Aeroplane Construction and Operation

by John B. Rathbun

CHAPTER XIII. CHASSIS CONSTRUCTION.

General Notes. The chassis or landing gear carries the weight of the aeroplane when resting on or running over the ground, and is subjected to very heavy shocks, especially when landing. It is provided with pneumatic tired wheels, an elastic shock absorbing device, and the structural members that connect the axle with the fuselage. In some forms of landing gear, the wheels are supplemented by long horizontal skids which serve to support the machine after the elastic shock absorbers are fully extended or when the wheels collapse. The skids also protect the aeroplane in cases where the wheels run into a ditch and also prevent the machine from nosing over in a bad landing. Since the skids and their structural members cause a high resistance they are now seldom used except on the larger and slower machines. In running over the ground, or in making a hard landing, part of the shock is taken up by the deflection of the tires and part by the deflection of the shock absorber. The greater the movement of the tires and absorber, the less will be the stress in the frame.

In the majority of cases, the shock absorbers consist of rubber bands or cords, these being wound over the axle and under a stationary part of the chassis members. Since rubber is capable of absorbing and dissipating a greater amount of energy per pound of weight than steel, it is the most commonly used material. Rubber causes much less rebound or "kick" than steel springs. The principal objection to rubber is its rotting under the influence of sunlight, or when in contact with lubricating oil. The movement of the axle tube is generally constrained by a slotted guide or by a short radius rod.

The design of a suitable chassis is quite a complicated problem, for the stresses are severe, and yet the weight and resistance must be kept at a minimum. In running over rough or soft ground for the "Get off," the shocks and vibration must be absorbed without excessive stress in the framework, and without disturbing the balance or poise of the machine. There must be little tendency toward nosing over, and the machine must be balanced about the tread so that side gusts have little tendency in throwing the machine out of its path. It must be simple and easily repaired, and the wheels must be large enough to roll easily over moderately rough ground.

Fig 1. "V" Type Chassis as Applied to "Zens" Monoplane. Courtesy "Flight."

Types of Chassis. The simplest and most extensively used landing gear is the "Vee" type shown by Fig. 1, and is equally applicable to monoplanes, biplanes or triplanes. Primarily, the Vee chassis consists of two wheels, an axle, a rubber shock absorber, and two sets of Vee form struts. The chassis shown by Fig. 1-a is that of the Hansa-Brandenburg and is typical of biplane chassis. The winding of the rubber cord and the arrangement of the chassis struts are clearly shown. The two struts are connected at the bottom by a metal fitting, and the rubber is wound over the axle and under this fitting. No guiding device is used for the axle, the machine being freely suspended by the chord. The struts are made as nearly streamline form as possible.

Fig 1a. "V" Type Chassis Used on Hansa-Brandenberg Biplane.

Fig. 2 is a front view of a typical Vee chassis, and Fig. 3 is side view of the same device, the same reference letters being used in each view. The vertical struts C run from the fuselage at F to the connecting axle guide plate G. The wheels W-W are connected with the steel tube axle A, and the struts are braced against side thrust by the cross-tube D and the stay wire braces B-B. In Fig. 3 the metal fitting G is provided with the guiding slot S for the axle A. The elastic rubber cord absorber passes over the axle and is fastened to the plate G by the studs I. Fig. 4 is a side view of the chassis of the Lawson trainer, which like many other primary training machines, uses a front pilot wheel to guard against nosing over. The rear two wheels (W) are elastically supported between the Vee struts C and F, while the front wheel X is attached to the fuselage by the vertical strut E, and to the rear wheel frame by the tube G. It will be noted that the front wheel is smaller than the rear main wheels, as this wheel carries but little load. The tail skid T is hinged to the fuselage and is provided with elastic cord at the upper end so that the shock is reduced when the tail strikes the ground. Fig. 5 shown directly above the Lawson trainer, is the complete assembly of the Hansa-Brandenburg already described. The tail skid of the Hansa-Brandenburg is indicated by T.