ESDU Engineer
Issue 14
ESDU Math-Model for Aircraft Tyres Operating on Contaminated Runways
An overview is given of work carried out at ESDU to develop a mathematical model for the longitudinal forces developed by free-rolling or braked aircraft tyres on contaminated runways.
ESDU’s Structures, Stress & Strength, and Vibration & Acoustic Fatigue Series were used in the design calculations. His comment remains, “I don’t know how we could have done it without ESDU”.
WHAT IS A CONTAMINATED RUNWAY?
In civil aviation a “contaminated runway” is one that is covered in a relatively deep layer of water, slush, loose snow, ice or compacted snow.
The direct effects on aircraft performance of such contaminants arise due to the additional drag of the contaminants on the tyres (and airframe) and the decreased braking friction available. The consequences are, as a result, mixed:
- During the take-off ground run the extra drag on the wheels reduces the aircraft’s ability to accelerate.
- During landing or the “stop” phase of an accelerate-stop, the reduced braking friction and increased drag on the tyres act in opposition to one another.
The presence of such contaminants can also affect severely the ground-handling capability of the aircraft – particularly in cross-wind conditions.
THE RESEARCH BACKGROUND
During the past four decades there have been numerous test programmes aimed at improving the ability either to predict aircraft performance in these conditions or to identify when such conditions exist. Major test programs have been conducted in the UK, USA, Scandinavia, Japan, France and Canada. What has been lacking, however, is a coherent effort to synthesise the various data in the form of a mathematical model to make sense of the many different factors involved.
Since the early 1990s there have been two major international projects related to the subject of contaminated runways.
- The EU-sponsored, “CONTAM RUNWAY”,
- The Canadian-sponsored Joint Winter Runway Friction Measurement Program (JWRFMP).
“CONTAM RUNWAY” led to the collection of existing, relevant test data and prediction methods from European civil-aircraft manufacturers. New tests, particularly on smaller types of aircraft, were conducted.
“JWRFMP” is an initiative supported by more than 30 organisations in 12 countries, including the USA, Canada, France,Germany, Norway and Japan. Extensive tests have been conducted using various aircraft and runway friction measuring devices – with particular emphasis on winter-contaminated runway surfaces.
ESDU INVOLVEMENT
Since 1970 the ESDU Performance Committee has issued a number of Data Items on aircraft tyre braking and drag forces and, by 1997, had acquired much significant experience in modelling aircraft and ground-test machine braking performances on wet runways. This led to an invitation to assist in the analysis of the test data produced by the JWRFMP. A contract, “Contaminated runways performance data analysis”, was agreed in April 1999 between ESDU and the Public Works and Government Services Canada (on behalf of Transport Canada). In addition to utilizing the JWRFMP test results, it was a feature of the contract that the work should seek to exploit the very significant body of test data accumulated over the previous 40 years so as to permit a generally-applicable mathematical model to be established for the widest possible range of runway surface conditions.
Work on this contract was undertaken by Ken Balkwill, a senior Member of the ESDU Performance Committee – here acting in the role of “consultant to ESDU”.
The immediate result of this work was the completion in 2004 of a Final Report, “Development of a comprehensive method for modelling performance of aircraft tyres rolling or braking on dry and precipitation-contaminated runways”, issued by Transport Canada as TP 14289E and available from www.tc.gc.ca/tdc/publication/pdf/14200/14289e.pdf.
Publication was timed to coincide with the “3rd International Meeting on Aircraft Performance on Contaminated Runways”, held in Montreal in November 2004. At that meeting Ken Balkwill gave a brief (8-slide) presentation of the (230 page) report. (see www/tc/gc/ca/tdc/events/imapcr2004.htm).
The next step in this work is to convert the mathematical model and its associated numerical coefficients into ESDU format, comprising a “parent” Data Item which will be accompanied by several “example” Data Items to illustrate the various ways of using the model.
THE CONTAMINATED RUNWAY MODEL
Both the technical work and the resulting mathematical model separate naturally into two major elements.
- Braking forces generated between the tyre and the contaminated surface – separate treatments are needed for water and for ice (or compacted snow).
- Drag forces due to the tyre rolling through a deep layer of contaminant – separate treatments are needed for fluid (water, slush) and for solid (snow) contaminants.
It has been shown that the relevant elements can be combined, as necessary, to deal with, for example, braking in deep snow.
The model incorporates data from experiments as diverse as blocks of rubber sliding on glass to a large transport aircraft braking on a runway covered with up to six inches of snow. The modelling is dependent on knowledge of eight independent variables. These are
1. Depth of runway surface macro-texture, dtex
2. Depth of contaminant, d
3. Relative density of contaminant, σ
4. Tyre forward speed
5. Tyre inflation pressure
6. Tyre vertical load
7. Nominal tyre width
8. Nominal tyre diameter.
Of these, only 1, 2 and 3 are related to the runway and its condition. All the other quantities are part of conventional ground performance calculations. Whilst it is not mentioned in the list, the mode of operation of the aircraft antiskid system is also needed; that is, the range of values of slip ratio over which it operates. This, too, is usually available or can be inferred from tests on a dry runway.
DIRECT APPLICATION OF MODEL TO AIRCRAFT PERFORMANCE CALCULATIONS
The simplest application of the model is to provide values of tyre braking and drag forces for inclusion in aircraft performance calculations – such as those required in the generation of aircraft flight manuals (AFM). It has been established already that the model performs in a manner that is consistent with currently-accepted methods used by manufacturers.
However, the reliable use of such performance data (for take-off, accelerate-stop and landing distances) depends very much on aircraft and airport operators having up-to-date information on runway conditions (i.e. runway texture, contaminant type, depth and relative density). In many cases these data are not available and it becomes necessary to make conservative assessments of the runway state so as to ensure safe operations.
COMBINED USE OF MODEL AND GROUND-TEST MACHINE MEASUREMENTS TO PREDICT AIRCRAFTOPERATIONAL PERFORMANCE
Sketch 1 illustrates, in a general fashion, how the ESDU model can be combined with ground-test machine measurementson the operational runway to permit reliable calculations of aircraft performance for those runway conditions.
In practice, each possible “route” through Sketch 1 involves a contribution from each of items (i) to (iv) in Table 1.
Clearly, the number of possible “routes” is large – although not as numerous as the product of all the options in Table 1 (for example, an ice-covered runway will involve the computation of braking force but not drag force).
No matter which route is involved, the mathematical model remains the same and is capable of taking information derived from a ground-test machine and generating relevant tyre-force data for an aircraft.
The practical use of the model in an operational context requires it to be programmed with the relevant data for any likely pair of ground-test machine and aircraft type. While computational times for the model will be short, significant co-ordination effort will be required to ensure reliable communication of the relevant information from the ground-test machine to the aircraft and any relevant airport operations centre.
Queries regarding this article should be directed to the Performance Group via esdu@esdu.com or the ‘feedback’ form at www.esdu.com.