sUAS Weight, Balance, and Performance Practice Exam

Principles of Weight and Balance for sUAS

• Remote PIC Responsibility: The Remote Pilot in Command (Remote PIC) is strictly responsible for ensuring the aircraft is operated within its weight and balance limitations. This requires using the most current weight and balance data available for the specific aircraft.

• Loading and CG Documentation: To ensure that the unmanned aircraft center of gravity (CG) limits are not exceeded, the Remote PIC must follow specific aircraft loading instructions found in the Pilot’s Operating Handbook (POH) or the UAS Flight Manual.

• Consequences of Improper Loading: Operating a small unmanned aircraft (sUA) outside of its specific weight and balance limitations may result in a total loss of control.

• Lateral Center of Gravity: While longitudinal CG (forward/aft) is frequently discussed, the weight and balance of an sUAS must account for all axes to maintain flight stability.

• Mounting External Equipment: When attaching cameras, sensors, or other equipment to an sUAS, the items must be mounted in a manner that does not adversely affect the center of gravity.

Performance Effects of Overloading and Weight

• Exceeding Maximum Allowable Weight: Loading an sUAS above its maximum allowable takeoff weight results in several performance degradations:

  • Shorter Endurance: The aircraft will exert more energy to maintain flight, reducing total flight time.

  • Decreased Rate of Climb: The aircraft will be unable to gain altitude as quickly as it would at a lower weight.

  • Increased Takeoff Distance: More power and runway (or launch area) are required to reach flying speed.

  • Decreased Speed: Overall cruise and maximum speeds are negatively impacted.

• Performance Data Sources: The manufacturer’s publications (manuals, performance charts, and specifications) are the best and most reliable source for sUAS performance data and information.

• Critical Launch Conditions: The performance of an aircraft during launch is most critical when there is a combination of:

  • High gross weight.

  • High altitude.

  • High temperature.

  • Unfavorable wind.

Center of Gravity (CG) Dynamics and Stability

• Aft (Rearward) CG Loading: If the center of gravity is located too far aft (toward the rear of the aircraft), the likely result is that the aircraft will have difficulty recovering from a stalled position.

• Forward CG Loading: While not explicitly detailed as a failure in the transcript, CG limits define the range within which the aircraft remains controllable. Moving the CG forward affects the stall speed and stability, but the transcript emphasizes the recovery danger of an aft CG.

Aerodynamics: Stalls and Angle of Attack

• Definition of Stall: A stall occurs when the smooth airflow over the wings or propellers of an unmanned aircraft is disrupted, causing lift to reduce (degenerate) rapidly.

• Cause of Stall: A stall is specifically caused when the wing or propeller(s) exceeds its critical angle of attack.

• Angle of Attack (AOA) Definition: The term "angle of attack" is defined as the angle between the wing chord line and the relative wind.

• Critical Angle of Attack Consistency: The specific angle of attack at which a wing or airfoil stalls will remain the same regardless of the gross weight of the aircraft. However, the airspeed at which that angle is reached can vary based on weight and load factor.

Load Factor and Structural Integrity

• Definition of Load Factor: Load factor represents the ratio of the total load supported by the aircraft's lifting surfaces to the actual weight of the aircraft and its contents.

• Maneuvering Effects: The load factor on the wings or rotors of an unmanned aircraft increases anytime the aircraft is subjected to maneuvers other than straight and level flight.

• Specific Maneuvers:

  • Level Turns: A basic flight maneuver that increases the load factor compared to straight and level flight.

• Consequences of Increased Load Factor:

  • The aircraft will stall at a higher airspeed.

  • Increased stress is applied to the aircraft structure.

• Structural Load Limits: The amount of excess load that can be safely imposed on the wing depends on the speed of the airplane and the abruptness at which the load is applied.

Load Factor Calculations (Reference to Appendix 2, Figure 2)

Calculations for structural load are based on the bank angle of the aircraft. As the bank angle increases, the load factor (measured in Gs) increases. The weight the structure must support is calculated as: $\text{Required Structural Weight Support} = \text{Actual Weight} \times \text{Load Factor}$.

• Case 1: $33\,\text{pounds}$ at $30^{\circ}$ Bank:

  • During a $30^{\circ}$ banked turn while maintaining altitude, the structure must support an approximate weight of $38\,\text{pounds}$.

• Case 2: $50\,\text{pounds}$ at $30^{\circ}$ Bank:

  • During a $30^{\circ}$ banked turn while maintaining altitude, the structure must support an approximate weight of $60\,\text{pounds}$.

• Case 3: $10\,\text{pounds}$ at $60^{\circ}$ Bank:

  • At a $60^{\circ}$ bank angle, the load factor is $2.0$.

  • The structure must support: $10\,\text{pounds} \times 2.0 = 20\,\text{pounds}$.

  • Result: $20\,\text{pounds}$.