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Aerospace propeller useful for diesel engines with extreme excitation of vibrations

Periodic Reporting for period 3 - ARGOS (Aerospace propeller useful for diesel engines with extreme excitation of vibrations)

Reporting period: 2018-09-01 to 2019-09-30

The 714030-ARGOS project is aimed at a new aerospace propulsion unit based on a compression ignition (diesel) engine. The specific problem solved by ARGOS is the design of a direct drive propeller capable to withstand the increased level of torsional vibrations of a piston engine producing high power on low rotational speed of crankshaft.
The main advantages of such a propulsion unit are low fuel consumption that allows significant increase of the flight radius of the aircraft without serious impacts to the airframe design, lower fuel flammability important during accidents and better availability of certified jet fuel worldwide compared to certified avgas fuel. One of the most important properties is also the low noise produced by the slowly rotating propeller.
Of course such a propulsion unit has also disadvantages given by the specific properties of piston engines with the diesel working cycle. The power produced by the engine is given by the torque moment and its fluctuation during one revolution of the crankshaft together with rotational speed of the crankshaft. The same power produced on lower rotational speed of a diesel engine requests higher torque moment peeks than the torque moment peeks produced by equivalent avgas engines working on higher rotational speed.
Simply said the diesel engines need propellers capable to withstand increased level of torsional vibrations of propeller shaft. The propellers originally developed and certified for avgas engines usually suffer by fatigue-life problems when adopted to diesel engines.
Fuel consumption reduction, lower noise emissions and low operational cost of the aerospace diesel engine propulsion unit and therefore of the aircraft using it are in great accordance with preferences of modern society who is gradually increasing its demand on transport activities.
The main objective of the project is to design a new propeller and a governor useful for diesel engines with optimized aerodynamics to provide maximum propulsion efficiency for the specific working conditions of the engine and also capable to work with the increased level of torsional vibration excitation without negative impacts to fatigue-life properties of the propeller.
The first project period was devoted to the comparison of equivalent avgas and diesel engines mainly from the point of view of excitation of torsional vibrations and to the corresponding response of the propeller.
The data measured on different engines prove significantly increased level of torsional vibrations and corresponding increase of cyclic components of stress measured on the propeller parts. On the other hand the increased level of vibrations is given mainly by the different rotational speed and number of cylinders. The influence of different work cycle (compression ignition - diesel) seems to be less significant. A more detailed analysis of this fact will be done in the next phases of the project because better understanding of the dependences would allow better exploitation of experimental data from different projects for future propeller development.
The main output of the first period is the definition of design specifications and development plan. The output provides basic rules for the propeller design and continuation of experimental work to meet the project objectives.

The second project period was aimed at design works and production of propeller and governor prototypes. The requirements for the design were defined in the previous period and confirmed by CDR organized by Topic Manger, Safran AE.
The propeller design is done so as to be able to absorb increased vibrational energy produced by engine in the propeller structure without an additional device, i.e. without a special energy absorber between the propeller and engine flanges. Also requirements for the minimum propeller weight and moment of inertia were respected with maximum attention.
The production phase was started after CDR and according to the project plan. The first propeller and governor prototypes were installed on the engine and successfully passed the first functional tests at the middle of August, 2018.
The work continues with preparation of complex test program of the propeller and governor prototypes and their parts.

The final project period was dedicated to the complex propeller and governor test program. The new propeller was a subject of long – term engine testing, including strain-gauge measurements of stress distribution. The goal was to verify that the real stress distribution corresponds to the theoretical results and to the initial assumptions created at the project beginning.
The propeller was also subjected to different methods of modal properties analysis including testing using PRODERA system.
All project objectives were accomplished and a plan of steps in the after project time period was prepared.
Important data files were obtained during the first period of the project. It is necessary to point out that most of the currently produced modern propellers were optimized based on avgas engine parameters corresponding to the state of art from the middle of the last century.
Typical direct drive avgas engine has maximum rotational speed between 2600 and 2750 RPM and it is known that some of the (direct drive) engine projects were working with the rotational speed exceeding 3000 RPM. Such development goes against the requirement to reduce noise emissions of new aircrafts and also limits the flight performances of aeroplanes. The high rotational speed lowers maximum flight velocity of aeroplanes because of the problems with the maximum permitted Mach number at the propeller blade tip.
The aerodynamic analysis done during the first reported project period proves that propeller blades optimized for the lowered rotational speed of diesel engines provide better propulsion efficiency than the blades originally optimized for avgas engines when simply adopted to the diesel conditions. Noise emission analysis also proves better parameters of the new propeller because of the lower speed of the blade tips, i.e. due to the lower blade tip Mach number, and also because of the lower frequency of emitted noise which is important for the results of noise expressed by means of filter “A” weighted values of pressure level. Filter “A” reflects the specific properties of human hearing, i.e. the ability to suppress some tones and to increase the influence of other tones to the life comfort of people.

The main expected project outputs are a new propeller prototype with a governor adapted to the specific conditions of the target compression-ignition engine. The first prototypes already passed initial engine testing to prove the proper functionality. The tests were accomplished successfully and also other requested parameters are met. The resulting mass properties of propeller and governor are below the values requested by the project call. Of course, the prototypes just entered the beginning of the test program and some parameters can be additionally influenced by test results but it seems likely that the propeller and governor optimized for the target compression-ignition engine will have the same or better mass properties as the equivalent propeller and governor useful for avgas engines. The design of propeller is ready for the future development of the propulsion unit, i.e. for increased engine power and for thrust reverse and blade feathering capability.
Propeller instrumentation, measurement data acquisition and evaluation was provided by VZLU.
An instrumented WOODCOMP propeller was tested on spark-ignition and compression ignition engines.
Tests of propeller control system
The idea presented by a CAD model is materialized by the first prototype
Theoretical results are supported by testing (materials, parts, subassemblies, etc.)
New propeller assembly, balancing and preperation for testing.
Parts of the new propeller were subject of FEA
JIHOSTROJ tested its propeller governors to obtain important data for the new application.
Design work and testing program preparation were initiated.
Propeller prototype no. 2 ready for s-g measurements on the engine
Based on obtained data the new propeller and governor design specifications were defined.
Propeller prototype no. 4 for long-term engine testing
New governor installed on the engine, ready for tests.