Obturator ring

An obturator ring was a type of piston ring used in the early rotary engines of some World War I fighter aircraft for improved sealing in the presence of cylinder distortion.

Purpose

The cylinders of rotary aircraft engines (engines with the crankshaft fixed to the airframe and rotating cylinders) suffered from uneven cylinder cooling as the side facing the direction of rotation received more cooling air which lead to thermal distortion. To keep weight down the cylinders on rotary engines had very thin-walls (1.5 mm)^[1] and some had no cylinder liners. On engine types without cylinder liners, obturator rings, made of bronze in the early Gnome engines,^[2] were fitted as these were soft enough to not damage cylinder walls and could flex to the shape of the cylinder.

In operation wear on the rings was considerable. Engines needed to be overhauled about every 20 hours.^[1] The reliability of Gnome engines license-built by The British Gnome and Le Rhone Engine Co. was improved with an overhaul life of about 80 hours being achieved, mainly as a result using a special tool to roll the 'L' section obturator rings.^[3] Clerget rotary aircraft engines also used obturator rings which were prone to overheating and seizure.^[4]

Le Rhône and Bentley BR1/BR2 rotary engines used cylinder liners and were sealed using conventional piston rings rather than obturator rings.^[5]^[6]

References

^ ^a ^b The Rotary Aero Engine, Andrew Nahum 1987, ISBN 1 900747 12 X, p.40
^ Aviation Engine Examiner, Major Victor W. Page, The Norman W. Henley Publishing Co., 2 West 45th Street, New York 1931, p.146
^ I Kept No Diary, Air Commodore F.R. Banks 1978, ISBN 0 9504543 9 7, p.63
^ Bentley, Walter Owen (1969). "5 - Aero Engines". My life and my cars. A S Barnes. ISBN 978-0498073427.
^ "Chapter II - The 80 and 110 Horsepower LeRhone Motor". Rotary Engines. United States: The War Department. 1919.
^ Lumsden, Alec (2002). British Piston Engines and their Aircraft. Airlife Publishing. p. 88. ISBN 1853102946.