189
edits
JSBSim Aerodynamics: Difference between revisions
Jump to navigation
Jump to search
→Forces
No edit summary |
(→Forces) |
||
| Line 36: | Line 36: | ||
== Forces == | == Forces == | ||
* '''Lift: CL''' 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 Reynolds | 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. | ||
* '''Drag: CD''' 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. | * '''Lift: CL''' Lift is the portion of the aerodynamic force that is at a right angle to the relative wind, and points up. | ||
* '''Side: CY''' | ** 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. | ||
* '''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. | |||
* '''Side: CY''' Side is the portion of the aerodynamic force that is at a right angle to the relative wind and points to the side. | |||
This system was developed by and for people using wind tunnels. Lift is to the top of the wind tunnel, drag is out the back and side is to the side. As a real aircraft in free space yanks and banks it becomes somewhat ambiguous which direction the forces are applied. For example, an aircraft is diving straight at the ground, 90 degrees nose down pitch. Its angle of attack, and the relative wind, is almost 0. 'Lift' is supposed to be perpendicular to the relative wind and point to the heavens, but there is not a vector that satisfies that description in this case. | |||
Enter the Axial, Normal, Side system. | |||
* '''Axial: CA''' Axial force is the portion of the aerodynamic force that is parallel to the aircraft's longitudinal (X) axis. For very small angles of attack it is analogous to Drag. | |||
* '''Normal: CN''' Normal force is the portion of the aerodynamic force that is parallel to the aircraft's vertical (Z) axis. For very small angles of attack it is analogous to Lift. | |||
* '''Side: CY''' Side is the portion of the aerodynamic force that is parallel to the aircraft's lateral (Y) axis. | |||
== Moments == | == Moments == | ||