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JSBSim Aerodynamics: Difference between revisions
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Forces, Lift: CL, minor additions (QBarUW)
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For purposes of the Aerodynamics section the entire aircraft creates a single, unified aerodynamic force. This force is split into three component vectors. The most common way of splitting this force into vectors is the Lift Drag Side method. Another, potential better way from the perspective of generating a full 360 degree capable [[FDM]], is the Normal, Axial, Side method. | For purposes of the Aerodynamics section the entire aircraft creates a single, unified aerodynamic force. This force is split into three component vectors. The most common way of splitting this force into vectors is the Lift Drag Side method. Another, potential better way from the perspective of generating a full 360 degree capable [[FDM]], is the Normal, Axial, Side method. | ||
* '''Lift: CL''' Lift is the portion of the aerodynamic force that is at a right angle to the relative wind '''and''' lies within the x-z-axis plane (usually the aircraft's symmetry plane) '''and''' is oriented towards the aircraft's upside. (During inverted flight, it points to the ground). | * '''Lift: CL''' Lift is the portion of the aerodynamic force that is at a right angle to the relative wind '''and''' lies within the x-z-axis plane (usually the aircraft's symmetry plane) '''and''' is oriented towards the aircraft's upside. (During inverted flight, it points to the ground). | ||
** Lift is a function of QBar * Wing Area * Cl<sub>lift</sub>. Cl<sub>lift</sub> is generally derived from a 2D table as a function of AoA. In the real world it is also a function of the Reynolds and Mach Numbers. | ** Lift is a function of QBar * Wing Area * Cl<sub>lift</sub>. Cl<sub>lift</sub> is generally derived from a 2D table as a function of AoA. In the real world it is also a function of the Reynolds and Mach Numbers. At least for unswept wings, using '''QBarUW''' is more appropriate for CL than the generic QBar. Otherwise, your aircraft will likely be prone to the issue [https://github.com/JSBSim-Team/jsbsim/issues/89 Pitch oscillations at rest, Brakes ON, with pure crosswind]. Swept wings might need a different treatment that we have not investigated further. ''However this has only been tested for the particular case of the c172, these are not universal laws but rather guidance or at the very least trouble shooting.'' | ||
* '''Drag: CD''' Drag is the portion of the aerodynamic force that is parallel to the relative wind. | * '''Drag: CD''' Drag is the portion of the aerodynamic force that is parallel to the relative wind. | ||
** It is important to ensure all coefficient functions in the drag section remain positive. When drag coefficient functions are negative, drag is effectively acting as thrust opposite the relative wind. | ** It is important to ensure all coefficient functions in the drag section remain positive. When drag coefficient functions are negative, drag is effectively acting as thrust opposite the relative wind. | ||