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After the Normandy invasion on the 6th June 1944. Temporary landing strips were made close to the landing beaches to operate the Hawker Typhoon, which was designed to give close air support to the advancing allied troops. These aircraft were known as tank busters and were to perform a vital role in the Caen offensive.


Within a few days of operating from these temporary landing strips the aircraft started to experienced problems with their engines. The Hawker Typhoon had a large air intake under the fuselage for the Napier Sabre II engine. This air intake allowed the coarse Normandy dust and sand in to the engine allowing the dust to cause wear in the cylinders, effectively grounding the aircraft. The rate of attrition was so high that the whole of the offensive was now relying on air support from the UK. The situation was serious and the Air Ministry contacted D Napier & Sons Flight Development Establishment at Luton. Napiers reacted swiftly to the challenge Mr Cecil Cowdrey, the General Manager of Napiers and Mr Bonar, designed, built and test flew a momentum type air filter that was 96% efficient in just ten hours.


This temporary filter was built and fitted to the aircraft in a matter of days, which permitted the Hawker Typhoons to be used in the offensive on Caen and the breakout through the Falaise Gap.  The filter was one of inventions critical in securing the success of the Normandy landings.


It is difficult to tie-down the momentum filters exact date into service after D-Day on 6th June 1944, but we understand that the first airstrip in Normandy was constructed on 13th June 1944.  Operations with Typhoons took one week to identify the dust problem in the Sabre engines (according to a report in the "AEROPLANE" magazine of 26th July 1946), so the fitment of the filters must have been from the end of June to early July 1944.  The magazine then reported D. Napier and Son, Luton – Flight test facility "Designed in the morning, made in the afternoon and the prototype flown the same night, and were fitted in a week to all the Typhoons in Normandy”.


As a result, a letter was sent to the Chairman of D Napier & Son Ltd by Air Marshall Sir Arthur Coningham, 2nd Tactical Air Force.  In it he thanked Mr Cecil Cowdrey and Mr Bonar and the staff at the Luton Flight Development Establishment in producing this deflector at such short notice. The letter further stated that it had been made possible for Typhoons to be operating once more from continental airfields and to give much stronger support to the British 2nd Army than at that time seemed possible.

Momentum Air Filter
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Prototype Momentum air filter in Typhoon carcass
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Momentum air filter fitted in Typhoon
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