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JSBSim Aerodynamics: Difference between revisions
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* Faking lift polars - If we assume a symmetrical airfoil that stalls at +/- 15 degrees (0.26 radians) AoA and assume the lift is linear between the two we can create a table like: | * Faking lift polars - If we assume a symmetrical airfoil that stalls at +/- 15 degrees (0.26 radians) AoA and assume the lift is linear between the two we can create a table like: | ||
<table > | |||
<independentVar lookup="row">aero/alpha-rad</independentVar> | |||
<tableData> | |||
0.26 0.26 | 0.26 0.26 | ||
-0.26 -0.26 | -0.26 -0.26 | ||
</tableData> | |||
</table> | |||
Beyond +/- 22.5 degrees AoA we'll use 0.26 * sin(2*AoA) to approximate lift, and linear interpolate between 15 and 22.5 degrees AoA. (These numbers are rather arbitrary.) The resultant table returns the same value as alpha-rad for the "stable" flight regime so the function will accept the [[aeromatic]] coefficient. Outside of the "stable" flight regime a better number will be used allowing more realistic flight behavior at high alpha or beta angles. | Beyond +/- 22.5 degrees AoA we'll use 0.26 * sin(2*AoA) to approximate lift, and linear interpolate between 15 and 22.5 degrees AoA. (These numbers are rather arbitrary.) The resultant table returns the same value as alpha-rad for the "stable" flight regime so the function will accept the [[aeromatic]] coefficient. Outside of the "stable" flight regime a better number will be used allowing more realistic flight behavior at high alpha or beta angles. | ||
<syntaxhighlight lang="xml"> | |||
<table > | |||
<independentVar lookup="row">aero/alpha-rad</independentVar> | |||
<tableData> | |||
-3.14 0.0 | -3.14 0.0 | ||
-2.62 0.2252 | -2.62 0.2252 | ||
| Line 85: | Line 85: | ||
2.62 -0.2252 | 2.62 -0.2252 | ||
3.14 0.0 | 3.14 0.0 | ||
</tableData> | |||
</table> | |||
</syntaxhighlight> | |||
[[File:fakelifttable.png]] | [[File:fakelifttable.png]] | ||
| Line 99: | Line 99: | ||
Stall Speed = 32 mph | Stall Speed = 32 mph | ||
Air Density = 0.00238 slugs/ft^3 | Air Density = 0.00238 slugs/ft^3 | ||
QBar = 2.62871(slug | QBar = 2.62871(slug / (ft•s^2)) | ||
We get | We get | ||
2688 lb = 2.62871 (slug | 2688 lb = 2.62871 (slug / (ft * s^2)) * 280 ft^2 *Cl | ||
Cl = 2688 lbf / (2.62871 (slug | Cl = 2688 lbf / (2.62871 (slug / (ft * s^2)) * 280 ft^2) = 3.65 | ||
{{JSBSim}} | |||