Drag and Propulsive Power

Aeronautics and Aerospace · CHAPTER 3 / 5

Level: introductory undergraduate · Study session: 15–20 minutes including practice.

Learning outcome: Distinguish drag force from power needed to overcome drag.

Before you begin: Complete earlier chapters in this discipline; revise algebra and units as needed.

1. Understand the idea

With fixed drag coefficient, density and reference area, drag increases with speed squared. Propulsive power associated with drag equals drag times speed, so it scales with speed cubed under those assumptions. Actual aircraft drag also includes lift-dependent effects; this simple curve is not a full drag polar.

2. Write the model

D = ½ρV²SCD; P = DV

Define the symbols from the worked case before substituting values. Keep units consistent, record the assumptions and round only after the calculation.

3. Follow a worked example

For ρ=1.225 kg/m³, S=10 m² and CD=0.03, drag at 20 m/s is 73.5 N and corresponding power is 1470 W.

4. Explore the model

Figure: calculated teaching cases; straight segments connect sampled points, not experimental observations. Read exact values below.

Illustrative scenario sweep — synthetic educational data
Speed (m/s)	Drag (N)	DV power (W)
20	73.5	1470
30	165.375	4961.25
40	294	11760

Double speed from 20 to 40 m/s. Drag increases fourfold and drag power eightfold in this fixed-CD model.

5. Reproduce it yourself

Copy the input cases into Excel or a notebook and name each quantity and unit.
Recalculate each result from the stated equation or rule. For qualitative cases, explain the state or partition decision instead.
Change one input, predict the direction of change, then calculate again.
Write one sentence explaining where the model stops being valid.

6. Check your understanding

Is DV the required engine shaft power?

Reveal the answer and check your work

Not directly; propulsion efficiency must also be accounted for.

Compare your result, units and reasoning. If they differ, return to the equation and test the worked case first.

7. Common mistake

This model omits induced-drag variation and compressibility.

Further study and academic note

Explore the discipline reference and further learning resources. This is an original IIMEEE introductory teaching note with synthetic examples; it is not a peer-reviewed research result or a professional design specification. Figures were constructed from the listed teaching cases. Resource links support further study and do not imply endorsement.