11.1 Weight & Balance, Center of Gravity & Performance

11.1 Weight & Balance, Center of Gravity & Performance

Exam Focus: Weight, balance, and center of gravity calculations govern aircraft stability, controllability, and electrical safety. You must master the manufacturer weight limit enforced through CAR 901.31, the 250 g to 25 kg operating-weight boundary that CAR 900.01 uses to define a small remotely piloted aircraft, the aerodynamic trade-offs of forward vs. aft CG in fixed-wing aircraft, the severe hazards of an off-center CG in multirotors, and the mandatory 20% battery reserve rule.

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Weight Limitations & Regulatory Boundaries

In aviation, weight represents the downward gravitational force acting on aircraft mass. Every remotely piloted aircraft (RPA) operates within certified structural and aerodynamic weight envelopes.

Maximum Take-Off Weight (MTOW) & CAR 901.31

Under CAR 901.31, no pilot shall operate an RPAS except in accordance with the applicable operating manuals, and every manual publishes a maximum take-off weight. Flying above that published MTOW is therefore a regulatory breach, not merely a technical one:

• Certified MTOW: The maximum gross weight at which an aircraft is certified by the manufacturer to safely launch, sustain design flight loads, maneuver, and land.
• Gross Weight Elements: MTOW includes the airframe, motors, flight battery, and all removable payloads (gimbals, cameras, LiDAR pods, delivery cargo, and auxiliary lights).

The Small Basic Operational Category Ceiling

Transport Canada divides RPAs into weight categories with distinct operational privileges:

• Micro RPAs: Gross weight under 250 grams.
• Small RPAs (CARs Part IX): 250 grams up to and including 25.0 kg (55.0 lbs).
• Medium / Large RPAs: Greater than 25.0 kg.

Critical Exam Rule: The Basic Pilot Certificate permits operations exclusively for small RPAs (250 g to 25 kg) in uncontrolled (Class G) airspace. If auxiliary equipment increases an RPA's weight from 24.5 kg to 25.1 kg, the drone exceeds the statutory boundary. Flying it under a Basic certificate is illegal; it requires an Advanced Certificate or a Special Flight Operations Certificate (SFOC-RPAS) under CAR 903.01.

Structural, Electrical & Aerodynamic Payload Penalties

Mounting excessive payload degrades system margins across three critical areas:

1. Structural Overload: Heavy payloads increase mechanical strain on motor arms and spars, risking structural fracture in wind gusts.
2. Elevated Motor & ESC Workload: To hover, rotors must generate thrust equal to gross weight ($T = W$). Added mass forces motors to spin at higher continuous RPM, drawing high current (Amperes).
3. ESC Overheating & Voltage Sag: High current spikes internal heat ($P = I^2 R$) within motor coils and Electronic Speed Controller (ESC) MOSFET switches, risking thermal shutdown. Heavy drain also accelerates LiPo voltage sag, triggering premature low-battery autolandings.

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Center of Gravity (CG) Principles & Stability

The Center of Gravity (CG) is the theoretical point through which the resultant gravitational force acts on the aircraft—the balance point of all airframe mass and payloads.

Fixed-Wing CG Dynamics

Fixed-wing RPAs balance pitch through the distance between the CG and the wing's Center of Lift:

• Forward CG (Nose-Heavy):
  • Longitudinal Stability: Enhances pitch stability; the aircraft naturally resists stalls.
  • Stall Speed Penalty: To counter the heavy nose, the horizontal stabilizer must produce downward aerodynamic force (negative lift). This downforce increases total wing loading, which directly raises the stall speed.
  • Control Authority: Sluggish elevator response at low airspeeds, reducing landing flare authority and risking nose-gear impacts.
• Aft CG (Tail-Heavy):
  • Longitudinal Instability: Highly dangerous. The aircraft becomes hyper-sensitive in pitch and loses natural pitch-restoring moments.
  • Violent Stalls & Flat Spins: In a stall, the tail-heavy condition prevents the pilot from lowering the nose to recover airspeed. The RPA pitches up uncontrollably and enters an unrecoverable flat spin, crashing ballistically.

Multirotor CG vs. Center of Thrust (CT)

Multirotors achieve attitude equilibrium purely through differential rotor thrust.

• Center of Thrust (CT): Located at the exact geometric intersection of diagonal lines connecting opposite motor shafts.
• Alignment Rule: The physical Center of Gravity must align directly over the geometric Center of Thrust along both lateral and longitudinal axes.

Consequences of an Off-Center CG in Multirotors

When an asymmetric payload shifts the CG away from the Center of Thrust:

• Motor Saturation: Motors under the heavier side must spin near 100% capacity continuously just to hold a level hover, while opposing motors run at low RPM.
• Loss of Gust Authority: With overloaded motors near maximum throttle, the flight controller has no reserve thrust margin to counteract wind gusts from the heavy side.
• ESC Thermal Burnout: Overloaded ESCs suffer continuous thermal stress, leading to desynchronization or mid-air component failure.
• Control Asymmetry: Pitch and roll responses become sluggish toward the heavy side and violently twitchy in the opposite direction.

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Flight Performance & Battery Reserve Planning

Power vs. Weight Scaling

In rotary-wing flight, required hover power ($P$) scales non-linearly with total weight ($W$):

$P_{\text{hover}} \propto W^{\frac{3}{2}}$

A 25% increase in gross weight demands nearly a 40% increase in power output. Consequently, adding oversized auxiliary batteries yields diminishing returns, as added dead weight consumes more power than the extra capacity delivers.

The Mandatory 20% Battery Reserve Rule

Transport Canada safe operating principles mandate planning every flight to land with at least a 20% battery reserve:

• Landing Target: Plan touchdown when cells reach approximately 3.50V to 3.60V under load (roughly 3.70V resting nominal voltage).
• Operational Cushion: The 20% reserve protects against unexpected headwinds on the return leg, holding delays over the landing zone, and cold-weather voltage sag.
• Cell Protection: Discharging LiPo cells below 20% triggers chemical degradation, internal lithium plating, permanent capacity loss, and cell puffing.

Environmental Performance Modifiers

• Density Altitude: High temperatures, high field elevations, and high humidity reduce air density. Propellers produce less thrust per revolution, forcing motors to draw higher current and shortening endurance.
• Headwind Planning: When flying downwind from home, groundspeed drops sharply on the return flight. Pilots must calculate the Point of No Return (PNR) using groundspeed, initiating the return well before reaching half battery capacity.

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Performance Data, Turn Geometry and the Flight Manual

TP 15263 ticks three further Basic performance items that a weight-and-balance calculation alone does not cover.

Use of the Aircraft Flight Manual

Every RPAS ships with an operating manual that publishes the numbers no formula will give you: maximum take-off weight, maximum wind, operating temperature range, maximum altitude, and the endurance curve for a given payload. CAR 901.31 makes those numbers legally binding — the system must be operated in accordance with the applicable operating manuals. Two discipline points follow: carry the manual (or a readable copy) to the site, and re-read the relevant page whenever the payload, the battery type or the season changes.

Hover Ceiling and Maximum Altitude

• The hover ceiling is the density altitude at which the aircraft can just maintain a hover with no margin left to climb or manoeuvre. It is a performance limit, not a legal one, and it falls as weight, temperature and field elevation rise.
• The service ceiling or maximum operating altitude in the manual is usually expressed as a density altitude or as a field elevation limit, and it exists because propeller thrust falls with air density.
• The legal ceiling is entirely separate: 400 ft AGL under CAR 901.25(1)(a).

The Mountain Trap: A drone rated to 6,000 ft density altitude launched from a 4,500 ft mountain airstrip on a 30 °C afternoon may already be near its hover ceiling on the ground. It will lift off, hover unsteadily, and then fail to climb or to arrest a descent — long before it approaches the 400 ft legal limit. Always check the hover ceiling against the density altitude of the launch site, not its elevation.

Bank Angle, Rate of Turn and Radius of Turn

Two relationships govern every turn, and they pull in opposite directions:

The operational consequence for a fixed-wing RPA working inside a confined survey block is direct: to turn inside a tight boundary, slow down and bank more — but every degree of extra bank raises the load factor (see section 9.2) and therefore the stall speed, so there is a floor below which you cannot go. For a multirotor this appears as the trade between a fast, wide, energy-efficient orbit and a slow, tight orbit that keeps the subject in frame but drains the battery.

External Loads

An external load — a slung sensor, a spray tank, a delivery package, a tether — changes three things at once:

1. Performance: total weight rises, so hover ceiling and endurance both fall.
2. Stability: a load suspended below the aircraft can develop a pendulum oscillation that the flight controller may amplify rather than damp, because the controller is correcting for a mass it cannot sense directly.
3. Drag and CG: a bluff external load adds parasite drag and shifts the centre of gravity, often aft or to one side.

Legally, CAR 901.43(1)(d) prohibits a payload attached by a line unless the operation is conducted in accordance with the applicable operating manuals, and CAR 901.50 prohibits creating a hazard to persons or property by dropping an object in flight. Fly slung loads slowly, avoid abrupt inputs that start the pendulum, and plan a jettison-free flight path clear of people.

Weight, Balance & Performance Configuration Matrix

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Practical Exam Scenarios

Scenario 1: Auxiliary Payload Exceeding 25 kg Boundary

A pilot operates a 24.2 kg quadcopter under a Basic Pilot Certificate. A client requests attaching an 850 g thermal camera, bringing takeoff weight to 25.05 kg. The manufacturer MTOW is 26.0 kg.

• Analysis: While within manufacturer MTOW (26.0 kg), the 25.05 kg weight exceeds the 25.0 kg statutory ceiling for small RPAs under CARs Part IX Basic category.
• Action: The pilot must refuse the flight under Basic certification. The operation requires an Advanced Certificate, an SFOC-RPAS, or lightening the payload below 25.0 kg.

Scenario 2: Lateral Payload Offset & ESC Burnout

An operator mounts an off-center inspection spotlight on the right landing gear of a hexacopter without counterweighting the left side. During a steady hover in a 14-knot crosswind, the right rear ESC overheats and fails.

• Analysis: The offset CG forced right-side motors to spin near 100% throttle to keep the aircraft level. Sustained high current draw triggered ESC thermal destruction.
• Action: Payloads must align with the geometric Center of Thrust. Never rely on flight controller electronic trim to counteract physical center of gravity offsets.

Scenario 3: Downwind Flight & Depleted Reserve Margin

A pilot flies 1.5 km downwind with a 15-knot tailwind, consuming 45% battery. The pilot decides to turn back at 50% battery capacity.

• Analysis: Facing a 15-knot headwind, return groundspeed is cut in half. The return journey takes over twice as long, depleting the battery to 0% 400 metres short of the pad.
• Action: The pilot failed to maintain the mandatory 20% battery reserve cushion. Return flights against headwinds must be initiated early to guarantee landing with ≥ 20% capacity.