The Lovable Underdog of Turbomachinery

Everyone knows that APUs need love too…..

For Valentine’s Day, we want to look at an underdog of turbomachinery. A machine that is often overlooked, and not really in the limelight the way some of its larger cousins are, nor is it given the trendy position of being the “technology of the future” like its smaller cousins. Without this technology, airplanes would be entirely reliant on external power plants to maintain an electric power supply on the ground, and to start the main engines. So, what is this underappreciated machine?

APU plane
Okay one last hint – you can see its exhaust port.

If you haven’t been able to guess it, our Valentine this year is the aircraft auxiliary power unit, or APU for short. Although these are not present on all aircraft, they are typically used in larger airplanes such as commercial airliners. This allows aircraft to rely less on ground services when the main engines are not running. As a result, less equipment, manpower, and time are required to keep the plane in standby mode, and the aircraft can also service airports with less available resources in remote locations.

Where this Underdog Started

The aircraft auxiliary power unit can be traced back to the First World War, as they were used to provide electric power onboard airships and zeppelins. In the Second World War, American bombers and cargo aircraft had these systems as well. APUs were small piston engines, as the gas turbine had yet to be developed. These engines were typically V-twin or flat configuration engines, similar to what you might find on a motorcycle, and they were called putt-putts. These two-stroke engines usually put out less than 10-horsepower, but that was all that was required to provide DC power during low-level flight.

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Aircraft Fuel Systems

The airplane is a complex technical object. Like a human or other organisms, it consists of numerous vital systems; with one of the more critical ones being the fuel system. It is important part of any vehicle, let alone aircraft, aside from  the newest electric powered vehicles.

An aircraft’s fuel system provides fuel that is loaded, stored, managed and transported to the propulsion system of the vehicle[1, 2]. As aviation fuel is liquid, this system can be considered as hydraulic. Therefore, it’s able to be mapped out and modeled for analysis in a program like AxSTREAM NET™.

The Typical Fuel System of a Narrow-body Passenger Plane

For an example of a conventional aviation fuel system, consider a typical narrow-body airliner with two engines. Some of the popular planes in this category include the Boeing 737, the Tupolev Tu-204, Airbus A320, Comac C919, Sukhoi Superjet 100, Bombardier CRJ, Embraer E-Jet and Mitsubishi Regional Jet[3].

The storage fuel system is shown in figure 1 is for the Boeing 737-300. The fuel is kept in an integral tank that is divided to five separate subdivisions. They are the central, wing (main) and surge tanks[4].

Storage fuel system of a Boeing 737
Figure 1 – Storage Fuel System of a Boeing 737-300 [4]
The hydraulic scheme of the Boeing 737’s fuel system is shown in Figure 2. For fueling and defueling the storage system there are ports on the starboard wing. The system does not have pumps to onboard fuel, so fuel is pumped into the plane via a fuel truck. The other critical part of the fuel system is the line which delivers fuel to the two engines and the auxiliary power unit. In this line there are two boost centrifugal pumps by each engine.
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Centrifugal Compressor Reverse Engineering and Digital Twin Development

Centrifugal Compressors are the turbomachines also known as turbo-compressors, and belong to the roto-dynamic class of compressors. In these compressors the required pressure rise takes place due to the continuous conversion of angular momentum imparted to the working fluid by a high-speed impeller into pressure. These compressors are used in small gas-turbines, turbochargers, chiller units, in the process and paper industries, oil & gas industries and others.

The design and manufacturing of such compressors are always challenging because of its 3-dimensional shapes, high rotational speeds that interact with different loss mechanisms, and stringent working environments. In many circumstances, it is necessary to analyze an existing compressor, with the end goal being to redesign it, enhance its performance, or to use it in completely different applications. In order to meet such requirements, reverse engineering is a viable option. With reverse engineering, one can review competitor’s design to remain in market competition.

Reverse Engineering

Reverse engineering allows us to collect incomplete or non-existing design data and manufacture an accurate recreation, safely, of the original product or component.

Sometimes, it is also referred to as back engineering, in which centrifugal compressors or any other product are deconstructed to extract design information from them. Oftentimes, reverse engineering involves deconstructing individual components like the impeller or diffuser of larger compressors. End-users often use this approach when purchasing a replacement impeller or any other compressor part from an OEM is not an option. In some cases, where older impellers that have not been manufactured for 20 years or more, the original 2D drawings are no longer available.  When this is the case, the only way to obtain the design of an original compressor is through reverse engineering.

Reverse engineering requires a series of steps to gather precise information on a product’s dimensions. Once collected, the data can be stored in digital archives. Figure 1 (left) shows the typical process of reverse engineering. In figure 1 (right), one can see the scanning process of the centrifugal impeller using a laser scanner.

Figure 1 (left) Reverse Engineering Process (right) Scanning of impeller
Figure 1 (left) Reverse Engineering Process (right) Scanning of Impeller. Source

To reverse engineer an impeller or any other part of compressor, an organization will typically acquire the component and take it apart to examine its internal mechanisms. This way, engineers can unveil information about the original design and construction of the product. One can start by analyzing the dimensions and attributes of the impeller and make measurements of the blade widths, diameters and angles, as these dimensions often relate to the compressor’s performance. Read More

Hans von Ohain – The Other Father of Jet Engines and the Gas Turbine

The question of who invented the jet engine is often met with two different answers, and neither is really wrong. In fact, we posed this question on our LinkedIn page, and got the same mixed results seen elsewhere.  Both Sir Frank Whittle and Hans von Ohain were responsible for inventing the turbojet engine at the same time. While Dr. von Ohain knew of Sir Frank’s work, he did not draw information from, while Sir Frank was unaware that anyone else was designing a turbojet engine.  While we’ve covered Sir Frank Whittle before, today we’ll be looking at the life of Hans von Ohain, his invention of the turbojet, and his contributions to turbomachinery engineering.

Dr. Hans von Ohain
Dr. Hans von Ohain

Dr. Hans Joachim Pabst von Ohain was born on December 14, 1911 in Dessau, Germany. He went to school at the University of Göttingen where he received his PhD in Physics and Aerodynamics in 1935. During his studies and following his graduation, he was captivated by  aviation and airplane propulsion, with a specific interest in developing an aircraft that did not rely on a piston-driven propeller. According to the National Aviation Hall of Fame, he “conceived the idea for jet propulsion in 1933 when he realized that the great noise and vibrations of the propeller piston engines seemed to destroy the smoothness and steadiness of flying”. (1) Read More

Modern Challenges in Aviation Propulsion Systems


Aviation is coming into a new age of carbon free energy, similar to what is being explored with ground transportation. Currently, aviation transportation generates about 2.5% of the global CO2 emission [1]. Several countries have introduced targets to achieve net-zero emissions by 2050 [2].

Many aerospace teams have joined the great engineering challenge to change the future of aviation.  With this, a number of different types of aircraft with different types of propulsion systems have been proposed.

Hybrid Propulsion System

The first step to a carbon-free propulsion system is hybrid technology. This kind of power plant increases efficiency, decreases emission of greenhouse gases and uses a traditional engine to produce electricity and electric motors to drive the fans or propellers [3].

The chemical engines operate at optimal conditions at any mode of a route. On the other hand, the electric motor is able to work in generator mode, using the kinetic energy of the vehicle during deceleration.

Hybrid propulsion energy system
Figure 1 – Hybrid propulsion energy system

The hybrid aircraft is classified by several attributes. Using thrust devices, we will consider the two base types of propellers and fans. Read More

Rotor Dynamics Study of 4-Stage Compressor – from Theory to Application

Rotating machines have huge and important roles in our daily life although we may rarely think about them. Steam turbines at electrical power plants rotate the electrical generator shafts which produce electricity coming into our homes and offices. Driving to or from work, the reciprocating cycle in your vehicle’s internal combustion engine results in rotation of the transmission and the wheels of vehicles, while the electric car wheel operation is a result of induction motor rotation. If you get on an airplane, rotation of the turbo reactive gas turbine engine produces the effective thrust to sustain flight by moving, compressing and throwing the gas behind the plane. We can even find the useful effects of rotation in our kitchens when we are blending the food or washing our closes.

Although these rotating machines are different, the approaches to modelling their rotor dynamics are pretty much the same, since similar processes occur in rotating parts which differ in their vibrations from the non-rotating machines.

Do you remember the example of rotating washing machine? Have you ever seen it jumping on the floor trying to squeeze out your closet? We bet you have. This is the simplest example of the increased unbalance affecting the amplitudes of machine vibrations. Washing machines are designed to experience these noticeable vibrations during their operation without breaking. But the steam turbine or compressor rotors which have the tight clearances between the impellers and the casing can not boast of that leeway. In addition to that, the excessive vibrations significantly influence the machine’s useful life due to the increased fatigue.
This is why the rotor dynamics predictions are one of the most important parts of rotating machine analyses. And although they may seem easier than comprehensive stress-strain investigations of machine components, in some cases the rotor dynamics analysis can be trickiest part.

Usually, the rotor dynamics analyses are divided into lateral and torsional stages depending on the nature of rotor response to be used. They are discussed in different types of standards (API [1], ISO [2], etc.). Let’s consider the example of the lateral vibrations of a 4 stage compressor rotor with an operational speed of 8856 rpm.

Fig. 1 - 4 Stage Compressor Rotor
Fig. 1 – 4 Stage Compressor Rotor

This rotor rotates in the 4 pad tilting, pad oil film journal bearings. The characteristics of these bearings should be determined carefully to ensure that there will not be an excessive wear, heat generation or friction in them. Read More

Considerations for Electric Aircraft Fan Design

Due to concerns about air travel’s impact on climate change, research and development into electric aircraft has been ongoing for several years. Within the last decade several startups as well as larger corporations have been developing electric or hybrid electric aircraft (Ros, 2017). The ultimate goal is to Conduct long (>500 miles), full-electric commercial flights with large aircrafts capable of carrying 100’s of passengers, but this will require at least 5-10 more years of development. Luckily, smaller electric aircraft designed for short-range flights (<500 miles) with anywhere from 1-20 passengers have already been tested successfully utilizing electric batteries, a hybrid-electric system and even a hydrogen fuel cell.  With these advances, emission-free air travel is closer than you think.

Electric Aircraft

Examples of full-electric aircraft designs include the Airbus E-Fan 1.0 and E-Fan 1.1 (Airbus Group), shown in Figure 1. These two-person aircraft utilize two ducted, variable-pitch fans, shown in Figure 2. Each fan is powered by a 30-kW electric motor. The motors are powered by several lithium-ion battery packs stored in the wings. While the aircraft only provides an hour of flight time, the batteries can recharge in approximately one hour and can be easily be swapped in and out.

Figure 1: Airbus E-Fan (Airbus Group)
Figure 1: Airbus E-Fan (Airbus Group)
Figure 2: E-Fan ducted fan (Varmin, 2014)
Figure 2: E-Fan ducted fan (Varmin, 2014)

There are several reasons besides climate change why electric aircraft should be developed from a business perspective (Figure 3). Short and mid-range regional flights make up a significant portion of all flights around the world.  The current flight range of electric aircraft is limited to these short and mid-range fights. Additionally, shorter flights spend relatively more time taking off and landing than cruising at high altitudes, which makes shorter trips less energy efficient. While short, regional flights are economically unattractive for large commercial aircraft, a smaller aircraft with less fuel consumption may provide a valuable alternative. Read More

Notable Military Jet Engines

As a special tribute this Veterans Day, we decided to have a look at some of the most notable engines that have been used to propel military vehicles throughout history.

PW F135

Kicking off our list is the Pratt & Whitney 135 turbofan engine. The pride and joy of Pratt & Whitney’s military engine lineup, the 135 powers the US Military’s F35 Lightning II. Presently, two variants of the F135 are used in several different variants of the F35, although it should be noted that the F135 was developed specifically for the F35. The 3 engine variants are known as the F135-PW-100, the F135-PW-600, and the F135-PW-400, each for a different application of the F35. The 100 variant is used in the conventional take off and landing F35A, the 600 is used in the F135B for short take off and vertical landing F35B, and the 400 uses salt corrosion-resistant materials for the Naval variant F35C.

A Lockheed Martin F35A in fight, and an F35C taking off from the USS Abraham Lincoln

The F135 is capable of 28,000 lbf of thrust with the afterburner capability pushing thrust all the way to a whopping 43,000 lbf of thrust, making the Lightning II a supersonic STOVL aircraft suited to a wide variety of applications, as seen in the above illustrations. At the heart of the Pratt F135 are 3 fan stages, 6 compressor stages, and 3 turbine stages. In the STOVL variant, the F135-600 uses the same core components, but is also coupled to a drive shaft which connects the engine to the lift fans which were originally developed by Rolls-Royce, and give the Lightning the ability to hover, perform short distance takeoffs, and vertical landings.

A Royal Air Force RAF F35B Lightning II performing a vertical landing on a Royal Navy carrier.
A Royal Air Force RAF F35B Lightning II performing a vertical landing on a Royal Navy carrier.

The F35 by Pratt & Whitney and in turn the F35 Lightning II by Lockheed Martin represent the cutting edge in military aviation, and are the centerpieces of Pratt and Lockheed respectively. The Lightning variants and this line of turbofan engines will be in service with several branches of the US military and its allies around the world for the foreseeable future, with more iterations of the F135 to come. Read More

Hydrogen Energy: History, Applications, and Future Developments

A Brief History Of The Discovery Of Hydrogen 

The release of combustible gas during the interaction of metals and acids was observed as early as the 16th century. That is, during the formation of chemistry as a science. The famous English scientist Henry Cavendish had studied the substance since 1766, and gave it the name “combustible air”. When burned, this gas produced water. Unfortunately, the scientist’s adherence to the theory of phlogiston (the theory that suggested the existence of a fire-type element in materials) prevented him from coming to the correct conclusions.

Henry Cavendish (1731 – 1810)
Henry Cavendish (1731 – 1810) Source:

In 1783 the French chemist and naturalist A. Lavoisier, together with the engineer J. Meunier, and with the help of special gas meters carried out the synthesis of water, and then its analysis by means of decomposition of water vapor with hot iron. Thus, scientists were able to come to the correct conclusions, and dismantle the phlogiston theory. They found that “combustible air” is not only a part of water but can also be obtained from it. In 1787, Lavoisier put forward the assumption that the gas under study is a simple substance and, accordingly, belongs to the number of primary chemical elements. He named it hydrogene (from the Greek words hydor – water + gennao – I give birth), that is, “giving birth to water”.

de Lavoisier (1743 – 1794). Source:

A Little About The Properties Of Hydrogen 

In a free state and under normal conditions, hydrogen is a gas, and is colorless, odorless and tasteless. Hydrogen has almost 14.5 times mass less than air. It usually exists in combination with other elements, such as oxygen in water, carbon in methane, and organic compounds. Because hydrogen is chemically extremely active, it is rarely present as an unbound element. Read More

The Top 5 Coolest Turbomachinery Inventions (According to Us!)

As the leading authority on turbomachinery design, redesign, analysis, and optimization, we work with a wide range of machines from small water pumps and blowers to massive steam turbines, jet engines, and liquid rocket engines. While all of these machines have a certain “cool factor” to them since, after all, we’ve proven they make the world go round; some machines take coolness to the next level. Today, we’re taking a look at 5 of the coolest specific turbomachinery inventions, according to us.

Number 5 – The Arabelle Turbines

Starting with number 5, we have a pair of steam turbines, each known as “Arabelle”. You may be asking yourself “So what, steam turbines are everywhere.” You would be right, but these two have a bit of a size advantage. In fact, they’re the largest steam turbines in the world.

Designed and built by General Electric in France, these turbines are, according to GE, “longer than an Airbus 380 and taller than the average man. A pair of them, each capable of producing 1770 megawatts, is now set to cross the English Channel to provide energy for generations” (1).

They’ll be installed in a new nuclear power plant known as Hinkley Point C in Somerset. Their 1.7 gigawatt output will be enough to power 6 million homes, which is 7% of the UK’s power consumption. (1) The output and sheer size of the turbines aren’t the only large number either, the project costs nearly 24 billion US dollars.

A CAD model of the Arabelle steam turbines, image courtesy of General Electric.
A CAD model of the Arabelle steam turbines, image courtesy of General Electric.

The sheer size and performance figures have earned GE a place on our list of top 5 cool turbomachines!

Number 4 – The Garrett 3571VA Variable Geometry Turbocharger

This is one only gearheads and diesel-fans may recognize, but even then, it’s an obscure one. This Garrett turbocharger was a game changer for diesel engines used in light and medium duty trucks, specifically the Navistar International VT365, also known as the Ford 6.0 Liter Powerstroke engine. Read More