The curved yellow lines represent the maximum lift that the glider can generate at different airspeeds as Gs.
Condition 1 identifies the maximum speed at which the pilot can use full elevator up authority without damaging the glider.
Up to this point along the curve and to the left at lower speeds, the glider would stall before the pilot exceeds the design
load limit shown on the diagram. At higher speeds (to the right of condition 1), lift can exceed the maximum design load
factor before a stall occurs. Such operation may impose damaging loads to the structure.
Condition 2 represents the speed at which the pilot can use full down elevator authority and not create a negative load that
damages the glider. Above this speed the pilot can impose damaging loads to the structure.
Vertical Gusts During High-Speed Cruise
In high-speed cruise pilots should pay attention to load factor limitations. An encounter with an abrupt updraft during
wings-level high-speed cruise increases the angle of attack, bends the wings upward, briefly increases the G-load, and
stores elastic energy in the wing spars. As the wings release this energy, the wing spars spring downward and loft the
fuselage higher. As the fuselage reaches the top of this motion, the wing spars, now bent downward, move upward again
to release the stored energy. Since a negative G-load can occur as the fuselage drops downward, the seat belt and shoulder
harness can prevent the pilot's head from banging against the top of the canopy.
During these excursions, the weight of the pilot’s hand and arm may inadvertently move the control stick forward or aft.
Positive G-loading and the increased apparent weight of the pilot’s arm tend to move the control stick aft and further
increase the angle of attack and G-load. Negative G-loading and the decreased apparent weight of the pilot’s arm tend to
move the control stick forward and further decrease the angle of attack and G-load.
To minimize the intensification of vertical gusts and avoid high-speed pilot induced oscillations (PIOs), the pilot should
reduce speed when cruising through turbulent air. The pilot may also brace both arms and use both hands on the control
stick to prevent unwanted input. Some glider designs incorporate a parallelogram control stick linkage to reduce the
likelihood of PIOs during high-speed cruise.
Weight & Balance
The pilot should understand proper weight and balance management and the consequences of overloading or improperly
loading the glider.
Weight & Balance Information
The GFM/POH provided by the manufacturer gives information about the weight and balance of the glider. Since addition
or removal of equipment, such as radios, batteries, flight instruments, or airframe repairs affect the CG position, aviation
maintenance technicians (AMTs) record changes to the weight and balance data in the GFM/POH and glider airframe
logbook. They also update weight and balance placards.
Center of Gravity
Longitudinal balance affects stability around the lateral axis of a glider. To achieve satisfactory pitch attitude handling,
manufacturers position the center of gravity (CG) of a properly loaded glider forward of the center of lift (CL) and publish
the glider CG limits in the GFM/POH.
On most gliders, the horizontal stabilizer and elevator provide a down force to balance the CG and center of lift arrangement.
As the airspeed changes the pilot adjusts the trim, and the tail-down force exactly balances the forward CG. A glider in
this configuration tends to resume its previous pitch attitude after an upset about the lateral axis. Should an upset occur
that pitches the nose upward, the resultant slower airspeed and decrease in tail-down force lowers the nose and allows
the airspeed to return toward its pre-upset value. Conversely, if the upset places the aircraft in a nose-down attitude, the
increase in airspeed increases tail-down force and raises the nose toward the pre-upset condition. This arrangement creates
positive stability. However, if the tail stalls, this stabilizing action will not begin until the tail begins producing down force.
Problems Associated with CG forward of the Published Limit
Loading the glider with the CG forward of the limit makes it difficult to raise the nose on takeoff and requires considerable
back pressure on the controls to regulate pitch attitude. At low airspeeds the tail may stall or not provide sufficient down
force. Any tail stall results in a sudden nose-down pitch change and potential for a slow recovery. The pilot may not have
sufficient elevator authority to perform the landing flare due to nose heaviness. Inability to flare could result in a nose-first
hard landing.
A CG forward of the limit might occur for these reasons:
• The pilot weight exceeds the maximum permitted.
• Installed ballast weights added to the weight of the pilot exceed the maximum permitted.
Problems Associated with CG forward of the Published Limit
Loading a glider with the CG location behind the aft limit creates a tail-heavy condition. Tail heaviness can make pitch
control of the glider difficult or impossible.
A CG aft of the limit might occur for these reasons:
• The pilot weighs less than the specified minimum pilot seat weight without necessary ballast installed in the glider.
• Tailwheel dolly not removed prior to flight.
• A heavy, non-approved tailwheel or tail skid installed on the aft tail boom of the glider.
• Foreign matter or debris (water, ice, mud, sand, or nests) accumulation in the aft fuselage.
Sample Weight & Balance Problems
Some glider manufacturers provide weight and balance information in a graphic presentation. A well-designed graph
provides a convenient way to determine whether the glider is within weight and balance limits.
Sample figure 5-18 indicates that the minimum weight for the front seat pilot is 125 pounds (the lowest number on the
x-axis) to a maximum of 250 pounds (the highest number on the x-axis). It also indicates a maximum rear seat pilot weight
of 225 pounds (the highest number on the y-axis). If each pilot weighs 150 pounds, the intersection of pilot weights falls
within the envelope. Therefore, the glider load falls within the envelope for safe flight. If each pilot weighs 225 pounds,
the intersecting lines intersect in the yellow portion of the graph and indicate a load outside of weight and balance limits.
Rear seat pilot weight (lb)
Front seat pilot weight (lb)
125 150 175 200 225 250
Within weight and balance limits
Out of weight and balance limits
Figure 5-18. Sample weight and balance envelope.
Weight along the longitudinal axis of the glider affects the CG location. Pilots calculate the CG using of the arm or distance
of known weights from a specific point (datum) on the longitudinal axis. The GFM/POH supplies the arm from the datum
for the empty glider, each occupant seat, and for any cargo storage.
The pilot can determine the CG position using the following formulas:
• Weight × Arm = Moment.
• Total Moment ÷ Total Weight = CG Position (in relation to the datum).
The computational method involves the application of basic math functions as follows:
Given:
Maximum gross weight: 1,040 lb
Empty weight: 669 lb
CG range: 14.8–18.6 in
Front seat occupant: 180 lb
Rear seat occupant: 200 lb
To determine the loaded weight and CG, follow these steps:
1. List the empty weight of the glider and the weight of the occupants.
2. Enter the moment for each item listed. Remember, weight × arm = moment.
3. Total the weight and moments.
4. To determine the CG, divide the total moments by the total weight. [Figure 5-19]
Note: The weight and balance records for a particular glider provide the empty weight and moment, as well as the
information on the arm distance. [Figure 5-19]
Item
Empty weight
Front seat pilot
Rear seat pilot
Weight (pounds)
1,039 total weight
Arm (inches)
+93.7
+43.8
+74.7
+81.58
Moment (inch·pounds)
+62,685
+7,884
+14,193
+84,762 total moment
Figure 5-19. Sample weight and balance: front and rear seat pilot weights and moments.
In Figure 5-19 above, the weight of each pilot appears in the appropriate block in the table. For the front seat pilot,
multiplying 180 pounds by 43.8 inches yields a moment of 7,884 inch-pounds. For the rear seat pilot, multiplying 190
pounds by 74.7 inches yields a moment of 14,193 inch-pounds. The next step is to find the sum of all weights (980 pounds)
including the empty weight of the glider. Then, find the sum of all moments (+84,762 inch-pounds). To determine the CG
position of the loaded glider, divide the total moment by the total weight to in inches from the datum: 84,762 inch-pounds
÷ 1039 pounds = 81.58 aft of the datum.
For the final step the pilot determines whether total weight and CG location values are within acceptable limits. The GFM/
POH lists the maximum gross weight as 1,040 pounds. The operating weight of 1039 pounds does not exceed the 1,040
pounds maximum gross weight. The GFM/POH lists the approved CG range as between 78.2 inches and 86.1 inches from
the datum. The operating CG of 81.58 inches from the datum falls within these limits. Therefore, the calculation shows the
glider within operating limits if loaded as planned.
Ballast
Ballast includes nonstructural weight added to a glider. In soaring, ballast weight serves two purposes. Trim ballast adjusts
the location of the CG of the glider to remain within acceptable limits. Performance ballast improves high-speed cruise
performance.
Trim Ballast
Removable trim ballast weights, often made of metal, attach to a ballast receptacle incorporated in the glider structure.
These weights compensate for a front seat pilot who weighs less than needed to maintain the CG within acceptable
operating limits. The ballast weight mounted well forward in the glider cabin can move the CG within permissible limits
with the minimum addition of weight.
Whenever an approved POH or Glider Flight Manual limitation section includes specific instructions on the use of trim
ballast, pilots must follow the approved method stated in the GFM/POH for placement of that ballast. For gliders without
limitations or placards regarding placement of trim ballast, pilots may consider using a seat cushion with sand or lead shot
sewn into the unit to provide additional weight. Since this type of ballast may shift position during maneuvering, pilots
should not rely on seat cushion ballast during acrobatic or inverted flight. The pilot should develop a means to verify the
presence, absence, weight, and appropriateness of any trim ballast before a flight.
Trim ballast may also include water in a tail tank in the vertical fin. Water weighs 8.35 pounds per gallon. Because of
its far-aft location, the pilot can use a small amount of water in the tail tank to offset the moment of any main wing tank
performance ballast. Even though a tail tank generally holds less than two gallons of water, a calculation error leading to
excess water in the tail could result in flight with a CG aft of the limit.
Performance Ballast
Adding weight enhances high-speed performance in gliders. Increasing the operating weight of the glider increases the
optimum speed to fly during wings-level cruising flight. The resulting higher ground speed provides an advantage in cross-
country soaring and in glider racing.
Manufacturers commonly install water tanks in the main wing panels. That water acts as performance ballast. Personnel
add clean water through fill ports in the top of each wing. The amount of water introduced depends on the pilot’s choice
of operating weight. After adding water, replacement of the filler caps prevents water from sloshing out of the filler holes.
Vents in the filler caps allow air to enter the tanks to replace the volume of water drained from the tanks. [ Figure 5-20]
Pilots should ensure that the vents work properly to prevent wing damage when draining water ballast.
Figure 5-20. Water ballast tank vented filler cap.
Drain valves fit to the bottom of each tank, and the pilot controls the valves from inside the glider. [Figure 5-21] The pilot
can fully or partially drain the tanks with the glider on the ground to reduce weight prior to launch. The pilot can also
manipulate the valves to drain the ballast tanks partially or completely in flight—a process called dumping ballast, which
normally occurs prior to landing. The long streaks of white spray behind an airborne glider indicate water draining in the
air.
Drain valve closed Drain valve open
Figure 5-21. Water ballast drain valve handles.
Pilots should check the drain valves for correct operation prior to flight. Water ballast should drain from each wing tank
at the same rate. Unequal draining leads to a wing-heavy condition that makes inflight handling, as well as landings, more
difficult. If the wing-heavy condition becomes extreme, the pilot may lose control of the glider.
Water ballast should drain into the air outside the glider rather than leak into the fuselage. Water trapped in the fuselage
may flow through or over bulkheads, causing unmanaged changes to the glider CG. Sufficient CG movement could lead
to control difficulty or even total loss of control.
The flight manual provides guidance regarding the length of time it takes for the ballast tanks to drain completely. When
preparing for landing, the pilot should dump ballast early enough to give the ballast drains sufficient time to empty the
tanks.
Use of water ballast in low ambient temperatures can result in water freezing the drain valve, making dumping ballast
difficult or impossible. If only one valve freezes, uneven dumping may occur as discussed above. If water in the wings
freezes, serious wing damage may occur because water expands while freezing. The resulting increased volume can deform
ribs and other wing structures or delaminate glued bonds. In cold weather or when expecting cold flight conditions, pilots
should not use water ballast unless adding antifreeze to the water. The GFM has information on antifreeze compounds
approved for use in the glider.
A glider carrying large amounts of water ballast has noticeably different handling characteristics than the same glider
without water ballast. Water ballast:
• Reduces the rate of acceleration of the glider at the beginning of the launch due to the increased glider weight.
• Increases the length of ground roll prior to glider liftoff.
• Increases stall speed.
• Reduces aileron control during the takeoff roll, increasing the chance of uncontrolled wing drop and resultant
ground loop.
• Reduces rate of climb during climb-out.
• Reduces aileron response during free flight. The addition of large amounts of water increases lateral stability
substantially. This makes quick banking maneuvers difficult or impossible to perform.
The pilot routinely dumps water ballast before landing to reduce the weight of the glider. Dumping ballast:
• Decreases stall speed.
• Decreases the optimum airspeed for the landing approach.
• Shortens landing roll.
• Reduces the load that glider structures must support during landing and rollout.
While performance advantages from ballast occur during strong soaring conditions, pilots should consider that ballast
degrades takeoff performance, climb rate, and low-speed handling. Before committing to a launch with water ballast
aboard, the pilot should review operating limitations to ensure safety of flight.
Chapter Summary
Factors that affect all glider flights include temperature, atmospheric pressure, humidity, wind, and operating weight. Pilots
should consider the design of the glider and its operating characteristics and know the expected performance before flight.
Pilots should only fly when weight and balance conditions remain within limits since these conditions affect stability and
control. Glider polars indicate the performance speeds that pilots can expect at different weights and under different wind
conditions and can assist in maximizing performance. Glider pilots flying models that use water ballast should understand
how to verify proper system operation before flight and know when to drain any water when necessary or before landing.