November 04, 2016, ©. Leeham Co: We will now start to go through a modern turbofan airliner engine and look at the technologies which are used and what are their technical challenges. We will start today with the engine intake and the fan.
To make things concrete, we will use a GasTurb simulation of a Rolls-Royce Trent XWB 84k engine. This will provide us with realistic example data for the different parts of the engine. I want to stress that all values are assumed as typical for such an engine. I have no specific knowledge of the Trent XWB and will not use any data outside what is public information.
The GasTurb cross section of a three-shaft turbofan is shown in Figure 1. We will use the station numbers in the figure to navigate the engine and the data from the simulation to understand modern airline engines and their typical data.
The intake of the nacelle (2) provides distortion-free air to the engine’s fan. At static takeoff mode, it provides around 1350kg/s (3000lb/s) of air with minimum pressure loss. To do this, the lips of the nacelle must allow the air being sucked into the intake from wide angles around the intake mouth without separation as the air curves around the intake lips.
At the same time, the nacelle provides around 450kg/s (1000lb/s) of air at cruise with maximum pressure recovery from the M0.85 free stream air. Now the lips must allow the air to spill over the sides because not all air hitting the intake area can be consumed, once again with minimum drag (no separation of flow).
The air entering the intake at cruise altitude of 35kft (10.7km) has a speed of M0.85, a density of 0.38kg/m3, a pressure of 238hPa and a temperature of -55°C. This compares with M0.0, 1.225kg/m3, 1013hPa and +15°C for a standard ISA sea level day.
The engine fan does not like air with an axial velocity of M0.85. Preferred speed is around M0.5 (otherwise the relative velocity to the rotating fan blades will be too high). The intake provides this by expanding the flow in a diffusion section before the fan, (2) in Figure 1.
Expansion of the air slows its speed, raises the pressure and the temperature. This changing of the air’s state by changing the duct dimensions will be done at several stages of the engine. Figure 2 shows the principle with a practical example in the engine on the right.
The effect of the divergent intake (a diffuser section) is to raise the air pressure by about 1.5 times at cruise speed. With the intake diffuser section, we can adapt the axial velocity of the air to the desired ~M0.5. The final axial air speed will be a combination of the intakes influence and how much the engine is consuming air through the fan. In our example engine, we have a static takeoff air speed of M0.42, Top of Climb (final climb to initial cruise altitude) air speed of M0.51 and cruise M0.47.
The making of a good nacelle is not easy. The intake must feed the engine at vastly different speeds without causing air distortion before the fan and cruise drag. The design of intake lips is an art.
Today the intake section is also made with perforated walls to dampen the supersonic shock waves that is created by the fan at full takeoff RPM (the see-saw buzz that one hears). And it shall all weigh an absolute minimum.
The purpose of the fan is to accelerate the air entering the engine’s intake to an overspeed (speed difference between aircraft and engine exhaust) appropriate for the thrust needs of the aircraft the engine is mounted on. As described in a previous Corner, the lower the overspeed is, the higher the propulsive efficiency will be for the engine.
At the same time, the thrust of the engine is overspeed times accelerated air mass. If we reduce the overspeed, we either reduce thrust or we must increase the accelerated air mass. The role of the fan is therefore to accelerate a large mass of air to a modest overspeed, sufficient to create the thrust the aircraft needs for takeoff and its climb to initial cruise altitude, the so-called Top-of-Climb thrust (ToC).
The thrust needed for cruise is lower than that needed for Top of Climb. For four engine airliners, the ToC thrust determines the flow aerodynamics through the engine. This is the aerodynamically most demanding point in the flight envelope. For airliner twins, the one engine inoperative thrust requirement after takeoff (the V2 case) can also be aerodynamically dimensioning for the engine.
Fan aerodynamic design
The fan for our example engine has a diameter of 118in and rotates with up to 2600rpm for max. takeoff thrust. This gives a blade speed at the tip of 413m/s, which at the sea level takeoff case shall be compared with a sound velocity of 340m/s. The blade tips therefore move with M1.2 at maximum RPM.
We also saw that the axial velocity of the air into the fan was M0.42. The two speeds vector combine when the air curves around the blade tip which results in an air speed relative to the blade of around M1.5. At the same time, the inner sections of the fan have blade velocities of around 145m/s, which corresponds to M0.4. The relative air velocity lies a bit higher but still in the subsonic range.
At different throttle settings and combinations of altitude and temperature, the fan will have different parts which are subsonic, transonic and supersonic (the speed of sound varies with temperature and to some extent on pressure).
The fan blade designer then needs to design a blade which can work efficiently at these different speeds. At the tip, it will have a profile that is efficient at supersonic speeds (thin profile with sharp edges), in between a transonic design and at the root a subsonic design.
During the engine’s different flight phases, the boundaries between these flow types will move up and down the blade. The design will therefore focus on having max efficiency for cruise RPM conditions, with acceptable function for max thrust and idle cases.
A modern fan has a conversion efficiency of the shaft hp (around 80,000 at takeoff thrust) to thermodynamic power (pressure, around 1.4-1.5 times, and increased air speed, average 280m/s at takeoff thrust) of around 90%.
Fan blade evolution
The historical fan blade had the same form as a compressor blade, Figure 3 and 4. The weight of the solid titanium blade stopped a wider cord design. The long and slender shape made it difficult to get the blades stable enough to avoid flutter (aero-elastic resonant deformation), therefore mid-blade clappers were needed. These caused efficiency losses due to shock waves that formed around the clappers.
As fan blades have a nacelle wall that stop tip streams, they don’t have to have a high aspect ratio to be efficient. As hollow titanium blades were developed, fewer wide-chord blades could be used. These are more robust and therefore could be made without clappers. Gradually the form was adapted to the different flow states over the blade.
The reduced number of blades also increases the flow area between the blades, thereby increasing the air mass throughput for the fan.
Fan blade construction
The historical fan blades were made of solid titanium. Hollow titanium blades as pioneered by Rolls-Royce use different honeycomb or web structures as internal stiffeners. GE pioneered the use of prepreg Carbon Fibre Reinforced Plastic (CFRP) construction for the GE90 blade, later also used for the GEnx.
CFM and Rolls-Royce has taken CFRP blades to the next level by using dry fiber tows that are placed in a blade form, Figure 5. The CFRP resin is then injected when the form is closed. This method allows a high volume of fan blades to be produced at acceptable cost.
Pratt & Whitney has developed an aluminum based wide cord fan for their GTF engine series. These blades are made as a hollow core with aluminum foam filler, with aluminum sheets bonded around the core. As for CFRP blades, a Titanium leading edge protects the blade from erosion and bird strike impacts.
The fan section has several difficult challenges. First is to design a fan blade form which is efficient over a wide RPM and flight envelope. To achieve high efficiency the blade must be thin. To achieve the desired strength is must be thick. The result is highly sophisticated designs with air filled cores or CFRP designs with difficult to bond aluminium and titanium covers/leading edges.
The shear size of the fan blades makes them heavy. These heavy fan blades must be contained by the fan casing in the event of a blade off at full RPM. The fan casing therefore has a sturdy kevlar lined section outside the fan. The heavier the blade, the heavier the fan case. And modern turbofans are already to heavy. The quest for a lighter fan blade to turn the tide is the fan designer’s daily challenge.
In the next Corner, we will look at compression of the air in the engine core.