Armstrong Whitworth A.W.166

This project started, like other Armstrong Whitworth aircrafts, as a research airplane on mach speeds.

Made out of light alloys, Armstrong Whitworth stated that a steel wing would be lighter due to the substantial loss in effective depth of the spar box when using a very thick light alloy flanges.
As it was designed the airplane would only reach Mach 1,5 at sea level unless it was made entirely of steel. Armstrong Whitworth preferred to base it's proposals on the more familiar design and fabrication of light alloy to reduce riveting and stiffening, but steel was chosen in the parts of the all-moving tail for stiffness.

The specification ER.134T required a top speed about twice of the current fighter prototypes with a wing around half the present thickness/chord ratio. Armstrong Whitworth's 4% needed moving surfaces on both leading and trailing edges and the high wing loading was considered to be an advantage in obtaining very high speed at moderate altitude, but, on the downhill, it also meant that a standard low maneouvrability had to be accepted.
The wing was originally drawn tapered 10º on both edges. A later proposal made the inner wing straight but the outer one swept 55º. It would've been powered by two Armstrong Siddeley Sapphire Sa.7 engines mounted in underslung mid-wing nacelles with two position nozzles and variable centerbody intakes, with reheat providing 25% static thrust augmentation.
The AW.166 was going to have a bicycle undercarriage with the wheels folding into the engine nacelles and a 900gl (4091liters) fuel tank would've been placed in the fuselage. That amount of fuel was considered more than enough by the company.
The aircraft would've reached Mach 2 between 10973 and 15240m high for the required 10 minutes.
Armstrong Whitworth considered Mach 2 at sea level to be easy to achieve apart from cockpit cooling and conversion to all steel and, in spite of the difficulties meeting the ER.134T specification, Armstrong Whitworth was satisified as it seemed that this design would achieve speeds of Mach 2 and beyond with barely any difficulty.

A small unrequested brochure was presented for an AW.166 for Australia, powered by two Armstrong Siddeley Sapphire Sa.10,equipped with an Ai.18 collision course interception radar, a fatter nose and unspecified wing tip missiles (most probably the Firestreak or the Red Top).

Sources:
1. Midland Publishing - British Secret Projects - Fighters since 1950