engineering

Meredith effect

Meredith effect is a engineering 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 Meredith effect rather than just read about it. In short: The Meredith effect is a phenomenon whereby the aerodynamic drag produced by a cooling radiator may be offset by careful design of the cooling duct such that useful thrust is produced by the expansion of the hot air in the duct. The effect was discovered in the 1930s and became more important as the speeds of piston-engined aircraft increased over the next decade.

Key takeaways

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

Reference excerpt

The Meredith effect is a phenomenon whereby the aerodynamic drag produced by a cooling radiator may be offset by careful design of the cooling duct such that useful thrust is produced by the expansion of the hot air in the duct. The effect was discovered in the 1930s and became more important as the speeds of piston-engined aircraft increased over the next decade.

History F. W. Meredith was a British engineer working at the Royal Aircraft Establishment (RAE), Farnborough. Reflecting on the principles of liquid cooling, he realized that what was conventionally regarded as waste heat, to be transferred to the atmosphere by a coolant in a radiator, need not be lost. The heat adds energy to the airflow and, with careful design, this may be used to generate thrust. The work was published in 1935. The phenomenon became known as the "Meredith effect" and was quickly adopted by the designers of prototype fighter aircraft then under development, including the Supermarine Spitfire and Hawker Hurricane whose Rolls-Royce PV-12 engine, later named the Merlin, was cooled by ethylene glycol. An early example of a Meredith effect radiator was incorporated in the design of the Spitfire for the first flight of the prototype on 5 March 1936. Many engineers did not understand the operating principles of the effect. A common mistake was the idea that the air-cooled radial engine would benefit most, because its fins ran hotter than the radiator of a liquid-cooled engine, with the mistake persisting even as late as 1949. A significant debate on the origin and effectiveness of the principle arose as a result of an article written by Lee Atwood in the 1990s. The article generated rebuttals in both the AAHS Journal and the Friends Journal of the Air Force Museum Foundation.

Explanation The Meredith effect occurs when air flowing through a duct is heated by a heat-exchanger or radiator containing a hot working fluid. Typically the fluid is a coolant carrying waste heat from an internal combustion engine.

The duct must be travelling at a significant speed with respect to the air for the effect to occur. Air flowing into the duct meets drag resistance from the radiator surface and is compressed due to the ram air effect. As the air flows through the radiator it is heated, raising its temperature slightly and increasing its volume. The hot, pressurised air then exits through the exhaust duct which is shaped to be convergent, i.e. to narrow towards the rear. This accelerates the air backwards and the reaction of this acceleration against the installation provides a small forward thrust. The air expands and decreases temperature as it passes along the duct, before emerging to join the external air flow. Thus, the three processes of an open Brayton cycle are achieved: compression, heat addition at constant pressure, and expansion. The thrust obtainable depends upon the pressure ratio between the inside and outside of the duct and the temperature of the coolant. The higher boiling point of ethylene glycol compared to water allows the air to attain a higher temperature increasing the specific thrust. If the generated thrust is less than the aerodynamic drag of the ducting and radiator, then the arrangement serves to reduce the net aerodynamic drag of the radiator installation. If the generated thrust exceeds the aerodynamic drag of the installation, then the entire assemblage contributes a net forward thrust to the vehicle. The Meredith effect inspired the early American work on the aero-thermodynamic duct or ramjet, due to the similarity of their principles of operation. In more recent times the phenomenon has been utilised in racing cars by mounting the engine cooling radiators in tunnels.

Illustrations

Worked examples

Example 1 — a first encounter with Meredith effect

Start with the simplest possible case. Write down what Meredith effect claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, 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 Meredith effect 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 Meredith effect 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 Meredith effect

In research: Meredith effect appears in engineering 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 Meredith effect 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: Meredith effect is common in secondary-school and first-year university syllabi. It links to neighbouring topics 1930s aircraft piston engines, Aerospace engineering, Aircraft aerodynamics, so understanding it makes those chapters shorter.
In everyday life: Look for Meredith effect 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 Meredith effect in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Meredith effect 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 Meredith effect out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Meredith effect in simple terms?

Why does Meredith effect matter?

Because it connects several engineering 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 Meredith effect?

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 Meredith effect.