Adverse yaw is a fundamental aerodynamic phenomenon where an aircraft's nose tends to yaw in the opposite direction of an initiated roll. It happens primarily due to the difference in lift and drag generated by each wing during a turn.
Understanding Adverse Yaw
When a pilot wants to roll an aircraft, they use ailerons, which are control surfaces on the trailing edge of the wings. To initiate a roll to the right, for example:
- The aileron on the right wing deflects upward.
- The aileron on the left wing deflects downward.
This action causes the left wing to generate more lift, rising, and the right wing to generate less lift, lowering, thus creating the desired roll. However, this difference in lift comes with an associated difference in drag.
The Role of Lift and Drag Imbalance
The core of adverse yaw lies in the aerodynamic forces at play:
- Increased Drag on the Rising Wing: When the aileron on the left wing (the one lifting) deflects downward, it increases the wing's camber and its effective angle of attack. This generates significantly more lift, but critically, it also creates a substantial increase in induced drag on that wing. Induced drag is the part of the drag that is a byproduct of lift generation.
- Decreased Drag on the Lowering Wing: Conversely, the aileron on the right wing (the one lowering) deflects upward. This decreases its camber and effective angle of attack, reducing lift and, consequently, reducing induced drag on that wing.
This imbalance means the wing generating more lift (and rising) experiences considerably more drag than the wing generating less lift (and lowering). The increased drag on the rising wing pulls it backward, while the relatively lower drag on the lowering wing causes it to move forward. This differential drag creates a yawing moment that pulls the aircraft's nose opposite to the direction of the roll.
Consider the example of a right roll:
| Wing Section | Aileron Position | Lift Effect | Drag Effect (Induced) | Yaw Contribution |
|---|---|---|---|---|
| Left Wing | Down | Increase | Increase | Pulls Backward |
| Right Wing | Up | Decrease | Decrease | Pulls Forward |
This backward pull on the left wing during a right roll causes the nose of the aircraft to yaw to the left—adversely to the intended right turn.
Impact on Flight Control
Adverse yaw is considered an undesirable characteristic because it complicates coordinated turns. If not corrected, it can lead to:
- Skidding: The aircraft's nose points outside the direction of the turn.
- Reduced Efficiency: The aircraft flies less aerodynamically, increasing drag and fuel consumption.
- Pilot Workload: Pilots must constantly anticipate and correct for this tendency.
Solutions and Mitigation Strategies
Aircraft designers and pilots employ several techniques to counteract adverse yaw:
1. Rudder Input (Pilot Technique)
The most common and fundamental solution is for the pilot to apply coordinated rudder input. When initiating a roll, the pilot simultaneously applies rudder in the same direction as the intended roll. This creates a yawing moment that counteracts the adverse yaw. For example, during a right roll, right rudder is applied.
- Coordination: This skill is critical for smooth, efficient turns and is a fundamental part of flight training.
- Resources: For more on coordinated flight, consult resources like the FAA's Pilot's Handbook of Aeronautical Knowledge.
2. Differential Ailerons
This design feature causes the upward-moving aileron to deflect through a greater angle than the downward-moving aileron.
- Mechanism: The upward-moving aileron (which reduces lift) creates more drag than a symmetrically deflected aileron would.
- Result: This increases drag on the descending wing, helping to balance the drag on the ascending wing, thereby reducing the adverse yawing moment.
3. Frise Ailerons
Frise ailerons are designed so that when the aileron on one wing deflects upward, its leading edge protrudes into the airflow below the wing.
- Mechanism: This protrusion creates additional drag on the wing with the upward-deflected aileron (the descending wing).
- Result: Similar to differential ailerons, this added drag helps to equalize the drag between the wings, minimizing adverse yaw.
4. Coupled Controls
In some aircraft, particularly older or more basic models, the aileron and rudder controls may be mechanically linked or "coupled."
- Mechanism: Moving the control stick (for ailerons) automatically applies a small amount of rudder in the appropriate direction.
- Result: This simplifies coordinated turns for the pilot, though it may limit fine control.
By understanding the underlying cause—the difference in lift and drag between wings during a roll—pilots and designers can effectively manage and mitigate adverse yaw, ensuring safer and more efficient flight.
