The French Connection
Gresley had kept an eye on his contemporary across the Channel, Andre Chapelon, who had designed a 4-8-0 compound locomotive for similar work. Gresley did not use compounding but he incorporated the Kylchap (Kylala-Chapelon) blast pipe and chimney plus enlarged, streamlined steam passages and ports. The French influence was continued by the fitting a ACFI feedwater heater.
As the boiler was air smoothed to the height of the loading gauge the chimney could not project above this so tests were carried out using models in wind-tunnel experiments at the City and Guilds (Engineering) College, South Kensington. These determine the ideal shape of the deflectors to ensure that the steam/smoke would not drift down to obscure the drivers’ vision. Thus the striking appearance of Cock o’ the North was arrived at.
The locomotive had the usual Gresley preference of three cylinders; 21in by 26inch stroke, all driving the second coupled wheels, but he fitted this locomotive with Lentz rotary cam poppet valves. The boiler used was similar to the one used on the A3’s but extended to 6ft 5in diameter and had an enlarged grate of 50sq ft.
Testing
Oliver Bulleid took a keen interest in this locomotive and was highly delighted by the performance of Cock o’ the North when during a test in June 1934 it sailed over Stoke summit at 57mph at 30% cut-off with 19 carriages plus the dynamometer car in tow, 650tons in all. During this testing between Kings Cross and Grantham the cut-offs available were 10, 12, 15, 20, 22, 25, and 30 per cent but when it went into service in Scotland there were only three steps available, 18, 25 and 35 per cent, leading to a lot of fire throwing when steaming hard.
Bulleid accompanied Cock o’ the North when it was sent to the new French testing plant at Vitry. It was also tested on the Paris to Orleans main line where on one run it sustained 1,910 drawbar horsepower at 68mph for 35 minutes. During this time ‘Cock o’ the North’ was displayed at Gare du Nord where the locals thought it more chic than the local product.
During testing several lessons were learnt and applied but one was not! Bulleid had noted that the locomotive produced more smoke than the French ones on test, so deduced that there was insufficient air for satisfactory combustion. When he mentioned to his chief that grate and damper alterations were required Gresley decided not to implement them, probably due to the cost.
Valve Gear Conversion
By this time they had already decided that Cock o’ the North would have to be converted to Walschaerts-Gresley valve gear as the Lentz gear, as mentioned above, had a limited choice of cut-offs and was wasteful in coal and water due to the large clearance volumes. Also by this time reports were coming from Scotland that although the locomotives did the job, they were not only wasting coal but were running hot boxes and spreading the track. Their long trains were also hindering station operations! |
