Longitudinal Stability and Control
Effective Date: 15 June 98
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.
Longitudinal Stability and Control
Longitudinal Stability refers to the ability to control the aircraft pitch axis. The primary control surface in the longitudinal axis is the elevator. Control of the elevator is provided to the pilot through forward and aft motion of either a control stick for smaller fighter type aircraft, or a control column in larger transport type aircraft.
Static Stability
REQUIREMENT
Static stability refers to the ability of the aircraft to return to the trimmed flight conditions after it has been disturbed from the trim condition:
For Levels 1 and 2 there shall be no tendency for the airspeed to diverge aperiodically when the airplane is disturbed from trim with the cockpit controls fixed and with them free.
The requirement above is extracted from Mil-8785C (Reference 2). This requirement is satisfied if the variations of the pitch control force/position with airspeed are smooth and the local gradients are stable in 1g trimmed level flight. A stable gradient here means that forward (-ëPS) stick deflection yields an increase in airspeed and backward (+ëPS) yields a decrease in airspeed.
TEST PROCEDURE
There are two basic test methods for determining static stability. The first allows the altitude to vary at a constant PLA setting while using ëPS to increase and decrease speed around the trim point. The second method maintains constant altitude and varies PLA and ëPS to obtain an increase and decrease in airspeed around the trim point. Which method is used depends on aircraft susceptibility to thrust effects and Mach number. The two methods are described below:
1) Stabilize and trim at desired flight condition.
2) Without changing power or trim, use pitch stick input to first decrease speed to VTRIM-15%, and then increase speed to VTRIM+15% allowing altitude to vary slightly. Maintain small flight path angles during the run.
3) Repeat test over the desired speed, altitude, weight and cg range.
METHOD 2
1) Stabilize and trim at desired flight condition.
2) Reduce PLA gradually to decelerate to VTRIM-15%. Use aft pitch stick input as required to maintain nearly constant altitude.
3) Then increase PLA gradually to accelerate to VTRIM+15%. Use forward pitch stick as required to maintain nearly constant altitude.
4) Record data during all runs and repeat test over the desired speed, altitude, weight and cg range.
DATA REQUIREMENTS
Trim Conditions:
1) Configuration,
2) Weight,
3) Center of Gravity,
4) Pressure Altitude.
Test Variables:
1) CAS,
2) Elevator and longitudinal control position,
3) Control Force.
4) PLA
DATA ANALYSIS
1) At each test point a plot of ëPS versus CAS should be made. From this plot, the local slope of ëPS versus CAS should be determined (figure 2-2.)
2) The stick force gradients are then plotted versus cg location with different symbols for each of the different Mach ranges being tested. By showing that dëPS/dV is negative at all test points, we have shown that positive stability exists throughout the flight envelope.
3) To determine the location of the aircraft neutral point, extrapolate the trend of dëPS/dV versus cg location, the neutral point for each Mach range can then be determined.