UAS 250 Module 5 Study Notes: sUAS Loading and Performance

ACS Area of Operation: Loading and Performance

• ACS Document Context: This lesson navigates through the five major Airman Certification Standards (ACS) Areas of Operation (U.S. DOT, FAA, 2021).
  • I. Regulations
  • II. Airspace
  • III. Weather
  • IV. Loading and Performance
  • V. Operations
• Task IV.A Knowledge Elements: The lesson covers the key concepts from Task A, "Loading & Performance," which include:
  • General Loading & Performance
  • Effects of Loading Changes
  • Balance, Stability, and Center of Gravity
  • Use of Performance Data
• Study Guide Integration: The materials in the ACS and the FAA Remote Pilot sUAS Study Guide (U.S. DOT, FAA, 2016) complement each other. Area IV, Task A of the ACS corresponds directly to Chapter 4 of the Study Guide.

Fundamental Principles of Weight and Gravity

• Definition of Weight: Weight is the force with which gravity attracts a body toward the center of the Earth. It is defined as the product of the mass of a body and the acceleration acting on it.
• Definition of Gravity: Gravity is the acceleration that tends to draw all bodies toward the center of the Earth.
• Formula for Weight:     $\text{Weight (force)} = \text{Mass} \times \text{Gravity (acceleration)}$
• Lift and Weight Relationship: Lift is the upward force on the wing, acting perpendicular to the relative wind and the aircraft’s lateral axis. This force counteracts the aircraft's weight.
  • Equilibrium: In level, stabilized flight, when lift equals weight ($\text{Lift} = \text{Weight}$), the aircraft is in equilibrium with no upward or downward acceleration.
  • Lift < Weight: The aircraft's vertical speed decreases.
  • Lift > Weight: The aircraft's vertical speed increases.

Center of Gravity (CG) and Center of Pressure (CP)

• Center of Gravity (CG): This is the specific point where the aircraft's weight is concentrated. If an aircraft were supported at this single point, it would remain balanced in any position.
  • Importance: Its position significantly impacts the stability of small unmanned aircraft.
  • CG Limits: The acceptable location of the CG is determined by the specific design of the aircraft. Manufacturers establish rear and forward limits.
• Center of Pressure (CP): While weight is concentrated at the CG, aerodynamic lift forces act at the Center of Pressure (CP).
• Pitching Moments:
  • Nose-Down Pitch: Occurs when the CG is positioned ahead of the CP.
  • Nose-Up Pitch: Occurs when the CP is positioned ahead of the CG.
• Design Considerations: To maintain flight stability, aircraft designers typically position the rear limit of the CG forward of the CP at relevant flight speeds.

Stability, Maneuverability, and Controllability

• Stability: The inherent ability of an aircraft to counteract disturbances that affect its equilibrium. This quality allows the aircraft to return to or maintain its original flight path and is primarily a function of design.
• Maneuverability: Defined as "the quality of an aircraft that permits it to be maneuvered easily and withstand the stresses imposed by maneuvers." It is governed by weight, inertia, size and location of flight controls, structural strength, and the powerplant.
• Controllability: Defined as "the capability of an aircraft to respond to the pilot’s control, especially concerning flight path and attitude." It is the quality involving the response to control applications regardless of stability characteristics.

Effects of Weight and Balance on Performance

• Remote Pilot-In-Command (PIC) Responsibility: Before any flight, the PIC must verify the aircraft is correctly loaded by determining its weight and balance condition. Compliance with manufacturer or builder restrictions is critical to flight safety.
• Excess Weight Conditions: Taking off at maximum gross takeoff weight is not always safe. Performance-reducing factors include:
  • High elevations.
  • High air temperatures.
  • High humidity.
  • Density Altitude: High elevations, temperatures, and humidity lead to high-density altitudes which negatively impact takeoff and climb.
• Launch Area Considerations: Runway or launch area length, surface type, slope, surface wind, and obstacles may necessitate weight reduction before takeoff.
• Weight Changes During Flight:
  • The most common change is fuel consumption. As the aircraft becomes lighter, performance typically improves, though balance may shift.
  • In sUAS, weight may change due to diminishing payloads or jettisonable items.
• Adverse Balance: Improper weight distribution can be as detrimental as excess weight. As load items are moved or consumed, the CG may shift. If the CG falls outside allowable limits during flight, adjustments or weight reductions are required.

Performance Deficiencies of Overloaded Aircraft

• General Impact: Increased total weight is detrimental to performance and compromises structural integrity. Overloading may cause an aircraft to struggle during takeoff or exhibit poor flight characteristics once airborne.
• Phase-Specific Performance Issues:
  • Takeoff Phase: Longer takeoff run and higher required takeoff speed.
  • Climb Phase: Reduced rate of climb, reduced angle of climb, and lower maximum altitude (service ceiling).
  • Cruise/Mission Phase: Shorter range, reduced cruising speed, reduced maneuverability, and a higher stalling speed.
  • Landing Phase: Higher approach and landing speeds, and a longer landing roll.
• Safety Margins: Excessive weight reduces safety margins, which is particularly dangerous during emergencies (e.g., engine failure) or adverse weather.

Aerodynamic Load Factors (Pulling Gs)

• Definition: The load factor is the ratio between the lift and the total weight of the aircraft. It is a measure of the stress or "G-forces" on the aircraft structure.
• Measurement: Load factor is measured in Gs. A 3 G load factor means the total load on the structure is three times its weight ($3 \times \text{Weight}$).
• Importance of Load Factors:
  1. A pilot can impose a dangerous overload on aircraft structures.
  2. Increased load factor increases the stalling speed of the aircraft.
• Bank Angle and Load Factor: In a coordinated level turn, the load factor results from centrifugal force and weight. As the bank angle increases, the load factor rises rapidly:
  • $60^{\circ}$ Bank: Produces a load factor of $2.0\,Gs$.
  • $80^{\circ}$ Bank: Produces a load factor of nearly $6.0\,Gs$.
  • $90^{\circ}$ Bank: Theoretically, the load factor becomes infinite because the lift vector is entirely horizontal, with no vertical component to counteract weight.

Load Factor and Stall Speed

• Stall Definition: An aerodynamic stall occurs when the airflow over the wings or propeller is disturbed, significantly reducing lift. Weight then overcomes the remaining lift, leading to a rapid descent.
• Impact of High Stall Speed: A higher stall speed reduces the safe speed range for flight, especially during maneuvers, takeoff, or landing, and may leave insufficient altitude for recovery.
• Mathematical Relationship: Stall speed increases in proportion to the square root of the load factor.
  • Example: If an aircraft has a normal stall speed of $50\,knots$, and a $4\,G$ load factor is applied, the stall speed doubles to $100\,knots$ because $\sqrt{4} = 2$.
• Accelerated Stall Speed Calculation (Example):
  • Scenario: An aircraft with a normal unaccelerated stalling speed of $45\,knots$ enters a $70^{\circ}$ bank angle turn.
  • Step 1 (Determine Load Factor): Using the "Bank Angle vs Load Factor" chart (Fig 4-2), a $70^{\circ}$ bank identifies a load factor of approximately $3\,Gs$.
  • Step 2 (Determine New Stall Speed): Using the "Load Factor Changes Stall Speed" chart (Fig 4-3), find the line for a $3\,G$ load factor. Trace it to the curve for an unaccelerated stall speed of $45\,knots$ (halfway between the $40$ and $50\,knot$ lines).
  • Step 3 (Result): Following the point down to the horizontal axis reveals an accelerated stall speed of approximately $75\,knots$.
  • Rule for Pilot: The airspeed must be maintained above $75\,knots$ (rather than $45\,knots$) during the turn to prevent a stall.

Questions & Discussion

• Question: What may be considered as a point at which all the weight of the aircraft is concentrated?
  • Answer: Center of Gravity (CG).
• Question: What is the term for the capability of an aircraft to respond to the pilot’s control, especially regarding flight path and attitude, regardless of stability?
  • Answer: Controllability.
• Question: Conditions that negatively affect takeoff and climb (high elevation, high temp, high humidity) may require what in terms of weight?
  • Answer: A reduction in weight.
• Question: Operation with the center of gravity (CG) outside approved limits results in what?
  • Answer: Control difficulty.
• Question: What are the most important performance deficiencies of an overloaded aircraft?
  • Answer: Reduced maneuverability, longer takeoff run, reduced rate of climb, higher stall speed, and shorter range.