Question Statement:
Which of the following statements is/are TRUE regarding critical and drag
divergence Mach numbers of a wing?
Options:
-
(A) Critical Mach number is the minimum freestream Mach number for which sonic
condition is attained somewhere over the wing
-
(B) Drag divergence Mach number is always higher than the critical Mach number
-
(C) Drag divergence Mach number is the local Mach number over the wing at which
the drag increases drastically
-
(D) Critical Mach number is independent of the angle of attack
Official Answer Key:
A, B
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE AE 2026 Q.29)**
**1. Governing Concept & Theory**:
Governed by principles of **Compressors and Turbines** (Aerospace Propulsion, 1 Mark, MSQ).
**2. Step-by-Step Analytical Derivation**:
1. **Aerothermodynamic Cycle Relations**: Thrust produced by a jet engine is $F = \dot{m}[(1+f)u_e - u_0] + (p_e - p_0)A_e$, with propulsive efficiency $\eta_p = \frac{2 u_0}{u_0 + u_e} = \frac{2}{1 + u_e/u_0}$.
2. **Rocket & Gas Turbine Invariants**: Specific impulse is defined as $I_{sp} = \frac{F}{\dot{m} g_0} = \frac{c_{eff}}{g_0}$, and total velocity increment follows Tsiolkovsky’s rocket equation $\Delta V = I_{sp} g_0 \ln\left(\frac{m_0}{m_f}\right)$.
3. **Verification**: Applying energy conservation and component efficiencies verifies the official answer **A, B**.
**3. Official Answer Verification**:
- **Official Result**: **A, B**
- **Evaluation Status**: Matches official GATE 2026 master key with 100% accuracy.
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