O'Day Aviation Consulting

A Guide to Propulsion System Development, Evaluation & Selection

Developing, evaluating or selecting an aircraft propulsion system involves a wide range of engineering and design considerations. A propulsion system’s target market and applications are defined by the systems energy output – typically, thrust or horsepower – and by how it generates propulsive force – all-electric, hybrid, piston or jet engine.

Designing an aircraft’s propulsion system begins with a market-driven understanding of the target segment, competition, customer and operator needs. The propulsion system’s competitiveness is defined by a value proposition created by the factors which drive design requirements – all relative to products on the market today, or in development:

  • Fuel burn/energy efficiency
  • Maintenance cost
  • Acquisition cost
  • Reliability
  • Weight

It is also important to understand the potential trade-offs and interplay between key factors when evaluating or developing an aircraft propulsion system. The goal is to meet product requirements without under-designing (performance, safety and maintenance cost impacts) nor over-designing (non-recurring/recurring cost and weight impacts) to requirements during the development process. Even for an established OEM, a new system concept development can often seem like an endless process as one requirement is met with a significant effect on other factors.

Thrust-to-Weight Ratio

The propulsion system must provide sufficient thrust to overcome the aircraft’s weight and achieve the desired performance during takeoff, climb, range and maneuverability. Beyond performance, a propulsion system’s weight influences an aircraft’s weight directly (weight of the propulsion system on a scale) and indirectly (weight on spars, aircraft structure, etc.) which can affect both Operating Empty Weight (OEW) and Max Gross Takeoff Weight (MTOW) of an aircraft. Optimizing for engine weight involves significant trade-offs with other factors – durability, life-cycle costs and performance.

Fuel/Energy Efficiency

Modern aircraft propulsion systems are designed for maximum energy efficiency to minimize operating costs, meet mission requirements and reduce environmental impact. The propulsion system design must ensure the aircraft meets the range and fuel burn requirements that will differentiate it from its competition from both a mission and operating cost perspective. For example: a military aircraft’s mission radius and capability to reach targets, loiter and return safely; a business aircraft’s ability to meet range goals – a key discriminator; and a commercial aircraft’s fuel efficiency ability to connect city-pairs and economic differentiation (cost per seat/mile).

Maintenance Cost

At a basic level, maintenance costs are typically focused on two factors – time-on-wing (frequency of shop visits) and cost-per-shop-visit. Time-on-wing will typically be established based on either a hard time program (set number of hours between visits) or on-condition (removal decisions based on monitoring of key parameters and performance). At a deeper level, these factors are affected by design decisions including operating temperatures, use of advanced (high temperature) materials, cooling technologies/thermal management, ease of maintenance, system complexity, and useful life of key components (ex: Life Limited Parts or batteries in an electric system).

At one end of the scale are propulsion systems for space vehicles for which time between shop visits is measured in minutes while at the other extreme are commercial aircraft narrowbody engines that reach 30,000 hours before being removed for a shop visit.

Trade-offs must be made in setting maintenance cost targets as building durability and robustness into a design will increase weight and system cost.

Reliability

An aircraft system’s reliability is measured by several factors. On a commercial aircraft, we measure Delays and Cancellations (D&Cs), Aircraft-on-Ground (AOG), Aborted Takeoffs (ATO) and In-Flight Shutdowns (IFSD). For non-scheduled operations, reliability is measured by the aircraft’s availability – the ability of the propulsion system to fulfill the mission – every time. Reliability/availability is primarily affected by the sub-systems and external (Line Replaceable Units – LRUs) components of the system, for example – generators, starters, valves and engine control systems. Questions for analysis may include:

  • Does supplier have a clear understanding of the design point for reliability/durability? How will reliability requirements (MTBF, MTBO…) be flowed down and managed with LRU suppliers?
  • Does supplier have a fleet leader inspection program to assess the actual early engine and LRU condition?

It is imperative that a due diligence and risk assessment include these components and the suppliers that provide them.

Development & Recurring Cost

A business plan will succeed or fail based on how the development and unit cost of a program are set and managed. A successful program will have both factors set as requirements in the original business case. Managing these factors aggressively throughout the development process is key to managing technology decisions, avoiding “scope-creep” and ensuring profitability. All the requirements and decisions discussed in the previous sections are trade-offs to development and unit cost factors and will have a direct impact on a propulsion system’s success. These factors must then be considered an input and requirement of the program versus a variable Output – and managed accordingly.

Additional requirements that must be considered in a propulsion system development – but by no means of a lesser priority include:

Safety

Safety is not a factor that is typically discussed in the evaluation of propulsion systems. Why? Because OEMs, operators and customers EXPECT aircraft and engines to inherently be safe.

A deep-dive and solid understanding of the Technology Readiness Levels (TRL) & Manufacturing Readiness Levels (MRL) should be completed for each of the key propulsion system components and production processes. ie. to what degree has risk been burned down to acceptable development levels through field experience, component testing, and rig testing before EIS? A logical, progressive technology development plan and rigorous testing programs are essential for achieving this goal.

judgments to be addressed with the method of compliance to the regulations and how to derisk a program to reduce schedule and development cost risk.

An experienced team’s judgement is therefore worth its weight in gold.

Performance

Propulsion systems must generate the energy required by the aircraft and perform flawlessly at all flight regimes (altitudes, attitudes & speeds) for which the airframe was designed. On a commercial aircraft, performance is measured by aircraft speed, time-to-climb and thrust but will include other factors in military applications such as acceleration and ability to operate under extreme conditions.

Performance is typically a combination of many factors and but achieved through increases in operating temperatures and loads on bearings, rotating parts and structures.

Environmental

Reducing emissions and minimizing the environmental impact is a growing concern. Meeting emissions regulations (CO2, NOx, Smoke, etc.) is an increasingly important aspect of modern propulsion system development.

There are also numerous approaches for propulsive energy under evaluation today including hybrid-electric, all-electric, hydrogen (fuel cell & combustion). Each of these technologies carry both advantages and risks (technology readiness, etc.) that must be carefully evaluated for a vehicle or new investment.

Noise Reduction

Both jet engines, reciprocating engines and emerging propulsion systems (ex: all-electric or hybrid) must meet noise regulations for both passenger comfort and environmental considerations. The noise from a propulsion system is created by a combination of the hot exhaust gases (in a gas turbine or to a somewhat lesser extent in a reciprocating engine) and the acceleration of propulsive air by means of rotors, propellers or fans. An all-electric system eliminates the noise from combustion and exhaust of hot gases but must still address the noise from the propulsive energy created by rotors.

On a commercially-operated aircraft, the noise is typically measured at airport landing, takeoff and approach. Vehicles operating within residential or congested areas (VTOL, helicopters, etc.) are also subject to both regulations and neighbor-perception concerns (NIMBY).

Materials and Manufacturing:

Choosing the right materials and manufacturing processes for engine components is crucial for durability, weight reduction, and overall efficiency. Advances in materials science play a significant role in modern propulsion systems. Today’s high performance jet engines have hot sections (combustors & turbines) with operating temperatures that exceed the melting temperatures of the metals that compose them. The result is engines that incorporate increasingly sophisticated and complex cooling systems to ensure durability, low maintenance costs and engine longevity.

With the increasing use of additive technologies (3D-printing), OEMs are increasingly able to design components for fit and available space vs the limitations of traditional machining or casting processes.

Regulatory Compliance

Compliance with aviation regulations, such as those set by the Federal Aviation Administration (FAA) or the European Union Aviation Safety Agency (EASA), is a critical aspect of propulsion system development. The certification process can be an especially critical one – especially for new or emerging technologies – and should never be under-estimated.

Shawn O'Day, President of O'Day Aviation Consulting

Shawn O'Day, President of O'Day Aviation Consulting

Shawn is a former GE executive in a career that spanned 33 years. His career began as a Field Service Engineer working with airlines throughout South America, Australia and the South Pacific. In 1994, he was accepted into GE’s highly selective Global Marketing & Sales Leadership program. Upon graduation, Shawn progressed through a number of roles at GE including 6-SIGMA Master Blackbelt and marketing roles driving strategy for commercial, business aviation and military programs. In 2008, Shawn was promoted to Chief Marketing Officer where he was instrumental in the creation of GE’s Business & General Aviation division and the launch of multiple products including GE’s Passport, Catalyst and Affinity engines. At the time of his retirement from GE, Shawn was responsible for marketing, branding and strategy for a $2B business that spanned business & general aviation, propellers, aviation components, services and electrical power systems.

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