Even a person who has not seen the sea for some time probably knows the farewell word: “Seven feet under the keel”. And there are no questions. The keel of the ship is the most important structural part on which many details of its hull are mounted. But does anyone know where the keel of the plane is located and what it is for?
What it is?
This is the "organ" of stability, which allows the
aircraft to maintain a given course. Unlike ships, the keel of the aircraft is an integral part of the tail tail. At the bottom of the fuselage, aircraft have no keel! But there is one subtlety. The fact is that this part is tightly connected to the power elements of the fuselage, but because there is still something in common in the sea and air terms. So where is the keel of the plane? Simply put, this is the vertical part of the tail.
It is placed motionless, fixed at three points symmetrical to the centerline of the aircraft. In appearance, this part has the shape of an ideal trapezoid. As a rule, the keel of the aircraft consists of spars, ribs and skin. This is a classic scheme that has not changed much since the appearance of the first aircraft. The front side member is placed obliquely (as a rule).
Layouts
Most often, the keel is single, but in some cases it is made double or even triple (on helicopter bombers). In the latter case, this is required to ensure high directional stability of a heavy machine. By the way, all aircraft at the keel location are divided into three types:
- Built according to the normal scheme. Such, for example, is the keel of the A321.
- "Ducks", that is, aircraft in which the horizontal tail of the keel is located in front of the wings.
- Tailless. From the keel, only vertical plumage remains, horizontal ailerons are completely absent.
Of course, the last two varieties are more characteristic of the “community” of military aircraft, since such a keel placement is necessary to give the aircraft particularly high maneuverability.
In some cases, even more complex designs are used. For example, nodule ridges (they are also the fuselage keels). They are used on some supersonic aircraft, where maintaining perfect stability during flight is vital. Thus, under the keel of the plane (this is where, we already found out) there is an additional and massive influx. More common is the situation when the horizontal tail plumage in general has to be transferred to the very top of the keel. This happens if the engines are installed in the aft of the aircraft. A similar scheme, for example, can be seen on domestic cargo and passenger aircraft Il.
What is it for?
As you know, calm weather is an incredible rarity that does not happen more often than a couple of times a year. In most cases, there is wind, and its strength and direction can radically differ. When the plane flies, gusts of wind can greatly affect direction and course. The aircraft must be designed to return to a stable position on its own. Only in this case a safe flight is possible.
Main purpose
The main rule for constructing the keel is to place it so that it does not, under any circumstances, fall into a satellite stream from the wing. Otherwise, a sharp violation of directional stability is possible, and in the most difficult situations - physical deformation and destruction of the entire tail. So, the main purpose of the keel is to maintain track stability.
The design of many aircraft is such that this part is movable. By adjusting the magnitude of the deflection of the keel, the crew controls the course direction. An exception is military aircraft, in which engines with a controlled thrust vector are responsible for changing the direction of flight. In their case, making a mobile keel of an airplane (there is a photo of it in the article) is stupid, since overloads during maneuvering are such that it simply collapses.
What types of stability are provided by the keel?
There are three types of stability, for the sake of which the keel is included in the aircraft structure:
- Track.
- Longitudinal.
- Transverse.
We will deal with all these varieties in more detail. So, road stability. It should be remembered that in case of loss of longitudinal stability of the fuselage in flight, the aircraft will still continue to fly forward for some time due to inertial force. After this, the air flow begins to run onto the rear of the aircraft, which lies behind the center of gravity. The keel in this case prevents the occurrence of a rotational force, forcing the aircraft to rotate around its axis.
Longitudinal stability. Suppose a plane flies in normal mode, the center of gravity coincides with the center of application of pressure to its fuselage. At this moment, multidirectional forces are also acting on its fuselage, which tend to deploy the body of the aircraft. Lift and gravity act simultaneously. The keel of the aircraft (you will see a photo of this detail in the article) provides balance, which in this particular case is very unstable. Normal flight without tail, keel and stabilizers is impossible.
Other types of sustainability
Lateral stability. In general, this factor is a logical continuation of the previous property. When multidirectional forces act on the wing and transverse stabilizers of the keel, they "try" to overturn the plane. The shape of the wings counteracts this: if you look at them from a distance, they resemble the letter “U” with the highly divided upper “horns”. This form provides an independent correction of the position of the aircraft in space. The keel helps maintain lateral stability.
Note that for aircraft with a reverse sweep of the wing, the need for a keel is not so great ... at high speeds. If it falls, then the increase in reaction forces occurs exponentially. Therefore, for these machines, the most durable and light keel, which can withstand such high loads, is very important. And how can I get it? We will tell about this.
Features of the creation of modern aircraft
Currently, the specialists of the Federal Air Transport Agency and their foreign colleagues are focusing on the creation of aircraft parts (including keel) from large-sized parts made of the latest composite materials.
The share of these compounds in the design of modern aircraft is steadily growing. According to information from experts, their volume fraction already reaches from 25% to 50%, and small non-profit aircraft can even consist of plastic and composites by 75%. Why is this approach so widespread in aviation? The fact is that the same keel of a Boeing aircraft made of polymer "alloys" has a very low weight, very high strength and a resource that, using standard materials, is simply unrealistic to achieve.
Main materials
The most justified is the use of composites in the design of not only the tail, but also the wings and power elements of the fuselage, which should be not only very durable, but also quite flexible. Otherwise, the probability of structural failure under the influence of flight loads is not excluded.
But it was not always so. So, the pride of the Soviet aircraft industry, the Tu-160 aircraft, also known as the White Swan or Black Jack, has a keel of ... titanium alloys. Such a specific and extremely expensive material was chosen because of the huge loads on the design of this machine, which to this day reserves the title of the heaviest bomber in service. But nevertheless, such a cardinal approach to creating a keel is a rarity, but because today, designers often have to deal with simpler composite materials.
What tasks do you have to solve when creating a composite keel?
In the process of development, domestic designers had to solve a whole range of complex problems:
- Created the creation of large parts of the keel and other carbon fiber equipment infusion method.
- We also had to almost completely rethink and re-equip the main stages of production, which were not designed for the use of composite materials.
Other features
The latest software (FiberSim) was introduced into the production process, which allows to achieve the highest possible degrees of automation. In addition, now the keel of the
aircraft, the design of which is described in the article, can be manufactured using technologies where drawings are practically absent. The manufacture of this part with this approach is as follows:
- Design or selection of the finished model. Today, the keel (mainly) is designed in a fully automatic mode, without the participation of "human" developers.
- Cutting of used materials, also conducted automatically.
- In automatic mode, the calculation of the raw materials used to create the keel and its structural parts is performed.
- Laying of layers is carried out by robotic mechanisms controlled by a computer program.
In addition, a modern approach to the production of keels involves the following:
- Continuous production of prototypes that are tested in the most severe conditions.
- Non-destructive testing technologies are being developed that allow continuous monitoring of the condition of the keel on an airplane.
Advanced methods for creating the tail of the MS-21 aircraft
In the not so distant past, the aviation industry was literally stunned by the statement of domestic developers that they were developing an entirely new aircraft, the MS-21. Its unusual feature is that for almost the last three decades it is the first domestic machine for domestic flights. During its manufacture, many latest technologies were tested, which largely affected the innovative features of the keel and the entire tail.
By developing and releasing the keel caisson of the MS-21 aircraft, domestic specialists were able to achieve the following:
- Full automation of cutting of all parts and raw materials used in production. Due to this, it was possible to achieve at least 50% reduction in the total cost of the entire tail, and especially the keel.
- In the production of tail units, the ProDirector program is used, which allows us to achieve perfect accuracy in the processing of parts. This makes it possible to create not only durable, but also extremely light keels.
- Also, the keel of a modern aircraft is created using double curvature techniques. Thanks to them, it is possible to achieve multidirectional thickness in those areas where additional reinforcement of the structure is necessary (under the keel of the aircraft).
- Even large keel parts today can be "fried" in special autoclaves. The result is extremely durable and tough components that can withstand loads of any degree.
- Control of the geometry of parts also takes place under the control of complex computerized systems.
Other features
Thanks to the use of new technologies and techniques, the laboriousness of creating the tail and keel was reduced by 50-70%. Today, more than four thousand details of the fin and tail have passed state tests.
The main achievement - a reliable and simple technology for producing keel caisson parts measuring 7.6 x 2.5 m was developed. Currently, they have already begun to be delivered to the Irkutsk Aviation Plant. They are made of modern composite materials, and the features of this process have already managed to interest leading foreign manufacturers of aircraft.
Contemporary issues
Why did we spend so much time discussing modern ways of developing and building a keel? The fact is that since the 60s of the last century it has become completely clear that a further increase in the speed performance of aircraft is possible only if their strength is increased and completely new varieties of polymer materials are introduced into production. The problem with aircraft of recent generations is that their design (and keel in particular) is highly susceptible to "fatigue." Because of this, by about the 70s of the last century, numerous techniques for monitoring the state of the wing and tail were developed.
Production requirements are also high. Each batch of parts is subjected to the most severe overloads on vibration stands, and is tested with temperatures and pressure. And this is not surprising, since the smallest crack subsequently is fraught with the death of hundreds of passengers.
So you found out where the keel is and what it is for!