In the Spring of 1957 the Hawker Research and Development (R&D) Department carried out ground vibration testing of the Hunter.

For the first trial the aircraft stood on its undercarriage with tyre pressures reduced. The results were reasonable but the analysts wanted further tests to be conducted at lower vibration levels which would require some form of low frequency support. I was given the job of sorting this out.

What was needed was a flexible member between the aircraft and its supports and here I struck lucky. The tyre industry was conducting trials with tubeless tyres on electric milk floats. I thought these tyres would be just the job so we obtained details which enabled us to proceed with design work.

The aircraft supports at the fuselage and wing jacking points were to be air cylinders with tubeless tyres providing flexibility. Each support was based on a steel cylinder about 18 inches in diameter and 36 inches high. A thick, oversize, baseplate was welded to the bottom of the cylinder and a flanged ring, drilled with a ring of bolt holes, was welded to the top. One sidewall of a tyre rested on this drilled ring flange and was clamped to it by means of a ring plate, with bolts welded to it, placed inside the tyre. A second, similar ring plate with bolts facing upwards was also placed inside the tyre to clamp it to a thick top plate. A suitable sealant was applied to all rubber-to-metal clamping surfaces.

The top plate had a steel tube passing through it, the lower longer end going down inside the cylinder to be located at the bottom by three roller bearings allowing free vertical movement but preventing lateral movement. The top end of the tube, braced by gussets welded to the top plate, carried a standard aircraft jacking ball which was to engage with a jacking socket on the aircraft. Since all the support cylinders were the same size those for the wings, mid mounted on the Hunter, had steel stools bolted to the baseplate. A Schraeder air valve connector was fitted to the cylinder wall for pressurisation, as was a simple visual indicator to show tyre movement. A 2 BA bolt in the top plate was used for depressurisation - no Health & Safety in those days! With the design complete Charles Plantin and I went to see Roy Chaplin to seek his agreement. I know he was worried about the scheme but he agreed to sign-off our low frequency support design provided that the cylinder wall thickness was increased. This was done.

The recording equipment, borrowed from RAE Farnborough, utilised photo-sensitive paper requiring normal darkroom photographic processing. This posed a bit of a problem since the paper was in 6 inch wide 50 foot rolls. I had to set to and design a cradle and tanks in stainless steel to allow this huge roll to be developed and fixed. We felt sorry for Alison, the girl nominated to do the processing in the darkroom, for the equipment was heavy and cumbersome, but she did a great job. The processed rolls were hung over the hangar roof structure to dry, looking like Christmas decorations.

With the equipment made we descended on the Experimental Hangar at Dunsfold and stood by as Alan Wigginton’s fitters prepared the aircraft, a Hunter Mk4. We were to use Jumbo Betteridge’s Instrumentation Department facilities: the photo processing laboratory upstairs and the instrumentation room downstairs for analysis and plotting. The Hunter was jacked up on standard aircraft supports and the fuselage and wing contour boards put in place. The nose support was removed and replaced with one of our low frequency supports (LFSs). Shop air was used to inflate the support very carefully. The procedure was repeated with the wing units and when we were all satisfied the contour boards were lowered, but not removed. Alan couldn’t believe what we had done to his aircraft and we caught him in the tea break giving the Hunter a gentle prod to watch it floating like a jelly on a plate. In contrast to today’s A400M testing we were limited to one exciter and a couple of accelerometer pick-ups. The exciter was joined to the tail bumper position via a strain gauged rod, and we were ready for some exploratory tests. So far, so good. Because the aircraft was ‘floating’ it was easy to make mistakes. When we arrived the next morning we found the LFS tubes had bottomed due to the hangar cooling overnight, so that evening the contour board were raised to just touch the skin. One fitter, not realising how sensitive the system was, managed to raise a wing support too far causing the central tube to pop out of its roller bearing fitting. Fortunately, no harm was done and we got it back in place. Every night we had to disconnect the strain gauged rod from the rear fuselage.

When Rochefort realised just how many of his staff would be required down at Dunsfold for a number of weeks he directed that we were to use mathematician and tracer girls for the bulk of the work, leaving four of us to run the tests. John Barker was in charge supported by Graham Galton, Derek Simms and myself. One girl was put on photographic processing, another, with Derek, fitted the accelerometer pick-ups in turn at the many recording locations, and four others analysed the traces and plotted the results which were returned to Kingston daily. I was put in charge of these girls; my first experience in handling women. What a challenge!

Using the correct procedure with the accelerometer pick-ups was critical. They had only to be placed lightly on the structure and a lot of care was necessary when moving them round the aircraft symmetrically, measuring both vertical and lateral motion at the same time. As we scanned the vibration levels looking for the resonant frequencies of different parts of the aircraft we happened to hit on the right frequency for the tailplane. As it vibrated the noise was unbelievable, stopping all work in the hangar. We did not persist in this mode so the structure was undamaged. Overall the tests were very successful and as we gained confidence we also tested with the undercarriage retracted and with both 230 gallon ferry tanks and 90 gallon drop tanks fitted. We were pleased that were no accidents whilst testing and when it was all over the equipment was put away in R&D’s Nissen hut store, never to see the light of day again until it was scrapped.

I think that Bill Turner, responsible for the Experimental Hangar, had a ‘thing’ about allowing photography indoors because, sadly, no pictures were taken of this very interesting series of tests.