CG
CG
Arm
Pilot induced pitch moments
Pilot input force
Pilot induced pitch moments
Pilot input force
Arm
Figure 2-27. Pilot actuated pitching moment.
1.5
1.0
0.5
5 10 15 20 25
Angle of Attack
Root
at minimum
controlled
airspeed
Tips
at minimum
controlled
airspeed
Figure 2-26. Example of AOA versus C L for wing at minimum
controlled airspeed.
1.5
1.0
0.5
5 10 15 20 25
Angle of Attack
Root
Tip
.12
.3
Example of
Increasing AOA
3 degrees above trim
Example of trim AOA
Figure 2-28. Example AOA versus CL showing the wing increasing
AOA three degrees with the tip CL increasing more than the root.
of change of the CL is much less at higher AOA at the root
chord. Therefore, an increase in AOA for the wing results in
the tips creating a greater proportion of the lift and moving
the center of lift behind the CG, creating a negative pitching
moment to lower the nose at high AOA.
Based on the same principle, when the wing AOA is lowered
below the trim position, the tip chords’ C L decreases more
than the root chord and the center of lift for the wing moves
forward creating a positive moment to raise the nose at
lower AOA.
In situations where the pilot is fl ying in severe/extreme
turbulence, wind sheer, or the pilot is exceeding the limitations
of the aircraft, the WSC aircraft can get into a situation where
the root chord is at a negative AOA and not producing lift.
This could result in an emergency vertical dive situation, as
discussed later in the Whip Stall-Tuck-Tumble section. When
at very low angles or negative angles of attack, the WSC wing
is designed so that the wing has positive stability or a nose-
up aerodynamic moment. This is accomplished by a number
of different systems (washout struts, sprogs and refl ex lines)
further explained in chapter 3 that simply keep the trailing
edge of the wing up in an emergency low/negative AOA
dive situation. As shown in Figure 2-29, the root area of the
wing has refl ex which creates a positive pitching moment
for the root chord to rotate the nose up towards a level fl ying
attitude. At the same time, the tips are at a negative AOA
producing lift in the opposite direction as usual, creating a
moment to bring the nose/root chord up to a positive AOA
to start producing lift and raising the nose to a normal fl ight
condition. The negative lift or downward force as produced
at the tips and root as shown provide a positive moment to
raise the nose back to a normal fl ying attitude.
Dihedral from wing flex
Anhedral built into frame
Figure 2-30. Wing front view example showing anhedral in the
middle of the wing and dihedral at the outboard section of the wing
because of leading edge flex.
Relative Wind
Flightpath
Trailing edge at center
of wing held up by
reflex lines.
Reflex lines used
to hold up the
trailing edge
Aerodynamic force at tips
Wing root at negative AOA
producing no lift
Trailing edge of tips
held up with washout
struts at negative
angle of attack
Tip moment arm
-LT
Reflex force moment arm
Reflex in
airfoil creates
force near
trailing edge
of root area
Positive moment
caused by
aerodynamic
force at tips
plus reflex in
airfoil at root
-LR
Rel
ative Wind
Reflex
Figure 2-29. Emergency vertical dive recovery for a WSC wing.
Refl ex also provides a stable pitch up moment for an airfoil
when it is fl ying at normal fl ight angles of attack. The greater
the refl ex, the greater the nose up moment of the airfoil. This
is used in some WSC airfoil designs and also for trim control
as discussed in Chapter 3.
Carriage Moments
The wing design is the main contributing factor for pitch
stability and moments, but the carriage design can also
infl uence the pitching moment of the WSC aircraft. For
example, at very high speeds in a dive, a streamlined carriage
would have less drag and, therefore, a greater nose-up moment
because of less drag. The design of the carriage parts can have
an effect on aerodynamic forces on the carriage, resulting in
different moments for different carriage designs.
The drag of the wing in combination with the drag of the
carriage at various airspeeds provides a number of pitching
moments, which are tested by the manufacturer—a reason
the carriage is matched to the wing for compatibility. Each
manufacturer designs the carriage to match the wing and takes
into account these unique factors.
Pitch Moments Summary
Overall, the amount of sweep, twist, specifi c airfoil design
from root to the tip, and the carriage design determine the
pitching moments of the WSC aircraft. Some have small
pitching moments, some have greater pitching moments.
Each WSC model is different with a balance of these
aerodynamic parameters to accomplish the specifi c mission
for each unique carriage and wing combination.
Roll Stability and Moments
As described in the Pilot’s Handbook of Aeronautical
Knowledge, more dihedral or less anhedral in a WSC wing
creates more roll stability. More roll stability might be helpful
for a training wing or a fast wing made for long cross-country
straight fl ight, but most pilots want a balance between roll
stability and the ability to make quicker turns and a sport
car feel for banking/turning. Therefore, a balance between
the stability and the instability is achieved through anhedral
plus other important wing design features such as nose angle,
twist, and airfoil shape from root to tip.
An aerodynamic characteristic of swept wings is an “effective
dihedral” based on the sweep of the wing and angle of attack.
The combination of the physical anhedral in the wing and the
effective dihedral due to wing sweep provides the balance of
stability and rolling moments for a particular wing design.
The design of the wing can have actual dihedral or anhedral in
the wing. Even with anhedral designed in the inboard section
of the wing, the outboard sections of the wing could have
some dihedral because of the fl ex in the outboard leading
edges. As the wing is loaded up from additional weight or
during a turn, the tips fl ex up more creating more dihedral
and a roll stabilizing effect when loaded. [Figure 2-30]
Generally, it is thought that the wing remains level and the
weight shifts to the side to initiate a turn. Another way to
look at how the WSC wing rolls is to examine the carriage
and the wing moment from the carriage point of view. For
example, the CG hangs far below a wing weighing ⅛ of
the carriage weight. When the control bar is moved to the
side, creating a moment about the carriage/wing hang point,
the carriage stays vertical and the wing rotates around the
carriage. Therefore, there are two rolling moments that both
contribute to the WSC rolling into a bank:
• The pilot creating the force on the control bar rotating
the wing about the wing/carriage hang point.
Smaller area
creating loss of drag
Larger area
creating more drag
Yawed to the left, the wing
automatically rotates
into the relative wind
Relative Wind
Flightpath
Figure 2-32. Yaw correction about the vertical axis.
CG
Arm
Pilot input roll moments
Pilot input force
Resultant roll moments about CG
Figure 2-31. Pilot induced moments about wing/carriage hang point
and resultant CG rolling moment.
Figure 2-33. Keel pockets and vertical stabilizers are additional
tools designers use for yaw stability on the wing.
• Shifting weight to one side of the wing, thus warping
the wing to aerodynamically change the lift on each
side, as in airplane roll control. [Figure 2-31]
Carriage Moments
Carriage weight and resultant CG are the main factors that
contribute toward increasing the roll moment for the carriage.
Carriage aerodynamic forces are not typically a factor for
rolling moments.
Roll Stability Summary
Overall, roll stability and moments are a manufacturer/
make/model balance between dihedral/anhedral, wing twist,
nose angle, airfoil shape from root to tip, and leading edge
stiffness. Some designs are stable, others neutral, and others
can be designed to be slightly unstable for quick side-to-side
rolling.
Yaw Stability and Moments
There is no signifi cant turning about the vertical axis because
the WSC wing is designed to fl y directly into the relative
wind. Any sideways skidding or yaw is automatically
corrected to fl y straight with the swept wing design. An
airplane uses the vertical tail to stabilize it to fl y directly into
the relative wind like a dart. The unique design of the WSC
aircraft performs the same function through the swept wing
design, but also the wing twist and airfoil shape from root to
tip assists in the correction about the vertical axis. A simple
way to understand the yaw stability is to see that any yawing
motion is reduced simply through the increased area of the
wing as it rotates about its vertical axis. [Figure 2-32]
There is a slight amount of adverse yaw similar to an
airplane that can be noticed when a roll is fi rst initiated.
The amount varies with the specifi c manufacturer’s design
and make/model. In addition, the wing can yaw side to side
to some degree, with some different manufacturer’s make/
model more than others. The higher performance wings
with less twist and a greater nose angle are noted for less
yaw stability to gain performance. These wings also require
more pilot input and skill to minimize yaw instability through
pitch input. An addition to the wing planform, twist, and
airfoil shapes to minimize yaw, some wings utilize vertical
stabilizers similar to these in airplanes and others use tip fi ns.
[Figure 2-33] Generally, the WSC wing is yaw stable with
minor variations that are different for each wing and can be
controlled by pilot input, if needed.
Carriage Moments
The wing is a signifi cant factor in the design of yaw stability,
but the carriage can be a large factor also. If the area in front
of the CG is greater than the area in back of the CG, and
the wing yaws to the side, then the front would have more
Thrust line below the center of gravity
Thrust line above the center of gravity
CG
Thrust line
CG
Thrust line
Figure 2-35. Thrust line moments.
Figure 2-34. Wheel fins for carriage yaw stability.
drag and create a moment to yaw the WSC aircraft further
from the straight fl ight. Therefore, fi ns are sometimes put
on the carriage as needed so the carriage also has a yawing
aerodynamic force to track the WSC aircraft directly into the
wind. [Figure 2-34]
Since the carriage has such a large effect on yaw stability,
the carriage is matched to the wing for overall compatibility.
Each manufacturer designs the carriage to match the wing and
takes into account these unique factors of each design.
Yaw Stability Summary
These factors make the WSC aircraft track directly into
the relative wind and eliminate the need for a rudder to
make coordinated turns. Designs and methods vary with
manufacturer and wing type, but all WSC wings are designed
to track directly into the relative wind.
Thrust Moments
WSC aircraft designs can have different moments caused by
thrust based on where the thrust line is compared to the CG.
This is similar to an airplane except the WSC aircraft has no
horizontal stabilizer that is affected by propeller blast.
If the propeller thrust is below the CG [Figure 2-35, top],
this creates a pitch-up moment about the CG when thrust is
applied and a resultant decrease in speed. When reducing
the throttle, it reduces this moment and a nose pitch down
results with an increase in speed.
If the propeller thrust is above the CG [Figure 2-35, bottom],
this creates a pitch-down moment about the CG when thrust
is applied and a resultant increase in speed. When reducing
the throttle, it reduces this moment and a nose pitch up results
with a decrease in speed.
With the thrust line above or below the CG producing these
minor pitch and speed changes, they are usually minor for
most popular designs. Larger thrust moments about the CG
may require pilot input to minimize the pitch and speed
effects. Most manufacturers strive to keep the thrust as close
as possible to the vertical CG while also balancing the drag of
the carriage and the wing for its speed range. This is why the
carriage must be matched to the wing so these characteristics
provide a safe and easy to fl y WSC aircraft.
Stalls: Exceeding the Critical AOA
As the AOA increases to large values on the wing chord,
the air separates starting at the back of the airfoil. As the
AOA increases, the separated air moves forward towards the
leading edge. The critical AOA is the point at which the wing
is totally stalled, producing no lift—regardless of airspeed,
fl ight attitude, or weight. [Figure 2-36]
Phase 1 Whip StallPhase 2Phase 3Phase 4
Wing comletely
stalled and very
high pitch angle
Pitch attitude for
normal dive recovery
Vertical dive
ose rotates dow
Nose is tucked under
Tumble
Figure 2-37. Whip stall to tumble phases and sequence.
Wing stalls due
to an excessive
angle of attack
Laminar airflow
Turbulence
Figure 2-36. Stall progression for an airfoil chord as the angle of
attack is increased.
Because the AOA of the WSC wing root chord/nose is so much
higher than the AOA of the tips, the nose stalls before the tips.
It is similar to stalling with the airplane canard in which the
nose stalls fi rst, the main wing (or tips for the WSC aircraft)
continues to fl y, and the nose drops due to lack of lift.
In most normal situations, the root chord/nose stalls fi rst
because it is at a much higher AOA. The tips continue to fl y,
making the WSC wing resistant to a complete wing stall. A
pilot can even bring the aircraft into a high pitch angle stall
attitude and keep the nose high. The nose stalls and rotates
down because of the loss of lift, while the tips keep fl ying
and maintain control of the aircraft.
If fl ying within the operating limitations of the aircraft and
the WSC reaches a high AOA, the nose stalls, but the tips
continue fl ying. However, it must be understood that there are
many wing designs with many types of stall characteristics
for each unique design. For example, high-performance
wings could have less twist to gain performance, which
could cause the wing to stall more abruptly than a training
wing with more twist.
Whip Stall–Tuck–Tumble
A WSC aircraft can get to a high pitch attitude by fl ying
outside the its limitations or flying in extreme/severe
turbulence. If the wing gets to such a high pitch attitude and
the AOA is high enough that the tips stall, a whip stall occurs.
[Figure 2-37]
In a WSC wing, most of the area of the wing is behind the CG
(about three-quarters). With the tips and aft part of the wing
having the greatest drag, and the weight being forward, an
immediate and strong nose-down moment is created and the
WSC nose starts to drop. Since both the relative wind and the
wing are rapidly changing direction, there is no opportunity
to reestablish laminar airfl ow across the wing.
This rotational momentum can pull the nose down into a
number of increasingly worse situations, depending on the
severity of the whip stall. Figure 2-37 shows a whip stall and
the phases that can result, depending on the severity.
Phase 1—Minor whip stall results in a nose-down pitch
attitude at which the nose is at a positive AOA and the
positive stability raises the nose to normal fl ight, as
described in Figure 2-25C.
Phase 2—If the rotational movement is enough to
produce a vertical dive, as illustrated in Figure 2-29,
the aerodynamic dive recovery might raise the nose
to an attitude to recover from the dive and resume
normal fl ight condition.
Phase 3—The rotational momentum is enough to bring
the nose signifi cantly past vertical (the nose has tucked
under vertical), but could still recover to a vertical dive
and eventually resume a normal fl ight condition.
Phase 4—The rotational momentum is severe enough
to continue rotation, bringing the WSC wing into a
tumble from which there is no recovery to normal
fl ight, and structural damage is probable.
Avoidance and emergency procedures are covered in Chapter
6, Basic Flight Maneuverers, and Chapter 13, Abnormal and
Emergency Procedures.
Weight, Load, and Speed
Similar to airplanes, sailplanes, and PPCs, increasing weight
creates increases in speed and descent rate. However, the
WSC aircraft has a unique characteristic. Adding weight to
a WSC aircraft creates more twist in the wing because the
outboard leading edges fl ex more. With less lift at the tips, a
nose-up effect is created and the trim speed lowers.
Therefore, adding weight can increase speed similar to other
aircraft, but reduce the trim speed because of the increased
twist unique to the WSC aircraft. Each manufacturer’s
make/model has different effects depending on the specifi c
design. As described in the Pilot’s Handbook of Aeronautical
Knowledge, the stall speed increases as the weight or loading
increases so some manufacturers may have specifi c carriage/
wing hang point locations for different weights. Some require
CG locations to be forward for greater weights so the trim
speed is well above the stall speed for the wing.
WSC aircraft have the same forces as airplanes during normal
coordinated turns. Greater bank angles result in greater
resultant loads. The fl ight operating strength of an aircraft
is presented on a graph whose horizontal scale is based on
load factor. The diagram is called a VG diagram—velocity
versus “G” loads or load factor. Each aircraft has its own VG
diagram which is valid at a certain weight and altitude. See
the Pilot’s Handbook of Aeronautical Knowledge for more
details on the VG diagram. Load factors are also similar to
the VG diagram applicable to WSC.
Basic Propeller Principles
The WSC aircraft propeller principles are similar to those
found in the Pilot’s Handbook of Aeronautical Knowledge,
except there is no “corkscrewing effect of the slipstream”
and there is less P-factor because the carriage is generally
fl ying with the thrust line parallel to the relative wind. The
wing acts independently, raising and lowering the AOA and
speed. This was introduced at the beginning of this chapter
when angle of incidence was defi ned.
The torque reaction does have a noticeable effect on the WSC
aircraft. With the typical left hand turn tendency (for right
hand turning propellers), turns are not typically built into the
wing. As in airplanes, some cart designs point the engine
down and to the right. Others do not make any adjustment, and
the pilot accounts for the turning effect through pilot input.
It should be noted that many of the two-stroke propellers turn
to the right, as do conventional airplanes. However, many
four-stroke engine propellers turn to the left, creating a right
hand turn. Consult the POH for the torque characteristics of
your specifi c aircraft.
Chapter Summary
Basic principles of aerodynamics apply to all aircraft;
however, the unique design of the wing and the separate
fuselage/carriage provide a simplistic and effi cient aircraft.
The following provide a summary of the unique aerodynamics
for the WSC wing:
• The WSC wing is pitch stable without a tail because
of the combination of airfoil design from root to tip,
sweep, twist, and planform.
• WSC wing fl exibility allows the wing to twist from
side to side by shifting the weight providing the control
to roll the aircraft without control surfaces.
• The WSC wing only has two axes of control, pitch
and roll, while no yaw control is needed because it is
yaw stable.
• The WSC wing is stall resistant because under normal
fl ight conditions the tip chord is still fl ying while the
rest of the wing is stalled—similar to the airplane
canard system.