The thesis lays the theoretical foundations for understanding the interactions of the hydraulic system, the bootstrap tank and the axial piston hydraulic pump. Before your eyes lies the thesis "Bootstrap Reservoir and Hydraulic Pump Interaction in the Aircraft Hydraulic System", which was written to fulfill the thesis requirements of the PhD program in Engineering Sciences at Tampere University of Technology.
M ILITARY A IRCRAFT H YDRAULIC S YSTEMS
In central hydraulic systems, the aircraft's hydraulic system is divided into two or more separate systems, each of which is divided into multiple separate circuits. The airworthiness of the aircraft is highly dependent on the airworthiness of the hydraulic system.
M OTIVATION AND J USTIFICATION FOR THE R ESEARCH
Unfortunately, Tumarkian & Casey's studies have never been published in full or in scientifically relevant media (Casey, 2007).
O BJECTIVE OF THE T HESIS
S CIENTIFIC C ONTRIBUTION
In the model used in the simulations, the hydraulic pump and bootstrap reservoir with connecting pipes are modeled using analytical physics equations. Additionally, the systematic methodology that uses computer modeling and simulation along with laboratory and field testing to eliminate the need for flight testing or the use of an iron bird is new in the aerospace context.
R ESEARCH M ETHODS
Other parts of the hydraulic system are modeled using empirical black box-type empirical or semi-empirical models. Laboratory measurements and field tests with a real fighter aircraft on the ground are performed to verify the simulation model.
S TRUCTURE OF THE T HESIS
Dobchuk, et al., 2000) determined that friction on the swashplate affects the system's natural frequency and dynamic response. An accurate model of the piston-slipper relationship was developed in the work of Yi, et al.
M ODELLING AND S IMULATION M ETHODOLOGY
Although they generally lack fidelity compared to more advanced methods, they have features that other models do not have and offer options not available in other models. Their main advantage is good interoperability - they can easily be connected to other models of the same type, as well as to models based on other methodologies.
H YDRAULIC P UMP M ODEL
- Structure and Operation Principle of the Swashplate-Type Axial Piston Pump
- Piston Pressure Force
- Swashplate
- Rotating Group
- The Portplate
- Pressure Compensator
The amplitude of the linear motion is defined by the swashplate angle as the piston/slipper assemblies slide on the swashplate surface. The cross angle in the swashplate causes top dead center and bottom dead center to be functions of the swashplate angle (Figure 3-6). The port plate controls the opening and closing of the piston chambers to the pressure and supply lines of the pump.
B OOTSTRAP R ESERVOIR M ODEL
Casing pressure is modeled as a variable volume using a compressibility equation (for example, Eq. 3-53) whose volume changes as the control piston moves. Casing flow originates from pump leakage (sections 3.2.2 and 3.2.5) and compensator operation, which is explained in this chapter. The outflow from the enclosure is modeled using a tube model and turbulent orifice models as a heat exchanger and filter (further explained in Section 3.5).
H YDRAULIC L OAD M ODEL
The load regulation is set by the control input (C1) and gives a pump capacity demand equal to the total capacity demand of the system at the current duty point. The flow through the flow variation throttle valve can be in either direction depending on the current system pressure and the pressure on the other side of the throttle valve. The pressure and throttle settings are set by the control input (C2) to match the flow variation of the current duty point.
I NTEGRATED H YDRAULIC P UMP AND S YSTEM M ODEL
Capacity calculation is based on effective bulk modulus and chamber volume (LMS Imagine.Lab Amesim® User's Guides, 2014). These pipes have been modeled with pipe models to include pressure losses in each pipe and to study the effect of sizing these pipes. It makes the following assumptions: the bulk modulus of the liquid with no air/gas content is constant, which means that the corresponding density varies exponentially with pressure; and is the viscosity of the fluid with no air/gas content.
I NITIAL S ETTING OF M ODEL P ARAMETERS
The inner workings of the model cannot be verified on the basis of direct measurements because there are no applicable ways to measure phenomena occurring inside the pump. However, the stress on the hydraulic system in aerodynamically unstable aircraft such as most modern ones is still significant even during cruising. Certain operating points likely to induce adverse system-level effects can be identified on the basis of these studies.
L ABORATORY T ESTS FOR P UMP M ODEL V ERIFICATION
Test System
From these operational points possible on the ground, tests with real aircraft were selected to be used as verifiers. The maximum number of sensors in use at the same time is 16 analog sensors (pressure, flow, force, torque, etc.), 8 temperature sensors, and 16 digital sensors (rotational speed, etc.). The following sensors and transducer were used: a DC drive torque sensor (D) (HBM); pressure, discharge and supply line pressure transducers (A) (Kulite); discharge and discharge line flow sensors (B) (Force); drive speed sensor (E) (Honeywell); reservoir temperature sensor (C) (generic PT100).
Test measurements
As can be seen from all the curves, a slight vibration of the pressure regulator occurs after the flow rate changes. It can also be observed that the system pressure in this case rises well above the rated pressure, but is well within the maximum value specified in the maintenance manuals and SAE-AS5440 and MIL-H-5440H (SAE International, 2011) (U.S. Department defense sciences, 1999). Comparing Figure 4-3 and Figure 4-4, it can be seen that a significant increase in system pressure occurs only in situations of zero output flow, which is practically impossible in a real aircraft.
Discussion
However, based on laboratory test findings, some pressure control vibration is likely to occur in the aircraft as well. In field measurements, it is important to detect whether these vibrations occur so that the hydraulic capacity of the model can be matched to the actual system in system-level model verification. The flow points measured in the tests were five to six times higher than the nominal discharge flow of the pump being studied.
F IELD T ESTS FOR S YSTEM M ODEL V ERIFICATION
Test Setup
To this end, simulations were made using the pump model only with constant pressure supplies as supply and drain line pressure sources and a single orifice as load. Once a good match was found, the parameter value was tested at other operating points. The selected parameter values for the final pump design are those that give a satisfactory match at all operating points studied.
Test Operating Points
It can be seen that both increasing and decreasing flow in the system cause pressure transients to the supply and drain lines. The notable difference between Figure 4-9 and Figure 4-10 is a small constant oscillation of the drain line pressure in the right-hand plot. In the beginning, with the return movement of the air brake actuator, the supply pressure drops momentarily to zero.
Discussion
There are significant differences in the magnitude of the pressure transitions in the discharge and supply line between this operating point and other tested operating points. Setting and tuning of parameters was done by varying a single parameter at a certain operating point and within certain limits. The selected parameter values for the final system model are those that give a satisfactory match at all operating points studied.
C OMPARISON OF S IMULATED AND M EASURED O PERATION OF THE S YSTEM
The phenomenon is due to the volume of the pump casing changing as the control piston moves. A comparison of the pump's measured and simulated static performance characteristics is shown in Figure 4-18. The figure shows the measured and simulated delivery pressure, drive torque and drain flow as a percentage of the maximum value with the function delivery flow indicated as a percentage of maximum delivery.
D ISCUSSION
Furthermore, since the inlet pressure can in some cases drop below atmospheric pressure, cavitation in the pump is an inevitable phenomenon. As it was also shown in the field tests, these fluctuations are also reflected in the pressure in the pump housing, which, on the other hand, is also affected by the operation of the pump compensator. Due to the drain pipe filter and heat exchanger and the pressure losses in the drain pipe line, these flow transients cause the casing pressure to fluctuate.
E FFECT OF THE B OOTSTRAP R ESERVOIR AND S UPPLY L INE
The effect of the design parameters of the tank and the pump supply line can be evaluated in terms of the step response characteristics of the supply system. Changing the piston diameter changes the piston travel because the reservoir volume is kept constant. Lift time, however, also depends on piston travel and is thus more affected by variation in piston diameter.
E FFECT OF D RAIN L INE P RESSURE L OSSES
As described earlier, the drain flow has two components: the base drain flow due to pump leakage and the flow caused by the operation of the compensator. To some extent, the drain flow issue is a contradiction in terms: cooling and lubrication would benefit from a larger drain flow, which would require larger diameter pipes and larger filters and heat exchangers to keep pressure losses within acceptable limits. In addition, filter performance in the drain line is strongly affected by flow transients and would benefit from a steady flow (Multanen, 2002) (Multanen, 2000).
E FFECT OF THE D ECREASED B ULK M ODULUS C AUSED BY F REE A IR IN THE R ESERVOIR
The simulations show that as the air volume is reduced and the system becomes stiffer, the delay, rise time and settling time are reduced and the amplitude of the vibrations is reduced. It should be noted that here the absolute magnitude of these results is affected by the choice of the fluid model and its parameters (discussed in Chapter 3.5). Furthermore, the exact amount of free and dissolved air at any operating point cannot be known, and the stiffness of the system may have dynamics if significant air leakage occurs at the operating point.
D ISCUSSION
I MPROVING THE D YNAMIC R ESPONSE OF THE R ESERVOIR AND THE S UPPLY L INE
Piston diameter and supply line diameter are also tightly coupled to the aircraft's layout requirements and can only be changed within the limits set by them. The reservoir piston seal friction, on the other hand, can only be changed within the limits of available seal types and also within the limits set by seal compatibility and lifetime issues. Based on these findings, a general rule can be stated: the reservoir piston diameter and supply line diameter should both be as large as possible.
M INIMISING P RESSURE L OSSES IN THE C ASE D RAIN L INE
It pays to use a pipe with as large a diameter as possible, even if the pipe causes only part of the total pressure loss in the drain pipe.
M INIMISING THE F REE A IR C ONTENT OF THE R ESERVOIR
The second research question was proved by studying the effect of the drainage line pressure losses. The sensitivity of the system and component design variables that have an effect on these phenomena was studied. The system's sensitivity to free air in the reservoir was evaluated in terms of the characteristics of the supply side dynamics, and was found to be significantly more significant than any of the design parameters studied.
S UGGESTIONS FOR F URTHER R ESEARCH
Leakage Characteristics Of The Hydraulic Slipper Bearing In Swashplate Type Axial Piston Motor At Starting And Low Speed. The study of the gap characteristics between the valve plate and cylinder in swash plate type axial piston engine. Analysis of the flow dynamics characteristics of an axial piston pump based on the computational fluid dynamics method.
HYDRAULIC SYSTEM OF LOCHEED MARTIN F-16A
System B also contains a sump sump that provides hydraulic pressure to the shunt system in the event of failure of hydraulic system B. Each hydraulic system has an FLCS accumulator that is isolated from the main system by check valves. The EPU automatically activates when both hydraulic system pressures drop below 1000 psi or when the main generator is disconnected from the bus system.
