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Load Factor, Flight Envelope & Lift Augmentation
Load factor and manoeuvre stall speed, the V-n diagram and manoeuvring speed, and flaps/slats as lift-augmentation devices.
Load factor & flight envelope — summary notes
FreeMain ideas
- Load factor n = L/W, expressed in 'g'. Straight and level flight is 1 g; in a balanced level turn n = 1/cos φ, so a 60° bank needs 2 g — the wing (and everything in the aircraft) effectively weighs double.
- Raising the load factor raises the stall speed: V_s(n) = V_s1g·√n. At 2 g the stall speed is √2 ≈ 1.41 times the 1 g value, so a steep turn can provoke an accelerated stall well above the normal stalling speed.
- The V-n (flight-envelope) diagram plots airspeed against load factor. It is bounded below the manoeuvre by the aerodynamic stall (C_Lmax line), above by the structural limit load factor (e.g. +2.5 g / −1.0 g for a transport), and at high speed by V_NE and gust lines.
- Manoeuvring speed V_A is the speed at which the aircraft reaches limit load exactly as it stalls; below V_A full control deflection makes the wing stall before it can overstress the structure, so V_A is a protective speed — above it, full deflection can break the airframe.
- Lift-augmentation devices lower the take-off and landing speeds: trailing-edge FLAPS add camber to raise C_Lmax and lower the stall speed (also adding drag); leading-edge SLATS and SLOTS re-energise the boundary layer to delay separation to a higher AoA; combined, they let the wing fly slower.
- ⚠ Exam trap: flaps raise C_Lmax and lower the stall speed but do NOT increase the maximum angle of attack — slats/slots do that. And exceeding V_A with full/abrupt control inputs can overstress the airframe; the stall only protects you below V_A.
Key formulas
- Load factor
- n = L / W
- Load factor in a level turn
- n = 1 / cos φ
- Stall speed under load
- V_s(n) = V_s1g · √n
Solved examples
An aircraft stalls at 60 kt in level flight. What is its stall speed in a steady 60° banked level turn?
A 60° bank gives n = 1/cos 60° = 2 g. Stall speed rises by √n = √2 ≈ 1.41, so V_s = 60 × 1.41 ≈ 85 kt. The extra lift needed to turn brings the wing to its stalling angle at a much higher speed — the accelerated stall.
Load factor & envelope concept map
FreeLoad Factor & Flight Envelope
Load factor & flight envelope quiz
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