AEROSANI: WHERE TO BEGIN?

The editorial office of our magazine continues to receive numerous letters from amateur designers: interest in this type of snowmobile equipment among the most diverse age groups continues unabated. But the nature of the letters often suggests that many builders begin to make snowmobiles, not having a very clear idea of ​​what they want to build, how they see the future car, where to start work. Naturally, because of this, they make annoying mistakes, and as a result, they get a design without the desired operational and technical characteristics.

Many years of experience in the design of a wide variety of machines shows that in order to successfully complete the work, it is necessary to adhere to a strict sequence at all its stages - from the birth of an idea to its embodiment in metal.

Amateurs who independently build snowmobiles, as a rule, make a serious mistake, ignoring one of the first mandatory moments in the design process - the development of technical specifications for the design they create.

The technical task (or technical requirements) is the starting document for all further work: calculations, detailed design of the structure, and its subsequent manufacture. It records what the designer would like to receive from the machine, unit, part. That is, a specific design goal is set, which must be achieved in the process of work.

It should be emphasized that it is very difficult to pose the problem correctly, taking into account the real possibilities of its subsequent solution, but without the necessary initial data. However, this can be done if you first study the experience accumulated by others, familiarize yourself in detail with previously built, tested and showing good results designs.

A certain baggage of theoretical knowledge will also be required, especially on the operation of the machine and its individual units - and without this it is impossible to outline an exact program of action.

Correctly drawn up technical requirements largely determine not only the scheme of the future machine, but also its most important elements. The shape and dimensions have not yet been worked out, the design of individual units has not been indicated, and the task already allows you to have all the basic data that will be required for calculations and design.

Each designer is individual in its creative solutions. Therefore, snowmobiles created by amateurs - even of the same type, similar in purpose, the power of the installed engine - can be different both in technical characteristics and in solving the general scheme and individual units. This also largely depends on a number of particular reasons, which are determined by the availability of materials and finished units, production capabilities, the availability of a workshop, machine tools, the necessary tools and even local operating conditions.

In the technical requirements for an amateur-built snowmobile, the following issues must be recorded and preliminarily resolved.

PERFORMANCE CHARACTERISTICS

A set of requirements for it depends on a clear definition of what a car is created for: for example, the cross-country ability of a snowmobile on winter off-road, the general scheme and many other characteristics. If, for example, a sled will be used for hunting, the machine must have a high degree of cross-country ability.

The degree of cross-country ability of snowmobiles, their running characteristics are determined by the dynamic coefficient of "quality", which is found by the formula:

where K is the dynamic coefficient, T is the thrust force of the propeller in kgf, developed by it when working on site, that is, without the translational motion of the snowmobile, Gx is the running weight of the snowmobile in kgf.

The value of the coefficient K for an amateur-built snowmobile ranges from 0.2 to 0.3. But it should be remembered that the larger this value, the more passable in winter off-road conditions the car will turn out. The designer chooses the value of the dynamic coefficient, which should strive to be at least 0.2. The above formula K can also be used to determine T and Gx.

T = Gx K; and Gx = T / K,

These values ​​for preliminary calculation can be determined as follows:

where N is the engine power in hp. sec., 0.8 - coefficient ensuring the reliability and service life of the engine; Tu is the specific thrust of the propeller in kgf, that is, the force it develops in kgf per one liter. with. engine power.

Specific thrust Tu depends on the diameter of the air vane and the engine power. It is determined by the load on the area swept by the propeller, decreasing with increasing this load. With engine power up to 12 liters. with. - 4.5 kg / l. s., 20 p. with. - 4.0 kg / l. s., 50 l. with. - 3.5 kg / l. s., 100 l. with. - 3.0 kg / l. s., 260 l. with. - 2.6 kg / l. with.

More precisely, the tractive effort is determined by the formula:

T = (33.25 h D N) 2/3

where 33.25 is the coefficient; h is the efficiency of the propeller (for home-made ones, it fluctuates, depending on the correctness of the calculation and the quality of manufacture, from 0.65 to 0.82, or from 65 to 82%); D is the propeller diameter in meters, and N is the engine power.

The Gx value can be determined based on the fact that the running weight should not exceed 12 ÷ 15 kgf per one horsepower of the engine power.

LOADING CAPACITY OF AEROSANES

It includes the weight of the crew, passengers, transported cargo and fuel supply.

This load is given for a snowmobile with skis on a sole made of ordinary carbon steel, the coefficient of crunching of which on the snow is taken as 100%. When materials with a lower coefficient of friction are used instead of steel - brass, low-pressure polyethylene, fluoroplastic-4 - this value can be increased. In this case, G1x is determined from the expression

G 1 x = Gx 100 / f,

where f is the ratio of the coefficient of friction of the initial material to the coefficient of friction of carbon steel.

For the materials used on the soles of skis, the friction coefficients will be as follows: carbon steel 1.0, or 100%, wood (ash) 0.97, arktilite 0.935, stainless steel 0.810, duralumin 0.790, polyethylene 0.735, fluoroplastic 0.730, brass 0.710.

TRAVEL RANGE

The distance that the snowmobile must cover on the available fuel and oil supply in the tanks, without refueling on the way.

FUEL STOCK

The above-mentioned fuel reserve, in turn, determines the required capacity of the fuel tanks (plus 5% of the emergency reserve).

SPEED OF AEROSANES

In the technical requirements, two values ​​of the speed of movement of the snowmobile are usually recorded: Vt - technical, that is, the speed of the actual movement along the route, excluding the time spent on parking along the way. Vmax is the maximum possible, that is, the speed that the snowmobile can develop when the engine is running at maximum power and at full (calculated) load. The maximum speed is determined at a measured distance when driving on virgin snow at an outside air temperature of -5 to -20 °, in other words, when skis glide well on the snow. The measured distance should be straight, without obstacles from snow sastrugs, hummocks; its length is not less than 0.5 km.

TURNING RADIUS

It is measured on the inner track of the snowmobile skis at a speed that ensures the safety of its performance, usually 5 km / h.

ACCEPTABLE LIFT

The hoist land available for overcoming by a machine with a normal weight load is indicated.

ADDITIONAL REQUIREMENTS

In this section of the technical assignment, the snowmobile builder writes down what, in his opinion or on the basis of the study of previous experience, should be reflected in the design of the future machine. This includes limiting the size of the car or, conversely, increasing the length of the body for the transportation of some oversized cargo. Some requirements may reflect the designer's view of the desired machine layout or its individual elements - based on the study of existing experience. For example, "a snow sled should be made according to a four-ski scheme - as more stable and having less resistance to movement compared to a three-ski sled", etc.

SAFETY

The propeller is one of the most dangerous (for the surrounding) structural elements. High revolutions make the circle described by him transparent, the human eye does not perceive its "plane". Therefore, it is absolutely necessary:

SCREW GUARD

It is usually painted with bright red paint to alert people to danger.

BRAKING SYSTEM

It is most often provided by lever-pin or scraper devices.

ENGINE STARTING SYSTEM

Starting the engine most often used by most amateurs by turning it by the propeller by hand (with the ignition on) is strictly prohibited. I need a starter.

OUTDOOR LIGHTING

In winter, the daylight hours are very short, and in the northern latitudes the polar night sets in, therefore, headlights should be installed on the snowmobiles, allowing the driver to see the road well.

LAYOUT DRAWING OF AEROSANIA

After drawing up the technical requirements, you can proceed to the next stage of work - preliminary calculation and execution of the layout drawing of the snowmobile. These works are carried out in parallel. At the same time, the terms of reference will be a guideline: it will be clarified in the process of calculations.

Such a drawing is the basis for designing a future machine. It determines the location of all structural elements, the driver's and passengers' seats, outlines the general contours of the body, the location of doors, windows, hatches, and coordinates all basic dimensions.

Many lovers of technical creativity, trying to speed up work, omit such an important stage as the production of machine drawings.

This is a misguided practice. Experience shows that the time spent on the execution of drawings, the development of parts and dimensions on them, pays off a hundredfold due to the reduction in the cost of linking and fitting parts during construction, not to mention the fact that careless designers lose more due to the forced rework of individual parts ...

The layout drawing is carried out in three projections on a scale of at least 1: 5, if necessary, with additional sections, cuts and leaders of individual nodes.

During the assembly process, attention should also be paid to safety requirements. The main ones are as follows.

On an open-type aerosled, the distance from the driver's back to the plane of rotation of the propeller must be at least 0.8 m.

Rotating drives located behind the driver's back - pulleys, chains, belts - must be covered with safety guards.

In a closed type case, it is important to ensure reliable fastening and ease of use of doors and hatches. Not the least in these requirements is the need for a good view of the road from the driver's seat: the angle of view through the windshield should provide visibility already from 5 m from the toes of the front skis.

It is useful to protect the windshield from freezing - for example, by installing double glazing, as well as provide windshield wipers.

Fuel tanks and storage batteries must be separated from the cab by a sealed partition.

The cab should be heated using autonomous heaters: it is impossible to lay exhaust pipes along the body for these purposes.

The propeller across the width of the machine should not protrude beyond the outer edges of the skis; the distance from the end of the blade to the bottom of the hull is at least 50 mm, and to the soles of the skis - at least 250 mm. The ends of the rear skis must not fall into the plane of rotation of the propeller.

And what is the preferred exterior color of the snowmobile? Any, but always bright, clearly visible in the snow from a distance.