---
title: "Pressure drag, friction drag, streamlined aerofoil"
subject: "Fluid Mechanics"
chapter: 9
key: "semester_3/fluid_mechanics:pressure-vs-friction-drag-aerofoil"
source: https://www.bctengineer.com/q/semester_3/fluid_mechanics/pressure-vs-friction-drag-aerofoil
site: BCT Engineering
---

# Pressure drag, friction drag, streamlined aerofoil

Fluid Mechanics · Chapter 9: Flow past through submerged bodies

- Asked in: 2074 Asoj, 5 marks
- Video: [Friction and Pressure Drag](https://www.youtube.com/watch?v=9m_8P2bYXZQ) — Rick Sellens
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/fluid_mechanics/pressure-vs-friction-drag-aerofoil
- Whole chapter: https://www.bctengineer.com/md/semester_3/fluid_mechanics/chapter-9

## Question

Distinguish between pressure and friction drags. Explain with sketches, why the aerofoil is designed as streamlined body.

## Short answer

Pressure drag is caused by the front-rear pressure difference from a separated wake, acting normal to the surface. Friction drag arises from viscous shear tangential to the surface. An aerofoil's streamlined shape delays separation, yielding a narrow wake, so pressure drag is very small and total drag is mostly low skin friction, giving high lift-to-drag ratio.

## Answer

Total drag on a body in a fluid is the sum of pressure (form) drag and friction (skin) drag.

#### Pressure drag vs friction drag

| Point | Pressure (form) drag | Friction (skin) drag |
|---|---|---|
| Cause | Pressure difference between front and rear, due to a wake after separation | Viscous shear stress on the surface |
| Acts | Normal to the surface | Tangential to the surface |
| Depends on | Shape of body, position of separation | Surface area, roughness, boundary layer type |
| Large for | Bluff bodies (disc, cylinder, sphere) | Long streamlined bodies, plates parallel to flow |
| Reduced by | Streamlining | Smooth surface, keeping BL laminar |

#### Why an aerofoil is a streamlined body

```text
 flow ->   bluff body            streamlined body
           ___                     ______
          |   |  wide wake       /      \____
  ---->   |   | ~~~~~~          (               >  narrow wake
          |___|                   \______/----
```

- A bluff body causes the boundary layer to separate early (the adverse pressure gradient on the rear is steep). This gives a wide, low-pressure wake and large pressure drag.
- An aerofoil has a rounded nose and a long, gradually tapering tail. The rear pressure rises slowly, so the adverse pressure gradient is mild.
- The boundary layer stays attached almost to the trailing edge, the wake is very narrow, and the pressure drag becomes very small.
- The total drag is then mostly friction drag, which is small, so the aerofoil gives high lift with low drag ($C_L/C_D$ is large).

## Check yourself

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

1. What causes pressure (form) drag on a body?
   A. Viscous shear stress along the surface
   B. A pressure difference between front and rear because of a wake after separation
   C. Surface roughness of the body
   D. The weight of the fluid displaced by the body
   Answer: B. Pressure drag comes from the low-pressure wake behind the body after the boundary layer separates. Shear stress causes friction drag instead. (Key point: Cause of pressure drag)

2. How does friction (skin) drag act on the surface?
   A. Normal to the surface, from the wake
   B. Tangential to the surface, from viscous shear stress
   C. Normal to the surface, from pressure
   D. Tangential to the surface, from separation
   Answer: B. Friction drag is due to viscous shear, which acts tangentially. Pressure acts normal to the surface. (Key point: Friction drag direction)

3. Which body has mostly pressure drag rather than friction drag?
   A. A well-streamlined ship hull
   B. An aerofoil at small angle of attack
   C. A circular disc held normal to the flow
   D. A long thin plate parallel to the flow
   Answer: C. Bluff bodies such as a disc, cylinder or sphere have large wakes so pressure drag dominates. Plates parallel to flow are friction dominated. (Key point: Where each drag dominates)

4. Why does an aerofoil have very small pressure drag?
   A. Its sharp nose removes all viscous shear stress
   B. Its gradual tapering tail gives a mild adverse pressure gradient, so the boundary layer stays attached and the wake is narrow
   C. Its flat tail makes the rear pressure very high
   D. Its smooth surface makes the boundary layer separate early
   Answer: B. A long taper raises the rear pressure slowly, delaying separation and shrinking the wake. This leaves mostly small friction drag. (Key point: Why aerofoil is streamlined)

## Related in Fluid Mechanics

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- [Aeroplane angle of attack and take-off power](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Aaeroplane-angle-of-attack-power) — Fluid Mechanics, chapter 9

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