December 2013

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

Issue link: http://experimenter.epubxp.com/i/234576

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Page 33 of 40

F li g h t Te s t in g Te c hn i q u e s EAA stafers are currently installing the control system on the Zenith CH 750 that employees are building. We will bring you further updates on this project in an upcoming issue of Experimenter. Flight Control Forces Breakout and Friction By Ed Kolano An airplane's flight control system is the physical connection between the pilot and the plane. It provides the mechanism to maneuver the plane while constantly communicating with the pilot through forces and displacements. This tactile feedback has a major influence on the pilot's opinion of the airplane. Heavy control forces make the plane feel stiff while light forces can give the impression of oversensitivity. A similar argument can be made for control displacements, but pilots rely more on control forces (unless displacements are large) when judging an airplane's responsiveness. Most of us fly planes with reversible control systems. Move the elevator during your walkaround, and the stick moves—reversible. That's why you have to exert a force on the stick to deflect the elevator away from its trimmed position. Dynamic pressure exerts a force on the deflected elevator that creates a hinge moment, which becomes the force you feel on the stick. Some flight control systems employ springs, which create a stick force when the stick is displaced. Some have bob weights, which create a stick force when the plane is flown at other than 1g. Some have dampeners that create a force proportional to how fast the stick is moved. Servo and anti-servo tabs also augment the stick force. Flight control system gizmos abound. And there's friction. There's always friction. 34 Vol.2 No.12 / December 2013 Friction Friction is the resistance encountered when two objects in contact move or attempt to move relative to each other. To overcome this resistance, the force applied must be greater than the opposing friction force, and friction always opposes the applied force. This relative motion can be sliding, such as dragging a chair across the floor. It can be rolling, such as, well, rolling a bowling ball down the lane. Flight control system friction sources include bearings, cables in pulley channels, pushrods through guides, cable–bell crank connections, and control surface hinges. Zero friction is impossible, but the less friction, the better. Before a flight control surface can be deflected, the pilot must apply enough force to the cockpit control to overcome any friction. As the control is moving, friction must still be overpowered by the pilot in addition to the force of the air load on the surface and any other forceproducing components such as springs in the system. Friction always adds to the required cockpit control force, and it works in both directions. Friction causes a hysteresis effect. Let's say there's a constant 2 pounds of friction in the elevator system. The pilot begins pulling the stick back; but nothing will happen until his pull force exceeds 2 pounds, then the elevator begins to move. So now let's say the pilot has 3 pounds of stick-pull applied to maintain the

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