Influence of oil service life on selected performance parameters of an aircraft piston engine

This article presents an analysis of the influence of oil service life on the performance parameters of an aircraft piston engine lubrication system used in an ultralight aircraft. The ageing of oil between oil changes causes a change in its parameters (such as density, viscosity...). These parameters have a strong influence on the level of protection of the lubricated components. Currently, in aircraft, oil changes are carried out according to a time schedule – oil is changed every fixed period (residual life) regardless of its actual condition. The task of this article is to test the possibility of an indirect assessment of oil condition based on analysis of changes in selected parameters of engine lubrication system operation during normal operation. The oil warm-up speed during the pre-start procedure and the dependence of oil pressure on engine speed were assumed for the analysis. The study was conducted on an ultralight rotorcraft during normal operation. Selected first daily flights directly after oil change, and after 17, 32, 50 and 66 hours of operation were analysed. It was shown that the warm-up rate changes in the samples analyzed, but that this change may also be due to factors other than oil operating time. In the case of the oil pressure vs. speed characteristics, different characteristics were shown for different operating time, but no specific dependencies were found.


Introduction
From the first years of the XXI century, the market of ultralight aircraft mainly gyroplanes, is growing rapidly. On the base of the registry conducted by the Polish Aviation Authority, the number of registered gyroplanes raised from 0 in 2008 [17] up to 105 pcs. in 2022 [15].
These aircraft cover mostly the needs of hobbyists but, since 2013 (the year of the law change in Poland [1]) more and more jobs can be done using this type of aircraft. It is widely used as a platform for power lines or gas pipes scanning. In geodesy [4], it is used to prepare maps and photographs of longitudinal objects. In the agro industry it is used for both biological and chemical crops and forests protection. It is used in the field of surveillance and professional photography. For the last few years, it can be also seen in agro works, in monitoring the state of agrocenosis [5].
According to that, the market for maintenance has grown as well. It can be seen that not all of the owners and operators are careful enough with their aircraft. As safety is the biggest value in aviation, allthe manufacturers, operators, mechanics, owners and pilots are looking for the way how to make it safer. That is why the last decade was the decade of "electronic transformation" in the ultralight aircraft market. Most of the manufacturers moved away from analog avionics in favor of electronic ones. It opened new possibilities for system development. We can now see all kinds of different devices that could be connected to onboard avionics. It is to help the pilot or to modify the aircraft to suit the owner needs. One of these, which can improve safety the most, is a Flight Data Recorder that collects all the data during the flight. It collects air data (e.g. Airspeed) and engine data (e.g. rpm, fuel consumption).
As in modern aircraft Flight Data Recorders are fitted as standard a lot of analysis can be done after or even during every flight. What is common in airlines using QAR [10,22] (quick access recorder) can be now done in every small aircraft.
It is known from the automotive market [6,9,14] that the analysis of the oil degradation based on the record of the engine parameters can be done. Such a system of Remaining Useful Life of engine oil prediction is being onboard of many modern vehicles. The authors of this article think that it is the right time to introduce such a system in ultralight aircraft.
Till now, all of the oil changes were conducted according to total flight/operation time in ultralight aircraft. Most commonly every 100 hours [2,18]. Such a condition is not optimal in two ways. For the user, it would be best to replace the oil just before the moment that it lubricates the engine not enough, not to lower the expected life time of the engine [8,21]. It is expected that this point is sometimes earlier than 100 hoursif the pilot uses the aircraft extensively or later if the aircraft is being used gently, e.g. only on long flights with good weather.
The topic of the operation of objects based on condition analysis is widely described in the scientific literature [7,11,19,20,23]. It has been shown that in the case of internal combustion engines, it is possible to diagnose their condition on the basis of selected parameters of operation, and one of the most important factors determining the operating condition of the engine is the condition of the lubricating oil. These studies have shown a significant link between operating conditions and oil consumption rate. This allows prediction of oil condition degradation in on-board diagnostic systems and, based on this, suggests to the user the moment of oil change. This research work was carried out mainly for engines that powered vehicles or stationary engines. Operating conditions for aircraft engines are sig-Influence of oil service life on selected performance parameters of an aircraft piston engine nificantly different, which can significantly affect these correlations.
This article is the first step in developing a method for diagnosing oil condition based on aircraft operating conditions. It will allow assessment of oil condition and prediction its degradation. Authors think that it would be best for the owner of the aircraft and for the environment to replace oil just in the time when it is needed [3,16].

Research object
The research object is a Tercel Carbon gyroplane   [12] Its empty weight is 332 kg, and the MTOW has not changed and is 560 kg. The rest of its performance and technical data is shown in Table 1.

Scope of research
The scope of the research was to analyze the oil temperature and oil pressure records and check if there is any dependence on the total flight time of the gyroplane after the oil change. It was assumed that changes in temperature rise time or differences in the relation between oil temperature and engine rpm could be noticed. The data was collected by the

Methodology
The results were obtained by analyzing data collected from gyroplane's FDR. Data from the time between oil changes was chosenfrom 400 to 500 hoobs time (a measure of the total aircraft operation time). The last oil change was done after 400 hours. As a representation for further comparison 5 flights were chosen Table 2. To determine the changes in the oil parameters, a statistical analysis was performed. The analysis was divided into two parts. The first part was to research in the field of oil temperature rise time. The second part was to research the dependencies between engine rpm and oil pressure. Figure  3 shows the course of these parameters. COMBUSTION ENGINES, 0000;XXX(X)

Analysis of oil temperature rise time
The first results were obtained in the scope of oil temperature time rise. Figure 4 shows the oil temperature change during each flight. It can be seen that the starting point depends on the initial temperaturein this case, it is the outside temperature (this was confirmed using another independent sensorthe intake temperature). The slope of the temperature rise up to 60C can be recognized as similar in every case. Then there is a significant rise in temperature due to the maximum engine rpm achieved at takeoff. Then the temperature decreases and stays at the same average level for the whole flight.  Then the rise time at each flight was shown in the graph Fig. 6. It appears that no significant dependencies could be observed in this field.

Analysis of oil pressure record
Many analysis of oil pressure has been performed Fig.  7-9. No dependencies were found when comparing the full record of oil pressure during every flight. Comparing the oil pressures in the state of the rising engine rpm gave no results. This is due to the fact that the distribution of this correlation is heavily affected by the difference in flight parameters (the way the pilot performs the flight). The type and speed of maneuvers significantly affect the dynamics of changes in speed and oil pressure. Thus, it is impossible to draw conclusions from these distributions. Therefore, only selected operating points corresponding to steady-state flight and engine operation were analyzed. Figure 10 shows the analysis of the oil pressure in a steady state at about 4300 rpm engine set. With slight variations in engine rpm (±1%) chosen from every flight in the way that it was during a steady flight, not during takeoff, landing, or hard maneuvering. The achieved oil pressures are shown in figure 10. Each record in the graph shows the achieved pressure for a steady engine rpm. Steady states chosen from each flight vary between each other by no more than 10%. There is no upward or downward trend in relation to increasing hoobs time. For 405.1 h hoobs time, the achieved pressure was 3.21 bar at 4419 rpm, then there was 3.52 bar at 4301 rpm and then there was a drop to 3.46 bar at 4195 rpm. After that, there is a drop at 455.9 hoobs time for 3.01 at 4144 rpm, and rise again to 3.19 bar at 4528 rpm after 471.4 h hoobs time. Due to that, no relations could be found in this case. Then another approach was taken. Maximum oil pressure during takeoff at maximum reached rpm was analyzed in Fig. 11. In this case, some dependence could be noticed. Achieved maximum oil pressure during every takeoff raised up to about 5.5 bar. However, in order to recognize this relationship as appropriate, the same analysis should be performed for another aircraft in a similar range of total flight time. The maximum oil pressure during takeoff is a single point in a record, and it should not be taken as a proof for further inference.

Conclusions
No significant dependencies in the field of this research were found. 1. It is assumed that in the analysis of the oil temperature time rise, outside temperature has a significant impact. Comparing the time rise without taking it in consideration can not give results that could lead to conclusions in the field of oil degeneration. 2. The research in the field of oil pressure shown that the dependence between engine rpm and oil pressure is not changing in any particular way with rising hoobs time.
This factor can not be then taken to give any conclusions about oil degeneration as well. 3. In the future, it is planned to conduct research which will take into consideration not only the recorded data but also data obtained during laboratory oil examination [13]. 4. The changes in the maximum oil pressure achieved during takeoff have to be checked using records from other aircraft with similar hoobs time.

Acknowledgements
This work was co-financed under the 6th edition of the Minister's of Education and Science programme entitled "Implementation doctorate". Influence of oil service life on selected performance parameters of an aircraft piston engine