A scale model investigation of spin-up loads on aircraft wheels

Abstract

This project is essentially an investigation of the horizontal forces which act on aircraft wheels as they ‘spin-up’ on initial impact with the runway. The effects of different landing conditions on these forces were studied with the aid of a scale model and test-rig. The results obtained are described here and compared with predictions from simple theory. The implications that these results have for the design and operation of aircraft are also examined. Spin-Up and Spin-Up Load: Just before an aircraft touches down on a runway its wheels are stationary. The impact with the moving runway causes the wheels to accelerate to full speed in a short time. This process is known as SPIN-UP and is clearly recognizable by the puff of smoke and screeching sound produced. During spin-up a horizontal force acts on the lower surface of the wheel (this is transmitted to the aircraft through the axle). This force is known as the spin-up load and its value depends on a large number of factors such as aircraft horizontal speed, vertical speed, wheel inertia, friction coefficient between runway and tyre, stiffness of wheel suspension etc. The spin-up load or force is impulsive i.e. it acts for a very short time. Its magnitude, however, is considerable and it presents a design problem to the engineer who must know its exact value under different conditions before the aircraft is actually built. Aims of Project: There were three main aims of this project: 1. To design and construct a test-rig to apply and measure spin-up loads on a model landing gear. 2. To investigate the way in which various factors affect the spin-up load on the model, and to examine the implications for a full-size aircraft. 3. To conclude whether scale model testing is realistic or not, to identify problems involved in scale testing and to suggest the best possible test-rig design. Types of Aircraft Landing Gear: The landing gear or undercarriage is that part of the aircraft which supports it while it is on the ground. Before describing the particular type of landing gear studied in this project a brief review will be made of the various types in common use. Basic Forms: Aircraft landing gears may be divided into three basic forms which are (see fig. 1.1): (a) Tricycle gear, w-lth two mainwheel units and one nosewheel unit. This is the predominant form of landing gear on modern aircraft. (b) Conventional gear, with two mainwheel units and a tailwheel or tail skid. This type was common on early aircraft. It is no longer used on large aircraft but still finds application on certain smaller types. (c) Unconventional gear, which includes floats, skis, skids, pontoons and any other gear used to support the aircraft on awkward or unusual terrain (sand, snow, water etc.). A further division may be made into retractable and fixed undercarriages. A retractable undercarriage is stowed inside the aircraft during flight to reduce drag. Types of Landing Gear Support: Early aircraft had their wheels supported on rigid frame-type structures (fig. 1.2). These did not incorporate any means of shock absorbing so landings were quite bumpy. Modern aircraft landing gears usually contain struts which deflect or compress under load. This serves to cushion the aircraft against the landing impact and also to make taxying smoother and more comfortable. Light aircraft often use ‘spring-type’ struts (fig. 1.3) so called because one end of-the strut is built into the aircraft fuselage while the other end which holds the wheel is springy and can flex under load. This system, being elastic, has no means of energy dissipation and as a result the aircraft will usually bounce back up after touchdown. To prevent this pouncing or ‘rebound’ effect larger aircraft use some type of damper,' the most common type being the oleo-pneumatic strut or oleo for short. The main component is a vertical shock strut which is a type of shock absorber filled with compressed air and oil. The compressed air gives the strut its elasticity while the oil is constrained to pass through an orifice thus acting as a damper. Oleo-type undercarriages vary greatly in complexity, as can be seen from figs. 1.4 and 1.5. Light aircraft usually have one wheel on each undercarriage unit but heavier aircraft may use one or more pair of wheels. These may be arranged in different ways (fig. 1.6). The type of layout chosen depends on many different considerations such as aircraft weight, fuselage space available for stowage of retracted gear, braking effectiveness, tyre wear etc.