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Aerodynamics: Real vs Game

How regular sails (Create SailBlock) and symmetric sails (Simulated Symmetric Sail) produce lift and drag in this game, and why real-world aerodynamics cannot be copied over. The formulas and coefficients come from the Sable / Simulated sources (BlockSubLevelLiftProvider.sable$contributeLiftAndDrag()), the same model behind the Flight Management Computer tools solveSailLift / solveSailDirectionlessDrag.

Game aero ≠ Real aero: three fundamental differences

Keep these three in mind — every "placement rule" follows from them, not from aviation theory:

  1. Force directions are body-fixed; they do not flip with the airflow

    Real life: wing lift is perpendicular to the airflow and continuously redirects with angle of attack. Game: regular sail lift always points along the sail normal +n, symmetric sail drag along ±n, directionless damping along -v. → The sail's orientation decides the force direction: upside down, wing lift pushes downward.

  2. Force magnitude ∝ local airflow speed (linear), not v²

    Real life: dynamic pressure q = ½ρv². Game: each sail F ≈ k·P·Δt·v. → Aero forces decay linearly at low speed; at zero speed there is no aero force at all.

  3. The angle-of-attack (α) effect comes only from the (n·v) term, and only symmetric-sail normal drag is an "odd function"

    Regular sail lift decreases with (n·v)² (even function, symmetric in ±α); symmetric sail normal drag is the linear (n·v) term (odd function — nose-up and nose-down perturbations produce equal and opposite restoring forces).

→ Corollary: the real-world rule "center of gravity ahead of the aerodynamic center → static stability" relies on wing lift growing with α. In the game, regular sail lift is symmetric in ±α and shrinks, so that rule cannot be copied. True two-way static stability comes from an odd-function surface (symmetric sail normal drag) placed behind the center of gravity — it acts like a weathervane / α-spring that pulls the body back into alignment with the airflow.

Common skeleton: computed per sail per physics substep

Every physics substep Δt, for every sail, the algorithm has the same shape (parameters differ — see the next two sections):

  1. Normal n: regular sail = FACING opposite direction; symmetric sail = AXIS positive direction. Inside a contraption, first rotate it via localPose into the sub-level frame.
  2. Local airflow speed (taken at the sail block center): 
    v = R⁻¹( body linear velocity V + angular velocity ω × (block center − sub-level origin) )
    → Sails far from the COM "feel" the rotation and automatically generate aero damping.
  3. Air pressure P at that position: DimensionPhysicsData.getAirPressure(...), decaying with altitude per the dimension pressure curve (overworld ~300 m ≈ 0.39).
  4. Normal drag (along the sail normal): magnitude = k1·|n·v|·P·Δt, applied to the body negated → cancels the velocity component along the sail normal.
  5. Directionless drag (linear damping, opposite v): magnitude = k2·|v|·P·Δt, always cancels velocity.
  6. Lift (regular sails only — see below): direction always along +n, magnitude = k3·|TEMP|·P·Δt.
  7. Application point = sail block center (pos+0.5); moment τ = (application point − COM) × force → the farther a sail is from the COM, the larger the moment from the same force.

Regular Sail (Create SailBlock): lift + drag

Injected by a Create-compat mixin (SailBlockMixin), all parameters at Sable defaults:

Parameter Value Meaning
Normal n FACING opposite Axis of lift / normal drag
Lift coefficient k3 0.475 Only regular sails produce lift
Normal drag coefficient k1 0.75 Drag perpendicular to the sail face
Directionless drag coefficient k2 0.06888202261 Linear damping ((−0.75+√(0.75²+0.475²))/2, the minimum damping that exactly suppresses default-lift divergence)

Per physics substep:

  1. Normal drag (k1 = 0.75): F_par = n·(n·v)·0.75·P·Δt, applied to the body negated → cancels the normal component; magnitude = |n·v|·0.75·P·Δt.
  2. Directionless drag (k2): opposite v, magnitude = |v|·0.06888·P·Δt.
  3. Lift (the regular sail's core output, k3 = 0.475):
  4. First remove the part already eaten by normal drag: TEMP = v − F_par vector
  5. Magnitude = |TEMP|·0.475·P·Δt (≈ grows with local airflow speed)
  6. Direction = always along n (FACING opposite), with no (n·v)-style sign flip — the sail is always pushed toward the n side.
  7. Application point = sail block center; moment = (application point − COM) × force.

Implications for design:

  • A regular sail is a lift-first lifting surface. Lift direction is fixed in the sail's own frame (the n side) and rotates with the body — upside down, n points down and lift pushes down (into the ground); only right-side-up with the n side up is it reliable lift.
  • Lift/drag magnitude both ∝ local airflow speed |v| (linear velocity + angular velocity × lever arm) → the farther a sail is from the COM, the larger the force and moment from the same body motion (damping, trim and control all rely on this).
  • All forces scale with P (dimension base pressure × altitude curve).

Symmetric Sail (SymmetricSailBlock): drag only

Simulated's SymmetricSailBlock computes no force itself — it is an implementation of Sable's BlockSubLevelLiftProvider interface, with overridden parameters:

Parameter Symmetric sail Regular sail (default) Meaning
Normal n AXIS positive FACING opposite Sail face normal
Lift coefficient k3 0 (overridden) 0.475 Symmetric sails produce no lift
Normal drag coefficient k1 1.75 (overridden) 0.75 Drag perpendicular to the face (2.3×)
Directionless drag coefficient k2 0.06888202261 (not overridden) 0.06888202261 Linear damping

Per physics substep:

  1. Normal drag (the symmetric sail's main output): magnitude = |n·v|·1.75·P·Δt, direction along normal n, sign following (n·v), applied to the body negated → it always "cancels the velocity component along the sail normal".
  2. When n·v = 0 (airflow along the sail face) this term is 0 — the sail "has no effect"; drag only appears once deflected → this is what lets control surfaces / stabilizers produce control moments.
  3. Directionless drag (k2): opposite v, magnitude = |v|·0.06888·P·Δt (always cancels linear velocity).
  4. Lift = 0 → symmetric sails only produce drag (that is where the name comes from).
  5. Application point = sail block center; moment = (application point − COM) × force.

Implications for design:

  • A symmetric sail is a pure drag / damping surface: the larger the tail and the farther from the COM, the stronger the pitch/yaw damping and the more "stable" the aircraft — but also the more sluggish.
  • Symmetric sail = "weathervane α-spring": normal drag (n·v) is an odd function, so nose-up and nose-down perturbations produce equal and opposite restoring forces — placed behind the COM, a symmetric-sail tail alone gives two-way static stability, no "lift center behind COM" needed. Stiffness ∝ k1·P·V·lever arm (automatically weaker at altitude / low speed).
  • 1.75 is 2.3× the regular sail's normal coefficient (0.75), with zero lift → deflecting a symmetric sail redirects its drag and produces a control moment (official ponder: a tail symmetric sail + rotating bearing at 30° is a "rudder").
  • All drag scales with P: editing the dimension dimension_physics datapack's pressure/altitude curve scales the drag of all sails (and the lift of regular sails) together.

Don't forget: universal drag

The above covers the "sail" forces. Separately, Rapier applies constant velocity damping to every physics body (default d = 0.09), directly decaying body velocity without going through force groups — invisible to both the diagram and the flight recorder. The equivalent force:

F = −m·d·v   (proportional to mass and speed, NOT scaled by pressure P)

At cruise it is usually the largest drag term (nearly double the sail drag), so it must be added when balancing forces (thrust − sail drag − universal drag ≈ 0). See the "universal drag" section and the getUniversalDragForce tool in the Flight Management Computer.