L AGUNAS DIAPÍRICAS SOMERAS DE APORTACIÓN MIXTA SEMIPERMANENTES FLUCTUANTES .140

Modern airplanes are almost all of the full can-

tilever type. This type has no external struts or brac-

ing, or are braced with a minimum of struts, gener- ally of a fixed length. Earlier airplanes, especially biplanes, were braced by a maze of wires and struts, and their assembly and rigging was a time-consum- ing process and required much skill on the part of the technician.

The assembly of a cantilever airplane consists of fol- lowing the instructions in the manufacturer's main- tenance manual in detail. No attempt should ever be made to assemble an aircraft without this vital infor- mation. In general, the fuselage is leveled in the manner specified in the maintenance manual, and the wings are installed and the attachment bolts are torqued to specification. The fixed horizontal and vertical tail surfaces are installed and all of their attachment bolts are torqued, again to the specified values. After all of the fixed surfaces are installed, the movable surfaces are installed and the control actuating mechanism is attached and adjusted. WING ALIGNMENT

Cantilever wings have very little adjustment potential, as this is all taken care of when the airplane is built. Some airplanes have either a cam arrangement or a serrated washer at the rear spar attach- ment bolt, and a few degrees of wash-in may be set in the wing to correct for a wing-heavy flight condition.

Strut-braced wings using V-struts normally have

provisions for adjusting both the dihedral angle and the incidence angle of the wings. Install the wing and check the fuselage to be sure that it is level both longitudinally and laterally, and adjust the fittings in the end of the front struts to get the correct dihe- dral. This is determined by using a dihedral board that has a specific taper. It is held against the main spar on the bottom of the wing at the location spec- ified by the manufacturer, and the fitting in the end of the strut is screwed either in or out until the bot- tom of the dihedral board is level. Some aircraft,

rather than measuring the dihedral with a dihedral board, use a string stretched between the wing tips at the front spar. When the dihedral is correctly adjusted, there will be a specific distance between the wing root fitting and the string. [Figure 1-67]

Figure 1-67. When the bottom of the board is level, the wing has the proper dihedral. Before this check can be made, the aircraft must be leveled according to specifications.

When the dihedral is correctly adjusted, the wash-in or wash-out may be set. This is normally done by adjusting the length of the rear strut. An incidence board similar to a dihedral board is held under a specified wing rib, and the strut length is adjusted until the bottom of the board is level. On airplanes having this adjustment, the initial setting will likely have to be changed after the first flight to trim the airplane for straight and level hands-off flight. Increasing the angle of incidence, that angle between the chord line of the wing and the longitudinal axis of the airplane, is called

"washing the wing in" and it increases the lift. Washing out a wing is done by rigging it with a

lower angle of incidence to decrease its lift. [Figure 1-68]

Figure 1-68. An incidence board is used to check for wing warpage and for the proper wash-in or wash-out.

AILERON INSTALLATION

After the wing is installed and aligned, and all of the attachment bolts torqued and safetied, the ailerons may be installed and rigged. It is important that the hinges be shimmed in exact accordance with the manufacturer's specifications, and that the control

rods or cable be attached to the horns with the proper type of bolt. The attachment must be free to pivot, yet have no excessive looseness. The cable tension must be adjusted, and the stops checked to be sure that the travel is that specified in the Type Certificate Data Sheet. Level the aileron and install a protractor such as a universal propeller protractor or a special control surface protractor. Zero the protrac- tor when the aileron is in the exact trail position, and then deflect it upward until it contacts its stop and measure its travel. Next, deflect it downward until it contacts the stop, and measure its travel. The travel must be within the specified range. If it is not, the stops must be adjusted. When the stops have been reset, move the ailerons through their full travel from the control wheel to be sure they travel through the entire range and that they are stopped by the stops in the wing and not by those at the control wheel. Check to be extremely sure that the control wheel moves in the proper direction for the aileron action. Serious accidents have been caused by ailerons being rigged backward. [Figure 1-69]

Figure 1-69. A universal propeller protractor or a special control surface protractor are common tools used to set the correct control surface travel. In all cases, follow the manu- facturer's recommendations for both the tools and the pro- cedures to be used for any rigging operation.

Some ailerons are required to have a few degrees of droop. This means that when there is no airload they should both be a few degrees below the trailing edge of the wing. If the ailerons you are rigging have this requirement, be sure that they are properly drooped. The final check in rigging the ailerons is to be sure that all of the turnbuckles and any bolt or connector in the entire system are properly adjusted and safetied. Leave nothing to chance. Start at the control wheel, and systematically check every connection in the system to the aileron, through the balance cable to the other aileron and back to the control wheel.

FLAP INSTALLATION

The flaps are connected to their hinges and actuator rods in a manner similar to that of the ailerons. However, Fowler flaps are normally mounted on rollers that ride in tracks, and these must be adjusted so they ride up and down smoothly with no binding or interference.

There are a number of actuation methods for wing flaps. The simplest flaps are actuated by either cables or a torque tube directly from a hand lever in the cockpit. Other airplanes use electric motors to drive jacks crews that move the flaps up or down, and many of the larger aircraft use hydraulic actua- tors to provide the muscle to move the flaps against the air loads.

In a single-engine airplane flap system, the flaps are moved by cables from an electric motor-driven jackscrew. Limit switches shut the motor off at the full up and down position, and a cam-operated fol- low-up system allows the pilot to select various intermediate flap positions. When the flaps reach the selected deflection, the motor will stop. [Figure 1-70]

EMPENNAGE INSTALLATION

The tail surfaces on almost all modern airplanes are of the cantilever type and are bolted to fittings in the fuselage. Special care must be exercised that all the bolts used have the proper part number and that all of them are tightened in the proper sequence and to the correct torque.

Some lighter aircraft have wire-braced tail surfaces. When securing the horizontal stabilizer with streamlined wires, those having a flattened oval cross section, it is important that the wires be adjusted to the proper tension and that they be streamlined in the line of flight to minimize vibration.

Figure 1-70. A typical small aircraft flap actuation system uses a series of cables, bell cranks and push rods and may be actuated manually by a hand lever in the cockpit or by an electric motor.