Vibration During Take-Off in the Cockpit of Concorde

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.

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.

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. 3

Journal of Aeronautical History Paper 2017/01 many peaks exceeding 0.5 ‘g’. Much of the vibration in the cockpit was caused by bending of the fuselage at 2.28 Hz (cycles per second)*.

The reason for the higher than expected accelerations was not hard to find. The friction for the main undercarriage was so high that the undercarriage barely moved during the take-off roll, although the end load in the undercarriage varied substantially during the take-off (Figure 5). The amount the end load could vary without the sliding leg moving indicates the value of the sticking friction at that weight. The very flexible aircraft was supported on high pressure tyres, with effectively no springing or damping between the tyres and the airframe. The bending of the fuselage was being forced by the main undercarriages, with the nose undercarriage contributing virtually nothing. When the correct level of friction was included in the calculation, the stick-slip motion of the undercarriage matched real life, but both the Aerospatiale and RAE calculations over-estimated vibration levels by about 25%. The criterion used to obtain a single value for the vibration during a take-off was the incremental acceleration (positive or negative) exceeded by the ten largest peaks. This was referred to as ‘delta n ten’, Δn 10 , and is used in Figure 6 to show the effect of changing the oleo stiffness and friction during take-off at San Francisco. Figure 1 shows how the value of Δn 10 compares with the peak incremental acceleration during typical take-offs.

Improving the undercarriage

At RAE we looked at possible modifications to the main undercarriage, to improve the ride during taxying and take-off. This involved increasing the pressure in the air spring, which reduces the stiffness of the spring at take-off weight. This in turn required a small second stage oleo to provide the initial deflection of the undercarriage during landing, because increasing the spring pressure increased the breakout force needed to start the leg moving during a compression stroke. This was desirable anyway, because with the original oleo, touch down felt harsh. Fortunately, there was space available in the existing leg for the second stage oleo, because some of the early designs had proposed a double spring. RAE then ran take-offs on the computer from all the runways for which we had profiles, testing the effect of the undercarriage stiffness, friction and damping on vibration levels. Figure 6 shows the results for take-off from runway 28 at San Francisco, one of the most uneven runways for which we had a profile. The critical characteristic was the friction; if that could not be reduced by more than a half, changes to stiffness and damping were not effective, because the oleo did not move much. The modification to the undercarriage to include a two stage oleo was explored theoretically by RAE, using computer modelling to study its performance during take-off and landing, and formed the basis for the revised undercarriage proposed by BAC in September 1974 after tests of the production aircraft at Singapore (see below).

As well as supporting the aircraft while taxying, the undercarriage has to absorb the energy of the aircraft as it settles on the runway during landing. This is normally the dominant design case for an undercarriage. The stroke of the undercarriage and the maximum allowable load during landing were ______* Different parts of an aircraft vibrate naturally at different frequencies. In an airliner in turbulence, the tips of the wings can be seen to be bending at about 4 Hz. The fuselage tends to bend at a similar frequency, so the vibration combines to be wing and fuselage bending. 4

Journal of Aeronautical History Paper 2017/01 already set, so we had to juggle stiffness and damping to provide an almost constant load on the undercarriage as it compressed to full stroke during a landing at a sink rate of 10 feet/sec.

Throughout this analysis, RAE was working in close co-operation with the dynamics groups of British Aircraft Corporation at Filton and Weybridge, and Aerospatiale at Toulouse. By early 1970, the calculations had shown what changes to the undercarriage were needed, and a major report had been written (Mitchell, 1970a); later in 1970 the author moved on to other work. Having established what was causing the problem and in principle how it could be solved, the task of detailed re-design of the main undercarriage could be passed to the manufacturers of the airframe and the undercarriage, to carry out the detailed design work necessary. This included further theoretical analyses by BAC Weybridge, and taxy tests with the prototype 002 to measure the effect of reducing the stiffness of the air spring by increasing the air pressure in the oleo, for test only.

That the BAC predictions were correct did not receive confirmation until the British Prototype 002 went on a tour of the Far East in 1972. Europe, Middle East, Far East and Australia were covered and measurements of cockpit response confirmed that a problem existed. The manufacturers had cables back that the pilots had had a rough take off ride in particular from Athens, Bangkok, Singapore and Tokyo. There was an excellent recording system on board the aircraft and once these were received back in the U.K. it was realised that there was definitely a problem for the pilot. There was no structural airworthiness problem and nor was there any problem for the passengers. BAC’s approach was to make predictions and adjust the mathematical model to get agreement with the measurements.

Making the case to fit improved undercarriages

It was agreed to run some tests with the French prototype 001 and this went to Athens in January 1973. The conclusion was that there was a possible problem of pilot comfort, but Aerospatiale, as design authority for the main undercarriage, considered the production aircraft would be so different from the prototype that any modifications should be delayed until experience with the production aircraft had been obtained.

In September 1974 the first British production aircraft 202 was flown to Singapore. Pilots from BAC, Aerospatiale, BA, CAA and CEV (the French airworthiness authority) were on board. As BAC expected (and had predicted), the accelerations in the cockpit were as high, or even higher, than those measured on the prototype. After a whole series of test flights (including visits to Kuala Lumpur) the conclusion was reached that this high acceleration at the flight deck could interfere with emergency procedures. BAC had a solution (introducing a two stage oleo) which meant changing the design of the main undercarriage. Athens had already re-laid their runway after the Concorde trials there, and a decision on the undercarriage was delayed while attempts were made to persuade other airports to re- surface runways that caused problems for Concorde. Singapore agreed and finished the work in May 1975, which was immediately followed by more tests with aircraft 202. Unfortunately they had only put a thin skimming of tarmac on the surface and the big undulations, which excited the Concorde fuselage, had been untouched. A further resurfacing was undertaken and yet more trials with Concorde proved that this time the job was satisfactory. BA started a service from London to Singapore in conjunction with Singapore Airways but this only ran for a couple of years. 5

Journal of Aeronautical History Paper 2017/01

In parallel with the Concorde work, BAC had been running a series of measurements on British Airways VC10 aircraft and had developed a method of extracting runway data which could be used to calculate the accelerations on Concorde. This method was then used to estimate what would happen on other BA routes. The major routes were across the North Atlantic. This meant Dulles, Washington, and of course JFK, New York. The runways at Dulles were fine, but all indications were that JFK was rough. BA in the meantime had taken a number of legal actions in order to get permission to use JFK. If BA used JFK and if the BAC calculations were correct, there would be a necessity to modify the undercarriage as there was no way that JFK would modify their runway.

BAC had been discussing with Aerospatiale for some time how to modify their undercarriage and a two stage test undercarriage was produced by Messier, the original manufacturer, and fitted to Concorde 201, the first production aircraft. It was not possible to go to Athens or Singapore as these runways were now nice and smooth. However it had been discovered that Casablanca, Gander in Newfoundland and Caracas in Venezuela were all very rough.

The aircraft was taken to Gander and a series of tests were conducted over a week or more, including a day visit to Caracas. The modification worked, as expected, and the aircraft flew into JFK for the first time in October 1977 for a series of tests. These were partially to establish the special take off technique necessary to avoid built up areas and of course to try out the new undercarriage, which worked well. This undercarriage was then fitted to the whole fleet. The modified undercarriage reduced vibration levels by about 25% at a weight of 175 tonnes. At Musée l'Air et l'Espace at le Bourget, a prototype and a production Concorde are displayed side by side, and it is surprising how different the two undercarriages are in detailed external appearance. Figure 7 shows photographs of the prototype and production main undercarriages.

Because the RAE author had moved on to other work, he did not get feedback from aircrew on the effect of the improved undercarriage suspension. But 40 years later, while helping organise a conference for the Royal Aeronautical Society, he met several British Airways crew members who had flown the aircraft with the original and modified undercarriage. They reported that the improvements made a real difference for them. With the original undercarriage, the vibration was very bad, particularly at Singapore, where early crew training was done. David Rowland (Fleet Manager Concorde) remarked that fitting the modified undercarriage made things much more comfortable during take-off.

Structural fatigue

RAE went into the study of vibration during take-off because of concern over the working environment for the crew and implications for flight safety. As test results came back, it became clear that the vibration was also causing oscillatory structural loads that could well increase fatigue damage. Before the fatigue test of the airframe it was not possible to quantify this damage, but it provided another incentive for improving the situation.

Journal of Aeronautical History Paper 2017/01

Lessons from the study

The study was reported in detail in RAE Technical Report TR 70173 (Mitchell, 1970a) and a summary paper in the Aeronautical Journal with no scale on the graphs of aircraft responses (Mitchell, 1971). Both reports drew some general conclusions regarding undercarriages for supersonic transports. The most important is that for a SST, the undercarriage should be designed as much as a suspension system for taxying as it is designed to absorb energy during landing. This contrasts with the procedure for most aircraft, where the requirements for energy absorption dominate, and the suspension characteristics are rather left to come out as they will. The reason that SSTs are different is that the long flexible fuselage is sensitive to excitation by the main undercarriage, and this seems to be inherent in the geometry necessary for a supersonic airliner or bomber. The lower frequency of the bending mode of the fuselage is affected by undulations of the runway that have longer wavelengths than those that affect subsonic transports, and these usually have greater amplitudes. To achieve a vibration level during taxying comparable with that of a subsonic airliner, a SST needs an undercarriage with a much lower specific stiffness (stiffness/static load) than a subsonic transport.

This was an exciting study to be involved in, knowing that we were making a real contribution to the success of the Concorde project. If the undercarriages had been left in their original condition, the consequential vibration could have reduced the aircraft’s structural fatigue life. Perhaps more importantly, the vibration environment in the cockpit could have prevented the use of the aircraft on some uneven runways.

References

Kirk S. Irwin and William H. Andrews (1969) Summary of XB-70 airplane cockpit environmental data NASA TN D-5449, National Aeronautics and Space Administration, Flight Research Center, Edwards, California.

C G B Mitchell (1970a) A theoretical analysis of undercarriage loads and taxying vibration on a supersonic transport aircraft with experimental comparisons and an assessment of modifications to reduce ground loads Technical Report TR 70173, Royal Aircraft Establishment, Farnborough.

C G B Mitchell (1970b) Vertical acceleration in the cockpit of a subsonic transport aircraft during take-off measured during airline operation Aeronautical Research Council Current Paper CP No 1120, Ministry of Technology, HMSO, London.

C G B Mitchell (1971) Some measured and calculated effects of runway unevenness on a supersonic transport aircraft Aeronautical Journal, Vol.75, pp339-343, May 1971, Royal Aeronautical Society, London.

Journal of Aeronautical History Paper 2017/01

E Marjorie Owen (1971) Civil Aircraft Airworthiness Data Recording Programme – Achievements in recording and analysis of civil aircraft operations 1962-1969 Aeronautical Research Council Current Paper CP No 1181, Ministry of Technology, HMSO, London.

J.K. Zbrojek (1965) The need for research on the cockpit vibrational environment of the Concorde R.A.E. Technical Memorandum Aero 910, Royal Aircraft Establishment, Farnborough. Acknowledgement

This paper is based on one written by C G B Mitchell for a Royal Aeronautical Society conference Concorde: The Supersonic Achievement - 40th Anniversary Concorde Conference, 8th April 2009, ISBN 1 85768 267 X, conference reference 601, and extracts from the conference paper are published with the permission of the Society. Proceedings of the Concorde Conference are available from the Conference Department, Royal Aeronautical Society, 4 Hamilton Place, London W1J 7BQ.

Journal of Aeronautical History Paper 2017/01

‘delta n ten’ Δn 10

Figure 1 Comparison of cockpit vertical vibration during take-off for subsonic airliners in service and Concorde 002 at Fairford (RAE TR 70173)

[Type text]

Figure 2 A typical take-off record from ARC Current Paper CP 1120 (Mitchell, 1970b)

[Type text]

Figure 3 Sketch of the main undercarriage leg shock absorber (oleo) and leg shortening mechanism

(Mitchell, 1970a)

[Type text]

Figure 4 Vertical acceleration in the cockpit and near the centre of the passenger cabin of 001 during take-off at Toulouse (Mitchell, 1970a)

[Type text]

Figure 5 Measured main undercarriage loads and deflections during three takeoffs at Fairford (RAE TR 70173, Mitchell 1970a)

[Type text]

g Cockpitacceleration relative that to withthe standard undercarria

Figure 6 Variation in the vertical acceleration in the cockpit during takeoff on runway 28 at San Francisco as the oleo stiffness and friction are varied

[Type text]

Leg shortening mechanism

Oleo (inside sliding leg) Second oleo, inside leg below original oleo Sliding leg

Prototype main undercarriage Production main undercarriage

Figure 7 Concorde prototype and production main undercarriages, as displayed at Musée l'Air et l'Espace, le Bourget