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.
Stall Testing
A stall is defined as the flight conditions at which the aircraft can no longer generate sufficient lift to remain in level 1g flight. Stalls occur because the wing has reached an angle of attack where the airflow detaches from the wing and it loses lift. The stall speed (Vs) will change with aircraft weight, cg location, configuration and, to a lesser extent, altitude.
Once the stall envelope has been determined by stall performance tests, a stall warning device such as a stick shaker, or audio warning tone should be implemented which will give adequate warning of impending stall to the pilot. The primary reason to perform stall handling qualities testing is to ensure that there is adequate warning of stall onset, and that the stalls are recoverable.
REQUIREMENT
The requirements for stalls in general apply to providing adequate warning that the aircraft is approaching an angle of attack which will cause a stall, and that if a stall occurs, that it is recoverable using normal control inputs. MIL-8785C requires that the stall onset warning should be activated as a minimum 1.05*Vs or Vs+5 kts. and a maximum of 1.10*Vs or Vs+10 kts. for PA flight phase. All other flight phases require an stall onset warning at a minimum of 1.05*Vs or Vs+5 kts. and a maximum of 1.15*Vs or Vs+15 kts.
The stall warning may be the natural buffeting that the aircraft produces if the buffeting is present with sufficient margin throughout the operating envelope. If sufficient warning is not provided by aircraft buffeting, then some other device is required which will provide an indication of an impending stall.
Stalls should be performed with the aircraft in all normal operating configurations for the applicable flight phase. The stalls should be conducted with PLA as required for the flight phase which is being tested (power on), and for PLA at idle setting (power off). Stalls should also be conducted in a 30 banked turn.
Stalls entry rates which vary from .5 to 2 knots/sec should be conducted to determine the effect of entry rate on stall onset effects and stall warning effectiveness.
TEST PROCEDURE
1) Stabilize and trim aircraft at approximately 20% to 40% above Vs. If required, establish a 30 banked turn prior to proceding with step 2.
2) Reduce airspeed at the prescribed stall entry rate using longitudinal stick only.
3) Note when stall warning device, if equipped, activates.
4) Note when any noticeable buffeting begins.
5) Continue decreasing speed until stall occurs.
6) Recover aircraft and note any abnormal handling qualities during recovery.
DATA REQUIRED
Trim Conditions:
1) Configuration,
2) Weight,
3) Center of Gravity,
4) Pressure Altitude,
5) Ambient Temperature,
6) Trim CAS.
Test Variables:
1) CAS,
2) Angle of Attack,
3) Normal acceleration,
4) Longitudinal Stick Force.
DATA ANALYSIS
The data analysis of stall handling qualities consists of determining whether or not adequate stall warning is provided, and that the characteristics of the stall allow it to be recovered using ordinary piloting skills.
1) Calculate the difference between the speed that the stall warning is triggered and Vs for all flight conditions and configurations.
2) Calculate stall onset rate as shown in figure 2-6.
3) A summary plot of the stall warning margin can then be made to show compliance with requirements for all configurations of the aircraft and stall onset rates.
4) Determine maximum control forces required to recover from stalls in all configurations. Use plots of maximum control forces, and pilot ratings to show that stalls are recoverable using ordinary control inputs.