physics

Relative wind

Relative wind is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Relative wind rather than just read about it. In short: In aeronautics, the relative wind is the direction of movement of the atmosphere relative to an aircraft or an airfoil. It is opposite to the direction of movement of the aircraft or airfoil relative to the atmosphere.

Key takeaways

Relative wind belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Relative wind to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Relative wind from memory before moving on to harder problems.

Reference excerpt

In aeronautics, the relative wind is the direction of movement of the atmosphere relative to an aircraft or an airfoil. It is opposite to the direction of movement of the aircraft or airfoil relative to the atmosphere. Close to any point on the surface of an aircraft or airfoil, the air is moving parallel to the surface; but at a great distance from the aircraft or airfoil, the movement of the air can be represented by a single vector. This vector is the relative wind or the free stream velocity vector. The angle between the chord line of an airfoil and the relative wind defines the angle of attack. The relative wind is of great importance to pilots because exceeding the critical angle of attack will result in a stall, regardless of airspeed.

In freefall Relative wind is also used to describe the airflow relative to an object in freefall through an atmosphere, such as that of a person's body during the freefall portion of a skydive or BASE jump. In a normal skydive, the vertical descent of the skydiver creates an upward relative wind. The relative wind strength increases with increased descent rate. The relative wind is directly opposite the direction of travel. When a skydiver exits a forward-moving aircraft such as an aeroplane, the relative wind emanates from the direction the aeroplane is facing due to the skydiver's initial forward (horizontal) momentum. As aerodynamic drag gradually overcomes this forward momentum and gravity simultaneously attracts the skydiver downward, the relative wind alters proportionally into an upward (vertical) direction. This creates an arc of travel for the skydiver similar to water flowing from a low pressure hose held horizontally and creates a variation in the angle of the relative wind from horizontal to vertical. When exiting from a forward-moving aircraft (as distinguished from a hovering aircraft, such as a balloon or a helicopter in hover mode) during a normal belly-to-earth skydive, the skydiver must arch his body in the direction of travel which is initially horizontal. If the skydiver continues to arch, his belly will gradually alter pitch until he is belly-to-earth. This section of the jump is commonly referred to as "the hill". Relative wind differs from the wind in meteorology in that the object (e.g.., the skydiver) moves past the air, as opposed to the air moving past the object.

References

Worked examples

Example 1 — a first encounter with Relative wind

Start with the simplest possible case. Write down what Relative wind claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Relative wind before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Relative wind ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Relative wind

In research: Relative wind appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Relative wind in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Relative wind is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aerodynamics, Aviation stubs, so understanding it makes those chapters shorter.
In everyday life: Look for Relative wind outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Relative wind in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Relative wind means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Relative wind out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Relative wind in simple terms?

Why does Relative wind matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Relative wind?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Relative wind.