Editing LMS Coronation Class (section)

== Design history ==
[[File:6229 Duchess of Hamilton at the National Railway Museum.jpg|thumb|left|Streamlined version as originally built]]
Although the prior introduction of the [[LMS Princess Royal Class|Princess Royal]] class had provided the [[London Midland and Scottish Railway]] (LMS) with more powerful locomotives to be used on the main line between [[Euston railway station|London Euston]] and [[Glasgow Central station|Glasgow Central]], the board of directors were persuaded in 1936 that more such locomotives would be needed, particularly as they were being asked to approve the introduction of a new non-stop service between those cities, designated the ''[[Coronation Scot]]''.<ref>{{cite book |last=Evans |first=Martin |author-link=Martin Evans |title=Pacific Steam: the British Pacific Locomotive |year=1961 |publisher=Percival Marshall |location=London |chapter=5}}</ref> Initially, the Chief Mechanical Engineer, [[William Stanier]], planned to build five more Princess Royals, but the Chief Technical Assistant and Chief Draughtsman at the LMS [[Derby Works]], Tom Coleman, argued that it would be preferable to design a new class of locomotive that was more powerful, more reliable and easier to maintain. Stanier was convinced and the drawing office commenced designing the new class.<ref name="Roden">{{cite book |first=Andrew |last=Roden |year=2008 |title=The Duchesses: The Story of Britain's Ultimate Steam Locomotives |publisher=[[Aurum Press]] |location=London |isbn=978-1-84513-369-6 }}</ref>{{rp|18–19}} When Stanier was called on to perform an assignment in India, Coleman became responsible for most of the detailed design in his absence.<ref name="Bellwood">{{cite book |last1=Bellwood |first1=John E. |last2=Jenkinson |first2=David |title=Gresley and Stanier: A Centenary Tribute |date=May 1976 |publisher=[[The Stationery Office|Her Majesty's Stationery Office]] |location=London |isbn=0-11-290253-7 |author-link2=David Jenkinson}}</ref>{{rp|73}}

Compared to the Princess Royal Class, there were important differences which would lead to an improved performance. Increased power was obtained by adopting a bigger boiler with greater steam-raising capacity; this included a firebox heating surface of {{convert|230|sqft|abbr=on}} versus 217 sq ft, a flue heating surface of {{convert|2,577|sqft|abbr=on}} versus 2,299 sq ft, superheater surface area of {{convert|830|sqft|abbr=on}} (some sources say 822 sq ft) versus 598 sq ft and a grate area of {{convert|50|sqft|abbr=on}} versus 45 sq ft. Also, the steam passages were better streamlined for greater efficiency and, most importantly, the piston valves went up in size from 8 inches to 9.5 inches. In order to allow higher speeds, the diameter of the driving wheels was increased to {{convert|6|ft|9|in|abbr=on}} (from 6&nbsp;ft 6 in) and the cylinder diameters were increased by {{convert|1/4|in|abbr=on}}.<ref>{{cite book |last=Haresnape |first=Ken |title=Stanier Locomotives |year=1974 |orig-year=1970 |publisher=[[Ian Allan Publishing|Ian Allan]] |location=Shepperton |isbn=0-7110-0108-1 |section=11}}</ref> The outside cylinders were moved forward with rocking shafts operating the inside cylinders. Finally a coal pusher was incorporated into the tender so the fireman did not have to "bring the coal forward", significantly cutting his workload which was particularly important on the long runs from Euston to Glasgow.

Just as the new design was approaching finalisation, the LMS marketing department created a difficult problem. The [[London and North Eastern Railway]] (LNER) had recently introduced its streamlined [[Class A4]] locomotive which had captured the imagination of the public, and the marketing department persuaded the board that the LMS's new locomotives should be streamlined too. This was problematic in that the new design was so large that it only just conformed to the maximum loading gauge for the main line; moreover, it was sufficiently heavy that it was close to the Civil Engineer's maximum weight limit. Nevertheless, Coleman managed to design a streamlined steel casing that hugged the locomotive so tightly that it could still meet the loading gauge. The casing weighed some {{convert|5|LT|ST t|abbr=on}}, but Coleman managed to save an equivalent weight in the locomotive itself.{{r|Roden|pp=23–25}}<ref name="Haresnape">{{cite book |last=Haresnape |first=Brian |title=Railway Liveries 1923–1947 |year=1989 |publisher=Ian Allan |location=Shepperton |isbn=0-7110-1829-4}}</ref>{{rp|123}}

The casing was tested in a wind tunnel, and retained after it was found to be as good as other forms of streamlining.{{r|Bellwood|p=73}}<ref name="Peacock">{{cite journal |last=Peacock |first=D. W. |title=Railway wind tunnel work |journal=Journal of the Institution of Locomotive Engineers |date=1951 |volume=41 |at=606–61, Paper 506}}</ref> After introduction it was subsequently found that its aerodynamic form failed to disturb the air sufficiently to lift the exhaust from the chimney, thus obstructing the driver's vision with smoke.{{r|Bellwood|pp=73–74}}