16 Rotary Wing

16.1 Principal Aeroderivatives

16.1.1 Control Power

Derivative Common Name Principal Contributors Typical Sign
\(M_{B_1}\) Pitch control power
  • MR thrust vector
  • Mast bending moment
  • Control gearing
  • Rotor type
  • Effective hinge offset
-
\(L_{A_1}\) Roll control power
  • MR thrust vector
  • Mast bending moment
  • Control gearing
  • Rotor type
  • Effective hinge offset
-
\(N_{\theta_{\text{TR}}}\) Yaw control power
  • TR thrust
  • TR moment arm
  • Control gearing
-
\(Z_{\theta_C}\) Heave control power
  • MR thrust
  • Control gearing
-

16.1.2 Static Stability

Derivative Common Name Principal Contributors Typical Sign
\(M_u\) Speed stability
  • MR flap back
  • Mast bending moment
  • Horizontal tailplane
+
\(M_w\) Static / Incidence / Angle of Attack stability
  • MR flap back
  • Mast bending moment
  • Horizontal tailplane
  • Fuselage
 
\(L_v\) Lateral static stability (dihedral effect)
  • MR ‘flap back’
  • TR vertical moment arm
  • Fuselage
-
\(N_v\) Directional static stability (weathercock effect)
  • TR thrust
  • Vertical tailplane
  • Fuselage
+

16.1.3 Damping

Derivative Common Name Principal Contributors Typical Sign
\(X_u\) Drag damping
  • Rotor drag
  • Fuselage drag
-
\(Y_v\) Side force
  • Rotor drag
  • Fuselage drag
-
\(Z_w\) Heave damping
  • MR characteristics
-
\(L_p\) Roll damping
  • Main rotor
  • Effective hinge offset
-
\(M_q\) Pitch damping
  • Main rotor
  • Effective hinge offset
  • Horizontal tailplane
-
\(N_r\) Yaw damping
  • Tail rotor
  • Vertical tailplane
  • Fuselage
-

16.1.4 Cross Coupling

Derivative Common Name Principal Contributors Typical Sign
\(L_{\theta_{\text{TR}}}\) Tail rotor roll
  • Tail rotor vertical position
+
\(M_{\theta_C}\) Pitch change with power
  • Forward speed
  • Main rotor
+
\(N_{\theta_C}\) Torque reaction
  • Torque
 
\(Y_{\theta_{\text{TR}}}\) Tail rotor drift
  • Tail rotor
 

16.2 Forward Flight Static And Dynamic Stability 

Stability Characteristic Principal Influences Typical Test Role Relation
Longitudinal Static Stability
  • \(M_w\)
  • \(M_u\)
  • \(M_{\theta_C}\)
  • \(M_{\theta_{TR}}\)
  • Trimmed flight control positions
  • Trimmed flight control positions - collective
  • Apparent static stability
  • Collective fixed static stability
  • Control margins
  • Control inputs progressive, predictable, and in correct sense
  • Speed selection
  • Speed maintenance
Manoeuvre Stability
  • \(M_w\)
  • \(M_q\)
  • \(M_{\theta_C}\)
  • Apparent manoeuvre stability
  • Collective fixed manoeuvre stability
  • Pull-ups/push-overs
  • Aggressive turning and manoeuvring flight
Longitudinal Dynamic Stability
  • \(M_w\)
  • \(M_u\)
  • \(M_q\)
  • Excitation of dynamic long term mode
  • Natural turbulence, release to trim, pulse input
  • IMC flight
  • Transit
  • Nuisance mode
Lateral-Directional Static Stability
  • \(L_v\)
  • \(N_v\)
  • \(Y_v\)
  • Trimmed flight control positions
  • Steady heading sideslip (SHSS)
  • Control margins
  • Control inputs progressive, predictable, and in correct sense
  • Sideforce cues
  • Maintaining balanced flight
Lateral Static Stability (Dihedral)
  • \(L_v\)
  • SHSS
  • Turns on one control – pedal
  • Transit
  • Lateral and out-of-wind transitions
  • Instrument approaches
Directional Static Stability
  • \(N_v\)
  • SHSS
  • Turns on one control - cyclic
  • Transit
  • Instrument approaches
Lateral-Directional Dynamic Stability Oscillations (Dutch Roll Mode)
  • \(L_v\)
  • \(N_v\)
  • Excitation of LDO via doublet, pulse, or SHSS release to trim
  • IMC flight
  • Transit
  • Nuisance mode
Lateral-Directional Dynamic Stability Spiral Stability
  • \(L_v\)
  • \(N_r\)
  • \(N_v\)
  • \(L_r\)
  • Turns on one control – cyclic
  • Time to half/double bank angle
  • IMC flight
  • Turns
  • Lateral gust response

16.3 References

Cooke, A., Fitzpatrick, E., (2002), Helicopter Test and Evaluation, Wiley Blackwell, UK.
Leishman, J.G., (2006), Principles of Helicopter Aerodynamics, 2nd Edition, Cambridge University Press, UK.
Padfield, G.D., (2007), Helicopter Flight Dynamics, 2nd Edition, Blackwell Publishing, UK.