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.
Asymmetric Thrust Handling and Control
In all aircraft with more then one engine, most specifications contain requirements which pertain to the stability and control or handling qualities when one or more engine has failed. All specification documents should be reviewed to determine the requirements for engine failure. Two of the most critical requirements will be discussed in the following paragraphs.
Airborne Minimum Control Speed
REQUIREMENT
Airborn minimum control speed is defined as the lowest safe speed which is required to maintain straight line flight at a constant altitude when the most critical outboard engine has failed. Limits on the allowed control forces and bank angle are typically specified. This must be demonstrated for several flight conditions, weights, and configurations.
In cruise configuration it should be possible to balance out the yawing moment produced by the loss of thrust using bank angle only (rudder free). In power approach configuration, any combination of bank angle or sideslip may be used up to limits on rudder pedal and roll stick forces which may be restricted by the specification. MIL-8785C requires that rudder pedal force not exceed 180 lbs, and roll stick forces not exceed values which depend on aircraft class and controller type.
TEST PROCEDURE
Power Approach Configuration:
1) Put the aircraft into takeoff configuration.
2) Trim for symetric flight.
3) Operate engines at maximum take-off power.
4) Simulate failure of most critical outboard engine.
5) Maintain a steady straight path without using more than the specified bank angle and without reducing thrust on live engines or operating propellor or trim controls.
6) Repeat tests at sucessively lower speeds until one of the following conditions has been reached:
maximum rudder deflection,
maximum specified rudder pedal force,
a dangerous attitude,
stall.
Power On Clean Configuration:
1) Trim aircraft in clean configuration with normal rated power applied.
2) Simulate failure of most critical outboard engine.
3) Maintain straight flight by banking and sideslipping with the rudder pedals left free.
4) Repeat at various flight conditions throughout the flight envelope.
DATA REQUIRED
Trim Conditions:
1) Configuration,
2) Weight,
3) Center of Gravity,
4) Trim CAS
Test Variables:
1) CAS,
2) Sideslip angle ( ),
3) Bank Angle (è)
4) Aileron Deflection (ëa),
5) Rudder Deflection (ër),
6) Lateral Stick Force (FA),
7) Rudder Pedal Force (FR)
DATA ANALYSIS
1) Time histories of all data for the transients should be presented.
2) Crossplots showing bank angle, rudder pedal force, roll stick force, aileron deflection, and rudder deflection versus airspeed should be made for each configuration and altitude.
3) Determine VMC from criteria listed above.
4) Summary plots showing VMC as a function of altitude for all configurations should be made.
Ground Minimum Control Speed
Ground minimum control speed (VMCG) is the equivalent to the air minimum control speed for a refused takeoff. This is essentially the maximum speed at which the pilot can maintain control of the aircraft within specifications while it is still on the ground.
REQUIREMENT
The requirement for VMCG is very similar to that of VMC. The pilot must be able to maintain control of the aircraft and bring it to a stop on the runway with the most critical engine failed at some velocity during the takeoff roll. There are typically criteria set on rudder pedal force and offset from the center line of the runway. MIL-8785C requires that the aircraft be able to be brought to a stop with no more than 180 pounds of rudder pedal force and a deviation from the intended path of no more than 30 feet. Depending on the specification used, use of differential braking and nose wheel steering may or may not be allowed to maintain runway alignment.
TEST PROCEDURE
1) Configure aircraft for takeoff.
2) Begin takeoff run and accelerate up to the intended test speed.
3) At the speed to be tested, fail the most critical engine.
4) Using controls as required within limits specified, attempt to maintain aircraft on center line of runway.
5) Repeat as required until the speed has been reached where control can be maintained within the limits required.
DATA REQUIRED
Initial Conditions:
1) Configuration,
2) Weight,
3) Center of Gravity,
4) Trim CAS
Test Variables:
1) CAS
2) Rudder Pedal Force
3) Runway centerline offset
DATA ANALYSIS
1) Prepare time histories of data and determine when the specified conditions have been exceeded for VMCG.
2) Tabulate VMCG for all flight configurations, weights, and cg loadings.