Ground Roll

4

The calculation of the ground roll in landing follows along the same lines used for a takeoff ground roll, but with different parameters and initial conditions. The braking coefficient of friction varies from approximately 0.4 to 0.6 on a hard, dry surface to 0.2 on wet grass or 0.1 on snow. With spoilers the lift is essentially zero. Also, with flaps and spoilers the parasite drag coefficient may be higher.

Beginning with an initial value of VA, the equations of motion can be numerically integrated, accounting for the variation with V of any reverse thrust, drag, and possibly lift. One can also use the approximate relationship

derived previously, equation 7.6. In the case of landing, this becomes

where a is the magnitude of the deceleration evaluated at VАЫ 2.

As an example, consider the 747-100 at its maximum landing weight 2500 kN (564,000 lb). Assuming a fi of 0.4, an / of 80 m2 (260 ft2) and VA equ to 65 m/s (126 kt) gives

a = 4.05 m/s2

Therefore, the ground roll distance is estimated to be,

s = 522 m (1710 ft)

This gives a total estimated FAR landing distance, including the factor of 1.667, <

s(total) = 1670m (5480ft)

This compares favorably with the distance of 1880 m (6170 ft) quoted Reference 5.11. The difference is easily attributable to uncertainties in tl approach speed and the braking friction coefficient.

Lighter aircraft, except on an instrument approach, tend to descend at г angle steeper than 3°. With their lower wing loadings, light aircraft also touc down at much lower speeds. Hence, their landing distances are significant less than those for a jet transport. A Cherokee Arrow, for example, touchir down at approximately 65 kt can be stopped with moderate braking withi 300 m (1000 ft).