Stability Analysis with XFLR5
[Pages:70]About stability analysis using XFLR5
XFLR5
Revision 2.1 ? Copyright A. Deperrois - November 2010
Sign Conventions
The yaw, such that the nose goes to starboard
is >0
The pitching moment nose up is > 0
Revision 2.1 ? Copyright A. Deperrois - November 2010
The roll, such that the starboard wing goes down
is > 0
The three key points which must not be confused together
Centre of Pressure CP = Point where the resulting aero force applies Depends on the model's aerodynamics and on
the angle of attack
Centre of Gravity CG = Point where the moments act;
Depends only on the plane's mass distribution, not its aerodynamics
Also named XCmRef in XFLR5, since this is the point about which the pitching moment is calculated
Revision 2.1 ? Copyright A. Deperrois - November 2010
Neutral Point NP = Reference point for which the pitching moment does not depend
on the angle of attack
Depends only on the plane's external geometry
Not exactly intuitive, so let's explore the concept further
The neutral point = Analogy with the wind vane
Wind vane having undergone a perturbation, no longer in the wind direction
Wind
CG
NP
CP
CG forward of the NP The pressure
forces drive the
vane back in the
wind direction Very stable wind
vane
CG slightly forward of the NP The pressure forces
drive the vane back in
the wind direction The wind vane is
stable, but sensitive to
wind gusts
CG positioned at the NP The wind vane
rotates indefinitely Unstable
CG behind the NP The wind vane is stable... in the wrong direction
The Neutral Point is the rear limit for the CG 2nd principle : Forward of the NP, the CG thou shall position
Revision 2.1 ? Copyright A. Deperrois - November 2010
A preliminary note : Equilibrium is not stability !
Unstable Stable
Both positions are at equilibrium, only one is stable
Revision 2.1 ? Copyright A. Deperrois - November 2010
Mechanical stability
Unstable
Stable
Fx0
Fx>0
Fx 0 the model flies !
Cm0
Cm
Cm = 0 = balance = plane's operating point
Negative slope = Stability The curve's slope is also the strength of the
stabilizing force High slope = Stable sailplane !
Revision 2.1 ? Copyright A. Deperrois - November 2010
For information only : Cm0 = Moment
coefficient at zero-lift
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