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.
Rolling Maneuver Stability and Control
Rolling maneuver stability and control involves several facets of controllability. Roll control effectiveness is the time required to roll the aircraft through a specified bank angle. The roll mode time constant relates to how fast the aircraft responds in the roll axis. Oscillations in roll rate and bank angle following a step roll input must be restricted. Excursions in pitch and yaw during a rapid roll maneuver should also be determined during the flight test program.
REQUIREMENTS
As mentioned in the introduction to this section, roll mode stability and control testing involves many different aspects. Generally, the aircraft must respond quickly enough to perform the tasks for which it was designed, and be controllable and upon recovery from the maneuver. Only three of the more basic specific roll performance requirements will be discussed in this section. Additional requirements which apply to roll coupling, roll rate oscillations, and bank angle oscillations also exist. Please refer to reference documents for additional discussion of subjects not presented here.
Roll control effectiveness is a measure of how well the ailerons work in providing rolling control power. Roll control effectiveness is stated by specifying a maximum time for the aircraft to roll through a specified bank angle. A bank angle change of between 25 and 90 degrees is used depending on the flight phase and type of aircraft. In addition there are requirements on the maximum stick force or wheel rotation required to obtain the roll performance specified. The specific requirements should be studied in detail for this section, as there are many different nuances to roll control effectiveness.
Roll "quickness" is specified with the roll mode time constant in MIL-8785C. The roll mode time constant is essentially a measure of roll acceleration. The derivation of the roll mode time constant is beyond the scope of this text. The roll mode time constant can be approximated by the time required for the roll rate to reach 63% of the maximum steady state roll rate. A maximum value is specified for roll mode time constant in MIL-8785C, however, practical experience indicates there should also be a minimum value. An airplane that accelerates too quickly in the roll axis tends to jerk the pilot around an is very uncomfortable.
There are typically restrictions on excursions into the yaw axis during a rudder free rapid roll maneuver. The requirement in MIL-8785C is stated in terms of the ratio of the total sideslip excursion to the parameter k. The parameter k is ratio of actual bank angle change attained to the required change in bank angle. Other requirements exist for sideslip excursions with small roll inputs.
TEST PROCEDURE
Two basic types of maneuvers are used to assess roll stability and control and handling qualities. The two basic roll maneuvers are the bank to bank turn and the 360 roll. These two maneuvers can be performed with full, half, or quarter lateral control deflection with rudder pedal fixed and free.
Bank To Bank Turn:
1) Stabilize at desired test conditions and trim all control forces to zero.
2) Roll aircraft to specified bank angle and stabilize in turn.
3) Using specified amount of lateral stick input, roll the aircraft back to the same bank angle in the opposite direction from the initial bank and stabilize. Roll inputs should be as crisp as possible.
4) For rudder free maneuver, keep feet off rudder pedals during roll. For rudder fixed maneuver, use rudder as required to maintain zero sideslip.
5) Return to level stabilized flight.
6) Repeat in opposite direction to check for control symmetry.
360 Roll:
1) Stabilize at desired test conditions and trim all control forces to zero.
2) Using specified amount of lateral stick input, roll the aircraft through 360 .
3) Use opposite lateral stick input to recover from roll such that the 360 roll is completed in minimum time. All roll inputs should be as crisp as possible.
DATA REQUIRED
Entry Conditions:
1) Configuration,
2) Weight,
3) Pressure Altitude.
4) CAS
5) Normal Acceleration
Test Variables:
1) Lateral Acceleration,
2) Angle of Attack
3) Sideslip Angle,
4) Roll Rate,
5) Yaw Rate,
6) Pitch Rate,
7) Bank Angle ,
8) Aileron Deflection,
9) Rudder Deflection,
10) Elevator Deflection,
11) Lateral Control Position,
12) Rudder Pedal Position,
11) Lateral Stick Force,
12) Longitudinal Stick Force,
13) Rudder Pedal Force
DATA ANALYSIS
Because of the many different aspects of roll control, it is not possible in the context of this material to cover all analysis requirements. Instead, the more common parameters which are extracted from roll data will be discussed. These parameters can then be used to determine whether the aircraft under test Ýsmeets the required specifications.
Each roll maneuver should be plotted as a time history of the parameters listed above. From the time histories the following data should be determined:
1) Average Fa
2) The time required to roll through 25 , 30 , 45 , 50 , 60 , or 90 depending on the aircraft class requirements. These will be referred to as t .
3) For rudder free rolls, the maximum sideslip excursion during the maneuver ( max).
4) k = èact/ èreq (ratio of actual bank angle change attained to the required change in bank angle)
5) Maximum sustained roll rate (pmax).
6) Maximum nz encountered during maneuver (nzmax).
7) Time required to reach 63.2% of pmax. This is an approximation of the roll mode time constant (çR).
The following summary plots should be made with the requirement limit and structural limits shown to demonstrate compliance throughout the flight envelope. Each plot should note the other flight parameters (i.e. configuration, altitude, Mach, entry nz, etc.) for the plot where applicable.
1) t versus Mach.
2) pmax versus ëa.
3) Fa versus ëa.
3) max/k versus Mach.
4) çR versus Mach.
5) pmax versus Mach
6) nzmax versus Mach
7) max versus Mach
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 maneuver.