Propulsion System Integration

The sophisticated and challenging design requirements of modern helicopters encourage the tight coupling between propulsion and airframe system design. Coupling between propulsion and other

engineering disciplines is unique in that a major component of the propulsion system, the engine, is traditionally designed separately from the airframe. Coupling engine design and airframe design in a multidisciplinary optimization is complicated by the fact that subject matter experts usually reside in separate companies. In helicopter design, engine performance is typically addressed with a black box, usually a cycle performance program, commonly referred to as decks, supplied by the engine company. In conceptual design, the deck is often “rubberized” to allow engine growth with helicopter gross weight.

At the conclusion of conceptual design an estimate of rotor power and auxiliary system requirements is known with sufficient certainty that when combined with a technology readiness requirement (usually driven by non-engineering considerations like cost and schedule) results in a list of candidate engines, i.e. the approximate size and number of engines are known. High level decisions are made concerning whether the engine is an existing production engine, a lightly modified existing engine, a heavily modified existing engine, or a new design requiring testing and certification. The list of candidate engines may also be scrubbed with other operational requirements such as the level of icing certification. A candidate engine, or list of candidate engines, becomes a fixed design parameter in helicopter MDO at the preliminary design level.

Once an engine is down-selected, helicopter designs have little or no influence on the uninstalled engine performance. Still, a tightly integrated MDO with propulsion considerations is warranted because the propulsion system, that is the engine’s installed performance, strongly influences a number of key design objectives: Hover and take-off performance objectives are limited by total installed power. Fuel volume and therefore take-off gross weight is driven by installed engine specific fuel consumption. Handling quality and configuration layout objectives are influenced by the location of the engine. Maintenance access requirements are influenced by the location and shape of the engine cowling. Architecture and allocations of the fuel inserting system, environmental control system, electrical power system, transmission efficiency, and hydraulic system must be designed for the candidate engines selected during the conceptual phase.

The engine supplier guarantees minimum performance of an uninstalled engine. The airframe designers apply ambient conditions, flight speed, recovery curves, customer bleed schedules, power extraction, inlet pressure losses, and exhaust pressure losses to obtain the installed engine performance. Thus at the conclusion of conceptual design and throughout preliminary design, propulsion system optimization targets inlet losses, exhaust losses, accessory power extraction, transmission losses, and bleed flow.

For propulsion systems especially, cost is comprised of non-recurring engineering, certification, recurring, reliability & maintainability, fuel, and number of engines. Schedule is driven by development, certification, and production. Major performance metrics for the propulsion system include installed engine specific fuel consumption, installed propulsion system weight, and installed power available. By preliminary design, the size and number of engines has already been determined and therefore many of the decisions that greatly effect cost and schedule have been made. Therefore propulsion system optimization is centered on performance.

Figure 24 illustrates major performance goals and how they are interrelated within the propulsion system. The ultimate system metrics are highlighted in ovals and are strongly influenced by the elements directly beneath them and connected by solid lines. Elements in the hexagon shapes are associated with the engine and cannot be changed without modifying the engine. Airframe designers have some influence

dependency.

Propulsion Performance Metrics

Controlled by Airframe Manufacturer Controlled by Engine Manufacturer Installed Propulsion

System Weight Installed Fuel Burn

Power to Weight Ratio Uninstalled SFC Uninstalled Power Aircraft Speed Power Extraction Bleed Extraction Inlet Pressure Loss Trans - mission Efficiency RAM Recovery Exhaust Pressure Loss Weight of Fuel System Weight of Engine Mounts & Structure Weight of Airframe Mounted Exhaust System Weight of Engine Ancilliaries Installed Power Available to Rotor

Figure 24. Propulsion performance metrics

Some of the propulsion goals can be met by designing a uniform pressure and steady flow across the face of the inlet. One approach to achieving this goal is to tailor the flow field in front of the inlet by modifying upstream geometry (Figure 25) or angling the placement of the engine. High fidelity computational fluid dynamic analysis is ideal for verifying inlet flow quality. Tied to an optimization scheme that modifies influential geometry, CFD can improve inlet flow quality. Similarly to the fuselage aerodynamics problem, shape parameters would be design variables, which makes the specific variables highly configuration dependant. Inclusion of the rotor flow field to this design problem has been increasingly feasible with the use of CFD. This development may lead to prediction of flow problems that cause engine surge such as ingestion of hot exhaust gas, and unsteady rotor or bluff body wake in various flight conditions.

Candidate area for optimization for inlet flow quality

Figure 25. Regions of influence for improving engine installation

Table 27 Propulsion system objectives

Objective Notes

Minimize Installed power losses Improves A/C performance Minimize Mission fuel burn Ties in to sfc target Minimize Propulsion System weight Ties in to weight allocation

Table 28 Propulsion system design variables

Design Variable Number of design variables

Type Notes

Engine bleed & power extraction

2 Discrete or continuous Trade-off for energy requirements for accessories Inlet shape & inlet

separator design 6-20 Continuous Control inlet recovery and flow quality Exhaust geometry: tail

pipe or ejector length and area

2-6 Continuous Control exhaust losses

and temperature Estimated total 10-28 approx

Table 29 Propulsion system design constraints

Description Number of constraints

Surge margin 1 Ensure engine installation doesn’t adversely affect surge margin.

Install power 1 Power requirements from aero performance Installed sfc 1 Minimum sfc required for valid performance

assumptions