8.6 Weight, Power, Wind and Climb Performance

Four Factors on Launch and Recovery

Schedule 4 topic 2 asks for the effect of four specific changes on RPA performance during launch, landing and recovery.

Weight. Every gram must be lifted. More weight means:

• Higher hover power and current draw, so shorter endurance.
• Reduced climb rate and a smaller thrust margin above the hover.
• Longer stopping distance, because inertia rises with mass.
• Less capacity to recover from a gust — the spare thrust that would arrest an unexpected sink is already committed to holding the aircraft up.

Power. Power available falls with battery state of charge, motor and ESC temperature and component age. An aircraft that hovers at 55 per cent throttle on a fresh pack may hover at 70 per cent on the same pack at 30 per cent charge, because pack voltage sags as it discharges. That is why endurance planning uses an 80 per cent usable-capacity rule and why the last minutes of a flight have the least margin.

Ground effect. Within about one rotor diameter of the surface the ground restricts the downwash, reduces induced flow and boosts thrust for a given power. Launch is therefore easier than the climb that follows, and landings tend to float as the aircraft descends into the cushion.

Wind. Wind cuts both ways. A hover into wind gains translational lift and is more efficient than a hover in still air. But gusts demand rapid power changes, and the aircraft must tilt into the wind to hold position, committing part of its thrust to a horizontal component rather than to lift. Above the airframe's published wind limit the aircraft can be unable to make headway against the wind at all — the classic scenario where a drone at the far end of a survey line cannot get back.

Climb Performance

Schedule 4 topic 8 lists seven factors affecting climb rate and angle.

The last two combine into density altitude, and their combined effect is larger than pilots expect. An aircraft that climbs at 5 m/s at a coastal site on a 15-degree morning may manage 3 m/s at an inland site 500 m above sea level on a 38-degree afternoon — with the same battery and the same payload.

Headwind and the angle-versus-rate distinction

This is worth being precise about. Rate of climb is height per unit time and is a property of excess power, so wind does not change it. Angle of climb is height per unit of horizontal ground distance, and a headwind reduces the ground distance covered, so the angle steepens. That is why launching into wind is the preferred technique when an obstacle sits ahead: the aircraft clears it with less ground run.

Turning: Banked Versus Flat

Schedule 4 topic 9 distinguishes banked turns from rotations or flat turns, and asks about limitations on steep turns.

A banked turn is a turn in which the aircraft rolls, tilting the thrust vector so that its horizontal component provides the centripetal force that curves the flight path. It is the natural way to change direction in forward flight, and on a multirotor it is what the aircraft does when you feed in roll while flying forward.

The aerodynamic cost is the same as on any aircraft: once the thrust vector is tilted, only its vertical component opposes weight. Total thrust must therefore increase to hold altitude, and the increase grows sharply with bank angle:

A 60-degree banked turn requires twice the thrust of level flight. A heavily loaded multirotor on a hot day may simply not have it, and will descend through the turn.

A flat turn (rotation) is a pure yaw with the airframe kept level. The aircraft rotates about its vertical axis without changing its flight path or its thrust requirement. On a multirotor this is trivially easy — it is just an unbalancing of rotor torques — and it is the standard technique for a camera pan, for repositioning a sensor's field of view, or for turning on the spot in a confined space.

Because flat turns are so easy on a multirotor, they are usually the right answer for a camera-carrying aircraft: yaw the aircraft to point the sensor, rather than banking and losing altitude.

Limitations on steep turns

Three limits apply:

1. Thrust margin. As the table shows, steep turns demand rapidly increasing thrust. An aircraft near its weight or density-altitude limit does not have it.
2. The flight controller's tilt limit. Most controllers cap maximum attitude angle in normal modes — often around 30 to 35 degrees — precisely to keep the thrust demand manageable. A "sport" mode raises the cap and removes that protection.
3. Airframe and payload loads. A steep, fast turn increases the load factor on arms, motor mounts and gimbal, and can exceed a gimbal's mechanical travel, producing a visible jolt in the footage and mechanical stress on the mount.

Descending, Landing and Recovery

A controlled recovery brings these threads together:

• Land into wind wherever possible. The aircraft has translational lift, arrives at a lower groundspeed, and needs less tilt to hold position over the landing point.
• Descend at a moderate rate, not vertically at speed. A fast vertical descent risks descending into the aircraft's own downwash — the vortex ring condition covered in the next section.
• Expect the ground-effect cushion. As the aircraft enters ground effect it will tend to float. Reduce power progressively rather than chopping it.
• Run pre-landing checks. Confirm the landing area is still clear (people move), confirm battery reserve, confirm wind at the surface, and confirm the aircraft is over a firm, level surface clear of loose debris that the downwash will pick up.
• Keep the aircraft in sight through the final metres. Most landing damage happens in the last two metres, when the pilot's attention has already moved on to packing up.