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Testing Times

Testing Times Aircraft Engineering THE MONTHLY ORGAN OF THE AERONAUTICAL ENGINEERING PROFESSION VOL XXVII No 313 MARCH 1955 easy to be wise after the event but we now know that it is not possible to assess accurately stress concentrations from approximate calcula­ Y a coincidence, the report of the Court of Inquiry into the two tions and tests on sections of fuselages. Similarly, we now know that Comet accidents in January and April 1954 is published just as it is not enough to take a length of the fuselage and test it without Bwe are on the point of going to press with this issue which con­ reference to the rest of the structure. The stress in the fuselage tains M R PEACOCK'S Stress Analysis Group Conference paper on the imposed by the dynamic forces on the nose and tail sections in gusts flight flutter tests on the Javelin. and manoeuvres has also been shown to be so large that it may not be neglected. Seven years ago designers had to be content with ap­ Analogous Cases proximations in many instances where detailed calculations would In the case of the latter the aeroplane shed its elevators owing to have been a matter of months—almost years. The development of flutter in conditions where theoretical calculations had not shown the computers to the stage they have now reached means that the thought possibility of this phenomenon developing. The accepted view of the of such calculations is no longer so daunting as it was and such in­ cause of one, and probably both, of the Comet disasters is failure of vestigations can now be undertaken that even two or three years ago the fuselage structure as a result of fatigue which theoretical calcula­ would have been considered quite impracticable. tions in the design stage had not foretold accurately. In both instances, subsequent practical tests—in the one case in flight and in the other in a ground rig—have proved the existence of conditions which theory The Implication had not revealed. That is the link between the two investigations. Even so, the net result of the Court's inquiry is that more, not less, The Gloster tests mark an important step forward in being the practical tests of civil aircraft are necessary. This will involve the first time that such a series of practical ad hoc investigations has been allocation for tests to destruction of at least two airframes of every made in flight. We must pay a tribute to the ingenuity of the methods new type of commercial aeroplane intended to operate at high devised to reproduce the supposed conditions by artificial means and, altitudes. particularly, the careful manner in which the safety of the pilot was There will have to be simultaneous tests of extreme severity in successfully ensured in the gradual stepping up of the speeds of the flight and on the ground, in a tank of the type developed at the test flights. R.A.E. for the Comet investigation. The cost of these two experi­ In considering the Comet accidents, it is important to remember mental machines will of course increase the price of those available that this aeroplane was under development so long as seven years ago. for sale to operators; which means, in effect, that the number of those Since then many hours have been devoted by many inquirers all over put into production must be larger. On the question of the tank test, the world to the study of the stresses in airframes and especially of it is desirable to point out that even this will leave out of account fatigue and its effect on metals. There is no doubt that MESSRS DE one parameter. A test of this character cannot include the effect of HAVILLAND followed the best engineering practice of the time. The temperature and changes of temperature on the structure; which is only criticism can be of the design standards of 1948, which we now one of the most important points. Knowledge of this can only be know were below what they should have been to ensure adequate obtained, so far as we can see, by flight trials, unless a test method of strength in the pressurized fuselages of aeroplanes. MESSRS DE simulating conditions can be devised. HAVILLAND may indeed be said to have been unlucky in having to pay the penalty for their own enterprise in producing a design of aeroplane so far in advance of their competitors. Future Design Technique To sum up, the Report reinforces the conviction which has been Modern Knowledge growing for many years among aeronautical engineers, that to ensure the safety of the modern aircraft it will be necessary to carry Unfortunately, the methods used in those days to check the sound­ out design calculations to the full extent that current theory permits, ness of conclusions arrived at from theoretical calculations were not to test more and more thoroughly each assembly, and the complete adequate since the theory was based on insufficient practical know­ airframe, under conditions which approximate more and more ledge. There can be little doubt, for instance, that a complete examin­ ation of the fuselage in flight tests by fitting strain gauges at many closely to those which the aircraft will meet on service, and to pursue in ever greater detail the instrumentation and techniques which will points would have indicated that in some places stresses were being enable flight testing to give the greatest possible amount of informa­ attained greater than those envisaged from theoretical considerations tion. based on the knowledge existing at that time. It is, of course, only too http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Aircraft Engineering and Aerospace Technology Emerald Publishing

Testing Times

Aircraft Engineering and Aerospace Technology , Volume 27 (3): 1 – Mar 1, 1955
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Publisher
Emerald Publishing
Copyright
Copyright © Emerald Group Publishing Limited
ISSN
0002-2667
DOI
10.1108/eb032531
Publisher site
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Abstract

Aircraft Engineering THE MONTHLY ORGAN OF THE AERONAUTICAL ENGINEERING PROFESSION VOL XXVII No 313 MARCH 1955 easy to be wise after the event but we now know that it is not possible to assess accurately stress concentrations from approximate calcula­ Y a coincidence, the report of the Court of Inquiry into the two tions and tests on sections of fuselages. Similarly, we now know that Comet accidents in January and April 1954 is published just as it is not enough to take a length of the fuselage and test it without Bwe are on the point of going to press with this issue which con­ reference to the rest of the structure. The stress in the fuselage tains M R PEACOCK'S Stress Analysis Group Conference paper on the imposed by the dynamic forces on the nose and tail sections in gusts flight flutter tests on the Javelin. and manoeuvres has also been shown to be so large that it may not be neglected. Seven years ago designers had to be content with ap­ Analogous Cases proximations in many instances where detailed calculations would In the case of the latter the aeroplane shed its elevators owing to have been a matter of months—almost years. The development of flutter in conditions where theoretical calculations had not shown the computers to the stage they have now reached means that the thought possibility of this phenomenon developing. The accepted view of the of such calculations is no longer so daunting as it was and such in­ cause of one, and probably both, of the Comet disasters is failure of vestigations can now be undertaken that even two or three years ago the fuselage structure as a result of fatigue which theoretical calcula­ would have been considered quite impracticable. tions in the design stage had not foretold accurately. In both instances, subsequent practical tests—in the one case in flight and in the other in a ground rig—have proved the existence of conditions which theory The Implication had not revealed. That is the link between the two investigations. Even so, the net result of the Court's inquiry is that more, not less, The Gloster tests mark an important step forward in being the practical tests of civil aircraft are necessary. This will involve the first time that such a series of practical ad hoc investigations has been allocation for tests to destruction of at least two airframes of every made in flight. We must pay a tribute to the ingenuity of the methods new type of commercial aeroplane intended to operate at high devised to reproduce the supposed conditions by artificial means and, altitudes. particularly, the careful manner in which the safety of the pilot was There will have to be simultaneous tests of extreme severity in successfully ensured in the gradual stepping up of the speeds of the flight and on the ground, in a tank of the type developed at the test flights. R.A.E. for the Comet investigation. The cost of these two experi­ In considering the Comet accidents, it is important to remember mental machines will of course increase the price of those available that this aeroplane was under development so long as seven years ago. for sale to operators; which means, in effect, that the number of those Since then many hours have been devoted by many inquirers all over put into production must be larger. On the question of the tank test, the world to the study of the stresses in airframes and especially of it is desirable to point out that even this will leave out of account fatigue and its effect on metals. There is no doubt that MESSRS DE one parameter. A test of this character cannot include the effect of HAVILLAND followed the best engineering practice of the time. The temperature and changes of temperature on the structure; which is only criticism can be of the design standards of 1948, which we now one of the most important points. Knowledge of this can only be know were below what they should have been to ensure adequate obtained, so far as we can see, by flight trials, unless a test method of strength in the pressurized fuselages of aeroplanes. MESSRS DE simulating conditions can be devised. HAVILLAND may indeed be said to have been unlucky in having to pay the penalty for their own enterprise in producing a design of aeroplane so far in advance of their competitors. Future Design Technique To sum up, the Report reinforces the conviction which has been Modern Knowledge growing for many years among aeronautical engineers, that to ensure the safety of the modern aircraft it will be necessary to carry Unfortunately, the methods used in those days to check the sound­ out design calculations to the full extent that current theory permits, ness of conclusions arrived at from theoretical calculations were not to test more and more thoroughly each assembly, and the complete adequate since the theory was based on insufficient practical know­ airframe, under conditions which approximate more and more ledge. There can be little doubt, for instance, that a complete examin­ ation of the fuselage in flight tests by fitting strain gauges at many closely to those which the aircraft will meet on service, and to pursue in ever greater detail the instrumentation and techniques which will points would have indicated that in some places stresses were being enable flight testing to give the greatest possible amount of informa­ attained greater than those envisaged from theoretical considerations tion. based on the knowledge existing at that time. It is, of course, only too

Journal

Aircraft Engineering and Aerospace Technology – Emerald Publishing

Published: Mar 1, 1955

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