Journal of Aeronautical History Paper 2017/01 VIBRATION DURING TAKE-OFF IN THE COCKPIT OF CONCORDE C G B (Kit) Mitchell* and Brian W Payne** * Formerly Structures Department, Royal Aircraft Establishment, Farnborough ** Formerly Chief Dynamics Engineer, British Aircraft Corporation Ltd, Weybridge and Filton Abstract Before Concorde flew, there was concern that vibration in the cockpit during take-off could be severe. This proved to be the case for the prototypes, made worse by undercarriage oleos with high friction that stuck and moved very little during the take-off roll. RAE, in cooperation with BAC and Aerospatiale, developed a computer programme that predicted vibration and undercarriage behaviour and allowed the effect of design changes to be explored. The characteristics needed to reduce vibration were identified as reduced friction and a softer air spring at take-off weight. This could be achieved while still absorbing the energy of landing by using a 2-stage oleo, for which space was available. Once the required characteristics had been identified, the project was passed to industry for further analysis and the necessary design changes. The path to developing revised undercarriages was tortuous, involving many tests, until undercarriages with 2-stage oleos were fitted in 1977 and reduced vibration by about 25 percent. Introduction One of the interesting, though specialised, technical challenges set by the Concorde project was reducing the level of vibration in the cockpit during the take-off roll, caused by the unevenness of the runway exciting bending of the long flexible fuselage. It was an issue that was not reported publicly, but if it had not been successfully overcome, could have prevented the use of the aircraft from some uneven runways and also reduced its structural fatigue life. As this account describes activities which took place over 40 years ago, reliance has had to be placed on memory, as well as on the documents still available. The authors therefore apologise in advance for any detailed errors or omissions Before the prototypes flew, RAE was expecting that vibration in the cockpit during take-off would be a problem (Zbrojek, 1965). The prototype North American XB-70 supersonic bomber was shaking its crew during take-off at levels that NASA rated unacceptable (Irwin and Andrews, 1969). Calculations in the USA of vibration during high speed taxying for several hypothetical SSTs predicted high acceleration levels in the cockpit (peak vertical accelerations of more than +/- 2g). Calculations by SNIASx for Concorde on runway 28 at San Francisco, an abnormally rough runway for which a profile was available from NASA, predicted a peak cockpit acceleration of 3.1g. But this was probably pessimistic, because when calculations of vibration during take-off for subsonic aircraft were compared with flight test measurements, the measured accelerations were typically half to two-thirds of those calculated. _____________________________________________________ x Société Nationale Industrrielle Aérospatiale - Aerospatiale. 1 Journal of Aeronautical History Paper 2017/01 At the same time, as the first generation of big jets started flying in Africa, it was found that on some runways the vibration levels in the cockpits were borderline unacceptable. In extreme cases, this was probably interfering with the ability of the crew to read instruments during the take-off roll. Some of this information was from reports by pilots, some from data recorded on airliners that had been instrumented as part of the Civil Aircraft Airworthiness Data Recording Programme (Owen, 1971). Establishing acceptable levels of vibration in the cockpit RAE had cockpit vibration measurements from a VC-10 and a Boeing 707 in airline service with British Airways (Mitchell, 1970b), and was reasonably confident that the crew complained if the incremental vertical acceleration in the cockpit exceeded about 0.5 - 0.6g (Figure 1). The curves for VC-10 and Boeing 707 in Figure 1 were obtained by analysing film records of take-offs and landings such as Figure 2, which shows a take-off from New York JFK. The magnitudes of the acceleration peaks were measured by hand and plotted as exceedance curves (a count of the numbers of peaks exceeding different values). On subsonic aircraft, much of this vibration was caused by structural vibration of the fuselage at about 4 Hz. The structure of Concorde is more flexible, so the first vibration mode for the fuselage occurred at 2.28 Hz. This means that fuselage bending is excited by undulations on the runway with a typical wavelength of about 40 metres, which tend to be of larger amplitude than the undulations of about 20 metre wavelength that affect subsonic transports. RAE calculated a typical acceleration history for the cockpit during take-off, and the late Geoff Rowland in Engineering Physics Department used the former TSR-2 simulator at Weybridge to replicate this history, to determine whether this would provide an acceptable cockpit environment for the flight crew. We could not make the ride in the simulator quite as rough as the calculations predicted, because the simulator motion system was not powerful enough. Indeed, we could not match the worst environments that pilots were already experiencing in big jets in Africa. A number of the BAC Concorde test crew tried the simulator, and commented that they did not believe an airliner would ever behave like that, and that if it did, it would be wholly unacceptable. So when the flight test instrumentation showed that at Fairford the prototype Concorde 002 was vibrating rather more than we had achieved with the simulator, we were surprised not to be getting reports of crew complaints (measured peak vertical acceleration at the pilot's seat was around 0.6 - 0.7g at 2¼ Hz, and was a little higher than that being experienced in the XB-70). It was really only when the first accelerate- stop tests were done that the crews seemed to become aware of any difficulties caused by the vibration environment. However, during the route proving flight to the Far East by 002 mentioned below, the Concorde prototype encountered runways more uneven than Fairford, Toulouse and Heathrow. During one take-off, peak vertical accelerations of +/- 1.3g at 2¼ Hz were experienced at the pilot's seat, and the crew were not slow to complain. 2 Journal of Aeronautical History Paper 2017/01 Concorde vibration - measured and calculated Royal Aircraft Establishment initially used a very complicated computer programme to calculate vibration in a VC-10 during take-off at Boscombe Down, to compare with experimental measurements. We found the programme was seriously overestimating the vibration experienced. The programme took a long time to run, and was so complicated that it was difficult to use for parametric studies to understand the causes of the effects observed. RAE then developed a much simpler computer programme, which represented the flexible aircraft supported by main and nose undercarriages during a take-off along a runway with a defined profile. The springs provided by the tyres and the undercarriage air springs are non-linear - that is, the force is not proportional to deflection - and were represented by equations that fitted characteristics measured on test rigs. Most importantly, both the static and sliding friction of the oleo (the oil/air spring unit that provides stiffness and damping) were included, because it usually takes a bigger force to unstick a moveable object than to keep it sliding*. On the prototype undercarriage the static friction was large, which meant that small changes in end load on the undercarriage did not move the sliding leg, because it was stuck by friction. A contributor to the high friction was a mechanism that shortened the main undercarriage leg during the retraction cycle, but which trapped the sliding lower part of the leg between the outer structural casing and the interior oleo mechanism, acting as a friction lock, Figure 3. The values of the static and sliding friction were deduced from drop tests for the undercarriage. The measured end load was different for compression and extension of the leg, and the difference was twice the sliding friction. Initially the static friction was guessed as slightly larger than the sliding friction, but once flight test results were available, it was possible to measure how much the undercarriage end load could vary without the sliding leg moving. The RAE studies concentrated on vertical vibration in the cockpit. The level of lateral vibration was lower, but possibly as disturbing, because pilots are more sensitive to lateral acceleration. Symmetric motion was easier to analyse and required less data, so it became the priority topic. The army surveyed the profiles of a number of runways, including the Concorde flight test base at Fairford, and we calculated the vibration levels we should expect. One of the reasons for a visit to Heathrow by prototype 002 early in the test programme was to measure the vibration at a major airport, though we did not have the runway surveyed. Once the prototypes started flying, to our surprise we found that vibration levels during taxying were higher than predicted rather than lower, as had occurred for every other aircraft where measurements and calculations were compared. Figure 4 shows the history of the vertical acceleration in the cockpit and in the passenger cabin during take-off from Toulouse by the French prototype 001. The peak acceleration in the cockpit is about 0.75 ‘g’, with ____________________________________________________________ * Friction is the force needed to move an object horizontally along a level surface, and is proportionate to the weight of the object. The static friction is the force needed to start an object moving, the sliding friction is the force needed to keep it moving once it has started. Static friction is almost always larger than sliding friction, and this difference can cause vibration such as chalk screeching on a blackboard, tyres screaming in a skid or a machine tool cutter juddering.