Aboukir air filter on Spit V - performance?

[DavidIsby]

Of all the kit that has been hung on Spitfires, nothing seems to have attracted as much dislike as the Aboukir filter.

Does anyone know of a source for the details of how badly it cut Spitfire V performance? What was the ram recovery ratio (what ratio of possible air intake actually makes it into the supercharger – 50% for a basic Spitfire IA)? What did it add to overall drag?

Thanks,
David Isby

[Graham Boak]

You are thinking of the Vokes filter: the Aboukir version was a local Mediterranean fit which was thought to give less drag. I have never seen any specific aerodynamic information quoted for the Aboukir filter, but it certainly had less surface area which would reduce external drag. The total penalty would come from the sum of the increased external drag and the "internal drag" of the system, which would be expressed in terms of a loss in power of the engine. Data showing the performance effects of the Vokes filter can be seen in Hawker Hurricane from MacDonalds (many years ago) - contact me offline. I believe that these are predicted and/or test figures, and it seems that the performance loss was greater in service.

I don't recognise your figure of ram air recovery. This is normally expressed in terms of the pressure loss, or perhaps rather the pressure gain from receiving air in motion. This will only be a few % - 2% comes to mind but I don't recall the context. The Vokes filter effectively stopped the air still whilst passing through it, removing any such benefit. The later Aerovee filter had a bypass so that away from the ground the air passed freely.