Does The Concorde Expand By A Few Centimetres In Length Because Of The Heat Generated By Friction As It Passes Through The Atmosphere At High Speeds?

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 the Concorde. The temperatures on the airframe vary from 128 deg C at the nose to 90 deg C at the tail, with the average fuselage temperature being around 95 deg 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's 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's 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 cutouts 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.

Published in The Hindu on Nov 29, 2001.