Wings under test (1947)
A film produced for C.S.I.R.’s Division of Aeronautics on the equipment and methods used in the testing of aircraft wings.
[Music plays and Coat of Arms appears on screen with text: Council Scientific & Industrial Research]
[Image changes to show a plane flying with text: Wings Under test]
[Image changes to show text: Produced by Information Service, C.S.I.R in collaboration with C.S.I.R Division of Aeronautics]
[Image changes to show a plane taking off from the runway]
Narrator: In Australia today 30,000 route miles of airlines are in continuous operation and air travel is accepted as part of our everyday life. The safety factor is still however a much-discussed topic of travellers.
[Image changes to show people boarding a plane]
Probably because first they are not aware that on a mileage basis air travel is safer than road travel and secondly they have not been fully informed of the efficient organisation technical skill and scientific endeavour which has and is being applied to ensure freedom from mishaps.
[Image changes to show a plane taking off from the runway]
Operational value is the paramount consideration in the design of the aircraft for the Royal Australian Air Force but the safety of the air crew is provided for to the maximum possible extent.
[Image changes to show stationary planes and a plane performing different manoeuvres in the sky]
Enormous loads are imposed on the wings by violent manoeuvres an adequate strength is insured by careful design and the most searching tests so that accidents such as this shall not happen.
[Image changes to show a plane flying. The wing breaks away from the main part of the aircraft and the plane plummets to the ground and explodes]
[Image changes to show an outside shot of the Council for Scientific and Industrial Research building]
To investigate problems associated with aircraft and to prevent such accidents the Council for Scientific and Industrial Research has established the Division of Aeronautics at Fishermans Bend, Melbourne. In order to give a clear idea of the methods used in the laboratory for applying test loads to aircraft wings let us first use this model to demonstrate the method of investigating the strength of wings.
[Image changes to show a model aircraft]
The loads in normal flight act upwards on the wing surfaces but with a large wing it’s often more convenient in the laboratory to turn the wing upside down.
[Image changes to show a man demonstrating on the model aircraft what the narrator is saying]
To support it centrally from the test frame at two points and to apply the loads downwards. Loading frames bearing on specified positions are placed around the wing. To produce the desired loading distribution the frames are connected by a lever system and then by cables to hydraulic loading units bolted to the floor, which is designed to resist the maximum test load. The loading units are worked by oil pressure. The oil being supplied through a common pipeline from a single pumping unit. The pressure and therefore the load can easily be controlled by one operator. Now that we have the idea, let us follow in some detail the procedure required for testing an actual wing in the laboratory.
[Image changes to show a truck with what appears to be an aircraft wing driving off. Image then changes to show the wing being prepared for testing in a test bay]
At the end of World War Two, Australia was producing large numbers of the renowned Mosquito aircraft and it’s fitting that we tell of the testing of the wings of these famous fighter bombers. The wings are tested in a test bay having a reinforced concrete floor 120 feet in length and capable of withstanding a uniformly distributed load of 300 tons. However, before the wings are placed in test to determine their strength and confirmation to airworthiness requirements much preliminary planning must be carried out, a considerable portion of it in the drafting room.
[Image changes to show a roomful of people seated at tables and sketching]
The first thing is to consider the design of the wing. Then to determine the test loads and finally to plan the loading system. The laboratory workshops are fully equipped to manufacture the wide range of specialised testing equipment required.
[Image changes to show inside a laboratory where parts are being prepared and assembled for testing]
A good example of the precision work necessary is the manufacturer of the hydraulic loading jacks, the rams of which must be ground to very close tolerances. The jacks are of the packless ram type and the sliding parts must have a very high surface finish to eliminate friction because the actual test loads applied to the wing are determined by measuring the oil pressure in the hydraulic system.
[Image continues to show the preparation for testing going on in the laboratory]
Meanwhile the wing has been prepared for testing.
[Image changes to show the wing being lifted and turned over]
It is turned upside down. This position being a convenient one in that the simulated air loads may be applied downwards by the loading units attached to the test floor. In interpreting the following sequence, the inverted position must be borne in mind. As part of the plan method to obtain correct load distribution between the fuselage and wing, a dummy fuselage is attached.
[Image changes to show two men putting the covers in place and attaching them to the test wing]
The tank bay covers are part of the stress structure and are carefully screwed into position. While still on trestles, the loading stations are marked out in accordance with the loading plan. After as much work as convenient has been carried out with the wing at ground level it is hoisted up and moved into the test position.
[Image changes to show the wing being lifted up]
This wing is one of a series of Mosquito wings being tested and therefore the lower sections of the loading frames, by means of which the loads will be applied are already in approximate position above the hydraulic jacks. Great care is taken to ensure that no damage is done to the wing. This is particularly important because the Mosquito wing derives much of its strength from the outer skin of plywood.
[Image continues to show the test wing being put in position and then further preparations being carried out on it]
There is a double skin on the compression surface and a single skin on the tension surface. To the center section at the wing steel structures are attached representing the front and rear fuselage section. As mentioned earlier these dummies structures are necessary to ensure the correct loading conditions at the wing fuselage joints. The front and rear sections are connected by steel cross members and the whole structure suspended from a rigid test frame by two stud links. One at the front and one at the rear. The wing and its dummy fuselage are therefore free to roll about the central cord wise axis. In order to represent the engine loads it is necessary to apply loads to the engine mounts.
[Image changes to show the engine mounts being applied to the test wing]
These loads are approximately opposite in direction to the air loads therefore in the inverted test position the engine loads must be applied upwards. The engine mounting is connected by a ball and socket joint with a stout steel column. The column transmits the upward load from a bank of five hydraulic jacks. On the previously marked positions pivoted loading blocks covered with thick felt are placed along the wing. One set on a front spar and one set on the rear spar. On these blocks the loading frames will rest.
[Image continues to show the preparations being carried out on the test wing as being described by the narrator]
The frames for applying the air loads are now placed in the approximately correct positions along the span of both port and starboard sides. The frames may be put on singularly or if the wing under test is one of a series of the same type of wing, in a group which is more convenient and saves considerable time. The lower part of each loading frame is then raised and attached to the opposite member. These lower sections are not in direct contact with the wing but transmit the load to the upper frames by front and rear connections. The testing equipment applies loads to these lower beams by a lever system. The position of the lever connecting links is carefully adjusted to obtain a correct test load distribution from frame to frame. The load is applied to the lever system by flexible steel cables connected to hydraulic jacks some of which we saw being manufactured in the workshops.
[Image continues to show the preparations being carried out on the test wing as being described by the narrator]
The jacks are actuated by oil supplied by pumps capable of delivering at pressures as high as 3,000 pounds per square inch. The loading units are bolted to the specially constructed test floor. As the wing supports over its span the weight of the test rig as well as the superimposed load, the rig is weighed and the required allowances made. It is usually necessary then to add weights to parts of the rig so that the rig weight everywhere will be a uniformed percentage of the design ultimate test load. Deflections of the wing during the test are measured by fine steel wires attached at desired positions along both front and rear spars.
[Image changes to show the steel wires being attached]
The wires pass around pulleys clamped to the floor and then over a deflection measuring scale attached to an indicator board. One board being at each wing tip. In this way deflection of the wing during the test can be measured quickly and accurately. As the wing is suspended at only two points it will, if the loading is slightly asymmetrical, tend to roll to the most highly loaded side. It is therefore necessary to take measurements at the center of the wing of the amount of roll in order to make corrections to the deflection readings. A final check-up is now made of the equipment. The loading frames and their positions are examined.
[Image changes to show scientists looking back over the equipment applied to the test wing]
As are the hydraulic jacks and associated cables and levers, the pumping units and all the other many parts of the equipment. Slings are attached to the overhead electric hoists in order to catch the fractured wing section when ultimate failure occurs. The wing is now ready for test. The first phase of the test is to apply a proof test load. The oil pumps are started and the load maintainer valve opened. The load maintainer controls the supply of oil at constant pressure to the two outer jacks which apply a balancing load to each engine mount Sufficient balancing pressure is now applied to all the loading jacks to take out the slack in the rig. The jacks are so weighted that all float at the same pressure. Unlike the outer engine jacks, which exert a constant force throughout the test the inner engine jacks apply the varying test load on the engine mounts.
[Camera pans over the pump operator at his equipment and all the equipment applied to the test wing]
The pump operator calls for all the recording equipment to be set at zero. And he sets his oil pressure gauge at fifty pounds per square inch which is the calibrated pressure at which the jacks float. The zero readings of the dial indicators at the wing’s centre are recorded. The wing deflection indicators are also set at zero. The first loading is to be 25 percent of the design ultimate load. The load is gradually applied in planned increments. At the previously directed levels in the loading the pump operator holds the hydraulic pressure constant, calls the loading, in this case 25 percent, and directs that all deflection readings be recorded.
[Image changes to show people recording at different areas of the test wing]
The deflection markers give a direct reading of the deflection at the wing at specified positions along its whole length. The dial indicators at the wing’s centre record any slight roll of the wing to either side. To check that the hydraulic system and levers are transmitting the calculated load, proving loops and also strain gauge links have been incorporated. After all readings have been noted the load is increased in set stages until the load has been built up to the test proof load. The test proof load which is arbitrarily selected and specified by the Air Worthiness Authority must be applied for at least one minute during and after which the wing must remain in an airworthy condition. All the Mosquito wings used in these particular investigations were proof loaded to 90 percent of the ultimate design load.
[Image changes to show the scientists thoroughly inspecting all different areas of the test wing]
Having withstood the first proof loading the wing is now subjected to repeated loadings to 90 percent.
[Image changes to show the pump operator starting the pump again]
All pumps are switched on. Failure might occur from fatigue or if the wing does not fail under a specified number of loadings its ultimate static strength may have been influenced. Repeated load testing is in its infancy. The staff at the Division of Aeronautics being the first to carry out such work on large aircraft structures. In this particular case the peak load of each cycle is over 50 tons. No small load to be applied and released up to six times per minute.
[Image changes back to show the testing being carried out on the test wing]
The repeated load control permits the load to cycle between any desired limits. A counter records the number of cycles. The wing has now withstood 5,000 repeated loadings without any apparent damage. The final test, the determination of the ultimate strength of the wing, necessitates testing to destruction. The load is to be applied in scale increments up to 90 percent of the design ultimate load and then gradually increased until ultimate failure occurs. This crucial test of the wing’s strength is witnessed by representatives of the organisation for whom the test is being carried out, in this case the Royal Australian Air Force and by the senior officers of the Division of Aeronautics.
[Image changes to show the pump operator receiving instructions and then back to the testing being conducted on the test wing]
At 90 percent, to which point the wing has previously been proof loaded the pump operator directs the recorders to make readings.
[Image changes to show readings being taking by scientists from the test equipment]
Then to remove the dial gauges from the centre of the wing and finally to stand clear. The load will now be gradually increased until failure occurs. The loading has increased to 100 percent of design ultimate load. From previous experience it’s considered the failure will be in this area.
[Image changes back to the test wing and sounds of the pumps can be heard. The test wing collapses and scientists move in closer to inspect the collapsed wing]
The failure is typical of this type of wing. The ultimate strength of the wing was in excess of the designed ultimate load. It represented a force of approximately nine times gravity acting on a fully loaded Mosquito aircraft weighing over eight tons.
[Image changes back to show the pump operator recording the figures from the test equipment]
Upon removal of the loading frames the collapsed wing is more clearly revealed.
[Image changes to show the frames being removed to reveal the collapsed wing]
As part of the complete record of the test the various failures are photographed and some of these photographs used to illustrate reports.
[Image changes to show people taking photos of the collapsed test wing]
To investigate the cause of failure it’s necessary to cut the wing up for thorough examination.
[Image changes to show the wing being sawn-off and removed]
Material test coupons are also removed and from these the laboratory workshops prepare test specimens.
[Image changes to show the collected specimens being tested]
In the Materials Strength Testing Laboratory these test specimen are examined to determine the basic strength properties of the material built into the wing. Routine specification tests are made of impact strength or toughness. Tensile strength is also an important characteristic. As is strength in compression.
[Image continues to show the collected specimens being tested]
With wooden constructions the moisture content of the specimens is an important factor. Besides all these routine tests the laboratory is well equipped for the numerous non-standard tests often required in research work.
[Image changes to show a man seated at a desk writing notes]
Finally, having obtained all the test results, it is necessary to analyse them and prepare a report. This report covers the wing tested but similar reports are issued covering the numerous applied and fundamental problems associated with the aircraft industry and carried out by the Division.
[Image changes to show the report being printed and compiled]
[Image changes to show people seated inside a plane]
The design of faster and more efficient aircraft and the safety and comfort of all who travel by air are dependent, to a considerable extent, upon aeronautical research and the wide field of general scientific research being carried out in Australian and other laboratories throughout the world.
[Image changes to show a plane flying]
[Music plays and Coat of Arms appears on screen with text: The End C.S.I.R]
[Music plays and CSIRO logo appears with text: Big ideas start here www.csiro.au]