The basis of air movement on the principles of aerodynamics is the presence of a force that counteracts the air resistance in flight and gravity. On all modern aircraft, with the exception of gliders, there is an engine whose power is converted to this force. The mechanism that converts the rotation of the shaft of the power plant into traction is the aircraft propeller.
Propeller Description
Aircraft propeller is a mechanical device with blades, rotated by the engine shaft and creating traction for the movement of the aircraft in the air. Due to the inclination of the blades, the screw throws air back, creating a region of reduced pressure in front of itself and increased pressure behind it. Almost all people on earth at least once in their life had the opportunity to see this device, so numerous scientifically-based definitions are not required. The screw consists of blades, a sleeve connected to the engine through a special flange, balancing weights placed on the sleeve, a mechanism for changing the pitch of the screw and a cowl covering the sleeve.
Other names
What else is called an airplane propeller? Historically, there are two main names: the propeller itself and the propeller. However, in the future, other names appeared that emphasized either the design features or additional functions assigned to this unit. In particular:
- Fenestron. A screw inserted into a special channel in the tail of the helicopter.
- Impeller. Screw enclosed in a special ring.
- Rotor fan. These are arrow-shaped, or saber-shaped screws in two rows with a reduced diameter.
- Wind blower. Emergency backup system for electricity from the incoming air flow.
- Rotor. This is sometimes called the rotor of the helicopter and some others.
Screw theory
At its core, any airplane propeller is a kind of mobile wings in miniature, living according to the same laws of aerodynamics as the wing. That is, moving in the atmospheric environment, the blades, due to their profile and inclination, create a stream of air, which is the driving force of the aircraft. The strength of this flow, in addition to a specific profile, depends on the diameter and frequency of rotation of the screw. At the same time, the thrust dependence on revolutions is quadratic, and on the diameter, even to the 4th degree. The general thrust formula is as follows: P = α * ρ * n 2 * D 4 , where:
- α - propeller thrust coefficient (depends on the design and profile of the blades);
- ρ is the air density;
- n is the number of revolutions of the screw;
- D is the diameter of the screw.
It is interesting to compare with the above formula, another, derived from the same theory of the screw. This is the required power to ensure rotation: T = Β * ρ * n 3 * D 5 , where Β is the calculated screw power factor.
A comparison of these two formulas shows that, by increasing the speed of the aircraft propeller and increasing the diameter of the propeller, the required engine power grows exponentially. If the thrust level is proportional to the square of revolutions and the 4th degree of diameter, then the required engine power is already growing in proportion to the cube of revolutions and the 5th degree of the diameter of the screw. With increasing engine power, its weight also increases, which requires even more traction. Another vicious circle in the aircraft industry.
Propeller Specifications
Any propeller mounted on an airplane has a set of characteristics as follows:
- Screw diameter.
- Geometric stroke (step). This term refers to the distance that a screw would travel by crashing into a theoretical solid surface in one revolution.
- Tread - the actual distance traveled by a screw in one revolution. Obviously, this value depends on the speed and speed.
- The blade angle is the angle between the plane and the actual inclination of the screw.
- The shape of the blades - most modern blades have a saber-like, curved shape.
- Profile of the blades - the section of each blade has, as a rule, a wing shape.
- The middle chord of the blade is the geometric distance between the leading and trailing edges.
In this case, the main characteristic of the aircraft propeller remains its thrust, that is, for the sake of which it is generally needed.
Advantages
Aircraft using a propeller as a propulsion device are much more economical than their turbojet “brothers”. The efficiency reaches 86%, which is an unattainable value for jet aircraft. This is their main advantage, which actually put them back into operation during the oil crisis of the 70s of the last century. At short flight distances, speed is not decisive in comparison with profitability, so most regional aircraft are screw.
disadvantages
Disadvantages of an airplane with a propeller are also available. First of all, these are purely “kinetic” cons. During rotation, the aircraft propeller, having its own mass, affects the aircraft body. If the blades, for example, rotate clockwise, then the body tends to rotate, respectively, counterclockwise. The turbulence created by the propeller actively interacts with the wings and plumage of the aircraft, creating various flows on the right and left, thereby destabilizing the flight path.
And finally, the rotating propeller is a kind of gyroscope, that is, it seeks to maintain its position, which complicates the process of changing the flight path for an aircraft. These shortcomings of the aircraft propeller were known for a long time, and designers learned to deal with them by introducing a certain asymmetry in the design of the ships themselves or their control surfaces (rudders, spoilers, etc.). In fairness, it should be noted that jet engines have similar "kinetic" shortcomings, but to a slightly lesser extent.
The minuses include the so-called locking effect, when an increase in the diameter and frequency of rotation of the aircraft propeller to certain limits ceases to produce an effect in the form of an increase in thrust. This effect is associated with the appearance in some parts of the blades of air flows at near- or supersonic speeds, which creates a wave crisis, that is, the formation of shock waves in the air. In fact, they overcome the sound line. In this regard, the maximum speed of aircraft with a propeller does not exceed 650-700 km / h.
Perhaps the only exception was the Tu-95 bomber, developing a speed of up to 950 km / h, that is, almost sound speed. Each of its engines is equipped with two coaxial screws rotating in opposite directions. Well, the last problem with propeller airplanes is their noise, the requirements for which by the aviation authorities are constantly being tightened.
Classification
There are many classification options for aircraft propellers. They are divided into groups depending on the material from which they are made, on the shape of the blades, their diameter, quantity, as well as a number of other characteristics. However, the most important is their classification according to two criteria:
- The first - the screws come with variable pitch and fixed pitch.
- The second - the screws are pulling and pushing.
The first is installed in the front of the aircraft, and the second, respectively, in its rear. A propeller-driven plane arose earlier, but then was forgotten for a while and only relatively recently reappeared in the sky. Now this arrangement is widely used on small aircraft. There are even completely exotic options, equipped with both pulling and pushing blades at the same time. Aircraft with a propeller at the rear has several advantages, the main one of which is its higher aerodynamic quality. However, due to the lack of additional airflow from the wing with the air flow from the propeller, it has the worst takeoff and landing characteristics.
Variable pitch screws
Almost all modern medium and large aircraft are equipped with variable pitch propellers. With a large pitch of the blades, a large thrust is achieved, but if the engine speed is quite low, speed will be extremely slow. This is very similar to the situation with the car, when in higher gears try to move off.
High speed and a small pitch of the screw create a risk of flow disruption and traction falling to zero. Therefore, during the flight step is constantly changing. Now this is done by automation, and before the pilot himself had to constantly monitor this and manually adjust the angle of inclination. The mechanism for changing the pitch of the screw is a special sleeve with a drive mechanism, turning the blades relative to the axis of rotation by the desired degree.
Modern development in Russia
Work on improving devices has never stopped. Currently, tests are underway for the new aircraft propeller AB-112. It will be used on the Il-112V light military transport aircraft. This is a 6-blade propeller, with a 87% efficiency, 3.9 meters in diameter and a rotation speed of 1200 rpm and a variable pitch propeller. A new blade profile has been developed and its design is facilitated.