Whether you’re a prospective aircraft owner or just an aviation enthusiast, you probably already know that there are different types of aircraft engines. But what you may not know is that in the 1930s, piston engines were the only type of engines available to aircraft designers. While pistons in their various forms (rotary, air-cooled, etc.) served their purpose, they had their own limitations. These limitations and the need for high-performance aircraft gave birth to what we now know as jet engines. In this article, we’ll be comparing two common types of jet engines—the turbojet and the turboprop.
In very practical terms, the major difference between the turbojet engine and the turboprop engine is that a turboprop adds a propeller in the front right before the inlet into the engine. Put simply, the turboprop is simply a modified variation of the turbojet which is the original type of jet engine that was designed by Sir Frank Whittle in 1930.
This design modification directly impacts how each engine type works. While the power generated in a turbojet’s turbine drives only the compressor, a turboprop’s turbine drives both the compressor and the propeller.
Because turboprops and turbojets are built for different working conditions, it is quite impossible to say which of them is better than the other. While the turbojet is more efficient at higher altitudes, the turboprop is designed for flights at lower altitudes. In other words, these two gas turbine types have different advantages and disadvantages and these will be considered in detail in this article.
Table of Contents
How Do Turbojets Work?
Turbojets are the simplest type of gas turbine engines. They consist of the basic components you’ll find on all jet engines. A turbojet operates based on the principle of compressing, expanding, and releasing air at a high velocity. The basic components of the turbojet engine include the inlet, compressor, combustion chamber, turbine, and exhaust nozzle. The inlet and compressor are generally referred to as the cold section. These are the components where the temperature of the air is relatively low. The other 3 components make up the hot section.
The inlet is the first part right in front of the turbojet engine. Here, air is sucked from the atmosphere into the engine and driven at a certain angle toward the compressor blades. The inlet regulates the speed of airflow into the engine which is necessary to ensure the engine works optimally.
In the compressor, the air from the inlet is compressed by the interaction between the compressor’s rotor and stator blades. This compression is needed to increase the pressure of the air which is later sent to the combustion chamber. A compressor’s rotor is powered by the energy transmitted from the turbine.
The combustion chamber is the section where the air from the compressor is burned. The fuel in this part of the engine burns the high-pressure air and energy is released in the process. The high pressure of the air at the entrance is converted to high velocity at the exit by means of fuel combustion.
The turbine performs a reverse of the process that goes on in the compressor. Also using its rotor and stator blades, the turbine expands the air that passes through it. The high-velocity air it produces finally escapes through the exhaust nozzle. As a result, sufficient thrust is generated to power the engine and the plane as a whole.
How Do Turboprops Work?
As earlier mentioned, turboprops contain the same components as turbojets but with a large fan called a propeller attached in front. The rotation of the propeller is controlled by a gearbox which is fixed in between it and the main engine’s inlet. Both the propeller and gearbox are connected to the rest of the engine (gas generator) using a long connecting shaft.
Unlike the turbojet, the turboprop’s turbine does not only power the compressor but also the propeller. A large portion of the energy generated by the turbine is transmitted to the propeller to induce its rotation.
The speed of the propeller’s rotation is controlled by the gearbox between it and the inlet. In other words, the propeller is attached in a way in which its speed is independent of the speed of the turbine. This regulation is achieved by connecting the gear with the lowest speed directly to the propeller.
As a result, the rotational movement of the propeller generates thrust to power the whole airplane. So, unlike the turbojet, the turbine and exhaust are directly responsible for only a small percentage (about 10%) of the engine’s thrust.
Application: Turbojets vs. Turboprops
In a nutshell, turbojets are perfectly suited for flights at higher altitudes and high supersonic speeds. They become less efficient when used for other mission requirements. Turboprops, on the other hand, work more efficiently at lower altitudes and subsonic speeds. The application of turbojet and turboprop engines can be compared based on the following factors:
Altitude
Turbojets are built to be more fuel-efficient at higher altitudes. Altitudes from 41,000 feet to 45,000 feet are considered ideal. It has been earlier established that turbojet engines generate all their thrust through the compress-expand process that happens in the engine’s core.
Turbojets are more efficient at higher altitudes because the air at these altitudes is cooler. A combustion engine’s thrust generation process relies solely on the speed, temperature, and pressure of the air that enters it. Cool air expands better and therefore is more efficient than the warm air that is characteristic of lower altitudes.
Another factor to consider is the density of the air. The air at higher altitudes is much thinner and of lesser density. This type of air, therefore, causes less drag and allows the aircraft to fly faster at high altitudes.
Conversely, turboprops’ application is more suited for low-altitude flights. One can expect optimum fuel efficiency from them at altitudes below 25,000 feet. Turboprops can further reach a ceiling altitude of 30,000 feet.
Turboprop engines are built to generate thrust primarily through the rotation of the propellers. These propellers work more efficiently at lower altitudes where the air is denser. This is because they can push a larger amount of air and generate more thrust in the process.
Because turbojets fly at higher altitudes, they are less susceptible to crashes resulting from changing weather conditions than turboprops.
Speed
Turbojets are highly optimized for high-velocity flight. This is because they rely fully on the air flowing through the engine’s core to generate thrust. The faster the speed of the incoming air, the more the pressure that can be generated by the compressor. Turbojets are perfect for flights with cruising speeds between 370 and 450 knots (425 – 520 miles per hour). This makes them optimal for covering longer distances over a shorter period of time.
The range of turbojets can typically go as high as 8 nautical miles as demonstrated in the case of the Gulfstream 650ER. This also reduces the overall cycles of the aircraft.
However, turboprops are ideal for flying at low speeds. The propeller in front of the engine becomes less efficient with faster-moving air. Turboprop engines fly at speeds 20 to 30% lower than that of turbojets in the same category. This makes them excellent for short-range flight. Their major advantage is their high fuel efficiency under optimal conditions.
Runway
Turbojets are characterized by their low take-off thrust. They, therefore, require a longer concrete runway to attain the necessary speed for takeoff. On average, turbojet aircraft need a paved runway with a minimum length of 5,000 feet. This is why turbojets only do well taking off and landing in standard airports.
By contrast, turboprops are a more versatile option. A turboprop engine’s propeller has the capacity to generate more thrust at takeoff. This, therefore, allows turboprops to land and take off from shorter and unpaved runways. Dust and other objects do not impact the health of a turboprop as much as that of the turbojet. A turboprop aircraft can often land on runways 3,200 feet long.
Turboprops also have lower landing speeds making it possible for them to land on a short runway. A very good example is the Twin Otters which can land on runways as short as 1,000 feet in length. So, if you’re looking to land your aircraft on an unimproved grass airstrip, a turboprop aircraft is more suited for the job. Turboprops are used predominantly for charter flights.
Cabin Conditions
One last condition to consider is the cabin condition. Due to the large external blades rotating in front of the aircraft, cabins of turboprop aircraft experience higher noise and vibration levels. Executives and aircraft owners looking to get some work done while on the trip may want to consider the turbojet. Though it is common knowledge that turbofans are the quietest of all engine types.
However, turbojets in the same category are much quieter and vibrate less. This is as a result of the fact that all their blades are located inside the core of the engine. Passengers will, therefore, have a much quieter cabin experience. Most of the noise audible from the cabin of turbojets comes from air passing over the fuselage.
Acquisition Costs: Turbojets vs. Turboprops
If we were to compare turbojets and turboprops in the same category, turbojet engines would generally incur more acquisition costs. Most turbojets are bigger, fly more people and therefore cost more.
For example, a brand new Gulfstream G450, a long-range turbojet aircraft with an executive seating capacity of 14 may cost $40 million. An alternative acquisition option is to purchase a pre-owned model. This can go for as little as $14 million depending on the aircraft’s age and condition. Another example is the Bombardier Challenger 605 worth about $27 million.
A turboprop aircraft very similar to the Gulfstream G450 is the Viking Air DHC-6 Twin Otter. The Twin Otter is a medium multi turboprop that has a seating capacity of about 19 people. A brand new unit costs $6.5 million approximately. The aircraft is a very popular option for commercial skydiving operations.
Maintenance and Operating Costs
Turboprops are known to be more fuel-efficient and consequently more cost-effective, especially over short distances. As a result, turbojets generally require more operating costs than their turboprop counterparts. Here is a list of the operating cost per hour of some common turbojet and turboprop aircraft. The engine model is specified in brackets:
Turbojets
Airbus ACJ319 (CFM56-5B7) – $5369.42
Bombardier Challenger 650 (CF34-3B MTO) – $2,724.25
Cessna Citation CJ4 (FJ44-4A) – $1,540.13
Dassault Falcon 7X (PW307A) – $3,070.62
Gulfstream G650 (BR 725 A1-12) – $3,790.25
Turboprops
Beechcraft King Air 250 (PT6A-52) – $1,180.48
Cessna 208 Caravan (PT6A-114A) – $508.73
Daher-Socata TBM 930 (PT6A-66D) – $673
Piper M600 (PT6A-42A) – $535.84
Viking Air DHC 6-400 Twin Otter (PT6A-34) – $968.40
These estimated costs were collected from a Conklin and de Decker analysis provided exclusively to AOPA Pilot in 2016. The analysis’ data were obtained over a 10-year time period.
Safety: Turbojets vs. Turboprops
There is a pretty common myth that turboprops are less safe than other types of jet engines because of the conspicuous propellers attached in front of them. But there are several factors that affect the safety of an aircraft. One of the factors that put turboprops at a great disadvantage is weather. Because turboprop aircraft fly at lower altitudes, they are more likely to be affected by turbulent weather conditions.
On the other hand, turbojets are also more liable to damage if a foreign object finds its way into the engine’s core (e.g. a bird strike). But it also depends on the size of the foreign object. A large bird will cause almost the same extent of damage to both turbojets and turboprops.
It is therefore hard to give a final verdict as to which aircraft is safer than the other. However, turboprops have been proven to record fewer and less fatal landing accidents due to their low landing speed.
Here is a youtube video where the safety of both options is discussed in more detail:
Pros (Turbojets)
- Relatively simple design.
- Less number of connecting parts reducing losses due to friction.
- Relatively low cost of production and maintenance.
- Absence of unbalanced forces. This means they are free from vibrations which are a great source of losses.
- Capable of reaching very high speeds.
Cons (Turbojets)
- High fuel consumption.
- Poor performance (low thrust) at takeoff and low speeds.
Pros (Turboprops)
- Highly efficient at lower altitudes (in dense air).
- Require shorter runways.
- Propellers can be feathered to minimize drag in the event of engine failure.
- High fuel efficiency.
Cons (Turboprops)
- Propellers lose efficiency at high altitudes.
- Gearing systems are heavy and prone to breakdown.
- High vibration levels which may lead to passenger discomfort.
Which Is the Best Choice?
It is clear from the content of this article that the right choice of aircraft type between these two largely depends on the mission requirement.
Conclusively, turbojets are great for long-range, high-speed flights while turboprops are perfect for charter which generally require transportation over a shorter range.
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