Britain came out of the Second World War with a captured German idea it did not fully understand. The delta wing promised high speed and a strong, light structure, and the Germans had been drawing them for years. Nobody in Britain knew what one actually felt like to fly.

So the Air Ministry commissioned aircraft whose only purpose was to find out. The Boulton Paul P.111 was one of them: a small, broad-bodied jet with no tailplane, swappable wingtips, and controls so sensitive that its test pilots had to be protected from them.

It was later painted bright yellow, and the people who flew it started calling it the Yellow Peril.

An Aeroplane You Could Change the Shape Of

The clever part was not the delta. It was the wingtips. The P.111 was built with removable tips so that researchers could bolt on different shapes and effectively turn one airframe into several different wings — a highly desirable feature when the entire point of the aircraft is aerodynamic investigation rather than performance.

The Rolls-Royce Nene had to live inside the fuselage, and a Nene is not a slim engine. The result was a body far broader than the aircraft’s size suggests, which gives the P.111 its slightly startled, unorthodox look. It also carried a Martin-Baker ejection seat and a fully powered flight control system, both still novel in 1950.

The Controls Were the Problem

It flew on 10 October 1950, and the flight test programme immediately ran into the thing that plagued early powered controls: the aircraft responded far too eagerly. The flight controls were highly sensitive. Worse, major trim shifts occurred every time the landing gear went down or came up, so the aircraft changed its mind about where it wanted to point at exactly the moment a pilot has the least spare attention. It also needed a relatively high landing speed.

That combination produced a minor landing accident, and after it the aircraft was substantially modified: fuselage-mounted airbrakes were added, the undercarriage doors were redesigned to reduce the trim change, and — most significantly — a rudimentary feedback arrangement was fitted to the flight controls.

That last item is worth pausing on. A fully powered control system with no feedback gives the pilot no sense of how hard the air is pushing back, which is precisely how you over-control an aircraft. The fix the P.111 received in the early 1950s is the ancestor of the artificial feel systems in every fly-by-wire aircraft flying today.

The Yellow Peril

The modifications were extensive enough that the aircraft was redesignated P.111A. A long nose-mounted probe carrying a pitot head was added, and the whole machine was repainted. The original P.111 had been either unpainted or silver; the P.111A came out bright yellow overall, and that is where the nickname came from.

The rebuilt aircraft first flew from RAF Boscombe Down on 2 July 1953. Shortly afterwards the anti-spin parachute on the rear port fuselage was strengthened — which tells you something about what the test programme was doing with a tailless delta.

What It Was Actually For

The P.111 never entered service, never carried a weapon, and never set a record. Judged as an aeroplane it is a footnote. Judged as an instrument it did exactly what it was built to do: it told British designers what a tailless delta does when you deploy the gear, when you slow down, and when you ask too much of the controls.

The P.111 should not be confused with the later Boulton Paul P.120, which shares considerable similarities. Both were part of the same effort to answer the delta question before committing to an operational design.

A closer look at the configuration and what the programme was trying to establish.

Its final test flight came in 1958, after which it was transported to the Cranfield College of Aeronautics to be used as a training airframe. That unglamorous fate is why it still exists: aircraft sent to colleges get kept, and aircraft sent back to the manufacturer get cut up.

Britain built a small fleet of these single-purpose research aeroplanes in the decade after 1945, flew them at Farnborough, learned what it needed, and then mostly forgot them. Here is what that era looked like at the time:

Sources: Air Ministry Specification E.27/46 programme records, Boulton Paul Aircraft company histories, Midland Air Museum, Cranfield College of Aeronautics records.