Hinge line definition does not cover all control surface architectures

[MarAlder]

Current state

• the hinge line is specified in the path element by an innerHingePoint and and outerHingePoint
• its xsi and eta coordinates are both defined in the wing component segment as it is the parent of the control surface
• the eta position of the innerHingePoint and outerHingePoint is determined by the outerShape element -> innerShape/outerShape -> etaLE (etaTE does not influence this coordinate, i.e. no interpolation between LE and TE position)
• the xsi position (hingeXsi element) is measured from the leading edge of the parent wing structure
• the z-position of the hing line is specified by the hingeRelHeight element which refers to the local thickness of the wing at the hinge point position (0 = lower skin; 1 = upper skin)

Problem description

The hinge line cannot be outside of the parent wing geometry, because:

• eta, xsi and hingeRelHeight may only take values between 0 and 1 and must therefore be within the parent wing geometry

Certain configurations are not possible to model with the current definition, e.g.:

• tracks issue Kinematic description in controlSurfaceTrackTypeType for different steps along path #605 (@raedma): A configuration like trackType 2 subType2 is not possible

• similar example [Strüber 2014]:
  

• Canard wings: hinge line is out of the parent wing geometry and the hinge line could be better described by a hinge point and direction vector [Quora; accessed 17-04-2020]
  

Solution

... will be documented in this issue. Any suggestions welcome.

Current problems to consider in this context:

• if possible: remain backwards compatible (i.e., additional elements, avoid changes to current definition)
• in case of an additional offset vector:
  • implicitly fix the origin hinge line to reduce ambiguity?
  • relative coordinates (less problems with scaling of geometry) or absolute coordinates to be consistent with hinge line translation in the step definition?
• is an additional eta translation necessary for the hinge point? (could solve the problem to define a hinge line which is inside of a canard wing geometry)