Horizontal tail of an aircraft. Airplane stabilizer

25.11.2019

The effectiveness of the tail depends largely on its location on the aircraft. It is desirable that, in all flight modes, the empennage does not fall into the flow zone inhibited by the wing, engine nacelles, fuselage or other parts of the aircraft. The relative position of its parts VO and GO also has a great influence on the efficiency of the plumage.

Behind the aircraft wing, a zone of retarded flow is formed, called a wake. The dimensions of this zone depend on the flight speed, the angle of attack of the wing and its parameters. The exact boundaries of the wake are determined based on aerodynamic sweeps. In a co-current jet, the velocities are significantly reduced, the flow angles reach large values, and the zone is saturated with vortices.

For these reasons, placing a horizontal tail in a wake would lead to a decrease in its efficiency (due to a decrease in flow velocity), deterioration in stability characteristics (due to large bevel angles) and the occurrence of vibrations during intense vortex formation. When choosing the position of the horizontal tail, it is necessary that in all flight modes it does not fall into the wake.

Fig.4 Fig.5

The horizontal tail is located either above (Fig. 4a) or below (Fig. 4b) the wake.

When choosing the position of the horizontal tail, it is also necessary to ensure that it is sufficiently removed from the jet stream of the engines.

The relative position of the horizontal and vertical tails should be such that in flight one part of the tail shades the other as little as possible. When an aircraft flies at high angles of attack or when gliding, a certain part of the vertical tail falls into the aerodynamic shadow of the horizontal tail. An aircraft whose vertical tail and especially the rudder are heavily shaded has poor spin characteristics (it is difficult to recover from a spin).

Shadowing of the vertical tail can be reduced by placing the horizontal tail either behind, ahead of, or on top of the vertical tail.

Each of these options has its own advantages and disadvantages.

If the horizontal tail arm is correctly selected, then when placing the vertical tail in front of the horizontal tail, it is necessary to increase the area of the vertical tail to ensure its required efficiency, and this will lead to an increase in its mass and drag and to an increase in the torque of the fuselage. When placing the vertical tail behind the horizontal one, it will be necessary to increase the length of the fuselage, which will cause an increase in the mass of the fuselage and its resistance. When placing horizontal

vertical tail, the mounting design becomes more complicated and the keel loads increase.

Currently, on heavy transport and passenger planes with engines mounted on pylons on the sides of the rear fuselage, the T-tail design became widespread.

In this case, the horizontal tail is removed from the engine jet. The advantages of such a scheme also include increasing the efficiency of the vertical tail (in this case, the horizontal tail plays the role of an end plate) and reducing the possibility of its shading. A major disadvantage of this scheme is the possibility of the aircraft entering the so-called “deep stall” mode.

If the permissible values of the angle of attack are exceeded (this can happen accidentally during a strong vertical gust) and a stall occurs on the wing, the wake may cover the entire horizontal tail and the effectiveness of the rudder will be insufficient.

To increase directional stability and the efficiency of the vertical tail at high gliding angles, forks and ventral ridges are installed on aircraft (Fig. 6).

Finally, the issue of placing the tail on the aircraft and the relative position of its individual parts is decided on the basis of the results of purging, and then flight tests.

8.1. Justification of the aerodynamic design of the aircraft.

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 given 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 (TRJ). The tail is made rather in a T-shape, 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 conventionally identical wing planform) without a failure in the fuselage area, as in the low-wing and mid-wing designs. Moreover, the fact that a high-wing aircraft has interference resistance, although greater than that of a mid-wing aircraft, but less than that of a low-wing aircraft, 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 make it significantly 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 diverted away 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.	The location of the engine under the wing ensures the required distance of the propeller blades relative to the ground.
According to the location of the engines.	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 allows you to reduce its weight.
According to the location of the floor.	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.

What do we know about the aircraft stabilizer? Most people will simply shrug their shoulders. Those who loved physics at school may be able to say a few words, but, of course, specialists will most likely be able to answer this question most fully. Meanwhile, this is a very important part, without which flight is virtually impossible.

Basic structure of the aircraft

If you ask several adults to draw an airliner, the pictures will be approximately the same and will differ only in details. The layout of the aircraft will most likely look like this: cockpit, wings, fuselage, interior and the so-called tail. Someone will draw portholes, and someone will forget about them, perhaps some other little things will be missed. Perhaps artists will not even be able to answer why certain details are needed; we simply don’t think about it, although we see airplanes quite often, both live and in pictures, in movies and just on TV. And this, in fact, is the fundamental design of the aircraft - the rest, in comparison, is just trifles. The fuselage and wings actually serve to lift the airliner into the air, the cockpit is used for control, and the cabin contains passengers or cargo. Well, what about the tail unit, what is it for? Not for beauty, right?

Tail

Those who drive a car know perfectly well how to go to the side: you just need to turn the steering wheel, following which the wheels will move. But an airplane is a completely different matter, because there are no roads in the air, and some other mechanisms are needed to control it. This is where pure science comes in: a flying car is subject to a large number of different forces, and those that are useful are amplified, while others are minimized, resulting in a certain balance being achieved.

Probably, almost everyone who has seen an airliner in their life has paid attention to the complex structure in its tail section - the tail. It is this relatively small part, oddly enough, that controls this entire gigantic machine, forcing it not only to turn, but also to gain or lose altitude. It consists of two parts: vertical and horizontal, which, in turn, are also divided into two. There are also two steering wheels: one serves to set the direction of movement, and the other - the height. In addition, there is a part with which the longitudinal stability of the airliner is achieved.

By the way, the stabilizer of an aircraft can be located not only in its rear part. But more on this a little later.

Stabilizer

The modern aircraft design includes many parts necessary to maintain the safe condition of the airliner and its passengers at all stages of the flight. And, perhaps, the main one is the stabilizer located at the rear of the structure. It is, in fact, just a bar, so it is surprising how such a relatively small part can in any way affect the movement of a huge airliner. But it is really very important - when this part breaks down, the flight can end very tragically. For example, according to the official version, it was the plane's stabilizer that caused the recent crash of a Boeing passenger plane in Rostov-on-Don. According to international experts, a discrepancy in the actions of the pilots and an error by one of them triggered one of the parts of the tail, moving the stabilizer into a position characteristic of a dive. The crew was simply unable to do anything to prevent a collision. Fortunately, aircraft manufacturing does not stand still, and each subsequent flight provides less and less room for the human factor.

Functions

As the name suggests, an aircraft's stabilizer serves to control its movement. By compensating for and dampening some peaks and vibrations, it makes for a smoother and safer flight. Since deviations occur in both the vertical and horizontal axes, the stabilizer is also controlled in two directions - that’s why it consists of two parts. They can have a very different design, depending on the type and purpose of the aircraft, but in any case they are present on any modern aircraft.

Horizontal part

It is responsible for vertical balancing, preventing the car from “nodding off” every now and then, and consists of two main parts. The first of them is a fixed surface, which, in fact, is the aircraft’s altitude stabilizer. A second part is attached to this part on a hinge - the steering wheel, which provides control.

In a normal aerodynamic design, the horizontal stabilizer is located in the tail. However, there are also designs when it is located in front of the wing or there are two of them - in the front and behind. There are also so-called “tailless” or “flying wing” designs that have no horizontal tail at all.

Vertical part

This part provides the aircraft with directional stability in flight, preventing it from wobbling from side to side. This is also a composite structure, which includes a fixed vertical stabilizer of the aircraft, or fin, as well as a rudder on a hinge.

This part, like the wing, depending on the purpose and required characteristics, can have a very different shape. Variety is also achieved through differences in the relative position of all surfaces and the addition of additional parts, such as the foril or ventral ridge.

Shape and mobility

Perhaps the most popular in civil aviation now is the T-shaped tail, in which the horizontal part is located at the end of the fin. However, there are also some others.

For some time, a V-shaped tail was used, in which both parts simultaneously performed the functions of both the horizontal and vertical parts. Difficult management and relatively little effectiveness have prevented this variant from becoming widespread.

In addition, there are spaced vertical tails, in which parts of them can be located on the sides of the fuselage and even on the wings.

As for mobility, usually the stabilizing surfaces are rigidly fixed relative to the body. However, there are variations, especially when it comes to horizontal tails.

If you can change the angle relative to the longitudinal axis on the ground, this type of stabilizer is called adjustable. If the aircraft stabilizer can be controlled in the air, it will be movable. This is typical for heavy airliners that require additional balancing. Finally, supersonic aircraft use a movable aircraft stabilizer, which also serves as an elevator.

Load-bearing surfaces designed to ensure stability, controllability and balancing of the aircraft are called tail surfaces.

Providing longitudinal balancing, stability and controllability of a conventional aircraft is carried out by the horizontal tail; track balancing, stability and controllability - vertical; balancing and control of the aircraft relative to the longitudinal axis is carried out using ailerons or roll rudders, which represent a certain portion of the tail section of the wing. The tail usually consists of fixed surfaces, which serve to ensure equilibrium (balancing) and stability, and movable surfaces, the deflection of which creates aerodynamic moments that provide equilibrium (balancing) and flight control. The fixed part of the horizontal tail is called the stabilizer, and the vertical tail is called the keel.

The elevator, usually consisting of two halves, is hinged to the stabilizer, and the rudder is attached to the keel (Fig. 57).

In Fig. Figure 57 shows the principle of operation of the tail when the steering wheel is deflected. The tail (in the case under consideration, horizontal) is flown around by an air flow at a certain angle of attack α g.o, which is not equal to zero.

Therefore, an aerodynamic force R g.o arises on the tail, which, due to the large shoulder relative to the center of gravity of the aircraft, creates a moment that balances the total moment from the wing, engine thrust, and fuselage. Thus, the moment of the tail balances the aircraft. By deflecting the rudder in one direction or another, you can change not only the magnitude, but also the direction of the moment and thus cause the aircraft to rotate relative to the transverse axis, i.e. control the aircraft. The moment relative to the axis of rotation of the steering wheel, arising from the action of the aerodynamic force R p on it, is usually called the hinge moment and is denoted M w = R p a.

The magnitude of the hinge moment depends on the flight speed (Mach number), angles of attack and sideslip, rudder deflection angle, location of the suspension hinges and rudder dimensions. When deflecting the control levers, the pilot must apply a certain force to overcome the hinge moment.

Maintaining the effort required to deflect the rudder acceptable to the pilot is achieved by using aerodynamic compensation, which will be discussed below.

The effectiveness of the rudders can be assessed by the change in the values of the longitudinal moment, roll and yaw moments when the corresponding rudder is deflected by one degree. At low flight speeds, the effectiveness of the rudders depends little on the flight speed (Mach number). However, at high flight speeds, air compressibility, as well as elastic deformations of the structure, noticeably reduce the effectiveness of the rudders. The decrease in the efficiency of the rudder at high transonic speeds is mainly due to the elastic twist of the stabilizer, fin, and wing, which reduces the overall increase in the lifting force of the profile from rudder deflection (see Fig. 57).

The degree of elastic twist of the profile when the steering wheel is deflected depends on the magnitude of the aerodynamic moment acting on the profile (relative to the center of rigidity of the profile), as well as on the rigidity of the structure itself.

The small relative thickness of the tail of high-speed aircraft, which means low rigidity, can cause reverse control phenomena.

The decrease in the efficiency of rudders when flowing around them at supersonic speeds is caused by other reasons. In supersonic flow, the additional lift force when the rudder is deflected occurs only on the rudder; the fixed part of the tail (fin, stabilizer) does not take part in creating the additional aerodynamic force. Therefore, in order to obtain a sufficient degree of controllability, a greater deflection of the steering wheel or an increase in the area of the deflected surface is necessary. For this purpose, a movable, controlled stabilizer is installed on supersonic aircraft, which does not have an elevator. The same applies to the vertical tail. On supersonic aircraft it is possible to use a rotating fin without a rudder.

Changing the direction of flight is achieved by turning the stabilizer and fin. The deflection angles of the stabilizer and fin are significantly less than the deflection angles of the corresponding rudders. The deflection of steering-less surfaces is carried out using irreversible self-braking hydraulic or electrical power devices. The rudderless tail provides effective control and balancing of the aircraft over a wide range of speeds, from low subsonic to high supersonic, as well as over a wide range of alignments.

Ailerons (roll rudders) are located at the end of the wing (Fig. 58). The principle of operation of ailerons is to redistribute the aerodynamic load along the wing span. If, for example, the left aileron deflects down and the right aileron deflects up, then the lift of the left half of the wing will increase, and the right will decrease. As a result, a moment appears that tilts the plane. It is difficult to ensure sufficient efficiency of roll rudders in supersonic aircraft. The small thickness of the wing and especially its end sections lead to the fact that when the ailerons are deflected, the wing twists in the direction opposite to the ailerons deflection. This dramatically reduces their effectiveness. Increasing the rigidity of the wing tip sections leads to an increase in the weight of the structure, which is undesirable.

Recently, aircraft with so-called internal ailerons have appeared (Fig. 58, b). If conventional (Fig. 58, a) ailerons are installed along the end of the wing, then the internal ailerons are located closer to the fuselage. With the same aileron area, due to a decrease in the arm relative to the longitudinal axis of the aircraft, the efficiency of the internal ailerons when flying at low speeds is reduced. However, at high flight speeds, the inboard ailerons are more effective. Simultaneous installation of external and internal ailerons is possible. In this case, when flying at low speeds, external ailerons are used, and at high speeds, internal ailerons are used. The inner ailerons can be used as flaps during takeoff and landing.

Ailerons, occupying a relatively large share of the wing span, create difficulties in placing the wing mechanization along the entire span, as a result of which the effectiveness of the latter is reduced. The desire to increase the efficiency of mechanization tools led to the creation of interceptors. The spoiler is a small flat or slightly curved plate located along the wing span, which is hidden in the wing during flight. When in use, the spoiler extends upward from the left or right half of the wing, approximately normal to the wing surface, and, causing disruption of the air flow, leads to a change in lift and roll of the aircraft. Typically, the spoiler works in conjunction with the aileron and extends on the part of the wing at which the aileron deflects upward.

Thus, the effect of the spoiler is summed up with the effect of the aileron. The use of spoilers makes it possible to reduce the length of the aileron and thereby increase the span of the flaps, therefore increasing the efficiency of wing mechanization.

On some aircraft, spoilers are used as brake flaps and in this case are simultaneously deflected upward on both parts of the wing only after the aircraft has landed or during an aborted takeoff. On other aircraft, the spoilers extend for braking some part of the full travel, and the remainder of the travel can be used for lateral control. The height of the fully extended spoiler is 5-10% of the wing chord, and the length is 10-35% of the half-span. To maintain greater smoothness of the flow around the wing and reduce stall resistance, the spoilers are sometimes made not continuous along the span, but comb-shaped. The efficiency of such breakers is somewhat less than that of solid ones, but due to the weakening of stall phenomena, the accompanying shaking of the wing and tail is reduced.

Literature used: "Fundamentals of Aviation" authors: G.A. Nikitin, E.A. Bakanov

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