1. of Runway Friction Testing Part 4: different types of CFME
The Why and How
of Runway Friction Testing by
Delia Harverson
Findlay Irvine Ltd.
Midlothian, Scotland
and
Thomas J. Yager
NASA Langley Research Center
Hampton, VA, USA
Part 4: different types of CFME

2. ? design decision 1 braked wheel or slewed (angled) wheel
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.

4. ` 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.

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

6. car with integral measuring system
design decision 2
trailer or
car with integral measuring system ?
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

8. ? design decision 3 measure LOAD or assume it constant
FRICTION = DRAG / LOAD
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.

10. ? design decision 4 measure DRAG directly or
measure torque and calculate drag ?
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.

11. ? design decision 5 fixed slip or variable slip
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.

13. ? design decision 6 which slip ratio
free rolling wheel = 0% slip ratio
locked wheel = 100% slip ratio
slip speed = test speed x slip ratio
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.

14. ? design decision 7 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 7

15. Calibration and maintenance
design 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.