October 21, 2016, ©. Leeham Co: In our Corners on East bloc aeronautical industries, we could see that the hardest part to master in a new civilian airliner is the engine.
Both new airliners from Russia and China (Irkut MC-21 and COMAC C919) start their lives with Western engines.
Why is this so? What are the challenges that make engines harder to create than aircraft?
We will spend several Corners on the main reasons that airliner engines are harder to do than aircraft.
The technology challenge
A modern high-by-pass turbofan poses a combination of technical challenges to its designers that are exceeding in many ways those presented to civil airliner designers.
For starters, the aerodynamics are more complex. An airliner design presents subsonic and transonic aerodynamic problems. These are relatively straight-forward to model in CFD (Computational Fluid Dynamics) and to measure in windtunnels.
A modern high by-pass turbofan mixes subsonic, transonic and supersonic aerodynamics on a single fan/compressor blade. And the blades are running close to hard objects affecting the airstream, such as a fan case or within a stator structure for the compressor.
Add to that, that the blades are so closely spaced that their aerodynamics interact. It’s like flying a Baron von Richthofen Fokker Dr. I triplane through the sound barrier and cater for the interacting shock waves from each wing. As they are staggered, the interaction is not symmetrical.
Secondly the material stresses are higher. Not only shall a turbine blade endure 1,700 °C/3100°F but it shall do it at 1,7 MPa/250 Psi pressure and at a centrifugal force of 40,000N/ 9,000lbf for a 0.1kg turbine blade.
It shall be considered that the melting point of the metals in the engine is below these temperatures, let alone the point of plastic deformation. Hence it’s all about exotic alloys, sophisticated cooling schemes and a very special mechanical design.
But the challenges don’t stop there. The pressure rise over the compressor stages are in the order of two times. This means the air will find every little passage way to flow back. Efficient rotating seals must be invented, seals that can withstand rotating at 10-20,000 rpm in the harsh conditions for 10 years and 20,000 missions.
Seals must also be designed to keep oil where it should be, in the bearing areas. If oil seeps into adjacent hot areas, the catastrophe is programmed, as seen for the Qantas Airways Airbus A380 climbing out from Singapore or the Bombardier CSeries ground testing its engine.
Finally, is must all function in harmony, from the coldest day (-40°C) to the hottest (+50°C), from M0.00 to M0.85 and from idle to full thrust. Today, all this is controlled by a redundant set of control computers in the so called FADEC (Full Authority Digital Engine Control).
The challenge for the control is not to make the control program. It’s to find measurement methods for the parameters that one need for the control. There is no way to reliably measure the perhaps most critical parameter in the engine, the inlet temperature of the first turbine stage after the combustor.
Sensors that operate at 1,700C and 1,700 000 Pa for years and measure the temperature on an accuracy of degrees, in a vibrating environment with aggressive gases passing by at M0.5 are not available.
Therefore, control of the engine is done via indirect sensors like EGT (Exhaust Gas Temperature sensor) which sits at the end of the engine. Part of the development of the engine is to establish the reliable relationship between the parameter that you can measure and the one you wanted to measure.
A look in more detail
In subsequent Corners, we will look at these problem areas in more detail. We will do that as a journey through a typical high by-pass turbofan.
We will use the GasTurb engine modeling program to give us the typical data for a modern engine and we will point out the technological challenges the different engine parts pose.
We will also cover what is the present state of the art for the area and what the engine manufacturers are working on for future engine generations.