where --

*F*Hmax=maximum
rearward horizontal force acting on the wheel (in pounds);

*r*e=effective
rolling radius of wheel under impact based on recommended operating tire
pressure (which may be assumed to be equal to the rolling radius under a
static load of *n*j*W*e) in feet;

*I*w=rotational
mass moment of inertia of rolling assembly (in slug feet);

*V*H=linear
velocity of airplane parallel to ground at instant of contact (assumed to
be 1.2 *V*S0, **IN FEET PER SECOND);**

*V*c=peripheral
speed of tire, if prerotation is used (in feet per second) (there must be
a positive means of pre-rotation before pre-rotation may be considered);

*n*=equals
effective coefficient of friction (0.80 may be used);

*F*Vmax=maximum
vertical force on wheel (pounds)=*n*j*W*e,
where *W*e and *n*j are defined in
§23.725;

*t*s=time
interval between ground **CONTACT AND ATTAINMENT OF MAXIMUM VERTICAL
FORCE ON WHEEL (SECONDS). (HOWEVER, IF THE VALUE OF** *F*Vmax,
from the above equation exceeds 0.8 *F*Vmax, the latter
value must be used for *F*Hmax.)

(b) The equation assumes a linear
variation of load factor with time until the peak load is reached and
under this assumption, the equation determines the drag force at the time
that the wheel peripheral velocity at radius *r*e equals
the airplane velocity. Most shock absorbers do not exactly follow a linear
variation of load factor with time. Therefore, rational or conservative
allowances must be made to compensate for these variations. On most
landing gears, the time for wheel spin-up will be less than the time
required to develop maximum vertical load factor for the specified rate of
descent and forward velocity. For exceptionally large wheels, a wheel
peripheral velocity equal to the ground speed may not have been attained
at the time of maximum vertical gear load. However, as stated above, the
drag spin-up load need not exceed 0.8 of the maximum vertical loads.

(c) Dynamic spring-back of the landing
gear and adjacent structure at the instant just after the wheels come up
to speed may result in dynamic forward acting loads of considerable
magnitude. This effect must be determined, in the level landing condition,
by assuming that the wheel spin-up loads calculated by the methods of this
appendix are reversed. Dynamic spring-back is likely to become critical
for landing gear units having wheels of large mass or high landing speeds.