PropFan Integration Concept - Engine Mount Structure Assembly Preliminary Design

As previous two articles were dedicated to PropFan Integration Concept as Boundary Layer Splitted Ingestion Propulsion System as an introduction to the potential solution, the following article is going to put another brick in foundation of the concept as providing an another feature description.

Splitted Ingestion Essence

As described in the second article, splitted ingestion could be beneficial, as modification, to Boundary Layer Ingestion Propulsion System which presents certain kind of aircraft propulsion. This way of how to propel aircraft is very likely to bring so-called Green Thrust Production while putting fuel consumption into very low numbers. There have been a number of studies, research publications and experiments proving the benefits of this propulsion system. There is always focus on this field nowdays.

Splitted ingestion is to split/divide in-flowing boundary layer into two streams as one of them, consisting of much greater volume of boundary layer airflow mass than another one does, is ingested/sucked by gas generator (core) allowing a propulsor to work with more uniform airflow at an air inlet area entrance. This could lead to more efficient propulsor working and thrust production meanwhile mitigating pressure drag being generated behind flying aircraft. The concept benefits are provided in the second article.

Powerplant-to-Airframe Integration

To integrate powerplant with aircraft airframe got into attention of aerospace engineering many years ago as they realized potential benefits of this option. To get powerplant included within aircraft airframe means the powerplant system is part of airframe structure - fuselage. Some load-carrying structural members or other members of airframe and powerplant structure are shared together. Some of them might function both for airframe as well as powerplant purposes.

In most cases, where this integration might be utilized, propulsor is installed in aft part of fuselage in order to mitigate wake turbulence (pressure drag) effects. The powerplant pushes airframe through air as thrust is generated by the propulsor.

The powerplant(s) might be installed on upper fuselage section - e.g. D8 Double Bubble aircraft concept or on aft fuselage section when whole-surface boundary layer ingestion is utilized - e.g. described concept.

This configuration of powerplant being integrated with the airframe has got certain advantages and one of them is going to be pointed out herein. Most significant, within described concept, is: Boundary Layer Re-Energize = Refilling Wake Turbulence (Pressure Drag Mitigation). This factor presents essential and key one of entire described conceptual design.

PropFan-to-Airframe Integration - Engine Mount Structure Assembly

Installing of PropFan Propulsion System into aft fuselage section could present most efficient boundary layer ingestion process as pressure drag would be mitigated as efficient as possible. There has not been any work/study done in relation to integrate PropFan into airframe yet. The very first approach was introduced in my diploma thesis where elementary engine mount structure configuration was depicted. This Propfan-Airframe putting together leads to very challenging technical problem.

Engine Mount Structure

Commonly, engine mount structure purposes are:

• to transfer loads from engine to airframe,
• to create base for engine itself,
• to create interface between engine and airframe,
• to create mounting platform for other devices,
• to absorb vibrations coming from engine.

As the powerplant is installed in the aft fuselage section, there is one great difference in comparison to "tractor" version where propulsor pulls the aircraft. The difference is that the PropFan is going to push the aircraft, so negative-tension forces actuate on the engine mount structure elements in dominant way.

In this concept, the engine mount structure is going to be subjected to the following kind of loadings (the loading sources added):

• Negative-Tension Loading - thrust (whole structure) and within bend loading and inertial forces (some structural elements only),
• Bend Loading - static forces/stationary mass and inertial forces,
• Tension Loading - reversed thrust (whole structure), within bend loading and inertial forces (some structural elements only),
• Torsion Loading - reaction to rotary assemblies motion (whole structure).

Bend loading would actuate on whole structure itself, but it will not occur within the truss structure struts. The truss struts transfer the axial forces only, so each aforementioned loading will result in the tension and negative-tension loading only.

In fact, there is never one kind of loading actuating on the structure only. It is certain that any combination of aforementioned loadings would actuate on the structure. Gyroscopic forces would enter into the system as well. All of those kinds of loadings have to be transferred through the engine mount structure to the airframe with as minimal losses as possible as well as without any plastic deformations being present within the structure elements. Unwanted vibration loadings would have to be absorbed by vibration isolators where kinetic energy of vibration is dissipated.

Installing of PropFan Propulsion System into aft fuselage section could present most efficient boundary layer ingestion process as pressure drag would be mitigated as efficient as possible.

PropFan Mount Structure Assembly Architecture

Within the preliminary design consideration, the architecture is provided. PropFan as powerplant unit would be interfaced with the aircraft airframe by means of mount structure assembly which would be categorized into two sections: 1) Outer and 2) Inner. The mount structure assembly would consist of the following sub-parts:

• Conical Truss Mount Structure,
• Linear Truss Mount Structure,
• Interconnect Annulus Mount Structure,
• Conical Truss Mount Structure V-Fittings,
• Mount Thrust Struts.

All couplings among each sub-part of whole assembly would be realized by knuckle joints consisting of: 1) fork - double-eye or triple-eye, 2) single-eye, 3) securing pin or bolt-nut-washer assembly (those ones omitted within the pictures shown below).

Conical Truss Mount Structure

This sub-part of assembly would present a member of outer mount structure assembly as coupling interconnect annulus mount structure with fuselage aft pressure bulkhead. Coupling would be reached by means of V-Fitting which would be mounted to the aft pressure bulkhead as well as to the fuselage skin. Coupling of V-Fitting with an ambient structure could be realized by high-strength rivets or bolt-nut-washer assemblies. Over opposite side of conical truss mount structure, knuckle joint would be found as incorporating double-eye on the conical truss structure assembly and triple-eye being part of interconnect annulus mount structure.* The joint would be secured by securing pin or bolt-nut-washer assembly. Magnitude of preload applied to the joint would be a function of loadings and their combinations being transferred through the mount structure assembly.

*Triple-Eye would present safety redundancy factor within the case of one eye failure. The rest of the eyes, within the joint, would re-distribute the loadings to the airframe structure. In addition to this, the rest of the joints within assembly would be capable to transfer the thrust-loads and other loads while being re-distributed. It follows Fail-Safe design philosophy.

Four primary struts would be circled around central axis by 90°. V-shaped diagonal members - struts would fill the gaps among the primary struts in order to increase torsional stiffness of the structure as well as to re-distribute transferred loadings into the aircraft airframe structure. All those struts present axially-loaded members which means the axial forces might be transferred only - no strut bending being present.

Linear Truss Mount Structure

This sub-part would be a member of inner mount structure assembly as coupling interconnect annulus mount structure with gas generator (core) - low-pressure compressor support structure where triple-eye fittings would be mounted on. The low-pressure compressor support structure would be load-carrying member which would function to carry rotary assembly, to transfer the motion loads and to tranfer the thrust loads into the engine mount structure assembly. Those knuckle joints occupying area of linear truss mount structure-to-rotary support structure connection would be equipped by vibration isolators as they would be tasked to absorb and dissipate vibration kinetic energy. Those unwanted vibrations are not to be sent farther into the mount structure.

The first optional solution to the member of inner mount structure assembly is likely to be as following: conical shape would be applied rather than linear shape would, as the loadings would be distributed to larger area. Accompanied to this potential solution, the conical truss mount structure would have to be put on larger diameter within the coupling with the interconnect annulus mount structure. If considering the solution, the interconnect annulus mount structure would become lighter as joint location would get closer to an outer diameter of the ring and corners would be torn off. This would lead to getting the ring inner structure smaller.

The second optional solution is likely to be as following: the truss structure would be omitted as the load-carrying support structure would be installed between the gas generator and the interconnect annulus mount structure instead of the truss structure being mounted in-between. It would present quite similar support structure which is supposed to be found on the gas turbine engines commonly as being tasked to support the rotary assemblies. This potential solution could lead to the weight reduction in comparison to the first optional solution while being capable to carry the loads coming from the powerplant.

Interconnect Annulus Mount Structure

This sub-part of whole presented assembly means probably most interesting, significant as well as complex assembly element. As creating interface, it would be given the following purposes:

• to interconnect inner and outer mount structure assembly,
• to transfer thrust-loads from the powerplant,
• to create base for the mount thrust struts,
• to create entrance for the core air inlet area,
• to create splitting-area for incoming airflow mass,
• to create enviroment for line system routings*,
• to create enviroment for boundary layer active control system and others.

*The line system routings would incorporate all sub-systems such like: fuel system, electrical system, pneumatic system, hydraulic system. Pipes, hoses and wirings would be routed through the interconnect annulus mount structure inlet guide vanes.

There would be the inner structure incorporating the corners and bays as being circumferenced by the inner ring. The corners would accomodate the triple-eye fittings on both sides. The thrust-loads, being transferred by the thrust struts, would be carried by those corners as thrust force flow would continue to the inner mount structure.

Inlet guide vanes/Annulus struts would be tasked to those purposes:

• thrust-load-carrying members,
• in-coming airflow guiders,
• line system routings.

In fact, those annulus struts could get larger in thickness than depicted if needed. For thrust-load-carrying and line system routing, this change would be efficient but for in-coming airflow it could be quite bad as greater turbulent airflow would be added to airflow field.

At least four of those struts, those ones above the bays, would be hollowed in order to create path-through for the line system routings. There is possiblity to have each line system being routed through each so-called bay strut in order to have the safety redundancy in case of some strut structural failure which would be unlikely a lot. Moreover, the hot lines of pneumatic system coming from the core to the airframe would be capable to de-ice those struts, but it is questionable about the rest.

Manufacturing technology could get quite difficult as the part is complex in shape. Milling, drilling, welding and mounting process could be applied within the manufacturing process. As written above, the part modifications are very likely to occur later on while looking for the possible optimal configuration.

Conical Truss Mount Structure V-Fittings

Those V-shaped attach fittings would present essential elements within the load-transfer process while the load re-distribution would be provided by them. These ones would be absolute interface between the poweplant system and the airframe. They would attach the conical truss mount structure with the fuselage aft pressure bulkhead and fuselage skin. The loads would be transferred from the conical truss mount structure to the aft pressure bulkhead and fuselage skin. Attachment could be achieved by the high-strength rivets or bolts. In addition to this, there would be some kind of stiffening being placed in the area of the V-Fittings-to-skin attachment. It would mean for instance, the fuselage skin stringers or doublers would be added to the area. It would lead to more efficient load re-distribution to ambient structure.

The diverging shape would be applied to the Attach V-Fittings while being in touch with the fuselage skin. This feature would allow the load re-distribution to the skin structure in efficient way. The Attach V-Fitting would be connected with the skin doublers or stringers in order to achieve higher stiffening of the attachment area as mentioned above.

Mount Thrust Struts

The last ones to be described. These four load-carrying elements would connect the gas generator (core) with the interconnect annulus mount structure which would present so-called base for those mount thrust struts. One end of the strut (fork) would be fixed to one-eye being mounted or (part) on (of) the high-pressure turbine support structure case. Opposite end of the strut (fork) would be fixed to one-eye which would be part of the annulus structure.

The struts would be tasked to re-distribute the thrust loadings and therefore they would help the mount structure carry the powerplant system. One great advantage would be to have those struts being fixed nearby the powerplant centre of gravity. As depicted, they would be installed such way, there would be clear access to the powerplant sub-systems from the sides when uncovered.

Conclusion

The preliminary design consideration of the powerplant mount structure has been defined. It is the first approach to this problem. It is certain, some parts, elements and mount structure features are very likely to be modified while looking for the optimal configuration. 3D model is assumed to be changed and optimized in order to create optimal input for mechanical numerical computation.

Other features of this concept, while the design process is still in progress, will be presented in the next articles.

Written and designed by Ing. Filip Čorba.