Loading & Performance

Weight, CG & Performance

CG = balance point. Loading outside limits can reduce stability/control. Like carrying a loaded tray with one hand — the total weight hasn't changed, but if it shifts too far to one side you lose stable control of it even though nothing got heavier.

More payload → more weight → usually more required power → usually less endurance.

High density altitude + heavy payload = smaller performance margin.

A shifting load changes CG. Secure payloads and verify manufacturer weight/balance limits.

Stalls & load factor (fixed-wing)

A wing stalls when it exceeds its critical angle of attack — this can happen at any airspeed or attitude, not just "flying too slowly." Think of angle of attack like tilting a hand out of a car window: tilt it too steeply and the smooth airflow over it breaks up, no matter how fast the car is going.

Steeper turns and abrupt maneuvers increase load factor (the ratio of lift to weight), which raises the stalling speed for that maneuver — a wing can stall at a higher-than-normal airspeed if it's being asked for more lift than it can produce at that angle of attack.

Weight & Balance Math

The allowed-payload formula from the card above is the one piece of math this area actually tests. Work it both ways: solving for allowed payload, and checking whether a specific payload is legal.

Example 1 — solving for allowed payload. An aircraft has a max takeoff weight of 55 lb. The bare aircraft, battery, and mounted equipment together weigh 48 lb.

Adding an 8 lb payload would put total weight at 56 lb — 1 lb over the 55 lb max takeoff weight. Not allowed, even though it's "only" 1 lb over.

Example 2 — a lighter aircraft, same formula. A small quadcopter has a max takeoff weight of 4.4 lb. Aircraft + battery weigh 3.9 lb.

A 0.5 lb camera payload is exactly at the limit — legal. A 0.6 lb gimbal/camera combo is 0.1 lb over — not legal without removing weight elsewhere.

Example 3 — a CG shift, not just a weight problem. Suppose that same 0.5 lb payload in Example 2 is mounted entirely on one side of the airframe instead of centered underneath it. Total weight is still within limits, but the balance point has moved off-center. The aircraft may still fly, but with reduced control authority or an unwanted trim/drift tendency toward the heavy side — a reminder that "under max weight" and "balanced" are two separate checks, not one.

Density Altitude's Effect on Performance

This card is about the consequence for a loaded aircraft, not the atmospheric definition of density altitude (that's covered on the Weather page). The short version: thinner air means each rotor revolution or wing pass produces less lift and less thrust than it would in denser, cooler, lower-elevation air.

For a small UAS, high density altitude (hot day, high field elevation, or both) shows up as:

• Longer takeoff roll for fixed-wing aircraft, or reduced hover capability for multirotors — the aircraft needs more power to produce the same lift.
• Reduced climb rate — less excess power is available once level-flight power is subtracted out.
• Reduced battery endurance margin — motors draw more current to make up for the thinner air, so a flight that would be routine on a cool day at sea level can burn noticeably more battery on a hot day at a high-elevation site.

This is exactly why the earlier card warns that high density altitude + heavy payload = a smaller performance margin — the two effects stack: less available lift/thrust from the air, and more weight demanding it.

Battery Performance Under Load

The Weather page covers the general planning idea of not flying far downwind without reserving enough battery for the return trip against a headwind. This card builds a worked numeric picture of how load — not just wind — eats into that same reserve.

Three ordinary stressors stack instead of staying separate: a heavy payload draws more current to hover or climb; a headwind on the return leg means the motors work harder just to make progress over the ground; and cold temperatures reduce the chemical capacity a battery can actually deliver, independent of how it's flown.

None of those numbers are FAA-published or manufacturer-guaranteed — they're illustrative to make the point. The test-relevant takeaway is the planning principle: a comfortable single-factor margin (just wind, or just cold, or just payload) can look very different once all three are present on the same flight, so plan battery reserve for the worst realistic combination, not the best single case.

Manufacturer Operating Limits

Part 107 sets the floor — the blanket federal rules that apply to every small UAS operation. A specific aircraft's manufacturer-published operating limits — max demonstrated wind speed, operating temperature range, and maximum payload — are often more restrictive than anything Part 107 itself requires, and they govern that aircraft in practice.

If a question asks what takes precedence when the manufacturer's limits are tighter than the regulation, the manufacturer's flight-manual-style limits are what actually constrains that specific aircraft. Part 107 doesn't override a manufacturer's documented limit, and exceeding a manufacturer limit isn't excused just because Part 107 itself would still allow it.

Before any flight, check the aircraft's own documentation — not just the regulation — for its demonstrated wind limit, temperature range, and max payload, and treat whichever number is more restrictive as the real limit for that flight.

Load Factor, Explained

A walkthrough of load factor before the bank-angle/stall-speed table below.

Load Factor vs. Bank Angle

In a level, coordinated turn, load factor rises with bank angle, and stalling speed rises with the square root of load factor. These are standard fixed-wing aerodynamic values, not illustrative figures:

Example: an aircraft that stalls at 50 kt wings-level will stall at roughly 71 kt in a sustained 60° level turn (50 × 1.41 ≈ 71). The load factor more than doubles well before the bank angle does — the relationship isn't linear.

CG Shift Scenarios

Scenario 1 — uneven loading at takeoff. A payload (camera, delivery package, sensor pod) is mounted off-center or latched unevenly before launch. Total weight may be well within limits, but the CG has moved toward the heavy side before the aircraft ever leaves the ground. The result is reduced control authority in that direction — the aircraft may need constant correction, or fly with an unwanted drift/trim tendency, to hold the heading and attitude it would otherwise hold easily.

Scenario 2 — CG shifting during the flight itself. Fuel burn on a gas-powered aircraft, or uneven battery drain across multiple packs, gradually shifts CG over the course of a flight even if the aircraft was loaded correctly at takeoff. A flight that handled normally at launch can develop a CG-related control tendency well into the mission, purely from mass being consumed or redistributed in flight.