History of Aviation - Chapter 6
THE RIGGING OF AIRPLANES
Object.-The object of this chapter is to teach the elementary principles of correct rigging. It is not expected that the student will become an expert mechanic, but with this treatment as a basis and through practice he will be able to judge whether or not a maclime is correctly and safely rigged. In other words, he will not have to depend on someone else's judgment as to whether panels, wires, controls, struts, etc., of a machine are in good order, but he will be able to observe understandingly that they are. If the engine goes wrong he can land, if the rigging goes wrong he is in great difficulty. Moreover, if the rigging is wrong, speed is lessened and the stability is uncertain.
The first thing to be learned in rigging is a knowledge of the peculiar terms which have come into use in aeronautics defining different parts of the machines. Our present list of terms is derived, partly from French, partly from English, and partly from American terms. Thus different names may refer to the same part.
1. Tractor.-An airplane that is pulled through the air by a propeller situated in front of the machine, is called a tractor.
2. Pusher.-If the propeller is back of the main lifting pIane~ the machine is called a pusher.
3. Fuselage or Body.-The main body of the airplane in~ which the pilot sits and to which the landing gear, motor
controls, and sustaining surfaces are fixed. A small body,> especially in pusher types of machines, is called a Nacelle.
4. Cockpit.-The openings and space in the fuselage where. pilot or observer sits.
5. Streamline Body.-The shape of a body or part which permits a regular flow of air around and along it with the least resistance, in other words with minimum obstruction and eddying.
6. Fairing.-Building up a member or part of the plane with a false piece that it may have a stream-line body.
7. Wings, Planes, Panels.-The main supporting surfaces of an airplane are called wings, although the terms planes and panels are probably as frequently used and even preferred by many. The term panel refers properly to a section of the wings with the included struts and wires. The small panel directly above the body is called the engine section panel or the center panel, while the panels to the right and left of the body or fuselage are called the main panels. The main panels are the right and left panels as seen from the seat. Each main panel may be subdivided into the inner wing bay, the outer wing bay, and the overhang.
8. Landing Gear, Chassis or Undercarriage.-The wheels and the struts and wires by which they are attached to the fuselage.
9. Horizontal Stabilizer or Horizontal Fin.-The horizontal fixed tail plane.
10. Vertical Stabilizer or Vertical Fin.-The small vertical fixed plane in front of the rudder.
11. Rudder.-The hinged surface used to control the direction of the aircraft in the horizontal plane. As with a boat, for steering or "yawing" or changing its direction of travel.
12. Elevator or Flap; Flippers.-A hinged horizontal surface for controlling the airplane up and down, usually attached
to the fixed tail plane; for pitching the machine or "nosing up" and "nosing down."
13. Tail or "Empennages."-A general name sometimes applied to the tail surfaces of a machine.
14. Mast or Cabane.-The small vertical strut on top of the upper plane used for bracing the overhang.
15. Ailerons.-Movable auxiliary surfaces used for the control of rolling or baiiking motion. Other definitions are that they are for the lateral control or for maintaining equilibrium. When they are a part. of the upper plane they are sometimes called wing flaps.
16. Landing Wires or Ground Wires (Single).-The single wires which support the weight of the panels when landing or on the ground.
17. Flying Wires, or Load Wires (Double).-The wires which support the body or fuselage from the planes when in flight.
18. Drift Wires.-The horizontal wires which lead from the nose of the fuselage to the wings and thus keep them from collapsing backward. For the same reason the wings have interior drift wires.
19. Diagonal Wires.-Any inclined bracing wires.
20. Skids.-(a) Tail Skid.-The flexible support under the tail of the machine.
(b) Wing Skid.-The protection under the outer edge of the lower wing.
(c) Chassis Skids.-Skids sometimes placed in front of the landing gear.
21. Horns, or Control Braces.-The steel struts on the controls to which the control wires are attached.
22. Struts; Wing Struts.-The vertical members of the wing trusses of a biplane, used to take pressure or compression, whereas the wires of the trusses are used to take pull or tension. There are also fuselage struts and chassis struts.
23. Spar or Wing Bars.-Thc longitudinal members of the interior wing framework.
24. Rib (Wing).-The members of the interior wing frame. work transverse to the spars.
25. The Longerons or Longitudinals.-The fore and aft or lengthwise members of the framing of the fuselage, usually continuous across a number of points of support.
26. Engine (Right and Left Hand) .-Jn the ordinary tractor machine, when viewed from the pilot's seat a right-handed engine revolves clockwise and right-handed.
28. Pitch (Propeller).-The distance forward that the propeller would travel in one revolution, if there were no slip, that is, if it were moving in a thread cut at the same inclination as the blade. Pitch angle refers to the angle of inclination of the propeller blade.
29. Slip.-Slip is the difference between the actual travel forward of a screw propeller in one revolution and its pitch.
30. Dope.-A general term applied to the material used in treating the cloth surface of airplane members to increase strength, produce tautness, and act as a filler to maintain air and moisture tightness. Usually of the cellulose type.
31. Controls.-Since there are three axes or main directions about which an airplane may turn or rotate it follows that three controlling devices are required. These are: (1) the elevator for pitching; (2) the rudder for steering or yawing; (3) the ailerons for lateral, rolling or banking control.
The term controls is a general term used to distinguish the means provided for operating the devices used to control speed, direction of flight and attitude of the aircraft.
32. Cotter Pins.-Must be on every nut.
33. Castelled Nuts.-Admit cotter pins.
34. Turnbuckles.-Must be well and evenly threaded and locked with safety wires.
35. Safety Wires.-For locking turnbuckjes and hinge pins.
36. Shackle and Pin.-
37. Hinge Connections.-
38. Leading Edge or Entering Edge.-The front edge of a plane.
39. Trailing Edge.-The rear edge of a plane.
40. Stagger.-The horizontal distance that the entering edge of the upper wing of a biplane is ahead of the entering edge of the lower wing.
41. Dihedral Angle.-A term used to denote that the wings are arranged to incline slightly upward from the body toward their tips. the angle made with the horizontal by this inclination of the wing is called the dihedral angle.
42. Angle of Incidence.-The angle at which a wing is inclined to the line of flight.
43. Decalage.-Difference in angle of incidence between any two distinct aerofoils on an airplane.
44. Chord.-Distance between the entering edge and trailing edge of a wing measured on a straight line touching front and rear bottom points of a wing.
45. Camber.-The depth of the curve given to a sustaining surface such as a wing. Thus it will be observed that the planes are not straight in cross-section but are concave slightly upward. The depth of this concavity is the camber. Another way of expressing this is that camber is the greatest distance between the surface of a wing and its chord line.
46. Gap.-The distance between the lower and upper wings of a biplane.
47. Spread.-The distance over all from one wing tip to the other wing tip.
48. Aerofoil.-A general name applied to any wing or lifting surface of an airplane.
49. Deadhead Resistance.-Fach part of an airplane against which the wind strikes offers a resistance against being moved through the air. This is called the deadhead resistance or the parasite resistance. It is for the purpose of lessening this resistance that the parts of a machine are stream-lined. Remember that force or power must be applied to overcome this resistance and the lessening of such resistance decreases the power necessary. A parallel illustration is to think of the power necessary to push a board sideways through water.
50. Drift.-When the air strikes the inclined wing of an
airplane its force has two components. One part called the lift (see 52) acts up and tends to lift the machine. The other part, called drift, tends to push the machine backward. Thi5 drift must also be overcome by applying power enough to drive the machine forward.
51. Total Resistance.-Sometimcs called drag. (49) Deadhead resistance added to (50) drift, gives the total forces oppos.. ing the forward movement of the airplane. This is called the~ total resistance and is overcome by the thrust of the propeller.~
52. Lift.-(See 50). The upward or vertical part of the air~ pressure acting against the wings, and which is utilized to lift or support the airplane.
53. Center of Gravity.-The point of balance of an airplane which may be otherwise defined as the point through which the mass of an airplane acts. If the weight is too far forward the machine is nose-heavy. If the weight is too far behind the center of lift the machine is tail-heavy.
54. Aspect Ratio.-The ratio of span to chord of a wing or any other aerofoil.
55. Gliding Angle (Volplane).-The angle made to the horizontal by the flight path of an airplane with the engine shut off; e.g., an airplane is 1000 ft. high, when its engine fails. Suppose its gliding angle is 1 in 6. Therefore, in stifl air it can glide 6000 ft. forward. The general term glide refers to flying without power.
56. The Angle of Best Climb.-The steepest angle at which an airplane can climb.
57. Stability.-The property of an airplane to maintain its direction and to return easily to its equilibrium or balance with a minimum of oscillation. This is sometimes called dynamical stability. An airplane may have (first) inherent stability, which is the stability due to the arrangement and disposition of its fixed parts. It may also have stability with regard to any one of the three directions in which it may move. These arc named as follows: (1) directional stability, with reference to the vertical axis; (2) lateral stability with reference to the
longitudinal (or fore and aft) axis; (3) longitudinal stability, stability with reference to the lateral (or thwartship) axis.
58. Flying Position.-Refers to the position of the fuselage when flying. With the Curtiss J N 4 machines in this position the top longerons are horizontal and level both ways. The engine bearers are also level, and the wings have an angle of incidence of 20.
59. Capacity.-The weight an airplane will carry in excess of the dead loa~~dead load includes structure power plant and essential accessories).
60. Flight Path.-The path of the center of gravity of an aircraft with reference to the air.
61. Stalling.-A term describing the condition of an airplane which from any cause has lost the relative speed necessary for support and controlling, and referring particularly to angles of incidence greater than the critical angle.
62. Sweepback.-The horizontal angle (if any) that the leading edge of a machine makes with the crosswise or lateral axis of an airplane.
63. Nose Dive or Vol-pique.-A dangerously steep descent, head on.
The materials of construction for airplanes should be of such material, size and form as to combine4 greatest strength and least weight. With metal parts in particular it may be necessary to substitute less strong material for the sake of getting non-corrosive qualities, ability to withstand bending, ductility or ease of bending, etc. With wood, absence of warping is important as well. The materials which are considered are the following: wood, steel, including wires; special metals as aluminum, brass, monel metal, copper, etc., and also linen and dope.
Strength of Materials.-It is important in a general way to understand the terms used in speaking of strength of materials. Thus we may have strength in tension, strength in compression, or strength in shearing, bending and torsion. Some material fitted to take tension will not take compression, as for example wire; some material, as bolts, are suited to take shear, etc.
In general all material for airplanes has been carefully tested and no excess material is used above that necessary to give the machine the necessary strength.
Tension.-This means the strength of a material which enables it to withstand a pull. Thus wires are used where strength of this kind is required.
Compression.-This refers to strength against a pressure. Wire has no strength for this purpose, and wood or sometimes steel is used.
Shearing.-Refers to strength against cutting off sideways. Thus the pull on an eyebolt tends to shear the eyebolt, or the side pull on any bolt or pin tends to shear the pin.
Bending.-In bending material the fibres on the. outside tend to pull apart; those on the inside tend to go together. Thus on the outside we have tension, and on the inside compression. Along the center line there is neither tension or compression, it is the "neutral axis.
Torsion.-Torsion is a twisting force, such as an engine propeller shaft receives.
Testing for Strength.-If a wire is an inch square in cross-section and breaks when a load of 150,000 lb. is hung on it, we say that the strength of the wire is 150,000 per square inch. Smaller wires equally strong have a strength of 150,000 lb. per square inch also, but they in themselves will not support a load of 150,000 lb. but only the fraction of that, according to the fraction of a square inch represented by their cross-section.
In the same way, a square inch of wood under a compressive load may break at 5000 lb. If, how- ~ ever, the piece of wood is long in proportion to its thickness, it will bend easily and support much less
weight. For example, a perfectly straight wall cane could perhaps have a ton weight put on it without breaking but if the cane were not ~. squarely or if it started to bend it would immediate break under the load.
These cases illustrate the importance of having struts perfectly straight, not too spindling an evenly bedded in their sockets. Some training machines are built with a factor of safety of 12. That is to say, the breaking strength of any part i&~. twelve times the ordinary load or stress under which the piece is placed. It should be remembered, however, that under any unusual condition in the _ air, such as banking, etc., extra strains are placed on I the parts and the factor of safety is much less than
12. Factor of safety of 12 thus does not mean exactly what it does in other engineering work, where allowances are made for severe conditions. The so-called factor of safety of 12 in airplane work is probably no greater than a factor of safety of 2 or 3 in regular engineering work.
There are three all-important features in the flying machine construction, viz., lightness, strength and extreme rigidity. Spruce is the wood generally used for parts when lightness is desired more than strength, oak, ash, hickory and maple are all stronger, but they are also considerably heavier, and where the saving of weight is essential, the difference is largely in favor of the spruce. This will be seen in the following condensed table of U.
S. Government Specifications.
Weight per Modulus of Compression
Wood cubic foot, rupture, strength,
pounds (15% pounds per pounds per
moisture) square inch square inch
Hickory 50 16,300 7,300
White Oak 46 12,000 5,900
Ash 40 12,700 .6,000
Walnut 38 11,900 6,100
Spruce 27 7,900 4,300
White Pine 29 7,600 4,800
A frequently asked question is: "Why is not aluminum or some similar metal, substituted for wood?" Wood, particularly spruce, is preferred because, weight considered, it is much stronger than aluminum, and this is the lightest of all metals. In this connection the following table will be of interest.
Weight in Tensile
Material - cubic feet, strength per Compression
pounds sq. in strength per sq. in
Spruce 27 7,900 4,300
Aluminum 162 15,000 12,000
Brass (sheet) 510 20,000 12,000
Steel (tool) 490 100,000 60,000
Nickel steel 480 100,000*
Copper (sheet) 548 30,000 40,000
Monel metal 540 90,000 30,000
Wood.-Present practice in airplane construction is to use wood for practically all framing, in other words, for all parts which take pressure or corn
* But has very high elastic limit.
pression. Although wood is not as strong for its size as steel and therefore offers more air resistant for the same strength yet the fact that frame part~ must not be too spindling, in other words, that they must have a certain thickness in proportion to their unsupported length, has led to the use of wood in~ spite of the greater strength of steel. Some airplanes, however, as the Sturtevant, are constructed with practically a steel framing.
It should be borne in mind that any piece or kind of wood will not answer for framing, and more especially for repair parts. There is a tremendous difference in the strength and suitability among different woods for the work. For instance, a piece of wood of cross or irregular grain, one with knots, or even one which has been bored or cut or bruised on the outside, may have only. half or less the strength of the original piece. Air drying doubles the strength of green wood, proper oven drying is better yet.
Notice how the ends of each piece are ferruled, usually with copper or tin. This is to prevent the bolt pulling out with the grain of the wood, and also prevents splitting and end checking and gives a uniform base on which the pressure comes.
It is generally advised not to paint wood as it tends to conceal defects from inspection. So varnish only.
Wrapping wooden members with linen or cord tightly and doping this, both to make waterproof and to still further tighten, increases the resistance to splitting. The absence of warping tendencies determine often what wood to choose.
The selection of lumber and detection of flaws is a matter of experience and should be cultivated. It is, however, nothing more than the extension of the knowledge that leads a man to pick out a good baseball bat.
Woods.-1. Spruce.-Should be clear, straight grained, smooth and free from knot holes and sap pockets, and carefully kiln-dried or seasoned. It is about the lightest and for its weight the strongest wood used. It is ordinarily used as a material for spars, struts, landing gear, etc., as it has a proper combination of flexibility, lightness and strength.
2. White Pine.-A very light wood used for wing ribs, and small struts.
3. Ash.-Springy, strong in tension, hard and tough, but is considerably heavier than spruce. Used for longerons, rudder post, etc.
4. Maple.-Used for small wood details, as for blocks connecting rib pieces across a spar or for spacers in a built-up rib.
5. Hard Pine.-Tough and uniform and recommended for long pieces, such as the wooden braces in the wings.
6. Walnut, Mahogany, Quarter-sawed Oak.-The strength, uniformity, hardness and finishing qualities make these woods favorites for propeller construction.
7. Cedar Wood.-Is used occasionally for fusel
age coverings or for hull planking in hydroplanes as it is light, uniform and easily worked. Veneers, or cross-glued thin layers of wood, are sometimes ~ used for coverings.
Laminated or built-up wooden members have been much used for framing and for ribs and spars. The engine bearers are always of wood on account of vibration and are also laminated. In lamination the wooden strut is built up of several pieces of wood carefully glued together. The grains of the different layers run in different directions, consequently a stronger and more uniform stick often
is secured. The objection to laminated pieces ~ comes from The weather causing ungluing. Laminated pieces should be wrapped in linen or paper and freshened with paint or varnish from time to time.
Forms.-Attention should be called to the hollowed form of many of the wooden members. In any beam or strut, material at the center of the cross-section is of far less value in taking the load than the material away from the center. Therefore, to secure greatest strength with least weight, it is permissible to lighten wooden members if done understandingly.
Steel.-There is a tremendous difference in the strength, wearing and other desirable qualities among different steels and irons. For airplane work none but the best qualities are allowed. For this reason the use of ordinary iron bolts (as stove bolts) or metal fastenings or wire not standardized
and of known qualities should not be permitted. the airplane is no stronger than its weakest fitting.
This does not mean that the hardest and strongest steel must necessarily be used, as ease of working and freedom from brittleness may be just as important qualities, but the steel on all metal fittings should be of high-grade uniform stock. A ductile, not too easily bent, mild carbon steel is usually recommended for all steel plate, clips, sockets and other metal parts. If any parts are required to be tempered or hardened it must be remembered that they become brittle and can not afterward be bent without annealing or softening. Tool or drill steel is a name given to uniform or rather reliable grades of steel adapted to heat treatment as tempering or annealing. Often the bolts, clips, nuts, pins, devices and other fittings are of special heat-treated nickel steel which must not be heated locally for bending or for attachment. Such work seriously weakens the steel. The steel is often copper- or nickel-plated and enameled to prevent rusting. Do not forget that the proper material may be twice as strong as other material which looks the same but which has not received special treatment.
Wires. Only the highest grade of steel wire, strand and cord is allowable. Manufacturers, as Roebling of Trenton, N. J., manufacture special aviator wire and cord, which is given the highest possible combination of strength and toughness, combined with ability to withstand bending, etc.
Steel wire ropes for airplane work are divided into three classes as follows:
1. The solid wire = 1 wire (as piano-wire grade) and known as aviation wire.
2. The strand stay, consisting either of 7 or
wires stranded together and known as "aviator strand." Flying and landing wires on Curtiss.
3. Cord or Rope Stay.-Seven strands twisted together forming a rope, each strand being of 7 or 19 wires and known to trade as aviator cord. The wires are either tinned or galvanized as protection against rust, etc. Ordinarily galvanizing is used, but hard wires and very small wires are injured by the heat of galvanizing and they are therefore tinned.
No. 1. The single wire is the strongest for its weight. Single wires will not coil easily without kinking and are easily injured by a blow, therefore their use is confined to the protected parts of the machine such as brace wires in the fuselage and in the wings.
The strand stay (No. 2) of 7 or 19 wires is generally used for tension wires, as it is more elastic (can be bent around smaller curve) without injury, as the flying and landing wires on the Curtiss. The smaller strands usually have 7 wires, the larger ones 19 wires.
No. 3. The Tinned Aviator Cord.-The 7 by 19 cord is used for stays on foreign machines. It is 1~j times as elastic as a solid wire of the same material. On the Curtiss it is used for control wires. For steering gear and controls extra flexible
aviator cord is also recommended. This has .a cotton center which gives extra flexibility and is used for steering gear and controls. It is 2j~ times as elastic as a single wire.
Although wire strands or cords are not quite as strong for the same size as a single wire they are preferred for general work, being easier to handle and because a single weak spot in one wire does not seriously injure the whole strand.
Especial care is necessary to avoid using common steel wires, or strands which have a frayed or broken wire, or wire that has been kinked and then straightened or wire that has been locally heated o~r wire that has been bruised. All these factors weaken steel rope much more than is supposed ordinarily.
Wire Fastening or Terminal Connections.-Wire terminals are of four classes:
1. Ferrule and dip in solder, then bend back the end. With or without thimble; used on single wires or on strand; 50 to 94 per cent. as strong as the wire.
2. Thimble and End Splicing.-The splice must be long and complete. Used on cable; 80 to 85 per cent. as strong as the strand; breaks at last tuck in the- splice.
3. Socket.-Nearly 100 per cent. strong.
4. End Wrap and Solder.-Simple and serviceable; not used for hard wire.
Present practice is rather toward elimination of acid and solder, imperfect bends, flattening of cable on bends, and toward care in avoiding all injury as
kinking to wire, strand and cord due to unskillful handling of material in the field.
Other Metals.-Other metals as aluminum, brass. bronze, copper, monel metal (copper and nickel)~ are used for certain airplane fittings for the reasons of lightness, non-corrosive qualities, or ease of' bending, etc. The trouble with these metals is that they are not uniform and reliable in strength and in an important part the great strength combined with minimum weight given by steel ~ is not equaled by any of these metals. Aluminum is used on the engine hood and also for control levers ~ and for the backs of the seats. In other words, for V parts and castings which require light metal construction, but which are under no particular stress. Tin and copper are used for ferrules of wire joints and for tankage. Copper or brass wire are used for safety wires. Special Tobin bronze is used for turnbuckles as the part must not only be strong but free from any tendency to rust. Monel metal (nickel 60 per cent., copper 35 per cent., iron 5 per cent.) is strong and has the special property of being acid- and rust-resisting. It has been used for metal fittings and even for wires and for the water jacket of the motor. Until more strength tests show greater uniformity of strength, it is to be recommended with caution.
In dealing with metals like steel, it should be remembered that they are subject to crystallization and fatigue.
Repeated jarring may cause a bar of steel to break easily at a particular point, when the metal is said to have crystallized there.
Fatigue of a metal may be defined as Loss of springiness which may come from repeated bending and which lessens the strength of metal. Above all, however, corrosion of steel must be guarded against.
The above points should be clear, as in airplane work you are dealing with a structure which is safe with perfect materials and workmanship. The factor of safety, however, is not great enough to permit carelessness, or defective material.
Linen.-The almost universal wing covering is fine, unbleached Irish linen, stretched rather loosely ~oiled wing frames and then treated with dope.
The linen used weighs 3~_i to 4~ oz. per square yard, and should have a strength with the length of the cloth or "warp" of at least 60 lb. per inch of width. The strength in this direction is slightly greater than that taken crosswise of the cloth or on the filler or weft. There is a gain of strength and tautness by varnishing or "doping."
In general, it is desirable to have wing material which will not sag easily and have the fabric yield rather than break. This often reduces stress and saves complete failure.
Dope.-Tec linen must be coated with a more or less waterproof dope. Some form of cellulose acetate or nitrate with more or less softening material is used and to these some suitable solvent as acetone is added.
The cellulose acetate or nitrate in the dope acts as a waterproof sizing, shrinks the cloth tight, and prevents it from changing in tightness due to moisture. Spar varnish protects this layer from ~ peeling and makes the wing more waterproof. In service, varnish or dope must be applied every few weeks.
The U. S. Army practice calls for four coats of cellulose nitrate dope followed by two coats of spar varnish to prevent inflammability. Cellulose acetate is more elastic and durable than the acetate but is also more inflammable.
Commercial dopes with various desirable properties are: Cellon, Novavia, Emaillite, Cavaro, Titanine, etc.