Chapter 4
Loading and Performance

Introduction

The remote PIC is responsible for ensuring the sUAS is in a condition for safe operation. Part of this involves checking for proper loading, so that the device operates to the expected performance standards.

Prior to each flight, the remote PIC must ensure that any object attached to or carried by the small unmanned aircraft is secure and does not adversely affect the flight characteristics or controllability of the aircraft. For example, some sUA do not have a set holder or slot for the battery; instead, it is simply attached with hook-and-loop or other type of fastener. This allows some leeway on the lateral and longitudinal location of the battery on the sUA. Remote pilots should ensure the battery is installed in the proper location so it does not adversely affect the controllability of the aircraft. The attachments must be secure so the battery does not move during flight. Similar concerns exist and cautions advised if any external attachments are installed. Also be sure to close and lock (if applicable) all panels or doors.

Follow all manufacturer recommendations for evaluating performance to ensure safe and efficient operation. This manufacturer information may include operational performance details for the aircraft such as launch, climb, range, endurance, descent, and landing. It is important to understand the significance of the operational data to be able to make practical use of the aircraft’s capabilities and limitations. The manufacturer’s information regarding performance data is not standardized; availability and how this information is conveyed can vary greatly between sUAS types. If manufacturer-published performance data is unavailable, the remote pilot should seek out performance data that may have already been determined and published by other users of the same sUAS manufacturer model, and use that data as a starting point.

Check weather conditions prior to and during every sUAS flight and consider the effects of weather on aircraft performance.

Airplane flight control systems consist of primary and secondary systems. The ailerons, elevator (or stabilator) and rudder constitute the primary control system and are required to control an airplane safely during flight. Wing flaps, leading edge devices, spoilers and trim systems constitute the secondary control system and improve the performance characteristics of the airplane or relieve the pilot of excessive control forces. See Figure 4-1.

Figure 4-1. Aircraft flight controls

A helicopter has four flight control inputs: cyclic, collective, antitorque pedals, and throttle. The cyclic can vary the pitch of the rotor blades throughout each revolution of the main rotor system to develop lift (thrust). The result is to tilt the rotor disk in a particular direction, resulting in the helicopter moving in that direction.

Determining Speed and Altitude

The remote pilot must not exceed these regulatory limitations for operating an sUAS:

• Cannot be flown faster than a ground speed of 87 knots (100 miles per hour).

• Cannot be flown higher than 400 feet AGL unless flown within a 400-foot radius of a structure and is not flown higher than 400 feet above the structure’s immediate uppermost limit.

Some of the possible ways to ensure that 87 knots is not exceeded include:

• Installing a GPS device on the sUAS that reports ground speed information to the remote pilot, wherein the remote pilot takes into account the wind direction and speed and calculates the sUAS airspeed for a given direction of flight;

• Timing the ground speed of the sUAS when it is flown between two or more fixed points, taking into account wind speed and direction between each point, then noting the power settings of the sUAS to operate at or less than 87 knots ground speed;

• Using the sUAS manufacturer’s design limitations (e.g., installed ground speed limiters);

• Using a radar gun to measure speed of the sUAS;

• Using an anemometer coupled with inertial information from onboard sensors; or

• Using inertial sensors capable of detecting wind speed and velocity as well as sUAS movement to determine ground speed.

These navigation terms are used in aviation as related to ground speed:

• Dead reckoning is navigation solely by means of computations based on time, airspeed, distance, and direction. The products derived from these variables, when adjusted by windspeed and velocity, are heading and ground speed.

• Pilotage is navigation by reference to landmarks or checkpoints.

• The wind triangle is navigation using triangulation. The true heading and the ground speed can be found by drawing a wind triangle of vectors. One side of the triangle is the wind direction and velocity; another side is the true heading and true airspeed, the last side is the track, or true course, and the ground speed. Each side of a wind triangle is the vector sum of the other two sides.

Some possible ways for a remote pilot to determine altitude above the ground or structure are as follows:

• Install a calibrated altitude reporting device on the sUAS that reports the sUAS altitude above mean sea level to the remote pilot, wherein the remote pilot subtracts the MSL elevation of the CS from the sUAS-reported MSL altitude to determine the sUAS altitude above the terrain or structure.

• Install a GPS device on the sUAS that also has the capability of reporting MSL altitude to the remote pilot (note that there may be minor errors associated with using this type of altitude reporting).

• With the sUAS on the ground, have the remote pilot and VO pace off 400 feet from the sUAS to get a visual perspective of the sUAS at that distance, wherein the remote pilot and VO maintain that visual perspective or closer while the sUAS is in flight.

• Use the known height of local rising terrain and/or structures as a reference.

• If the CS has a measure of distance between it and the sUA, fly directly overhead until reaching 400 feet and note the visual perspective.

In addition to local resources, the Sectional Chart for that region should be consulted for information on the altitude of the terrain and structures. Towers and other known obstructions are depicted with their altitude noted. It is critical to understand that altitude dimensions of airspace depicted on Sectional Charts are in MSL; remote pilots must be careful to properly convert between AGL and MSL as applicable to their equipment, operation, and regulation restrictions. See Legend 1 in the CT-8080-2.

Loading

As with any aircraft, compliance with weight and balance limits is critical to the safety of flight for sUAS. An unmanned aircraft that is loaded out of balance may exhibit unexpected and unsafe flight characteristics. An overweight condition may cause problematic control or performance limitations. Before any flight, verify that the unmanned aircraft is correctly loaded by determining the weight and balance condition.

• Review any available manufacturer weight and balance data and follow all restrictions and limitations.

• If the manufacturer does not provide specific weight and balance data, apply general weight and balance principals to determine limits for a given flight. For example, add weight in a manner that does not adversely affect the aircraft’s center of gravity (CG) location—a point at which the sUA would balance if it were suspended at that point. Usually this is located near the geographical center of multi-copters but may vary along the centerline of the fuselage of fixed-wing and single-rotor sUA.

Although a maximum gross launch weight may be specified, the aircraft may not always safely take off with this load under all conditions. Or if it does become airborne, the unmanned aircraft may exhibit unexpected and unusually poor flight characteristics. Conditions that affect launch and climb performance, such as high elevations, high air temperatures, and high humidity (high density altitudes) as well as windy conditions may require a reduction in weight before flight is attempted. Other factors to consider prior to launch are runway/launch area length, surface, slope, surface wind, and the presence of obstacles. These factors may require a reduction in weight prior to flight.

Weight changes during flight also have a direct effect on aircraft performance. Fuel burn is the most common weight change that takes place during flight. As fuel is used, the aircraft becomes lighter and performance is improved, but this could have a negative effect on balance. For battery-powered sUAS operations, weight change during flight may occur when expendable items are used on board (e.g., a jettisonable load such as an agricultural spray). Changes of mounted equipment between flights, such as the installation of different cameras, battery packs, or other instruments may also affect the weight and balance and performance of an sUAS.

Adverse balance conditions (i.e., weight distribution) may affect flight characteristics in much the same manner as an excess weight condition. Limits for the location of the CG may be established by the manufacturer and may be covered in the pilot operating handbook (POH) or sUAS flight manual. The CG is not a fixed point marked on the aircraft; its location depends on the distribution of aircraft weight. As variable load items are shifted or expended, there may be a resultant shift in CG location. The remote PIC should determine how the CG will shift and the resultant effects on the aircraft. If the CG is not within the allowable limits after loading or do not remain within the allowable limits for safe flight, it will be necessary to relocate or shed some weight before flight is attempted.

Excessive weight reduces the flight performance in almost every respect. In addition, operating above the maximum weight limitation can compromise the structural integrity of an sUA. The most common performance deficiencies of an overloaded aircraft are:

• Reduced rate of climb;

• Lower maximum altitude;

• Shorter endurance; and

• Reduced maneuverability.

Prior to conducting a mission or extended flight, it is recommended to test-fly the sUA to determine if there are any unexpected performance issues due to loading. This testing should be done away from obstacles and people.

Load Factor

In aerodynamics, load is the force or imposed stress that must be supported by an sUA structure in flight. The loads imposed on the wings or rotors in flight are stated in terms of load factor. In straight-and-level flight, the sUAS wings/rotors support a load equal to the sum of the weight of the sUAS plus its contents. This particular load factor is equal to 1 G, where “G” refers to the pull of gravity. However, centrifugal force is generated which acts toward the outside of the curve any time an sUAS is flying a curved path (turns, climbs, or descents).

Unmanned aircraft performance can be decreased due to an increase in load factor when the aircraft is operated in maneuvers other than straight and level flight. The load factor increases at a significant rate after a bank (turn) has reached 45° or 50°. The load factor for any aircraft in a coordinated level turn at 60° bank is 2 Gs. The load factor in an 80° bank is 5.75 Gs. See Figure 4-2. The wing must produce lift equal to these load factors if altitude is to be maintained. The remote PIC should be mindful of the increased load factor and its possible effects on the aircraft’s structural integrity and the results of an increase in stall speed. These principles apply to both fixed-wing and rotor-wing designs, but in the case of rotor-wing unmanned aircraft, the weight/load must be supported by the lift generated by the propellers.

Figure 4-2. Load factor chart

As with manned aircraft, an unmanned aircraft will stall when critical angle of attack of the wing or rotors/propeller is exceeded. This can occur when an unmanned aircraft is turned too sharply/tightly or pitched up too steeply or rapidly. Remote pilots of rotor type unmanned aircraft should use particular caution when descending in a vertical straight line. In some cases, the turbulent downward airflow can disrupt the normal production of lift by the propellers as well as cause problematic air circulation producing vortices. These phenomena are referred to as vortex ring state or settling with power, and when they occur the aircraft can wobble, descend rapidly, or become uncontrollable. Recovery from this state of flight requires forward or rearward motion—counterintuitively, the addition of power to arrest the descent only makes the situation worse. Due to the low-altitude operating environment, consideration should be given to ensure aircraft control is maintained and the aircraft is not operated outside its performance limits.

Stalls

An airfoil is a structure or body that produces a useful reaction to air movement. Airplane wings, helicopter rotor blades, and propellers are airfoils. The chord line is an imaginary straight line from the leading edge to the trailing edge of an airfoil. In aerodynamics, relative wind is the wind “felt” or experienced by an airfoil. It is created by the movement of air past an airfoil, by the motion of an airfoil through the air, or by a combination of the two. Relative wind is parallel to, and in the opposite direction of the flight path of the airfoil. The angle of attack is the angle between the chord line of the airfoil and the relative wind. Angle of attack is directly related to the generation of lift by an airfoil. See Figures 4-3 through 4-6.

Figure 4-3. A typical airfoil cross-section

Figure 4-4. Chord line

Figure 4-5. Relative wind

Figure 4-6. Angle of attack

As the angle of attack is increased (to increase lift), the air will no longer flow smoothly over the upper airfoil surface but instead will become turbulent or “burble” near the trailing edge. A further increase in the angle of attack will cause the turbulent area to expand forward. At an angle of attack of approximately 18° to 20° (for most airfoils), turbulence over the upper wing surface decreases lift so drastically that flight cannot be sustained and the airfoil stalls. See Figure 4-7. The angle at which a stall occurs is called the critical angle of attack. An unmanned aircraft can stall at any airspeed or any attitude, but will always stall at the same critical angle of attack. The critical angle of attack of an airfoil is a function of its design therefore does not change based upon weight, maneuvering, or density altitude. However, the airspeed (strength of the relative wind) at which a given aircraft will stall in a particular configuration will remain the same regardless of altitude.

Because air density decreases with an increase in altitude, an unmanned aircraft must have greater forward speed to encounter the same strength of relative wind as would be experienced with the thicker air at lower altitudes. An easier way to envision this concept is to imagine how many molecules of air pass over an airfoil per second—thicker air at lower altitudes has more air molecules for a given area than the thinner air at higher altitudes. In order to successfully keep an aircraft aloft, a minimum number of air molecules must pass over the airfoil per second. As fewer molecules are available to make the journey as altitude increases, the only way to ensure that the aircraft can stay aloft is to increase its forward speed, thus forcing more air molecules over the airfoil each second.

Figure 4-7. Flow of air over wing at various angles of attack

Performance

Performance or operational information may be provided by the manufacturer in the form of POH, owner’s manual, or on the manufacturer’s website. Follow all manufacturer recommendations for evaluating performance to ensure safe and efficient operation. Even when specific performance data is not provided, the remote PIC should be familiar with:

• The operating environment.

• All available information regarding the safe and recommended operation of the sUAS.

• Conditions that may impact the performance or controllability of the sUAS.

Even when operational data is not supplied by the manufacturer, the remote PIC can better understand the unmanned aircraft's capabilities and limitations by establishing a process for tracking malfunctions, defects, and flight characteristics in various environments and conditions. Use this operational data to establish a baseline for determining performance, reliability, and risk assessment for your particular system.

The remote PIC is responsible for ensuring that every flight can be accomplished safely, does not pose an undue hazard, and does not increase the likelihood of a loss of positive control. Consider how your decisions affect the safety of flight. For example:

• If you attempt flight in windy conditions, the unmanned aircraft may require an unusually high power setting to maneuver. This action may cause a rapid depletion of battery power and result in a failure mode.

• If you attempt flight in wintery weather conditions, ice may accumulate on the unmanned aircraft's surface. Ice increases the weight and adversely affects performance characteristics of the small unmanned aircraft.

Due to the diversity and rapidly evolving nature of sUAS operations, individual remote PICs have flexibility to determine what equipage methods, if any, mitigate risk sufficiently to meet performance-based requirements, such as the prohibition against creating an undue hazard if there is a loss of aircraft control.