Quick questions on The four forces of flight: HSC Engineering Studies Aeronautical Engineering

In steady level flight (constant altitude, constant airspeed, no acceleration), both pairs of forces balance:

In a climb at angle $\theta$ at constant airspeed, the forces along the flight path and perpendicular to it must each balance:

In a descent at angle $\theta$ at constant airspeed:

If forces are not balanced, Newton's second law gives:

Qantas operates Boeing 737s on domestic routes and Boeing 787, Airbus A330 and Airbus A380 on international routes. The new Boeing 787-9 has a quoted cruise $L/D$ of about 21 (composite wing, advanced aerofoil design). The Royal Australian Air Force operates Boeing F/A-18F Super Hornets and Lockheed F-35A Lightning II, which trade $L/D$ for supersonic capability and manoeuvrability.

Only at constant speed. During acceleration (takeoff, climbout, descent), the net force is non-zero.

Always use $\sin\theta$ and $\cos\theta$ components when the flight path is not horizontal. :::