Drone loading and performance: density altitude, load factor and weight

Loading and performance is a small part of the Part 107 test, but it is easy points once you know three ideas: thin air hurts, weight hurts, and steep turns multiply loads.

Density altitude

Density altitude is the altitude in the standard atmosphere that matches the air density where you are. High density altitude means thin air and lower performance: propellers and rotors make less thrust, climb is weaker, and batteries drain faster.

Four things raise density altitude: high elevation, low pressure, high temperature and high humidity. Humid air is less dense than dry air because water vapor is lighter than air. A hot afternoon at a mountain site is the worst case.

The standard atmosphere at sea level is 29.92 inches of mercury and 15 °C (59 °F).

Weight and balance

Adding weight makes the aircraft work harder for the same lift. An overloaded drone climbs slowly, has less endurance and less maneuverability, and on a fixed-wing aircraft stalls at a higher speed. The first sign is often poor performance at takeoff.

The center of gravity is the point where the weight is concentrated. Moving a camera or battery moves the CG; outside the manufacturer's limits the aircraft becomes hard to control. Releasing a payload in flight also shifts the CG.

Load factor

Load factor is the ratio of lift to weight, measured in G. In a level turn it rises with bank angle: about 1.15 G at 30°, 2 G at 60°, and it climbs very steeply beyond that. A 10-pound drone in a 60° level turn loads its structure like 20 pounds.

Stall speed rises with the square root of the load factor, so at 2 G a fixed-wing drone stalls about 41% faster than in level flight.

Lift, drag and rotors

Lift comes from air flowing over a wing or rotor blade at an angle of attack. Exceed the critical angle of attack and the airfoil stalls, at any speed. Induced drag is highest at low speed; parasite drag grows with speed.

Multirotors yaw by speeding up one pair of opposite-turning rotors, and they move forward by tilting the whole aircraft. Near the ground they gain ground effect; descending fast into their own downwash can trigger vortex ring state.