Distributed propulsion

For distributed propulsion on rail see: Multiple unit

Distributed propulsion (DP) is a type of powered flight propulsion system for fixed-wing aircraft in which airflows and forces are distributed about a vessel. Its goal is to increase performance in fuel efficiency, emissions, noise, field length, and handling performance as compared to the use of a single large engine, jet, or propeller. DP is typically accomplished by spanwise distribution of partially or fully embedded multiple small engines or fans across the width of wing. It may instead employ ducting of exhaust gases along the entire trailing edge of a wing.

Definition

The following is the definition of distributed propulsion technology for subsonic fixed-wing aircraft.^[1]

Distributed propulsion technology refers to a propulsion concept in aviation where the means of propulsion consists of three or more propulsion units (engines/propulsors/thrusters/other propulsion units) arranged in two different configurations (Leader or Follower) and classified into five propulsion unit intensity classes (A–E) in addition to three thrust-to-weight ratio categories (I-III). Distributed within/above/around or across the flying vehicle’s wing(s)/fuselage(s) or airframe structure, these propulsion units all or partially contribute to enabling and maintaining flight. If the total propulsion thrust is distributed across a designated area of the fuselage (distributed exhaust), the minimum requirement of three or more propulsion units shall be waived and fulfilled with solely one minimum propulsion unit, only if the exhaust spread of the total propulsion thrust is ejected through at least three independent exhaust spaces or more. In addition to providing propulsion, distributed propulsion arrangements may also provide one or more of the following functions:

• Direct reenergizing of the boundary layer,
• Flow separation control,
• Powered lift/circulation control,
• Viscous drag reduction,
• Noise reduction/noise spectrum alteration,
• Vortex/vorticity control, including reduction in drag due to lift,
• Vehicle control/vectored thrust
• Reduced signatures,
• Redundancy, for safety and reliability, and
• Lower propulsion costs by achieving commodity production rates.

Distributed propulsion categories

The distributed propulsion categories can be divided into two major parts consisting of Leader and Follower arrangements. Leader arrangements refer to distributed propulsion concepts where all the propulsion units solely contribute to the propulsion thrust without driving a secondary propulsion unit(either entirely or partially), e.g., distributed engines. On the contrary, the Follower arrangement denotes a distributed propulsion concept where at least one propulsion unit is used as a secondary propulsion unit. This could for instance refer to a gas turbine arrangement where a power generator drives one or a multiple of fans, e.g. a turboelectric multi-fan distributed propulsion concept.

Distributed propulsion unit intensity classes

Intensity classes refer to a classification between different number of propulsion units. The propulsion intensity can further be interpreted as how packed the propulsion units are. These propulsion units include engines/propulsors/thrusters or any other conventional/unconventional propulsion units that may contribute to the total propulsion thrust and does include the minimum number of propulsion units to define a distributed propulsion concept. The following table shows the number of engines for each intensity class.

Distributed propulsion thrust-to-weight ratios

Three different categories define the thrust-to-weight ratios of the distributed propulsion concept. One legitimate approach that would take the aircraft weight into consideration is the choice to divide the total aircraft thrust produced by the distributed propulsion system with the Maximum Take-Off Weight (MTOW). The following table shows different categories of thrust-to-weight ratios(I–III). The choice of thrust-to-weight ratios based on the propulsion unit weight only is abandoned on the basis of establishing a broader impact of the distributed propulsion on the total aircraft.

Example depicting the implementation of the distributed propulsion definition

Implementation of the definition of distributed propulsion could be rather practical for communicating the specific propulsion system of interest. Thus, the following convention denotes a specific system:

DP(L/F)-INTENSITY CLASS(A-E)-THRUST-TO-WEIGHT CATEGORY(I-III)-(X)

DP(F/L) refers to distributed propulsion leader or follower configuration as discussed earlier. Following the dash sign, the intensity class is given with only one letter(A–E) based on the table above which describes different distributed propulsion intensity classes. Another dash line follows and then the thrust-to-weight ratio is presented as a Roman number based on the table above which describe different thrust-to-weight ration intensity classes. The last dashed line and the letter X shall only be inserted if the system is of a distributed exhaust configuration and completely omitted if not. As an example and based on the following data, 1945s Blohm and Voss 238 V1 aircraft would be denoted as DPL-B-I,since the aircraft employs six piston engines and has a thrust-to-weight ratio(based on MTOW) less than 0.10.

Hence, what is needed for categorizing a distributed propulsion system is the following:

• Information to determine Follower or Leader configuration
• The number of propulsion units
• Total propulsion thrust
• Maximum Take-Off Weight for establishing the Thrust-to-weight ratio
• Potential exhaust system

Types

Any fixed-wing aircraft with more than one propulsor can be considered a distributed propulsion aircraft. Nonetheless, incorrect references to distributed propulsion technology has triggered the definition above.^[2] The common modern usage DP describes a propulsion system scheme with distributed exhaust, a large number of distributed engines (typically fully or partially embedded within the wing), or a large number of distributed fans with a common core.^[3] These implementations are often proposed in conjunction with blended wing body (BWB) or hybrid wing body (HWB) aircraft.

Implementation approaches include jet flaps, transverse or cross-flow fans (CFF), multiple small engines (typically gas turbines), or multiple fans driven by a smaller number of engine cores. In the last case, the power transmission between the fans and engines may be linked by ducting hot gas,^[4] mechanical gears,^[5] or electric power lines.^[6]

While some of these concepts were tested on full scale aircraft in the 1960 - 1970s, such as the Hunting H.126, they were not fielded in production aircraft. More recently, several full-size and smaller unmanned aerial vehicle (UAV) projects have proposed DP approaches to meet noise abatement,^[7] fuel efficiency, and field length goals. Advancements in materials engineering, cryogenic cooling systems, novel fuels,^[8] and high fidelity computational fluid dynamics (CFD) modeling and analysis^[9] have been credited for the renewed interest in DP approaches.

Benefits

Recent analytic and experimental distributed propulsion studies suggest several improvements in aircraft performance.^[10] They include fuel consumption efficiency, noise abatement, steep climbing for short take off and landing (STOL), novel control approaches (in particular eliminating control surfaces for roll, pitch and yaw moments), and high bypass ratios. It has also been suggested that smaller propulsors will be cheaper to manufacture and easier to handle during assembly and maintenance.^[11]

References