Theory of Flight — the Four Forces
Balance of lift, weight, thrust and drag; the glide and glide ratio; climb, descent and the turn.
Theory of flight — summary notes
Free- Four forces act on an aircraft: lift (perpendicular to the relative airflow), weight (down through the CG), thrust (forward) and drag (rearward). In steady, level, unaccelerated flight they balance in two pairs: L = W and T = D.
- Lift and weight, and thrust and drag, act as couples; the tailplane produces the balancing moment, and the CG position sets how much download the tail must carry and how stable the aircraft feels.
- In a glide (no thrust) the aircraft trades height for distance. Glide ratio = L/D = horizontal distance ÷ height lost, so a 10:1 wing glides 10 units forward for every 1 unit down; best glide RANGE is flown at maximum L/D (V_IMD).
- Glide range for a given L/D is INDEPENDENT of weight — a heavier aircraft simply flies the same angle at a higher speed and higher rate of descent, covering the same still-air distance.
- In a climb the excess of thrust over drag lifts the weight: T = D + W·sin γ; in a level turn lift is banked so its horizontal component provides the centripetal force, and total lift must rise to L = W/cos φ.
- ⚠ Exam trap: glide DISTANCE depends only on L/D, not on weight; weight changes only the best-glide speed and the rate of descent. Also L acts perpendicular to the relative airflow, not straight up relative to the ground.
- Steady level flight
- L = W and T = D
- Glide ratio
- L/D = horizontal distance ÷ height lost
- Lift in a turn
- L = W / cos φ
An aircraft with a 12:1 glide ratio suffers a total engine failure at 6000 ft above the terrain in still air. Ignoring wind, roughly how far can it glide?
Glide distance = glide ratio × height lost = 12 × 6000 ft = 72 000 ft ≈ 13.6 statute miles (about 11.8 nm). It is flown at the best-L/D speed; a heavier aircraft would reach the same distance but fly faster and descend more quickly.
The four forces in flight
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Four forces concept map
FreeTheory of Flight — Four Forces
Theory of flight quiz
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