Longitudinal Static Stability
Theory of Longitudinal Static Stability
Stick Fixed Stability (tailless aircraft)
An aircraft that has elevons (combined ailerons and elevators) e.g., Concorde is governed by slightly different equations.
The effect of the tab angle is usually ignored as most tailless aircraft have powered controls and so no tabs. The lift equation therefore becomes
Moments about the cg give us:
For TRIM, the elevon angle to trim is [pic]
For STABILITY it is clear that the aerodynamic centre is also the neutral point for a tailless aircraft (hn = h0 and Kn = h0-h)
Control Hinge Moments
When we consider the aerodynamic moment acting about the hinge line on the control surface itself there are contributions coming from the incidence and camber of the aerofoil as well as from the deflections of the controls
The coefficient of hinge moment is given by:
[pic] where Sη = control surface area and [pic]= control surface mean chord – both of which are measured aft of the hinge line. So we can say that: [pic] and it is normal to neglect the tab term when dealing with elevons.
Exercises
1. The data below applies to an aircraft in steady level flight at 200kt EAS. Calculate the elevator angle required for longitudinal trim. Also obtain the stick-fixed neutral point and the static margin. NB: the cg is 0.61m aft of leading edge of mean chord
|Parameter |Value |
|Weight, W |30 kN |
|1 knot |0.515 m/s |
|ρ0 |1.225 kg/m3 |
|Wing area, S
Stick Fixed Stability (tailless aircraft)
An aircraft that has elevons (combined ailerons and elevators) e.g., Concorde is governed by slightly different equations.
The effect of the tab angle is usually ignored as most tailless aircraft have powered controls and so no tabs. The lift equation therefore becomes
Moments about the cg give us:
For TRIM, the elevon angle to trim is [pic]
For STABILITY it is clear that the aerodynamic centre is also the neutral point for a tailless aircraft (hn = h0 and Kn = h0-h)
Control Hinge Moments
When we consider the aerodynamic moment acting about the hinge line on the control surface itself there are contributions coming from the incidence and camber of the aerofoil as well as from the deflections of the controls
The coefficient of hinge moment is given by:
[pic] where Sη = control surface area and [pic]= control surface mean chord – both of which are measured aft of the hinge line. So we can say that: [pic] and it is normal to neglect the tab term when dealing with elevons.
Exercises
1. The data below applies to an aircraft in steady level flight at 200kt EAS. Calculate the elevator angle required for longitudinal trim. Also obtain the stick-fixed neutral point and the static margin. NB: the cg is 0.61m aft of leading edge of mean chord
|Parameter |Value |
|Weight, W |30 kN |
|1 knot |0.515 m/s |
|ρ0 |1.225 kg/m3 |
|Wing area, S