Lecture on Cruise Flight Dynamics

Introduction

• Cruise is a relatively low-effort phase of flight.
• Important for pilots as it involves maintaining direction and altitude.
• Despite its simplicity compared to takeoff and landing, there are key performance considerations.

Basic Principles

• Forces in Flight: Lift equals weight, thrust equals drag.
• In diagrams, these forces act through the aircraft's center.
• In real life, forces act at different points:
  • Lift: Acts through the center of pressure (CFP).
  • Weight: Acts through the center of gravity (CG).
• Typically, CG is in front of CFP, aiding stability in commercial planes.

Stability and Forces Interaction

• The lift-weight couple causes a nose-down pitching moment.
• Counteracting Nose-Down Moment:
  • Achieved by creating downforce at the tail.
  • Requires additional lift to counteract this.
• Engine and Drag Dynamics:
  • Engines below wings create a nose-up rotation.
  • Helps counteract lift-weight couple, reducing tail downforce needs.

Cruise Flight Balances

• Forces Equation:
  • Lift = Weight + Downforce
  • Thrust = Drag
• Moments must be balanced (nose-up vs. nose-down).
• Trim drag arises from the need to adjust horizontal stabilizer.

Fuel Efficiency Considerations

• Center of Gravity (CG) and Fuel Burn:
  • Forward CG leads to higher fuel consumption due to longer moment arm.
  • Rearward CG reduces fuel burn.
• Some aircraft adjust CG by moving fuel during cruise.

Cost Index and Cruise Speed

• Cost Index: Balances fuel burn and flight speed.
• Theoretical maximum speed is where thrust equals drag.
• Factors like altitude affect thrust available and thus cruise speed.

Summary

• Cruise involves balancing forces and moments.
• Position of CG significantly impacts fuel efficiency.
• Understanding interactions of thrust, drag, and moments is crucial for optimal cruise performance.