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.
Maneuvering Longitudinal Stability
Maneuvering stability is essentially the pilots ability to feel normal acceleration changes from stick inputs during turning flight. When nZ no longer changes as a function of stick input, the aircraft is said to be at the maneuver point. Maneuver point is a function of Mach, altitude, and nZ. Stick fixed and stick free maneuver points are defined as that cg position which yields zero values for dëPS/dnZ and dFS/dnZ respectively.
REQUIREMENT
In general, the gradients dëPS/dnZ and dFS/dnZ should be stable (negative in sign) throughout the aircraft flight envelope. In addition, it is normally required for the FS versus nZ to be linear for certain load factor ranges with allowable deviations from linearity up to a stated load factor. Requirements may also be stated as minimum and maximum values of dëPS/dnZ or dFS/dnZ. Additional requirements may be added for specific flight configurations or flight conditions.
TEST PROCEDURE
Three methods can be employed for obtaining the data required to assess maneuvering stability. The three methods use steady turn, wind up turn, or symmetrical pullup maneuvers to obtain data during turning flight.
Steady Level Turn:
1) Stabilize and trim in level flight at the desired altitude, Mach number, and cg position.
2) Enter a coordinated turn with the desired normal acceleration. Maintain the desired speed or Mach number while allowing altitude to vary as necessary (ñ2000 feet). Power may be added as required to avoid large altitude changes at higher load factors.
3) Maintain constant acceleration while recording data. Make note of any lightening of stick forces during maneuver.
4) Repeat procedure for the range of Mach numbers, altitude, and cg locations.
Wind Up Turn:
1) Stabilize and trim at the desired altitude and at the maximum speed desired for the test.
2) Enter a 2g turn and allow airspeed to decrease slowly (~2 knots/sec for low performance aircraft, and 10 knots/sec for high performance aircraft) by varying altitude (ñ2000 feet) and decreasing power until the low speed limit is reached.
3) Attain maximum speed again and repeat procedure for a series of load factors in order to cover the g range at each Mach number. Repeat for all desired altitudes.
Symmetric Pull-Up:
1) Stabilize and trim at the desired altitude and at the desired Mach and altitude for the test.
2) Without changing PLA, climb to a height slightly above the trim altitude.
3) Dive to regain approximately the test altitude and speed.
4) Execute a symmetrical pullup at constant desired nZ from an initial nose down attitude that is sufficient to maintain nearly constant acceleration and speed for several seconds.
5) Repeat for range of Mach, altitude, and load factor.
DATA REQUIRED
Trim Conditions:
1) Configuration,
2) Weight,
3) Center of Gravity,
4) Pressure Altitude.
5) Trim CAS
Test Variables:
1) CAS,
2) Pressure Altitude,
3) Normal Acceleration,
4) Longitudinal control surface position,
5) Longitudinal Control Force.
6) Stick Position.
DATA ANALYSIS
1) Using cross plots of ëPS versus nZ and FS versus nZ determine the gradients ëPS/ nZ and FS/ nZ for each test point.
2) From the data determined in step one ,and test point cg location, plot ëPS/ nZ and FS/ nZ versus cg in %MAC as shown in figure 2-4.
3) By extrapolating ëPS/ nZ and FS/ nZ plots created in step 2 above to the zero values, the stick fixed, and stick free maneuver points can be determined for each load factor at all Mach numbers and altitudes tested.
4) Summary plots can then be made of maneuver point versus Mach number for each level of load factor