Introduction to Aircraft Performance
In this section we discuss the factors that affect aircraft performance, which include the aircraft weight, atmospheric conditions, runway environment, and the fundamental physical laws governing the forces acting on an aircraft.
Weight and balance are critical components in the utilization of an aircraft to its fullest potential. The pilot must know how much can be loaded onto the aircraft without violating CG limits, as well as weight limits Additional weight may or may not place the CG outside of the CG envelope, but the maximum gross weight could be exceeded. The excess weight can overstress the aircraft and degrade the performance.
While it is difficult to exceed CG limits in multirotor aircraft, pilots should never overload an aircraft because overloading can cause structural damage and failures. On a multirotor aircraft, an out of balance condition requires the motors on the heavy side to work harder to keep the aircraft level. This extra power greatly reduces the aircraft endurance.
Most sUAS aircraft manufacturers do not provide weight and balance data. At best, they only provide a maximum gross weight.
To ensure that the unmanned aircraft center of gravity (CG) limits are not exceeded, follow the aircraft loading instructions specified in the Pilot's Operating Handbook or UAS Flight Manual. That is, if such a manual is provided by the aircraft manufacturer. Part 107 aircraft aren't certified under Part 23 or Part 25, and an AFM/POH is not mandated by the rules. Only some fixed-wing sUAS aircraft manufacturers provide CG information.
AFM/POH? Some aircraft come with a Pilot Operating Handbook and some come with an Aircraft Flight Manual. Why the different name, and is there a difference between them?
Both a POH and an AFM meet the "Operating Limitations" preflight requirement. The difference between the two is mainly in length and content: an AFM is usually a thinner document, satisfying the requirements of FAR 23.1581 and not much else, while a POH contains these required items plus other information like system diagrams The contents & format of a POH are standardized in GAMA's (General Aviation Manufacturers association) Specification.
Parts of the POH, like the Limitations section, are FAA-Approved (for certified aircraft), and serve as the required AFM, and both documents are typically associated with a specific airframe by serial number.
A better explanation might be this: The AFM is a regulatory document (it's contents are prescribed under the section of the regulations the aircraft was certificated under - Part 23, Part 25, etc). The POH is a GAMA-defined document whose contents meet the regulatory requirements of an AFM, and present other information in a standardized way so that a pilot can go from a Cessna to a Piper to a Mooney to a Socata and browse the book to learn about the airplane they're about to fly with all the information presented the same way no matter who the manufacturer is.
Weight and Balance
Thrust, drag, lift, and weight are forces that act upon all aircraft in flight. Understanding how these forces work and knowing how to control them with the use of power and flight controls are essential to flight. The four forces are defined as follows:
• Thrust- the forward force produced by the powerplant/ propeller or rotor. It opposes or overcomes the force of drag. As a general rule, it acts parallel to the longitudinal axis.
• Drag- a rearward, retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects. As a general rule, drag opposes thrust and acts rearward parallel to the relative wind.
• Lift- is a force that is produced by the dynamic effect of the air acting on the airfoil, and acts perpendicular to the flight path through the center of lift (CL) and perpendicular to the lateral axis. In level flight, lift opposes the downward force of weight.
• Weight- the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage. Weight is a force that pulls the aircraft downward because of the force of gravity. It opposes lift and acts vertically downward through the aircraft’s center of gravity (CG).
In steady flight, the sum of these opposing forces is always zero. Multirotor aircraft and helicopters are exposed to the same forces.
Forces Acting on the Aircraft
Load Factor and Turns
In aerodynamics, the maximum load factor (at given bank angle) is a proportion between lift and weight and has a trigonometric relationship. The load factor is measured in Gs (acceleration of gravity), a unit of force equal to the force exerted by gravity on a body at rest and indicates the force to which a body is subjected when it is accelerated. Any force applied to an aircraft to deflect its flight from a straight line produces a stress on its structure. The amount of this force is the load factor. While a course in aerodynamics is not a prerequisite for obtaining a pilot’s license, the competent pilot should have a solid understanding of the forces that act on the aircraft, the advantageous use of these forces, and the operating limitations of the aircraft being flown. For example, a load factor of 3 means the total load on an aircraft’s structure is three times its weight. Since load factors are expressed in terms of Gs, a load factor of 3 may be spoken of as 3 Gs, or a load factor of 4 as 4 Gs.
Load Factors in Steep Turns
At a constant altitude, during a coordinated turn in any aircraft, the load factor is the result of two forces: centrifugal force and weight. For any given bank angle, the Rate of Turn (ROT) varies with the airspeed- the higher the speed, the slower the ROT. This compensates for added centrifugal force, allowing the load factor to remain the same. An important fact about turns is that the load factor increases at a terrific rate after a bank 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 5.76 Gs. The wing must produce lift equal to these load factors if altitude is to be maintained.
Rate of Turn
The rate of turn (ROT) is the number of degrees (expressed in degrees per second) of heading change that an aircraft makes.
Why is this important to the sUAS pilot?
An aircraft with its center of gravity outside of the manufacturer's specification may be uncontrollable in flight or crash shortly after take-off. In a multirotor aircraft the motors and propellers on the "heavy" side have to work harder to maintain control. At the least, the flight time is reduced because of the extra energy directed to the heavy side of the craft. In a fixed wing aircraft the most likely problem is that the tail (or canard) surface does not have enough "authority" to overcome the out-of-balance condition. If the aircraft CG is aft of the CG limit specified by the manufacturer, no amount of down elevator will prevent the wings from pitching up into an aerodynamic stall. Likewise, if the CG is too far forward, no amount of up elevator will rotate the wings into an angle of attack that supports flight.
If an aircraft is dropping a payload, the pilot needs to be aware of any CG change before and after the drop.