Drone Weight and Balance, Center of Gravity and Load Factor, Explained

Loading questions on the Part 107 test come back to three ideas: how much the aircraft weighs, where that weight sits, and how maneuvers multiply it. Small unmanned aircraft loading is one of the knowledge areas the test covers (14 CFR 107.73(d)). The rules and figures below come from Part 107, the FAA's Remote Pilot study guide and its Pilot's Handbook of Aeronautical Knowledge (PHAK).

The 55-pound line

To count as a small unmanned aircraft under Part 107, the drone has to come in under 55 pounds at takeoff, and that total covers everything carried on it or attached to it (14 CFR 107.3). The payload counts, and so does the battery.

An example. A heavy-lift drone weighs 48.5 pounds with its batteries, and the client's camera package weighs 6.5 pounds. Together that is 55.0 pounds, which is not less than 55, so loaded like that it no longer meets the definition. A 5.5-pound camera brings the total to 54.0 pounds and back under the line.

Weight also brings a preflight duty: before flight, make sure every attached or carried item is secure and doesn't hurt the way the aircraft flies or answers the controls (14 CFR 107.49(e)).

Why extra weight costs performance

Only lift holds the weight up, and an airfoil can't make unlimited lift: its shape, angle of attack, airspeed and the density of the air all cap it. Load the aircraft past the lift it can make and it may not fly at all (FAA Remote Pilot Study Guide, p. 32). Short of that, weight still eats into the climb, because the extra angle of attack needed to carry it brings extra drag and leaves less spare thrust (FAA Remote Pilot Study Guide, p. 22).

The study guide's list of overloading penalties covers the whole flight. Takeoff needs more speed and more room. The climb is slower and shallower, and the aircraft can't go as high. Range, cruising speed and maneuverability all drop while the stalling speed goes up, and landing is faster and longer. Poor loading usually shows itself first on takeoff (FAA Remote Pilot Study Guide, p. 33).

Stability can change too, so a well-behaved aircraft may handle quite differently once it is overloaded. Making a habit of overloading can also add up to hidden damage that preflight checks miss, until a part fails later on an ordinary flight (FAA PHAK, p. 5-42).

Even the maximum takeoff weight can be too much on some days. FAA guidance lists hot, high or humid conditions, a short, sloping or poor launch surface, surface wind and obstacles as reasons to fly lighter (AC 107-2A, para. B.2.1.1).

Balance: where the weight sits

Think of the center of gravity (CG) as the balance point: treat all the weight as sitting there, and an aircraft held up at that spot would balance in any attitude (FAA Remote Pilot Study Guide, p. 29). Lift pushes up somewhere else, at the center of pressure, and where the CG sits relative to it sets the pitch tendency: a CG in front of it means nose down, a CG behind it means nose up (FAA Remote Pilot Study Guide, p. 29). Fly with the CG outside its limits and control becomes difficult (FAA Remote Pilot Study Guide, p. 32).

• Nose heavy: the nose is a struggle to control and lift, most of all on takeoff and landing (FAA PHAK, p. 10-3).
• Tail heavy: pitch stability gets worse, stalls are harder to recover from, and the controls feel so light that overstressing the aircraft becomes easy (FAA PHAK, p. 10-3).
• Heavy on one side: holding a correction against a lateral imbalance increases drag and reduces efficiency (FAA PHAK, p. 10-3).

The CG moves with the load. Shift a payload, or use something up, and the balance point can move with it. A load released in flight makes the aircraft lighter, which helps performance, but it can upset the balance (FAA Remote Pilot Study Guide, p. 29). So plan the change: if the CG would end up outside its limits, move or remove weight before takeoff (FAA Remote Pilot Study Guide, p. 29). And whatever you drop must not put people or property at undue risk (14 CFR 107.23(b)).

Load factor: what a turn does to the load

Load factor, in Gs, measures the stress on the structure whenever the aircraft is pushed off a straight line. A load factor of 3 means the structure carries three times the aircraft's weight (FAA Remote Pilot Study Guide, p. 30). Straight, unaccelerated flight is 1 G (FAA PHAK, p. 5-36).

Hold altitude through a coordinated turn and two forces combine to make the load factor: weight and centrifugal force. Speed doesn't change it, since at the same bank a faster aircraft simply turns at a slower rate (FAA Remote Pilot Study Guide, p. 31). The figures the study guide gives for any aircraft in a coordinated level turn (FAA Remote Pilot Study Guide, p. 31):

• 60 degrees of bank: 2 Gs.
• 80 degrees of bank: 5.76 Gs.
• After 45 or 50 degrees, the load factor climbs at what the FAA calls a "terrific rate."
• 90 degrees: a banked, constant-altitude turn isn't mathematically possible.

The study guide calls the relationship trigonometric (FAA Remote Pilot Study Guide, p. 30). Both figures match 1 divided by the cosine of the bank angle: the cosine of 60 degrees is 0.5, giving 2, and the cosine of 80 degrees is about 0.174, giving about 5.76. To hold altitude, the wing has to produce lift to match the load factor (FAA Remote Pilot Study Guide, p. 31), and the total load is the weight multiplied by the load factor (FAA Remote Pilot Study Guide, p. 30): at 2 Gs, a 10-pound aircraft's wings carry 20 pounds.

Load factor and the stall

Push the angle of attack too high and the airflow over the wing tears away, taking lift with it: that is a stall (FAA Remote Pilot Study Guide, p. 31). The angle where it happens is fixed for each aircraft. Speed, weight, load factor and density altitude don't move it (FAA PHAK, p. 5-25). Speed alone doesn't protect you, either: the study guide says any aircraft can be stalled at any airspeed, within its structural limits (FAA Remote Pilot Study Guide, p. 31).

The stalling speed is what changes, growing with the square root of the load factor (FAA Remote Pilot Study Guide, p. 31). Multiply the normal stalling speed by:

• About 1.41 at 2 Gs (the square root of 2).
• 2 at 4 Gs. The study guide's example: a 50-knot stall becomes a 100-knot stall (FAA Remote Pilot Study Guide, p. 31).
• 2.4 at 5.76 Gs (the square root of 5.76).
• 3 at 9 Gs: the same aircraft would stall at 150 knots (FAA Remote Pilot Study Guide, p. 31).

Turns aren't the only cause. Pulling up sharply, or anything else that loads the aircraft beyond 1 G, does the same, and the study guide notes that unexpected stalls in steep turns or sharp pull-ups close to the ground have caused many accidents (FAA Remote Pilot Study Guide, p. 32).

Worked example: a new sensor and a steep turn

Your fixed-wing mapping drone weighs 6 pounds. You fit a 1.5-pound sensor, so it now weighs 7.5 pounds, and loaded like that it stalls at 24 knots in level flight. During the survey you make a coordinated 60-degree turn at constant altitude.

1. Load factor: 2 Gs at 60 degrees (FAA Remote Pilot Study Guide, p. 31).
2. Load on the wings: 7.5 × 2 = 15 pounds. Without the sensor it would be 6 × 2 = 12 pounds, so the 1.5 pounds you added becomes 3 extra pounds of load in the turn. The PHAK puts the same idea in bigger numbers: at 3 Gs, each extra 100 pounds means 300 more pounds for the structure to bear (FAA PHAK, p. 5-42).
3. Stalling speed: 24 × 1.41 is about 34 knots. A speed that is comfortable in level flight can be too slow in the turn.
4. Takeaway: keep turns shallow and coordinated when the aircraft is heavy, and leave a wide speed margin whenever you bank.

Your preflight loading check

• Follow the weight and balance restrictions set by the manufacturer or builder; FAA guidance calls compliance with them critical to flight safety (AC 107-2A, para. B.2.1).
• Add up the takeoff weight with the payload and battery, and keep it under 55 pounds (14 CFR 107.3).
• Check that every attached item is secure and doesn't affect how the aircraft flies (14 CFR 107.49(e)).
• Look up the maximum weights, airspeed limits and prohibited maneuvers in the manual; FAA guidance encourages manufacturers to provide them (AC 107-2A, para. 8.12.2.5.1).
• On a hot, high or humid day, plan to carry less, even below the maximum.

Test yourself

Try the free loading and performance questions, then mix them with every other topic in the free 20-question Part 107 practice test. The key numbers cheat sheet lists the limits worth memorizing, including the 55-pound weight limit. Part 107 Complete ($39, one-time) adds lessons on weight and balance, load factor and performance, with timed mock exams. Thin air makes every load harder to carry, so read how density altitude affects drone performance next.

Questions people ask

What is the load factor in a 60-degree bank?

2 Gs, for any aircraft in a coordinated, constant-altitude turn. The wing must produce lift equal to twice the aircraft's weight to hold altitude (FAA Remote Pilot Study Guide, p. 31).

How much does the stall speed go up with load factor?

Stalling speed rises with the square root of the load factor. Under 4 Gs it doubles, so an aircraft that stalls at 50 knots in normal flight stalls at 100 knots (FAA Remote Pilot Study Guide, p. 31).

Does the payload count toward the 55-pound limit?

Yes. A small unmanned aircraft must weigh less than 55 pounds on takeoff, including everything that is on board or otherwise attached (14 CFR 107.3).

What happens if the center of gravity is too far aft?

Longitudinal stability suffers, it is harder to recover from stalls, and the control forces become very light, which makes it easy to overstress the aircraft (PHAK, p. 10-3).