Subtopic: Control Surfaces (Elevator, Aileron, Rudder)
Question Statement:
A general aviation airplane is in steady and level flight. The airplane is prone to
adverse yaw. Which one of the following options best describes the deflections of
aileron and rudder to achieve a coordinated right turn?
Options:
(A) Left aileron: down; Right aileron: up; Rudder: left
(B) Left aileron: down; Right aileron: up; Rudder: right
(C) Left aileron: up; Right aileron: down; Rudder: left
(D) Left aileron: up; Right aileron: down; Rudder: right
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Detailed Step-by-Step Solution (GATE AE 2025 Q.22)**
**1. Intuitive Physical Setup & Given Parameters**:
- Aircraft is in steady and level flight.
- Aircraft is prone to **adverse yaw** during roll initiation.
- The pilot desires to execute a **coordinated right turn**.
- We need to determine the proper deflections of the left aileron, right aileron, and rudder.
**2. Step-by-Step Flight Mechanics Derivation**:
1. **Aileron Deflection for Right Roll**:
- To bank (roll) to the right, lift must increase on the left wing and decrease on the right wing:
- **Left aileron deflects DOWN**: Increases effective camber and lift on the left wing.
- **Right aileron deflects UP**: Decreases effective camber and lift on the right wing.
2. **Understanding Adverse Yaw**:
- The downward-deflected left aileron increases lift, which simultaneously increases induced drag on the left wing ($C_{Di} \propto C_L^2$).
- The upward-deflected right aileron decreases lift and induced drag on the right wing.
- This differential drag creates a yawing moment that points the nose to the **LEFT** (opposite to the desired right turn), known as *adverse yaw*.
3. **Coordinating the Turn with Rudder**:
- To counteract adverse yaw and coordinate the right turn (preventing sideslip), the pilot must apply **right rudder** (rudder trailing edge deflected to the right).
- Right rudder creates a side force to the left on the vertical tail, generating a restoring yawing moment to the **right**.
4. Combined Control Surface Deflections:
- Left aileron: **down**
- Right aileron: **up**
- Rudder: **right**
**3. Comprehensive Options & Distractor Analysis**:
- **(A) Left aileron: down; Right aileron: up; Rudder: left**: ❌ **INCORRECT**. Rudder left would amplify the adverse yaw to the left.
- **(B) Left aileron: down; Right aileron: up; Rudder: right**: ✅ **CORRECT**. Banks right (left down, right up) and applies right rudder to eliminate adverse yaw.
- **(C) Left aileron: up; Right aileron: down; Rudder: left**: ❌ **INCORRECT**. This rolls the aircraft to the left.
- **(D) Left aileron: up; Right aileron: down; Rudder: right**: ❌ **INCORRECT**. Left roll with right rudder.
**4. Official Master Key & Exam Takeaway**:
- **Official Answer Key**: **B**
- **Exam Takeaway**: In any roll, the downgoing aileron creates higher induced drag causing adverse yaw towards the rising wing. Coordinated flight always requires rudder input in the direction of the turn.