Limit Load Factors WW2 Fighters eg. Spitfire, Mustang

[QJB]

Just wondering if anyone has an idea of the limit load factors for the famous WW2 fighters such as the Spitfire, P51, Zero etc. Guessing they didn't use g meters back then. Was there ever any cases documented of fighters breaking appart due to g loads rather than enemy fire?

J

[barit1]

A Grumman F8F-2 Bearcat had a wing failure during a low pass (http://www.ntsb.gov/ntsb/brief.asp?ev_id=60113&key=0) at an airshow. The wingman reported heavy turbulence.

[27/09]

No in flight break ups that I'm aware of.

In the case of the Spitfire the wing loading went from 24lb/sq ft on the Mk 1 to just over 42lb/sq ft on the Seafire 47. MK 1, 5820 LB TOW, the MK 47, 12,500 lb overload TOW. To quote Jeffery Quill who was responsible for much of the development test flying of the F.37/34, the equivalent of the MK 1 carrying 32 airline pax, each with 40 lb of baggage.

Yes there were differing wings fitted as the aircraft was developed but these figures sure are a testament to the original design.

[Brian Abraham]

British fighters of the era were built to a lower "g" tolerance than American. When North American built the lightweight Mustang (P-51H) they used British standards in order to keep the weight down. Many a Spitfire broke up in mid air as a result of overly enthusiastic maneuvering. All the records, with rare exception, report the pilot being killed, so no hint on what he may have been doing when things came unstuck.

[27/09]

Many a Spitfire broke up in mid air as a result of overly enthusiastic maneuvering. All the records, with rare exception, report the pilot being killed, so no hint on what he may have been doing when things came unstuck.

This made me go and lookup an incident Johnnie Houlton mentions in his book "Spitfire Strikes". This incident gives some idea what stress these aircraft could sustain and still bring their pilots home.

He had been testing out the supercharger aneroid control on a new Mk IX, and from 30,000 ft decide to do a Trim and Dive test on the way back down. He half rolled with 70% power and trimed into a vertical dive.

To quote;

"What followed next arose from a combination of 'finger trouble' and ignorance.....

.....when I started to pull out of the dive, the aircraft was flashing towards the earth at 1000 ft/sec. As I eased back on the stick and the elevator trim together, the nose of the Spitfire lifted about 20 degrees from the vertical, then the stick just "fell free" - I moved it quickly in all directions, and felt only 'snatching' here and there. Simultaneously the aircraft began to roll very slowly to the right, and I flicked on two quarter turns of nose-up trim on the trim-tab wheel....

.....The elevator sudddenly took over again and the Spitfire reared violently out of the dive, but the savage g force, forced my whole body downwards....... I was peering through a rain blurred window at a vague and wobbly image: "that's the instrument panel of a Spitfire . . . it's upside down . . . what am I doing in a Spitfire?" The panel flopped right side up, and I levered myself upright, to find the aircraft back at 9000 ft and still making sluggish upward rolls to the right. As I collapsed under the g force the split-type stick had folded across my right leg,. My goggles were over my mouth, my oxygen mask under my chin and I ached all over.....

.....The metal skin of the Spitfire wings were badly buckled and was 'oil canning' freely, and the wing tips were twisted....."

Imagine what stresses must have been applied to the aircraft that were lost through structural failure, though perhaps in some cases it could have been a culmination stress over a period of time.

[Jane-DoH]

I remember hearing about the Spitfire using a spar-boom design which actually consisted of five concentric tubes in which as you went back further, the outer tubes were progressively cut away.

Was this design feature lighter for the same strength than the structures used on US aircraft designs?

[rigpiggy]

not a tube but several laminations. I don't have the blueprints or pic of the spar under construction, but here is a good indication. in the fuselage pic there is a wing beside the aircraft
Spitfire Restoration Photo Gallery (http://www.fighterfactory.com/airplane-gallery/spitfire-restoration-photos.php)
and a main spar being rebuilthttp://www.maltaviationmuseum.com/images/spitfire/18b.jpg

[Brian Abraham]

Had a thumb through “Spitfire – The History”, Morgan & Shacklady and came up with the following.

Specification No. F 7/30 to which the Spitfire was designed.
Structural Strength

(a) The strength of the main structure when carrying the load specified in paragraph 3, plus 100 lb shall not be less than as specified hereunder

Load factor throughout the structure with the centre of pressure in its most forward position 9.0.

Load factor for wing structure with the centre of pressure in its most backward position in horizontal flight 6.0

Load factor in terminal nose dive 1.75

Inverted Flight

(1) Load factor at incidence corresponding to the inverted stall and with C.P. at 1/3 of chord 4.5

(2) Load factor at incidence appropriate to steady horizontal inverted flight and at the maximum speed of horizontal normal flight 4.5

(b) The alighting gear must be able to withstand an impact at a vertical velocity of 10 feet per second and at this velocity the load on the alighting gear must not exceed three times the fully loaded weight of the aircraft.

(c) When subject to the impact forces on alighting, as specified above, the load factor for the alighting gear must not be less than M/3, and for the remainder of the structure not less than 1- 1 /2. The load factor for the structure and the attachment fittings of the alighting gear must always be greater than that for the alighting gear itself by the margin indicated above.

(d) The maximum weight per wheel of the aircraft in pounds must not exceed 12 times the product of the wheel and tyre diameters in inches with the aircraft carrying the full load specified above.

(c) The above factors are to be determined by the approved official methods as published by the DTD and the detail requirements given in A.P.970 are also to be satisfied. With a view to minimising the risk of flutter, attention should be given to the recommendations of A.P. 1177, particularly as regards the static balance of ailerons.

(0 The wing is to be sufficiently rigid to withstand satisfactorily any torsional or other loads which may be encountered during service operations.

(g) Ribs (both mainplane and tail unit) are required to develop, on test, factors 20% greater than those specified for the aircraft as a whole.

Towards the end of 1942 Supermarine were investigating the means by which the aerodynamics of the wing could be improved. It had faults, the worst being the inherent aeroelasticity due to the light structure behind an immensely strong torsion box leading edge.

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.

Flight manuals of the period make no mention of “g” limits, they didn’t have “g” meters in any event. I guess they applied Rule 27 of the fighter pilots creed, “The aircraft G-limits are only there in case there is another flight planned for that particular airplane. If subsequent flights do not appear likely, there are no G-limits.”

Edited to add: A feature of the wing which contributed greatly to its success was an innovative spar boom design, made up of five square tubes which fitted into each other. As the wing thinned out along its span the tubes were progressively cut away in a similar fashion to a leaf spring; two of these booms were linked together by an alloy web, creating a lightweight and very strong main spar. Wing root piccie.

http://i101.photobucket.com/albums/m56/babraham227/s.jpg

[rigpiggy]

I stand corrected, my thoughts were more along line, of the round tube ala BD-4/5 fame

[Jane-DoH]

Brian Abraham

A feature of the wing which contributed greatly to its success was an innovative spar boom design, made up of five square tubes which fitted into each other. As the wing thinned out along its span the tubes were progressively cut away in a similar fashion to a leaf spring; two of these booms were linked together by an alloy web, creating a lightweight and very strong main spar.

I'm just wondering how strong it was compared to other aircraft designs for it's weight, particularly ones built around the same time in the United States

[DERG]

Yeah Robyn me too...would be really interesting to compare the Spitfire with the German ME 109...would it not? I think the Spitfire was lighter and stronger but not sure.

[27/09]

The first post mentioned the Zero.

It was apparently quite a formidable foe, it was light and very nimble and could out manoeuvre most of the American fighers in the Pacific arena, but there was one way many Allied pilots brought the Zero to it's doom and that was to dive away from the Zero and pull quickly out of the dive, the ensuing "G" forces were too much for the engine mount, or the structure it was attached to, on the Zero and the engine would "drop out".

In the end the Zero's light structure, while being a big advantage performance wise, was it's archilles heel.

[Landroger]

Yeah Robyn me too...would be really interesting to compare the Spitfire with the German ME 109...would it not? I think the Spitfire was lighter and stronger but not sure.


I seem to remember from Len Deighton's 'Fighter', that Bf109 pilot's would avoid direct high 'G' manoeuvres against the Spitfire because although the 109 could, aerodynamically, out turn its rival, Luftwaffe pilots were reluctant to press on that hard. This was because - however well the 109 was designed - the wings were removable at the roots; the undercarriage is fixed to the fuselage, not the wing as the Spitfire. And they would often demonstrate this weak point - dramatically.

The main advantage the 109 had over the Spitfire was the fuel injection/carburetter issue, allowing the Messerschmidt to push over into the dive, where the Spitfire had to half roll and pull.

Roger.

[bearfoil]

The F8F Bearcat had tips that were frangible. At "past critical" cantilever, the tips would separate chordwise, perhaps 36 inches inboard. One or the other, or both. It was thought that a pilot had a chance with most of a wing rather than none. It happened more than once.

I'm calling this a true story.......

[barit1]

True about the Bearcat wing - in fact there was a small explosive charge at the "structural fuse", cross-connected so if one side broke, the other would immediately be blown off. :eek:

However, after an accident (possibly Blue Angels?) they decided this wasn't so hot an idea, and reworked the wings to remove the "feature".

[henry crun]

Corky Meyer did wing tip separation test flying in the Bearcat, what follows is abbreviated from the full story in his book Flight Journal.

Grumman knew about the problem before the aircraft entered service, and carefully designed a joint in the wing and aileron about 3ft in from the tips so both would separate at the same time.
They were also required to demonstrate takeoffs and landings with both, then alternate tips removed.

However, once in service there was a fatal accident when only one tip separated on a dive bombing pullout.
It was after that acident that the idea of blowing tips off was tested, and after some problems was approved for service.
Prima cord was used at the joint, cross connected with micro switches so that if one was pulled off the other would blow off.

However they had not built in any safeguards against maintainance mistakes, and in service on a carrier there was a short circuit which blew off the tips and fatally injured a mechanic.
That was the end of the blown tips idea.

The tips were firmly fixed in place and the load limit was reduced to 4g; but then pilots were exceeding that limit and wings were coming off at the root.
This was corrected by beefing up the structure so that the G limit was increased to 7.5.

[Jane-DoH]

Does anybody have any guesstimates as to how much weight the Spitfire's spar-boom saved over a more conventional one?

[barit1]

The Spitfire's main spar was unconventional only in its fabrication details. Tapered spar caps were not unusual in wooden cantilever wings, and a tubular-element truss spar was used in the contemporary Spartan 7W personal aircraft (albeit with steel tubing vs aluminium).

The use of hollow tubing is particularly weight-efficient for compressive loads (e.g. the upper spar cap in positive-G loading).

Another efficient structure is the corrugated inner skin, riveted to the smooth outer skin, developed by Jack Northrop about 1930, and used in the DC-1/DC-2/DC-3 series. There is no heavy spar, only a series of several lightweight spar webs to maintain the airfoil shape.