January 13, 2017, ©. Leeham Co: The time has come to go through the reasons why some turbofan engines are designed with a gearbox between the fan and the low pressure shaft.
The principle design is shown in Figure 1. It’s a graphical representation of a geared turbofan from the engine analysis software GasTurb.
The base idea is to have the low pressure spool of the engine to run at a considerably higher RPM than the fan.
Speed is good
The reason that one wants the low pressure spool to run faster is that the low spool’s compressor and turbines get more efficient when the blades have a high tangential speed. The physics behind this are the same as for an aircraft wing: lift increases with forward speed.
When the RPM and the diameter of the blades bring the tips of the blades to supersonic speed, the aerodynamics get more complicated. The supersonic aerodynamics are connected with shockwaves, which can degrade efficiency.
Turbofan engine designers therefore try to keep only the tip of the blades in the supersonic region. As the fan has a very large diameter, the RPM should stay below 5,000 RPM for single aisle engines (typical fan diameters of 50- to 80-inch) and below 3,000 RPM for wide-body engines ( fan diameters from 75- to 135-inches).
At the same time, the booster and low pressure compressor have diameters of one third of the fan diameter. Consequently, you would like to see low spool RPMs of 9,000-15,000 RPM to get the blade speeds in the right window. The way to get there is to have a gear ratio of 1:3 or more between the fan and the low spool.
A gear between the fan and the turbo-machinery is nothing new in the aircraft engine domain. Turboprops have such a large diameter for the propulsive blades that a gearbox is necessary between the core and the propellers. It was when the high RPM cores made for turboprops or APUs are used for turbofans that a geared fan became necessary.
A gearbox is always a complication and a component that can give trouble. Therefore it was avoided for a long time on jet and turbofan engines which were designed from scratch. The designers added more stages to the compressors and turbines to compensate for the lower efficiency and they increased the diameters of the stages as much as possible. With the very high bypass ratio engines of today the mismatch has become so large that geared designs will be more common.
The first geared turbofan for civil use, Garett AiResearch’s (today Honewell’s) TFE731, was developed from the Douglas DC-10 APU, Figure 2.
It had quite a few gearbox problems in the first variants. The fan is a large gyro and the gearbox needs to be decoupled from the forces this generates. Garret learned this the hard way. The TFE731 is primarily used on business jets since its certification in 1972. It’s still popular today for smaller business jets.
Another early geared turbofan was designed by Lycoming from the T55 turboshaft helicopter engine. Once again, the high RPM of the core made a gearbox necessary for the ALF502, Figure 3.
This engine started life on some military projects but found wide use from its 1980 certification on the BAe 146 regional jet and the Challenger 600 Business jet.
I wrote “The first geared turbofan for civil use” above. The reason was that the first geared turbofan used on an aircraft was for a military aircraft. I learned to fly jets on the aircraft, the SAAB 105 jet trainer. It was equipped with a geared turbofan made by SAFRAN’s Turbomeca division. The engine (Figure 4) was based on the Bastan turboprop engine.
The engine ran for the first time in 1961 and was used on the SAAB 105 between 1966 and 1990.
Pratt & Whitney GTF
Pratt & Whitney (PW) realized the advantages of making a clean-sheet turbofan with a geared fan (a GTF for Geared Turbo Fan). PW also saw the problems Garrett had when applying a gearbox with a fan. PW’s application went far beyond the 7000hp that had been the highest gearbox rating historically. PW needed 30,000hp for its new single aisle engine family.
The company therefore took its time and researched the use of a planetary gearbox through different projects for 20 years. Certification of the first PW Geared turbofan, the PW1500G engine for the Bombardier CSeries aircraft, occurred in February 2013. First entry into service was on the Airbus A320 with the PW1100G, Figure 5.
The gearboxes used for turbofans are of the planetary type using Epicyclic gears, Figure 6. Such gearboxes are compact and can transfer high torques with high efficiency. The input from the high RPM low spool comes from the middle shaft (the yellow sun gear) and output to the fan can be taken through a rotating ring gear (pink) or planet gear carrier (green).
In either case, the other gear member has to be fixed to hold the gearbox. If the output is taken from the ring gear (pink), gear ratios are around 3:1 for practical geometries and from the planet gear carrier (green) around 4:1.
The PW GTFs use a ring gear which is connected to the fan; therefore the gear ratio is 3:1, Figure 7.