TM 1-410. Technical Manual, Airplane Structures 1941: Section 7 - Flight Control Mechanisms

SECTION VII: FLIGHT CONTROL MECHANISMS

20. General.-Flight controls of an airplane consist of the control stick (or column and wheel) and the rudder pedals with which the primary control surfaces are actuated, and the levers, wheels, and handles with which the secondary control surfaces are actuated. A complete set of flight controls is located in the pilot's cockpit and in some two-place airplanes in both cockpits. Controls installed in the cockpit are connected to control surfaces installed on wings and empennage by a system of cables, rods, bell cranks, etc., called flight control linkage. These assemblies for control of ailerons, elevators, rudder, tail wheel, and tabs are shown in figures 21 to 24, inclusive. Extra flexible steel cable consisting of 7 strands having 19 wires per strand is generally used in control linkage. These cables are guided through fair leads and over pulleys in such a manner as to avoid contact with other mechanisms and structural members of the airplane. Cables carrying relatively low initial tensions are often led through flexible steel conduit. Fair leads and pulleys are made of a nonmagnetic phenolic composition. Push and pull rods (or tubes) if of considerable length are supported and guided by rollers as shown in figure 25. All rods throughout the control linkage are adjustable at one or both ends.

21. Operation.-When the control stick (or column) (fig. 21) is pulled rearward, the trailing edge of the elevators is raised and when the stick is pushed forward the motion is reversed. When the control stick (fig. 22) is pushed to the right, the right aileron trailing edge is raised and that of the left aileron is lowered. The stick pushed in the opposite direction reverses this action. When the right rudder pedal is pushed forward the rudder trailing edge is swung to the right, and by pushing the left rudder pedal forward the rudder trailing

FIGURE 21 - Elevator control linkage

FIGURE 22 - Aileron control linkage 

 FIGURE 23 - Rudder and tail wheel control linkage 

 FIGURE 23 - Tab control linkage 

edge is moved to the left. As shown in figure 23, the tail wheel is subject to the same control as the rudder. In the case of tabs, the directions in the cockpit for moving the control are always stated in terms of effect on attitude of the airplane and not necessarily direction in which the tab is moved, as shown in figure 26.

22. Alinement-a. Each control has a neutral position. The neutral position of the control stick is slightly forward of the center of its range of movement in all -directions. The exact amount that the top of the stick should be forward of this midposition usually is specified in the Technical Order Handbook of the airplane. Rudderpedals are in neutral when they are in the center of their movement range.  Tab controls are marked to indicate their neutral position.

Caution.-Rudder pedals are also adjustable to suit leg length of each pilot, but this should not be confused with rudder cable adjustments.

FIGURE 25.-Wing flap and control rod and tube.

b. The neutral position of control surfaces are as follows:

(1) Rudder is in neutral when it alines with the center line of the fuselage; that is, lies in the plane of symmetry of the fuselage. In some few cases, neutral position of the rudder is specified as the position in which it streamlines with the vertical fin. The rudder movement is equal in both directions from neutral.

(2) Elevators are in neutral when they streamline with the horizontal stabilizer, the latter if adjustable being in its neutral position.

The range of elevator movement from neutral is greater upward than downward to provide adequate control for holding tail of the airplane down during landing, especially after the wheels touch the ground.

FIGURE 26.- Tab controls. 

(3) Ailerons are in neutral when they streamline with their respective wings, or in case droop is specified, they droop by similar amounts. An aileron droops when it hangs below the position in which it streamlines with the wing. Droop is measured as the distance of center of trailing edge of aileron below center of trailing edge of wing. Ailer

ons usually have a greater up travel than down. This causes a greater drag on the down wing (up aileron) than on the upper wing (down aileron) and assists the rudder in making correct turn for a given direction of bank. Difference in extent of travel of ailerons is accomplished by means of a differential incorporated in the aileron control linkage.

c. (1) The three phases in alinement of flight control systems are

(a) Synchronize controls and control surfaces in neutral.

(b) Establish proper tension in control cables.

(c) Limit movement of controls.

(2) In the first phase, the controls in the cockpit are placed and held in their neutral position. The control surfaces, each in turn, are then brought into their neutral position by means of adjustments provided in the control linkage. One turnbuckle is tightened and the opposing one loosened.

(3) In the second phase, tension of control cables is checked by means of a tensiometer in accordance with tensions authorized for the particular cable and airplane. In case tension of a cable is found to be incorrect, the two turnbuckles are each shortened or lengthened a given amount until correct tension is obtained. If these two turnbuckles are not adjusted in like manner, alinement of controls already attained will be altered. Where authorized initial tensions are not given, the tensiometer is not used and the tension is checked by feel of cables and action of controls. Some tab controls employ two main cables and one balance cable which extends directly from one tab to the other, each having one turnbuckle. In this case the simplest procedure is to aline to neutral one tab only by using the two main cable turnbuckles, ignoring the other tab and the balance cable turnbuckle. With this tab in neutral and proper tension in control cable, the other tab is then alined to neutral by using the balance cable turnbuckle and the turnbuckle in the main cable that extends to the second tab. Care should be taken not to alter adjustment of the other main cable turnbuckle and to let out and take up exactly similar amounts on the two turnbuckles controlling alinement of the second tab.

(4) In the third phase, each control in the cockpit is moved in turn longitudinally or laterally as far as it will go, and in each case the extent of movement of the control surface is checked. A protractor is commonly used for this purpose. The limiting stop shown in figure 27 is adjusted until extreme attitude of the control surface in that one direction is satisfactory. The control is then moved in the opposite direction and the procedure repeated. No readjustment of control cables is involved in this procedure.

It is extremely important that the pilot be able to move the surfaces through entire specified range. A reduction of only a few degrees in allowable movement may reduce the maneuverability of the airplane and result in poor recovery from spins. After a turnbuckle has been adjusted, it must be properly safetied in such a manner that the safety wire opposes the tendency of the turnbuckle to loosen.

23. Maintenance-a. Ball-bearing assemblies used on control mechanisms are usually of the sealed type and do not require an external application of lubricant.

FIGURE 27-Aileron travel limiting stop.

Some of the pivots where the load is light or the movement relatively slow are plain bearings. This type of bearing must be lubricated and fittings are always provided for this purpose.

b. Because of the number of wires in control cables, their failure is never abrupt but is progressive over periods of extended use. Many broken wires encountered in use show up soon after placing the cable in service due to the fact that some of the wires are under greater tension or are much harder than the rest. After these overstressed or overhard wires have broken, very few additional broken wires will be encountered in normal service for a considerable time. Control cables are generally considered serviceable unless there are more than six broken wires in any 1 inch length of cable. Close attention should be given to that portion of cable which passes over pulleys or through fair leads. Any indications of rust should be investigated with reference to extent of internal damage.

24. Precautions against fouling.-Serious hazard is present in the possibility that flight controls might be jammed when articles such as microphones, flashlights, oxygen mouthpieces, etc., are dropped to the floor and come in contact with the controls. All personnel are cautioned that each time microphones, oxygen tubes, or other accessories in aircraft are used, they are replaced and securely seated in the carrying hooks or other receptacles provided. In the case of microphones and oxygen mouthpieces, an extra length of wire and tubing is taped to a convenient part of the airplane in a manner that will prevent them from contacting any part of the controls if they are dropped.