Cable-control systems of GA1 aircraft and gliders will remain in use, owing to the use of the electrical or hydraulic systems being irrational due to the inappropriate costs and complex services involved. Currently, symmetrical loads and bending are used in the fatigue tests of airplane cable-control systems, just like in the tests of elevator ropes2. The cable and ropes tests are commonly conducted by bending them in the range of ±90° with a constant load. The mentioned tests are adequate for assessing the operational loads of cable transport systems such as cranes, lifts, vertical and horizontal cable transport, and so on (Brzęczek, 2020; Kubryn et al., 2018; Tytko, 2021). For the ropes and pulleys of transport systems, the described methods are used to assess the wear and for damage evaluation (Hankus & Hankus, 2006; Kubryn et al., 2018; Tytko, 1998, 2021). Typical cables test results are given in Figure 1. Kubryn et al. (2018) and Tytko (1998) could only partially point out to the real wear as a function of the operational measure and fatigue life of the aviation-control systems, and these do not correlate with the real deflections and loads.
The results of the KSAN cable (aviation cable) tests with a diameter of 3.5 mm up to 100,000 ± 90° bends (Kubryn et al., 2018).
The forces transferred by the control system should ensuring stability and control are limited by admissible control are results from surfaces deflections
Simplified system of forces and moments of the aircraft in the classical layout.
The given dependencies indicate the complexity of the issue with random loads and deformations (see Figures 8 and 9). The characteristics of variability are typical and similar for defined airplane categories, operation profiles, and the in-flight mass and CG position. Real loads of the control systems are modified by the required pretensioning (Certification Specifications for CS-23, 2003). The sum of the pretensioning and the real loads cannot reach a negative value for any permissible flight configuration, including loads generated by doubled control systems (Brzęczek, 2019; Certification Specifications for CS-23, 2003). Specifications (Certification Specifications for CS-23, 2003) also require that the designed systems ensure the positive gradients of the flying controls forces
Transmission of hinge moments,
In the present article, only the longitudinal control of the aircraft was analyzed (Figure 2). The resultant value of the force on the tail,
Simplified system of forces and moments ensuring the balance and longitudinal control of the classical layout of monoplane.
Classical horizontal tail unit aircraft or glider.
Required values of forces and moments depend on the current aircraft configuration (CG position,
Generalized aerodynamic data can be used for the estimation of
Loads and strain of the cable should be presented in conjunction with the changes in wrap angles
This approach with respect to the geometric configuration as given in Figure 2 and the required values of deflection for profiles of exploitation (Figures 4, 7, and 8), CG position, and mass changes in the flight should be used for analyses and tests and for programing the service life of aviation control-cable systems.
The values of the control systems forces with respect to surface deflection, From the measurements of the actual values of the forces on the passive and active cables of the aircraft operated in a specific operating profile, related to angle From the measurements of the control surface deflection angles,
Method (a) is closely related to the configuration of the aircraft (see Figure 1), the specific CG, and the profile of its operation. In this study, the second method (b) was analyzed. Data prepared in this way can be further generalized by taking into account the actual values of air speed, mass in flight, and the actual CG position. When converting data into the test loads spectrum, the maximum pre-tension values should be used as a conservative approach (Figure 13).
The sources and methods of the loads of the aircraft and the aircraft’s control system acquisition are presented in Figures 5 and 7.
Typical flight mission (Brzęczek, 2020).
Scheme of the elevator aircraft cable-control system.
Probability density function of flight time distribution for the aircraft commuter category (Kubryn et al., 2018).
All real or postulated missions of the selected airplane category allow to define a typical operating profile. The spectrum of the real loads of the aircraft’s control systems are divided into:
– controlled loads (depending on the pilot and his training, but determined by the category of the aircraft and their operational profile) are resulting from performed missions; – controlled loads resulting from the reaction of the pilot or autopilot on flight disturbances (e. g., after gust, engine asymmetry, etc.); – environmental loads (depending on the state of the atmosphere, the load of the power unit, and the condition of the runway); and – properties of the aircraft (mass and location of the CG, aerodynamic and power unit characteristics, and stiffness and deformations of the loaded airframe structure) (Brzęczek, 2019).
Actual load values of control system elements (Figure 6) during the flight mission (Figures 7 and 8) should be collected as the result of the operating load spectrum based on the operation aircraft’s profile.
Real deflection of the elevator during flight, sampling of 50 Hz. Elaborated based on tests data (Department of Avionics and Control Systems of Rzeszów University of Technology, 2019).
The loads of cables are combined with deformations on the pulleys or sliders, and real forces cause a complex state of stress in the strands and wires of the cables (Brzęczek, 2019).
The requirement is for a positive cable tension for each extreme load (Certification Specifications for CS-23, 2003); hence, the need to introduce the
From analyzing the stability and maneuverability of airplanes related to the geometrics and kinematics of cable control systems, we conclude: the deflections of the control surfaces are ±10° for >90% of the total surface deflection (see Figure 9).
Spectrum of rudder deflection. Elaborated on (Department of Avionics and Control Systems of Rzeszów University of Technology, 2019) data. Aerodrome traffic circuit flight. The average value depends on mass and CG location.
The above-mentioned control-cable deformations and displacements indicate that their fatigue lives are much more than those obtained in the tests (Kubryn et al., 2018; Pieróg, 2011).
The factor significantly influencing the fatigue life of the cables is a relatively low-value cable load. The method described below and the results of the related tests can be useful for predicting the fatigue life of the cables and defining the diagnostic and adjustment intervals.
The force values of the cable-control system can be presented in formulas (5) and (6):
An example of the
Below, the two possible methods of
The values of the hinge moments coefficients of the control surfaces determined by the data obtained from tunnel tests are given by formula (7):
– see Figure 11
– see Figure 12
An example of
An example of
Loads of the passive and active cables of the airplane control system should be additionally corrected by the friction forces of the systems (Brzęczek, 2019). The values of the deflections
The stochastic deflections of the control surfaces,
Idea of a cable test strand4. More than six points of cable testing and inspection. The strand enables the ongoing measurement of cable elongation. The markings in Figure 13 mean: 1 – spring or hydraulic system, simulation of nonlinear hinge change, 2 – cable lock, 3 – pulleys, 4 – cable A, 5 – wrap angle adjustment, 6 –turnbuckl, 7 – cable B, 8 – tensioner (pre-tension value of force), and 9 – stochastic angular displacement (
The main task involved in conducting the tests of airplane cable-control systems (Figures 6 and 13) is the use of stochastic non-linear loads correlated with cable deformation. The strand allows test cables with different wrap angles, but with the same variable (stochastic) load values, for two or more cables at the same time. The cable loads are applied by angular excitation, and the nonlinearity of the forces changes as a function of the deflection angles and is implemented by spring or hydraulic systems.
In addition, as emphasized in this paper, the magnitude of the used loads is in correlation with the cable deformation and displacement (Figure 6). Loads and operational spectrum are related to the specific type of airplane control system (Figures 5 and 7) moreover enables to define a critical point on the cable system by the wrap angle displacement (Figure 13). The critical point and fatigue test results should be used to plan the inspection and diagnostic intervals of the aircraft’s cable-control systems.
A measure of the symptoms of wear of the aviation control-cable systems can be conducted by the cable elongation, reduction of cable diameter, and the broken wires. The measurement test’s results will be used for coefficients ε – limit elongation of the cable or rope;
The assessment of the elongation of the cable can be determined by the measurement of the pre-tension (Brzęczek, 2019, 2020) as well. Based on the results of measuring the cable elongation, the term for the diagnostics intervals and the cable replacement can be predicted (Brzęczek, 2019).
The presented method for preparing and performing the fatigue tests of an airplane’s control-cable system is based on the real loads and deformations of the cable under random values of forces. The loads of the control cables with the associated complex stress of the individual wires, in the presented proposal, are determined by the real values of the hinge moment and other related factors such as the aircraft’s operational profile consequence. The proposed solutions are based on flight measurements of