Propulsion opportunities for future commuter aircraft

Circa 1990 propulsion improvement concepts are discussed for 1000-5000 SHP conventional turboprop powerplants including engines, gearboxes, and propellers. Cycle selection, powerplant configurations, and advanced technology elements are defined and evaluated using average stage length DOC for commuter aircraft as the primary merit criterion. The paper summari, ,.!os a series of five NASA-sponsored studies t,r4t addressed k this topic and assesses the significance of the resulting overall powerplant improvement potential relative to current production powerplants, engines now in development, and unconventional alternatives such as adiabatic diesels and regenerative turboprops. Text w The rapi d growth of air transportation since the start of the jet age has wrought W ever larger and faster aircraft to satisfy dense and long-range markets. Together with regulatory reform this trend created increasing demand for aircraft designed specifically for short haul markets (fig. 1). During the 1970's commuter passenger air traffic grew at an average annual rate of 14 percent. This rapid growth is expected to continue--spurned on by the abandonment of air service to small communities by the major airlines as a result of high fuel prices and deregulation, In 1980 commuter airlines carried about 4 percent of air carrier traffic, but by 1990 the commuter portion is estimated to increase to 10 percent. Since current turbofan powered airplanes are too large and inefficient for successful sho t-haul applications, this market will be satisfied by increased numbers a^d varieties of propeller-driven aircraft. In response to this increasing need, many new commuter airplane development programs have been launched as well as the development of several new turboprop powerplants in the 1200-2500 SHP class (e.g., CT7, PW100, TPE 331-15). These new airplanes and their powerplants will generally be somewhat larger than most of the current commuter airplanes, utilize rather conventional configurations, and employ technology advances already defined and well in-hand. There are, of course, longer term technological opportunities that can be envisioned. Such opportunities were the subject of recent NASA-sponsored studies that addressed potential improvements in airplane aerodynamics, structures, systems, and propulsion disciplines. These so-called STAT (Small Transport Aircraft Technology) studies were done initially by several airframe manufacturers who assumed potential powerplant improvementsas Tiee mainly on judgment, To provide substantive evidense for these powerplant assumptions, a series of complementary propulsion studies was then Initiated by NASA with the General Electric Company, Detroit Diesel Allison, and the Garrett Turbine Engine Company to identify and evaluate specific engine technologies for advanced conventional turboprops. To help focus these studies, guidelines were set forth (Table 1) that included specifying the technology timeframe to be 1988 readiness--i.e., technology brought to a level that is ready for commercial development. This implies that engines using such technology could enter service in the early 1990's. This was an important assumptiai because the new generation of 1200-2500 ,HP engines now in development will enter service in the mid-1980's. Each company selected both a current production engine and a mid-1980's production engine as baselines to measure the benefits of their advanced technology 1990's conceptual engines (Table ?). All of these er;gines, whether actual or hypothetical, were scaled during the studies to power conventional 30and 50-passenger twin-engine airplanes designed to fly Mach 0.45 for 600 N.M. However, Allison deleted the 30-passenger airplane and added a 50-passenger, Mach 0.70 airplane in order to complement Lockheed's STAT high-speed airplane designed for executive transport as well as for commuter use. The required takeoff power for these applications ranged from 1300 to 5000 SHP. For completeness, Hamilton-Standard and McCauley investigated advanced propeller technologies. Advanced Engine Technologies To enable a long list of candidate technologies to be screened according to potential value, 100 N.M. Dtage length DOC was selected as the single most meaningful criterion that properly reflects the relative importance of diverse characteristics-powerplant efficiency, weight, cost, maintenance requirements, size, etc. On this basis, fuel efficiency is the dominant driver since over 1/3 of airplane DOC is fuel cost (fig. 2) and the sensitivity of both DOC and block fuel to engine weight, cost, and maintenance cost is much less than for SFC (fig. 3)