---
title: "Terms associated with the aerofoil"
subject: "Fluid Mechanics"
chapter: 9
key: "semester_3/fluid_mechanics:aerofoil-terms-sketch"
source: https://www.bctengineer.com/q/semester_3/fluid_mechanics/aerofoil-terms-sketch
site: BCT Engineering
---

# Terms associated with the aerofoil

Fluid Mechanics · Chapter 9: Flow past through submerged bodies

- Asked in: 2072 Chaitra, 2 marks
- Video: [What is an Airfoil? | Understanding some Terms and Definitions related to an Airfoil!](https://www.youtube.com/watch?v=6wgrDBa-AXo) — JxJ AVIATION
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/fluid_mechanics/aerofoil-terms-sketch
- Whole chapter: https://www.bctengineer.com/md/semester_3/fluid_mechanics/chapter-9

## Question

Define the terms associated with the Aerofoil with neat sketch.

## Short answer

Chord line joins leading and trailing edges; chord length is the reference dimension. Camber is the maximum distance between the camber line and chord line. Angle of attack is the angle between chord line and freestream. Span is wing length perpendicular to flow. Stall angle is where lift suddenly drops.

## Answer

An aerofoil (airfoil) is a streamlined body shaped to produce lift with low drag when placed in a flow, as in wings and blades.

```text
              chord line  c
      leading  <------------------------->  trailing
       edge    ___________________          edge
         (    /       camber line  \___
          \  /_________ mean line ______>
           \/___________________________/
   angle of attack a: between chord line and flow
 flow ->
```

- **Leading edge**: the front, rounded edge of the aerofoil.
- **Trailing edge**: the rear, sharp edge where the flow leaves.
- **Chord line**: the straight line joining the leading and trailing edges.
- **Chord length ($c$)**: the length of the chord line; it is the reference length for the Reynolds number.
- **Camber line (mean line)**: the line midway between the upper and lower surfaces. Camber is its maximum distance from the chord line.
- **Thickness**: the maximum distance between upper and lower surfaces, usually given as a percentage of chord.
- **Angle of attack ($\alpha$)**: the angle between the chord line and the direction of the undisturbed flow.
- **Span ($b$)**: the length of the wing perpendicular to the flow.
- **Aspect ratio**: $AR = b^2/S$ (span squared over plan area).
- **Stall angle**: the angle of attack at which the flow separates and lift drops suddenly.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the chord line of an aerofoil?
   A. The line midway between the upper and lower surfaces
   B. The maximum distance between the upper and lower surfaces
   C. The line along the wing span
   D. The straight line joining the leading and trailing edges
   Answer: D. The chord line joins the leading edge to the trailing edge. The line midway between surfaces is the camber (mean) line. (Key point: Chord line)

2. How is the angle of attack defined?
   A. Angle between the chord line and the direction of the undisturbed flow
   B. Angle between the camber line and the chord line
   C. Angle between the upper surface and lower surface
   D. Angle between the span and the flow
   Answer: A. Angle of attack alpha is measured from the chord line to the undisturbed flow direction. (Key point: Angle of attack)

3. How is the aspect ratio of a wing defined?
   A. AR = c/b, chord over span
   B. AR = b^2/S, span squared over plan area
   C. AR = S/b^2, plan area over span squared
   D. AR = t/c, thickness over chord
   Answer: B. Aspect ratio is span squared divided by plan area. Thickness over chord is the thickness ratio. (Key point: Aspect ratio)

4. What happens at the stall angle?
   A. The drag becomes zero
   B. The flow separates and the lift drops suddenly
   C. The lift reaches zero smoothly
   D. The chord length is reduced
   Answer: B. Beyond the stall angle the flow separates from the upper surface, causing a sudden loss of lift. (Key point: Stall angle)