Center of Gravity
Longitudinal balance affects the stability of the longitudinal axis of the glider. To achieve satisfactory pitch attitude handling in a glider, the CG of the properly loaded glider is forward of the center of pressure (CP). When a glider is produced, the manufacturer provides glider CG limitations, which require compliance. These limitations are generally found in the GFM/ POH and may also be found in the glider airframe logbook. Addition or removal of equipment, such as radios, batteries, or flight instruments, or airframe repairs can have an effect on the CG position. Aviation maintenance technicians (AMTs) must record any changes in the weight and balance data in the GFM/POH or glider airframe logbook. Weight and balance placards in the cockpit must also be updated.
Problems Associated With CG Forward of Forward Limit
If the CG is within limits, pitch attitude control stays within acceptable limits. However, if the glider is loaded so the CG is forward of the forward limit, handling is compromised. The glider is said to be nose heavy. Nose heaviness makes it difficult to raise the nose on takeoff and considerable back pressure on the control stick is required to control the pitch attitude. Tail stalls occur at airspeeds higher than normal and are followed by a rapid nose-down pitch tendency. Restoring a normal flight attitude during stall recoveries takes longer. The landing flare is more difficult than normal, or perhaps even impossible, due to nose heaviness. Inability to flare could result in a hard nose-first landing.
The following are the most common reasons for CG forward of forward limit:
- Pilot weight exceeds the maximum permitted pilot weight.
- Seat or nose ballast weights are installed but are not required due to the weight of the pilot.
Problems Associated With CG Aft of Aft Limit
If the glider is loaded so the CG location is behind the aft limit, handling is compromised. The glider is said to be tail heavy. Tail heaviness can make pitch control of the glider difficult or even impossible.
The fundamental problem with a CG aft of the aft limit is the designed function of the horizontal stabilizer and elevator. Fixed wing aircraft are generally designed so that the horizontal stabilizer and elevator provide a down force to counter the slightly nose forward CG such that the aircraft tend to resume a level pitch attitude after an upset about the lateral axis. As the airspeed changes, the pilot changes the trim or trims the aircraft so the down force exactly balances the forward CG within limits. Should the aircraft be upset and the nose pitches upward, the resultant slower airspeed results in less down force produced by the horizontal stabilizer and elevator. This decreased down force lets the nose lower so the airspeed retains to the pre-upset value. This is called positive stability. Conversely, if the upset places the aircraft in a nose down attitude, the increased airspeed will increase the down force and raise the nose to the pre-upset balanced condition. However, if the control surface is in a stalled condition, this stabilizing action will not begin until the control surface regains un-stalled airflow and begins producing down force again.
The following are the most common reasons for flight with CG located behind permissible limits:
- Pilot weight is less than the specified minimum pilot seat weight and trim ballast weights necessary for the lightweight pilot are not installed in the glider prior to flight.
- Tailwheel dolly is still attached, far aft on the tailboom of the glider.
- Foreign matter or debris (water, ice, mud, sand, and nests) has accumulated in the aft fuselage of the glider and was not discovered and removed prior to flight.
- A heavy, non-approved tailwheel or tail skid was installed on the aft tail boom of the glider.
Sample Weight and 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 limitations.
In Figure 5-19, the chart indicates that the minimum weight for the front seat pilot is 125 pounds, and that the maximum is 250 pounds. It also indicates that the maximum rear seat pilot weight is 225 pounds. If each pilot weighs 150 pounds, the intersection of pilot weights falls within the envelope; the glider load is within the envelope and is safe for flight. If each pilot weighs 225 pounds, the rear seat maximum load is exceeded, and the glider load is outside the envelope and unsafe for flight.
The CG position can also be determined by calculation using the following formulas:
- Weight × Arm = Moment
- Total Moment ÷ Total Weight = CG Position (in inches aft of the reference datum)
The computational method involves the application of basic math functions. The following is an example of the computational method.
|Maximum gross weight||1,100 lb|
|Empty weight||600 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.
- List the empty weight of the glider and the weight of the occupants.
- Enter the moment for each item listed. Remember, weight × arm = moment. To simplify calculations, the moments may be divided by 100.
- Total the weight and moments.
- To determine the CG, divide the moments by the weight.
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 5-20]
In Figure 5-20, the weight of each pilot has been entered into the correct block in the table. For the front seat pilot, multiplying 180 pounds by +30 inches yields a moment of +5,400 inch·pounds. For the rear seat pilot, multiplying 200 pounds by –5 inches yields a moment of –1,000 inch·pounds. The next step is to find the sum of all weights (980 pounds) and record it. Then, find the sum of all moments (+16,400 inch·pounds) and record it. Now, find the arm (the CG position) of the loaded glider. Divide the total moment by the total weight to discover the CG of the loaded aircraft glider in inches from the datum:
+16,400 inch·pounds ÷ 980 pounds = +16.73 inches
The final step is to determine whether total weight and CG location values are within acceptable limits. The GFM/ POH lists the maximum gross weight as 1,100 pounds. The operating weight of 980 pounds is less than the 1,100 pounds maximum gross weight. The GFM/POH lists the approved CG range as between +14.80 inches and +18.60 inches from the datum. The operating CG is +16.73 inches from the datum and is within these limits. The weight and balance are within operating limits.