The Defence Evaluation & Research Agency (DERA) Farnborough UK, (formerly known as the Royal Aircraft Establishment), has an illustrious record of achievement in aerospace research and development. From its earliest beginnings as the site of the first flight in the UK back in 1908, through Frank Whittle’s innovative work on the jet engine, through aerospace structures such as the Concorde and the Harrier vertical take-off fighter. Today, the establishment employs 6 500 people engaged in a unique range of high technology services, which, although primary focused on military applications, are increasingly encompassing commercial activities.
The Aero-Structures Laboratory
The Aero-Structures Laboratory, just one of the DERA test facilities, is a long time user of LMS CADA-X. Dr. Graham Skingle gave an overview of the departments activities. “Our mission is to improve the safety, reliability and performance of helicopters and fixed wing aircraft, and to reduce their life cycle costs. We run independent assessments on new helicopter designs, predict the dynamic effects of structural modifications or different weapons configurations to the airframe, and perform structural design optimization for specified operation constraints.”
The modern facilities were constructed last year and can perform a modal test on a full size aircraft. A 64 channel SCADAS-based LMS system is used for structural testing, modal analysis and FE correlation. A typical modal survey will involve shaking the structure at four locations and acquiring data at several hundred triaxial DOFs, measured in patches of 60 channels at a time. Graham also makes extensive use of the LMS CADA-X User Programming capabilities to integrate his own processes into the acquisition procedures.
“By its very nature, our work can involve some very interesting projects, and sometimes we end up with unexpected applications. For example, we were once investigating an aluminium honeycomb panel for structural applications and discovered that, in reality, it was more suited to sound radiation. We have now licensed this technology to a loudspeaker manufacturer, NXT, who will use it to develop flat panel loudspeakers. One novel application in summer ‘97 was to perform the vibration survey of the ThrustSSC – the world’s first supersonic car.
The most powerful car ever built
The ThrustSSC is the most powerful, most extraordinary car ever built. Two Rolls-Royce Spey jet engines from the Phantom fighter, each producing 25,000 pounds of thrust (the equivalent of 134MW or 134 Formula One cars) propel the 10 ton vehicle from standstill to 100mph (161kph) in four seconds – and up to 600mph (1000kph) in a further 12 seconds. Travel from zero to Mach 1 and back to zero requires only 7 miles – an unimaginable “white-knuckle ride” that lasts approximately just one minute.
Testing the ThrustSSC, Dr. Graham Skingle on the LMS System, Ron Ayers (left), Chief Aerodynamicist, ThrustSSC.
Designing a supersonic car poses many questions. Above all, nothing is known about ground effect in the transonic region: transonic wind tunnels are not able to simulate ‘moving ground effects’ aerodynamics. But the resultant buffeting is likely to be very severe. It was predicted that if the vehicle is travelling at Mach 11and the pitch attitude was just 0.51 excessively nose-up (relative to correct trim angle), the vehicle would take off. It was proposed that, just before leaving to Black Rock Desert in Nevada, the complete vehicle would be resonance tested at DERA to determine its natural frequencies. During the trial runs, building up to Mach 1, the vehicle operation was monitored for convergence of the natural frequencies with the buffet forces. “The structural survey was completed in a day”, said Graham, “the major problems we had to overcome were the dynamic isolation of a heavy vehicle almost 15m long – and keeping the BBC television crew who were working on a article for a popular science program off the structure during data acquisition.” Eight modes were identified up to 32Hz: the first rigid body roll mode occurred at 7.1Hz with 2% damping, and the first pitch mode at 8.6Hz, damping 1.5%. All of which proved to be within expectations. In comparison, a Formula 1 first rigid body roll mode would typically occur at around 4-5Hz – showing just how stiff the ThrustSSC is.
The ThrustSSC achieved the first ever supersonic World Land Speed Record of 763mph (Mach 1.020) on 15th October 1997.
