Bjorn’s Corner: Aircraft engines in operation

By Bjorn Fehrm

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.

Figure 1. A principal picutre of a direct drive turbofan. Source: GasTurb.

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.

Typical engine cycle

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;

  • push back
  • taxing
  • take-off
  • climb
  • cruise
  • descent
  • and landing

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 simulation

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.

The mission

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.

Figure 2. Mission data for CFM56 simulated with GasTurb. Source: Leeham Co.

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.

7 Comments on “Bjorn’s Corner: Aircraft engines in operation

  1. Bjorn,
    Thanks for these articles. They are very interesting.

    Why is Gross Thrust even mentioned? It seems a bit fictitious to me as it is never realizable except at zero forward speed when it is equal to the Net Thrust. Also, saying that the engine produces the Gross Thrust even though aircraft only experiences the Net Thrust implies a force imbalance that doesn’t exist.

    • The imbalance is real and therefore it’s important to understand what is happening.

      The engines work (the hp the LPT has to produce to drive the fan) is hp to produce the Gross thrust. At the ToC the engine is producing 1/4 the thrust of TO but the engine has to produce 1/2 the work (Gross thrust is half while net thrust is 1/4). It means that as thrust declines (the Net thrust that is denoted Thrust in every day speak) the engine’s work does not correlate. It correlates with the Gross thrust.

      I have now included the work of the Low Pressure Turbine (LPT) in the table. Note that it correlates with the Gross Thrust, but not fully. Some of the gross thrust is coming from the remaining energy in the combustion gases after the LPT, going out as core thrust.

      The difference between Gross and Net thrust is Thrust lapse. Lapse increases with By Pass Ratio, which limits how a high BPR engine can function at high speeds. But Lapse is already a very important factor at V2 or the normal cruise speed of M0.8. The higher the BPR, the lower the thrust fraction that remains at V2 or that remains to fly with one engine out at cruise.

      You therefore have to plan with a route around high mountain chains in a high BPR twin (you always have to plan for loosing an engine) as the thrust that you will have remaining at higher speeds/altitude is limited. You then of course reduce the speed/altitude to the best compromise between drag/OEI thrust and make sure that this altitude is higher than the minimum altitude you need for ground clearance.

      We also saw the effect of Lapse in the SST article series. A high BPR engine does not work at M2.0, it has to work extremely hard to generate little thrust (a lot of Gross thrust for little Net thrust).

    • The CFM56-5B4/3 is the 27klbf version for the A320. I choose that deliberately so that I can demonstrate how an engine gets stretched to its ultimate version, the 32klbf version for the A321. So we will get there and we will discuss how the engine companies does this stretch, it will be part of the article series.

      • Do you plan to cover off the impact on maintenance cost of juicing the engine to operate larger aircraft?

        • Hi Bruce,

          absolutely. This is why I choose a “cool” engine to start with. We will be able to see what happens when one “throttle-push” this variant to higher thrust levels and what happens to time on wing.

  2. There was an interesting comparison between ATR and the Q400 in the regards to mountain flying and engine loss.

    Q400 had lots of power and was not very restricted (some places of course but not a lot of Himalayan height mountains in the world)

    ATR was restricted and much more and planning for that had to be done and its route were not as direct.

    It may not be as economical but there are significant advantage including safety to having plenty of power.

    I was with my brother in a Super Cub one day doing a landing and we got nailed with a nasty gust of cross wind.

    My experience said we were in serious trouble.

    We weren’t, he knew the airplane, he just added throttle and we popped up and out and well above trees in a couple of seconds.

    Over the years they kept adding more power versions of the 4 cylinder to that airplane and it was very obvious that it was not just short field work but significant safety if things started to go wrong.

    I drove motorcycles for many years and it was true there to. With excess power you could avoid issues. I ran a 400 CC machine on the freeway once and got caught in between and barely squeezed out of it. Never had that occur on the 700/750/800s.

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