JSBSim Aerodynamics: Difference between revisions

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JSBSim Aerodynamics (view source)

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* '''Stall''' - A 'stall' is generally regarded as a loss of lift due to flow separation over the top of a wing, however, examination of lift polar for an airfoil over a full 360 degrees shows that significant amounts of lift are NOT lost as the stall occurs. The biggest aerodynamic effect of a stall is a large and rapid increase in drag.
* '''Stall''' - A 'stall' is generally regarded as a loss of lift due to flow separation over the top of a wing, however, examination of lift polar for an airfoil over a full 360 degrees shows that significant amounts of lift are NOT lost as the stall occurs. The biggest aerodynamic effect of a stall is a large and rapid increase in drag.
* '''Spin''' - Spins are caused loss of stability in the Yaw Moment axis. A stock [[Aeromatic]] [[FDM]] yaw section does not take alpha into account when calculating the yaw moment.
* '''Spin''' - Spins are caused loss of stability in the Yaw Moment axis. A stock [[Aeromatic]] [[FDM]] yaw section does not take alpha into account when calculating the yaw moment.
* '''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:
<syntaxhighlight lang="xml">
<table >
  <independentVar lookup="row">aero/alpha-rad</independentVar>
  <tableData>
    0.26  0.26
  -0.26 -0.26
  </tableData>
</table>
</syntaxhighlight>
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
  -2.62  0.2252
  -2.36  0.26
  -2.09  0.2252
  -1.57  0.0
  -1.05  -0.2252
  -0.79  -0.26
  -0.52  -0.2252
  -0.39  -0.1838
  -0.26  -0.2588
    0.0    0.0
    0.26  0.2588
    0.39  0.1838
    0.52  0.2252
    0.79  0.26
    1.05  0.2252
    1.57  0.0
    2.09  -0.2252
    2.36  -0.26
    2.62  -0.2252
    3.14  0.0
  </tableData>
</table>
</syntaxhighlight>
[[File:fakelifttable.png]]
This table can drop directly into an [[aeromatic]] [[FDM]] to replace an instance of aero/alpha-rad or aero/beta-rad (changing the independent var of course).
* Calculating Stall Speed - For an aircraft in straight and level flight, lift is equal to the weight of the aircraft. If we plug some numbers into the equation
  lift = QBar * Sw-sqft * Cl
Using the Fi-156 Storch as an example
  Weight = 2688 lb
  Wing Area = 280 sq-ft
  Stall Speed = 32 mph
  Air Density = 0.00238 slugs/ft^3
  QBar = 2.62871(lbf / ft^2) or psf
We get
  2688 lb = 2.62871 (lbf / ft^2) * 280 ft^2 *Cl
  Cl = 2688 lb / (2.62871 (lbf / ft^2) * 280 ft^2) = 3.65
{{JSBSim}}
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