The wing has a structural frame that the sail fits over. Although the wing structure is rigid, it is designed to move and allow the sail to flex and the wing to deform or “warp,” to provide a simple control system with no pulleys, pushrods, hinges, control cables, or separate control surfaces. This simplifies maintenance and reduces the cost and weight of the wing. Each wing is built from high-quality aircraft parts including alloyed aluminum tubes, stainless steel cables, hardware, and specially designed sail cloth.
Wing Frame Components
The structural frame of the wing is composed of the leading edges, keel, crossbar, pilot control frame, king post and wires/ struts. The wing frame is a number of structural triangles formed by the wing components. These triangles, braced by wires and struts, provide a strong and lightweight frame to support the flexible sail. [Figure 3-2]
Leading edges are tube assemblies that are at the front of the wing, the leading edges of the wing airfoil. These are swept back to form the front shape of the wing and attached to each other with nose plates. The leading edges support the airfoil and are designed to flex as part of the wing structure.
The leading edges are each made up of two main sections, an inboard and an outboard section, as shown in Figures 3-2 and 3-3. Additional tubing “sleeves” are typically used for added strength where the leading edge attaches to the nose plates, and where the inboard and outboard tubes join at the crossbar attachment. This sleeving can be internal or external depending on the specific manufacturer’s design. Typically, additional sleeving is used throughout the leading edges at various locations to strengthen and vary the flex for the particular design of the wing. Each manufacturer and make/model uses different internal and external sleeving to accomplish specific strength and flex characteristics. Generally, the inboard sections are stiffer and the outboard leading-edge section flexes as part of the flexible wing design. Sleeving is commonly added throughout the aircraft where bolt holes are drilled through the tubing to reinforce it around the bolt hole.
The outboard leading edge sections can be removed to pack up the wing into a “short pack” which is commonly used for shipping. [Figure 3-3]
The wing keel is like that of a boat keel, the center of the wing, fore and aft. It attaches to the leading edges at the nose plate and performs a number of important functions. It is the structure where the carriage attaches to the wing, and it is the wing structure that connects the center section of the sail at the “keel pocket” (discussed later in this chapter in the sail section). The control frame and king post (if so equipped) also attaches to the keel. It also provides structure for the upper and lower wires (if so equipped) and a reference or anchor for the crossbar which needs some movement in relation to the keel for roll control.
The keel is rigid and is not designed to flex nor is it highly stressed like the leading edges except where the undercarriage attaches to the wing. Sleeving is normally added to strengthen this middle area as well at the nose attachment and rear cable attachments.
The crossbar is two aluminum tube sections hinged above the keel that attach to the leading edges. The crossbar is tensioned back with the crossbar tensioning cables, which pushes the leading edges forward to conform to the sail. These crossbar tensioning cables are attached at the rear of the keel when the wing is tensioned into flying position. [Figure 3-4]
These crossbar sections are under a compression load and designed to be stiff with no bending. A larger diameter tube is typically used to avoid any bending when the wing is flying. A ding, dent, or bend in the crossbar could spell disaster during flight because it is one of the main structural members that holds leading edges into position during flight.
For wing take down and packing, the crossbar haul back cables are released, the crossbar hinged center moves forward, and the leading edges rotate in toward the keel about the nose plates and come together, allowing the wing to fold down into a long tube for transport and/or storage.
The triangle-shaped control frame serves two main purposes. It provides the lower structure for the wing and is the control bar for the pilot. The control frame is bolted to the keel with two downtubes extending from the keel attachment to the horizontal base tube, which is the pilot’s control bar. [Figures 3-2, 3-5, and 3-6]
Control frame corner brackets at the bottom of the downtubes provide the wing structural attachments for the flying cables or struts that attach to each leading edge/crossbar junction, and secure the control bar fore and aft to the wing with the front and back wires attached to nose plates and the aft section of the keel. [Figures 3-5 and 3-6]
During flight, the downtubes are similar in compression to the crossbar and must be stiff and straight to maintain structural integrity. The base tube/control bar is under tension during flight.
Front and rear flying wires hold the control frame in place fore and aft. Side flying wires hold the control frame in place side to side and provide structure to hold the wings in place while flying. [Figures 3-2, 3-5, and 3-6] Strutted wings use struts in place of the side flying wires, which is discussed later in this chapter.
Training bars are added for dual controls so the person in back can fly the aircraft. These are typically used by an instructor for training but can be used by a passenger in the back also. [Figure 3-7]
King Post With Wires-on-Top Wing Design
Similar to the lower control frame holding the wing in position during flight, the king post is attached to the keel and supports the upper ground wires which hold the wing in position on the ground and negative loads during flight. [Figure 3-2] It also provides a structure for reflex lines which is discussed later in wing systems.
Topless Wings With Struts
Similar to airplanes with struts to support the wings, some WSC aircraft replace side flying wires with struts, eliminating the king post and ground wires on top of the wing. This provides a number of benefits, but primarily, no king post is needed because the struts can take a compression load and hold the wings upon the ground and also take the negative loads during flight. With struts, a WSC aircraft is much shorter in height allowing it to fit into hangars with lower doors and ceilings. This can make a big difference in finding a suitable storage for the aircraft if leaving it set up. [Figure 3-8]
Some strutted designs allow the wings to be folded back while still on the carriage. This can also be helpful when using a smaller space for storage by folding the wing up without taking it off the carriage. [Figure 3-9] It is also convenient for sea trikes since the aircraft does not have to be taken out of the water to fold up the wing.
Strutted wings have a clean upper surface with no holes required for the king post or wires to go through the top of the sail. This reduces interference drag on the top of the wing. Increasing overall efficiency, no holes in the sail also eliminates any high-pressure leakage from underneath the wing getting sucked up to the lower pressure on top of the wing. [Figure 3-10]
The sail is a highly refined design that integrates with its wing frame. Each sail and wing frame are designed for each other and are not interchangeable with other sails or wings. Modern sails are designed with complex geometry and sewn to precision to achieve a highly efficient design. Because of the flexibility of the wing frame and the modern techniques in sail design, the leading edge can have a curved shape which adds to the efficiency and stability of the wing. [Figure 3-11]
Battens and Leading Edge Stiffener
As discussed in the aerodynamics section, stiff preformed battens are the airfoil ribs that maintain the airfoil shape from the root to the tips. Additionally, a foam or mylar stiffener is inserted in a pocket at the leading edge to keep a rigid airfoil shape between the battens from the leading edge up to the airfoil high point. Double surface wings have additional ribs on the bottom surface that are straight or formed to maintain the bottom surface camber.
Sail Material and Panels
Sail material is a combination of polyester materials designed with different weaves, thickness, and orientation to fit the design mission of the wing. Panels are cut to different shapes and laid down at different angles to provide the stiffness and flexibility where needed for the specific wing design. Automated machines typically cut the fabric to precision tolerances and the panels are sewn together with high strength thread.
Pockets and Hardware
Pockets are added for battens and hardware is installed for the wing frame and wire attachments. Trailing edge line or wires are sometimes added for reinforcement and can be used for tuning. Battens are held in with a variety of batten ties or other methods unique to the manufacturer. [Figure 3-12]
Sail Attachment to Wing Frame
The sail is attached to the wing frame at the nose and the tips. A keel pocket towards the back of the sail secures the sail to the wing keel. [Figure 3-13]