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JSBSim Aerodynamics: Difference between revisions
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Moments, Pitch: Cm additions
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** Another roll moment source is main wing dihedral angle. If a dihedral wing encounters a sidewind component, let's say from the right, then the effective angle of attack will slightly increase for the right wing and decrease for the left wing. This is because - from the wing's perspective - the side wind component approaches the right wing from below (leading to an increase in AOA and more lift) and the left wing from above (leading to a decrease in AOA and less lift). Rolling the aircraft to one side will induce a side motion which leads to a resulting difference in lift, creating a roll moment counteracting the initial roll angle and providing roll stability. | ** Another roll moment source is main wing dihedral angle. If a dihedral wing encounters a sidewind component, let's say from the right, then the effective angle of attack will slightly increase for the right wing and decrease for the left wing. This is because - from the wing's perspective - the side wind component approaches the right wing from below (leading to an increase in AOA and more lift) and the left wing from above (leading to a decrease in AOA and less lift). Rolling the aircraft to one side will induce a side motion which leads to a resulting difference in lift, creating a roll moment counteracting the initial roll angle and providing roll stability. | ||
* '''Pitch: Cm''' The primary contributor to the pitching moment is the lift generated by the horizontal tail. The pitching moment of the main wing airfoil is a secondary contributor. Because of this fact it makes sense that the value for Cm will resemble the CL curve for the horizontal tail airfoil. The standard CL curve does not take into account the changes in tail Angle of Attack (AoA) and QBar due to main wing down-wash and rotational velocity around the Y axis. | * '''Pitch: Cm''' The primary contributor to the pitching moment is the lift generated by the horizontal tail. The pitching moment of the main wing airfoil is a secondary contributor. Because of this fact it makes sense that the value for Cm will resemble the CL curve for the horizontal tail airfoil. The standard CL curve does not take into account the changes in tail Angle of Attack (AoA) and QBar due to main wing down-wash and rotational velocity around the Y axis. At least for unswept wings, using '''QBarUW''' is more appropriate for Cm 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.'' | ||
* '''Yaw: Cn''' The primary contributor to the yawing moment is the lift generated by the vertical tail. The wind force on the fuselage is a secondary contributor. Because of this fact it makes sense that the value for Cn will resemble the CL curve for the vertical tail airfoil. Changes in QBar due to the vertical tail moving into the 'shadow' of a stalled main wing may need to be accounted for, as well. The source for vertical tail Angle of Attack (AoA) should be Beta and QBarUV is more appropriate for Cn than the generic QBar. | * '''Yaw: Cn''' The primary contributor to the yawing moment is the lift generated by the vertical tail. The wind force on the fuselage is a secondary contributor. Because of this fact it makes sense that the value for Cn will resemble the CL curve for the vertical tail airfoil. Changes in QBar due to the vertical tail moving into the 'shadow' of a stalled main wing may need to be accounted for, as well. The source for vertical tail Angle of Attack (AoA) should be Beta and QBarUV is more appropriate for Cn than the generic QBar. | ||