THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 88-GT-321 345 E. 47 St., New York, N.Y. 10017
The Society shall not be responsible for statements or opinions advanced in papers or 1n d'1s cussion at meetings of the Society or of its Divisions or Sections, or printed in its publications Discussion is printed only if the paper is published in an ASME Journal. Papers are available from ASME for fifteen months after the meeting. Printed in USA. Copyright © 1988 by ASME Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1988/79191/V002T02A027/2397578/v002t02a027-88-gt-321.pdf by guest on 27 September 2021 Evaluation of Potential Engine Concepts for a High Altitude Long Endurance Vehicle
EDWARD J. KOWALSKI Boeing Advanced Systems P. 0. Box 3707, M/S 33-18 Seattle, WA 98124-2207
ABSTRACT till a speed of Mach 0. 6 has been attained. The aircraft continues to climb at this speed till the A potential need has been identified for a High cruise altitude of 45,000 feet has been reached. The Altitude Long Endurance (HALE) aircraft to augment aircraft will continue to cruise at this flight current surveillance and engagement capability. HALE condition for a total of 200 nm. llhen the mission platforms offer mission flexibility and survivability radius has been reached, the aircraft will loiter for which can complement ground based surveillance and 24 hours at Mach 0.6 and an altitude of 45,000 ft. The engagement systems. Current mission requirements aircraft cruises back at Mach 0. 6 at an altitude of include a loiter altitude of 45,000 to 60,000 feet and 45,000 feet then descends to sea level and loiters for a loiter time of 12 to 24 hours. The HALE aircraft 30 minutes at Mach 0. 3 before landing. The aircraft is will also be required to carry a sensor payload weight required to maintain a minimum rate of climb capability between 50,000 and 100,000 pounds. This paper will of 100 fpm at the start of the 24 hour loiter segment. evaluate the potential of several propulsion system candidates. Engines to be examined include the "classical" turbofan engine with bypass ratios up to ------eight, the "ultra high bypass ratio" turbofan with ------'' ® bypass ratios up to 20, General Electric's Unducted Fan ---6� (UDF) and the turboprop in a pusher and tractor ,- --� configuration with single and counter rotation © prop fans.
INTRODUCTION
The Boeing Advanced Systems has had an Independent ® Research and Development ( IR&D) project entitled Propulsion System and Airframe Integration in existence for more than ten years. As one of its objectives, studies are conducted which will identify propulsion nm systems applicable to advanced military weapons CD 200 systems. Dialogue is maintained between BAS and engine G) companies in order to keep abreast of the latest engine company offerings. Takeoff at maximum power ® 2 In order to evaluate the potential of several @ Climb at q=200 lb/ft propulsion system candidates on a HALE vehicle, work was jointly done with Boeing Advanced Systems "Advanced Climb at Mn=0.6 Long Endurance Vehicle Technology" program. It is the © intent of this paper to understand the impact of high ® Cruise at Mn=0.6, ALT=45000 ft altitude surveillance mission requirements on aircraft design characteristics, engine cycle characteristics ® Loiter at Mn=0.6, ALT=45000 ft for 24ft hr and propulsion system concepts. (i)Cruise back at Mn=0.6, ALT=45000 Mission Description @ Descend to sea level The mission used for this analysis is depicted in Figure 1. The aircraft is required to takeoff at Loiter at Mn=0.3, ALT =0 ft for 1/2 hr maximum power then climb at a constant q of 200 lb/ft2
Presented at the Gas Turbine and AeroengineFigure 1. Congress High Altitude Long Endurance Mission Profile Amsterdam, The Netherlands-June
6-9, 1988 I
Aircraft Description "Classical" Turbofan Engine Description and Cycle Selection Figure 2 depicts a configuration designed to satisfy HALE mission requirements. This configuration The Pratt & llhi tney Aircraft parametric engine features a 50,000 pound sensor with near 360° viewing performance program (Ref. 2) was used to generate engine capability. The sensor which it carries consists of a performance, geometry and weight for a set of large phased array radar incorporated into the side of "classical" turbofan engines. the aircraft's fuselage. This sensor's field of view is to the side and from the horizon downward. Hence, A "first order" method was devised to determine the aircraft's wings and engines are mounted out of the the optimum engine cycle for a HALE application. Due Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1988/79191/V002T02A027/2397578/v002t02a027-88-gt-321.pdf by guest on 27 September 2021 sensor's field of view. Additional sensors are located to the 24 hour loiter requirement, it was assumed that in the nose and tail of this aircraft as well as in a the engine cycle which had the minimum combination of small turret on top of the fuselage. This fuel consumption (during the loiter segment) and engine configuration serves as the baseline for this study to weight would be the optimum engine cycle. Engine determine the impact of engine cycle and concept on the performance and weight were generated for engines with size of a HALE aircraft. The aircraft was redrawn for the following ranges of cycle characteristics: each propulsive concept: turbofan ("classical" and ultra high bypass) turboprop (pusher and tractor) and 4 < BPR < 8 UDF. 1900°F < T 4 < 3200°F 25 < OPR < 35 The subsonic inlet on the baseline configuration 1 < e < 1. 035 is designed with a relatively thin lip and has no boundary layer bleed or bypass system. Blow-in doors A power line of thrust versus specific fuel are used at takeoff and low speed. Axisymmetric consumption was generated at the mission loiter convergent nozzles were also used. condition, Mach=0. 6 and an altitude of 45,000 feet. It was assumed that the aircraft would loiter at the Aircraft Installation minimum specific fuel consumption (SFC) point. The thrust at that point was determined and then scaled to The Boeing Advanced Systems developed Engine a value required by the aircraft per engine at the Installation Analysis Program (Ref.1) was used to loiter condition. At the minimum SFC point, the amount account for the installation losses that occurred when of fuel burned by the engine during the 24-hour loiter the engines were integrated onto the airplane with its time was calculated. The engine weight was also scaled inlet and nozzle. to reflect the loiter thrust requirement. The fuel
�i:-
,,,.1, � / 1- - I o _j"T'T--16ft---- in ·------295 ft in0
80 ft in0 --)F1-. -1 .- /�-: --�rr��I I 84ft9in
, L_�-�� i 1 I ,, � c �;,:;,,m /
Figure 2. Aircraft Configuration for High-Altitude Surveillance 2 burned was added to the scaled engine weight. This Vehicle Sizing with "Classical" Turbofan Engines total weight will be referred to as the Propulsive System Yeight Parameter (PSYP). Figure 3 shows a For each of the three engine cycles described above a range of aircraft wing loaoverall pressure ratio (OPR) and maximum lb/ft ) and thrust-to-weight ratios (T/Y=0.28-0. 45) were examined. Figure 4 shows a performance map of the turbine inlet temperature (T4) for a throttle ratio of 1. 035. As the design BPR increased from 4 to 8, the resulting sized aircrafts for the BPR=8 engine. The same performance maps were generated for the aircrafts m1n1mum design T4 which could be run in Pratt & Yhitney's parametric engine performance program using the selected BPR 4 and 6 engines. Aircraft Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1988/79191/V002T02A027/2397578/v002t02a027-88-gt-321.pdf by guest on 27 September 2021 increased due to the deck's design limit on fan constraints and mission requirements were accounted pressure ratio (FPR) of 1. 5. Figure 3 also shows that for. On each plot, a wing span limit of 300 feet and a for the engines designed with BPRs of 6 and 8, minimum rate climb of 100 fpm were imposed. The resulting minimum TOGY and the corresponding Y/S and combination of minimum T4 and maximum OPR resulted in the overall minimum Propulsive System Yeight Parameter. T/Y for each engine are as follows: The hook which results for the set of engines designed BPR TOG\l (lb) Y/S T/Y with a bypass ratio=4.0 at a T4=1900°F occurs due to the parametric engine performance program's limit on 4 356,000 92 o.27 fan pressure ratio of 1. 5. 6 342,000 94 0.28 8 340,000 96 0. 29 For the design bypass ratios of 4, 6 and 8, the following engine design cycles were selected to size Propulsion System Yeight Parameter Method Validation the HALE vehicle due to their respective minimum values of the PSYP: To further understand the effect of PSYP on final aircraft takeoff gross weight and hence validate the 4 6 8 PSYP method as a screening tool in the preliminary 35 35 35 design of these types of aircraft, vehicle s1z1ng 2200 2200 2400 studies were performed on a total of nine engines from 1. 035 1. 035 1. 035 each of the groups of bypass ratio engines for a total of 27 sized vehicles. These engines are highlighted (large circle symbols) on Figure 3. For each of the 27
58
Legend:
57 • Engine used to size HALE aircraft Q Signify 27 engines used for method verification study 56 Altitude = 45,000 ft Mach= 0.6 Minimum SFC PT 55
£! 0 0 0 Q) 54 a;L- E ro en 53 .E Q(J)
w 52 ;;:E
u;>- Q)"' c 51
0:::J ·w 0::Q 0 50
49 BPR =
4 48
47 BPR = 6
Figure 3. Propulsion System Weight Parameter Versus Engine Cycle Characteristics 3 engine cycles, a range of airplane wing loadings (W/S=70-105 lb/ft2) and thrust-to-weight ratios (T IW=O. 28-0. 45) were examined. Carpet plots of the sized aircrafts were constructed for each engine. A wing span limit of 300 feet and a minimum rate of climb "Classical" Turbofan of 100 fpm were imposed. The resulting minimum TOGWs BPR =8 are displayed in Figure 5. The engines highlighted with a black symbol are those engines that yielded the minimum PSWP factor from Figure 3. '
,Q Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1988/79191/V002T02A027/2397578/v002t02a027-88-gt-321.pdf by guest on 27 September 2021 70 ' The results of this study show that the Propulsive 460 WIS System Weight Parameter provides a good preliminary 0 0 method for engine cycle select ions for a HALE 0 440 application (i. e. a long endurance cruise mission) . E 420 Turboprop/Propfan Selection "ill Ol::
This process of minimizing the PSWP was extended 400 eCJ) to include the use of turboprop engines on the HALE OlCJ) aircraft. Two turboprop engines were assessed: the 380 � 0.30 Pratt & Whitney STS742 (tractor configuration) and the 15 0.28 STS743 (pusher configuration) . Both the STS742 and 360 Q) STS743 are three spool engines. The high pressure f-.¥. ell spool consists of a high pressure compressor (four 340 stage axial plus one stage centrifugal) driven by a @ single stage high pressure turbine. The low pressure Selected 100 FPM rate-of-climb spool incorporates a four stage, low pressure design capability start of loiter compressor driven by a single stage, low pressure point turbine. Power output to the propeller gearbox is provided by a four stage, free turbine. Uninstalled engine performance was calculated for the STS742 and Figure 4. Takeoff Gross Weight Versus Wing STS743 using P&W's Steady State Engine Performance Computer Decks, Ref. 3 and 4, respectively. Loading and Thrust Weight
�0-.., § 380 .£ � £ 375 340 BPR =6 BPR :8 7 335 Legend: • 330 Engines chosen using PSEP prediction method Figure 5. MaximumTakeoff Gross Turbine Weight Inlet VersusTemperature Operating Pressure Ratio, Bypass Ratio, and 4 To understand the impact of integrating propeller and turboprop characteristics on mission performance, variables such as: the mission's maximum flight speed, number of propeller blades, propeller loading (SHP/02) Tractor STS 742 engine with and tip speed (UTrp), and engine performance must be 4X4 counter-rotating propfan optimized simultaneously. A "first order" computer program was utilized to access the impact of the entire 53.5 propulsive system (engine, gearbox and propeller) .n weight and fuel consumption on mission performance. As 0 0 51.5 Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1988/79191/V002T02A027/2397578/v002t02a027-88-gt-321.pdf by guest on 27 September 2021 outlined earlier for the "classical" turbofan engines, 0 the PSWP was determined. The turboprop engines were analyzed using different propfan configurations with Ci)....: Ea> 49.5 performance provided by Hamilton Standard (Ref. 3, 4 Disk loading � 2 and 5). These propfan configurations included: 6, 8 SHP/D and 10 bladed single rotation propfans and 4x4, 5x5 and 47.5 cu 6x6 bladed counter-rotating propfans. For each of the a...1:: turboprop propulsive systems, the PSWP was computed for .Ql a range of tip speeds (600-900 ft/sec) and loadings � 45.5 2 a> (5-40 SHP/0 ) at the loiter condition. Carpet plots of E PSWP versus tip speed and loading were examined to U5 43.5 determine the optimum combination. Figure 6 is an a> example plot for the STS742 turboprop engine operating (/) c>. with a 4x4 counter rotating propfan. In this example, 0 41.5 a pro fan tip speed of 800 ft/sec and a loading of 15 i2 SHP/O� were chosen as the "optimum" propfan characteristics due to its minimum value of the PSWP 2:J 39.5 Cl. factor. A tip speed of 900 ft/sec was discounted a... because this speed would cause supersonic tip 37.5 velocities. Similarly, for each of the twelve turboprop/ 35.5 propfan systems, minimum PSWP factors were determined. The results for both the tractor and pusher turboprop engines are tabulated in Figure 7 in terms of the minimum PSWP factor. This is done for each engine/ Figure 6. Propulsion System Weight Parameter Versus propfan combination. Tip Speed (ft/sec) and Disk Loading (SHP/D ) STS742 STS743 Propfan (Tractor Configuration) (Pusher Configuration) NUMBER U TIP PSWP U p PSWP 2 OF BLADES TYPE (FT/SEC) SHP/D2 (1,000 LB) (FTA SEC) SHP/D (1,000 LB) SR 800 8.0 37.2 8.0 38.6 6 800 8 SR 800 10.0 37.5 800 10.0 38.8 10 SR 800 11.5 37.8 800 12.0 39.3 4x4 CR 800 15.0 35.6 800 13.0 37.2 5x5 CR 800 17.5 36.1 800 17.0 37.3 CR 800 22.5 36.1 20.0 37.5 6x6 800 LEGEND: SR SINGLE ROTATION CR COUNTERROTATION Figure 7. Resulting Propfan Characteristics for Minimum Propulsion System Weight Parameter 5 Vehicle Sizing with Turboprop Engines For each engine/propfan configuration with the Tractor STS 742 engine with minimum PSWP values from the preceeding table, a 4X4 counter-rotating propfan vehicle sizing study was then performed. A range of ft aircraft wing loadings (70-100 lb/ft2) and thrust-to Span constraint= 300 weights (0. 28-0. 45) were examined. Carpet plots of the sized aircrafts were constructed for each 1 � engine/propfan configuration. A wing span limit of 300 335 Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1988/79191/V002T02A027/2397578/v002t02a027-88-gt-321.pdf by guest on 27 September 2021 feet and a minimum rate of climb of 100 fpm were 70 imposed . An example plot for the STS742 operating with 0.45 Wing loading (lb/ft2 ) a 4x4 counter rotating propfan is presented in Figure 325 8. The other engine/ propfan combinations (from the ,Q � preceeding table) were studied in a similar manner. g Minimum TOGW's for each turboprop engine/propfan configurations are tabulated in Figure 9. 0 315 E -� Unducted Fan Engine 305 The Undue ted Fan (Ref. 6) is a unique type of ::;: engine concept developed by General Electric with NASA. 2en Olen The UDF has a counter-rotating unducted fan directly driven by a power turbine. This design concept :g 295 eliminates the need for a reduction gearbox to transmit -t;lQ) power to the fan blades. f- 285 Vehicle Sizing with the Unducted Fan 30 The same baseline aircraft was again optimized for 275 t wing loading and thrust-to-weight to obtain minimum 10.28 TOGW for the Unducted Fan. A carpet plot of the sized aircrafts were constructed for the UDF. A wing span limit of 300 feet and a minimum rate of climb of 100 265 Selected design point fpm were imposed. Figure 10 shows this carpet plot for the UDF engine. The resulting minimum aircraft TOGW of Rate-of climb constraint= 100 ft 295,000 pounds occurred at a wing loading of 95 lbs/ft2 per min at start of loiter and a thrust-to-weight of 0. 27. Figure 8. Takeoff Gross Weight Versus Wing Loading and Thrust to Weight STS742 STS743 Prop fan (Tractor Configuration) (Pusher Configuration) NUMBER W/S W/S OF BLADES TYPE TOGW (LB) (LB/FT2) T/W TOGW (LB) (LB/FT2) T/W 6 SR 279,000 90 0.31 292,000 90 0.32 8 SR 283,000 90 0.31 295,500 90 0.31 10 SR 286,000 90 0.30 298,550 90 0.30 4x4 CR 271,000 90 0.31 282,500 90 0.31 5x5 CR 273,000 92 0.29 284,000 88 0.30 6x6 CR 276,000 93 0.28 286,000 90 0.29 LEGEND: SR SINGLE ROTATION CR COUNTERROTATION Figure 9. Resulting Minimum TOGW for Minimum Propulsion System Weight Parameter 6 STS749 Unducted Fan Ultra-High Bypass� Turbofan l � 440 Wing loading� ·f;/J 70 o.4s Wing :9 410 �Qj Loading 80 80 Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1988/79191/V002T02A027/2397578/v002t02a027-88-gt-321.pdf by guest on 27 September 2021 0 420 :9 0 0 390 .E 0 Ol 0 "(ii 0 3':: 400 .E 370 Cf) "(ii 0.... Ol Cf) 3':: CJ 380 - 350 a eCf) (BPR=14. 7) over this same "classical" turbofan Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1988/79191/V002T02A027/2397578/v002t02a027-88-gt-321.pdf by guest on 27 September 2021 A study was made to evaluate the potential of (BPR=4. 0) can be attributed to the low engine thrust several propulsion systems utilized on a High Altitude to-weight ratio of V2500 Superfan. Long Endurance (HALE) vehicle. Performance and weight characteristics of "classical" and ultra-high bypass turbofans, pusher and tractor configured turboprops and the Unducted Fan were used to size a HALE vehicle for a fixed mission radius of 200nm. In a second study, P" E 0 perturbations were made on engine weight and specific Ol V2500 Superfan fuel consumption to show their effect on aircraft size. ·a; 12 3: 0 STS749 8 L:J..UDF A "first order" method was utilized to determine 0 STS743/SR/6 BLD the optimum engine cycle of a "classical" turbofan rne D rn 0 STS742/SR/6 BLD (BPR=4,6,8). Engine performance and weights were (.'.) - STS742/CR/4x4 BLD generated for a set of turbofans over a range of design 0 Q) STS743/CR/4x4 BLD cycle characteristics. For each engine, its weight and .::t'. the amount of fuel burned during the HALE mission 24 nl 4 I- hour loiter time were added together. This total .C::: weight was referred to as the Propulsion System Weight Q) Parameter (PSWP). The three engines with the engine Ol 0 c -4 +10. design cycle characteristics which minimized this nl -10 ..c parameter (as a function of BPR) were used to size the 0 HALE vehicle. The validity of this method was then c proven when aircrafts were sized for three sets of Q) -8 2 engines with design cycle characteristics similar to Q) the three chosen cycles. a.. -12 Percent Change in Engine Specific Fuel Consumption 400 Figure 13. ofPercent Engine Change Specific in Fuel Aircraft Consumption TOGW as a Function ,...... 356K � 343K 342K 340K 0 0 350 0 333K � '-' w 3: (/) 295K Zv co CXl (J) oz a... a... a... v z v 0 Figure 14. HALE Vehicle TOGW Comparison 8 Figure 3 indicates a decreasing trend in the value The impact of varying engine weight on aircraft of the propulsion system weight parameter as the design size seems to result in the same distribution of overall pressure ratio of the "classical" turbofans propulsive systems. The two turboprops and the UDF had increased. Presently, the Pratt & \./hitney Aircraft an average of +1.8% variation in TOGW while the two parametric engine performance program (Ref. 2) has an ultra-high bypass turbofans had a +3.4% and a -1.9% upper limit of 35 in overall pressure ratio. Point variation. design engines with higher OPR (such as those engines which resulted from the NASA Energy Efficient Engine REFERENCES (E3) Program) should be evaluated. Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1988/79191/V002T02A027/2397578/v002t02a027-88-gt-321.pdf by guest on 27 September 2021 1. Atkins, R. A., E. s. Schreffler, "Engine Installation Analysis Program User's Manual", The "first order" method of minimizing the PS\.IP Boeing Document Dl80-26839-1, 1982. for the "classical" turbofan engines was extended to include the use of turboprop engines. Performance and 2. Pratt & Whitney, "User's Manual for Pratt & weight were generated for two turboprop configurations: \./hitney Aircraft JT69 Engine Family Performance pusher and tractor. The performance of the turboprop Model", CCD 1177-03.00, August 1981. propulsive systems was generated using three propfan configurations: 6, 8 and 10 bladed single rotation 3. Pratt & Whitney, "User's Manual for Steady State propfans and 4x4, 5x5 and 6x6 bladed counter-rotating Performance of the STS742 Study Engine", CCD prop fans. Propulsive sys tern weight included that of 0628-01.0, July 1985. the turboprop engine, gearbox and propfan. For each of the propulsive systems, the PSWP was computed for a 4. Pratt & Whitney, "User's Manual for Steady State range of propfan tip speeds and loadings. Those Performance of the STS743 Study Engine", CCD propulsive systems which minimized the PS\.IP were used 0629-01.0, August 1985. to size the HALE aircraft. 5. United Technologies Hamilton Standard, "6, 8 and Both the pusher configured turboprop (STS743) and 10 Bladed Propeller Parametric Data Package", the tractor configured turboprop (STS742) minimize SP08482, May 1982. aircraft TOGW when operating with a 4x4 counter rota ting prop fan. \./hen both turboprops are operating 6. General Electric, "Unducted Fan, GE36 Study B22" with a single-rotation propfan, TOG\./ is minimized with G008 1B, September 1985. a six bladed propfan. 7. Pratt & Whitney, "STS749 Ducted Prop Engine", The design characteristics of both the pusher July 1985. configured turboprop (STS743) and the tractor configured turboprop (STS742) were fixed by Pratt & 8. International Aero Engine, "V2500 Superfan", CCD \./hitney . Propfan characteristics (single versus 0683-00.00, February 1987. counter rotation, number of blades, tip speed and propeller loading) were optimized in this study for these fixed turboprop designs. A more accurate approach would be to vary turboprop and propfan design characteristics simultaneously in order to minimize the propulsion system weight parameter. This method could possibly result in a different choice of both the turboprop and propfan design characteristics. The UDF aircraft weighed 11.41% less than the STS749 ultra-high turbofan aircraft but weighted 8.9% more than the tractor turboprop aircraft operating with a 4x4 counter-rotating propfan. A sensitivity study was conducted to determine the effect on aircraft takeoff gross weight when engine weight and specific fuel consumption are varied +10%. As expected, the variation of engine specific -f uel consumption had a greater effect on aircraft takeoff gross weight than the variation of engine weight. This is primarily due to the mission's 24 hour loiter requirement. Varying specific fuel consumption has a lesser impact on TOG\./ for the two turboprops than that of the two ultra-high bypass turbofans. The change in specific fuel consumption of the STS742 and the STS743 using a six bladed single rotation propfan and a 4x4 counter rotation propfan, resulted in an average +8.1% and -7. 5% change in TOG\./. The STS749 and the V2500 Superfan produced changes in TOG\./ that ranged from +13 .1% to -10.1%. The UDF falls in between with a +9. 5% and a -8. 1% change in TOG\./. 9