Bjorn’s Corner: Aircraft engines in operation, Part 2

By Bjorn Fehrm

January 27, 2017, ©. Leeham Co: In the last Corner, we began looking at the in-service operation of a Turbofan. We covered how thrust and fuel consumption varies in the different phases of an airliner’s mission.

Now we will dig a little deeper into how a mission will stress the engine’s different parts.

Figure 1. Principal picture of a direct drive turbofan. Source: GasTurb.

With this knowledge, we will later look at how operators make sure their engines are safe and in good operational condition over the 20 years life of an aircraft.

Engine stress points

In the last Corner, we showed data from a simulated Airbus A320 misson with a CFM56-5B4 engine. The professional simulation software GasTurb tabulated what happens inside the engine during Take-Off, Climb and Cruise.

In Figure 2, we have expanded the table to show more in-depth what is happening inside the engine. It’s about 10% of the simulation output from GasTurb, but the additional data is for engine designers, not for us who want a general understanding what is going on.

It might seems like an lot of data, but we take it in steps and discuss each row and what it shows.

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

At the top blue section, we have as before our key mission data. We added the bleed air the aircraft needs for the air conditioning and to keep the cabin pressure. A single-aisle aircraft like the A320 consumes around 1lb/s an engine, taken from the engine’s compressor as described before. We have also 67hp power off-take to drive the aircraft’s hydraulic pumps and electrical generators mounted on the engine’s peripheral gearbox.

Both these off-takes drain the engine of energy and often the air supply is shut off for take-off so the engines can focus on delivering thrust.

The next additions are Engine mass flow (lb/s) and Specific thrust (ft/s). Specific thrust is another name for the air mass’ overspeed relative to ambient air. We can see the air is traveling on average at 1020ft/s at Take-Off. This is close to the speed of sound at 1126ft/s on a standard day. Turbulence created by high airspeed is the primary source of noise from Jet engines.

As engines are made with higher bypass ratios, the mass throughput increases and the specific thrust decreases for the same thrust. This makes high bypass engines less noisy.

Higher temperatures

Next, we have temperatures in the engine. Just before the air enters the combustor, it’s compressed to 28 times the value at the inlet. This increases the temperature to close to 600°C (T3, Figure 1). The last stages of the compressor must therefore be made with high temperature resistant Nickel alloys as discussed before.

The turbines are the power generators in the engine. The more fuel which is injected in the combustor, the higher the entry temperature to the turbines and the more energy there is in the gas that drives the turbines.

T41 is the temperature entering the first turbine rotor. 1,300°C is a lower value than the high pressure turbine can sustain in a CFM56-5B (around 1,400°C is max). The 5B4 version is a “derated” version of the strongest variant, the 5B3 at 32klbf.

The engine control computer, the FADEC, cuts of fuel supply earlier on the B4 version than the B3. Therefore the B4 runs cooler. The result is the engine does not spin to maximum allowed RPM on the low and high speed shafts during max thrust periods (5,200 and 15,200 are the maximum allowed RPM on 5B3). This subjects the rotors and blades to less mechanical stress. Combined with lower temperatures, this results in an engine that can stay longer on wing before needing an overhaul.

It’s difficult to measure the temperatures around the combustor and high pressure turbine. Thus, engines are often controlled via the temperature around the Low Pressure Turbine (LPT), T45, or even further back like T49.5 (last LPT stage) for the CFM56. The temperature range is here around 1000°C and it’s called the Exhaust Gas Temperature, EGT.

When an engine deteriorates, the FADEC will compensate deteriorated performance by injecting more fuel to restore performance. This raises the EGT. Gradually it will come close to a limit which triggers a scheduled overhaul.

Effect of higher temperatures

Figure 2 shows that the engine endures highest mechanical and thermal stress during Take-Off power at V2, when ambient temperature is 15° higher than normal ISA temperature. The net thrust is only 22klbf, but the engine is working hard. It’s developing 30klbf Gross thrust and working with the highest temperatures and RPMs during the mission.

The forward speed of M0.25 is taking away 8klbf of thrust and the high ambient temperatures is thinning the air, so mass flow goes down. The engine therefore increases RPM to increase specific thrust to compensate. This increases temperatures through the engine.

The V2 Take-Off thrust needed when one engine goes inoperative is often the dimensioning case for mechanical and thermal margins in the engine.

9 Comments on “Bjorn’s Corner: Aircraft engines in operation, Part 2

  1. Hello Bjorn

    I was told that top of the climb was the worst case sometimes
    I doesn’t look like this on the mission you modelled
    Is top of the climb the design point for some missions / planes?

    I remember 777-200LR having some difficulties to reach an efficient initial cruise level out of hot places

    Best regards

    • Hi Crise, good question.

      Top of Climb is normally the aerodynamic design point, i.e. it decides the size of the flow paths in the engine. In a previous Corner we showed the desired Mach numbers at different parts of the engine (0.5 in the compressor, 0.2-0.3 in the combustor etc) and ToC generates the highest Mach in the engine’s different sections. Therefore it decides the physical size of the engine’s flow paths. But it’s not the mechanically or thermally most stressing point, normally TO or the hot V2 point decides the mechanical dimensions and the thermal solutions.

  2. SORRY BJORN
    I did not understand the thrust lapse issue, in part 1
    “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).”
    What is the reason for my misunderstanding?
    Very simply that when the aircraft is moving at a given speed, the air is entering the engine at that precise speed.
    the overspeed over the ambient air should be exactly the same§
    could you clarify this issue?

    • Hi Flyer,

      there is a good explanation of thrust here:

      https://www.grc.nasa.gov/www/k-12/airplane/turbth.html

      In essence it’s about the momentum change of the air through the engine. The momentum of the air entering the engine has to be deduced from the momentum of the air leaving the engine. Some call the term you deduce “ram drag”. As it has the effect of reducing the value of the speed of air leaving the engine (the mass flow of air going in and out is the same, you can ignore the mass flow of the fuel that gets added, it’s a fraction of a percent) I shorthand the difference to “overspeed” over ambient air.

      Hence thrust is mass flow of air * (air speed out – air speed in) or mass flow * overspeed of air leaving the engine.

      The advantage of thinking in terms of “overspeed” is that it’s the same as “Specific thrust”, a term that all engine experts use because it says what type of engine it is. An engine with high Specific thrust is suitable for fast aircraft. A low specific thrust engine can only work on a low speed aircraft but is very effective there (high propulsive efficiency).

      Example low bypass turbofan for fighters (F100, F404..) = high specific thrust. High bypass turbofan (PW1100G, LEAP..) = low specific thrust.

      • Thank you Bjorn
        it is more clear now!
        there is certainly a lot of drag at the intake.
        Because inside the engine at full power, the air velocity in some parts is pretty near sound velocity, already at very low speed.
        if you simply add the aircraft velocity, the air velocity will go beyond limits
        the problem is that it is not easy to see what is causing that drag!

      • Generally low specific thrust engines like civil turbofans are large diameter and carried in pods. Military engines require a small diameter to fit inside the narrow fuselages preferred by fighter jets – except F-35!
        Strangely the early British bombers and airliners went for internal engines while the US ones went for external pods, eg NA Tornado through Boeing B47 and B52 and to B58 along with 707/DC8

        • Hi dukofuerl,

          question is what is cause and effect re engines inside the fuselage. The engine specific thrust is very much decided by the mission envelope. There is a good papers on that by the creator of GasTurb, Dr Kurzke ex MTU : https://www.sto.nato.int/publications/STO%20Educational%20Notes/RTO-EN-AVT-185/EN-AVT-185-02.pdf

          Re the Brittish engines in the wing versus the podded jet engine started by Boeing on the B-47. It would be great fun to dig out the complete discussion from the time re what was the better way to go.

          Sure one knows some of the arguments (and which way went pervasive) but it would be fun to revisit the complete discussion.

  3. Bjorn,

    I don’t see Table 2. Also EGT is around 650 C during cruise and 900 C at takeoff, not 1000 C.

    You mention 1 lbm/s cabin air bleed from LPC, but you did not say what the turbine cooling bleed air from HPC was under various conditions (TO, TC, CR).

    Net thrust = Gross Thrust – Ram drag
    Gross Thrust = mdot * exhaust velocity
    Ram drag = mdot * flight velocity
    Net thrust = mdot * (exhaust velocity – flight velocity)
    specific thrust = exhaust velocity – flight velocity

    where mdot is the mas flow rate of air. The mass flow rate of fuel is less than 2-3% of the mass flow rate of air and has been neglected in the above expressions. Also the nozzle is assumed to be perfectly expanded, meaning exit pressure is equal to ambient pressure. Hope this makes it more clear.

    • Hi Kant,

      should be Figure 2, fixed.

      Re “around 1000°C”, this was Turbofan EGTs in general, not the 27k derate version of the 32k CFM56-5B3, it runs cooler.

      Thanks for summarizing the thrust discussion.

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