9.4 Airframe Configurations, Stability & Flight Controls
Key Takeaways
- Longitudinal stability acts in pitch about the lateral axis and depends on the horizontal stabilizer and centre-of-gravity position; an aft CG reduces it toward uncontrollability.
- Lateral stability acts in roll and comes from wing dihedral, sweepback and keel effect; directional stability acts in yaw and comes from the vertical fin weathervaning the aircraft into the relative wind.
- A multirotor is inherently unstable and flies only because the flight controller makes thousands of corrections per second; an inherently stable fixed wing tends to return to its trimmed attitude on its own.
- Helicopter control is split three ways: collective changes the pitch of all main-rotor blades together, cyclic tilts the rotor disc to choose a direction, and the tail rotor counters main-rotor torque and controls yaw.
- Dynamic rollover occurs when a leg or skid is pinned and the aircraft pivots about that contact point; once past the critical angle, no amount of opposite control input will stop the roll.
9.4 Airframe Configurations, Stability & Flight Controls
Exam Focus: Section 7 of TP 15263 is ticked Basic almost end to end, and it is broader than the four forces. It asks you to identify aircraft parts and components for fixed-wing, multi-rotor and helicopter types, to describe stability about all three axes, to know wing planform, spoilers and flaps, to name the aeroplane and helicopter flight controls, and to explain multi-rotor dynamics including settling with power, recirculation and dynamic rollover, plus load factor from gusts. This section covers the parts that a multirotor-only pilot never meets in daily flying.
1. Aircraft Parts and Components
Fixed-Wing
| Component | Function |
|---|---|
| Fuselage | Primary structure; houses avionics, payload, fuel or battery |
| Wing | Generates lift; carries the ailerons and any flaps or spoilers |
| Empennage | Tail group: horizontal stabilizer (pitch stability) and vertical stabilizer/fin (yaw stability) |
| Control surfaces | Ailerons, elevator, rudder — and on RPAs often combined surfaces such as elevons (delta) or a V-tail |
| Landing gear | Tricycle, taildragger, skids, or none at all on catapult-launched, belly-landing designs |
| Powerplant | Tractor propeller (front) or pusher propeller (rear), or an electric ducted fan |
Multi-Rotor
Frame and arms, motors and electronic speed controllers, fixed-pitch propellers in counter-rotating pairs, the flight controller with its IMU, the power distribution board, landing gear, and usually a stabilized gimbal carrying the payload. A multi-rotor has no aerodynamic control surfaces at all — every control input is a change in relative motor speed.
Helicopter (Single Main Rotor)
Main rotor with variable-pitch blades, the swashplate that translates control inputs into cyclic and collective blade pitch, the mast and transmission, the tail boom with the tail rotor, and skids or wheels. A helicopter RPA is mechanically the most complex of the three and the least common at Basic level — but it is the configuration the exam uses to test cyclic, collective and tail rotor.
2. Stability
Stability is an aircraft's tendency to return to its original attitude after a disturbance without pilot input.
- Static stability is the initial tendency: positive (returns toward the original attitude), neutral (stays where the gust left it), or negative (diverges further).
- Dynamic stability describes what happens over time: the oscillation may damp out (positively dynamically stable), continue unchanged, or grow.
The Three Axes
| Stability | Axis of Rotation | Motion | Principal Source |
|---|---|---|---|
| Longitudinal | Lateral axis (wingtip to wingtip) | Pitch | Horizontal stabilizer and CG position |
| Lateral | Longitudinal axis (nose to tail) | Roll | Wing dihedral, sweepback, keel effect (high wing) |
| Directional | Normal/vertical axis | Yaw | Vertical fin weathervaning the aircraft into the relative wind |
The Effect of Centre-of-Gravity Position
Longitudinal stability is the one the exam presses on, because it is directly under the pilot's control through loading:
- Forward CG: more longitudinally stable, heavier pitch response, higher stall speed, shorter endurance (the tail carries a larger download).
- Aft CG: less stable, lighter and twitchier pitch response, and beyond the aft limit the aircraft becomes uncontrollable in pitch. On a multirotor, an off-centre CG means the controller must hold a permanent corrective tilt, so one pair of motors runs hotter and the whole aircraft drifts in a hover and loses endurance.
Inherent Stability and the Multirotor
A conventional fixed wing is inherently stable: disturb it and aerodynamics alone start the recovery. A multirotor is inherently unstable — it has no aerodynamic mechanism for self-correction and remains upright only because the flight controller reads the IMU and adjusts motor speeds thousands of times per second. This is why a saturated or mis-initialized IMU is not an inconvenience on a multirotor; it is a loss of the only thing keeping the aircraft level.
3. Design of the Wing
Planform
Planform is the shape of the wing seen from above.
| Planform | Characteristic |
|---|---|
| Rectangular | Simple and cheap; stalls at the root first, which preserves aileron authority — common on trainers and survey RPAs |
| Tapered | Lower induced drag than rectangular; better structural efficiency |
| Elliptical | Theoretically lowest induced drag; expensive to build; stalls across the whole span at once |
| Swept | Delays compressibility effects at high speed; adds lateral stability; poor low-speed behaviour |
| Delta | Very high structural depth and wide speed range; high induced drag at low speed |
Aspect ratio (span squared divided by area) is the other key number: a high-aspect-ratio wing — long and slender, like a sailplane or a long-endurance survey RPA — produces less induced drag and better glide and endurance, at the cost of roll rate and structural weight.
Flaps and Spoilers
- Flaps are high-lift devices on the trailing edge inboard of the ailerons. Extending them increases camber (and on some types area), which raises the coefficient of lift so the aircraft can fly at a lower speed, and increases drag so it can descend more steeply without accelerating. On an RPA, flaps let a heavy survey wing arrive slowly at a short strip.
- Spoilers are panels on the upper surface that, when raised, disrupt the airflow: they destroy lift and add drag. They are used to steepen a descent without gaining speed, to dump lift on touchdown so the wheels grip, and on some designs to provide roll control in place of ailerons.
Do Not Confuse Them: A flap adds lift and drag. A spoiler destroys lift and adds drag.
4. Aeroplane Flight Controls
| Axis | Motion | Primary Control | Located On |
|---|---|---|---|
| Longitudinal (nose to tail) | Roll | Ailerons (move in opposite directions) | Outboard trailing edge of the wing |
| Lateral (wingtip to wingtip) | Pitch | Elevator | Trailing edge of the horizontal stabilizer |
| Normal / vertical | Yaw | Rudder | Trailing edge of the vertical fin |
Trim relieves a steady control pressure so the aircraft holds an attitude hands-off. A crewed aircraft uses a trim tab on the control surface; an RPA uses electronic sub-trim in the control station to remove a persistent drift. Trim is not a substitute for fixing the underlying cause: if an aircraft needs increasing trim over a season, look for a warped surface, a shifted CG, or a damaged airframe.
5. Helicopter Flight Controls
| Control | What It Changes | Result |
|---|---|---|
| Collective | Pitch of all main-rotor blades together | Total rotor thrust — the aircraft climbs or descends |
| Cyclic | Blade pitch cyclically as each blade goes round, tilting the rotor disc | The thrust vector tilts, so the aircraft moves forward, back or sideways |
| Tail rotor (anti-torque pedals) | Thrust from the tail rotor | Counters main-rotor torque reaction and controls yaw (heading) |
| Throttle / governor | Engine power | Maintains constant rotor RPM as collective demand changes |
The torque relationship is the exam point: driving a main rotor in one direction pushes the fuselage in the opposite direction (Newton's third law). The tail rotor exists to cancel that torque. Lose the tail rotor and the fuselage spins. A multirotor solves the same problem with no tail rotor at all, by using counter-rotating pairs whose torques cancel — and it yaws deliberately by speeding up one rotation direction and slowing the other.
6. Multi-Rotor Dynamics
Settling With Power (Vortex Ring State)
Covered in detail in section 9.2: a near-vertical descent with power applied and little forward speed lets the aircraft sink into its own downwash. Recovery is lateral — fly out into clean air — not more throttle.
Recirculation
Recirculation is the related but distinct problem of the rotor re-ingesting air it has already disturbed, typically when hovering close to a wall, a parapet, a hangar face or a confined courtyard. The rotor is fed turbulent, already-accelerated air instead of clean air, so thrust becomes unsteady. The symptoms are a wandering hover, buffeting, and a sudden partial loss of lift as the aircraft is drawn toward the surface. Mitigation is procedural: keep at least one rotor diameter — preferably several — between the disc and any vertical surface, and approach walls from a stabilized hover rather than drifting in sideways.
Dynamic Rollover
Dynamic rollover is a ground-contact accident, and it is the reason a drone can flip during an apparently gentle take-off.
If one leg, skid or arm is pinned — caught in long grass, a crack, mud, a net, a cargo strap, or simply resting on a slope — the aircraft can no longer lift straight up. Instead it begins to pivot about that contact point. As it rolls, the thrust vector tilts with the airframe, so the rolling moment increases while the controller's ability to oppose it decreases. Beyond a critical roll angle, full opposite control cannot stop the rollover, and the aircraft flips with the rotors striking the surface.
Prevention:
- Launch from a level, firm, clear surface — use a landing pad on grass, gravel or snow.
- Lift vertically and positively to a hover; never "peel" one side off the ground.
- Watch for snagging — tall grass, netting, tie-downs, a camera strap, a tether.
- On a slope, if the aircraft starts to lean during lift-off, reduce power immediately and set it back down; do not try to fly out of it.
7. Load Factor from Gusts
Section 9.2 covers load factor in a banked turn. The second source in TP 15263 is gust loading.
A sharp vertical gust changes the relative airflow and therefore the angle of attack almost instantly. Lift rises in proportion, so the load factor spikes — with no control input from the pilot. Three consequences:
- Structural limitation: the airframe has a design limit load. Repeated gust spikes fatigue composite arms and wing spars, and a single severe one can break them.
- Power limitation: on a multirotor, the controller answers a gust with a burst of motor current. In gusty conditions the aircraft can hit its thrust ceiling and simply stop responding.
- Operational answer: slow down and get lower. Reducing speed reduces the load factor produced by a given gust, and landing before the gust front arrives removes the problem entirely.
An RPA is loaded with a heavy payload mounted well aft of its normal centre of gravity. What happens to its longitudinal stability?
What is the difference between a flap and a spoiler?
A remotely piloted helicopter is hovering. The pilot wants to climb vertically without changing heading or moving horizontally. Which control does this?
During take-off from long grass, one landing leg of a multirotor snags. The aircraft begins to lean and roll about that leg. What is happening, and what should the pilot do?