Adverse Yaw

Concept: Adverse yaw is the initial tendency of an aircraft to yaw in the direction opposite to the roll command when ailerons are deflected. For example, initiating a roll to the right with ailerons often causes the nose to initially yaw slightly to the left. This is primarily governed by the yawing moment due to aileron deflection derivative (Cn_da).

Physical Origin: When ailerons are deflected to initiate a roll (e.g., right aileron up, left aileron down for a right roll), the downward-deflected aileron (left wing) increases the lift on that wing, but also increases its induced drag. The upward-deflected aileron (right wing) decreases lift but also slightly decreases induced drag (or increases it less). This difference in drag between the wings creates a yawing moment that initially pulls the nose towards the upward-moving wing (left yaw for a right roll command). Wing sweep and other design factors also influence this effect.

Flight Test Proposal:

1. Setup:

   • Select an aircraft (results may be more pronounced on aircraft with less sophisticated aileron designs or significant wingspan).

   • Configure mission for cruise speed and safe altitude.

   • Set data recording window (e.g., 0-20 seconds).

   • Trim for stable, wings-level flight.

2. Excitation:

   • Apply a sharp, distinct aileron input (e.g., tap 'P' or 'O' firmly) and hold it for a few seconds.

3. Hands-Off (Rudder): Keep the rudder strictly neutral.

4. Observation & Recording: Carefully observe the initial yawing motion immediately after the aileron input. Does the nose swing opposite to the roll direction before eventually coordinating? Record the data.

Data Analysis:

1. Load Data: Use the visualization tool.

2. Key Plots: Focus on the first 1-3 seconds after the aileron input.

   • Yaw Rate (r_yaw_rate) vs. Time: Look for an initial "spike" or movement in the yaw rate opposite to the direction expected for a coordinated turn. (E.g., for a right roll command (roll_demand > 0 or negative depending on convention), look for an initial negative r_yaw_rate).

   • Roll Control (roll_demand_attained) vs. Time: To correlate the yaw motion with the aileron input timing.

   • Sideslip Angle (beta_DEG) vs. Time: Adverse yaw is often accompanied by an initial sideslip.

   • (Optional) Moment Coeffs (Control) Plot: If available, the CM_yaw_from_control trace should show the yawing moment generated by the ailerons.

3. Identify Parameters:

   • Sign of Initial Yaw Rate: Confirm if the initial r_yaw_rate is opposite to the direction of the commanded roll.

   • Magnitude: Qualitatively assess the strength of the adverse yaw effect. Compare r_yaw_rate magnitude to the p_roll_rate achieved.

References:

• Most standard aerodynamics and flight dynamics textbooks cover adverse yaw in sections on roll control and lateral-directional coupling.

• Nelson, R. C. (1998). Flight Stability and Automatic Control.

• Perkins, C. D., & Hage, R. E. (1949). Airplane Performance Stability and Control.