ARGOS: A new spin on Clean Sky innovation
Running propeller-driven aircraft in the SAT category on jet fuel rather than avgas – the conventional fuel of General Aviation – yields substantial operational and environmental benefits. But to do so requires a special type of propeller to contend with mechanical loading and the issue of noise. Clean Sky's ARGOS project is progressing through these challenges to bring energy efficiency and quieter air travel for Europe's regional aviation.
For short-haul flights with a dozen passengers or less – we're talking about the Small Air Transport (SAT) of aviation – it's hard to beat the operational economics of propeller-driven aircraft. But what if you could squeeze out even more energy efficiency and improve the environmental footprint too?
That's the premise underpinning Clean Sky's ARGOS (Aerospace propeller useful for diesel engines with extreme excitation of vibrations) project where the idea is to design and test an adaptive and innovative propeller and an adaptive governor on small aircraft in order to use a diesel cycle (compression ignition) engine for small aircraft instead of using the regular avgas. To make this viable the propeller and the governor both need to be adapted to cope with the forces that need to be applied – a reference to a key part of the challenge which is that aerospace propellers used on jet-fuel reciprocating direct drive engines are prone to extreme excitation of vibrations. Hence, the objective is to design and test a new propeller useful for such operational conditions. The 40-month project, which is scheduled to conclude this September, is being coordinated by Woodcomp Propellers s.r.o., based in the Czech Republic.
Woodcomp has designed a prototype propeller and governor, they've tested it and the first results were positive. The aim is to have lower fuel consumption because the propeller rotates slower compared to the avgas cycle, and there will be reduced noise production – those are the advantages.
Clean Sky colleague and Project Officer for Engine ITDs Jean-François Brouckaert adds that "running a piston engine on kerosene has the advantage that you run with unleaded fuel. Another advantage is the efficiency of the diesel cycle where you can get, potentially, between 30% to 60% of CO2 reduction by FFC, because you have to run full rich on take-off on a small aircraft and there you will gain a lot. And then, at cruise, you will gain a little bit less than 20% or 30% because of the difference in cycle efficiency. The topic leader for this is Safran Aero engines and they have already certified a 4 cylinder engine of 200 horsepower, but now they want to develop a 6 cylinder engine and for that the torque distribution is different to what they have on the 4 cylinder, and the torque on the piston engine is of course different to that on the gas turbine. So this is why you need a specifically designed propeller, and the opportunity has been taken to design it in an optimised aeroacoustic way to have low noise. The rotational speed is 1700 RPM or less, so as not to be too noisy". The propeller/engine combination will eventually be tested and validated until 'permit to fly’.
Woodcomp Propellers' R&D Manager and Project Coordinator for the ARGOS project Vilém Pompe says that "Compression ignition engines need robust design of propellers capable to dissipate increased level of torsional vibrations generated by the engine work compared to the avgas engines. The benefits of ARGOS project should be seen in the fact that the compression ignition engines need propellers optimised for lower rotational speed compared to the avgas engines which contribute to the reduction of noise of the aircraft. Aerodynamic optimization of the propeller will also contribute to the already improved efficiency of the engine and therefore to the reduction of fuel consumption".
"Robust propeller design will avoid fatigue problems of generally available propellers originally designed based on data about vibrations available from avgas engines when they are installed on compression ignition engines" says Pompe, adding that "The fact that the new propulsion unit, including our propeller, is designed and optimised for JET (kerosene-based) fuel contributes to the operational safety of general aviation aircraft by reduced volatility and reduced volume of fuel on-board due to the lower fuel consumption".
Two propeller prototypes have already been produced and have been subjected to laboratory and engine tests to establish that it is possible to design a propeller capable of dissipating the increased vibrational energy in its structure without the need of a special damper between the engine and propeller flanges. Two material versions of the propeller blades with different abilities to absorb vibrational energy have been produced and the tests should elicit insights related to the impacts of the different principles of fuel ignition (compression versus spark ignition), insights which will inform the propeller design and durability.
Regarding the importance of this project to European aviation, Pompe says that "a propulsion unit consisting of a reciprocating (piston) engine and an aerospace propeller represents the most efficient solution for specific categories of aircraft", especially in the general aviation segment of "1 to 6 passengers or for other corresponding payload with the maximum flight speed up to 500-550 km/h", where he says that in terms of the "consumption of energy, emissions, length and quality of needed runway, maintenance and other operational costs the piston engines with propellers are unbeatable. Fuel saving, operational economy and environmental protection are the key interests of modern civilization. Constantly increasing demand for transport of passengers and goods are typical symptoms of advanced civilization, therefore research and development in the field of advanced and efficient propulsion units is important for European aviation".