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Evolution towards the conventional helicopter we know today has been long and sometimes dangerous. The ability to take off, hover and land without the need for runways was always foreseen by engineers as an achievable goal having innumerable military and civilian roles otherwise impossible by fixed wing aircraft.
With so many potential roles, today’s helicopters are a platform for numerous types of equipment carefully selected and integrated to perform any chosen function or mission. For the designers and engineers, the multidisciplinary balancing act is between having a structure to carry the equipment, but without impacting on the aircraft performance, efficiency and longevity, whilst meeting all mandatory safety, maintenance and flight certification requirements.
During Altair’s Global ATC 2018, Airbus Helicopters1 (formerly Eurocopter), one of the largest producers of military and civilian rotary wing aircraft, gave us an insight into how they meet the diverse needs of their many and varied customers.
Birth of the Whirlybird
With Leonardo da Vinci’s air screw invention in the late 1400s, the seed of a vertical lift-off, rotary-wing powered flying machine was sown. In 1861, although it never flew, Gustave Ponton d’Amécourt baptized his steam-powered, working model made of aluminum – the new wonder material of the age – “hélicoptère” from the Greek “helix” plus “pteron” meaning wing.
From c. 1900, the race was on around the world. Foot by foot altitude increased, flights lasted longer, distances flown extended, and structures with their pioneering pilots resisted landings better. In Europe, the world’s first practical transverse twin-rotor helicopter, designed by Heinrich Focke, first flew in 1936 then set new world records the next year. Whereas in the US, Russian-born engineer Igor Sikorsky’s single lifting-rotor military helicopter design, having a smaller tail rotor mounted on the tail boom to counteract the torque produced by the single main rotor, in 1942 was the forerunner of Sikorsky’s large-scale, mass-produced helicopters. In 1946, Bell Helicopter certified a twin-blade, with weighted stabilizer bar design: the first US civilian helicopter, which remained popular for nearly 30 years.
1 Airbus Helicopters, formerly Eurocopter, arose from the merger of Germany’s Deutsche Aerospace and France’s Aerospatiale helicopter divisions in 1992, to be joined in 2000 by Spain’s CASA.
Today’s Multidisciplinary Approach for Success
Airbus Helicopters never deliver two the same. This puts pressure on having a means to quickly define the helicopter and applying an efficient, proven method when designing a new helicopter or modifying one for a specific version.
Hervé Dutruc, with his Altair Global ATC 2018 presentation “Airbus Helicopters – Impact of composite on Antenna radiation” took us through the results of an on-going study and described their experience and practice using Altair FEKO to optimize placement and performance of a weather radar.
First step: choose the antenna location to attain the final expected performances by considering the effects of surrounding equipment. However, antennas cannot be placed where they interfere with access for maintenance or compromise ground- or blade clearances.
The advantages composites offer to aerospace industries is well documented and largely proven by their expanding use: weight-saving, durability from fatigue and environmental factors. With the ease and affordability of manufacturing complex geometries, composites are being used to produce aerodynamic profiles, including radomes, that improve not only the aesthetics, but also fuel efficiency.
Whilst glass composite materials are considered transparent at radar frequencies, 10GHz typically, the surfaces of these complex geometry radomes can provoke high levels of reflection, unwanted side lobes on RF signal profiles leading to unwanted reflections from the ground. All these diminish the expected in-flight radar performances.
Altair FEKO is key within the optimization study which is part of Airbus Helicopters large investigation into the suitability of materials for radomes. A 2D infinite plane with an electromagnetic field generated at the surface was used to quantify levels of reflection and transmission. Both monolithic (solid laminates) and glass-skinned sandwich panels, having different thicknesses, were investigated using different angle of incidence in order to analyze two polarizations: perpendicular or parallel to the incidence plane, in TE and TM mode.
The philosopher’s stone of radome design would be a non-attenuating, single type of structurally-efficient material which has a consistent RF performance, in both TE and TM mode, irrespective of incidence angle. Dream on!
The relationship between thickness of monolithic glass laminate to the signal loss for different incidence angles showed that the ideal thickness, acceptable from an RF point of view, imposed an unacceptable weight penalty plus a shift in the helicopters center of gravity. In the search for lighter options, sandwich panels comprising glass-fiber composite skins on a honeycomb core showed that honeycomb thickness was a key parameter to optimization. However, in general, sandwich panels were shown unsuitable for areas of radomes experiencing high angles of incidence because of the signal loss by reflection, but also because of an uncompromising, wide difference between TE and TM mode.
The study then looked at positioning the radar antenna within the radome. Here, there is some margin to optimize the antenna with respect to the surrounding structures to reduce unwanted side lobes.
In this illustration, Altair FEKO – Altair’s comprehensive computational electromagnetics (CEM) software – is used within a multidisciplinary approach where material, structural and RF engineers work together to ensure each and every Airbus Helicopter – be they brand new or a modification – meet the customer’s expectations.
View the Altair Global ATC 2018 presentation “Airbus Helicopters – Impact of Composite on Antenna Radiation”: https://www.altair.com/resource/high-frequency-electromagnetics-playlist