Methods that ensure accurate assembly and correct trim in flight
AN AVRO TRAINING BIPLANE is a good example from which to illustrate the various items of the rigging of an aeroplane. During the process of rigging, the machine is lifted just clear of the ground on trestles. The height of the trestles is generally such that the machine is held in the same attitude as that in which it flies level. Correctness in the height of the trestles is most important, because, if the aircraft were rigged while in the wrong attitude, the machine would not fly properly.
THE type of aeroplane chosen to illustrate the sequence of operations followed in rigging an aeroplane is a small two-seater biplane fitted with dual control, of the type used by many flying schools and clubs.
The first thing to be done is to support the fuselage on trestles at front and rear. The front trestle will be placed so that it is clear of the undercarriage fittings and leaves sufficient clearance for the undercarriage when attached.
The undercarriage comprises two shock-absorbing legs, supporting struts and axles for the wheels. These members are assembled as a complete unit and then attached to the fuselage. After the attachment bolts have been secured, the undercarriage is checked for position.
Plumb-lines are dropped from corresponding points on either side of the fuselage and measurements are taken outwards from the plumb-lines to the ends of the axle. The measurements should be the same on either side. To check alinement of the wheels, measurements with a steel tape are taken from the extremities of the axle to the bottom of the rearmost fuselage member.
The undercarriage completed, the tail unit may next be attached. The horizontal surface is the tailplane and to its rear edge are hinged two flaps of slightly smaller area than the tailplane. These are the elevators. The vertical surface above the centre line of the tailplane is the fin and it has the rudder hinged to its rear edge.
The fin is bolted to fittings on the fuselage. The tailplane is attached at three points: two at the front which provide a hinging movement, and one at the rear to a vertical member inside the fuselage. This member is capable of vertical travel for a few inches when actuated by a handwheel inside the cockpit. Communication between handwheel and member is made by a light cable, chain and worm gear.
Apart from seeing that the handwheel and worm gear are in their correct relative positions (that is, wheel wound fully back, worm gear at the top of its travel), no further adjustments are made to the tail unit at this stage.
The centre section is the small top centre plane. This is now assembled with the struts which support it above the fuselage, and attachments are made and secured. It is important that the centre section shall be truly rigged, as on its correct disposition depends the setting of the upper wings.
THE UPPER AND LOWER WINGS of either side of the machine are joined together by their struts and wires before being attached to the aircraft. This is known as “boxing” the planes. The wings are placed with their leading edges on blocks on the floor during the process, and a temporary strut is put in to support the inner ends of the wings while they are being lifted into position.
The next step then is to put the fuselage into rigging position. The rigging position is specified by the manufacturers and details are obtained from the rigging diagram, which also quotes all the measurements. As an example, rigging position may be specified as being when a certain portion of the longerons (longitudinal fuselage members) are horizontal longitudinally and transversely.
To put the fuselage into rigging position, adjustable trestles are used under the fuselage at front and rear. A straight-edge is placed across the specified portion of the longerons, and a spirit level used on the straight-edge. Adjustments are made to the front trestle until the bubble of the spirit level is central. A check is next made in a similar manner longitudinally and the rear trestle is adjusted until the correct reading is obtained. A final check is made to ensure that the transverse level has not been upset. Rigging position on the ground generally corresponds to the normal flying position of the aeroplane.
To true up the centre section, plumb-lines are dropped from the front fittings which receive the upper wings. Measurements taken inwards from these plumb-lines to the top longerons are made to equalize by adjusting the diagonal bracing wires between the front pair of struts. The angle which the centre section from front to rear makes to the horizontal is measured by a straight-edge held against the under surface and parallel to a rib. A straightedge of greater length than the centre section is used, and on the projecting portion a clinometer, or similar instrument capable of measuring degrees and minutes, is placed. The instrument is first set to the angle specified on the drawing. The side diagonal struts which have one screwed end for adjustment are each adjusted an equal amount until the bubble of the spirit level, incorporated in the instrument, is central - thus showing that the angle of inclination of the base, which is resting on the straight-edge, is that to which the instrument was set.
The next step is boxing the mainplanes. Taking one side at a time, the upper and lower wings are placed with their leading edges on blocks on the floor and at right angles to the fuselage. The struts and bracing wires between the wings are put into place; and a temporary strut, called a jury strut, is put in to support the inner, or root ends of the wings, while they are being lifted into position.
When all is ready, one pair of wings is lifted bodily into position, and the bolts securing the root ends to the centre section and fuselage fittings are inserted. The jury strut is removed and the inner ends of the bracing wires are attached to their fittings. A trestle is placed under the lower plane to support the assembly while the operation is repeated on the other side.
The mainplanes are now ready for truing up. Various measurements, such as the angle of incidence, have to be taken into consideration, and the planes adjusted accordingly.
The angle of incidence for this purpose is the angle between a straightedge held against the underside of the main-plane parallel to a rib, and the horizontal. This angle, generally about 3 degrees, is calculated to obtain the best performance from the particular cross-section of wing used. Any error in setting this angle will cause the aeroplane to fly badly. If the angle is too large, the aeroplane will fly “tail heavy”; if too small, “nose heaviness” will result; and if there is a difference between the angles of the wings on right- and left-hand sides, the aeroplane will fly “one wing low”.
Dihedral angle is the angle between a straight-edge placed at right angles to the ribs, on the upper surface of the wing, and the horizontal. The function of dihedral is to give the aeroplane stability laterally when in flight. This is achieved in the following way. When the aeroplane is disturbed from normal flight by a gust under one wing and tilted sideways, the forces acting on the aeroplane cause it to slip in the direction of the downgoing wing. This sets up an airflow parallel to, but opposite to, the direction of motion. If there were no dihedral, the air would flow smoothly over the planes, but because of the greater tilt given to the upgoing planes by dihedral, the airflow meets the upper surface of those planes and forces the aeroplane back to an even keel.
THE DIHEDRAL OF THE WINGS is tested by means of a straight-edge resting on two blocks placed on one of the spars of a wing. On the straight-edge is placed a clinometer which enables the angle the wing makes with the horizontal to be correctly set. This angle is known as the dihedral of the main planes, which are higher at their tips than at the fuselage. The object of the dihedral is to provide the aircraft with lateral stability.
The horizontal distance by which the leading edge of the upper plane is in advance of the lower is known as “stagger”. As the greater part of the lift of a wing is obtained from its upper surface, it is necessary so to place the planes of a biplane that there is a minimum amount of interference between the airflow over the upper surface of the bottom plane and the lower surface of the top plane. By giving the planes stagger, this interference is considerably reduced and the pilot’s field of view is improved at the same time.
Before truing up the wings, a further check on the rigging position will be made to see that the attitude of the fuselage has not been disturbed during the attachment of the wings.
Each wing is built on two main members, which run throughout its length. These are the spars, and dihedral is checked by placing two hardwood blocks of equal dimensions on the spar, and a straight-edge, with a clinometer set to the specified angle, across the blocks. Adjustments are made on the front pair of wires between the interplane struts and fuselage. These wires are of bi-convex section, and the ends are round and threaded left- and right-hand. These threaded ends are received into corresponding fittings on the wings and fuselage. Thus, if the wire is turned in one direction it will slacken, and vice versa. The effect is to increase or decrease the angle of inclination of the wing according to the direction of rotation of the wire.
The wires which run from the centre section downwards and outwards to the bottom of the interplane struts are the landing or anti-lift wires, and they support the wings when the aeroplane is on the ground. The opposite wires from the top of the interplane struts to the bottom of the fuselage are the flying, or lift wires, and help to support the fuselage in flight. To increase dihedral the flying wire is first slackened, and the landing wire tightened.
The angle of incidence is next checked in the manner described for the centre section. The checks are made near the root and near the interplane struts, on the lower surface of the bottom plane. As the root end attachments are fixtures, the only adjustment will be made at the outer end of the plane, so that the angle of incidence is constant throughout. To increase the angle of incidence, the rear landing wire is slackened until the correct reading is obtained, and the rear flying wire tightened until the correct tension is obtained. Correct tension is a matter of experience and is judged by “feel”: but as a general rule, satisfactory tension will be obtained if the wire is given from a half to a full turn with the special tool, after having been tensioned as much as possible by hand.
Tolerance to Avoid Distortion
Stagger is checked by dropping plumb-lines over the leading edge of the top plane, in line with the interplane struts. The horizontal measurement is taken between the plumb-line and the leading edge of the bottom plane. Adjustments are made on the pair of wires, or diagonal strut, between the interplane struts, and on the rear landing wire.
As the rear landing wire is important in controlling the angle of incidence, it is necessary to adjust the incidence and stagger together and to make constant checks on each during adjustments. To avoid overtensioning the wires and possibly distorting the structure, a small tolerance is allowed on each measurement. On a biplane of this type, a tolerance of plus or minus 15 minutes on dihedral and incidence angles, and plus or minus ⅛-in on stagger is permissible.
After all adjustments have been made, the wings are checked for squareness with the fuselage. A steel tape is used and measurements are taken from the interplane strut fittings to the centre of the airscrew hub, and to the bottom of the rearmost fuselage member.
The tailplane is next trued up. The requirements are that it shall be laterally level; that the angles of incidence in the maximum, minimum and mean positions shall be correct; and that the tailplane shall be square with the fuselage.
Lateral level is checked by placing two blocks on the front spar and a straight-edge and spirit level on the blocks. Any error will be due to incorrect alinement of the fuselage members, and is likely to occur only if the fuselage is of the wire-braced type, when adjustments may be made.
Angles of incidence are checked with the tail incidence actuating wheel in the cockpit wound fully back, then fully forward, and finally in the mean position. If an adjustment is required it is made by resetting the positions of the chain and worm gear.
The function of the tailplane is to give longitudinal stability to the aeroplane by providing an upward or downward force at the rear of the fuselage when the attitude of the aeroplane is disturbed about its lateral axis. Alterations in the position of the centre of gravity affect the longitudinal balance of the aeroplane. Changes in the position of the centre of gravity may be caused by the aeroplane being flown solo, or on a long flight by the consumption of petrol. When such a change of position takes place the pilot would, if the incidence of the tailplane were fixed, have to “trim” the aeroplane by holding the control column slightly forward or back. As, however, the incidence of the tailplane is variable, it is possible to trim the aeroplane by the use of the handwheel in the cockpit. Use can also be made of the variable incidence in taking off and gliding, when the forces are altered by the reduction of the thrust. In other words, it may be said that slight nose heaviness or tail heaviness can be counteracted by the use of the tail-plane variable-incidence gear.
Truing Up the Controls
The final check of the tailplane for squareness with the fuselage is done by taking measurements from corresponding points at the ends of the tailplane, to points on, or equidistant from, the centre line of the fuselage; for example, from the tips of the front spar to the rear centre section strut fittings.
The next operation is truing up the controls.
To adjust the elevators the tailplane is set to its mean position and the control column is clamped firmly in the central position in the cockpit. Adjustments are made to the control cables until the elevators are in true prolongation with the tailplane. The clamp is removed from the control column and the full upward and downward travel of the elevators is measured, either with a clinometer placed on the elevator or with a straight-edge and rule held vertically against the trailing edge. The amount of movement must be in accordance with the maker’s specification.
The control column is again clamped central for truing up the ailerons. The same procedure is adopted, the trailing edge of the ailerons being set flush with or slightly below the trailing edge of the mainplane. After the setting, the angular movement is checked.
TESTING THE ANGLE OF INCIDENCE of the lower right wing. The procedure is similar to the test for dihedral angle, and is made with a clinometer on a straight-edge laid along one of the ribs of the wing. The angle of incidence is the angle to the horizontal at which the wing is tipped up towards its leading edge. Variation of this angle alters the lift of the wing.
Finally the rudder bar is set square in the cockpit and the rudder controls are adjusted until the rudder is pointing truly fore and aft. The travel on either side is checked by dropping a plumb-line from the trailing edge of the rudder just to touch the ground, and measuring the distance on either side of central when the rudder bar is pushed fully left or right. The tolerance on angular movement of control surfaces is plus only. Up to one degree would be permissible.
Before installation of the engine the engine bearers or engine-plate will be checked for alinement. The engine having been installed, the aeroplane is now ready for final inspection before flight test.
The inspector works systematically, starting from one point. First all fabric-covered surfaces will be examined for signs of damage which may have occurred during assembly. Small holes will be sewn up and patches placed over them. Next, leading and trailing edges, interplane struts and control surfaces will be inspected for any signs of distortion caused by overtensioning of wires when truing up. Detailed inspection of all attachments will be made to ensure that each bolt or pin used is of the correct size, and that bolts have not been overtightened. Fittings may be crushed into the wooden member to which they are attached by overtightening bolts. On the other hand, the nuts may run up to the ends of the thread on the bolt and, although no space may be showing, any play will be betrayed by shaking. The nuts must have all threads engaged and must be secured by an approved method against slackening.
The threaded ends of the bracing wires must be engaged to a certain depth in the barrels which receive them. To check this, each barrel is provided with a small inspection hole which must be “blind” for the wire to be in safety; that is, when a pin is inserted into the small hole, it must not pass through, but must be stopped by the threaded part of the wire.
The wires must be secured against rotation either by means of lockmits or by clamping them together where they intersect. This method also prevents chafing of one wire against another. In addition, the flying and landing wires must have their major axes parallel to the line of flight. The tension of all bracing wires is checked by feel.
Before lowering the aeroplane from trestles so that the correct functioning of the undercarriage and tail skid may be checked, the wheel brakes are tried by spinning the wheels and swinging the rudder bar to left and right, and noticing the differential effect on the braking. Tyre pressures are checked with a gauge, and the aeroplane is lowered to the ground and gently rocked from the wing tips to see that the telescopic shock-absorbing struts are working properly.
The air screw is checked for truth of track by turning it to a vertical position and placing a trestle fairly close in front of it. A measurement is taken from the trestle to the tip of the airscrew blade and noted. The airscrew is then turned through 180 degrees, and the measurement to the tip of the other blade is taken. It should not exceed the previous measurement by more than 3/16-in either way. Should it do so the hub bolts must be slackened off and tightened down evenly.
Inspection of controls must be duplicated and done by different individuals. Each control cable is traced through from the cockpit to the surface it actuates. All attachments must be secure, and adjustment points must be locked to prevent further tightening or slackening by vibration.
Test for Airworthiness
The hinges of control surfaces and. the pivots of operating levers must all be secure. With the control column and rudder bar central, all control surfaces should be neutral. Direction of movement is important, as in some instances it is possible to cross the control cables, causing a reversal of movement.
The cables must be clear of all parts against which they may chafe, and the movements must be free and smooth, without play or stiffness at any points. After the operation of controls from the cockpit has been checked, the instruments are cheeked for position and security. The engine is run up, oil pressure and full engine revolutions on the ground are noted, and the aeroplane is ready for a flight test.
During the test the behaviour of the aeroplane on the ground and in the air is carefully watched. Signs of vibration are looked for. All controls are centralized and the aeroplane should maintain an even keel. The time taken to climb to a specified height is noted with a stop-watch. Air speed and engine revolutions when climbing, cruising and flying at full throttle are compared with the figures specified by the manufacturers. If the pilot’s report is satisfactory, the aeroplane is certified as airworthy. This certification is valid for twelve months. At the end of this time a complete overhaul is carried out, worn or defective parts are replaced and the re-rigging is begun.
TO MEASURE THE STAGGER of an aeroplane, a plumb-line is dropped over the leading edge of the upper plane. The stagger is the distance from the plumb-line to the leading edge of the lower plane. The object of stagger is to prevent the airflow over the lower surface of the top plane from interfering with the airflow over the upper surface of the bottom plane. Generally, it also provides the pilot with a better view.