The vertical tail of an aircraft. Vertical tail (VT). See what “airplane plumage” is in other dictionaries

The tail is the airfoils located at the rear of the aircraft. They look like relatively small “wings”, which are traditionally installed in horizontal and vertical planes and are called “stabilizers”.

It is according to this parameter that the tail unit is divided, first of all, into horizontal and vertical, respectively, with the planes in which it is installed. The classic scheme is one vertical and two horizontal stabilizers, which are directly connected to tail part fuselage. This is the scheme most widely used on civil airliners. However, there are other schemes - for example, T-shaped, which is used on the Tu-154.

In a similar scheme horizontal tail attached to the top of the vertical, and when viewed from the front or rear of the aircraft, it resembles the letter "T", from which it gets its name. There is also a scheme with two vertical stabilizers, which are placed at the ends of the horizontal tail; an example of an aircraft with this type of tail is the An-225. Also, most modern fighters have two vertical stabilizers, but they are installed on the fuselage, since they have a fuselage shape that is somewhat more “flattened” horizontally compared to civil and cargo aircraft.

Well, in general, there are dozens of different tail configurations and each has its own advantages and disadvantages, which will be discussed below. It is not always installed in the tail of the aircraft, but this only applies to horizontal stabilizers.

The tail of the Tu-154 aircraft

The tail of the An-225 aircraft

The principle of operation of the tail unit. Main functions.

And now about the functions of the tail, why is it necessary? Since it is also called stabilizers, we can assume that they stabilize something. That's right, that's true. The tail is necessary to stabilize and balance the aircraft in the air, and also to control the aircraft along two axes - yaw (left-right) and pitch (up-down).

Vertical tail unit.

The functions of the vertical tail are to stabilize the aircraft. In addition to the two axes listed above, there is also a third - roll (rotation around the longitudinal axis of the aircraft), and so, in the absence of a vertical stabilizer, the roll causes the aircraft to sway relative to the vertical axis, moreover, the sway is very serious and completely uncontrollable. The second function is yaw axis control.

A deflectable profile is attached to the trailing edge of the vertical stabilizer, which is controlled from the cockpit. These are the two main functions of the vertical tail unit; the number, position and shape of the vertical stabilizers are absolutely unimportant - they always perform these two functions.

Types of vertical tail units.

Horizontal tail unit.

Now about the horizontal tail unit. It also has two main functions, the first can be described as balancing. In order to understand what's what, you can conduct a simple experiment. It is necessary to take a long object, for example a ruler, and place it on one outstretched finger so that it does not fall or bend either back or forward, i.e. find its center of gravity. So, now the ruler (fuselage) has a wing (finger), it doesn’t seem difficult to balance it. Well, now you need to imagine that tons of fuel are being pumped into the train, hundreds of passengers are boarding, great amount cargo

Naturally, it is simply impossible to load all this perfectly relative to the center of gravity, but there is a way out. It is necessary to resort to using the finger of the second hand and place it on top of the conditionally rear part of the ruler, and then move the “front” finger to the back. The result is a relatively stable structure. You can also do it differently: place the “back” finger under the ruler and move the “front” finger forward, towards the bow. Both of these examples show the principle of operation of a horizontal tail.

The first type is more common, when horizontal stabilizers create a force opposite to the lifting force of the wings. Well, their second function is control along the pitch axis. Here everything is absolutely the same as with the vertical tail. There is a deflectable trailing edge profile, which is controlled from the cockpit and increases or decreases the force that the horizontal stabilizer creates due to its aerodynamic profile. Here a reservation should be made regarding the deflectable trailing edge, because some aircraft, especially combat aircraft, have completely deflectable planes, and not just parts of them, this also applies to the vertical tail, but the principle of operation and functions do not change.

Types of horizontal tail units.

And now about why designers are moving away from the classical scheme. Now there are a huge number of aircraft and their purpose, along with their characteristics, is very different. And, in fact, here it is necessary to analyze a specific class of aircraft and even a specific aircraft separately, but to understand the basic principles, a few examples will be enough.

The first - the already mentioned An-225, has a double vertical tail for the reason that it can carry such a bulky thing as the Buran shuttle, which in flight would aerodynamically obscure the only vertical stabilizer located in the center, and its effectiveness was would be extremely low. The T-shaped tail of the Tu-154 also has its advantages. Since it is located even behind the rear point of the fuselage, due to the sweep of the vertical stabilizer, the force arm there is the largest (here you can again resort to a ruler and two fingers of different hands; the closer the rear finger is to the front, the greater the force required on it), therefore it can be made smaller and not as powerful as with the classical scheme. However, now all loads directed along the pitch axis are transferred not to the fuselage, but to the vertical stabilizer, which is why it needs to be seriously strengthened, and therefore heavier.

In addition, you also have to additionally pull the pipelines of the hydraulic control system, which adds even more weight. And in general, this design is more complex, and therefore less safe. As for fighters, why they use fully deflectable planes and twin vertical stabilizers, the main reason is to increase efficiency. After all, it is clear that a fighter cannot have excess maneuverability.

Provides longitudinal stability, controllability and balancing. The horizontal tail consists of a fixed surface - a stabilizer and an elevator hinged to it. For tail-mounted aircraft, the horizontal empennage is installed at the rear of the aircraft - on the fuselage or on the top of the fin (T-shape).

In the canard design, the empennage is located at the nose of the aircraft in front of the wing. A combined scheme is possible, when an aircraft with a tail tail is equipped with an additional front tail - a scheme with a front horizontal tail (front horizontal tail), which allows you to take advantage of both of these schemes. The “tailless” and “flying wing” designs do not have horizontal tail surfaces.

A fixed stabilizer usually has a fixed installation angle relative to the longitudinal axis of the aircraft. Sometimes provision is made for adjusting this angle on the ground. Such a stabilizer is called adjustable.

On heavy aircraft, to increase the efficiency of longitudinal control, the angle of installation of the stabilizer with the help of an additional drive can be changed in flight, usually during takeoff and landing, as well as to balance the aircraft at a given flight mode. Such a stabilizer is called movable.

On supersonic speeds flight, the effectiveness of the elevator drops sharply. Therefore supersonic aircraft Instead of the classic GO scheme with an elevator, a controlled stabilizer is used, the installation angle of which is adjusted by the pilot using the longitudinal control command lever or the aircraft’s on-board computer. In this case there is no elevator.

Vertical tail (VO)

Provides the aircraft with directional stability, controllability and balancing relative to the vertical axis. It consists of a fixed surface - the keel and a rudder hinged to it.

All-moving VO is used very rarely. The efficiency of the air defense can be increased by installing a forkeel - a forward influx in the root part of the fin and an additional ventral ridge. Another way is to use several (usually no more than two identical) keels.

Plumage forms

The shapes of the tail surfaces are determined by the same parameters as the shapes of the wing: aspect ratio, taper, sweep angle, airfoil and its relative thickness. As in the case of the wing, trapezoidal, oval, swept and triangular tails are distinguished.

The plumage pattern is determined by the number of its surfaces and their relative position. The most common schemes are:

a scheme with a central location of the vertical tail in the plane of symmetry of the aircraft - the horizontal tail in this case can be located both on the fuselage and on the fin at any distance from the aircraft axis. (The layout with the GO located at the end of the keel is usually called a T-shaped tail.)
scheme with a spaced vertical tail - its two surfaces can be attached to the sides of the fuselage or at the ends of the horizontal tail. In a two-beam fuselage design, the VO surfaces are installed at the ends of the fuselage beams. On canard, tailless, and flying wing aircraft, a spaced air defense is installed at the ends of the wing or in its middle part,
V-shaped tail, consisting of two inclined surfaces that perform the functions of both horizontal and vertical tail. Due to the complexity of control and, as a consequence, low efficiency, such plumage is not widely used. (However, the use of computer flight systems has changed the situation for the better. Current control of the V-shaped tail in those equipped with it the latest aircraft takes over the on-board computer - the pilot just needs to set the flight direction (left-right, up-down) with the standard control stick, and the computer will do everything that is needed for this.)

The required tail efficiency is ensured the right choice the shapes and location of its surfaces, as well as the numerical values of the parameters of these surfaces. To avoid shadowing, the tail organs should not fall into the wake of the wing, nacelles and other aircraft components. The use of computer flight systems has no less influence on the efficiency of the tail. For example, before the advent of fairly advanced aircraft on-board computers, the V-shaped tail was almost never used, due to its complexity in control.

The later onset of the wave crisis on the tail is achieved by increased sweep angles and smaller relative thicknesses compared to the wing. Flutter and buffeting can be avoided by known measures to eliminate these aeroelastic phenomena.

Aircraft landing gear- a system of supports for an aircraft that ensures its parking and movement across an airfield or water during takeoff and landing. Usually it consists of several wheels, sometimes skis or floats are used. In some cases, tracks or floats combined with wheels are used.

Basic chassis layout diagrams (English)Russian:

With tail wheel. The main legs or support are located in front of the center of gravity, and the auxiliary (tail) is behind (Douglas DC-3).
With front wheel. The front (nose) wheel is located in front of the center of gravity, and the main supports are behind the center of gravity. The strut in the forward fuselage usually accounts for 10-15% of the mass. They became widespread during the Second World War and in the post-war years (for example, Boeing 747).
Bicycle type. The two main supports are located in the fuselage, in front and behind the center of gravity of the vehicle. Two side supports are attached to the sides (Boeing B-52 Stratofortress, Myasishchev 3M, Yakovlev Yak-25,27,28).

The main elements of the aircraft landing gear are:

shock-absorbing struts to soften the shock that occurs at the moment of landing.
wheels (pneumatics) equipped with brakes to reduce the length of the post-landing run
a system of braces (rods) that perceive ground reactions and attach shock-absorbing struts and wheels to the wing and fuselage

Most aircraft After takeoff, the landing gear is retracted into the fuselage or wing. For small aircraft, the landing gear, as a rule, is not retractable and has a design that allows the wheels to be replaced with skis or

The design of the main parts of the tail - the stabilizer and the fin - is usually similar. The elevators and rudders are also identical in design. On large airplanes Stabilizers are usually made detachable. The fin can be manufactured integrally with the fuselage or as a separate part. The tail structure of modern aircraft is usually made of metal. The sheathing of the keel and stabilizer is usually rigid (duralumin). Rudders of small aircraft subsonic speeds sheathed with canvas, which reduces their weight and simplifies the design. On high-speed aircraft, the rudders, like the frame, are metal.

Keel and stabilizer. On small aircraft, the fin and stabilizer are most often made of two-spars. On heavy aircraft, the fin and stabilizer are usually of a monoblock design with working skin (Fig. 59).

The main elements of the strength set (spars, walls, stringers, ribs) are structurally designed in the same way as those of the wing and perform the same functions, i.e., bending is perceived by the spar belts, stringers and partially by the skin; lateral force is perceived by the walls of the side members; torsion - closed loop; sheathing - walls of side members. The stabilizer and fin are attached to the fuselage using units on the spars and frames. For mounting (suspension) of the rudders, the stabilizer and keel have special brackets with universal and single-axis hinges. In Fig. Figure 60 shows a typical steering wheel suspension assembly.

Rudders and ailerons (roll rudders).

Rudders and ailerons, as a rule, are single-spar with a set of stringers and ribs.

To increase the rigidity of the front part of the steering wheel, a wall (auxiliary spar) is sometimes installed.

In modern aircraft construction, three characteristic types of rudders are used for aircraft with different flight speeds: a rudder with a tubular spar, a rudder with a rigid nose, and a rudder with a rigid skin for high-speed aircraft. In any type of rudder, a set of ribs collects the air load from the surface of the rudder and transfers it to the spar and torsion contour, as well as to the rigid trailing edge.

In the first design, the rib ribs transfer the entire load they collect only to the spar, and since it is tubular, it can successfully work in both bending and torsion.

In the second scheme, forces from the ribs are transferred to the wall of the beam spar, loading it with transverse bending, and the moment from the ribs is transferred to the contour formed by the wall of the spar with a rigid toe. This circuit works for torsion. In this scheme, the functions are distributed as follows: transverse bending is perceived by the beam spar, and torsion is perceived by the contour of the power toe.

In the third scheme (Fig. 61) there is a similar distribution of functions, but the torque is transmitted here to the entire contour of the skin, and not just to the toe.

In accordance with one or another force transmission scheme, power connections between the steering elements are made. For the rudders of the first scheme, the ribs are connected only to the spar with rivets along its circumference.

The rudders of the second and third schemes have ribs connected to the walls of the side members and the torsion contour. This connection is provided by rivets, bolts and sometimes glue.

In order to better use the skin to absorb the bending moment and maintain the profile shape, handlebars with foam or honeycomb filler are used. They have high rigidity with low weight.

Trimmers(Fig. 62) are an auxiliary steering surface mounted on the rear of the main steering wheel. With the help of trimmers, the aircraft is balanced relative to all its axes when the alignment and flight mode change. The trimmer deflection is carried out independently of the rudder deflection, usually with the help of special irreversible self-braking electric mechanisms, activated at the right time by the pilot with a two-way push switch. The elevator trim is usually controlled using a cable-type mechanical device. The essence of the trimmer's operation can be explained with the following example. When one of the aircraft's engines fails, a turning moment appears, counteraction to which can be created by deflecting the rudder. Flying an airplane for a long time with the rudder tilted is tiring for the pilot. By deflecting the trimmer in the direction opposite to the rudder deflection, the load transmitted to the pilot’s legs can be reduced to any small amount. The compensating moment from the trimmer, which counteracts the hinge moment, arises due to the large arm of the force applied to the trimmer, although the force itself is small. The magnitude of the hinge moment can be written in the following form.

Consists of horizontal and vertical tail.

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General information

Basic requirements for plumage:

ensuring high efficiency with minimal drag and minimal weight of the structure;
possible less shading of the empennage by other parts of the aircraft - the wing, fuselage, engine nacelles, as well as one part of the empennage of another;
absence of vibrations and oscillations such as flutter and buffeting;
later than on the wing, the development of a wave crisis.

Horizontal tail (HO)

Rudders and ailerons

Due to the complete identity of the design and power operation of the rudders and ailerons, in the future, for brevity, we will talk only about the rudders, although everything said will be fully applicable to the ailerons. The main power element of the steering wheel (and aileron, of course), which bends and absorbs almost all the shear force, is the spar, which rests on the hinged supports of the suspension units.

The main load on the rudders is aerodynamic, which occurs when balancing, maneuvering an aircraft or when flying in rough air. Taking this load, the steering spar acts as a continuous multi-support beam. The peculiarity of its operation is that the rudder supports are fixed on elastic structures, the deformation of which under load significantly affects the force work of the rudder spar.

The perception of steering torque is ensured by a closed contour of the skin, which is closed by the spar wall in the cutout areas for the mounting brackets. The maximum torque acts in the section of the control horn to which the control rod fits. The location of the hog (control rod) along the span of the steering wheel can significantly influence the deformation of the steering wheel during torsion.

Aerodynamic compensation of rudders

In flight, when the control surfaces are deflected, hinge moments arise, which are balanced by the efforts of the pilot on the command control levers. These forces depend on the size and angle of deflection of the steering wheel, as well as on the speed pressure. On modern aircraft, the control forces are too large, so it is necessary to provide special means in the design of the rudders to reduce the hinge moments and the control forces that balance them. For this purpose, aerodynamic compensation of the steering wheels is used, the essence of which is that part of the aerodynamic forces of the steering wheel create a moment relative to the axis of rotation, opposite to the main hinge moment.

The most common types of aerodynamic compensation are:

horn - at the end of the steering wheel, part of its area in the form of a “horn” is located in front of the hinge axis, which ensures the creation of a moment of the opposite sign in relation to the main hinge;
axial - part of the steering wheel area along its entire span is located in front of the hinge axis (the hinge axis moves backward), which reduces the hinge moment;
internal - usually used on ailerons and consists of plates attached to the nose of the aileron at the front, which are connected by a flexible partition to the walls of the chamber inside the wing. When the aileron deflects, a pressure difference is created in the chamber above and below the plates, which reduces the hinge moment.
servo compensation - a small surface is hinged in the tail part of the rudder, which is connected by a rod to a fixed point on the wing or tail. This rod ensures automatic deflection of the servo compensator in the direction opposite to the steering deflection. Aerodynamic forces on the servo compensator reduce the steering joint moment.

The angles of deflection and the operating efficiency of such a compensator are proportional to the angles of deflection of the steering wheel, which is not always justified, since control forces depend not only on the angles of deflection of the steering wheel, but also on the speed pressure. More advanced is the spring servo compensator, in which, due to the inclusion of springs with pre-tensioning in the control kinematics, the deflection angles are proportional to the steering control forces, which best suits the purpose of the servo compensator - to reduce these forces.

Means of aerodynamic balancing of an aircraft

Any steady state of aircraft flight, as a rule, is carried out with the rudders deflected, which ensures balancing - balancing- the aircraft relative to its center of mass. The resulting forces on the controls in the cockpit are usually called balancing. In order not to tire the pilot in vain and save him from these unnecessary efforts, a trimmer is installed on each control surface, allowing the balancing forces to be completely removed.

The trimmer is structurally completely identical to the servo compensator and is also hingedly suspended in the rear part of the steering wheel, but, unlike the servo compensator, it has additional manual or electromechanical control. The pilot, deflecting the trimmer in the direction opposite to the rudder deflection, achieves balancing of the rudder at a given deflection angle with zero effort on the command lever. In some cases, a combined trimmer-servo compensator surface is used, which, when the drive is turned on, works as a trimmer, and when turned off, it performs the functions of a servo compensator.

It should be added that the trimmer can only be used in control systems in which the forces on the command levers are directly related to the hinge moment of the steering wheel - mechanical boosterless control systems or systems with reversible boosters. In systems with irreversible boosters - hydraulic boosters - the natural forces on the control edges are very small, and to simulate “mechanical control” for the pilot, they are additionally created by spring loading mechanisms and do not depend on the hinge moment of the steering wheel. In this case, trimmers are not installed on the steering wheels, and the balancing forces are removed by special devices - trimming effect mechanisms installed in the control wiring.

Another means of balancing an aircraft in steady flight mode can be an adjustable stabilizer. Typically, such a stabilizer is pivotally mounted on the rear suspension units, and the front units are connected to a power drive, which, moving bow stabilizer up or down, changes its installation angles in flight. By selecting the desired installation angle, the pilot can balance the aircraft with zero hinge moment on the elevator. The same stabilizer also provides the required efficiency of longitudinal control of the aircraft during takeoff and landing.

Means for eliminating flutter of rudders and ailerons

The reason for the occurrence of flexural-aileron and flexural-steering flutter is their mass imbalance relative to the hinge axis. Typically, the center of mass of steering surfaces is located behind the axis of rotation. As a result, during flexural vibrations of the load-bearing surfaces, the inertia forces applied at the center of mass of the rudders, due to deformations and backlashes in the control wiring, deflect the rudders by a certain angle, which leads to the appearance of additional aerodynamic forces that increase the flexural deformations of the load-bearing surfaces. As the speed increases, the rocking forces increase and at a speed called the critical flutter speed, the structure collapses.

A radical means of eliminating this type of flutter is to install balancing weights in the nose of the rudders and ailerons in order to move their center of mass forward.

100% weight balancing of the steering wheels, in which the center of mass is located on the axis of rotation of the steering wheel, ensures complete elimination of the cause of the occurrence and development of flutter.

Selection and calculation

The tail organs in flight are subject to distributed aerodynamic forces, the magnitude and law of distribution of which are set by strength standards or determined by blowing. Due to their smallness, the mass inertial forces of the tail are usually neglected. Considering the work of the tail elements when they perceive external loads, by analogy with the wing, one should distinguish between the general force work of the tail units as beams, in the sections of which shear forces, bending and torques act, and the local work from the air load falling on each section of the skin with its reinforcements elements.

Various tail units differ from each other in purpose and methods of fastening, which introduces its own characteristics into power work and affects the choice of their structural power schemes. The required efficiency of the tail is ensured by the correct choice of the shape and location of its surfaces, as well as the numerical values of the parameters of these surfaces. To avoid shading, the tail organs should not fall into the wake of the wing, nacelles and other aircraft components. The use of computer flight systems has no less influence on the efficiency of the tail. For example, before the advent of sufficiently advanced aircraft

8.1. Rationale aerodynamic design airplane.

A modern aircraft is a complex technical system, the elements of which, each individually and collectively, must have maximum reliability. The aircraft as a whole must meet the specified requirements and be highly efficient at the appropriate technical level.

When developing projects for new generation aircraft that will enter service in the early 2000s, great importance is attached to achieving high technical and economic efficiency. These aircraft must not only have good performance at the time of entry into service, but also have the potential to be modified to systematically improve efficiency throughout the entire production period. This is necessary in order to ensure the implementation of new requirements and achievements of technological progress with minimal costs.

When considering the diagram passenger plane For local airlines, it is advisable to study all previously created aircraft in this class.

The development of passenger aviation began actively after the Second World War. Since then, the design of aircraft of this class, gradually undergoing changes, has come to the most optimal for today. In most cases, this is an aircraft made according to a normal aerodynamic configuration, a monoplane. Engines are usually located under the wing (TVD), under the wing on pylons or on the wing (TRD). The tail unit is made rather in a T-shaped pattern, sometimes in a normal one. The fuselage section consists of circular arcs. The landing gear is made according to the scheme with a nose wheel, the main struts are often multi-wheeled and multi-supported, retracting either into the elongated engine nacelles of turboprop engines (for aircraft weighing up to about 20 tons) or into fuselage bulges.

The typical fuselage layout is a cockpit in the nose, a long passenger cabin.

Deviation from this established layout scheme can only be caused by some special requirements for the aircraft. In other cases, when developing a passenger aircraft, designers try to adhere to this particular scheme, since it is practically optimal. Below is the rationale for using this scheme.

The use of a normal aerodynamic design for transport aircraft is primarily due to its advantages:

Good longitudinal and directional stability. Thanks to this property, the normal scheme greatly outperforms the “duck” and “tailless” schemes.

On the other hand, this scheme has sufficient controllability for a non-maneuverable aircraft. Due to the presence of these properties in the normal aerodynamic design, the aircraft is easy to control, which makes it possible for pilots of any qualification to operate it. However, the normal scheme has the following disadvantages:

Large balancing losses, which, other things being equal, greatly reduces the quality of the aircraft.

The useful mass output of the normal design is lower, since the mass of the structure is usually greater (if only because the “tailless” tail has no horizontal tail at all, while for the “duck” it creates a positive lift force, working like a wing and, therefore, unloading the wing, which makes it possible to reduce the area of the latter).

The influence of the bevel of the flow behind the wing on the horizontal tail, although not as critical as the influence of the anti-aircraft propulsion of the “duck”, nevertheless, this has to be taken into account, spreading the wing and horizontal tail in height. You should also take into account the fact that aircraft made according to the “canard” and “tailless” configurations require large angles of attack during takeoff and landing, which makes it structurally almost impossible to use swept wings of large and medium aspect ratio, since the use of such wings and large angles attack is due to the very high height of the chassis. Because of this, the canard and tailless designs use only low aspect ratio wings that have a triangular, gothic, ogival or crescent-shaped planform. Due to the low aspect ratio, such wings have low aerodynamic quality in subsonic flight conditions. These considerations determine the feasibility of using canard and tailless configurations on aircraft whose main flight mode is flight at supersonic speed.

Comparing all the advantages and disadvantages of the three aerodynamic designs, we come to the conclusion that it is advisable to use a classic aerodynamic design on a subsonic passenger aircraft.

8.2. The location of the wing relative to the fuselage.

For passenger aircraft, the choice of wing layout relative to the fuselage is primarily related to layout considerations. The need for free volumes inside the fuselage does not allow the use of a mid-wing design, since on the one hand it is impossible to pass the wing center section through the fuselage, and on the other hand, using a wing without a center section, with the consoles connected to the power ring frame, is unprofitable in terms of weight.

Unlike the mid-wing aircraft, the high-wing and low-wing designs do not interfere with the creation of a single cargo compartment. When choosing between them, preference is given to the high-wing design, since the designed aircraft will be used at airfields of different classes, including unpaved runways where there are no access ramps. It allows you to minimize the height of the floor above ground level, which greatly simplifies and facilitates the boarding of passengers and loading of luggage through the entrance door-stairway.

From an aerodynamic point of view, a high-wing aircraft is advantageous in that it allows one to obtain a distribution of circulation on the wing that is close to elliptical (with a conditionally identical wing planform) without a failure in the fuselage area, as in the low-wing and mid-wing designs. Moreover, the fact that the high-wing aircraft has interference resistance, although greater than that of the mid-wing, but less than that of the low-wing, makes it possible to obtain high quality aircraft built according to this design. With a low wing position, drag (at speeds from M<0,7) больше, чем при среднем и высоком расположении. Ниже приведены поляры для трёх схем расположения крыла на фюзеляже, из которых видно, что
(at
) in the low-wing aircraft is greater than in the mid-wing and high-wing aircraft (Fig. 8.2.1.).

The high-wing design has the following layout and design disadvantages:

The landing gear cannot be placed on the wing, or (on small aircraft) the main landing gear legs are bulky and heavy. In this case, the landing gear is usually placed on the fuselage, loading it with large concentrated forces.

During an emergency landing, the wing (especially if engines are installed on it) tends to crush the fuselage and the passenger cabin located in it. To eliminate this problem, it is necessary to strengthen the structure of the fuselage in the wing area and significantly make it heavier.

During an emergency landing on water, the fuselage goes under the surface of the water, thereby complicating the emergency evacuation of passengers and crew.

8.3. Plumage diagram.

For passenger aircraft, there are two competing tail designs: normal and T-shaped.

Powerful propeller wakes adversely affect the conventional low-mounted horizontal tail and can impair aircraft stability in some flight conditions. The high-mounted horizontal tail significantly increases the stability of the aircraft, since it extends beyond the zone of influence of the wake. At the same time, the efficiency of the keel also increases. A conventional keel of equivalent geometry would have an area 10% larger. Since the high-mounted horizontal tail has a larger horizontal arm due to the rearward canting of the keel, to create the necessary longitudinal moment requires a force on the handle that is half that of a conventional horizontal tail. In addition, the T-tail provides a higher level of passenger comfort as it reduces structural vibration caused by propeller wakes. The weight of the regular and T-shaped tails is approximately the same.

The use of a T-tail increases the cost of the aircraft by less than 5% due to increased development and production tooling costs. However, the advantages of this plumage justify its use.

Among other advantages of the T-shaped tail are:

The horizontal tail provides an "endplate" for the vertical tail, which increases the effective extension of the fin. This makes it possible to reduce the area of the vertical tail and thereby lighten the structure.

The horizontal tail is retracted from the area where its structure is exposed to sound waves, which can create a danger of fatigue failure. The service life of the horizontal tail increases.

8.4. Selecting the number of engines and their placement.

The required number of engines for an aircraft's power plant depends on a number of factors, determined both by the purpose of the aircraft and its basic parameters and flight characteristics.

The main criteria when choosing the number of engines on an aircraft are:

The aircraft must have the required launch thrust-to-weight ratio;

The aircraft must have sufficient reliability and efficiency;

The effective thrust of the power plant should be as high as possible;

The relative cost of engines should be as low as possible;

With a formal approach, it is possible to provide the required starting thrust-to-weight ratio of the designed aircraft with any number of engines (depending on the starting thrust of one engine). Therefore, when resolving this issue, it is also necessary to take into account the specific purpose of the aircraft and the requirements for its layout and power plant. Help in choosing the number of engines can be provided by studying aircraft of a similar class already used on airlines.

With the development of passenger aircraft for local airlines, designers eventually came to the optimal number of engines on aircraft of this class - two engines. The refusal to use one engine is explained by the fact that there are great difficulties with its layout, and also one engine does not satisfy flight safety. The use of three or more engines will unjustifiably make the design heavier and more complex, which will result in an increase in the cost of the aircraft as a whole and a decrease in its combat readiness.

When choosing a location for installing the engines, several options for their placement were considered. As a result of the analysis, the choice was made on the scheme for mounting the engines under the wing. The advantages of this scheme are:

The wing is unloaded in flight by engines, which makes it possible to reduce its weight by 10... 15%

With this design of the control system, the critical flutter speed increases - the engines act as anti-flutter balancers, shifting the CM of the wing sections forward.

It is possible to reliably isolate the wing from the engines using fire barriers.

Blowing the wing mechanization with a jet from the propellers increases its efficiency.

The disadvantages of the scheme include:

Large turning moments when one engine fails in flight.

- Engines located far from the ground are more difficult to maintain.

Today, two types of engines are used on non-maneuverable subsonic aircraft - theater engines and turbofan engines. Cruising speed is of decisive importance when choosing an engine type. It is advantageous to use theater engines at flight speeds corresponding to M = 0.45...0.7 (Fig. 8.4.2.). In this speed range, it is much more economical than a turbofan engine (specific fuel consumption is 1.5 times less). The use of a turboprop engine at speeds corresponding to M = 0.7...0.9 is unprofitable, since it has insufficient specific power and an increased level of noise and vibration on the aircraft.

Taking into account all the above facts, and based on the initial data for the designed aircraft, we make the choice for the control system in favor of the theater.

8.5. Results of the analysis.

The above analysis shows that for a short-haul passenger aircraft two main schemes are applicable (Fig. 8.5.1.).

Scheme 1: Low-wing aircraft with low-mounted main engine, engines in the wing, and landing gear located in engine nacelles.

Scheme 2: High-wing aircraft with a T-shaped tail, engines under the wing and landing gear located in nacelles on the fuselage.

From the point of view of operation, aerodynamics and economics, the second scheme is the most profitable for this type of aircraft (Table 8.5.1.).

Table 8.5.1.
Options	According to the location of the engines.	When the engine is located on the wing, the propeller blades are close to the ground surface, which does not allow operation on unpaved runways.
Options	The location of the engine under the wing ensures the required distance of the propeller blades relative to the ground.	To service the engine you have to climb onto the wing.
To service the engine, you must use a stepladder.	According to the location of the chassis.	Due to the high height, the main landing gear strut has a large mass.
The lower height of the main landing gear makes it possible to reduce its weight.	The high floor makes it difficult for passengers to board and disembark without the use of access ramps.	The low floor and gangway door make it easier for passengers to board and load hand luggage.
By type of plumage.	The overall dimensions of the tail make it difficult to place the aircraft in hangars, but the low-mounted GO is easier to maintain.	Due to the smaller dimensions of the VO, it does not cause problems with placement in hangars, but the T-shaped stabilizer is more difficult to maintain.

8.6. Statistics of previously created aircraft of this class.

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