Air propeller efficiency. Classification of propellers. Dependence of efficiency on altitude and flight speed

Recently, there has been a certain wandering, and sometimes outright misleading, regarding the choice of propellers on hobbit aerobatic models, which, with certain assumptions, can also include training models. The reason here, it seems, is that in traditional sports areas, guidelines have long been developed and theoretical justifications for the optimal choice of propellers have been carried out - in high-speed, racing, timer models. In order to come to the correct criteria without going too deep into the wilds of the classical theory of the screw, the following material is proposed for discussion.

At first sight theoretician everything is simple. You take the external and throttle characteristics of the motor and the family of aerodynamic characteristics of commercially available propellers, using the latter you build a family of graphs of the required power in the same coordinates as the external characteristics of the motor. Then, in the desired speed mode, you find the intersection of the graphs - that's how you got the optimal screw. Everything is more difficult in life. If, with due diligence, the external characteristics of the motor can still be taken on the stand, then the blowing characteristics of model propellers are unlikely. Model firms, even giants, don't give them either. The output suggests itself as follows: for basic parameters generally accepted or recommended by the motor manufacturer are accepted, and then they are successively approximated in the direction desired by the designer. To do this, one must at least qualitatively understand how certain design parameters affect the characteristics of the propeller. This will be discussed further.

Let's start with the main provisions of the propeller theory, taking from it only a few formulas:

propeller thrust

Power required to rotate the propeller

Relative propeller pitch

propeller thrust ratio

propeller power factor

Air density

propeller revolutions

Screw diameter

aircraft speed

We will not take more formulas, otherwise many will not be interested.

Analytically, you can’t count much here, because the main thing is how the propeller thrust and power coefficients behave, as well as their ratio, which determines the propeller efficiency. These parameters are established empirically by taking the characteristics of specific propellers by blowing in a wind tunnel. Therefore, we will consider their qualitative change depending on different parameters. Let's start with efficiency. For a typical screw, the graph looks like this:

Please note that the relative step is a dimensionless value and is equal to one at a flight speed of 1 m / s, a propeller speed of 60 rpm and its diameter of 1 meter. Now we need to explain why the graph looks like this. At zero tread, the efficiency is zero, because the propeller does not do any work - the plane stands still. With a stroke of 1.6, this propeller also does no work, because its pitch is such that the blades move at zero angle of attack (i.e., perpendicular to the flow) and do not form any thrust. For screws with a different pitch general form the graphic is the same, but it is proportionally compressed (with a smaller step) or stretched (with a larger step) along the axis. With a slip of 20-30% (for a given screw in the region = 1.1 - 1.4), the efficiency of the screw is maximum and can reach a value of 0.8. This is the most advantageous area in terms of engine power utilization. Interestingly, in this region, the efficiency changes insignificantly; as the speed decreases in this range, the thrust proportionally increases, which has a positive effect on the speed stability of the flight. When slipping is less than 15 - 20%, the efficiency begins to drop sharply, because the angle of attack of the blade decreases, respectively, the propeller blade falls and its thrust decreases. In the relative pitch range from 0 to 0.9, the efficiency of the propeller depends almost linearly on the speed, which indicates its almost constant thrust !!!. Those. Contrary to popular belief, the thrust of a properly selected propeller in flight can be determined quite accurately from static thrust with slight corrections. If you look more precisely at this part of the graph, then it is somewhat convex in the left half. This is because the thrust of the propeller decreases somewhat with decreasing speed due to an increase in the load on the propeller B (see the formula, where the speed is in the denominator, and even squared). A typical dependence when changing B from zero to 10 looks like this:

The drop in the thrust coefficient is associated with a change in the nature of the air flow in front of the propeller with a decrease in speed. But it is not the reason that is important to us, but the fact that a properly selected propeller in statics gives thrust that is less than thrust at maximum efficiency, by no more than 15%.

Now about what a properly selected screw is. Let's go back to the efficiency chart. If you apply a family of propeller graphs on it, differing only in pitch, then they will resemble the existing one, but compressed, or stretched along the axis, as mentioned above. True, the maximum efficiency with decreasing step also decreases. The maximum value of 0.8 occurs if the optimal slip of the screw falls on a relative step of about one. This is one of the criteria for a properly selected screw.

To assess where the typical values ​​​​are, let's take a 40-volume engine with a power of 1.3 hp. at 14,000 rpm and calculate a propeller of size 250 by 150 typical for this case. At an aerobatic speed of 90 km / h, we get 0.43. With such a step, the maximum efficiency will not exceed 0.6. To obtain such an efficiency, the propeller pitch with a slip of 20% should be about 9 centimeters, and to realize the available power with such a pitch, the propeller diameter must be increased to 27 - 30 centimeters. With the above step, the efficiency will be no higher than 0.5. Such a low efficiency is obtained due to too high engine speeds at maximum power.

Let's see how the F3A professionals look in the light of the above. The vast majority of them fly OS MAX 140 RX with a 16 by 14 inch propeller at speeds of 90 - 70 km/h at about 9000 rpm. The 14 inch propeller is optimal at 25% slip at about 180 km/h. At 90 km / h, its efficiency will be 0.65, and at 70 km / h - 0.5. A simple calculation shows that in the speed range of 50 - 100 km / h, the thrust of this propeller does not depend on the speed at all, but is determined only by the engine speed. This is probably what professionals like, because. with this propeller in the flight speed range, there is a one-to-one relationship between the position of the throttle stick and the engine thrust. The optimal 18 by 8 propeller will give a thrust greater than twenty percent at 90 km / h, but it will depend not only on the engine speed, but also on the speed of the aircraft. The pros are willing to sacrifice this additive for better traction control.

The worst situation is for timer models. There, the motor turns up to 30,000 rpm, and the aircraft's lifting speed is low. With a very small screw diameter, the load on the screw turns out to be terrible. In the context of the foregoing, the remark of E. Verbitsky, mentioned in the 5th issue of the Ministry of Education and Science for 1999, sounds very plausible. It says that, according to his calculations, "... conventional F1C propellers with a diameter of 180 mm at a speed of 28000 rpm have an efficiency of about 40%. By reducing the speed to 7000 using a gearbox while increasing the diameter of the propeller, you can increase the propeller efficiency to 80%". The same results were obtained from the author of this material.

Here at radio racing - it's just the opposite. There, the speeds are such that for almost any speed you can calculate a propeller with an efficiency close to 0.8. Above, little attention has been paid to the power factor. This is no coincidence. The fact is that this parameter is important when calculating the extreme regime. If the propeller is designed for maximum thrust at maximum power, then in partial modes, which were mainly discussed, there is confidence that the engine power will be enough. Moreover, regardless of the external characteristics of the motor, because the revolutions in the formula for the required power are in the third power. So quickly, power cannot drop with a decrease in speed, even in engines with resonant exhaust and high-speed valve timing. For aerobatic models, it is not extreme modes that are more important, but the entire range of speeds and propeller loads.

A few lines about the width of the blade. It is widely believed that by reducing the width of the propeller blade, one can slightly increase its efficiency. This is true, but for high-speed modes with a relatively small load on the propeller. For a propeller with a narrow blade, the characteristic goes more steeply. So much so that at a heavy load, the efficiency of a propeller with a wider blade is higher. At the same time, this occurs in the region of small absolute values ​​of the efficiency.

For low flight speeds with high-speed motors, it is not possible to reduce the pitch and increase the diameter of the propeller indefinitely. When the angle of attack of the blade is less than the most favorable polar profile for a given profile, the thrust of a single element decreases faster than the swept area of ​​the propeller increases. Those. for slow flight there is a minimum step, beyond which the optimization of the propeller installation is possible only through the gearbox.

Which of the above mentioned lengthy reasoning can be concluded?

The first- a properly selected propeller will provide the pilotage with an approximately constant maximum thrust in a wide range of flight speeds, starting from the start.

Second- existing model engines, due to the high-speed external characteristics, do not allow for slow aerobatics current trends F3A use screws with good efficiency. By the way, from this conclusion follows the opinion widely presented in the articles of the Moscow Union of Artists and Artists about the importance for flight and training models of the cubature of the engine, and not its power, in particular, by the authors A. Sokolov and D. Dmitriev.

Third- for modern 3D aerobatics and on fan-fly aircraft, the use of a geared motor with a sharply increased propeller diameter can be considered promising. Only this way will dramatically (twice) improve the thrust/weight ratio of the power plant. Then you can count on a large margin of thrust at helicopter speeds and hovering. Now they hang on Diamante with 310 by 95 mm screws. This is the limit, it is no longer effective to reduce the step below.

And the last - about the variable pitch propellers. On aerobatic models, their use is impractical. VISH, of course, will allow at low speeds to give an increase in traction due to a higher efficiency, but this increase is not needed there. In addition, this increase will be less than the theoretical one due to the aerodynamic twist of the blade. Unlike helicopter propellers, aircraft propellers have decent twist, optimal only at one step. In large aviation, the VISH has become widespread mainly to ensure high efficiency of the engine installation, which does not play a role for models.

P.S. The material contains formulas and graphs from the monographs of Aleksandrov V.L. "Air screws" and Bolonkina A.A. "The theory of flight of flying models". In the efficiency calculations, a grid of aerodynamic characteristics of the English propeller from the latest work was used.

Part of the engine rotational energy is spent on rotating the propeller and is aimed at overcoming air resistance, swirling the ejected jet, etc. Therefore, the useful second work, or the useful traction power of the propeller, nb, there will be less engine power N e spent on the rotation of the propeller.

The ratio of useful propulsive power to the power consumed by the propeller (effective engine power) is called the coefficient useful action(efficiency) of the propeller and is denoted h . It is determined by the formula

Rice. 11 Power characteristics of the M-14P engine of the Yak-52 and Yak-55 aircraft

Rice. 12 Approximate view of the curve of change in available power depending on airspeed

Rice. 13 Altitude characteristic of the M-14P engine in modes 1 - takeoff, 2 - nominal 1, 3 - nominal 2, 4 - cruising 1; 5 - cruising 2

The value of the efficiency of the propeller depends on the same factors as the propulsive power of the propeller.

The efficiency is always less than unity and reaches 0.8 ... 0.9 for the best propellers.

The graph of the dependence of the available effective power on the flight speed for the Yak-52 and Yak-55 aircraft is shown in Fig. eleven.

Graph Fig. 12 is called the characteristic of the power plant in terms of power.

At V=0, Np=0; at flight speed V=300 km/h, Np==275 hp (for the Yak-52 aircraft) and V=320 km/h, Np=275 l. With. (for the Yak-55 aircraft), where Np- required power.

With increasing altitude, the effective power decreases due to a decrease in air density. The characteristic of its change for the Yak-52 and Yak-55 aircraft from the flight altitude H is shown in Fig. 13.

To reduce the speed of rotation of the propeller in the engine, a gearbox is used.

The degree of reduction is selected in such a way that in the nominal mode the ends of the blades are flowed around by a subsonic air flow.

VARIABLE PITCH SCREWS

To eliminate the shortcomings of fixed-pitch and fixed-pitch propellers, a variable-pitch propeller (VSP) is used. Vetchinkin is the founder of the VIS theory.

REQUIREMENTS FOR VISH:

VISH should set the most favorable angles of attack of the blades in all flight modes;

remove the rated power from the engine over the entire operating range of speeds and altitudes;

to maintain the maximum value of the coefficient of efficiency over the largest possible range of speeds.

The blades of the VISH are either controlled by a special mechanism, or are set to the desired position under the influence of forces acting on the propeller. In the first case, these are hydraulic and electric propellers, in the second - aerodynamic ones.

hydraulic screw- a propeller, in which the change in the angle of installation of the blades is carried out by the pressure of the oil supplied to the mechanism located in the propeller hub.

electric screw- a propeller, in which the change in the angle of installation of the blades is made by an electric motor connected to the blades by a mechanical transmission.

Aeromechanical propeller- a propeller, in which the change in the angle of installation of the blades is carried out automatically - by aerodynamic and centrifugal forces.

The most widely used hydraulic VISH. An automatic device in variable-pitch propellers is designed to maintain a constant set speed of the propeller (engine) by synchronously changing the angle of inclination of the blades when changing the flight mode (speed, altitude) and is called a speed constancy controller (RPO).

Rice. 14 Operation of V530TA-D35 variable pitch propeller at different flight speeds

RPO, together with the mechanism for turning the blades, changes the pitch of the propeller (the angle of inclination of the blades) in such a way that the revolutions set by the pilot using the VIS control lever remain unchanged (given) when the flight mode changes.

At the same time, it should be remembered that the revolutions will be maintained until the effective power on the engine shaft Ne is greater than the power required to rotate the propeller when the blades are set to the smallest angle of inclination (small pitch).

On Fig. 14 shows a diagram of the operation of the VIS.

When changing the flight speed from takeoff to maximum in level flight, the angle of installation of the blades j increases from its minimum value j min up to maximum j max (big step). Due to this, the angles of attack of the blade change little and remain close to the most advantageous.

The work of the VIS during takeoff is characterized by the fact that the entire engine power is used during takeoff - the greatest thrust is developed. This is possible provided that the engine develops maximum speed, and each part of the propeller blade develops the greatest thrust, having the least resistance to rotation.

To do this, it is necessary that each element of the propeller blade work at angles of attack close to critical, but without stalling the air flow. On Fig. 14, a shows that the angle of attack of the blade before takeoff (V=0) due to the flow of air at a speed DV slightly different from the angle of inclination of the blade by the value fmin. The angle of attack of the blade corresponds to the magnitude of the maximum lifting force.

The resistance to rotation in this case reaches a value at which the power expended on the rotation of the screw and the effective power of the engine are compared and the revolutions will be unchanged. With an increase in speed, the angle of attack of the propeller blades decreases (Fig. 14, b). The resistance to rotation decreases and the propeller becomes lighter, as it were. The engine speed should increase, but the RPO keeps them constant by changing the angle of attack of the blades. As the flight speed increases, the blades turn to a greater angle. j cf .

When flying to top speed VISH should also provide the maximum value of thrust. When flying at maximum speed, the angle of inclination of the blades has limit value pmax (Fig. 14, c). Therefore, with a change in flight speed, the angle of attack of the blade changes, with a decrease in flight speed, the angle of attack increases - the propeller becomes heavier, with an increase in flight speed, the angle of attack decreases - the propeller becomes lighter. RPO automatically translates the propeller blades to the appropriate angles.

As the flight altitude increases, engine power decreases and the RPO reduces the angle of inclination of the blades to facilitate engine operation, and vice versa. Consequently, the RPO keeps the engine speed constant with a change in flight altitude.

During landing approach, the propeller is set to a small pitch, which corresponds to the takeoff speed. This makes it possible for the pilot, when performing various maneuvers on the landing glide path, to obtain takeoff power of the engine with an increase in speed to maximum.

The bladed propeller of an aircraft, also known as a propeller or bladed machine, which is driven into rotation by the operation of the engine. With the help of a screw, the torque from the engine is converted into thrust.

The propeller acts as a propeller in aircraft such as airplanes, cyclogyros, gyroplanes, snowmobiles, hovercraft, ekranoplans, as well as helicopters with turboprop and piston engines. For each of these machines, the screw can perform different functions. In airplanes, it is used as a main rotor, which creates thrust, and in helicopters, it provides lift and taxiing.

All aircraft propellers are divided into two main types: propellers with variable and fixed pitch. Depending on the design of the aircraft, propellers can provide either push or pull thrust.

When rotating, the propeller blades capture air and produce its rejection in the opposite direction of flight. A low pressure is created in front of the screw, and a high pressure zone behind. The thrown air acquires a radial and circumferential direction, due to this, part of the energy that is supplied to the propeller is lost. The very swirling of the air flow reduces the streamlining of the apparatus. Agricultural aircraft, when working on fields, have poor uniformity in the dispersion of chemicals due to the flow from the propeller. A similar problem is solved in devices that have a coaxial screw layout, in this case compensation occurs using the operation of the rear screw, which rotates in the opposite direction. Similar propellers are installed on aircraft such as the An-22, Tu-142 and Tu-95.

Technical parameters of propellers

The most significant characteristics of the propellers, on which the thrust force and the flight itself depend, of course, are the pitch of the propeller and its diameter. Pitch is the distance a propeller can travel by being screwed into the air in one complete revolution. Until the 30s of the last century, propellers with a constant rotation pitch were used. Only in the late 1930s, almost all aircraft were equipped with variable-pitch propellers.

Screw parameters:

The diameter of the propeller circle is the size that the tips of the blades describe when rotating.

The pitch of the screw is the actual distance traveled by the screw in one revolution. This characteristic depends on speed and rpm.

The geometric pitch of the propeller is the distance that the propeller could travel in a solid medium in one revolution. It differs from the tread of the propeller in the air by the sliding of the blades in the air.

The angle of location and installation of the propeller blades is the inclination of the blade section to the real plane of rotation. Due to the presence of twist of the blades, the angle of rotation is measured along the section, in most cases it is 2/3 of the entire length of the blade.

The propeller blades have a front - cutting - and a trailing edge. The cross section of the blades has a wing-type profile. In the profile of the blades there is a chord, which has a relative curvature and thickness. To increase the strength of the propeller blades, a chord is used, which has a thickening towards the propeller root. The section chords are in different planes, since the blade is made twisted.

The propeller pitch is the main characteristic of the propeller, it primarily depends on the angle of the blades. Pitch is measured in units of distance traveled per revolution. The more pitch the propeller makes in one revolution, the more volume is discarded by the blade. In turn, an increase in pitch leads to additional loads on the power plant, respectively, the number of revolutions decreases. Modern aircraft have the ability to change the inclination of the blades without stopping the engine.

Advantages and disadvantages of propellers

The efficiency of propellers on modern aircraft reaches 86%, which makes them in demand by the aircraft industry. It should also be noted that turboprops are much more economical than jet aircraft. Nevertheless, the screws have some limitations both in operation and in the constructive plan.

One of these limitations is the “locking effect”, which occurs when the diameter of the screw increases or when the number of revolutions is added, and the thrust, in turn, remains at the same level. This is due to the fact that sections with supersonic or transonic air flows appear on the propeller blades. This effect does not allow aircraft with screws to develop a speed higher than 700 km / h. On the this moment most fast car with propellers is the domestic model of the Tu-95 long-range bomber, which can reach speeds of 920 km / h.

Another disadvantage of screws is the high noise level, which is regulated by ICAO world standards. The noise from the screws does not fit into the noise standards.

Modern developments and the future of aircraft propellers

Technology and experience allow designers to overcome some of the noise problems and increase traction beyond the limitations.

Thus, it was possible to bypass the locking effect due to the use of a powerful turboprop engine of the NK-12 type, which transmits power to two coaxial propellers. Their rotation in different directions made it possible to bypass locking and increase traction.

Thin saber-shaped blades are also used on propellers, which have the ability to delay the crisis. This allows you to achieve higher speeds. This type of propellers is installed on the An-70 aircraft.

At the moment, developments are underway to create supersonic propellers. Despite the fact that the design is being carried out for a very long time with considerable cash injections, it has not been possible to achieve a positive result. They have a very complex and precise shape, which greatly complicates the calculations of designers. Some off-the-shelf propellers of the supersonic type have been shown to be very noisy.

Enclosing the propeller in a ring - an impeller - is a promising direction of development, since it reduces the end flow around the blades and the noise level. It also improved security. There are some aircraft with fans that have the same design as the impeller, but are additionally equipped with an airflow direction apparatus. This greatly improves the efficiency of the propeller and the engine.

In flight, an airplane overcomes air resistance all the time. This work is performed by its power plant, consisting either of a reciprocating internal combustion engine and a propeller, or of a jet engine. We will briefly talk about the propeller only.

Each of us has been familiar with the propeller since childhood.

In villages, children often install a two-bladed windmill on the gates, which rotates so fast in the wind that its blades merge into a continuous circle. The windmill is the simplest screw. If you put such a screw on the axis, twist it tightly between the palms and release it, then it will fly up with a buzz.

The aircraft propeller is mounted on the engine shaft. When the propeller rotates, the blades run into the air at a certain angle of attack and throw it back, due to which, as if starting from the air, they tend to move forward. Thus, during rotation, the propeller develops an aerodynamic force directed along the axis of the propeller. This force pulls the aircraft forward and is therefore called thrust.

The propeller can have two, three or four blades. The profile (section) of the blade is similar to the profile of the wing.

The pitch of the propeller and the angle of the blade to the plane of rotation play an important role in the work on creating thrust.

The pitch of a propeller is the distance that the propeller would have to travel in one full revolution if it were screwed into the air like a bolt into a nut. In reality, during the flight of an aircraft, the propeller, due to the low density of air, advances a slightly shorter distance.

The pitch of the propeller is the greater, the greater the angle of installation of the blade to the plane of rotation (Fig. 17, a).

Thus, a propeller with a large angle of blades "walks" faster than a prop with a small angle of installation (similar to how a bolt with a large thread is screwed into a nut faster than a bolt with a fine thread). Therefore, a large pitch propeller is needed for high flight speed, and a small pitch propeller is needed for low speed.

The operation of the propeller blades is similar to that of a wing. But propeller movement is more complicated. Unlike a wing, propeller blades in flight not only move forward, but also rotate at the same time. These movements add up, and therefore the propeller blades move in flight along a certain helical line (Fig. 17, b). Let's see how the thrust force of the propeller arises.

To do this, we select a small element on each blade, limited by two sections (Fig. 17, a). It can be considered as a small wing, which in flight moves along a helix, running into the air at a certain angle of attack. Consequently, the element of the blade, like the wing of an aircraft, will create an aerodynamic force P. We can decompose this force into two forces - parallel to the axis of the propeller and perpendicular to it. Strength,

Directed forward, and will be the thrust force of the blade element, the second, small force, directed against the rotation of the screw, will be the braking force.

The elementary thrust forces of both blades in total will give the thrust force T of the entire propeller, as if fitted to its axis. The braking forces are overcome by the engine.

The thrust force of the propeller is very much dependent on the flight speed. It decreases with increasing speed. Why does this happen and what does it mean for the flight?

When the plane is on the ground and power point works, then the propeller blades have only one speed - circumferential (Fig. 17, a). This means that air runs onto the blade in the direction of arrow B, shown in the plane of rotation of the screw. The angle between this arrow and the blade profile chord will obviously be the angle of attack. As you can see, when the air is still, it is equal to the angle of the blade to the plane of rotation. Otherwise it turns out in flight, when, except for rotary motion, the screw also moves forward (together with the aircraft).

In flight, these movements add up, and as a result, the blade moves along a helical line (Fig. 17, b). Therefore, air runs onto the blade in the direction of arrow B1, and the angle between it and the profile chord will be the angle of attack. You can see that the angle of attack has become smaller than the installation angle. And the greater the flight speed, the smaller the angles of attack of the blades, and therefore the smaller the thrust force will become (at a constant number of revolutions of the propeller).

This disadvantage is especially inherent in a simple propeller, in which the angle of the blades, and thus the pitch of the propeller, cannot be changed in flight (a simple propeller has other disadvantages). A variable pitch screw is much more perfect (Fig. 18). Such a screw, thanks to a special sleeve device, changes its pitch without the participation of the pilot. When the pilot reduces the airspeed, the pitch of the propeller immediately decreases; when the pilot increases the speed, the propeller increases the pitch.

G. V. Makhotkin

Propeller design

Air propeller has gained a reputation as an indispensable propulsion for high-speed watercraft operated in shallow and overgrown water areas, as well as for amphibious snowmobiles that have to work on snow, ice and water. A lot of experience has already been accumulated both here and abroad. propeller applications on high-speed small craft and amphibians. So, since 1964, in our country, amphibious snowmobiles have been mass-produced and operated (Fig. 1) of the Design Bureau named after. A. N. Tupolev. In the United States, several tens of thousands of airboats, as the Americans call them, are operated in Florida.

The problem of creating a high-speed shallow-draught motor boat with a propeller continues to interest our amateur shipbuilders. The most available power for them is 20-30 hp. With. Therefore, we will consider the main issues of designing an air propulsion system with the expectation of just such a power.

Careful determination of the geometric dimensions of the propeller will allow full use of engine power and obtain thrust close to maximum with the available power. In this case, the correct choice of the propeller diameter will be of particular importance, on which not only the efficiency of the propeller, but also the noise level, directly determined by the circumferential speeds, depends.

Studies of the dependence of thrust on stroke speed have established that in order to realize the capabilities of a propeller with a power of 25 hp. With. it is necessary to have its diameter - about 2 m. To ensure the lowest energy costs, the air must be thrown back by a jet with larger area sections; in our particular case, the area swept by the screw will be about 3 m². Reducing the propeller diameter to 1 m to reduce the noise level will reduce the area swept by the propeller by 4 times, and this, despite the increase in speed in the jet, will cause a drop in traction at mooring lines by 37%. Unfortunately, this decrease in thrust cannot be compensated for either by the pitch, or by the number of blades, or by their width.

With an increase in the speed of movement, the loss in traction from a decrease in diameter decreases; thus, increasing speeds allows the use of smaller diameter propellers. For propellers with a diameter of 1 and 2 m, which provide maximum traction on the moorings, at a speed of 90 km/h, the thrust values ​​become equal. Increasing the diameter to 2.5 m, increasing the traction at the moorings, gives only a slight increase in traction at speeds over 50 km/h. In the general case, each operating speed range (at a certain engine power) has its own optimal propeller diameter. With an increase in power at a constant speed, the diameter optimal in terms of efficiency increases.

As follows from Fig. 2 graphs, the thrust of a propeller with a diameter of 1 m is greater than the thrust of a water propeller (standard) of an outboard motor "Neptune-23" or "Privet-22" at speeds over 55 km / h, and a propeller with a diameter of 2 m - already at speeds over 30 -35 km/h. Calculations show that at a speed of 50 km/h, the kilometer fuel consumption of an engine with a propeller with a diameter of 2 m will be 20-25% less than the most economical outboard motor "Privet-22".

The sequence of selection of propeller elements according to the given graphs is as follows. The propeller diameter is determined depending on the required traction on the moorings at a given power on the propeller shaft. If the operation of the motorboat is expected in populated areas or areas where there are noise restrictions, the acceptable (today) noise level will correspond to the circumferential speed - 160-180 m / s. Having determined, based on this conditional norm and the diameter of the screw, the maximum number of its revolutions, we will establish the gear ratio from the motor shaft to the screw shaft.

For a diameter of 2 m, the permissible noise level will be about 1500 rpm (for a diameter of 1 m - about 3000 rpm); thus, the gear ratio at an engine speed of 4500 rpm will be about 3 (for a diameter of 1 m - about 1.5).

With the help of the graph in Fig. 3 you will be able to determine the amount of propeller thrust if the propeller diameter and engine power have already been selected. For our example, the engine of the most affordable power was chosen - 25 hp. s., and the diameter of the screw is 2 m. For this particular case, the thrust value is 110 kg.

The lack of reliable gearboxes is perhaps the biggest hurdle to overcome. As a rule, chain and belt drives made by amateurs in artisanal conditions turn out to be unreliable and have low efficiency. The forced installation directly on the motor shaft leads to the need to reduce the diameter and, consequently, reduce the efficiency of the mover.

To determine the blade width and pitch, use the nomogram shown in Fig. 4. On the horizontal right scale, from the point corresponding to the power on the propeller shaft, we draw a vertical until it intersects with the curve corresponding to the previously found propeller diameter. From the point of intersection we draw a horizontal straight line to the intersection with a vertical drawn from a point lying on the left scale of the number of revolutions. The resulting value determines the amount of coverage of the designed propeller (aircraft manufacturers call the ratio of the sum of the widths of the blades to the diameter the coverage).

For two-bladed propellers, the coating is equal to the ratio of the blade width to the propeller radius R. Above the coating values, the values ​​of the optimal pitches of the propeller are indicated. For our example, we obtained: coverage σ=0.165 and relative pitch (ratio of pitch to diameter) h=0.52. For a screw with a diameter of 1 m, σ=0.50 m and h=0.65. A propeller with a diameter of 2 m should be 2-bladed with a blade width of 16.5% R, since the amount of coverage is small; a propeller with a diameter of 1 m can be 6-bladed with a blade width of 50:3 = 16.6% R or 4-bladed with a blade width of 50:2 = 25% R. Increasing the number of blades will give an additional reduction in noise level.

With a sufficient degree of accuracy, we can assume that the pitch of the propeller does not depend on the number of blades. We give the geometric dimensions of a wooden blade with a width of 16.5% R. All dimensions in the drawing fig. 5 are given as a percentage of the radius. For example, section D is 16.4% R, located at 60% R. The section chord is divided into 10 equal parts, i.e., 1.64% R each; the toe is broken through 0.82% R. The ordinates of the profile in millimeters are determined by multiplying the radius by the percentage value corresponding to each ordinate, i.e. by 1.278; 1.690; 2.046 ... 0.548.