Quote:
Originally Posted by Crumpp
Why do you think bob-weights were added to the design to fix the longitudinal stability?
Because they just felt like changing something? Maybe they were bored and had nothing else to do?
Or maybe, just maybe, there was a real engineering issue with the longitudinal stability that required a real engineering solution?
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Quill devotes an entire chapter to longitudinal stability (pages 229-241 Murray 1983) in his book "Spitfire a Test Pilot's Story" in which he goes into detail about the problems involving several Spitfire Vs which broke up
in 1942; Quill describes the problem of Spitfire Vs breaking up; he then goes on to describe the solutions which were a): to ensure that when new equipment was added that the loading was kept within limits and b):the design of bobweights, which were added to the elevator circuit, as well as the modified elevators fitted to later Spitfire marks. After the bob-weights were fitted, and the loading sorted out, the problem disappeared.
Quill
"
In general configuration the Mk I and Mk II production aeroplanes were almost identical to the prototype and so there was no problem with their stability." (231-232)
"The Mk III Spitfire did not go into production, but the success of the bobweight experiment in curing its instability...opened up the possibility of its use for later marks of Spitfire....which was just as well as we had to...respond to a nasty situation which developed in 1942.
The Mk V aircraft was...in full service with Fighter Command and,...a fair amount of additional operational equipment had gradually crept into the aircraft, most of it stowed within the fuselage.
The aftmost acceptable position for the aircraft's centre of gravity had been fixed in the mormal course of flight testing by the firm and by the A & AEE....Any rearward movement of the centre of gravity in service, for whatever reason, would begin to destabilise the aircraft. Therefore, for each sub-variant of the Mk V detailed instructions for the correct loading of the aircraft were issued to squadrons....However the importance of these loading instructions was not generally appreciated in squadrons and in the daily round of operational activity they tended to be disregarded." (pages 232-233 -Quill goes on to describe 65 Sqn's Spitfire Vbs which were found to be dangerously unstable)
"There was thus a real chance that, as of that moment, in almost every squadron in the Command Spitfires were flying in a dangerous state of instability....Up to that time there had been a distressing and increasing incidence of total structural failure of Spitfires in the air, which was causing great comcern in the MAP and especially at Supermarine." (pages234-235) Once the bobweights had been introduced and, in later marks, the modified mass balances on the elevators...it was statistically established that, as soon as the longitudinal stability of the Spitfire was thus brought under control, the problem of the unexplained breakings-up of aircraft in mid-air,...'softly and suddenly vanished away'." (page 238 )
So the bob-or inertia weights were introduced in 1942 to help solve bad loading or overloading of Spitfire Vs at Squadron level - it had nothing to do with NACA's report.