The propeller, or, as they said at the dawn of aviation, the propeller is experiencing its rebirth today. The reason for this is the appearance of hang-gliders and motor paragliders with very advanced propeller-engine systems. Pilots quickly realized that they could also be used on the ground.
It should be noted that trike power units are excellent for creating airboats in terms of power, reliability and efficiency, since the parameters of a motor with a propeller are no worse than those of traditional power units with propeller. Moreover, a boat with aeropropulsion is not afraid of shallow water, thickets of reeds, sedges and algae. In addition, the glider engine releases exhaust gases not into the water, like the outboard or stationary power unit of any boat (from the point of view of environmentalists, this method of muffling the exhaust does not stand up to criticism!), but into the air.
So, an airboat. The heart of his propulsion system is the Whirlwind outboard motor - a compact two-cylinder liquid-cooled engine with a power of about 25 hp. Unfortunately, its crankshaft rotation speed is too high to work in tandem with propeller, therefore the motor is equipped with a three-belt V-belt gearbox with a gear ratio of 1.6. V-belts are “Zhiguli”, from the “engine - pump - generator” system.
The driving and driven pulleys are machined from duralumin (D16T or AK4-1T) and, after adjustment, are subjected to hard anodizing. The drive pulley is attached to the flywheel with rivets.
1 - glider body (upper part); 2 - door; 3 - engine hood; 4 - power plant; 5 - air screw; 6 - propeller keel guard; 7 - steering device; 8 - glider body (lower part).
To install a driven pulley on the engine, it is necessary to install a spacer plate made of steel sheet 5 mm thick, and mount the cantilever axle of the driven pulley on it. The pulley itself rotates on an axis on two ball bearings 204 and one 205. Between the bearings there are duralumin spacer bushings. The pulley is fixed to the axle with a locking ring and a screw with a washer.
The spacer plate is bolted to the engine crankcase and to the brackets, and the latter are installed on adapter bushings, which are screwed instead of nuts onto the engine head mounting studs. To tension the belts, a mechanism is used, consisting of a bushing welded to a spacer plate and a bolt with a nut.
As already mentioned, the engine is cooled by liquid, using sea water supplied to the cooling jacket homemade pump, made on the basis of the impeller from the Kama electric pump. For supporting optimal temperature engine (80-85°C) standard car thermostat.
The engine is started using a cord, for which a pulley is installed between the propeller and the spinner, around which the cord is wound before starting.
The propeller of the airboat is wooden, monoblock, that is, made from a solid pine block. True, it is not easy to select such a block without knots and cross-layers, and in this case it makes sense to glue the workpiece epoxy resin from carefully trimmed plates about 10 mm thick. When selecting plates, you need to make sure that the layers of wood are positioned symmetrically relative to the gluing planes - this will save the propeller from possible warping in the future.
The manufacture of a propeller begins with the preparation of templates - plywood or, better yet, duralumin, which are made according to a carefully executed plaza drawing on a scale of 1:1. You will need the following templates: plan, side view (up to the axis of symmetry), as well as the upper and lower profiles of the screw.
To begin with, the workpiece is jointed on all sides in accordance with overall dimensions screw, after which axial lines are drawn on it and, using a template, the contours of the side view are drawn. Next, excess wood is removed - first with a sharpened hatchet, and then with a plane and rasp.
Next, the workpiece is marked using a planning template, which is secured with a small nail in the center of the future propeller, outlined with a pencil, after which the template is rotated 180° and the planned projection of the second blade is marked. Excess wood is removed with a bow or fine-toothed band saw.
The most important part of the work is giving the blades an aerodynamic profile. As can be seen from the drawing of the screw, one side is flat and the other is convex. In accordance with the position of the control sections, the installation locations of the templates are marked on the workpiece, and “beacons” are made with a semicircular chisel and a semicircular rasp - in accordance with the configuration of the upper and lower templates.
The main tool for processing propeller blades is a small ax made of good steel, literally sharpened to a razor sharpness. When removing wood, it is recommended to first make small cuts - this will avoid splitting the workpiece. This is followed by preliminary processing of the workpiece with a plane and rasp.
This is followed by final finishing in the slipway. The latter is a carefully trimmed board with a thickness of at least 60 mm, on which transverse cuts are made to a depth of 20 mm to install the lower profile templates of the propeller blade. The central rod of the slipway is machined from steel or duralumin; its diameter must correspond to the hole in the propeller hub. The rod is glued in the center of the slipway board strictly perpendicular to its surface.
Next, the working surfaces of the lower templates are rubbed with colored pencil or blue, the propeller blank is put on the central rod and pressed against the templates - first with one blade, and then with the other. In this case, traces of the templates will be imprinted on the workpiece in those places where they come into contact with the lower surface of the propeller. "Dirty" areas using a planer, plow, rasp or wooden block with the sandpaper glued to it, they are cleaned off, the workpiece is again placed in the slipway - and the processing of the propeller blades is repeated. When traces of a colored pencil are imprinted across the entire width of the blade, the processing of its lower surface can be considered complete.
The upper part of the screw is machined in a slipway using upper templates (they are also called counter templates). First, using a semicircular rasp, the blade is adjusted to the counter-templates (as professionals say, counter-templates are seated), as a result of which the template and the counter-template should come into contact along the parting plane, tightly enveloping the blade itself. Then the treated areas are rubbed with a colored pencil and the areas between the control sections are processed. IN in this case painting is necessary in order to prevent re-processing of the blade at the locations of the control sections. In this case, the correctness of processing is checked with a straight steel ruler applied to the one-percent points of adjacent sections. On a properly made blade there should be no gap between the ruler and the surface.
If during the work an awkward movement of the tool leads to chipping of the wood, this does not mean at all that the work is irreparably damaged. You can fix it with putty mixed with epoxy glue and small sawdust.
The finished propeller is carefully balanced. The best way to do this is to tightly insert a metal roller into the central hole and install the propeller on the balancing rulers. If one of the blades turns out to be lighter, it is recommended to load it with lead, for which small strips of this metal are first glued onto it, and when the propeller is balanced, the strips are melted and poured into a mold, for example, into a piece steel pipe. The resulting rod (or rods) is riveted into a hole drilled in the place of the blade where the lead strips were glued. The hole on both sides of the blade should be slightly countersunk.
Finishing the propeller consists of covering it with two layers of thin fiberglass, followed by sanding, final balancing, priming and painting with auto enamel.
The airboat hull consists of two large parts - upper and lower. It is better to start assembling it from the bottom. To do this, in accordance with the theoretical drawing of the hull and drawings, form-building frames are cut from plywood 12 mm thick, and stringers and keels are cut from slats with a cross-section of 20×20, 30×20 and 30×30 mm. The frame is assembled on a flat floor. The diametrical plane and the location of the frames are first marked on it. The frames are attached to the floor using wooden blocks and braces. The adjustment of the longitudinal slats is done “in place”, the fastening of the slats to the frames is done with epoxy glue with temporary fixation of the elements with safety wire. Curvilinear slats for the front part of the frame are obtained by first steaming them in boiling water and fixing them with wire on the frame. After the slats have dried, they are fixed to the frames with epoxy glue.
After thinning (leveling) the frame, the spacing is filled with blocks of construction foam, which are fixed using the same epoxy binder. After treating the foam surface (if necessary, it is puttied with the already familiar composition of epoxy glue and sawdust), the body is covered with two layers of fiberglass, puttied, sanded and painted with auto enamels. From the inside, the foam is cut flush with the frames and also covered with fiberglass.
A - frame assembly; B- filling spacing foam blocks; B – covering the body with fiberglass
A - marking the workpiece using a side view template; B - marking using a planning template; B - cutting of “beacons” and rough trimming of the blades; G - processing of blades with a plane; D – processing with rasp and sandpaper
1 - M10 bolt; 2 - washer; 3 - air screw; 4.17 - M8 bolts; 5 - lock washer; 6.7 - bearings 204; 8 - axis-console; 9,10 - spacer bushings; 11 - bearing 205; 12 - remote washer; 13 - retaining ring; 14 - M8 nut; 15-bolt belt tension mechanism; 16 - driven pulley; 18 - adapter bushings, 19 - gearbox bracket (2 pcs.); 20 - V-belt (4 pcs.); 21 - driving pulley; 22 - rivet d5 (steel, 10 pcs.); 23 - spacer plate; 24 - Whirlwind-30 engine.
1 - central rod; 2 – air screw; 3,4 - counter-patterns; 5 - slipway board; 6 - lower templates.
Making the upper part of an airboat is not much different from the lower part. True, the frame is assembled not from plywood frames, but from prepared curved slats, and not on the floor, but on the already finished lower part of the body.
The frame on which the engine motor mount is attached has an increased cross-section and reinforcements at the junction of the slats - plywood gussets. The frame itself is attached to a cross member made of a square steel pipe with a cross-section of 40×40 mm and is fixed with braces made of pipes with a diameter of 22 mm.
Shaping is also done using polystyrene foam followed by gluing with fiberglass.
The door glazing is made of plexiglass 4 mm thick, the windshield is from back door car "Moskvich-2141". Part of the door itself became an element of the cabin.
Airboat doors consist of wooden frame and plywood sheathing. They are covered with fiberglass inside and out. Door hinges are homemade, overhead. In the ceiling of the cabin (or, if you prefer, the deckhouse) there is a removable hatch cover made from a cut-out part of the roof.
To date active recreation, fishing and other activities related to movement in shallow water are quite common. However, large boats not only make it difficult to navigate such parts of the journey, but are also quite expensive. That is why many resort to making unusual watercraft on their own. How to make an airboat with your own hands? To answer this question, you first need to decide what it is. A vessel that moves with the help of a propeller or an aircraft turbine is an airboat (airboat). This type of vehicle is very suitable for moving through shallow waters, since its moving part (engine, turbine, etc.) is above the water. Therefore, the depth of the reservoir does not matter. The second feature is that the dimensions of such a vehicle are quite modest, which increases its advantage.
General information about the unit
So, let's start figuring out how to make an airboat with your own hands. Everyone understands that the most necessary parts of this watercraft are the hull and the engine. Here it is worth paying attention to the fact that you can choose several device options as the driving part. Experts say that the best option The engine is the power part of the trike. It is almost ideal in such parameters as:
- Power.
- Reliability.
- Efficiency.
Not bad additional characteristics It will also be that such a device copes well with overcoming thickets of reeds, sedges and accumulations of algae.
However, not everyone has such a power unit, and buying it is not always profitable. Therefore, you can use, for example, a motor from a Japanese motorcycle. A homemade airboat with this type of device will also be quite good.
Selecting a moving part
One very important feature The peculiarity of such unusual boats is that they discharge emissions from the operation of power elements not into the water, but into the air. Environmental experts say this is much better.
If a person decides to create such an airboat, then the first thing he needs to purchase is an engine. In the article, the Whirlwind outboard motor will be taken as an example. The characteristics of this unit are as follows: two-cylinder, liquid-cooled, and its power is about 25 hp. A rather pleasant bonus is that the device is compact in design. However, this does not mean that you should only use this type of engine. You can construct an airboat from a car engine with your own hands.
If we return to the consideration of "Whirlwind", then there is one nuance. In it, the frequency with which the crankshaft rotates is quite high. It is not suitable for direct connection to a propeller. To solve this problem, the motor is additionally equipped with a three-ribb V-belt gearbox with a gear ratio of 1.6. Models used in Zhiguli cars, where the “engine - pump - generator” system is used, are taken as V-belts.
Airboat pulleys
The next elements are two pulleys. One of them will be the leader and the other will be the follower. These two parts are also the main ones for assembling an airboat with your own hands. Pulleys are machined from a material such as duralumin. After this, they are adjusted and subjected to an operation such as hard anodizing. The first part, that is, the drive pulley, must be attached to the flywheel using rivets. To install the second pulley to the engine, you will have to place a spacer plate made of 5 mm thick steel on its front part. It is necessary to install the cantilever axle of the driven pulley on this plate. It will rotate on an axis using two ball bearings 204 and one 205. Between these elements there are spacer bushings, also made of duralumin.
Securing parts
To secure the pulley to the axle, a locking ring and a screw and washer are usually used. The spacer plate, which was used previously, is bolted to the engine crankcase and to the brackets. These elements, that is, brackets, are mounted on adapter bushings, which are screwed onto the engine head mounting studs instead of nuts. Next you need to move on to tensioning the belts. To perform this operation, you need to use a special mechanism, which consists of several elements. The first is a sleeve welded to the attachment plate, and the second is a bolt with a nut.
It was already mentioned earlier that the cooling in the design of homemade airboats of this type is liquid. It is important to note here that sea water is used, which is supplied to the cooling jacket. To draw liquid, a homemade pump is used, which is made on the basis of the impeller from the Kama electric pump.
The simplest car thermostat is used as a sensor that monitors the temperature and regulates it within the normal range (80 - 85 degrees Celsius). To launch an airboat with your own hands, a cord is used. The location of this element is between the propeller and the spinner. By pulling the cord, the engine starts, since there is a pulley inside, around which this part is wound before starting the device.
Air propeller
This is also one of the main details of the type of watercraft under consideration. To create a propeller for an airboat with your own hands, you need to understand its design. This element is wooden and monoblock. In other words, you need to use a solid block of wood to make the part. It is worth noting here that finding a timber that does not have defects in the form of knots or cracks is problematic. Therefore, you can do it differently. Designers suggest taking several plates, the thickness of which will be at least 10 mm, and gluing them together using epoxy resin.
Before you begin the gluing process itself, you need to make sure that the layers of wood are arranged symmetrically. This must be done in order to save the screw from possible deformations during further operation. The finished (glued) workpiece is marked according to a standard drawing, which is hung in the center of the block and nailed with a small nail. Next, you need to circle the existing drawing, and then turn it 180 degrees and circle it again. In this way, projections of both blades can be obtained.
Assembling the screw structure
It is very important to remove any excess wood that may interfere with the operation of the screw. To do this, use a fine-toothed bow or belt type. The most important part of the work when creating an airboat with your own hands is giving the propeller an aerodynamic profile. It is important to note here that one of the sides of this part should be flat, and the other convex. It is better to immediately note this on the drawing, since the error cannot be corrected later. You will have to create the entire structure again.
To process the propeller blades, you need to have a small ax that will be sharpened very well. This tool must be made of steel High Quality. When removing an excess layer of wood, you need to work quite carefully to avoid cracks. Experts recommend making small cuts - this is the safest option. After rough processing with an ax, you can begin to preliminary preparation, for which a plane and rasp are used. Final finishing is carried out using a slipway. Let's tell you what it should be like.
Slipway
To build an airboat with your own hands, you will definitely need this device. It is a carefully leveled board, the thickness of which is at least 60 mm. It is used to make cuts up to 20 mm deep. The lower profile templates of the propeller blade are inserted into the resulting recesses.
The slipway is machined from several parts. Its basis is the central rod, which is made of materials such as steel or duralumin. The diameter of the rod is determined by the hole in the screw hub. They must match each other. The resulting rod is located exactly in the center and strictly perpendicular to the slipway board.
Airboat hull
To create a working homemade airboat, you need to devote a lot of time to creating the hull. This is the main element, which is quite voluminous if made entirely. For this reason, experts recommend dividing it into two components - upper and lower. It is better to start assembling these two elements from the bottom. To do this, it is necessary to cut out form-building frames from plywood, the thickness of which is at least 12 mm. To prepare components such as keels and stringers, slats with dimensions of 20x20, 30x20 or 30x30 mm are used. The frame of the lower part of the boat must be assembled on a flat floor. Before starting the process of forming the lower part, you need to mark its diametrical plane, and also mark the places where the frames will be located.
Top part
If we talk about manufacturing the upper part of the case, then this process is practically no different from assembling the lower part. The only significant difference is that it is formed not from plywood frames, but from previously prepared curved slats. Note that the formation of the body is no longer carried out on the floor, but on the directly finished and assembled lower part of the body. It’s worth saying here that you can avoid this labor-intensive work if you assemble an airboat from a PVC boat with your own hands. The body of such models is already ready and represents a single structure.
Engine frame
Let's look at one more important detail. This is the engine motor mount. It is attached to one of the frames. The element to which the frame will be attached must be reinforced. Its cross section should be increased. It should also have reinforcement at the joints of the slats. This can be done using a plywood scarf. A steel pipe is used to secure the frame to the crossbar square section 40x40 mm. To fix this element, a brace is used, which is created using pipes with a diameter of 22 mm. For glazing of doors, if any, plexiglass with a thickness of 4 mm is used.
Depending on the reliability of the frame fastening and the planned use of the vessel, you can use various power elements. Some take the engine from the Ural for a homemade airboat. You can also achieve good power with this component.
A little about the benefits
Naturally, to gain popularity, it is necessary to have some advantages that other types of swimming equipment do not have. For an airboat, these qualities were the following few points. Firstly, engine leakage will accumulate not outside, but inside. Secondly, steering such a small boat brings quite a lot of adrenaline, since the speed it can reach is quite high. In addition, making homemade airboats with your own hands will bring a lot of joy to those who love to make things. For fishermen, the most significant advantage is that such a vehicle can plow almost any water expanse, and quiet operation allows you to silently swim to fish habitats.
Control
Today, such devices do not use direct control transmission, but belt or gear transmission. The advantages of both systems are that they adjust the fuel supply to the engine and steering movement.
It is also worth noting that some fishermen or simply those who like to travel in this way equip their airboat additional equipment. This could be glass, comfortable seats, spotlights, etc.
Universal device
An airboat can be used for more than just traveling on water. Some craftsmen have quite coped with the task of creating a small “amphibian” that can be used to move not only on water, but also on ice. If we talk about the characteristics of the resulting transport, its speed (with passengers) on hard surfaces is up to 90 km/h, and on water up to 45 km/h.
The basis for creating such an amphibian was the Yantar motorboat. The main difference from conventional airboats (besides the fact that it also moves on hard soil) is that a V-belt variator from a snowmobile is used as a transmitter from the gearbox to the propeller. This is what served as the main difference and the opportunity to create a real all-terrain vehicle.
It's not often you see a boat propelled by a propeller. And this is not surprising - the air density is 840 times less water. And since both the water propeller and the propeller operate on reactive principle, then the thrust and efficiency of the propeller depend mainly on how much air mass is thrown back and with what acceleration. The greater this mass and the higher the speed of air flow behind the propeller, the greater the thrust the propulsion develops. That is why it is necessary to make an air propeller of a much larger diameter than a water propeller, and to provide a much higher rotation speed in order to obtain comparable thrust. And even so, designers of boats with propellers rarely manage to achieve a sufficiently high efficiency of the propulsion system.
In addition to relatively low efficiency and large dimensions, propellers have other disadvantages. Thus, their operation is accompanied by increased noise, and the propeller must be protected with a grille and a reliable fence to prevent the possibility of injury to the driver or passengers. And yet, in some cases, a propeller may turn out to be the most convenient, if not the only, propulsion option for a boat. It's about about shallow or weed-filled rivers and lakes where even a jet boat cannot pass.
The article below by Yu. V. Shukevich is addressed primarily to amateur designers and builders of boats with propellers. In it, the author shares his experience in selecting a propeller for a small motorboat, and also cites materials he borrowed from a number of other sources on the approximate calculation of a propeller and the design of its profile.
It should be noted that propellers can be used not only on high-speed planing boats.
For example, a 3 HP engine. With. with a propeller D=1.4 m gives a thrust of about 20 kg. This thrust is quite enough to give a light boat a speed of 10-15 km/h, so for small displacement boats or catamarans, where good cross-country ability is needed, it is quite possible to install low-power motors with a propeller. In addition, making a small fixed-pitch propeller and installing it directly on the engine shaft is much easier than, for example, a water jet, and the boat’s cross-country ability will, of course, be better.
To build the motorboat (Fig. 1), I used sea sled-type contours, one of the projects of which was published in the 13th issue (Fig. 2). The hull, 4.0 m long and 1.4 m wide, is built on frames made of 10 mm plywood and a longitudinal set of pine slats. The bottom skin is made of BP-1 plywood, 3.5 mm thick, the sides - 2.5 mm thick. The exterior of the body is covered with fiberglass cloth and epoxy resin. Foam blocks are glued in the bow and stern.
The planned engine from the M-62 motorcycle could not be obtained. I had to assemble it from parts of the IZH Planet engine and the MP-800 motor pump. The power of this unit was about 30 hp. pp., assembled weight 42 kg.
The propeller shaft bearing housing, the shaft itself and the bushing are remade from the corresponding parts of the tail rotor of the MI-1 helicopter, which have served their purpose. I made the propeller blades from pine and covered them with nylon on ED-5 resin. The propeller with a diameter of 1.7 m is reversible, variable pitch. The transmission to the propeller from the engine is carried out by a chain from the IZH-56 motorcycle. The engine and propeller drive are mounted on a frame made of chromosil pipes.
Since the design already used finished parts, designed for significantly higher powers, the total weight of the installation turned out to be quite large - about 100 kg. To simplify the fuel system, the consumable 15-liter gas tank also had to be moved to the propeller frame (Fig. 3, 4).
During the engine run-in, the propeller thrust was measured in place - when the throttle was opened 2/3, it turned out to be equal to 80 kg.
The boat was tested on Lake Kenon. At full speed, it passed through continuous thickets of reeds and grass (the speed did not drop from this), walked along the shore at a depth of 8-10 cm, without touching the bottom. The boat moved well even on a fairly high wave, it was well controlled in planing mode, the speed with one driver reached 45 km/h, with two passengers - 42 km/h.
The boat was transported to the water on a trailer behind a motorcycle. If you start the engine of a boat on a trailer, it easily pushes a motorcycle with a sidecar in front of it. So it should go just as easily on ice.
Design flaws were also revealed. The chain vibrated strongly while driving (the number of sprocket revolutions was about 5000 rpm), and the water rudder turned out to be insufficiently effective, especially at low boat speeds. The chain was replaced over the winter V-belt drive, which operates silently and can withstand high speeds. To increase traction, a profiled nozzle with a gap of 6 mm was installed around the propeller. However, it did not provide an increase in thrust; when the gap was reduced to 2 mm during transient engine conditions, the ring began to vibrate and the propeller touched it. In the future, it is planned to increase the thrust of the propeller by increasing its diameter and installing a gearbox. However, the results obtained can be considered quite good. The speed of 45 km/h with a full load of 280-300 kg and good cross-country ability completely repay the efforts spent on construction.
The main difficulty that the builder of such a motorboat may encounter is calculating the propeller. Below is a series practical recommendations on the selection of the main elements of propellers, borrowed from a number of sources (a list of them is given at the end of the article).
Screw diameter
The desire to obtain the greatest thrust and efficiency of the propeller forces the use of large diameter propellers or an increase in the number of revolutions. But both ways have their limits: an increase in diameter, as a rule, is limited by design considerations (for example, it is undesirable for the edges of the blades to protrude beyond the width of the boat), and with increasing speed, the peripheral speeds of the ends of the blades approach the speed of sound and k. The propeller pressure decreases sharply. In this case, wooden screws reach the critical rotation speed faster than metal screws (Fig. 6 and 7).An increase in diameter also worsens the stability of the motorboat, its maneuverability through reeds and reeds, reduces seaworthiness, and increases the dimensions of the installation and weight.
Typically, the diameters of propellers, even with a powerful engine, do not exceed 2.5 m. To determine the diameter of the propeller, you can use the formula:
where W k is the peripheral speed of the blade tip, m/s;
n in - number of screw revolutions per minute;
N - engine power, l. With.;
n - number of propeller revolutions per second.
It is possible to increase the thrust force without increasing the diameter, by increasing the number of blades to 3 or even 4. However, the efficiency of multi-bladed propellers is somewhat reduced due to the operation of the blades in a more disturbed flow. When calculating a multi-blade propeller, a correction factor is introduced k 2 =0,9.
To calculate the diameter of a two-bladed propeller with blades of normal width, the coefficient k 2 =1.0 (with b max =0.08÷0.09); two-blade propeller with narrow blades k 2 =1,1 (b max =0.06÷0.07); slotted two-blade propeller with very wide blades k 2 =0.14÷0.2 (everywhere b max = b max/D; b max - maximum blade width).
Shape and dimensions of the cross section of the blade
Most often, flat-convex segmental and aviation profiles are used for propellers. The main geometric characteristics of profiles are the chord size b and profile thickness WITH(Fig. 8). The relative profile thickness is the ratio c=C/b; profiles are: thick ( c=0.21÷0.15), average ( c=0.12÷0.1) and thin ( cIncreasing the width of the blade does not give any gain - due to the increase in its weight, the efficiency of the propeller decreases; this is explained by the fact that as the width increases, the thickness of the blade also increases. Characteristic cross section The propeller blade is its section at a radius equal to 0.75 R. The value of the profile chord of this section is called the average chord of the blade b 0.75. To calculate it, we can recommend the formula:
Where k- number of blades;
C y is the average lift coefficient of a given profile, determined from the graph (Fig. 9).
Having calculated the value of the average chord of the blade b 0.75, it is necessary to determine its relative width: b rel = b 0.75 / D; for wooden screws this value should be in the range from 0.08 to 0.12. Wide blades with b 0.75 >0.12 will have lower efficiency. If it turns out that the relative width of the blade does not fall within the recommended limits, it means that the propeller parameters were not chosen well. In this case, you can change the width of the blade by changing the peripheral speed, or increase the number of propeller blades. It is better to make a propeller with the same blade width along the entire length with a rectangular wide end (Fig. 10).
The relative thicknesses of the blade profiles should be: at the hub - 0.18÷0.2, in the section at R 0.75 - 0.14÷0.13 and at the ends of the blades - 0.07÷0.1.
It is advisable to use large relative thicknesses on low-speed propellers with a blade tip peripheral speed of up to 180 m/s.
The pitch of the screw or the average installation angle of the section located at a radius of 0.75 R relative to the plane of rotation of the screw is determined by the formula:
The installation angles of the remaining sections φ n are determined by the relative value of φ taken from the graph (Fig. 11):
The propeller thrust can be determined by the formula:
where η is the propeller efficiency;
Δ - relative air density (at normal conditions numerically equal to 1);
D - screw diameter in m;
N is the power supplied to the propeller, in hp. With.
or
where K 1 for a two-bladed propeller is 7.5.
In conclusion, examples of propeller calculations made for the described motorboat are given.
Calculations are given for a wooden, reversible propeller with a chain gear (screw 1) and a metal propeller (screw 2) for installation directly on the engine shaft (the angle of the blades can be adjusted when the engine is stopped).
Initial data: engine power - 30 hp. With.; crankshaft speed - 3600 rpm; gear ratio - 2.
I. Selection of screw diameter. For wooden screw 1, I chose the peripheral speed Wк = 160 m/s, corresponding to the highest efficiency, then (1)
In the second case, I chose the diameter of propeller 2 for design reasons, equal to the width of the boat 1.4 m. We find the critical speed for a metal propeller with a diameter of 1.4 m from the graph in Fig. 7 n=4000 rpm, but actually 3600 rpm, therefore,
According to the graph (Fig. 6) we find the value of efficiency η = 0.6, which, of course, is less than for a wooden screw, but in this case there will be no power loss in the gearbox.
II. Determining the propeller thrust (7):
where N is taken taking into account losses in the gearbox;
This result almost coincides with dynamometer tests on mooring lines - the thrust turned out to be 80 kg.
III. Determining the width of the blade for these screws at a distance of 0.75 R (3):
In the second case, the blade turns out narrower, which is more advantageous.
IV. Determining the installation angle of the section along the blade at a distance of 0.75 R (4):
According to Fig. 11, you can determine the installation angles of the sections at any radius. If the propeller is variable pitch, then it is important to correctly only twist the blade, i.e. the angle of attack of the blade can be changed depending on the sailing conditions (I can change the pitch of a wooden propeller from -1 m to +1.5 m). If the propeller has a constant pitch, then an error in determining the pitch can lead to the fact that the engine will not pull such a propeller or will not operate at full power.
The weight of one blade of the first propeller is 2.5 kg. I cast the second screw from duralumin alloy. The weight of its blade is 3 kg.
Installing a propeller without a gearbox made it possible to reduce the weight of the propeller unit by 30 kg.
- 1. "Snowmobile". I. N. Yuvenalyev, ed. DOSAAF, 1962
- 2. Magazine “Modeler-Constructor”, No. 9, 1968, No. 11, 12 for 1970, ed. “Knowledge”, No. 11, 1967
- 3. Brochure of the “Transport” series, ed. “Knowledge”, No. 11, 1967
How to do homemade airboat. It should be noted that trike power units are excellent for creating airboats in terms of power, reliability and efficiency, since the parameters of a motor with a propeller are no worse than those of traditional power units with a propeller. Moreover, a boat with aeropropulsion is not afraid of shallow water, thickets of reeds, sedges and algae.
In addition, the glider engine releases exhaust gases not into the water, like the outboard or stationary power unit of any boat (from the point of view of environmentalists, this method of muffling the exhaust does not stand up to criticism!), but into the air. So, airboat. The heart of his propulsion system is the Whirlwind outboard motor - a compact two-cylinder liquid-cooled engine with a power of about 25 hp. Unfortunately, its crankshaft rotation speed is too high to work in tandem with a propeller, so the motor is equipped with a three-ribb V-belt gearbox with a gear ratio of 1.6. V-belts are “Zhiguli”, from the “engine-pump-generator” system.
The driving and driven pulleys are machined from duralumin (D16T or AK4-1T) and, after adjustment, are subjected to hard anodizing. The drive pulley is attached to the flywheel with rivets. To install a driven pulley on the engine, it is necessary to install a plate-spacer made of a steel sheet 5 mm thick on its front part, and mount the cantilever axle of the driven pulley on it. The pulley itself rotates on an axis on two ball bearings 204 and one 205. Between the bearings there are duralumin spacer bushings.
The pulley is fixed to the axle with a locking ring and a screw with a washer. The spacer plate is bolted to the engine crankcase and to the brackets, and the latter are installed on adapter bushings, which are screwed instead of nuts onto the engine head mounting studs. To tension the belts, a mechanism is used, consisting of a bushing welded to a spacer plate and a bolt with a nut. As already mentioned, the engine is cooled by liquid, using sea water supplied to the cooling jacket by a homemade pump made on the basis of an impeller from a Kama electric pump.
To maintain optimal engine temperature (80-85°C), a standard car thermostat is used. The engine is started using a cord, for which a pulley is installed between the propeller and the spinner, around which the cord is wound before starting. The propeller of the airboat is wooden, monoblock, that is, made from a solid pine block. True, it is not easy to select such a block without knots and cross-layers, and in this case it makes sense to glue the workpiece with epoxy resin from carefully planed plates about 10 mm thick.
When selecting plates, you need to make sure that the layers of wood are positioned symmetrically relative to the gluing planes - this will save the propeller from possible warping in the future. The manufacture of a propeller begins with the preparation of templates - plywood or, better yet, duralumin, which are made according to a carefully executed plaza drawing on a scale of 1:1. You will need the following templates: plan, side view (up to the axis of symmetry), as well as the upper and lower profiles of the screw. To begin with, the workpiece is jointed on all sides in accordance with the overall dimensions of the screw, after which axial lines are applied to it and, using a template, side view contours are applied.
Next, excess wood is removed - first with a sharpened hatchet, and then with a plane and rasp. Next, the workpiece is marked using a planning template, which is secured with a small nail in the center of the future propeller, outlined with a pencil, after which the template is rotated 180 degrees and the planned projection of the second blade is marked. Excess wood is removed with a bow or fine-toothed band saw. The most important part of the work is giving the blades an aerodynamic profile. As can be seen from the drawing of the screw, one side is flat and the other is convex.
In accordance with the position of the control sections, the installation locations of the templates are marked on the workpiece, and “beacons” are made with a semicircular chisel and a semicircular rasp - in accordance with the configuration of the upper and lower templates. The main tool for processing propeller blades is a small ax made of good steel, literally sharpened to a razor sharpness. When removing wood, it is recommended to first make small cuts - this will avoid splitting the workpiece. This is followed by preliminary processing of the workpiece with a plane and rasp. This is followed by final finishing in the slipway. The latter is a carefully planed board with a thickness of at least 60 mm, on which transverse cuts are made to a depth of 20 mm to install the lower profile templates of the propeller blade.
The central rod of the slipway is machined from steel or duralumin; its diameter must correspond to the hole in the propeller hub. The rod is glued in the center of the slipway board strictly perpendicular to its surface. Next, the working surfaces of the lower templates are rubbed with colored pencil or blue, the propeller blank is put on the central rod and pressed against the templates - first with one blade, and then with the other. In this case, traces of the templates will be imprinted on the workpiece in those places where they come into contact with the lower surface of the propeller.
The “stained” areas are cleaned off using a plane, plow, rasp or a wooden block with sandpaper glued to it, the workpiece is again placed in the slipway - and the processing of the propeller blades is repeated. When traces of a colored pencil are imprinted across the entire width of the blade, the processing of its lower surface can be considered complete. The upper part of the screw is machined in a slipway using upper templates (they are also called counter templates). First, using a semicircular rasp, the blade is adjusted to the counter-templates (as professionals say, counter-templates are seated), as a result of which the template and the counter-template should come into contact along the parting plane, tightly enveloping the blade itself.
Then the treated areas are rubbed with a colored pencil and the areas between the control sections are processed. In this case, painting is necessary in order to prevent re-processing of the blade at the locations of the control sections. In this case, the correctness of processing is checked with a straight steel ruler applied to the one-percent points of adjacent sections. On a properly made blade there should be no gap between the ruler and the surface. If during the work an awkward movement of the tool leads to chipping of the wood, this does not mean at all that the work is irreparably damaged. You can fix it with putty mixed with epoxy glue and small sawdust.
The finished propeller is carefully balanced. The best way to do this is to tightly insert a metal roller into the central hole and install the propeller on the balancing rulers. If one of the blades turns out to be lighter, it is recommended to load it with lead, for which small strips of this metal are first glued onto it, and when the propeller is balanced, the strips are melted and poured into a mold, for example, into a piece of steel pipe. The resulting rod (or rods) is riveted into a hole drilled in the place of the blade where the lead strips were glued.
The hole on both sides of the blade should be slightly countersunk. Finishing the propeller consists of covering it with two layers of thin fiberglass, followed by sanding, final balancing, priming and painting with auto enamel. Frame homemade airboat consists of two large parts - upper and lower. It is better to start assembling it from the bottom. To do this, in accordance with the theoretical drawing of the hull and drawings, form-building frames are cut from plywood 12 mm thick, and stringers and keels are cut from slats with a cross-section of 20x20, 30x20 and 30x30 mm. The frame is assembled on a flat floor. The diametrical plane and the location of the frames are first marked on it. The frames are attached to the floor using wooden blocks and braces.
The adjustment of the longitudinal slats is done “in place”, the fastening of the slats to the frames is done with epoxy glue with temporary fixation of the elements with safety wire. Curvilinear slats for the front part of the frame are obtained by first steaming them in boiling water and fixing them with wire on the frame. After the slats have dried, they are fixed to the frames with epoxy glue. After thinning (leveling) the frame, the spacing is filled with blocks of construction foam, which are fixed using the same epoxy binder.
After treating the foam surface (if necessary, it is puttied with the already familiar composition of epoxy glue and sawdust), the body is covered with two layers of fiberglass, puttied, sanded and painted with auto enamels. From the inside, the foam is cut flush with the frames and also covered with fiberglass. Making the upper part of an airboat is not much different from the lower part. True, the frame is assembled not from plywood frames, but from prepared curved slats, and not on the floor, but on the already finished lower part of the body.
The frame on which the engine motor mount is attached has an increased cross-section and reinforcements at the junction of the slats - plywood gussets. The frame itself is attached to a crossbar made of a square steel pipe with a cross-section of 40x40 mm and fixed with braces made of pipes with a diameter of 22 mm. Shaping is also done using polystyrene foam followed by gluing with fiberglass. The door glazing is made of 4 mm thick plexiglass, the windshield is from the rear door of a Moskvich-2141 car. Part of the door itself became an element of the cabin.
Airboat doors consist of a wooden frame and plywood skin. They are covered with fiberglass inside and out. Door hinges are homemade, overhead. In the ceiling of the cabin (or, if you prefer, the deckhouse) there is a removable hatch cover made from a cut-out part of the roof. At the rear of the airboat, two keels are mounted, which organize the air flow and also serve as a guard for the propeller.
Controlled homemade airboat using a steering wheel, on the shaft of which a steering drum is attached, connected by cable wiring to a traverse on the steering wheel stock box. Gas control - a lever located under the driver’s left hand. The cabin contains seats for the passenger and driver. The seat and back frames are glued together from wooden slats and sheathed with 4 mm plywood. Pillows are made of foam rubber and artificial leather.
