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.
Trim Characteristics
REQUIREMENT
It shall be possible to trim the aircraft to zero control forces throughout the aircraft operational flight envelope. This requirement in general, and applies to both longitudinal, and lateral-directional trim devices.
In addition to the ability to trim the aircraft at all flight conditions, there are normally requirements placed on transients in trim forces required when aircraft configuration changes are made in flight:
The pitch trim changes caused by operation of secondary control devices shall not be so large that a peak pitch control force in excess of 10 pounds for center stick controllers or 20 pounds for wheel controllers is required when such configuration changes are made in flight under conditions representative of operational procedure.
The above requirement was extracted from MIL-8785C (reference 2). MIL-8785C contains a list of configuration changes that are required and the flight conditions under which they should be demonstrated.
Other requirements include use of secondary control systems, trim irreversibility, trim rates, changes due to armament operation, release of stores, drag devices, and sideslip.
TEST PROCEDURE
The ability to trim the aircraft throughout the flight envelope is normally a fall out of other testing. As such there will be no discussion of a test technique for normal trim tests.
Many of the configuration changes which should be tested for effects on trim change also take place during normal flight operations and do not need to be tested specifically. For those transients which are not occur during normal test operations and should be set up specially. The following procedure should be used for transient trim change tests:
1) Stabilize at desired flight condition and trim the control forces to zero.
2) Execute the specified configuration change and hold the required parameter constant.
3) Repeat test for each different configuration change in Table 1.
DATA REQUIRED
Trim Conditions:
1) Configuration,
2) Weight,
3) Center of Gravity,
4) Pressure Altitude.
5) Trim CAS
Test Variables:
1) Longitudinal Control Force,
2) Configuration Variable.
DATA ANALYSIS
Plots should be prepared showing the amount of control surface deflection required to trim the aircraft in all flight configurations throughout the envelope.
Data should be summarized in a table showing the changes in stick force for each of the transients tested. If more than one test case is available for each transient, an average value and a standard deviation should be supplied if applicable.