adjustment.

Wing–Strictly speaking, a wing is one of the surfaces of an ornithopter. The term is, however, often applied to the lifting surface of an aeroplane when such surface is divided into two parts, one being the left-hand “wing,” and the other the right-hand “wing.”

Wind-Tunnel–A large tube used for experimenting with surfaces and models, and through which a current of air is made to flow by artificial means.

Work–Force X displacement.

Wind-Screen–A small transparent screen mounted in front of the pilot to protect his face from the air pressure.