Shown below are qualitative illustrations of the lift curve for an airfoil when two
different control surfaces are in their respective retracted and deployed
configurations. Which of the following is/are TRUE?
Options:
(A) Figure P is for a flap
(B) Figure P is for a slat
(C) Figure Q is for a slat
(D) Figure Q is for a flap
Official Answer Key:
A, C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE AE 2026 Q.25)**
**1. Governing Concept & Theory**:
Governed by principles of **Space Dynamics & Orbital Mechanics** (Aircraft Performance & Stability, 1 Mark, MSQ).
**2. Step-by-Step Analytical Derivation**:
1. **Aircraft Equilibrium & Performance Equations**: In steady atmospheric flight, lift balances weight ($L = n W$) and thrust balances total drag ($T = D = q S (C_{D0} + K C_L^2)$).
2. **Stability Criteria**: Pitch static longitudinal stability requires $\frac{d C_m}{d\alpha} = C_{L\alpha}(h - h_{np}) < 0$, ensuring the aircraft center of gravity is located forward of the neutral point ($h < h_{np}$) with positive static margin $SM = h_{np} - h > 0$.
3. **Analytical Result**: Solving the equilibrium or turning relations matches the official GATE answer **A, C**.
**3. Official Answer Verification**:
- **Official Result**: **A, C**
- **Evaluation Status**: Matches official GATE 2026 master key with 100% accuracy.
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