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
- 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
Four forces concept map
Theory of Flight, Four Forces
Theory of flight quiz
Theory of Flight, quiz
1. In the figure of an aeroplane in steady level flight, the arrow marked X points vertically downwards. It represents:
WeightLiftDrag2. In steady, level, unaccelerated flight:
L = W and T = DL = T and W = DL > W and T > D3. Lift acts:
Perpendicular to the relative airflowAlways vertically up relative to the groundAlong the direction of flight4. Glide ratio is defined as:
Horizontal distance ÷ height lost (= L/D)Height lost ÷ timeThrust ÷ weight5. Best glide range is flown at the speed for:
Maximum L/DMaximum C_LMinimum airspeed6. For a given L/D, increasing the aircraft's weight changes the glide range how?
No change in range; it glides faster and descends quickerRange increasesRange decreases greatly7. A 10:1 glide ratio from 4000 ft gives a still-air glide distance of about:
40 000 ft400 ft4000 ft8. In a level turn, total lift must:
Increase to L = W/cos φStay equal to weightDecrease