Turbofan:Blades

Blade design and manufacturing

This page explains how blades should be designed for efficiency, and how can a simple and low-cost manufacturing be relevant.

Blade fixation

The root/fixation/insert of the blade is discussed on the related subsystem design page: compressor, turbine or fan. In summary, real engines use a fir tree shape to hold the blade centrifugally, while keeping them free axially. They are generally fixed using a locking screw. This design allows for easy replacement of damaged blades, but is quite complex to manufacture, and requires lots of parts. Since our engine will be smaller, using fixing screws may not be feasible, and blades are subject to less centrifugal stress. Simpler blade fixation mechanism should be relevant.

Blade designs for efficient air flow

Blades have to be carefully designed, because the overall efficiency of the engine largely depends on their design.

Stages. They are not only made of blades on the rotor, but also blades on the stator, generally called vanes. A stage is then a pair of a rotor stage and a stator stage, in this order. The stator prevents a rotating air flow to form inside the engine (swirl), driven by the action of the rotor blades. Stator vanes redirect the airflow in the more appropriate direction for the next rotor stage. They increase the energy of the gas (enthalpy) by removing the swirling effect that impairs it.

Blade shape. Most basic design of a fan has flat-shaped blades. Twisted blades with a flat section are an improvement, taking into account the difference in apparent airflow velocity and torque all along the blade. Next step is to have a non-flat section, but an airfoil section. This has been proved to be the only design meeting efficiency requirements of the turbine engines, in 192X by XXX. Finally, modern engines are designed with curved edges for the fan, for optimal known efficiency as well as for noise reduction.

Rotor/stator gaps. Highest efficiency is reached in turbofans when gaps are reduced between blades and the stator, or between the rotor and stator blades. As always, good efficiency means good high precision and higher cost. Anyway, the precision of blades will have to be very good if we don't want it to dislocate when it reaches the high rotations-per-minute achieved by the engines.

Design of stages is linked to the energy the blades have to give (compressor and fan) or take (turbine) to the air flow. To better define and understand that energy, we will use standard thermodynamic parameters of gas, a.k.a state variables of a gas, on which are based quantities like the enthalpy: temperature, volume, pressure. We will also use the velocity because the actual work given by a turbofan engine is related to the mass flow rate of the gas expelled by the engine, which relates to velocity of this gas and the state variables.

Fan design

Main goal of the fan is to increase the mass flow rate, mainly by increasing the velocity. The mass flow is linked to the area of the fan blades and the angular speed of the fan. To increase the velocity, simplest way is to reduce volume. The fan duct will thus have to act as a compressor on the aft-end. On the front-end, it is generally designed as an expander, to increase the pressure, allowing more efficient work on the air flow.

Compressor design

Main goal of the compressor is to increase pressure. Due to friction of the gas on the blades and guide vanes mainly, temperature is increased too. Thus, volume has to decrease, intake area will be greater than compressor discharge area.

Turbine design

Main goal of the turbine is to extract energy from the hot and fast gas discharged by the combustion into mechanical (rotational) work. Pressure and temperature may remain constant through the turbine, and high velocity and pressure will provide better efficiency [to be verified].

Mechanical constraints

Blades on all three parts of a turbofan engine undergo heavy mechanical constraints due to high rpm achieved by the rotor, the high temperature in the turbine section and non-negligible temperature in the end of the compressor section, and the high pressure of the gas on which work is performed.

Fan blades are not made of plain metal in real engines. In the eighties, they were made in honeycomb composite sandwich material, they are now made in triangular sandwich structure. For an engine of the size we are targeting, fan blades may be built with plain carbon fiber, like the Schuebeler HST high quality R/C engine.

Compressor blades are made of titanium alloys, providing high strength and rigidity at these temperatures.

Turbine blades are made of nickel alloys, better sustaining the high temperature, and still at higher strength than steel.

Manufacturing propositions

Hot pressing is used to manufacture real-engines' blades, and hot isostatic pressing possibly too, as explained on the How are made turbine blades video. I believe that a hot forging press can be done cheaply considering the small size of our blades. For the main fan, it thus may not be used.

The above picture lacks the airfoil section of the blade, because the CAD software we use doesn't currently support it.

Work-hardening and head-treating have to be studied, and depend on the crafting method used in the first place.

The high-pressure turbine blades have to face very high temperature and pressure. On real engines, they are made of titanium and nickel-based superalloys. Since the required lifetime is lower in our case, we may achieve a working engine with cheaper metals, like steel or nickel-rich alloy for the turbine blades. For the compressor blades, aluminum alloys are probably be a good solution.

Don't forget that the blade insert will have to be milled at some point.