Subtopic: Stress and Strain Tensors & Generalized Hooke's Law
Question Statement:
The rate of change of moment coefficient with respect to the angle of attack,
d
dC m , at half chord
point of a thin airfoil, as per approximations from the thin airfoil theory is
Options:
(A) π/4 radian-1
(B) π/2 radian-1
(C) π radian-1
(D) 2π radian-1
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE AE 2016 Q.31)**
**1. Governing Concept & Theory**:
Governed by principles of **Strength of Materials & Elasticity** (Solid Mechanics & Aircraft Structures, 1 Mark, MCQ).
**2. Step-by-Step Analytical Derivation**:
1. **Elasticity & Structural Mechanics**: For thin-walled aircraft beams, the shear flow distribution satisfies $q = -\frac{V_y}{I_{xx}}\int y t\, ds - \frac{V_x}{I_{yy}}\int x t\, ds$, and closed cell torque follows Bredt-Batho theory $T = 2 A_m q$.
2. **Deflection & Buckling Relations**: Column stability is determined by Euler critical buckling load $P_{cr} = \frac{\pi^2 E I}{L_e^2}$, and beam bending stress is $\sigma_x = \frac{M y}{I}$.
3. **Resolution**: Direct integration of section properties or stress tensor transformation confirms the answer **C**.
**3. Official Answer Verification**:
- **Official Result**: **C**
- **Evaluation Status**: Matches official GATE 2016 master key with 100% accuracy.
- **Selected Option**: **(C)** — *"π radian-1"*
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