Private Pilot Groundschool Session4 Weight & Balance, Aircraft Performance (PHAK Chapters 9&10)

Weight & Balance Increased weight requires more lift and power, and thereby decreases available excess power for acceleration and climb In order to be controllable, an aircraft must not only be within the maximum allowed GROSS WEIGHT, but the balance point, or CENTER OF GRAVITY must be within a specified range, or CG ENVILOPE defined by A FORWARD LIMIT and an AFT LIMIT. An aircraft which is out of CG may be uncontrollable, and/or unable to recover from a stall. Aft CG will cause an airplne to be less stable about the lateral axis

Calculating CG To determine whether an aircraft is within the specified limits of its CG envelope, the balance point must be calculated. MOMENT is the weight of an object multiplied by it’s ARM or distance from a reference point. In order to find out if two weights balance, their moment must me determined. Objects which have the same moment will have the same effect on balance. Weights forward of the reference point will have a negative arm, and therefore a negative moment, whereas weights aft of the reference point will have a positive arm and moment.

Calculating CG (continued) To calculate CG, divide the TOTAL MOMENT by the TOTAL WEIGHT. The total moment is the sum of the moments of all objects in the aircraft, and the aircraft itself. Objects with negative moment will decrease this number. The CG and total weight can be calculated in either a table or a graph. Pilot’s Operating Handbooks will vary somewhat in their presentation, but the overall information contained is the same. Some manuals will give a graph of moment to weight, eliminating the need to divide the moment by the weight. In these cases, simply use the total weight and total moment to determine if the aircraft is within weight and balance limitations.

Aircraft Performance In order to determine whether a flight can safely be completed, the performance of an aircraft at the weights, altitudes, temperatures, and airports expected on the flight must be calculated. Pilot’s Operating Handbooks provide charts and data to calculate these. Takeoff performance varies with aircraft weight, altitude, temperature, runway surface condition, and wind Some manuals give this information in table form, while others give it in chart form. Cruise performance is dependent on altidude temperature, and aircraft weight. However, the cruise speed derived from the information in the POH must still be corrected for wind. Landing performance is largely dependant on the same variables as takeoff, however runway surface condition plays a larger role, as brakes are used to slow the aircraft on landing.