Stall Speed Determination

Concept: The stall speed is the minimum steady airspeed at which an aircraft can maintain level flight. Below this speed, the wing's angle of attack (AoA) exceeds the critical angle, causing airflow separation, a dramatic loss of lift, and usually an increase in drag. Stall speed isn't a single fixed value; it increases with weight, load factor (e.g., in turns), and altitude (as True Airspeed, TAS, though Equivalent Airspeed, EAS, stall speed is less affected by altitude). This test focuses on the 1g stall speed in level flight (Vs).

Physical Origin: Lift is generated by the pressure difference created by airflow over the wing's airfoil shape, which is dependent on airspeed and the angle of attack. For a given weight in level flight, a specific amount of lift is required. As airspeed decreases, the pilot must increase the angle of attack (by pulling back on the controls/elevator) to maintain the required lift. However, every airfoil has a critical angle of attack (typically 15-20 degrees) beyond which the airflow can no longer follow the wing's upper surface smoothly. The flow separates, turbulence increases drastically, lift drops sharply, and drag often increases significantly. This condition is the aerodynamic stall. The speed at which this occurs while trying to maintain level flight at 1g is the stall speed.

Flight Test Proposal:

1. Setup:

   • Select an aircraft model.

   • Configure mission for a safe starting altitude (e.g., 1500m or higher) to allow recovery room.

   • Set data recording window (e.g., 0-60 seconds or longer if deceleration is very slow).

   • Start at a moderate speed, well above the expected stall speed.

2. Procedure (Careful Deceleration):

   • Trim the aircraft for stable, level flight (maintain constant altitude).

   • Gradually reduce thrust using the number keys (e.g., step down from '6' to '5' to '4'...).

   • As the aircraft decelerates, smoothly increase pitch attitude using the elevator ('A' key) as needed to maintain level flight (keep Vertical Speed Indicator VSI_ms close to zero). This is the crucial part – maintain altitude while speed decreases.

   • Continue reducing speed slowly while maintaining altitude.

3. Stall Recognition & Recovery:

   • Observe the aircraft's behavior and the data readouts (especially AoA and speed). The stall may be indicated by:

     • A sudden drop of the nose (pitch break).

     • Buffeting or shaking (not simulated visually, but implied by physics).

     • A rapid increase in descent rate (VSI_ms becomes significantly negative) even with full up elevator.

     • Angle of attack (alpha_DEG) reaching its maximum value or exceeding typical critical angles (e.g., > 15-18 deg).

     • Lift Coefficient (CL) reaching its maximum value (CL_max) and potentially decreasing afterwards.

   • As soon as stall is clearly identified, initiate recovery: Apply full thrust and reduce the angle of attack (neutralize or apply forward elevator 'Q') to regain airspeed and control. While recovery isn't the focus of data analysis here, it's good practice.

4. Recording: Ensure data is recorded throughout the deceleration and stall entry.

Data Analysis:

1. Load Data: Use the visualization tool.

2. Key Plots:

   • Altitude (ALTITUDE_m) vs. Time: Verify that altitude was kept reasonably constant during the deceleration phase leading up to the stall.

   • True Airspeed (TAS) or Equivalent Airspeed (EAS) vs. Time: Observe the gradual decrease in speed.

   • Angle of Attack (alpha_DEG) vs. Time: Watch the AoA increase steadily as speed decreases, then potentially increase rapidly or fluctuate at the stall.

   • Lift Coefficient (CL) vs. Time: Should increase as AoA increases, reach a peak (CL_max), and then potentially drop or plateau at the stall.

   • Vertical Speed (VSI_ms) vs. Time: Should be near zero during the approach and then show a distinct increase in descent rate at the stall.

   • Pitch Rate (q_pitch_rate) vs. Time: May show oscillations or a sharp nose-down pitch rate at the stall.

3. Identify Stall Speed:

   • Pinpoint the time at which the stall occurs using indicators like the peak CL, peak alpha_DEG, sudden increase in descent rate, or pitch break.

   • Read the Equivalent Airspeed (EAS) value from the data at that precise moment. This is the best representation of the 1g stall speed (Vs), as it's less affected by density changes (altitude) than TAS. If EAS is unavailable, TAS can be used but should be noted along with the altitude/density.

References:

• Standard aerodynamics textbooks covering lift, angle of attack, airfoils, and stall characteristics.

• Pilot training manuals often have sections on stall recognition and recovery.

• Anderson, J. D. (2016). Fundamentals of Aerodynamics. McGraw-Hill Education.

• Dole, C. E., Lewis, J. E., Badick, J. R., & Johnson, B. A. (2017). Flight Theory and Aerodynamics: A Practical Guide for Operational Safety. Wiley.