Tire Friction with Ground: Rolling and Braking Friction Coefficient

Ground movement would experience friction between the tire and the ground. Dur­ing the takeoff run, this friction is considered drag that consumes engine power. Figure 7.15 is a representation of the ground-rolling friction coefficient, л, versus aircraft speed for various types of runways. Conceptual studies use the value for the friction coefficient, л.

• A Type 3 runway (concrete pavement) = 0.02 to 0.025 (0.025 is recommended for coursework)

• A Type 2 runway = 0.025 to 0.04 (0.03 is recommended for coursework)

• A Type 1 runway = 0.04 to 0.3, depending on the surface type, as follows:

hard turf = 0.04

grass field = 0.04 to 0.1 (0.05 is recommended for a maintained airfield) soft ground = 0.1 to 0.3 (not addressed in this book)

The braking friction coefficient, ль would be much higher depending on the run­way surface condition (e. g., dry, wet, slush, or snow – or ice-covered) (Table 7.7). A typical value is лЬ « 0.5. Locked wheels skid that wear out a tire to the point of a possible blowout. Most high-performance aircraft that touch down above 80 knots

Table 7.7. Average braking coefficient,

Aircraft speed (mph)

20

40

60

80

100

Dry concrete runway, pb

0.85

0.77

0.67

0.57

0.46

Wet concrete runway, pb

0.56

0.44

0.35

0.28

0.23

Iced runway

0.1 to 0.2

0.1 to 0.2

0.1 to 0.2

0.1 to 0.2

0.1 to 0.2

have an antiskid device when the p, b value could be as high as 0.7. Slipping wheels are not considered during the conceptual study phase. Tire-tread selection should be compatible with the runway surface condition (e. g., to avoid hydroplaning). The braking friction coefficient, p, b = 0.45 to 0.5, is the average value used in this book. The tire load is based on a brake coefficient of 0.8.

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