LANDING AND GROUND LOADS

The most critical loads on the landing gear occur at high gross weight and high rate of descent at touchdown. Since the landing gear has requirements of static strength and fatigue strength similar to any other com­ponent, overstress must be avoided to prevent failure and derive the anticipated service life from the components.

The most significant function of the landing gear is to absorb the vertical energy of the air­craft at touchdown. An aircraft at a given weight and rate of descent at touchdown has a certain kinetic energy which must be dis­sipated in the shock absorbers of the landing gear. If the energy were not absorbed at touchdown, the aircraft would bounce along similar to an automobile with faulty shock absorbers. As the strut deflects on touchdown, oil is forced through an orifice at high velocity and the energy of the aircraft is absorbed. To have an efficient strut the orifice size must be controlled with a tapered pin to absorb the energy with the most uniform force on the strut.

The vertical landing loads resulting at touch­down can be simplified to an extent by assum­ing the action of the strut to produce a uni­formly accelerated motion of the aircraft. T^ landing load factor for touchdown at a consta rate of descent can be expressed by the follow­ing equation:

n= landing load factor—the ratio of the load in the strut, F, to the weight, W

ROD = rate of descent, ft. per sec.

g= acceleration due to gravity = 32 ft. per sec.2

S= effective stroke of the stmt, ft.

As an example, assume that an aircraft touches down at a constant rate of descent of 18 ft. per sec. and the effective stroke of the strut is 18 inches (1.5 ft ). The landing load factor for the condition would be 3-37; the average force would be 3-37 times the weight of the aircraft. (Note: there is no specific correlation between the landing load factor and the indication of a cockpit mounted flight accelerometer. The response of the instrument, its mounting, and the onset of landing loads usually prevent direct correlation.)

This simplified equation points out two im­portant facts. The effective stroke of the strut should be large to minimize the loads since a greater distance of travel reduces the force necessary to do the work of arresting the ver­tical descent of the aircraft. This should

emphasize the necessity of proper maintenance of the struts. An additional fact illustrated is that the landing load factor varies as the square of the touchdown rate of descent. Therefore, a 20 percent higher rate of descent increases the landing load factor 44 percent. This fact should emphasize the need for proper landing technique to prevent a hard landing and over­stress of the landing gear components and associated structure.

The effect of landing gross weight is two­fold. A higher gross weight at some landing load factor produces a higher force in the landing gear. The higher gross weight re­quires a higher approach speed and, if the same glide path is used, a higher rate of descent results. In addition to the principal vertical loads on the landing gear, there are varied side loads, wheel spin up and spring back loads, etc., all of which tend to be more critical at high gross weight, high touchdown ground speed, and high rate of descent.

The function of the landing gear as a shock absorbing device has an important application when a forced landing must be accomplished on an unprepared surface. If the terrain is rough and the landing gear is not extended, initial contact will be made with relatively solid structure and whatever energy is ab­sorbed will be accompanied by high vertical accelerations. These high vertical accelera­tions encountered with a gear-up landing on an unprepared surface are the source of a very incapacitating type injury—vertical compres­sion fracture of the vertebrae. Unless some peculiarity of the configuration makes it inadvisable, it is generally recommended that the landing gear be down for forced landing on an unprepared surface. (Note: for those prone to forget, it is also recommended that the gear be down for landing on prepared surfaces.)