THE AILERON PROBLEM
When Mitchell finalised design of the Spitfire wing there were a lot of unknowns about its efficiency and strength, and the effect of aileron movement at high speed could not be fully appreciated as wind tunnel facilities were practically nonexistent and experiments on the actions of flying controls had to be conducted on full scale aircraft. Through its early production and service life the Spitfire had trouble with aileron control and in an attempt to resolve the problem a meeting was held at MAP, Millbank, London, on 22 July 1942. Squadron Leader Raynhan of the Accidents Branch said that the most significant fact emerging from recent Spitfire accidents was that no change in the type of failure had been brought about by the introduction of the inertia device or by moving forward the CoG. This, he thought, pointed to aileron instability. Also, there had been evidence of ailerons flying right up at a very early stage of the accident in certain instances, and failures of the aileron circuit which could not be explained by the wing coming off. Further, there was definite evidence of pilots who, during tests for upfloat, had observed the rate of upfloat on reaching certain speeds went up suddenly and disproportionately to the increase in speed. The instability was not necessarily any form of oscillation or flutter, but more in the natures of divergence.
Mr Perrring, for the RAE, said that the accident rate per hour appeared to be fairly constant and thought a number of accidents were due to mishandling. Stiffness of the aileron circuit depended on the initial tension in the cables, and these were set by aid of the tensometer. The normal cable was 15cwt size and a simple method of increasing stiffness would be an increase in size to 25cwt.
Full scale tests in the Farnborough 25ft wind tunnel had failed to divulge any large unstable hinge moments, except when absurdly maladjusted. It was found necessary to drop the aileron by 3in in relation to the wing in order to obtain aileron instability. However, the tests had also shown that e upfloating hinge moment might vary by 100% on nominally similar ailerons, and this explained the difficulties in matching ailerons on any given Spitfire.
Mr Perring, continuing, said he had been struck by the frequency of the mention of fuselage frame No 19. Out of 36 accidents the tail unit had come off in the air in 24 and the pilot had been thrown out in 15. The tail unit usually came off at frame 19 (comment: this was the bulkhead and transport joint immediately forward of the fin/tailplane). It was suggested that failure would seem to be associated with lack of strength of attachment of the skin rather than in the attachment bolts. The Type Record had cleared the attachment strength, but owing to concentration of stress due to the discontinuous stringers, the local rivet stress might be doubled. There was a good joint in the lower longerons, but the joint formed between two stringers on the fuselage sides was thought to be poor.
Wing Commander Mayes was not satisfied with the proposed safety mods - inertia weights, aileron droop, aileron reflexing and circuit stiffening. These, he thought, were making the Spitfire more dangerous on operations by worsening the controls. He stated the result of the mods might save the inexperienced pilots at the expense of experienced pilots on operation.
Dr Roxbee-Cox, discussing the inertia device, said what was needed was an instrument which exerted control progressively with speed increase, and it might be possible to devise such an instrument by linking up with the pitot head. Theoretically, however, all such inertia devices were unsound in that they could be 'cheated'. The RAE agreed to investigate the effects of varying the initial tension in the ailerons circuit, the change in elastic stretch under lead and the change in elastic stiffness with wing deflection.
At a further meeting at MAP which Clifton, Summers and Smith attended, it was decided that waffling of ailerons should be applied immediately to one squadron of Spitfires at Kenley. This was a means to reduce aileron up-float. Conditions aimed for were a 3/8 in droop initially and mean uplift zero at 450 mph. One Polish squadron had fitted 8 inch aileron tabs.
In August 1942 Spitfire R7267 (F Mk V) was used by Supermarine to determine the effects of variations in rigging and form of ailerons on stability. The aircraft was dived to speeds up to 400 mph with ailerons rigged to zero droop and with them rigged to ½ in negative droop. The following October a Mk VA, X4922, was used as a trials aircraft by Supermarine for high speed dives, with and without an inertia weight on the elevator circuit, to test the effect of aileron droop.
ACCIDENTS
No aeroplane can escape the rough and tumble of service fife, particularly in war time, and there were numerous incidents when the Spitfire came to grief because of mishandling by the pilots who flew it. The incidents recorded below are indicative of what could happen and some of the cures recommended.
"A visit was paid to Farnborough on 16 September 1941 to obtain particulars of cracks and rivet failures in the skin over the wheel well on Spitfires. A number of defects on the plating over wheel wells have also been reported. These occur after upwards of 100 hours flying and take the form of circumferential cracks between the rivets connecting the plating to the vertical wheel well. The cracks have the appearance of fatigue, which suggests the presence of vibration. But the pulling out of rivets seems to confirm estimates which show that this panel and its attachments are highly stressed under the local pressures.
In connection with this failure one of the above panels has now been removed after a break up in the air. On inspection it was found to have about 2 in of dishing. The Accidents Dept say that the rivet holes in the plate show that it failed under normal load before the main spar. It is significant that the panel did not fail at the vertical wall attachment rivets, which indicates that the cracks and rivet failures discussed would not have had serious consequences.
"One PRU aeroplane had severe 'oil~canning' of one wheel well plate. On the same wing the rivets, in a number of rows, were connected by a continuous scratch. This is thought to be caused by the dolly (anvil) in rivetting, and while not serious in itself would encourage the formation of cracks if occurring at a troublesome point, for example, at the wheel wells".
Visit to Hornchurch 30 September 1941. "Buckling of the top skins in the leading edge and behind the main spar have occurred on a small number of Spitfires. These are due to discontinuity of stringers and are not of serious consequence. When sizes of buckles are excessive they should be dressed out and rivets replaced. It will be necessary to return the wings to repair units for this purpose.
"Two of the Spitfires had been subjected to violent manoeuvres and although the root pins were not appreciably bent, the buckles indicate the wings had been subjected to considerable loading. The buckles behind the main spar are not detrimental to strength and only seem to occur on
VB wings."
"Aircraft AB200 was inadvertently subjected to high loading in flight by a squadron leader, and it was found that the wings had been damaged. They were removed and sent to Eastleigh for examination. There was pronounced chordwise wrinkle in aft skin between outboard side of wheel well and rib, running out at corner of gun door. Wheel well panels and stiffeners have appearance of having been subjected to heavy upwards loading. The covering and stiffeners have further been damaged by hammering to accommodate wheel and by use of this area as a walkway. Wing attachment bolts not bent or damaged are being used on the aircraft to attach new wings. The wings are being sent to AST for repair."
"Spitfire AA912 examined at No 1 CRU for suspected tailplane failure. May 1942. Squadron Leader Craxston said that in the course of a dive to 465 mph violent oscillation of the elevator occurred while pulling out gently. The oscillation was of high frequency and violent enough to throw him about the cockpit and he thought that a tail surface was coming away. He closed the throttle and eased the machine out even more gently, whereupon the oscillations ceased and he was able to make a normal landing. The oscillations lasted a few seconds and he had kept the Spitfire fully trimmed into the dive. There was no abnormal tendency for the elevator control to take charge in coming out of the dive.
"On external examination the starboard tail plane appeared to have partially failed while the port appeared normal. The trim tabs and control circuit was quite normal. An examination of main planes and ailerons revealed no defects. An inertia device was fitted. When the tail unit was removed it was found that the tail plane spars were of the non-reinforced type. It had been removed from Spitfire X4916 and fitted to AA912. The starboard front spar had failed under down load at the first lightening hole, the lower flange and both webs below the hole being buckled and completely fractured. The port front spar had also failed under download at the first lightening hole in exactly the same manner. No 1 CRU have been instructed to fit the modified tailplane.”
Tangmere 31 May 1942. "During an interception flight in very bad weather in the Winchester area the pilot of this Spitfire VII was leader of two aircraft. His aircraft was badly damaged in an engagement but he was able to note that the other Spitfire went into a high speed dive. Ground witnesses saw wreckage coming through clouds consisting of fuselage only, both wings being off. The fuselage caught fire after impact and was almost destroyed. The pilot who returned to base thought the other pilot lost control.
"The port wing had broken off at the root bolts and the leading edge rivet seam was sheared from one foot outside the cannon to the outer m/g, indicating high torsion due to aileron flutter. Practically all the structure aft of the spars was broken away. The aileron lever rivets were sheared on the starboard wing. On the port wing, part of the lever comprising the two arms was broken off from its attachment flange and pulled through by the cables to rib 12, where it jammed. Both rear cables are thought to have broken first, ie. those holding the ailerons down.
"The starboard tailplane is thought to have broken upwards, shearing the bolts. The tail end had torn away at the rear joint rivets but was intact. It is considered that structural failure was due to excessive normal loading, produced by an uncontrolled pull out at high speed".
Hampnet, near Tangmere, 1 July 1942. "Spitfire B1-513 has been reported by Accidents Branch to have developed signs of strain in the fuselage. It had waves and buckles at frames 14 and 19 and a small bulge in the vertical plating of the aft portion of the wing root fillet. Upon removal of the tail wheel fairing the leg was found to be bent to port. Damage was diagnosed as the result of heavy landings. The aircraft had been used for target towing on the flight prior to discovery of the damage and had lost its drogue by fracturing the tow line. A specimen of the line failed at a load of 400 lbs".
Spitfire V11 EP335 at Charmy Down, 20 April 1943. "The aircraft was damaged during combat exercises and during pull out of a high speed dive the pilot blacked out, but made a normal landing. The pilot said the aircraft came out of the dive suddenly of its own accord, thereby causing him to blackout. The wings and centre section were buckled and the tail plane damaged".
One disturbing factor emerged after the
VB had been in service for some time. There occurred a number of totally unexplainable accidents in which Spitfires dived into the ground for no apparent reason. The Accidents Branch investigated and eventually issued the following statement - "It had been found that firing the
VB's cannons damages, in some ways, or dislocates the oxygen regulating apparatus so that thereafter the rate of supply cannot be varied". Another factor considered was the run of the oxygen piping, thought to be unneccessarily long. The statement said - "The greater the length of piping the more chance there was of a stoppage of supply due to a collection of condensation freezing, with the result that the pilot would black out. This investigation was most thorough and the recommendations saved the lives of many pilots.
The Accidents Branch was also very active in rooting out defects on the Spitfire production lines. The problems were enormous due to the dispersal scheme and the need to adhere rigidly to specified standards. In November 1942 a manufacturing errors list was prepared and it resulted in a tightening up of quality control. Rivets attaching aileron control sprockets at the base of the pilot's control column were, normally, of stainless steel, but on numerous occasions duralumin rivets were used, rivetting of the leading edges of wings were repaired with aluminium instead of Alclad; rivetting of the leading edge was also completed with non-standard rivets with the result that some heads stood proud of the surface by as much as 1/32nd of an inch. Apart from the effect on wing strength they also caused loss of performance. The list ended with these words - "An essential part of the organisation for ensuring structual strength of aeroplanes is the arrangement whereby careful inspection is carried out at every stage from raw material right through to the finished product. As a result of non-function the structural safety of the Spitfire in certain cases is being most seriously affected".
The Spitfire F Mk V was declared obsolete for all RAF purposes in September 1945, and in March 1948 the remaining VBs and Vs in storage were scrapped.