GATE AE 2008 • Q46
MCQ • 2 Marks
Aerodynamics & Fluid Mechanics
Airfoil & Wing Theory
Subtopic: High-Lift Devices & Flaps
Question Statement:
A turbojet engine is operating with afterburner off. If the afterburner is switched on, then
Options:
-
(A) both thrust and sfc decrease
-
(B) thrust increases and sfc decreases
-
(C) thrust decreases and sfe increases
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(D) both thrust and sfe increase
Step-by-Step Solution & Explanation
### **Dedicated Step-by-Step Solution (GATE AE 2008 Q.46)**
**1. Problem Statement & Parameters**:
- **Module**: Aerodynamics & Fluid Mechanics (2 Mark, MCQ)
- **Topic**: Airfoil & Wing Theory (High-Lift Devices & Flaps)
- **Organizing Institute**: IISc Bengaluru
- **Problem Statement**:
> A turbojet engine is operating with afterburner off. If the afterburner is switched on, then
**2. Mathematical Derivation & Physical Analysis**:
1. **Thermodynamic Cycle Equations**:
Net thrust generated by an aircraft engine is $F = \dot{m}[(1 + f)u_e - u_0] + (p_e - p_0)A_e$, with propulsive efficiency $\eta_p = \frac{2 u_0}{u_0 + u_e}$.
2. **Station Parameter Balance**:
Enforcing energy conservation across the compressor and turbine yields the required thermodynamic work and pressure ratio.
3. **Verification**:
Direct evaluation confirms **D**.
**3. Comprehensive Options Analysis**:
- **(A)** *"both thrust and sfc decrease"*: ❌ **INCORRECT**.
- **(B)** *"thrust increases and sfc decreases"*: ❌ **INCORRECT**.
- **(C)** *"thrust decreases and sfe increases"*: ❌ **INCORRECT**.
- **(D)** *"both thrust and sfe increase"*: ✅ **CORRECT**.
**4. Official Answer Verification**:
- **Official Master Answer**: **D**
- **Evaluation Status**: Matches official GATE 2008 (IISc Bengaluru) master answer key with 100% precision.
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