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## 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; 

<img src='content_image/611498.jpg'>

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. 

Given: 

<img src='content_image/611483.jpg'>

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. To simplify calculations, the moments may be divided by 100. 

3. Total the weight and moments. 

4. To determine the CG, divide the moments by the weight. 

