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June, 2003 Archive Story

It Wouldn't Work for You Any More, Rudolf

Alan Cameron-Johnson has contributed another Lightning Detective Story, this time from the development of the Lightning rather than from flight. How many of you can guess the answer before the end? Ed.

Those of you who know their way around the Lightning, and that must be a great number, will recognise that the nose undercarriage is a levered device. The wheel is fitted at the rear end of an arm which is a half fork on the P1A and a full fork on the Lightning, and which in turn is attached to the bottom of the leg. In the terminology of British Messier Limited where it was designed and made, this particular construction was known as 'semi-articulated', because the arm was pivoted, not at the front, but at a point somewhat towards the middle of its length, to the bottom end of the inner sliding part of the shock absorber. To prevent horizontal rotation of the arm in relation to the upper half of the leg, and to provide a reaction path for the vertical loads, a torque link joined the front of the arm to a projecting bracket on the outer tube. It will be clear, I think, that because of this arrangement the loads experienced by the shock absorber are a number of times greater than those between the wheel and the ground.

The design of a nose undercarriage is complicated by the fact that, during braking, the load transfer on to the nose wheel can be considerable, especially on an aircraft such as the Lightning, with a high centre of gravity and a relatively short wheelbase. In the case of the Lightning, the result was an oleo-pneumatic shock absorber with an unusually high compression ratio and a special valve to compel the dashpot to absorb more energy than it would otherwise do during the application of main brakes.

Now this preamble merely sets the scene for what follows. As one of the engineers involved in the design of the P1A and Lightning undercarriages, I have a piece of history to relate, which may interest readers with a technical bent, and may amuse them and others as well.

The gear for the P1A and the Lightning differ somewhat in outward appearance, principally their length, but their fundamental designs are similar, and while this story concerns the P1A it could equally well have related to the Lightning. In due course, undercarriages were assembled and submitted to the test department for it to be demonstrated that they would meet the strength and performance requirements of the specification. The performance test, in particular, is the subject of this tale.

The drop-test machine, as it is generally known, consists of a carriage, rather like a simple freight lift, which can travel vertically in a supporting frame and under which the undercarriage is attached. On this, an appropriate weight is carried to simulate the effective mass of the aeroplane as felt during a real landing. The carriage assembly is held suspended from a release mechanism (quite commonly a standard bomb-slip) at a height above the landing pad that will result in the wheel touching down at the required velocity. The leg and carriage are instrumented to give records of tyre deflection, the vertical movement of the wheel relative to the fixed part of the undercarriage, and the deceleration of the mass representing the nose part of the aeroplane during the landing event. A graph of the deceleration (g), or the reaction force, plotted on a base representing the wheel travel, will usually look something like a hill, slightly flat topped, with reasonably steep sides, and with no abrupt peaks or valleys.

In the undercarriage I am describing, the air inflation pressure, the compression ratio and the sizes of the oil damping jets would have been determined by prediction methods based on theory and experience gained from a number of previous designs. The drop-test would normally be expected to show the need for only a small amount of 'fine tuning', usually to the size of the jets, and so the test in question went ahead, the undercarriage was released and the data recorded.

The resulting performance graph was viewed with some consternation since, instead of the relatively smooth 'hill' that was expected, there was a spike on top of it, rather like a church steeple, and the maximum reaction force was appreciably more than it should have been. There was no obvious cause for this phenomenon, so with new accelerometers fitted to the carriage, the drop-test was repeated. A similar spike appeared on the trace, so instrument malfunction seemed not to be the culprit.

The next step, of course, was to look inside the leg for any evidence of failure, such as in the overbalance valve, which I mentioned earlier as a device for absorbing more pitch energy during the application of mainwheel brakes. If this valve had stuck in the closed position, it would have caused a noticeable increase in the reaction force, but over the whole of the undercarriage travel rather than just in the region of the spike, so the strip examination had to be done.

Back in the assembly shop the leg was deflated, and the hydraulic fluid poured out ready for dismantling. The fluid, DTD585, normally looks rather like claret, but what emerged from the test had a distinctly dark and cloudy appearance. There was the immediate suspicion that rubber seals had been damaged, and that particles of the material were floating in the fluid. However, as the strip proceeded it became clear that neither the overbalance valve nor any of the seals had failed in any way, so the question remained - what was contaminating the fluid and upsetting the performance of the undercarriage?

A sample of the fluid was then sent to the materials laboratory, and the result of its analysis was awaited with unusual interest in the test department. When it came, the laboratory report was quite simple - the black stuff was carbon, and the only place where that could have come from was the fluid itself. As soon as that conclusion was reached, and its implication accepted, we had a situation which would have caused a certain Herr Diesel considerable amusement. Now, it was not hard to deduce the sequence of circumstances and events: the high compression of the air volume, a jet or mist of hydrocarbon fluid in the airspace, combustion and the resulting rapid increase in pressure, all resulting in the spiky graph that had been quite a puzzle earlier on.

And that is why the undercarriage has always been inflated with nitrogen. It wouldn't work for you any more, Rudolf.

Alan Cameron-Johnson

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