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LESSONS · AERODYNAMICS · 10 MINUTES · AUSTRALIA AND NZ

Turns and load factor.

In a level turn the wing has two jobs: hold the aircraft up and pull it round. Doing both takes more lift than flying straight, and more lift means more load and a higher stall speed.

PREDICT FIRST

You steepen a level turn from 30° of bank to 60°. What happens to the load factor?

Lift has to do two jobs

Straight and level, all of the lift holds the aircraft up. Bank the wings and the lift tilts with them. Part of it still holds the aircraft up, and part of it pulls the aircraft round the turn. To stay level, the part holding you up must still equal the weight, so total lift must grow as the bank steepens.

That extra lift is what you feel pressing you into the seat. Load factor is the total load on the aircraft's structure compared with its weight, in G: a load factor of 2 means the structure carries twice the aircraft's weight.

Because the part holding you up must equal the weight, load factor in a level turn is 1 divided by the cosine of the bank angle: about 1.41 at 45° and 2 at 60°.

The load factor for any aircraft in a coordinated level turn at 60° bank is 2 Gs. The load factor in an 80° bank is 5.76 Gs.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 5, Load factors in steep turns

TRY IT

Bank it and watch the load build

The dashed lines split lift into the part that holds you up, which never changes, and the part that turns you, which grows fast past 45°.

holds you up: 1 Gturns you: 0.58 Gweight 1 Glift 1.15 G
01234560°20°40°60°80°Bank angleLoad factor, G
Load factor
1.15 G
Stall speed
54 kt
An 80 kg pilot feels
92 kg

The shaded band starts at 60°, the approximate maximum bank the FAA handbook gives for the average general aviation aircraft in a level turn.

Why steep turns near the ground catch pilots out

The extra load strains the structure, and it also raises the speed at which the wing stalls.

An increased load factor increases the stalling speed and makes stalls possible at seemingly safe flight speeds.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 5, Load factors

This sudden, unexpected loss of control, particularly in a steep turn or abrupt application of the back elevator control near the ground, has caused many accidents.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 5, Load factors and stalling speeds

PREDICT FIRST

An aircraft stalls at 50 kt wings level. You pull into a manoeuvre that puts 4 G on it. What is its stall speed now?

In a level turn at 60° the load factor is 2, so stall speed is multiplied by the square root of 2, about 1.41: a rise of about 41 percent. The example stall speed in the simulation runs from 40 to 65 kt. If your aircraft's wings-level stall speed from its flight manual falls in that range, set it and see where it lands.

The FAA handbook also gives a practical ceiling for level turns:

For a coordinated, constant altitude turn, the approximate maximum bank for the average general aviation aircraft is 60°.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 5, Load factors in steep turns

The yield factor of the average light plane is reached at a bank of approximately 70° to 75°, and the stalling speed is increased by approximately one-half at a bank of approximately 63°.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 5, Turns

Lock it in.

94 Australian and 94 New Zealand practice questions on aerodynamics. The ones you miss come back sooner.

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SOURCES FOR THIS LESSON

Source-checked, awaiting pilot review. Learning only. Your aircraft's flight manual and your instructor come first. Sources and licences