4.6 Axes, Attitude and Flight Control Principles

The Three Axes

Every aircraft rotates about three mutually perpendicular axes, and all three pass through the centre of gravity. Move the CG and you move all three axes with it — which is the deeper reason weight and balance matters.

A useful memory aid: you roll along the longitudinal axis, pitch across the lateral axis, and yaw around the vertical axis.

The multirotor column is worth dwelling on, because it is where the RPA syllabus diverges from crewed aviation. A multirotor has no control surfaces at all. Every input is a change in relative rotor speed:

• Roll — speed up the rotors on one side, slow the other. The airframe tilts, and the thrust vector tilts with it, translating the aircraft sideways.
• Pitch — speed up the rear rotors relative to the front (or the reverse). Same mechanism, different axis, producing forward or rearward translation.
• Yaw — speed up all the clockwise rotors relative to all the counter-clockwise ones. The torque reaction no longer cancels, and the airframe rotates about its vertical axis.
• Climb and descent — change the speed of all rotors together. This is the multirotor equivalent of collective on a helicopter.

Notice what this means: on a multirotor, translation and attitude are inseparable. The aircraft cannot move horizontally without tilting, because tilting the airframe is the only way to tilt the thrust vector. This is why a multirotor hovering into wind sits nose-down and why holding position in a strong wind costs endurance.

Slip and Skid

Slip and skid describe flight where the aircraft's longitudinal axis is not aligned with its flight path.

• Slip — the aircraft is yawed into the direction of turn, or is descending sideways toward the low wing. The relative airflow strikes the aircraft from the inside of the turn.
• Skid — the aircraft is yawed out of the turn; the tail slides toward the outside. The relative airflow strikes from the outside of the turn.

In crewed aviation a balance ball shows this directly. On an RPA the pilot sees it from outside: a fixed-wing RPA in a skidding turn presents more of its side to you than it should for the bank angle, and a slipping aircraft presents less. Both conditions increase drag, reduce climb performance, and — in a fixed-wing — raise the stall risk on the inner wing during a skidding turn near the ground. For a multirotor the equivalent condition is flying sideways or backwards at speed, which puts the airframe and any exposed sensor pod into a high-drag attitude and can disturb airflow over the rear rotors.

Power, Attitude, Speed and Climb

Schedule 4 topic 8(d) asks for the effect of changes in power on vertical and horizontal speed, and the answer differs by platform.

On a multirotor: power (total rotor rpm) controls vertical speed. Attitude (tilt) controls horizontal speed. Add power in a level hover and the aircraft climbs; tilt forward and it accelerates, but it also begins to descend unless power is added to make up for the thrust now being spent horizontally. That coupling is why a multirotor accelerating hard from a hover often sinks a little.

On a fixed-wing: the classic relationship applies — attitude controls airspeed, power controls rate of climb. Pitch the nose up without adding power and the aircraft slows, eventually to the stall. Add power without changing attitude and it climbs at roughly the same airspeed.

Angle of Climb Versus Rate of Climb

These two are constantly confused and the difference is operationally important.

• Angle of climb — height gained per unit of horizontal distance travelled. Best angle of climb gets you over an obstacle in the shortest ground distance.
• Rate of climb — height gained per unit of time, normally quoted in feet per minute or metres per second. Best rate of climb gets you to altitude soonest.

They occur at different speeds on a fixed-wing: best angle is flown slower, best rate faster. The distinction decides real scenarios. A fixed-wing RPA launched from a short strip with a treeline 200 m ahead needs the best angle — maximum height per metre of ground covered. The same aircraft climbing to survey altitude over open ground wants the best rate, because time is battery.

For a multirotor the geometry is simpler — it can climb vertically, so its angle of climb can be 90 degrees — but the rate matters enormously in one specific case: a downdraft. On the lee side of a ridge or a large building, the descending air can exceed a small multirotor's maximum climb rate. The aircraft is at full power, showing a positive climb command, and still descending. The escape is horizontal, out of the downdraft, not vertical.

Factors that change climb performance

Climb performance degrades with:

Trim

Trim is any means of holding a control input without the pilot maintaining pressure. On a crewed aircraft it is a trim tab or a moveable stabiliser. On an RPA it appears in two forms:

• Transmitter trims — small offsets applied to a stick channel so that centre stick produces zero commanded rate. On a modern GNSS-stabilised multirotor these should normally be at neutral; a drifting hover is a sensor or calibration problem to be fixed, not trimmed out.
• Autopilot trim and control-loop offsets — the flight controller continuously trims for wind and CG offsets in the background. On a well-set-up aircraft you never see it. On a badly balanced one you see it as a persistent tilt in the hover, and as one motor running hotter than the others after landing.

That last symptom is a genuinely useful pre-flight and post-flight habit: a motor that is consistently warmer than its neighbours after a hover is telling you the aircraft is out of balance, or that a propeller or motor is degrading. Investigate before the next flight.