Effective Date: 15 June 98
Test and Analysis Technique 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.
Dynamic Longitudinal Stability
Longitudinal dynamic stability has two modes of free motion. One is a long period mode commonly referred to as the phugoid mode which is oscillation of airspeed and pitch attitude at a nearly constant angle of attack. The phugoid mode oscillates with a period of approximately 20 to 50 seconds. The short period mode is an oscillation of pitch attitude and angle of attack at a nearly constant airspeed. The short period oscillation is normally well damped with a period of less then 4 seconds in conventional aircraft, and considerably less in aircraft employing a full time flight control system.
Normally, and almost always in an aircraft with a flight control system, the phugoid mode is not a stability problem. If present, the phugoid is usually more of an annoyance then anything else. However, this mode must still be tested for and characterized if it is found to be present in the aircraft under test. Typically the phugoid mode is more apparent at aft cg loadings, therefore the testing for the phugoid should be concentrated at the aft cg test points.
The short period mode is of more importance to the stability problem because of the pilot in the loop. The period of the short period mode falls in the frequency range of normal pilot handling qualities tasks such as tracking a target, or glideslope adjustment during power approach. Because the frequency of the short period mode is so close to that of typical handling qualities, this mode should be well damped. However, over damping of the short period mode can in itself cause deficient handling qualities.
Both short and long period modes are tested with fixed and with free controls. Fixed controls, or fixed stick, mean that the pilot maintains hands on the control stick with positive control. Controls free, or stick free maneuvers are performed by first upsetting the aircraft from steady flight, then allowing the aircraft to continue flying without any input from the pilot. In stick free maneuvers the control surfaces are free to deflect due to airloads that change as the airplane oscillates.
Phugoid Mode Testing
REQUIREMENT
Phugoid or long period oscillation requirements may be specified as a damping ratio, or a oscillation period. In some cases it may only be stated that the long period phugoid oscillation should not be objectionable. Some specification documents may require a specific value for the stick force gradients in terms of FP/ V (lbs/Knot). Refer to specific requirements documents for specific requirements.
TEST PROCEDURE
1) Stabilize the aircraft at the desired flight conditions and trim control forces to zero.
2) Start data recording.
3) Increase speed a small amount by pushing forward on control stick.
4) Stick fixed and stick free stability:
a. For stick fixed stability, return stick to neutral and hold fixed,
b. For stick free stability, release stick.
5) After stick is released, it may be necessary to maintain wings level using small changes in lateral trim.
DATA REQUIRED
Trim Conditions:
1) Configuration,
2) Weight,
3) Center of Gravity,
4) Pressure Altitude.
5) Trim CAS
Test Variables:
1) CAS, 2) Angle of Attack,
3) Normal Acceleration
4) Longitudinal control surface position,
5) Longitudinal Control Force.
6) Stick Position.
DATA ANALYSIS
1) A time history of a typical phugoid oscillation should be presented.
2) If the Phugoid mode is persistent and objectionable, damping ratios should be calculated for several airspeeds at all cg locations tested to verify that specification requirements have been met. Stick free and stick fixed damping ratios should be presented separately.
Short Period Oscillation Testing
REQUIREMENT
Short period oscillation requirements are typically stated as limits on damping ratio ( ) and undamped natural frequency ( N). Some requirements specify limits on and N as a function of the ratio of normal load factor (nZ) to angle of attack (à) nZ/à. In addition to the requirements placed on N and , there are sometimes additional restrictions on the residual oscillations.
TEST PROCEDURE
1) Stabilize the aircraft at the desired flight conditions.
2) For 1g test conditions, trim control forces to zero.
3) Start data recording.
4) Abruptly deflect longitudinal control to obtain a change in normal acceleration of approximately 1g from trim nZ.
5) For stick fixed stability, return stick to neutral and hold fixed.
6) For stick free stability, release stick ( this is normally done only for 1g maneuvers).
7) Responses should be obtained for both positive and negative changes in acceleration.
8) If the aircraft is equipped with a pitch damper, the maneuvers should be done with the damper on and off.
DATA REQUIRED
Trim Conditions:
1) Configuration,
2) Weight,
3) Center of Gravity,
4) Pressure Altitude.
5) Trim CAS
Test Variables:
1) CAS,
2) Angle of Attack,
3) Normal Acceleration
4) Longitudinal control surface position,
5) Longitudinal Control Force.
6) Stick Position.
DATA ANALYSIS
1) Determine and N from time histories of nZ versus time.
2) Calculate nZ/à for each flight condition tested from a cross plot of nZ versus à.
3) Plot and N versus Mach number for various altitudes and load factors to show Mach effects.
4) If required, plot and N versus nZ/à for various altitudes and load factors.
5) Plots should be created for each different configuration (CR, PA, etc.)
Note: For aircraft with full time flight control systems, it may not be possible to determine and N. In this case it may be necessary to show compliance with the requirements through the use of equivalent model matching techniques.