Thrust Measurement flies forwards

David Viney of Nobel Systems discusses new systems for the measurement of jet engine thrust.

It goes without saying that when an aircraft taxis to the end of the runway, the engines must be producing enough thrust for take off, or lives are at risk. Indeed, thrust is one of the four major parameters that need to be known for every jet engine after manufacture, whilst other important measurements are fuel consumption, speed (rpm) and pressure.

But it is not only after manufacture that the thrust needs to be measured – after major servicing all jet engines have to be run and must have a full performance test.

Jet engines are usually tested in the mode they most commonly operate – horizontally – and there are various ways that this can be carried out.

To measure thrust, the body of the engine must be supported but it must also be allowed to move freely in the horizontal plane. This is usually done by mounting it in a horizontal rectangular frame or thrust cradle. Force measuring devices, working in tension or compression, are then fixed between the cradle and ground to measure the thrust.

To support the cradle and engine weight without horizontal friction can be difficult. Historically, many types of bearings have been tried and found to have too much friction and stiction. The most common solution is to hang the engine test cradle on vertical flat flexures or diaphragms.

These rely on the fact that they are essentially hanging springs, and have a known spring rate and require a force to move them. This force is a loss in the system and a well-designed flexure system relies on the fact that these losses are low, linear and repeatable and can therefore be calibrated out.

THRUST SENSING TECHNIQUES

A number of techniques have been used to sense the jet engine thrust. For example, lever arms and knife-edges connected to a large dial – a sort of horizontal mechanical weighbridge. Hydraulic load cells have been tried – essentially a sealed capsule and diaphragm or flexible sac under a load platen with a pressure gauge connected to the output.

However, later solutions use strain gauge based load cells working in compression or tension. The most common design of cell that is used is the column or canister cell where a full wheatstone bridge of strain gauges is bonded on a steel column (or ring) to measure its deflection.

Sometimes, jet engine thrust is required in both directions, either when an engine is in an Altitude Test Facility (ATF) or, with the modern engine, deflectors built in to supply reverse thrust for braking purposes on landing.

One method of doing this is to use bi-directional load cells, ie, cells that can be used in tension or compression. A problem here is that load cells can have a different sensitivity when going through zero from tension to compression and one solution is to use compression cells preloaded against each other.

NO FLEXURE SUPPORTS

Another solution for jet thrust measurement, which has the added advantage of no flexure supports and in some cases no thrust cradle, is to use the Shear Pin Design of load cells such as the Nobel KIS.

These cells measure the principle strains caused by shear force either side of a web machined into a round beam (Figure 2a). Shear force, unlike bending, is independent of the point of application of applied load.

This beam is then doubled back on itself with an outer beam of equal stiffness, so that a load can be applied in front or behind the gauges (see Figure 2b). When a load is applied above the gauges, then the bending moments are zero.

This design of load cell, because it measures shear, allows movement of the loading point along the beam and it will withstand 100% side load without error and can sustain 100% safe overload.

It can be rotated so its measuring axis is horizontal and, because of its 100% side load capability, it can support the jet engine on its side axis without the need for flexures. In some cases, there is no need for a thrust cradle and it can support the engine directly.

Early jet engine thrust systems had another big problem – that of taking the measurements, which were written down by hand.

Pressure measurements were taken either on dial gauges or on mercury column manometers. Speed and thrust were also on gauges and a jet engine thrust run consisted of many people with clipboards or pads trying to take simultaneous gauge readings.

These disadvantages were overcome when strain gauge load cells started to be used – with electronic digital displays, logged automatically.

Modern requirements are for a narrow bandwidth, high-resolution digital output and display as well as a simultaneous wide bandwidth analogue output for transient and dynamic measurements.

This has usually been accomplished using an analogue amplifier wired in parallel with the digital system. These are fed with the same load cell inputs as the digital system and produce a 0-10V analogue thrust output at wider, independently adjustable, bandwidth.

More recently, digital load cell jet engine thrust measuring instrumentation has become very sophisticated.

The world’s first microprocessor based force and weight indicator was produced by Bofors over 25 years ago. The latest microprocessor based Instrumentation available (such as E-3-TAD and AST3) have digital as well as analogue outputs in the same units, each of which can have their bandwidths set independently.

So the jet engine industry now has available high accuracy load cells for its measurements, and these are coupled with sophisticated microprocessor based instruments for display and logging.

Nobel Systems Tel: 01234 220800