Monday, 6 December 2010

Vintage Aircraft Nuts [3]: Bristol Freighters

This post is exclusively about Bristol Frightners. Here in NZ we have about the last taxiiable Frightner as far as I know in the world. As we know, Safe Air was still operating them in NZ into the mid 80s. There were a few more commercial operations after that, including several in Canada where they were flying them into mining strips in the remote outback of British Columbia. From this company, Hawkair, the last flight of a Bristol Freighter in the world took place in about 2005 when they flew their sole survivor to a Canadian museum. The Poms had a go at preserving one in flying order, but they pranged it at an airshow and that was the end of it unfortunately.

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This is a map of the Bronson Creek airstrip which served the nearby Snip Mine which was a goldmine in British Columbia. It closed down in 1999. The mine camp and the airstrip are still used for recreation in the area and a proposal for a new mine called Bronson Slope is under consideration. Hawkair and its predecessor Trans Provincial Airways were obviously low cost operators with just a few aircraft, old piston engined planes that were cheap to buy and run, important to contain the costs of a mining operation where everything and everyone had to be flown in or out.

Here are a couple of promo clips for DVDs that hardcore enthusiasts can buy:

In particular note in the second clip the colours of the first aircraft shown, which are the Safe Air colours obviously with the name painted out. This particular aircraft had been flown to Canada from New Zealand when Safe Air closed up its Bristol operations here.

Now going forward to recent years, there are several Freighters preserved in NZ, one of which is at Omaka Aerodrome. About two years ago an effort was made to restore the engines to working order and it has since been taxiied at one or two airshows and the like. Here are some clips of it.

This is one of their first taxi runs. The means of turning the propellers by hand on the engines was done in order to check there was no hydraulic lock in any of the cylinders. As mentioned in my previous post, all inverted engines (the lower cylinders on a radial, inline engines which are inverted such as the Gypsy Queen on the Tiger Moth) are susceptible to fluid (generally lube oil) draining into the inverted cylinders after the engine is shut down. This ends up collecting in the combustion space where it can create a hydraulic lock situation (“hydraulicing”) when the engine is turned over for start up, because fluid is not compressible this would severely damage the engine. The procedure for starting such engines therefore encompassed either the hand turning shown, or turning several revolutions on the starter with the ignition off to ensure there was no lock.

This clip shows engine start and adjustment. Like the Rolls Royce Merlin, Bristol Hercules engines were comparatively noisy and were not popular on passenger aircraft. However the Hercules was a good solid engine that had one of the highest production levels of all time in the UK – over 57,000 of them were produced, many for WW2, and there were even some fitted to the mighty Lancaster bomber. It was really the peak of Bristol’s sleeve-valve technology – the later more powerful engines such as the Centaurus were not so numerous because of the development of turbine engines was so rapid after the war.

Another engine run, one of the better quality clips out there.

To finish off here is a clip of a preserved Hercules engine mounted on a trailer for operational display in the UK. This engine has quite a different exhaust system from the Freighter helping to account for the different engine sound.