Flight Model

PFC_FlightModel is where the aircraft is simulated. This page explains how each part works; the Reference page lists every tunable attribute with its default.

Per-surface aerodynamics

The model is force-based: instead of one set of stability derivatives, the aircraft is described as a list of aero surfaces (PFC_AeroSurfaceDef entries in the prefab). Each tick, for every surface:

1. The aircraft's airflow (velocity relative to the air mass) is transformed into the surface's local space. The GetSurfaceAirVelocityLS() hook decides what airflow a surface sees — the core passes every surface the CoM airflow; a variant can add the rotational component (ω × r) for physical pitch/yaw/roll damping.
2. PFC_AeroSurface.CalculateForce() computes lift and drag from the local angle of attack.
3. The resulting force is applied as an impulse at the surface's position via ApplyImpulseAt, so it produces the correct moment about the centre of mass (a tail surface pitches, a wingtip aileron rolls).

Lift & drag (Khan & Nahon 2015)

PFC_AeroSurface uses a linear lift curve pre-stall, blended into a flat-plate model post-stall:

• Pre-stall: Cl = liftSlope · (α − α₀) between the negative and positive stall angles.
• Post-stall: blends to Cl = 2·sin(α)·cos(α) over a 15° window past the stall angle.
• Drag: skin friction + induced drag Cl²/(π·AR·e) + a post-stall sin²(α) term.

Control surfaces add their deflection to the effective angle of attack scaled by m_fFlapFraction (the fraction of the chord that is the moving control), so a larger flap fraction = more control authority per degree of deflection.

Control surfaces

Each surface's m_iControlAxis decides what drives its deflection:

Axis	Driven by	Deflection
NONE	-	fixed (e.g. main wing, fuselage side)
ELEVATOR	pitch input	−pitch · maxDef
AILERON	roll input	−roll · maxDef (mirrored panel flips sign)
RUDDER	yaw input	−yaw · maxDef
FLAPS	flap angle	m_fCurrentFlapAngle

maxDef is m_fMaxControlDeflectionDeg (default 25°).

The axis→deflection mapping lives in the GetSurfaceDeflection() hook. A variant can override it to scale authority (e.g. high-speed control stiffening) or to serve extra axes added via modded enum PFC_ControlAxis (e.g. an airbrake); returning false leaves a surface undriven.

No flap controller in the core

The minimal core never deploys flaps - the GetFlapTargetAngle() hook returns 0, so m_fCurrentFlapAngle stays 0. A FLAPS surface still resolves correctly; a variant overrides the hook to drive the flap angle and the core handles the deploy ramp (m_fFlapDeployDurationSeconds) and the FlapAngle signal.

Engine & thrust

The engine is always on. Each tick the per-engine RPM ramps toward idle + throttle·(max−idle) at m_fRPMRate (fraction of max RPM per second), then thrust is:

thrustFraction = clamp((RPM − idle) / (max − idle), 0, 1)   // 0 at idle, 1 at max
thrust = thrustFraction · m_fMaxThrustPerEngine · numEngines · healthMul

Thrust is applied along the aircraft's forward axis at the centre of mass. Damage scales power down to m_fMinHealthThrustFraction of full at zero hull health; a destroyed aircraft makes zero thrust and its RPM snaps to zero.

Both halves are hooks: UpdateEngineSpool() owns the RPM ramp and ComputeThrustMagnitude() owns the thrust formula (with GetThrustHealthMultiplier() for the damage scaling), so a variant can substitute e.g. turbojet spool lag, idle residual thrust, or altitude thrust lapse without touching the rest of the tick. The fuselage drag term is likewise GetFuselageDragArea() — a variant can grow it with speed (transonic drag rise) or an airbrake.

Angular damping

A speed-scaled angular-rate damping bleeds angular velocity each tick:

speedFactor = clamp(speed / 80, 0.1, 1.5)
angVel *= 1 − min(m_fAngularDamping · speedFactor · dt, 1)

Real aerodynamic damping scales with airspeed; a constant damper would kill low-speed authority yet be too weak when fast, so the factor scales it with speed. This is the core's stability aid - keep it modest so it doesn't fight control inputs.

Wind & gusts

Aerodynamic forces act on airspeed (velocity relative to the air mass), computed by subtracting the wind from ground velocity. Consequences:

• A crosswind produces sideslip and weathervaning.
• A head/tailwind separates indicated airspeed from groundspeed.

Wind is assembled on the authoritative peer only (so it never needs replicating):

• Steady wind is read from the weather manager a few times a second, scaled by m_fWindScale.
• Near-ground gradient eases the wind from m_fWindSurfaceFraction at the deck up to full at m_fWindGradientHeight AGL - a boundary layer, so the crosswind shears as you descend on approach.
• Gusts add smooth time-varying turbulence scaled by m_fGustIntensity (and m_fGustVerticalScale for the vertical component), so calm air stays calm.

Instrument signals

When the owner is registered for replication, UpdateSignals() publishes the standard vanilla gauge signals (names match VehicleGauge_*.conf expectations). These drive HUDs and cockpit instruments and replicate to all peers. See the Reference for the full list.

Signal compression

Raw-unit signals (airspeed in km/h, altitude in m, climb in m/s) must use SignalCompressionFunc.None. Range01 clamps the value to [0,1] over the network, which silently pins every raw signal at 1 on the receiving side. Only already-normalized 0..1 signals (throttle, normalized RPM) may use Range01.

Robustness guards

A Game-Master lift/teleport or a solver penetration spike can hand the rigid body a non-finite or enormous velocity in a single frame. Left in the body, that overflows the solver next step and hard-crashes the engine. PFC_FlightModel therefore:

• Resets velocity + angular velocity and bails the tick on a clearly-broken (NaN / absurd-magnitude) value.
• Clamps a finite-but-implausible teleport spike (body speed > 600 m/s, angular > 40 rad/s, airspeed > 400 m/s) before it reaches the solver, leaving normal flight untouched.
• Sanitizes every value written to a signal (SafeNum), since a NaN reaching a gauge trips !BadFloat() asserts in every consumer and can take down the whole HUD.