Moments applied by a propeller to an aircraft
In addition to the production of thrust, a spinning propeller applies two moments to an aircraft:
- A torque which, according to Newton's 3rd law, tends to roll the aircraft in the direction opposite to the propeller rotation.
- A moment perpendicular to the propeller spinning axis and to the aircraft rotational movement a.k.a the gyroscopic effect.
When a moment is applied along the input axis (green), the propeller tends to rotate around the output axis (blue). The resulting moment around the output axis is sometimes refered to as the gyroscopic effect.
As was reported in the bug [#110], JSBSim computation of the sign of the gyroscopic effect was erroneous and has been fixed in the commits [6b7a21] and [efef74]
Since this bug has existed for a long time in JSBSim, it has been decided to fix the problem by adding a version attribute to the propeller section of the aircraft XML definition. When the version attribute is specified to a value strictly higher than 1.0, JSBSim computes the correct sign of the gyroscopic effect. Otherwise JSBSim uses the historical erroneous value of the gyroscopic sign.
<propeller name="aPropeller" version="1.01">
<!-- propeller definition -->
</propeller>Of course, it is strongly recommended to specify the version attribute to the propeller definition so that the correct sign of the gyroscopic effect is used.