4.1 Weight, Balance, and Center of Gravity

Why Loading and Performance Matters

Understanding how weight and balance affect a drone is a safety skill and a tested skill. The Loading and Performance area accounts for roughly 7-11% of the 60-question Part 107 knowledge test, so a handful of questions ride on these concepts. They are among the most concrete and learnable topics on the exam — closer to arithmetic than to judgment — which makes them reliable points for a prepared candidate.

The 60-question test gives you 2 hours, requires a 70% score (42 of 60 correct) to pass, costs $175, and is delivered at PSI testing centers. Missing easy loading questions is a wasteful way to drift toward the 18-question failure margin.

Maximum Takeoff Weight and the 55-Pound Rule

A small unmanned aircraft system (sUAS) under 14 CFR Part 107 is defined as one that weighs less than 55 pounds on takeoff, including everything on board or otherwise attached. That figure is the regulatory ceiling, not a target — your drone's maximum takeoff weight (MTOW) from the manufacturer is almost always far lower.

The governing limit is whichever is lower — your manufacturer MTOW or 55 pounds. A DJI-class quadcopter might have an MTOW of 5 to 6 pounds, so the 55-pound rule never comes into play for it. Exceeding the manufacturer MTOW can cause motor overheating, an inability to hover or climb, drastically reduced flight time, and structural failure.

What Counts Toward Takeoff Weight

Exam trap: A question may give you an aircraft that weighs 54 pounds empty and ask whether a 2-pound payload keeps it under Part 107. It does not — 56 pounds exceeds the 55-pound ceiling, so the operation falls outside Part 107.

Center of Gravity and How Payload Moves It

The center of gravity (CG) is the single point at which the aircraft's mass balances. For stable, controllable flight the CG must fall within the manufacturer's published limits. Every time you add, remove, or reposition a payload, the CG moves.

CG Position and Its Effects

Mounting Payload Correctly

1. Mount the payload as close to the CG as possible — usually the airframe's geometric center.
2. Secure it firmly — a payload that shifts in flight changes CG dynamically and can destabilize the aircraft.
3. Test-hover in a safe, open area before any real mission with a new payload.
4. Stay within the manufacturer's payload capacity, not just total MTOW.

Worked example: A drone with an empty weight of 2.8 lb carries a 0.5-lb battery, a 0.8-lb camera/gimbal, and 0.3-lb prop guards. Total takeoff weight = 4.4 lb. With a 5.5-lb manufacturer MTOW, you have 1.1 lb of margin and you are far under the 55-lb Part 107 ceiling. Mounting that 0.8-lb camera on the nose, however, drags the CG forward and forces the rear motors to compensate — draining the battery faster even though you are within weight limits.

Weight versus Performance

Every gram added to a multirotor reduces its margin, and the exam expects you to know the direction of every effect. As takeoff weight increases, the following all change for the worse, because the motors must spend a larger share of their available output simply holding the aircraft up rather than doing useful work:

• Thrust-to-weight ratio falls — there is less excess power available for maneuvering, climbing, and recovering from a gust. This is the root cause behind every other item on this list.
• Flight time drops — the motors draw more current just to hover, so the battery is depleted faster and usable mission time shrinks.
• Climb rate declines — climbing requires thrust beyond what hovering needs, and a heavy aircraft has less of that surplus to spare.
• Maximum speed declines — more of the available thrust is committed to staying aloft, leaving less to accelerate forward.
• Maneuverability degrades — the aircraft responds more sluggishly and feels heavy on the sticks.
• Stall speed rises for fixed-wing sUAS — a heavier wing must fly faster to produce the lift it needs, so the speed at which it stalls increases.

Pre-Flight Weight Worksheet

Before each flight, total every contributor and compare against the lower of your manufacturer MTOW and the 55-pound regulatory ceiling:

Total Weight = Empty Weight + Battery + Payload + Accessories

  Empty weight:        2.8 lb
  Battery:             0.5 lb
  Camera + gimbal:     0.8 lb
  Prop guards:         0.3 lb
  ------------------------------
  Takeoff weight:      4.4 lb
  Manufacturer MTOW:   5.5 lb
  ------------------------------
  Remaining capacity:  1.1 lb  (within limits)

Common Exam Traps in This Section

• Confusing the manufacturer MTOW with the 55-pound regulatory limit — both apply, and the lower one governs the flight.
• Forgetting that the 55-pound figure includes everything attached, not just the airframe and battery.
• Assuming weight only affects flight time, when it actually degrades climb rate, speed, and maneuverability as well.
• Believing the drone's flight controller will silently compensate for a poorly placed payload; it can trim a small offset but cannot fix a CG outside limits.

For the exam: Remember three anchors — (1) the Part 107 ceiling is less than 55 pounds including attachments, (2) adding or moving payload shifts the CG, and (3) excess weight reduces flight time, speed, climb rate, and maneuverability. These three ideas underpin nearly every loading question you will see.