TM 1-410. Technical Manual, Airplane Structures 1941: Section 6 - FLIGHT CONTROL SURFACES

SECTION VI: FLIGHT CONTROL SURFACES AND WING FLAPS

17. General.-a. Flight, control surfaces are hinged or movable airfoils designed to be rotated or otherwise moved by the pilot in order to change the attitude of the airplane during flight. They consist of

(1) Primary group made up of ailerons, elevators, and the rudder, by which the airplane is moved about its various axes.

(2) Secondary group composed of trim tabs, balance tabs, and servo tabs used for reducing force required to actuate primary control surfaces, or for trimming and balancing airplane in flight.

(3) Wing flaps which are intended to reduce landing speed of fast airplanes, shorten length of landing roll, and to facilitate landing in small or obstructed areas by permitting gliding angle to be increased without appreciably increasing gliding speed.

b. Rudder and elevators are members of the empennage, and ailerons are hinged sections of the main wings. The rudder controls the airplane directionally about its vertical axis, the elevators control the airplane longitudinally about its lateral axis, and the ailerons control the airplane laterally about it's longitudinal axis. To reduce the effort required by the pilot to operate the controls, these primary control surfaces are aerodynamically balanced. This involves hinging the surface so that a portion of the area is forward or ahead of the hinge line. As a result the forces acting on this area tend to balance some of the forces acting on the area behind the hinge line. Control surfaces also may be balanced statically by addition of lead weights in front of the hinge line. This is done to prevent any tendency of the control surface to flutter.

18. Structure-a. Primary control surfaces are movable surfaces and usually consist of an aluminum alloy structure built around a single spar member or torque tube. Ribs are fitted to the spar at the leading edge and at the trailing edge are joined together with a suitable band or metal strip. The leading edge or nose portion

FIGURE 1 8.-Rudder framework with solid ribs.

of the surface is covered with thin aluminum alloy sheet back to the spar member, thus forming the front part of the structure. The above construction is shown in figures 18 and 19. The former shows a rudder framework which has solid stamped ribs while the latter shows an elevator framework made up of ribs having lightening holes. Both are equipped with secondary control surfaces. Many of the primary control surfaces are fabric-covered. Such surfaces, being lighter than those with metal covering, require addition of less weight to produce proper static balance.

b. Secondary control surfaces are relatively small airfoils attached to or recessed into the trailing edge of the primary control surfaces and consist of trim tabs, balancing tabs, and servo tabs.

(1) Trim tabs are used to control the balance of an airplane in flight so that it will maintain straight level flight without pressure on the control stick or rudder pedals. This is accomplished by moving or deflecting the tab in the direction opposite to that in which the primary control surface is to be moved. The airflow striking the, tab causes the main surface to move to a position that will correct the unbalanced condition of the airplane. Trim tabs may be controlled from the cockpit or are of the ground adjustable type. In the latter case the tab is simply a strip of metal attached to the trailing edge of the primary control surface. In this case the tab is bent up or down as required to obtain desired result.

(2) Balancing tabs are similar in appearance to and are hinged in approximately the same location as controllable trim tabs. The essential difference between the two tabs is that the balancing tab is so connected to the wing structure by a rod that when the control surface is moved in any direction, the tab is rotated in the opposite direction. The result is a decrease in effort required by the pilot to move and to hold the control surface in any given attitude. When the control is released, the control surface will return to its neutral position. In case the rod between the tab and the wing structure is adjustable, the tab also becomes a ground adjustable trim tab.

FIGURE 19-Elevator framework with ribs having lightening holes.

(3) Servo tabs are used primarily on large airfoils to aid the pilot in moving primary control surfaces. The tab control linkage is crossconnected in parallel with the primary control linkage which is also connected in parallel with a. spring-loaded cartridge. The purpose of the spring is to allow the servo tab movement to precede the movement of the primary control surface. The airflow striking the tab moves the primary surface in the opposite direction, thus all the pilot does is move the tab, but should the airflow not be sufficient, further movement of the controls by the pilot will compress the spring and move the primary control surfaces manually.

c. Wing flaps are relatively large airfoils hinged to the wing structure. As a rule, the lower surface of the rear portion of the wing extending from the aileron inward toward the fuselage becomes the flap. When closed, the flap constitutes a section of the lower surface of the wing, and usually swings downward to open as shown in figure 20. Wing flaps are operated hydraulically or by means of an electricmotor. A mechanical system which can be operated by a hand crank in the cockpit is generally provided for emergency use. In taxying, the flaps should be fully retracted to protect them from stones or other objects thrown back by the propellers or kicked back by the wheels.

FIGURE 20-Wing flaps in down position.

19. Maintenance.-a. All movable parts and bearings must be kept well lubricated as specified in the handbook of service instructions for the particular airplane involved. Care must be exercised when cleaning around metal-shielded ball and roller bearings as they are packed with grease at time of assembly and under normal conditions this is sufficient for the life of the bearings. Washing with grease solvent would remove the lubricant and necessitate replacement of the bearing assembly. Such bearings are disassembled and lubricated only by repair depots during the overhaul of the airplane. Ground adjustable trim tabs should be bent to desired setting by using a clamp or wooden blocks so that entire surface of tab is adjusted evenly.