ATTT Open Rotor Engines — Still an As the Open Question? Turbine The open rotor aircraft engine, with its eye-catching two rows of contra- rotating scimitar-like propeller blades, has been under intermittent study and development since the 1980s. Holding the promise of the speed and performance Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/140/12/S46/6352674/me-2018-dec9.pdf by guest on 24 September 2021 Turns... of a turbofan, it is designed to provide the fuel economy of a turboprop engine. The open rotor is essentially a turbojet engine that drives twin exterior nacelle mounted propeller/fans at the rear of the engine. Since its conception in the #36 DECEMBER 2018 1970s at United Technologies Hamilton Standard Division (in conjunction with NASA), it has had a miscellany of names: Propfan, unducted fan (UDF), advanced turboprop and more recently, contra-rotating open rotor (CROR), or open rotor for short. (Some use counter in place of contra, while in propeller terminology the latter refers to same axis rotation only, and the former denotes different rotational axes.) PROPULSION EFFICIENCY To better understand open rotor engines, we need to look at the propulsion efficiency of jet engines in general. The first commercial jetliners were powered by turbojets - jet engines in which all thrust was provided by gases that went Lee S. Langston through the engine from inlet to exhaust nozzle, exiting in a single high velocity jet. The resulting momentum increase provides the thrust force necessary for Professor Emeritus flight, but kinetic energy is “wasted” in the exiting jet. University of Connecticut Mechanical Engineering A more efficient design is the turbofan jet engine, so-named for a ducted fan Department mounted in the front. Air drawn into the fan is divided, with some flowing out of the fan into the jet engine itself and the remainder bypassing the engine. The lower velocity bypassed air and the higher velocity engine air combine downstream to produce thrust with a larger mass flow at an average velocity lower than the high velocity jet flow. With a large frontal area, the commercial aircraft turbofan is designed to produce peak thrust at takeoff, with most of thrust produced by air drawn in by the fan and bypassing the jet engine core itself. Bypass ratios - the mass of fan air for every unit mass of air through the engine - currently can be as high as 9:1, as in General Electric’s 100,000 pound thrust GE90 engine that is used on Boeing 777s, and 12:1 in Pratt & Whitney’s PW1400G 30,000 pounds thrust geared fan engine for the Airbus 320. For subsonic flight, the propulsive efficiency,η p, of a turbofan is higher than that of a turbojet. This efficiency is defined as the useful propulsive power (the product of thrust and flight velocity, Vo) divided by jet power (rate of change of the kinetic energy of gases through the engine). This simplifies to [1], 2 η p = (1) Ve/Vo + 1 where Ve is a suitable average of the lower velocity bypass air and the higher velocity jet exhaust. In a straight forward and elegant way, Equ. (1) shows that to maximize propulsion efficiency, an engine must produce thrust by moving air through it with as little a change in velocity as possible. Thus, a turbojet engine with a higher value of Ve/Vo , has a lower propulsion efficiency, than a same-thrust 46 turbofan engine with its lower velocity ratio. Also, for turbofans we see from Equ. (1), that the higher the bypass ratio, the greater is The propeller/fan blades are rotated by the turbine of the propulsion efficiency. turbojet, by either direct drive or by a gearbox. Direct drive is gotten from contra-rotating, statorless turbine stages, WHY AN OPEN ROTOR? which means both rows contra-rotate at the same speed. A gearbox adds weight, but allows for the flexibility of As we have seen, a major factor in the continued control of different speeds for each row. domination of turbofan engines for economic airlines propulsion is the ability to increase bypass ratios (see A comparison of turbofan and open rotor (propfan) engines Equ. (1)). Currently, these are as high as 12:1 in the Pratt & has been given by Hendricks and Tong [3]. The comparison Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/140/12/S46/6352674/me-2018-dec9.pdf by guest on 24 September 2021 Whitney geared fan engines with Rolls-Royce planning a to similar thrust sized engines is summarized in Table 1, new geared fan engine at 15:1. taken from [3]. However, increasing fan diameters to go even higher in One can note in the table, the predicted reduced fuel bypass ratios increases nacelle weight, aerodynamic drag consumption of the 13.8 foot diameter propeller/fan open and duct losses, countering the efficiency gains predicted rotor engine, compared to the much smaller diameter by Equ. (1). turbofans. Also, the open rotor weight is highest but an aircraft so powered, will require less fuel to complete its Open rotor engines can significantly increase effective mission. This leads the authors [3] to deduce that overall bypass ratios to more than 30:1, to yield high values of aircraft takeoff gross weight would be similar between propulsion efficiency (Equ. (1)). By eliminating the need for turbofan and open rotor powered aircraft. (However, it a fan duct, studies show that open rotor engines can have a should be noted, aircraft weight might be added if fuselage 25-35% fuel consumption saving, compared to most current hardening is required for safe open rotor operation.) turbofan engines in service, and a 10-20% saving for the newer turbofans, recently introduced into the market. Geared Direct Drive Open Cruise Mach numbers, M, of airliners range from 0.7 to 0.9. Engine Parameter Turbofan Turbofan Rotor For the higher end of this range, propeller losses increase markedly (shocks forming in tip regions) so open rotor OPR 42 42 42 Top of Climb Thrust (lbf) 5000 5000 4960 engines are typically being designed to run at a maximum (M0.80;35000ft) of about M=0.78, taking full advantage of swept scimitar TSFC (lbm/hr/lbf) 0.502 0.525 0.441 shaped blades to mitigate compressibility effects [2]. OPR 32.7 33.8 29.4 Sea Level Static Thrust (lbf) 23400 22700 27300 (M0.0; 0ft; ISA+27°F) OPEN ROTOR FEATURES TSFC (lbm/hr/lbf) 0.257 0.290 0.158 Gas turbine aerodynamicists strive to have engine gas path Fan/Propeller Diameter (ft) 6.2 5.5 13.8 flows move axially wherever possible, to minimize losses Nacelle Maximum Diameter (ft) 7.6 6.7 5.6 and maximize axial momentum changes. Thus, open rotor engines have two rows of pitch controlled, contra-rotating Total Engine Pod Weight (lbm) 6630 6100 9220 propeller/fan blades, with the second row taking out the swirl from first, so that its exit flow is in a near axial Table 1. Comparison of an open rotor engine with direction. (This two row configuration also means that turbofans (Hendricks and Tong [3]). the aft row chops through blade wakes from the first row, which can generate siren-like noise.) OPEN ROTOR PROGRESS With its Hamilton Standard inception and the Arab oil embargo induced fuel price hikes of the 1970s, open rotor projects commenced in earnest in the 1980s. A Pratt & Whitney/Allison 578-DX open rotor engine was flight tested on a McDonnell Douglas MD-80 passenger aircraft in 1989. It had propeller/fan blades of diameter 11.6 ft., driven by a 13:1 gearbox. With falling fuel prices, the program ended, with a team member stating [4], “The operation was successful, but the patient died.” Starting in 1986, General Electric Aviation tested their GE36 open rotor engine (trade named Unducted Fan, or UDF) on both MD-80 and Boeing 727 aircraft. With ll.6 ft. diameter Figure 1. GE’s Unducted Fan Engine (open rotor) from the 1980s, mounted on an MD-80 aircraft. 47 propeller/fan blades, it had a direct turbine drive. The fiber 15 ft diameter propeller/fan blades. Recently plans GE36 demonstrated a 15% fuel burn reduction, compared for flight testing the engine on an Airbus aircraft have to contemporary turbofans [2], based on some 281 hours of been halted, with reports Airbus interest in the fuel-saving flight testing. Faced with a market where fuel prices were engine have been put on hold. dropping, the program was ended in 1989. In the words of a GE manager, “The signs were there the year before, that at 65 cents per gallon, the fuel price was too low to justify OPEN ROTOR CHALLENGES the UDF. If fuel were at a buck or so a gallon they’d be Van Zante [2] has outlined three technical challenges clamoring.” that remain to be addressed for successful open rotor In this new century, as fuel prices rose dramatically in the propulsion: early 2000s, the price of a barrel of oil continues to be a 1. Noise is a problem. Additional noise reduction deciding factor for the fate of open rotor engines. Added to beyond what has been accomplished to date, is Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/140/12/S46/6352674/me-2018-dec9.pdf by guest on 24 September 2021 this, is the consideration of climate change and reduction necessary. of greenhouse gas emissions by aircraft engines. 2. Airframe integration, with open rotor propeller/fan Recent development efforts include Rolls-Royce, which blade diameters of 12-15 ft.