杏吧原创

Stresses and strains

Question: I have heard that during flight Concorde expands by a few
centimetres in length because of the heat generated by friction as it passes
through the atmosphere at high speeds. This must surely put great strain on its
superstructure. What design features were incorporated to compensate for this
expansion?

Answer: Due to kinetic heating caused by supersonic airflow over the skin,
the thermal effects on the airframe were a major consideration in the design of
Concorde.

The temperatures on the airframe vary from 128 掳C at the nose to 90
掳C at the tail, with the average fuselage temperature being around 95
掳C. A passenger on Concorde can feel this effect by touching the cabin
window, which feels distinctly warm due to heat transfer through the multiple
panes. Because the aluminium airframe heats up so much, Concorde鈥檚 62.2-metre
fuselage expands by approximately 12.5 centimetres during supersonic cruise.

The manufacturers had to select a suitable aluminium alloy that would
maintain its mechanical properties at the aircraft鈥檚 design temperature for the
whole life of the airframe. Conventional aluminium alloys were unsuitable, so
they selected a material derived from an alloy created by Rolls-Royce. The
majority of the airframe was constructed from derivatives of this aluminium
alloy, known as CM001, CM002 and CM003. In areas where severe heating occurred,
titanium and steel were used.

Transient temperatures built up during acceleration or deceleration create
uneven temperature gradients throughout the whole airframe. This is particularly
noticeable in areas such as parts of the fuselage which are masked by the wings.
Other features that cause similar problems are the thermal effect of the
diminishing mass of fuel during the flight. These create stresses on the
structure in addition to the flight and inertia loads experienced by subsonic
aircraft. Design features to counter these thermal loads include corrugated spar
webs, diamond-shaped cut-outs in the internal structure and slotted lug
attachments that allow movement between structural items experiencing thermal
gradients.

To substantiate the design, extensive component testing was carried out,
followed by a complete airframe fatigue test. This took place at the Royal
Aircraft Establishment in Farnborough, where as well as the usual flight loads
additional thermal loads were applied using heat lamps and forced cooling to
simulate real flight conditions.

Sharon Maycock

Media Relations Executive

British Airways

Middlesex

A lighter note

Question: The back of the older English 拢20 notes depicts a lecture at
the Royal Institution. The banknote suggests the lecture took place in 1826, but
the apparatus on the demonstration bench looks more modern.

Above the bench is a triangular object, which appears to be projecting a cone
of light onto the bench. What is it? Is it a genuine lighting system, and how
does it work? Or does the banknote contain an anachronism?

Answer: The picture on the back of the old 拢20 notes is based on a
painting by Alexander Blaikley showing Michael Faraday delivering a Christmas
lecture at the Royal Institution on 27 December 1855 before the Prince Consort,
the Prince of Wales and Prince Alfred (not pictured on the note). The caption
makes it quite clear that the Christmas Lecture series started in 1826.

The triangular object above the bench is gas lighting (a very widespread
technology by the 1850s) which is depicted in the original painting, although
the cone of light appears only on the note.

Frank James

Royal Institution Centre for the History of Science and Technology

Royal Institution of Great Britain

London

Answer: The portrait of Michael Faraday (on the right-hand side of the
reverse of the note) comes from a photograph that was taken in 1857 by
Maull and Polybank. The original photo can be seen at the Royal Institution in London.

The lecture image on the note is simplified from the original, but a
few points might be of interest.

The number of items on the bench has been reduced, although all are in
keeping with the late 1800s.

The figures in the front row of the audience, clearly recognisable in the
original, include Darwin, Huxley, Crookes, Tyndall, Wheatstone, Daniels and
Frankland, although there is no evidence that they ever attended a lecture
together.

The item in Faraday鈥檚 right hand is a coil of wire used to demonstrate
magnetic induction (the original is on display at the Royal Institution).

The wall behind the bench contains a list of some of the chemical terms that
were coined by Faraday during his career.

Bryson Gore

Freelance Lecturer of Physics and Chemistry
at the Royal Institution of Great Britain

London

This week鈥檚 questions

Juice joust: How is concentrated orange juice made, how does it differ from
freshly squeezed orange juice and what are the health benefits and pitfalls of
drinking each type?

Rie Inoue

London

Sex secret: Why do women live longer than men?

Derek Dunn

University of Manchester