The Why and How of Runway Friction Testing

by

Delia HarversonFindlay Irvine Ltd.

Midlothian, Scotland

and

Thomas J. YagerNASA Langley Research Center

Hampton, VA, USA

Part 4: different types of CFME

A braked measuring wheel reflects rolling friction better and more directly than a slewed measuring wheel.

The MuMeter was designed with slewed measuring wheels because, in the 1960s, strain gauges were much

less advanced than they are today.

braked wheel or

slewed (angled) wheel

??

design decision 1 design decision 1

`̀̀̀Most CFMEs today are of the braked wheel, fixed slip type.

A single measuring wheel is forced to skid by a chain transmission linked to the drive wheel.

Instrumentation on the measuring axle allows the forces to be recorded

A pulse encoder measures distance.

The measuring system of the SARSYS car is a very similar to that of the GripTester

In heavy winter conditions, the integral measuring system has some operational advantages

but

Separating the measuring system from the automotive system gives greater flexibility and much lower cost of

ownership

trailer or car with integral measuring system

??

design decision 2 design decision 2

Whatever the weight of the tester, assuming the vertical force (LOAD) on the measuring wheel to be constant,

instead of measuring it, can lead to inaccurate readings.

If the CFME measures LOAD continuously, the accuracy and reliability of its readings will not compromised by a

light weight on the test wheel.

measure LOAD or

assume it constant

??FRICTION = DRAG / LOAD

design decision 3 design decision 3

The advantages of measuring DRAG and LOAD directly, by instrumenting the measuring axle, are

highlighted by a recent US FAA “CERTALERT” warning of the effects of loose surface contaminants on

devices using torque measurement.

measure DRAG directly or

measure torque and calculate drag

??

design decision 4 design decision 4

From an academic point of view, variable slip is attractive. From a practical point of view, it has been

found to be awkward and unreliable.

fixed slip or

variable slip

??

design decision 5 design decision 5

The slip ratio must be chosen with reference to the intended test speeds. For the ICAO specified speeds of

65km/h and 95km/h, a slip ratio of 15% gives maximum performance.

free rolling wheel = 0% slip ratio

locked wheel = 100% slip ratio

slip speed = test speed x slip ratio

which slip ratio

??

design decision 6 design decision 6

a PC or a specialised data logger

??

with a PC

… archiving is easy

… as computer design advances, options such

as colour displays can be made available

… data storage is virtually unlimited

… printout is on standard paper rather

than on paper rolls

design decision 7design decision 7

design decision 8design decision 8design decision 8design decision 8

Calibration and maintenance

• A badly maintained or wrongly calibrated CFME is worse than no CFME

• The field calibration process should be simple and straightforward

• The CFME should be given annual maintenance and calibration by a trained technician working in a suitable environment.