Principles of Flight
What the exam tests
Principles of flight tests lift, drag, stability, control, high-speed flight and load factor. Iran CAO questions often use simple numerical relationships.
Core rules
- Lift = CL × ½ρV²S; induced drag falls with speed, parasite drag rises with V².
- L/D max occurs where induced drag equals parasite drag; it gives maximum glide range.
- Stall speed rises with √(weight): +21% weight → +10% Vs.
- Load factor n = L/W; accelerated stall speed rises with √n.
- Swept wings delay Mach critical but need high-lift devices; Mach tuck and shock stall are high-speed traps.
Worked example
An aircraft stalls at 100 kt at 2,000 kg. At 2,420 kg, Vs = 100 × √(2,420/2,000) = 100 × √1.21 = 110 kt. In a 45° bank, n = 1/cos45° = 1.41, so Vs = 110 × √1.41 ≈ 131 kt. Reasoning: both weight and load factor increase the lift required; the square-root relationship is the quick check.
Traps
Using linear weight scaling for stall speed; confusing Vx and Vy; assuming L/D max changes with weight (the speed changes, the ratio does not); ignoring Mach effects at high altitude; treating induced drag as constant with speed.