Introduction
Weight-shift control (WSC) aircraft come in an array of shapes
and sizes, but the basic design features are fundamentally the
same. All WSC consist of a fl exible wing made with a sail
fi tted over a rigid airframe. A separate carriage is the fuselage
which consists of the fl ight deck, propulsion system, and
landing gear. [Figure 3-1]
Components
and Systems
Chapter 3
Figure 3-1. Wing and carriage of WSC aircraft.
CARRIAGE
Propulsion System
Flight Deck
Landing Gear
WING
Wing
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 fl ex and the wing to deform or “warp,”
to provide a simple control system with no pulleys, push
rods, hinges, control cables, or separate control surfaces. This
simplifi es 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 fl exible sail. [Figure 3-2]
Leading Edges
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 fl ex 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 specifi c manufacturer’s design.
Typically, additional sleeving is used throughout the leading
edges at various locations to strengthen and vary the fl ex for
the particular design of the wing. Each manufacturer and
make/model uses different internal and external sleeving
to accomplish specifi c strength and fl ex characteristics.
Generally, the inboard sections are stiffer and the outboard
leading edge section fl exes as part of the fl exible 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]
Leading Edge Tube
Leading Edge Tube Nose Plates
Keel
Crossbar
Upper Cables—Ground Wires King Post
Downtubes Control Bar
Control Frame
Crossbar
Lower Cables—Flying Wires
Crossbar Tensioning Cables
Crossbar Tensioning
Cables Attachment
Figure 3-2. Wing frame components.
Wing Tip
OutboardInboard Crossbar attaches here
Nose plate attaches here
Internal Sleeve External Sleeve
Outboard leading edge
release for short pack
Figure 3-3. Leading edge assembly.
Keel
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 fl ex nor is it highly
stressed like the leading edges except where the under
carriage attaches to the wing. Sleeving is normally added to
strengthen this middle area as well at the nose attachment
and rear cable attachments.
Front Flying Wires
Rear Flying Wires
Side Flying Wires
Control Bar
Control Frame
Corner Brackets
Down Tubes
Figure 3-6. Control frame with downtubes, control bar, and corner
bracket with flying wing wires, and control frame fore and aft
wires.
Side flying wires. Some
designs use two flying
wires for each side
Front Flying Wire Downtube
Rear Flying Wire
Control frame corner bracket Control Bar
Figure 3-5. Control frame corner bracket with wire attachments.
Notice the thick structural ⅛ -inch flying wires that support the
wing and smaller ⁄-inch cables holding the control frame in
place fore and aft.
Leading Edge Crossbar
Crossbar Tensioning Cables
Crossbar Tensioning Cables
Attachment to Rear of Wing Keel
Figure 3-4. View looking inside left hand wing from the tip showing
crossbar tensioned and pushing the leading edges into the sail.
Notice the slight bending of the leading edges to fit into the sail
(top). Crossbar tensioning cables attached to rear of keel in flying
position detail. See specific location on airframe with figure 3-2.
(bottom).
Crossbar
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 fl ying 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 fl ying.
A ding, dent, or bend in the crossbar could spell disaster
during fl ight because it is one of the main structural members
that holds leading edges into position during fl ight.
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.
Control Frame
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 fl ying 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]
Figure 3-8. Strutted wing on WSC aircraft carriage.
Figure 3-7. Passenger using training bars which are also used by the instructor during training.
Training bars used
for steering from
the back seat
During fl ight, 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
fl ight.
Front and rear fl ying wires hold the control frame in place
fore and aft. Side fl ying wires hold the control frame in place
side to side and provide structure to hold the wings in place
while fl ying. [Figures 3-2, 3-5, and 3-6] Strutted wings use
struts in place of the side fl ying wires, which is discussed
later in this chapter.
Training bars are added for dual controls so the person in back
can fl y 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 fl ight, 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 fl ight.
[Figure 3-2] It also provides a structure for refl ex 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 fl ying wires with struts, eliminating
the king post and ground wires on top of the wing. This
provides a number of benefi ts, but primarily, no king post is
needed because the struts can take a compression load and
hold the wings up on the ground and also take the negative
loads during fl ight. With struts, a WSC aircraft is much shorter
in height allowing it to fi t into hangars with lower doors and
ceilings. This can make a big difference in fi nding a suitable
storage for the aircraft if leaving it set up. [Figure 3-8]
Figure 3-10. Clean upper surface of strutted wing.
Figure 3-9. A strutted wing folded back so it can fit into a trailer for
storage and easy transport (top). Strutted wing with wings folded
back for easy storage (bottom).
Figure 3-11. Curved leading edge sail design.
Leading Edge
Leading Edge
Leading Edge
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 effi ciency, 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]
Sail Components
The sail is a highly refi ned 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 effi cient design. Because of the
fl exibility of the wing frame and the modern techniques in sail
design, the leading edge can have a curved shape which adds
to the effi ciency and stability of the wing. [Figure 3-11]
Figure 3-13. Keel pocket.
Figure 3-12. Trailing edge of the sail showing reinforcement panels,
trailing edge line, and batten ties with attachment hardware.
Figure 3-14. Crossbar tensioning junction attachment example.
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 fi t the
design mission of the wing. Panels are cut to different shapes
and laid down at different angles to provide the stiffness
and fl exibility where needed for the specifi c 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]
Cables and Hardware
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
fi ttings. Figure 3-5 shows detail of typical swaged fi ttings.
A variety of hardware is used for attaching these swaged
cable fi ttings to the airframe. Each manufacturer has different
hardware for wing components. [Figures 3-14 and 3-15]
Washout Strut
Sprog
Figure 3-18. Sprogs for strutted wing.
Figure 3-16. Reflex cables.
Washout Strut
Figure 3-17. Washout struts.
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
Refl ex 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 refl ex
systems used to accomplish this in emergency situations.
Refl ex cables—most wings with a king post use cables to
hold the trailing edge up at unusually low or negative angles
of attack. These refl ex 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
refl ex at high speeds because the drag of the wires pulls up
the trailing edge, creating more refl ex at these higher speeds.
[Figure 3-16]
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]
Sprogs—for wings using struts with no king post, sprogs
are used to keep the inboard trailing edge up in place of the
refl ex cables. A wire attached to the top of the leading edge
holds the sprog up in place. [Figure 3-18]
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