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Cables and Hardware – Weight-Shift Control Aircraft

Filed Under: WSC Components and Systems

Cables are used throughout the wing frame and sail to hold components in place and act as structure to carry loads. Flight and ground cables are stainless steel and attach to components with tangs or other hardware depending on the application. Cables are secured at each end with thimbles and swaged fittings. Figure 3-5 shows detail of typical swaged fittings. A variety of hardware is used for attaching these swaged cable fittings to the airframe. Each manufacturer has different hardware for wing components. [Figures 3-14 and 3-15]

Figure 3-14. Crossbar tensioning junction attachment example.
Figure 3-14. Crossbar tensioning junction attachment example.
Figure 3-15. View inside wing showing top wire coming though sail that is reinforced, being attached to the crossbar by a tang, an aircraft bolt, washers, and lock nut.
Figure 3-15. View inside wing showing top wire coming though sail that is reinforced, being attached to the crossbar by a tang, an aircraft bolt, washers, and lock nut.

Wing Systems

Reflex Systems

As discussed in the aerodynamics section, the trailing edge near the root and the tips must stay up during unusually low or negative angles of attack [Figure 2-29] to maintain a positive pitch stability for the aircraft. There are a number of reflex systems used to accomplish this in emergency situations.

Figure 2-29. Emergency vertical dive recovery for a WSC wing.
Figure 2-29. Emergency vertical dive recovery for a WSC wing.

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Reflex cables—most wings with a king post use cables to hold the trailing edge up at unusually low or negative angles of attack. These reflex cables are secured to the top of the king post and attach to several positions on the trailing edge where the battens are located. Different manufacturers have different positions where these are attached, depending on the design of the wing. Refl ex cables also provide additional reflex at high speeds because the drag of the wires pulls up the trailing edge, creating more reflex at these higher speeds. [Figure 3-16]

Figure 3-16. Reflex cables.
Figure 3-16. Reflex cables.

Washout struts—tubes near the tips that keep the tip trailing edge up during very low or negative angles of attack. They can be inside or outside the double surface of a wing. The refl ex cables may not go to the wingtip, so washout struts are used to hold up the trailing edge at the tip at very low and negative angles of attack. [Figure 3-17]

Figure 3-17. Washout struts.
Figure 3-17. Washout struts.

Sprogs—for wings using struts with no king post, sprogs are used to keep the inboard trailing edge up in place of the reflex cables. A wire attached to the top of the leading edge holds the sprog up in place. [Figure 3-18]

Figure 3-18. Sprogs for strutted wing.
Figure 3-18. Sprogs for strutted wing.

Pitch Control System

The pitch control system is a simple hinge on the keel at the hang point that allows the pilot to push the control bar out and pull the control bar in to control pitch. This wing attachment is different for each manufacturer, but all designs have this hang point wing attachment so the control bar is always perpendicular to the longitudinal axis of the aircraft. This raising and lowering of the nose is the pitch control system for the WSC aircraft. [Figures 2-7 and 3-19]

Figure 2-7. Angle of incidence.
Figure 2-7. Angle of incidence.
Figure 3-19. Hang point wing attachment.
Figure 3-19. Hang point wing attachment.

Roll Control System

Control bar movement from side to side controls the roll about the longitudinal axis. The wing attachment hang point allows the carriage to roll around the wing keel. Thus, it can also be looked at from the carriage point of view, when the control bar is moved side to side, the wing rotates around the wing keel relative to the carriage. [Figures 2-31 and 3-19]

Figure 2-31. Pilot induced moments about wing/carriage hang point and resultant CG rolling moment.
Figure 2-31. Pilot induced moments about wing/carriage hang point and resultant CG rolling moment.

It would first appear that moving the control bar to one side, thus shifting weight to the opposite side, could alone bank the aircraft. It is true that shifting weight to the right would naturally bank the aircraft to the right and put it into a right-hand turn. However, the weight alone is not enough to provide adequate roll control for practical flight.

As weight is moved to one side, the keel is pulled closer to that side’s leading edge. The actual keel movement is limited to only 1 to 2 inches each side of center. However, this limited keel movement is sufficient to warp the wing, changing the twist side to side (as discussed earlier in the aerodynamics section) to roll the aircraft [Figure 2-24] by changing the lift side to side. Simply, the shifting of weight from side to side pulls the keel toward the leading edge on that side and warps the wing to roll the aircraft.

Figure 2-24. Shifting weight to one side warps the wing by increasing the twist on the loaded side and decreasing the twist on the unloaded side.
Figure 2-24. Shifting weight to one side warps the wing by increasing the twist on the loaded side and decreasing the twist on the unloaded side.

Besides the keel shifting relative to the leading edges and crossbar, overall roll control is adjusted by the designers to fit the mission of the wing through sail material/stiffness, leading edge stiffness/flexibility, amount of twist, amount of travel the keel is allowed, airfoil shape, and the planform of the wing. [Figures 3-20 and 3-21]

Figure 3-20. Shifting weight to the right pulls the keel to the right (or lets the crossbar shift to the left) and increases twist on the right side for roll control.
Figure 3-20. Shifting weight to the right pulls the keel to the right (or lets the crossbar shift to the left) and increases twist on the right side for roll control.
Figure 3-21. Crossbar travel limiter.
Figure 3-21. Crossbar travel limiter.

Trim Systems

There are a number of trim systems to relieve the control pressures for pilots to fly at different “hands off” trim speeds. Ground adjustable trim allows the pilot to adjust the trim speed of the wing on the ground and remain at one speed during flight, while flight adjustable trim systems can change the trim speed in flight.

Ground Adjustable Trim Systems

The most common ground adjustable trim system, and typical of most aircraft, is moving the wing attachment hang point forward for faster trim speeds and aft for slower trim speeds. Each manufacturer has different hardware, but the basics of sliding the carriage wing hang point forward and backward on the keel is similar for all. As an example, moving the hang point at the furthest aft position to the furthest forward position could speed the wing up 20 knots. This in turn moves the control bar position back to a new “hands off” trim speed.

Another less commonly used method of increasing trim speed is to increase tension on the crossbar by pulling it back further, slightly increasing the nose angle and reducing twist. This increases the angle of attack (AOA) of the tips producing more lift, and it lowers the nose to a higher trim speed. This is a typical in-flight trim adjustment for high performance hang gliders. The roll control is diminished with this faster and stiffer wing.

Ground adjustable trim systems are described in the Pilot’s Operating Handbook (POH) for each aircraft. Different loads may require different pitch settings.

Inflight Adjustable Trim Systems

Being able to adjust the trim systems in flight has a number of advantages as discussed later in the flight sections. A number of inflight adjustable systems are available with different manufacturers. A common in-flight adjustable trim system is raising and lowering the trailing edge. Raising the trailing edge increases airfoil reflex and slows the wing. Lowering the trailing edge decreases airfoil reflex and speeds up the wing. Typically, a crank on a downtube controls a wire that runs up the downtube to the top of the wing. As a result of moving the crank, the trailing edge wires are raised and lowered and the trim speed changed. [Figure 3-22] Hydraulic or electrical systems can move the hang point on the wing for other inflight trim systems. [Figure 3-23]

Figure 3-22. A crank on the downtube of the control bar that adjusts the trailing edge reflex during flight.
Figure 3-22. A crank on the downtube of the control bar that adjusts the trailing edge reflex during flight.
Figure 3-23. Hydraulic inflight trim systems that move the hang point in flight controlled by the pilot.
Figure 3-23. Hydraulic inflight trim systems that move the hang point in flight controlled by the pilot.

Another pilot-actuated trim system in flight is an elastic system in which the pilot increases tension on the elastic system which raises the nose for climb and slower flight. [Figure 3-24]

Figure 3-24. More tension on elastic pulling down on the rear of the wing keel reduces the trim speed and is controlled by the pilot in flight.
Figure 3-24. More tension on elastic pulling down on the rear of the wing keel reduces the trim speed and is controlled by the pilot in flight.

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