ART DECO STREAMLINING: FAD OR FUNCTION?




















The huge piece of industrial art pictured above is an amazing 
machine named after a Duck, Mallard. Mallard is a steam locomotive built in the UK in 1938 and today, even after almost 90 years, Mallard still holds the world record as the fastest steam locomotive in history having reached 126mph during a brake test in 1938.

A modern observer might not attach too much weight to the effectiveness of this rather fanciful looking streamlining - in an age when we're used to Japanese bullet trains that look like they could be formed of pure liquid and just slink through the air - literally like a bullet - Mallards beautiful art-deco curves look more like something out of a Jules Verne novel than intelligent engineering.
Perhaps this is true, given the huge publicity surrounding locomotives like Mallard when they were built you could be forgiven for thinking the appearance of these engines was more for show than anything else. But when you actually look at how the locomotive worked it is surprising to find just how much science over fiction there is in this elegant monster.

Wind resistance to a fast moving train is actually a reasonably small factor compared to other parts of a locomotive that produce friction - such as the wheels against the rails, but, as small as it is, wind resistance increases with the speed of the train. Therefore, to locomotives built to run at high speed over long distances, like Mallard, it becomes an important factor.

Mallards streamlining centers around its horizontal wedge at the front of the locomotive, a design pioneered in France by Bugatti.

The secret to its effectiveness in reducing air resistance at high
speeds is that it directs the majority of the air flow over the top of the locomotive rather than along the sides.

Side winds are much more of a braking force than head winds directed over the top as the accumulative surface area of the sides of the locomotive are greater than the front and top.

At the highest speeds, especially when there was a headwind, streamlining such as Mallards did produce a slightly higher speed for the same amount power, the following table gives an overview of the power savings compared to an un-streamlined A3 like Flying Scotsman:



Another benefit of Mallards streamlining, recognised at the time, was its startling reduction on the slipstream effect of the trains hauled by locomotives of Mallard's class .

When a train travels in the open air it displaces the air around and
over it forming a slipstream alongside the train and a wake behind. The air at the surface of the train moves at the speed of the train, whilst far from the train the air moves at the ambient air speed.
Therefore, there is a region near the train sides where the air can be moving at speeds comparable to that of the train.

In this region, the air is very turbulent and after the train passes there is a wake flow, which decays as the train moves away.
These adverse effects increase with the aerodynamic roughness, the speed of the train and roughness elements and discontinuities in the train surface, such as exposed bogies, inter-car gaps and gaps between containers.

Mallards streamlining has the benefit of being extremely smooth which reduced the slipstream effect drastically. This was also aided by the coaches that made up the Coronation service that Mallard pulled:



They were articulated, which almost completely eliminated the gap between coaches.

The last coach, an observation car used for summer months, was also tapered at the end which reduced the impact of the wake behind the train.


Locomotives like Mallard were truly the pioneers of today's 'High Speed Trains' where streamlining gives substantial fuel savings, and the technique really justifies itself.