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aircraft:tmd:collision

# Collision

A collision happens when a part of the aircraft contacts the ground surface. When the part is made of rubber and can rotate this usually just fine and preferred over all other parts of the aircraft

When the part passes through the ground a force is generated which is proportional to the depth below the ground (in meters) times the factor K (N/m) plus the relative velocity (in meters per second) times the factor D (N per m/s). The force acts on the rigidbody assigned in the `Body` attribute.

A critical crash with any part that is not a wheel can cause the aircraft to be reset, when the designer of the aircraft, us, flags the part as important and sets the `CrashOnContact` property to true.

### wheel

Wheels are geometric shapes at a given position `R0`, with a `Radius` and `Width` all in meters, the `RimRadius` is the inner radius of the wheel, where the rubber stops (the rim), also in meters.

The `Inflation` is the pressure of the tire in pascals.

K and D are factors that define the strength of the forces generated per meter below ground and meter per second relative velocity respectively.

```            // GA size
<[wheel][NoseWheelHull][]
<[string8][Body][NoseGear]>
<[tmvector3d][R0][ 0.0 0.0 0.0 ]>
<[float64][Width][0.14]>
<[float64][Inflation][213737.56]>
<[float64][K][400000.0]>
<[float64][D][10000.0]>
>

// airliner size
<[wheel][FrontWheelHullLeft][]
<[string8][Body][NoseGearLower]>
<[tmvector3d][R0][ 0.0 0.0 0.0 ]>
<[float64][Width][0.243]>
<[float64][Inflation][1372057.24]>
<[float64][K][1000000.0]>
<[float64][D][100000.0]>
>```

### collisionhull

Main collision part, uses the mesh of the `Geometry` from the 3D model to compute collisions with the ground. The coefficients are optional lines of code that can be used to fine tune the friction of the part when skidding across the ground (kinetic) or when stopped (static).

Setting `CrashOnContact` to false allows the part to contact the ground without an taking damage. This is typically desired for the fuselage to allow the flight to continue after a tailstrikes or a gear up landing and also on the engine cowlings and (lower) wings, when the aircraft lands with gear up or the wing tip just briefly contacts the ground. Parts that contact the ground only in upset attitudes like the vertical stabilizer or upper wings in case of a biplane should always cause a crash on contact.

```            <[collisionhull][CollisionFuselage][]
<[string8][Body][Fuselage]>
<[string8][Geometry][Fuselage]>
<[float64][K][65000000.0]>
<[float64][D][520000.0]>
<[bool][CrashOnContact][true]>
<[float64][StaticFrictionCoefficient][1.0]>
<[float64][KineticFrictionCoefficient][0.8]>
>```

### collisionsphere

A spherical object at position `R0` with a set `Radius` that repels the `Body` from the ground. Used for small skids or a tail gear without a wheel.

```            <[collisionsphere][LeftWingWheel][]
<[string8][Body][LeftWing]>
<[tmvector3d][R0][ -0.011895 6.741012 0.065757 ]>
<[float64][K][150000.0]>
<[float64][D][9000.0]>
>```

```            <[collisionblade][CollisionPropellerLeft][]