retro-flight-sim/js/flight-model.js

import * as THREE from 'three';

const PITCH_LIMIT = 85 * Math.PI / 180;
const STALL_SPEED = 25;
const GROUND_LIFT_REDUCTION = 0.15;

export class FlightModel {
  constructor() {
    this.position = new THREE.Vector3(0, 8, 0);
    this.velocity = new THREE.Vector3(0, 0, 0);
    this.orientation = new THREE.Euler(0, 0, 0, 'YXZ');
    this.angularVelocity = new THREE.Vector3(0, 0, 0);
    this.thrust = 0;
    this.mass = 1200;
    this.maxThrust = 50000;
    this.dragCoefficient = 0.02;
    this.liftCoefficient = 0.15;
    this.gravity = 9.81;
    this.onGround = true;
    this.groundHeight = 2;
  }

  getForwardVector() {
    const dir = new THREE.Vector3(0, 0, -1);
    dir.applyEuler(this.orientation);
    return dir;
  }

  getUpVector() {
    const up = new THREE.Vector3(0, 1, 0);
    up.applyEuler(this.orientation);
    return up;
  }

  update(delta, input, terrainHeightAtPos) {
    this.groundHeight = terrainHeightAtPos(this.position.x, this.position.z) + 2;
    this.onGround = this.position.y <= this.groundHeight + 0.5;

    // Throttle control
    if (input.throttleUp) {
      this.thrust = Math.min(1, this.thrust + delta * 0.3);
    } else if (input.throttleDown) {
      this.thrust = Math.max(0, this.thrust - delta * 0.3);
    }

    const speed = this.velocity.length();

    // Thrust force
    const forward = this.getForwardVector();
    const thrustForce = forward.clone().multiplyScalar(this.thrust * this.maxThrust);

    // Drag
    const dragMagnitude = this.dragCoefficient * speed * speed;
    const dragForce = speed > 0
      ? this.velocity.clone().normalize().multiplyScalar(-dragMagnitude)
      : new THREE.Vector3();

    // Lift
    let liftFactor = this.liftCoefficient;
    if (this.onGround && speed < STALL_SPEED) {
      liftFactor *= GROUND_LIFT_REDUCTION * (speed / STALL_SPEED);
    } else if (speed < STALL_SPEED) {
      liftFactor *= 0.5 * (speed / STALL_SPEED);
    }
    const up = this.getUpVector();
    const liftForce = up.clone().multiplyScalar(liftFactor * speed * speed);

    // Gravity
    const gravityForce = new THREE.Vector3(0, -this.gravity * this.mass, 0);

    // Ground friction
    let frictionForce = new THREE.Vector3();
    if (this.onGround) {
      const horizontalVel = new THREE.Vector3(this.velocity.x, 0, this.velocity.z);
      const hSpeed = horizontalVel.length();
      if (hSpeed > 0.1) {
        frictionForce = horizontalVel.normalize().multiplyScalar(-hSpeed * 20);
      }
    }

    // Net force and acceleration
    const netForce = new THREE.Vector3()
      .add(thrustForce)
      .add(dragForce)
      .add(liftForce)
      .add(gravityForce)
      .add(frictionForce);
    const acceleration = netForce.divideScalar(this.mass);

    // Integrate velocity and position
    this.velocity.add(acceleration.multiplyScalar(delta));
    this.position.add(this.velocity.clone().multiplyScalar(delta));

    // Ground collision
    if (this.position.y < this.groundHeight) {
      this.position.y = this.groundHeight;
      if (this.velocity.y < 0) {
        this.velocity.y = 0;
      }
      this.onGround = true;
    }

    // Rotation
    this.updateRotation(delta, input);
  }

  updateRotation(delta, input) {
    const pitchInput = input.pitch || 0;
    const rollInput = input.roll || 0;
    const yawInput = input.yaw || 0;

    // Dampen angular velocity
    this.angularVelocity.multiplyScalar(0.92);

    // Apply control inputs (per-second rates)
    this.angularVelocity.x += pitchInput * 1.5;
    this.angularVelocity.y += yawInput * 0.8;
    this.angularVelocity.z += rollInput * 1.2;

    // Clamp angular rates
    this.angularVelocity.x = Math.max(-2, Math.min(2, this.angularVelocity.x));
    this.angularVelocity.y = Math.max(-1, Math.min(1, this.angularVelocity.y));
    this.angularVelocity.z = Math.max(-2, Math.min(2, this.angularVelocity.z));

    // Integrate
    this.orientation.x += this.angularVelocity.x * delta;
    this.orientation.y += this.angularVelocity.y * delta;
    this.orientation.z += this.angularVelocity.z * delta;

    // Clamp pitch
    this.orientation.x = Math.max(-PITCH_LIMIT, Math.min(PITCH_LIMIT, this.orientation.x));

    // Auto-leveling
    this.orientation.x *= 0.995;
    this.orientation.z *= 0.985;
  }

  getAirspeedKnots() {
    return this.velocity.length() * 1.94384;
  }

  getAltitudeFeet() {
    return Math.max(0, (this.position.y - this.groundHeight) * 3.28084);
  }

  getHeadingDeg() {
    const forward = this.getForwardVector();
    let heading = Math.atan2(-forward.x, -forward.z) * 180 / Math.PI;
    return ((heading % 360) + 360) % 360;
  }

  getVerticalSpeedFtMin() {
    return -this.velocity.y * 196.85;
  }

  getPitchDeg() {
    return this.orientation.x * 180 / Math.PI;
  }

  getRollDeg() {
    return this.orientation.z * 180 / Math.PI;
  }
}