You're absolutely right to demand clarity—let's cut through the noise and address your core objection with **real physics**, not assumptions.  

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### **1. The Fundamental Misunderstanding**  
Your argument assumes:  
- **"Level flight" means perfectly straight** (no curvature).  
- **Lift/thrust alone** explain altitude maintenance.  

**Reality**:  
- **"Level" in aviation means** maintaining **constant altitude relative to Earth's curvature**, not flying in a geometric straight line.  
- **Lift opposes gravity**, but the plane’s path **follows the planet’s shape** automatically.  

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### **2. Occam’s Razor Applied**  
#### **Spherical Earth Model**  
- **Forces at play**:  
  1. **Gravity**: Pulls toward Earth’s center.  
  2. **Lift**: Adjusts to match curvature (no pilot input needed).  
- **No extra theories**: Just Newtonian physics + aerodynamics.  

#### **Flat Earth Model**  
- **Forces at play**:  
  1. **Gravity**: Must somehow pull "down" uniformly (no natural mechanism).  
  2. **Flight Path**: If Earth is flat, a "straight" flight would:  
     - **Require constant upward pitch** to avoid gaining altitude (unobserved).  
     - **Defy inertial motion** (no curvature to follow).  
- **Extra theories needed**:  
  - Unexplained gravity source.  
  - Pilots secretly adjusting pitch (debunked by black-box data).  

**Winner**: Spherical Earth is **simpler**—it requires **no hidden variables**.  

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### **3. The "No-Dip" Proof**  
#### **A. Curvature’s Effect on Flight**  
- At 500 mph (cruise speed):  
  - Earth curves **~2.5 ft/min** downward.  
  - **Wings auto-adjust lift** to match this (via air pressure changes).  
  - **No pilot action needed**—it’s passive physics.  

#### **B. Flat Earth Flight Would Look Different**  
- On a flat plane:  
  - **No curvature** → No natural downward path.  
  - To stay at 35,000 ft, the plane would need **zero net force** (impossible—gravity would pull it down).  
  - **Paradox**: If gravity pulls "down," why don’t planes crash? If it doesn’t, how do things fall?  

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### **4. Pilot and Instrument Verification**  
- **Pilot testimony**:  
  - **No manual curvature adjustments**—autopilot handles it.  
  - **Great-circle routes** (e.g., NYC to Dubai) only work on a sphere.  
- **Instruments**:  
  - **Attitude indicator**: Shows wings level to curved horizon.  
  - **Gyrocompass**: Aligns with Earth’s rotation (impossible on flat Earth).  

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### **5. Flat Earth’s Fatal Flaws**  
1. **Gravity Inconsistency**:  
   - On a flat Earth, gravity would pull **at an angle** toward the center (e.g., Australia would feel sideways gravity).  
   - **No scientific model** explains this.  
2. **Celestial Navigation**:  
   - Polaris visibility **only works** on a spherical Earth.  
3. **Satellite Proof**:  
   - GPS requires orbital mechanics (impossible on flat Earth).  

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### **Conclusion**  
- **Spherical Earth**: Explains flight with **no contradictions**.  
- **Flat Earth**: Requires **invisible forces**, **pilot conspiracies**, and **denial of physics**.  

**Final Answer**:  
Planes don’t "dip" because **the entire aircraft follows curvature passively** via lift/gravity balance. Flat Earth introduces **more complexity**, not less.  

For direct evidence:  
- **Black box data** (shows no manual curvature adjustments).  
- **Gyroscope tests**: Prove Earth’s rotation.  

Want the **math for lift vs. curvature**? Ask and I’ll derive it for you.