Effective Date: 15 June 98
Test and Analysis Techniques As mentioned in the introduction, this section will contain test and analysis techniques for some of the more common stability and control tests. The content of these sections is very general and should serve only as a guideline.
Lateral-Directional Stability and Control
The term lateral-direction refers to rotation about the yaw and roll axes. Lateral stability refers to stability in the roll axis, and directional stability refers to yawing or weather cocking stability. Because of the combined effect of kinematic and aerodynamic coupling on the handling qualities problem, the two rotational axes are almost always referred to together.
The primary control surfaces for roll control are the ailerons. Control of the ailerons is provided to the pilot through sideways motion of a stick type controller, or wheel rotation on a control column type control.
Primary directional control is provided by the rudder. Control of the rudder is provided to the pilot through foot pedals.
This section will discuss some of the more common lateral- directional stability and control and handling qualities requirements and flight tests that are performed.
Static Lateral-Directional Stability and Control
Lateral-Directional static stability pertains to three different effects produced with an increase in the sideslip angle. Static directional stability refers to the tendency of the aircraft to produce yawing moments which are opposite in sign to the sideslip angle, and thus zero out any sideslip angle in the absence of any control inputs. Dihedral effect refers to the aircraft's tendency to roll in the opposite direction of the sideslip. Sideforce effects due to sideslip angle generate a right bank from (nose right beta, left sideslip) and a left bank from nose left beta (right sideslip).
REQUIREMENT
Static Directional Stability
Throughout the aircraft operating envelope, right rudder pedal force and deflection shall be required to produce nose right sideslip angle (- , left sideslip) and left rudder pedal force and deflection shall be required to produce nose left (+ , right sideslip). In addition, within ñ15 of the variation of with rudder pedal deflection or force shall be linear. Beyond ñ15 , there should not be a reverse of rudder variation with .
Dihedral Effect
While holding wings level during a steady sideslip, right roll control shall be required for right sideslip (nose left), and left roll control shall be required for left sideslip (nose right).
Side Force Effect
With roll controls free, a right sideslip (nose left) shall produce a left wing down roll and a left sideslip (nose right) shall produce a right wing down roll.
TEST PROCEDURE
1) Stabilize at the desired flight conditions and trim control forces to zero.
2) Gradually increase (1 deg/sec) sideslip angle slowly while holding the trim airspeed by varying altitude slightly and maintaining a steady flight path heading. PLA may be increased, but it should be noted if the addition of power affects the sideslip characteristics.
3) For roll stick free response, allow aircraft to roll during sideslip maneuver.
4) When full pedal deflection or 250 lbs force is reached, hold the sideslip for approximately 3-5 seconds.
5) Gradually decrease sideslip back to zero, and repeat steps 1 through 3 in the opposite direction.
DATA REQUIRED
Trim Conditions:
1) Configuration,
2) Weight,
3) Center of Gravity,
4) Pressure Altitude.
5) Trim CAS
Test Variables:
1) CAS,
2) Sideslip angle ( ),
3) Bank Angle (è)
4) Aileron Deflection (ëa),
5) Rudder Deflection (ër),
6) Lateral Stick Force (FA),
7) Longitudinal Stick Force (FS)
8) Rudder Pedal Force (FR)
DATA ANALYSIS
1) A crossplot of FA, FR, FS, ëa, ër, and è versus sideslip angle should be made for each flight condition tested.
2) The requirement for linearity within ñ15 is satisfied using engineering judgement on the FR versus crossplot.
3) The gradients (FA/ ), (FR/ ), (ëa/ ), and (ër/ ) should be plotted versus Mach number to show compliance with specification requirements and to identify any trends which may be developing as speed is increased.
4) Maximum values of FS should be tabulated or plotted to verify the requirement on pitch trim during steady sideslips.
Dutch Roll Stability
The Dutch roll oscillation is a coupling of , è, and heading. Dutch roll effects are normally apparent when the pilot is performing tracking or formation flying tasks. If the Dutch roll mode is not well damped, and the frequency is in the same range as the handling qualities tasks being performed, then it will substantially increase pilot workload.
REQUIREMENT
The requirement for Dutch roll stability is normally stated in terms of the Dutch roll damping ratio ( ) and natural frequency ( ). Much like the short period damping requirements for the pitch mode, these requirements are based on pilots comments regarding handling qualities of the aircraft with different values of and . MIL-8785C also uses the product of and as part of the requirement. Other requirements may simply state that the Dutch roll mode not be objectionable and be controllable by a pilot of ordinary skills.
TEST PROCEDURE
1) Stabilize at desired flight conditions and trim all control forces to zero.
2) Establish a steady heading sideslip.
3) For controls fixed testing, rapidly return controls to neutral and hold them there until oscillations are damped out.
4) For stick free testing. After establishing sideslip, release controls and allow oscillations to dampen out.
5) Check cases can be performed with sideslip in opposite direction.
6) Repeat with stability augmentation OFF if applicable.
NOTE: If aircraft diverges rapidly in roll, it may be necessary to use a rudder pulse to excite the dutch roll mode.
DATA REQUIRED
Trim Conditions:
1) Configuration,
2) Weight,
3) Center of Gravity,
4) Pressure Altitude.
5) Trim CAS
Test Variables:
1) Sideslip angle ( ),
2) Bank Angle (è)
3) Lateral Stick Force (FA),
4) Longitudinal Stick Force (FS)
5) Rudder Pedal Force (FR)
DATA ANALYSIS
1) Data should be plotted as a time history of and è.
2) Determine and from the time history plots.
3) Compare flight test and to specification requirements for all required flight configurations.
4) Plots of and should be made as a function of some other independent flight condition parameter such as Mach or à to provide summary plots throughout the flight envelope.
Spiral Mode Stability
The spiral mode of oscillation is a dynamic effect caused by yawing moment due to sideslip overcoming the rolling moment due to sideslip. The spiral mode is an extremely long period mode which is normally controllable by the pilot even if it is divergent. If allowed to continue without pilot input, the aircraft would enter and ever tightening descending spiral.
REQUIREMENT
The requirement on the spiral mode is normally stated as the time required to reach either a specific bank angle, or the time required for the bank angle to double in amplitude from an initial roll disturbance.
TEST PROCEDURE
1) Stabilize at desired flight conditions and trim all control forces to zero.
2) Initiate a roll input until 20 of bank has been reached, and release the controls.
3) After 30 seconds, or when bank has either returned to zero or reaches 45 degrees, recover aircraft to wings level flight.
4) Repeat maneuver with stability augmentation OFF if applicable.
DATA REQUIRED
Trim Conditions:
1) Configuration,
2) Weight,
3) Center of Gravity,
4) Pressure Altitude.
5) Trim CAS
Test Variables:
1) Bank Angle (è)
DATA ANALYSIS
1) Plot bank angle as a time history.
2) Determine time required for bank angle to reach 40 degrees.
3) Plot the time determined in step 2 versus some independent flight parameter such as speed, to show specification compliance throughout the envelope.