more difficult to dampen. As airspeed decreases in a glider loaded aft of the permissible CG limit, the nose of the glider
might rise uncontrollably and lead to an unrecoverable stall or spin. A forward CG increases induced drag and reduces
performance. A glider with a CG ahead of the published forward limit may not provide enough pitch control to raise the
nose during a landing. Chapter 5, Glider Performance, contains further discussion of proper loading of a glider and the
importance of CG.
Lateral Stability
Lateral stability describes the glider’s tendency to return to wings-level flight following a displacement. [ Figure 3-13]
For example, due to a gust of wind, the glider may start to roll. The angle of attack increases slightly on one wing as it
moves down, which causes the lift to increase on that wing. On the rising wing, the opposite effect decreases lift. This lift
differential tends to dampen the rolling motion but does not bring the glider back to wings-level.
Direction of rotation
Angle of attack reduced, lift reduced
Angle of attack increased, lift increased
Figure 3-13. Lateral stability.
Wing dihedral, the upward angle of the wings from horizontal, adds to lateral stability. As the glider rolls, the descending
wing has a larger vertical lift component than the rising wing. This difference in the vertical lift component between each
wing tends to roll the glider back toward level flight. [Figure 3-14]
Dihedral angle Dihedral angle
Figure 3-14. Dihedral angle.
Flutter
Due to the flexibility of a glider wing design, the wing may oscillate rapidly or flutter at high airspeeds. Additionally,
looseness in the control surfaces can also result in flutter of flight control surfaces near maximum speed. Improper balance
of the control surfaces may also cause flutter. Since prolonged flutter may cause structural failure, the pilot should reduce
the airspeed sufficiently to stop any flutter.
Pilot Induced Oscillation (PIO)
Pilot-induced oscillations (PIOs) can occur when the pilot applies excess control pressure that causes an overshoot of the
desired flight attitude. If the pilot repeatedly moves the controls back and forth using excess pressure, the glider oscillates
continuously past the desired attitude. The oscillations can increase in amplitude and may cause loss of control. While PIOs
most likely occur as pitch oscillations, roll and yaw induced PIOs may also occur.
Although PIOs can occur at any time, they often arise during primary training. Experienced pilots may also induce PIOs
when flying an unfamiliar make and model glider. For that reason and before flying an unfamiliar glider, all pilots should
review and understand the flight characteristics of that glider.
If encountering PIOs, the pilot should remember that changes in flight attitude take time. The pilot should begin to ease
flight control pressure as the glider begins to respond in the desired direction. As the glider nears the desired attitude, the
pilot centers the appropriate flight control so that overshooting does not occur.
During the first moments of the takeoff roll, as airflow begins to impact the control surfaces, it takes considerable
displacement of the flight controls to affect the glider’s flightpath. The pilot also experiences a higher control lag time
due to reduced control effectiveness at low speed. As the glider accelerates, aerodynamic response improves, lag time
decreases, and PIOs become less likely.
Turning Flight
Any moving object continues in a straight line until a force causes a change in direction. A pilot creates turning force by
using the ailerons to roll the glider so that the direction of the total lift vector inclines. When a glider rolls away from
wings-level, lift divides into two components. The vertical component opposes weight, while the other acts horizontally to
oppose centrifugal force. [Figure 3-15]
Vertical component
of lift (effective lift)
Weight
Resultant force
Centrifugal force
Lift
Figure 3-15. Forces in a banked turn.
When using the horizontal component of lift to turn the glider, a reduction in the vertical component of lift will occur unless
the pilot increases back pressure on the control stick, increasing the angle of attack and total lift produced by the wings.
Increasing total lift can restore the vertical component of lift to that required to counteract weight. Otherwise, the descent
rate in the airmass increases.
Load Factor
Any force applied to a glider to deflect its flight from a straight line produces a stress on its structure; load factor describes
this force. The load factor is the ratio of the total air load acting on the glider to the weight of the glider. Load factor may be
positive or negative and depends on the current flightpath. Load factor units often use “G,” a gravitational force equivalent.
A load factor of one, or 1 G, represents conditions in which the lift is equal to the weight. A glider in flight with a load
factor of one does not necessarily mean the glider is in straight-and-level flight, but rather that the total lift equals that of
unaccelerated straight-and-level flight.
When subjecting a glider to added Gs in a pull up from a dive, anyone in the glider feels a sensation of pressing into the
seat with a force equal to the number of Gs times the person’s weight. In addition, the person's extremities require added
muscular force to resist the downward force. Added Gs can affect blood flow to the brain, affect cognitive ability, and cause
disorientation.
Load factor increases rapidly as the angle of bank increases during a turn when the pilot increases lift to prevent a change
in vertical speed. From an aerodynamic perspective, load factors concern the pilot for two distinct reasons:
1. Below a certain airspeed, a stall occurs before the pilot can create a dangerous overload on the glider structure.
2. Above a certain airspeed, known as the maneuvering speed or V*A, a pilot can generate enough lift to create a
dangerous overload on the glider structure.
In a turn at constant speed, the pilot pulls back on the stick to furnish the extra lift necessary to maintain a constant vertical
speed in the airmass. The load and stall speed increase significantly as bank angles exceed 30 degrees. [ Figure 3-16] and
[Figure 3-17]
Load factor (Gs)
Angle of bank (degrees)
0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100°
Figure 3-16. The loads placed on a glider while maintaining a constant rate of descent with respect to the surrounding air increase as
the angle of bank increases.
Percentage increase in stall speed
Angle of bank (degrees)
0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100°
Figure 3-17. A 60° angle of bank causes a 41 percent increase in the glider’ s stall speed.
Rate of Turn
Rate of turn refers to the amount of time it takes for a glider to turn a specified number of degrees. If flown at the same
airspeed and angle of bank, every glider turns at the same rate. If airspeed increases while the angle of bank remains the
same, the rate of turn decreases. Conversely, a constant airspeed coupled with an increased angle of bank results in an
increased rate of turn.
Radius of Turn
The horizontal distance an aircraft uses to complete a turn depends upon the radius of turn. The radius of turn at any given
bank angle varies directly with the square of the airspeed. Therefore, if the airspeed of the glider were doubled, the radius
of the turn would be four times greater. The radius of turn also depends on a glider’s angle of bank. If the angle of bank
increases and the airspeed remains the same, the radius of turn decreases.[Figure 3-18] When flying in thermals, a smaller
turn radius enables a glider to fly closer to the fastest rising core of the thermal and gain altitude more quickly.
30°
45°
60°
TAS 40 MPH
TAS 60 MPH
TAS 80 MPH
feetTurn radius 7
feetTurn radius 0
feetTurn radius 8
feetTurn radius 5
feetTurn radius 7
feetTurn radius 0
feetTurn radius 2
feetTurn radius 9
feetTurn radius 7
Level stall airspeed at gross weight is 38 knots
Figure 3-18. A glider’ s radius of turn as compared to angle of bank.
Turn Coordination
When rolling into a turn, the lowered aileron on the outside wing produces more lift for that wing. Since induced drag is
a byproduct of lift, the outside wing also experiences more drag than the inside wing. This causes adverse yaw, a yawing
tendency toward the outside of the turn. Appropriate use of rudder corrects for any adverse yaw caused by aileron drag.
Because glider wings provide a long lever arm for adverse yaw, the pilot may need to use substantial rudder pressure
during a coordinated turn. The amount of adverse yaw may surprise a pilot transitioning from an airplane, and the amount
of adverse yaw will be much greater than experienced in a typical airplane.
Slips
While uncoordinated flight decreases performance, pilots can use different slipping techniques to steepen the descent angle
or to counteract a crosswind during landing.
Because of the location of the pitot tube and static vents, airspeed indicators in some gliders may have considerable error
when the pilot places the glider in a slip. The pilot should recognize this error and know how to perform slips based on
secondary indications such as the attitude of the glider, the sound of the airflow, and the feel of the flight controls.
A pilot normally coordinates rudder and aileron inputs during a turn. Using too little rudder, or if rudder is applied too late,
results in a slip. Too much rudder, or rudder applied before aileron, results in a skid. Both skids and slips expose a side of
the fuselage to the relative wind, creating additional parasite drag.
Forward Slip
During a forward slip the glider’s horizontal path over the ground remains unchanged. [ Figure 3-19] This slip uses
uncoordinated flight to dissipate energy which increases rate of descent without increasing the glider’s forward speed.
Pilots sometimes use forward slips during a landing approach over obstacles or for short-field landings when necessary to
dissipate altitude no longer needed for a margin of safety during the approach.
Forward Slip Slideslip
Figure 3-19. A comparison of a forward slip to a sideslip.
To enter a slip from straight flight, the pilot lowers the wing on the side toward which the slip occurs using the ailerons.
Simultaneously, the pilot yaws the aircraft's nose in the opposite direction by applying enough opposite rudder so that the
glider’s longitudinal axis no longer aligns with its flightpath. The pilot should yaw the glider such that it maintains the
original ground track and raises the nose sufficiently to prevent the airspeed from increasing. In crosswinds, initiating a
slip by lowering the wing on windward side of the glider provides more stable path control. The pilot discontinues a slip by
leveling the wings and by smoothly and simultaneously releasing the rudder pressure, while readjusting the pitch attitude
for a normal glide.
Note: Forward slips with wing flaps extended should not occur if the manufacturer’s operating instructions prohibit such
operation.
Sideslip
During crosswind landings, a sideslip can counteract wind drift and allows the glider to touch down with its longitudinal
axis parallel to the direction of motion. The pilot uses rudder pressure to keep the glider’s longitudinal axis parallel to the
desired ground track, but the path over the ground can change depending on the amount of bank. To perform a sideslip,
the pilot lowers the upwind wing and simultaneously applies sufficient opposite rudder to maintain the nose alignment.
Stalls
A stall occurs whenever the angle between the chord line and relative wind exceeds the critical AOA. [Figure 3-20] A stall
results in a reduction in lift, although the wings still support some of the aircraft's weight during a stall. Stalls may occur
at any airspeed and in any flight attitude.
Separation starts Separation moves forward Airfoil stalls
Figure 3-20. A stall occurs when the angle of attack exceeds the critical angle of attack.
Many factors affect the stall speed of a glider, including weight, load factor due to maneuvering, wing contamination, and
CG location. As the weight or the load factor of the glider increases, flight at any given airspeed relies on an increased
AOA—closer to the critical angle of attack. A higher load factor or glider weight causes the glider to reach the critical angle
at a higher speed. The distribution of weight also affects stall speed. For example, a forward CG requires more tail-down
force to balance the aircraft. This requires the wings to produce more lift than with the CG further aft. Therefore, a more
forward CG also increases stall speed.
Environmental factors also affect stall speed. Snow, ice, or frost accumulation on wing surfaces can increase the weight
of the wing and disrupt airflow, both of which increase stall speed. Turbulence has an impact on the stall speed of a glider
because the vertical gusts change the direction of the relative wind and abruptly increase the AOA. During landing in gusty
conditions, pilots normally increase the approach airspeed by half of the difference between the steady wind and gust value
to maintain a safe margin above stall. For example, if the winds were 10 knots gusting to 16 knots, it would be prudent to
add 3 knots ((16 – 10) ÷ 2 = 3) to the approach speed.
Spins
A spin develops from an aggravated stall that results in the glider descending in a helical or corkscrew path. A spin may
develop as a complex, uncoordinated flight maneuver in which one wing becomes more stalled than the other. Upon
entering a spin, the more completely stalled wing usually drops before the other, and the nose of the aircraft yaws in the
direction of the low wing. In this spin scenario, the ascending wing experiences more lift and less drag. [Figure 3-21] The
opposite wing moves down and back due to less lift and increased drag.
Increasing CL and CD
Coefficient of lift (CL)
Coefficient of drag (CD)
Descending wing
Stall
Ascending wing
Increasing AOA
Figure 3-21. The relative coefficients of lift and drag for each wing during a spin, which generate differential lift and drag and induces
roll and yaw.
Spins may occur after a glider stalls in uncoordinated flight with unequal airflow over the wings. Any resultant spin usually
occurs in the direction of rudder application. The entry, wing form, and CG usually determine the type of spin that results
from an uncoordinated wing stall. Glider pilots should understand the stall characteristics for any glider flown, and spin
recovery techniques as described in either the GFM, if applicable, or in Chapter 8, Abnormal and Emergency Procedures.
Spin classification includes three categories, as shown in Figure 3-22. The entry, wing form, and CG usually determine
the type of spin that results from an uncoordinated wing stall. The most common type of spin is the upright or erect spin,
which maintains a slightly nose-down rolling and yawing motion in the same direction. A second type of spin, an inverted
spin, involves the aircraft spinning upside down with the yaw and roll occurring in opposite directions. In a third type of
spin, the flat spin, the glider yaws around the vertical axis at a pitch attitude nearly level with the horizon. A flat spin often
has a very high rate of rotation with a difficult or impossible recovery. A properly loaded glider should not enter a flat spin.
Flat spins can also be inverted.
Erect spin Inverted spin Flat spin
Figure 3-22. Three types of spins.
A stall and spin entry near the ground may prove fatal because a pilot may not have adequate altitude to recover. Therefore,
pilots should avoid errors that could lead to a stall/spin accident. For example, during the approach and landing phase
with a tailwind on the base leg, a pilot might try to tighten the turn to final using rudder, or to make a steep turn to prevent
overshooting the final approach course. A skidding turn could lead to the lower wing exceeding its critical AOA before the
upper wing and could result in a spin. An excessively steep turn could also result in an accelerated stall and spin.
Chapter Summary
This chapter focuses on the four forces of flight, which include lift, drag, thrust, and weight. Components of one force
can contribute to another. When lift offsets gravity and thrust offsets drag, unaccelerated flight occurs. The chapter also
introduces the concept of ground effect, which results from a reduction in induced drag near the ground. A glider has 3 axes
of rotation and moves about each axis because of natural disturbances and pilot input. Roll occurs around the longitudinal
axis, pitch around the lateral axis, and yaw around the vertical axis. Stability refers to the ability of the aircraft to return to
its original path and orientation after an upset. Pilots unfamiliar with the effects of control inputs and stability can induce
oscillations. This chapter covers turning flight and the effect of forces on rate of turn, radius of turn, turn coordination, and
load factors. The chapter also discusses stalls and spins.