January 20, 2017, ©. Leeham Co: We have now covered the technology around airliner turbofans. Now it’s time for the real stuff: their operational life. Most decisions that an engine designer does is about how the engine shall function in practice.
To understand a typical cycle of an airliner engine and the stresses it endures, we will follow an engine during a typical mission.
We chose a single aisle mission because most flights are with single aisle aircraft and the cycle these fly is the most stressful for an engine.
We use GasTurb to simulate a typical short- to medium-range hop with a CFM56- equipped aircraft. Why the CFM56? Because it’s the world’s most used aircraft engine and it’s such a commodity today that we would not reveal any sensitive data with a simulation.
The stresses the engine goes through during a mission is determined by the aircraft’s flight mission profile. It consists of;
Of these, the take-off phase is the most stressful, followed by the climb. For long haul engines, the cruise phase is so long that it will also determine wear and tear on the engine components.
GasTurb is the best-known non-OEM software for gas turbine simulations. It’s used by specialized consultants like us, but also by engine and airframe OEMs. Engine OEMs use it in addition to their own specialized simulation tools.
GasTurb can be used on an PC and has excellent user interface and graphic presentation capabilities. An OEM’s own tools can do more adapted/deeper simulations but require specialized users to handle it. For our purposes, GasTurb is a perfect tool to show how things work.
Before we can run a typical single aisle mission with our CFM56 engine, it has to be created in GasTurb. I have spent the time to build a model of the CFM56-5B, which is accurate enough for our purposes.
We will fly a typical Airbus A320ceo mission, using two CFM56-5B4/3. It’s the 27klbf version of an engine which goes between 22klbf and 32klbf thrust.
The data from GasTurb will show where the engine has to work hard and what this means for different parts of the engine. We will later use this to understand how a mission influences an engine, its operational use and maintenance needs.
GasTurb allows the simulation of the key points of a mission and then lists all the key parameters of the engine at these points. The list covers all relevant data (pressures, temperatures, air speeds …) at the different stations that are shown in Figure 1.
GasTurb delivers 180 data-points for each mission point. We will select a few of these to understand what is going on. We will take it in steps and explain what we are seeing.
There is not much action at engine start and later taxi, so we begin our mission from Take-Off. In Figure 2, we have the mission and the first high level data from GasTurb’s output. The first four rows covers the missions flight data.
After take-off, we climb past the V2 point (M0.25, 400ft), hopefully with both engines running. Should only one be running, it must produce a certain net thrust at this point, so the aircraft can climb out safely from the airport on one engine. Here, the engine produces 22klbf net thrust which should be enough for the A320.
After the second climb segment we climb over the FL100 (10,000ft) point where Air Traffic Control (ATC) releases us from the 250kts ATC speed limit and we can accelerate to our climb speed. At Top of Climb (ToC) this speed is M0.76.
We have three Cruise mission points, the initial cruise weight point at 35,000ft, then the higher average cruise weight point and the final cruise step climb point at 37,000ft. As fuel burns off, induced drag goes down and we need less engine thrust to keep our cruise speed of M0.78.
Note that we have two thrust values. One that the engine is producing (Gross thrust) and one that the aircraft is experiencing (Net Thrust). Net thrust is what drives the aircraft forward. The thrust that is lost from Gross thrust is the loss due to forward speed, or thrust lapse.
At standstill, the two thrusts are equal (there is no forward speed). As soon as we are rolling, we are experiencing thrust lapse. Remember that thrust is: Mass of air which is pushed back by the engine times the overspeed of that air over the ambient air.
When the aircraft moves forward, the overspeed decreases. The reduction in overspeed generates the thrust loss ((lapse). This loss is critical to One Engine Inoperative (OEI) performance for airliners and is an important specification point for aircraft engines for airframe OEMs. Boeing underlines this by basing its thrust rating of the engine on this point, the BET (Boeing Equivalent Thrust) that we wrote about two weeks ago.
On the next line we have fuel consumption as specific fuel consumption. This means we show fuel consumption in lb per hour per generated pound of thrust. Note that the fuel consumption at no or low speed and dense air (sea level) has a lower value than at high forward speed/thinner air. The engine has a harder job at forward speed and thinner air.
When fuel consumption for an engine is given as around 0.3- to 0.4 lb/lbf and hr, it’s static sea level data and of almost no value. Cruise values, where the engine spend the long time and therefore consumes the fuel, is always above 0.5 lb/lbf and hr. Normal values are between 0.5 to 0.8 lb/lbf and hr.
Note also that we have delta Temperature from the standard atmosphere ISA as part of the mission data. We will later look at what happens to the engine when the temperature goes up over the ISA temperature profile. It increases the stress on the engine, it finds it harder to produce the thrust expected of it.
Next week we will dive deeper in the data.