Swashplate
The swashplate is an essential control element in helicopters , which provides the fixed and linear control inputs to the rotor, i. H. transfers to the rotating rotor blades. The swash plate consists of a rotatable and a fixed part. It is mounted around the rotor mast, can be moved axially to the rotor mast and tilted transversely to the rotor mast. The name swash plate comes from the fact that the upper part, which rotates with the rotor, shows a wobble movement when tilted.
It is used on the one hand for collective blade adjustment , i.e. changing the angle of attack of all main rotor blades and thus the lift, and on the other hand for cyclical blade adjustment to control the lateral and longitudinal movement.
Flight and control of the helicopter
A helicopter hovers and moves as a rotary wing through the lift and thrust of the main rotor. In the aerodynamic sense, this is a wing that rotates around the rotor mast and can be moved mechanically ( flapping and swivel joints ) or elastically within certain limits . The helicopter flies in any direction due to a corresponding inclination of the rotor plane (assumed surface on which the rotor blades rotate). The rotor creates lift to support the helicopter. In addition, due to the inclination, it generates the thrust for horizontal movement.
With the collective pitch , the pitch , the pilot changes the pitch of all blades evenly, resulting in the rise or fall of the helicopter. With the cyclic control (stick), the setting angles of the rotor blades are changed during the rotation of the rotor (cyclically) and the flow to them varies. The load-bearing capacity of the individual rotor blade thus varies during its revolution. Due to the changing aerodynamic lift and the lifting or lowering of the rotor blades per revolution, the rotor plane is brought into the desired inclination to the side (roll) or forwards / backwards (pitch).
In the system of the swash plate, both control inputs are mixed and the desired settings of the wing profile are transmitted to the rotating individual rotor blades by raising, lowering or tilting the swash plate through the blade adjustment levers.
Description using the example of Bölkow Bo 105
The swash plate of a Bölkow Bo 105 consists of two ring-shaped parts that can be rotated against each other via a ball bearing ring, referred to in the picture as rotating and stationary. Both parts are in turn movably attached to the sliding sleeve via a cardanic suspension (ball joint, joint bearing), which allows the unit to be tilted up to approx. ± 10 ° from the horizontal. Furthermore, the complete unit can be moved up and down by approx. ± 30 mm via the sliding sleeve on the carrier sleeve fixed to the gearbox to the rotor mast. The rotor mast runs in the interior of the support sleeve and goes through the center of the swash plate and carries the flange for fastening the rotor head at the upper end . The upper rotating ring of the swash plate is connected to the rotor flange via the articulated drive lever and thus follows the rotations of the rotor. The joints in the levers allow the swashplate to be tilted and to slide up and down axially.
The collective control rod moves the swash plate axially up or down depending on the movement of the pitch by the pilot. The cyclic control rods adjust the inclination of the swash plate via the differential lever depending on the control movement in the longitudinal direction and / or transverse direction of the stick. The differential levers mounted on the sliding sleeve ensure that cyclical and collective control inputs do not influence each other. When moving axially, the center pivot point of the differential lever goes along with it; the cyclical setting is not changed.
From the outer rotating ring of the swash plate, the pitch control rods lead to the pitch control levers for setting the angle of the rotor blade. Axial displacement of the swash plate and the blade adjustment rods thus produces the same angle setting on all rotor blades. An inclination of the swash plate produces a different angle setting on each rotor blade, since the pitch control rods are pulled down on one side according to the inclination and pushed up on the other side. When the rotor rotates, the angular setting of the rotor blade changes periodically per revolution from its minimum to maximum value, which is predetermined by the inclination of the swash plate.
Animations
- Click picture
Swashplate in neutral position
cyclic control
collective control raised
Inclination of the swash plate and the rotor plane
The movement of a helicopter forwards or backwards (nick) or left or right (roll) is initiated by a corresponding inclination of the rotor plane.
So that the rotor plane z. For example, if the control input "Roll to the right" also tends to the right, the corresponding rotor blade also runs exactly on the right side in the transverse axis at the lowest point in the rotor plane when the rotor is tilted sideways, it must be activated well before the transverse axis is reached become. The reason for this is that, from an elastomechanical point of view, a rotor blade basically behaves like a second-order oscillator, i.e. a spring-mass-damper system . The angle between the control point and the actual point of action in the rotor plane is called the lag angle, pilot angle or sometimes also the lead angle and is dependent on the natural flapping frequency of the rotor blade. The natural flapping frequency of the rotor blade in turn depends on the flapping hinge distance. In the case of a construction with a central flapping joint and without a mainspring, such as B. the Robinson R22 , the natural beat frequency is identical to the rotor rotation frequency, which is why the phase lag is exactly 90 °. A flapping hinge distance greater than zero increases the natural frequency, which means that the phase lag is less than 90 °. With the Bo 105, for example, which has a hingeless rotor that can be modeled with a replacement flapping hinge at 15% of the rotor radius, the lag angle is approximately 78 °. The direction of inclination of the swash plate is therefore not identical to the direction of inclination of the rotor plane and the desired direction of flight. If the lag angle of the rotor is z. B. 78 ° (as with Bo 105), then with the control input "Roll to the right" the rotor blades moving forwards and backwards must already be controlled 78 ° before reaching the transverse axis.
The graphic on the right illustrates this using the example of a left-hand rotating two-bladed main rotor with a pilot angle of 78 °. The rotor blade running backwards (light green) is activated shortly after passing the longitudinal axis with the maximum necessary positive angle of attack. This gives it more lift and rises to the highest point of the rotor plane until it reaches the transverse axis. At the same time, the rotor blade (light blue) moving forward is controlled with the maximum required negative angle of attack shortly after passing the longitudinal axis. As a result, the lift is reduced or even downforce is generated, so that the rotor blade moving forward sinks to the lowest point of the rotor plane until it reaches the transverse axis. This causes the rotor plane to tilt to the right. These movements are limited by the flapping hinge or the elasticity of the rotor blade.
In the meantime, the swash plate has controlled the rotor blades, which are now on the transverse axis, in such a way that they are briefly in the zero position. The rotor blade moving further backwards (light green) is controlled with less and less angle of attack so that it reaches the maximum required negative angle of attack shortly after passing the longitudinal axis, while the rotor blade moving forward (light blue) is controlled in such a way that it is shortly after passing the Longitudinal axis reached the maximum required positive angle of attack. This is repeated cyclically with each rotor blade revolution. Eventually the helicopter cell follows the movement of the rotor plane and the helicopter flies sideways to the right.
Further development
The construction of the swash plate Bo 105 shown dates from 1964. In principle, nothing significant has changed even with the current state of the art, as the picture using the example of EC 155 shows. The mixture of the collective and cyclical control takes place here only at a different point of the control linkage and not, as with Bo 105, by the differential lever directly on the sliding sleeve.
As is already the state of the art in hydrofoil construction, the future belongs to the fly-by-wire for helicopters . This means electronic control via data lines without mechanical connections such as control rods and therefore also without mechanical connection to the swash plate. In order to do without the swash plate completely, more advanced techniques are necessary.
For example, the manufacturer ZF has developed electrically longitudinally adjustable blade adjustment rods (IBC = Individual Blade Control) that are shortened or lengthened during rotation in response to electronic signals and thus adjust and angle the rotor blades. The system was tested by NASA in the wind tunnel and will be tested from 2006 in Germany at the Wehrtechnische Dienststelle 61 in Manching on a CH-53G and in the United States on a Sikorsky UH-60 .
Web links
- Helicopter control on helicopterflug.de
Individual evidence
- ↑ Student information 1/99. (PDF; 545 kB) German Aerospace Center eV, July 21, 1999, accessed on May 17, 2010 .
- ↑ Berend Gerdes van der Wall: Fundamentals of helicopter aerodynamics . Springer Vieweg, Berlin 2015, ISBN 978-3-662-44399-6 , pp. 248 ff .