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Aeronautical engineering

In flight impact

A study helps to forecast the resistance limit of an airplane?s fuselage on impacting with foreign objects

MIGUEL BOYAYANTest at USP: impact of a metal sphere against an aeronautical aluminum sheetMIGUEL BOYAYAN

During 2005, the Ministry of Aeronautics tallied up 480 incidents involving the collision of birds with airplanes in the country, mainly during landings and take-offs. The problem, in fact, reaches across all of the planet, even because bird populations are increasing in many countries as has been demonstrated by some studies – a positive factor for the environment, but one that brings concern for the aircraft manufacturers, as is the case with the Brazilian manufacturer Embraer. The company has just adopted an advanced system of structure analysis of aerplanes, developed in the laboratories of the Polytechnic School of the University of São  Paulo (Poli-USP). The system is composed of a mathematical model that verifies the resistance limits of materials used in the manufacture of the jets’ fuselages and wings, as well as forecasting when a rupture will occur because of an impact with foreign objects.

An undesirable impact marked one of the most celebrated airplanes of all time, the legendary Anglo-French supersonic Concorde, which had a tragic end. On the 25th of July 2000, seconds after taking off from Roissy-Charles de Gaulle airport, on the outskirts of Paris, it caught fire and came down from the sky killing all 109 passengers and crew aboard and a further four people on the ground. The accident shortened the airplane career, which was retired from service three years later. The tragedy was caused by a series of fatal incidents. At the moment of take-off, one of the aircrafts tires burst on passing over a piece of metal lying on the runway, which had fallen off another aircraft. Pieces of rubber from the tire were launched against the wing, perforated the fuel tank and led to the explosion of the jet.

Although not all accidents brought about by a collision with foreign objects, such as a piece of metal, rubber, stones, hailstones or birds, result in the crashing of the airplane, as happened with Concorde, these events always bring danger and are more common than one can possibly imagine. “The rupture of these materials is very difficult to forecast by the very complexity of the phenomenon, since it involves many variables, such as the properties of the material used in the manufacture of the aircraft, the size and format of the impacting object, the velocity and angle of collision, among others”, explains the mechanical engineer Marcílio Alves, a professor at USP, who headed the project.

“The information provided by our mathematical model will be essential in improving the development of Brazilian manufactured aircraft, since it will reveal data about the resistance of the materials used in their aeroplanes”, affirmed professor Alves, who is also the head of the Mechanics of Solids and Impact in Structures Group from the Mecatronic Engineering and Mechanical Systems Department of Poli. The mathematical model was transformed into computer coding and linked onto a commercial program for structural analysis used by the manufacturer. The expectation is that it begins to be used during this year.

The idea of developing the tool came from the Embraer company itself. “Our first contact with the company happened in 2000. Two years later we started the work and for this we were able to count upon financing from FAPESP by way of the Partnership for Innovation in Science and Aerospace Technology Program (Picta)”, reveals the researcher. The Picta is directed towards projects in the aerospace sector and makes up part of the Partnership for Technological Innovation Program (PITE), set up to finance projects in partnerships between universities, research institutes and companies. According to Alves, Embraer had already been able to count upon a process of analyzing impact problems, but one that was simple and limited.

“When they sought us out, one of the company’s concerns was to reduce the number of experimental tests of bird impacts (birds of a breed the size and weight of a vulture, for example) and rigid materials in their planes, substituting them with numerical simulations. With our tool, it?ll be possible to eliminate some tests, principally in the development phase of new aircraft.” These impact tests are a requirement of the organs that issue aircraft certificates.

MARCÍLIO ALVES / USPSystem simulates collisions of metal sphere on an aircraft’s structureMARCÍLIO ALVES / USP

An important aspect of the project was the work of describing the aluminum alloys used in the fabrication of the fuselage and of the aircraft wings. The material was tested under statistical and dynamic conditions, when the behavior of the fuselage under in flight impact situations is measured or under temperatures that vary from   -70o Celsius to +150o Celsius. The dynamic tests submit the materials to rapid changes of format that occur in a similar manner, with a duration of microseconds, in small aircraft shocks. This description is important because it reveals information about the materials? resistance and gives indications about their parameters of deformation and rupture. “Knowledge of the material’s dynamic properties is fundamental for the analysis of structures under impact loads”, highlighted the engineer. In order to carry out these tests, the researchers needed to develop equipments specific for dynamic tests, without any equivalent in Brazil.

Gun for testing
Another apparatus projected and built during the program was a projectile launcher, also known as a gas gun. Its function is to launch small objects, such as steel spheres and pieces of rubber, at velocities comparable to those of an airplane in situations of impact with birds – between 100 and 600 kilometers per hour (km/h) – against aeronautic structures, such as fuselage metal sheeting, simulating the collision of the plane with an object. The device is made up of a compressed gas cylinder, a tube of 6 meter in length and 50 to 100 millimeters in thickness and a holding device for the placement of the material that will be impacted by the projectile.

Infrared measuring devices and laser micro-displacements measure the velocity of the launched object and the deformation suffered by the structure. “These tests are important in order to validate our mathematical model. With these we can carry out a comparison of theory and practice”, explains Alves. During the three years that the project lasted, completed in July of this year, the team, which also included professors Larissa Driemeier and Sérgio Proença, from USP?s Engineering School of São Carlos, and the doctorate student Giancarlo Barbosa Micheli, from the Poli, as well as the engineer Carlos Eduardo Chaves, from Embraer, was able to count upon the collaboration of researchers abroad at the Mechanics Institute of the Bulgarian Academy of Sciences, the Impact Research Center of the University of Liverpool in the United Kingdom and from the University of Dortmund in Germany. “The international cooperation was essential for the realization of the project”, says Alves. “As well as this, with the knowledge acquired by the group from Poli over the last ten years, We offered training to 25 Embraer engineers about impacts on aeronautical structures.”

The Project
Behavior of materials  and aeronautical structures subject to impact (nº 02/11313-5); Modality Partnership for Innovation in Science and Aerospace Technology (Picta) and Partnership for Technological Innovation (PITE); Coordinator Marcílio Alves – USP; Investment R$ 367,896.00 (FAPESP) and R$ 210,000.00 (Embraer)

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