Which of the following statements about adverse yaw of an airplane is/are correct?
P. It is caused by the difference in induced drag between the two wings when ailerons are deflected.
Q. It causes the airplane to yaw in the direction of the roll.
R. It can be counteracted using differential ailerons or Frise ailerons.
Options:
(A) P and Q only
(B) Q and R only
(C) Q only
(D) P and R only
Official Answer Key:
D
Step-by-Step Solution & Explanation
### **Detailed Step-by-Step Solution (GATE AE 2019 Q.38 / AE Q.28)**
**1. Intuitive Physical Setup & Given Parameters**:
- Adverse yaw is an aerodynamic phenomenon occurring during roll initiation via aileron deflection.
**2. Step-by-Step Aerodynamic Derivation**:
1. **Mechanism of Adverse Yaw (Statement P)**:
- When rolling, the downward-deflected aileron increases wing camber and lift ($C_L$).
- Induced drag coefficient $C_{Di} = \frac{C_L^2}{\pi e AR}$ increases significantly on this rising wing.
- The upward-deflected aileron decreases lift and induced drag on the descending wing.
- The resulting drag differential produces a yawing moment directed *opposite* to the intended turn direction (hence termed **adverse** yaw). Thus, statement P is **CORRECT**.
2. **Direction of Yaw (Statement Q)**:
- Adverse yaw yaws the nose away from (opposite to) the roll direction, NOT in the direction of the roll. Thus, statement Q is **FALSE**.
3. **Design Remedies (Statement R)**:
- Adverse yaw is counteracted by:
- Rudder coordination.
- Differential ailerons (up-going aileron deflects more than down-going aileron).
- Frise ailerons (nose of up-going aileron protrudes into airflow beneath wing, adding parasite drag).
- Thus, statement R is **CORRECT**.
4. Combining statements: **P and R only**.
**3. Comprehensive Options & Distractor Analysis**:
- **(A)** P and Q only: ❌ Incorrect because Q is false.
- **(B)** Q and R only: ❌ Incorrect because Q is false.
- **(C)** Q only: ❌ Incorrect.
- **(D)** P and R only: ✅ **CORRECT**. Both P and R accurately state the aerodynamic cause and design solutions.
**4. Official Master Key & Exam Takeaway**:
- **Official Answer**: **D**
- **Exam Rule**: Adverse yaw = Yaw opposite to roll, caused by differential induced drag, alleviated by Frise/differential ailerons and rudder.