NASA PROPELLER TECHNOLOGY PROGRAM

Daniel C. MikkelsonNational Aeronautics and Space Administration

Lewis Research Center

The vast majority of general-aviation aircraft manufactured in the United

States are propeller powered (approximately 98 percent in 1978). Most of these

aircraft use propeller designs based on technology that has not changed signif-

icantly since the 1940's and early 1950's. This older technology has been ade-

quate; however, with the current world energy shortage and the possibility of

more stringent noise regulations, improved technology is needed. Studies con-

ducted by NASA and industry indicate that there are a number of improvements in

the technology of general-aviation propellers that could lead to significant

energy savings. New concepts like blade sweep, proplets, and composite mate-

rials, along with advanced analysis techniques have the potential for improving

the performance and lowering the noise of future propeller-powered aircraft that

cruise at low speeds. Current propeller-powered general-aviation aircraft are

limited by propeller compressibility losses to maximum cruise speeds near Mach

0.5. The technology being developed as part of NASA's Advanced Turboprop Proj-

ect offers the potential of extending this limit to at least Mach 0.8. At these

higher cruise speeds, advanced turboprop propulsion has the potential of large

energy savings compared with aircraft powered by advanced turbofan systems.

This paper summarizes NASA's program on propeller technology applicable to

both low and high speed general-aviation aircraft, and outlines the overall pro-

gram objectives and approach.

EFFICIENCY TRENDS

The free-air propeller is the propulsive device that has the highest level

of inherent efficiency for subsonic aircraft. A comparison of the installed

cruise efficiency of propeller-powered and turbofan-powered propulsion systems

is shown in figure i for a range of cruise speeds. The installation losses in-

cluded with the propeller-powered systems are nacelle drag and internal cooling

airflow losses. For the turbofan-powered systems the losses include fan cowling

external drag and the internal fan airflow losses associated with inlet recovery

and nozzle efficiency. The installed efficiency available with current technol-

ogy propeller-powered general-aviation (GA) aircraft ranges from about 70 to 75

percent for reciprocating powered applications to slightly over 80 percent for

turboprops. The reciprocating system performance is slightly lower due to high-

er nacelle drag and large internal cooling airflow losses (ref. I). The in-

stalled performance of the current lower speed turboprop systems remains high to

about Mach 0.5; about this speed, efficiency falls off significantly because of

large propeller compressibility losses. These propellers are generally designed

with blades of thickness to chord ratios (at 75 percent radius) that range from

about 5 to 7 percent. These rather thick blades, when operated at relatively

high tip helical Mach numbers, are the main cause of these losses.

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The advanced, high-speed turboprop shownin figure I is a new propulsionconcept that has the potential of eliminating or minimizing compressibilitylosses at flight speeds to Mach0.8. The level of potential installed effi-ciency projected for the advanced turboprop is considerably higher than thatavailable with comparable technology high-bypass turbofan systems. At Mach0.8the installed efficiency of turbofan systems would be approximately 65 percentcomparedwith about 75 percent for the advanced turboprop. This large perfor-manceadvantage for the advanced turboprop may offer the potential for someat-tractive energy savings for future high performance business aircraft.

ADVANCED,HIGH-SPEEDTURBOPROP

To achieve the performance potential of the advanced, high-speed turboprop,several new concepts and advanced technologies are required. (See fig. 2.)Thesenew concepts and advanced technologies are discussed in references 2 and3. The advancedpropeller would be powered by a large, modern turboshaft engineand gearbox to provide the maximumpower to the propeller with a minimumenginefuel consumption. Propeller efficiency would be kept high by minimizing com-pressibility losses. In the outboard part of the propeller blading, theselosses would be minimized by using sweepand thin blade sections (2.4 percentthickness to chord ratio at 75 percent radius). Blade sweepwould also reducepropeller source noise both during takeoff and landing and during high-speedcruise. In the inboard region an area-ruled spinner, in combination with anintegrated nacelle shape, would be used to reduce the local velocities throughthe propeller to minimize losses in this region. A power loading (shaft horse-power divided by propeller diameter squared) about five times higher than thatin current GA turboprops would be used to minimize propeller diameter andweight. Eight or ten blades would be required to maximize ideal efficiency dur-ing high-altitude, high-speed cruise. In addition to these advancedconcepts amodernblade fabrication technique would be used to construct the thin, highlyswept and twisted blades.

The program that NASAhas underway to address the technology requirementsof the advanced turboprop is shownin figure 3. The advanced turboprop project,part of NASA'sAircraft Energy Efficiency (ACEE)program, has the goals of a 15to 30 percent fuel saving relative to turbofan-powered aircraft, a significantreduction in turboprop propulsion-system-related operating costs, and a cabinride quality equivalent to the best turbofan-powered aircraft. The four majorelements of the advanced turboprop project are shownin figure 3.

In the first, propeller and nacelle, technology work is currently under-way in propeller aerodynamics, acoustics, and blade structures. The use ofadvanced aerodynamics concepts in the design of high-speed propellers is dis-cussed in reference 2, and somerecent wind tunnel results are presented in ref-erence 4. A photograph of an advanced high-speed propeller model is showninfigure 4. This model, along with three others, was tested in the Lewis 8- by6-foot wind tunnel. High performance and somesignificant noise reductions wereobtained during high-speed cruise testing of these models. Somedesign studyresults on advanced propeller acoustics and blade structures are contained inreferences 5 and 6.

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The secondmajor project element, cabin environment (fig. 3), concerns theaircraft fuselage, which maybe in the direct noise path of the propeller. Thefuselage will have to adequately attenuate this noise source if the cabin en-vironmental goals are to be achieved. Somerecent analytical studies on fuse-lage interior noise control for high-speed turboprops are presented in refer-ences 7 and 8. Fuselage vibration is also an important cabin environmental con-sideration, and future advanced turboprop aircraft will have to be designed tominimize or control any undesirable vibrations.

The third major element, installation aerodynamics, is concerned with theaccelerating, swirling, propeller slipstream passing over a wing. The technol-ogy challenge here is to design the overall aircraft to achieve the best combin-ation of propulsion system performance and airplane lift-to-drag ratio, whilemaintaining adequate aircraft stability and control. Results from a recentexperimental investigation of propeller slipstream wing interactions at cruisespeeds near Mach0.8 are presented in reference 9.

The final major element is the key mechanical components. Advanceddesignand packaging technology for the core engine, gearbox, and propeller will berequired if an advanced turboprop is to reduce maintenance costs and improvereliability. A study of current-generation turboprop reliability and main-tenance costs is presented in reference i0 along with an estimate of the poten-tial improvementsavailable from advanced technology.

Becausethe four major elements of the advanced turboprop project arehighly interrelated, aircraft trade-off studies are being madeto insure thatthese technologies are properly integrated and that progress is being madetoward achieving the overall project goals. A summaryof the progress madeunder NASA'sadvanced turboprop project is contained in reference 3. Also,papers that were presented at this conference by Gatzen, Jeracki, and Bober showthe potential and someof the recent aerodynamic advances madeas part of thisproject.

LOW-SPEEDPROPELLERS

Current-generation propeller-powered GAaircraft operate at cruise speedsof Mach 0.5 and below. The technology trends that are projected for the pro-pellers of these lower speed aircraft are shownin figure 5. The sketch in thelower left of this figure depicts current-technology propellers. These pro-pellers are designed based on a trade-off of the four factors enclosed in thecenter circle. For manyapplications performance is traded off to meet noiseand cost goals. Solid aluminum blade construction is used in most designs, andthis can result in a potential weight penalty whencomparedwith someof thefuture advancedmaterials which are being studied. The design trade-off onfuture advanced technology low-speed propellers maybe altered becauseof twokey factors - outside drivers and technology opportunities. The outside driversare the high cost and fuel availability problems due to the energy shortage, thepossibility of more stringent government noise regulations, and the need to re-tain or improve aircraft safety. In the technology opportunity area several newconcepts are currently under study that showconsiderable performance and noise

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benefits. Advancedanalysis techniques will make it possible to better under-stand propeller and nacelle aerodynamics and acoustics for additional benefits.Also, lightweight high-strength composites show considerable promise. Whenthese outside drivers and technology opportunities are applied to future ad-vanced technology propellers, the design may be altered to optimize performanceto a lower noise goal using lightweight composite blades of advanced shape.This may result in a small propeller cost increase; however, this penalty shouldbe more than overcomeby the large potential performance and weight advantages.An advanced technology low-speed propeller mayresemble the design depicted inthe sketch on the upper right of figure 5. Someof the advanced features in-corporated in this design are blade sweep, proplets (propeller tip device), ad-vanced airfoils, composite blades, and improved integration of the propeller andnacelle.

The NASAresearch program that addresses the projected technology trends oflow-speed GApropellers is summarizedin figure 6. A cost-benefit study thatevaluates the effect of advanced technologies on low-speed propellers is beingconducted by the McCauleyAccessory Division of the CessnaAircraft Company.The NASALangley Research Center is sponsoring two grant programs on reducingpropeller source noise. MIT is evaluating several approaches for reducing noisewhile optimizing performance (refs. ii and 12). In a complementary program OhioState University is evaluating an alternative noise reduction approach in aflight test program. In the propeller performance area, Purdue University, un-der a NASAgrant, is involved in a program to investigate several advanced con-cepts that showconsiderable potential for improving propeller performance. Inaddition, the current low-speed propeller aeroacoustic design methodology isbeing evaluated and enhancedthrough a cooperative program betweenOhio StateUniversity and Lewis.

General-aviation propellers are usually designed as separate propulsioncomponentswithout properly accounting for the aerodynamic interaction betweenthe nacelle (or fuselage) and the propeller. A program is underway at Missis-sippi State University to develop technology that will allow the propeller andnacelle to be designed in a more unified approach. This program should lead toimproved overall propulsion system performance for general-aviation aircraft.

Propeller dynamics and aeroelastics can be serious design limitations forboth current and future advanced low-speed propellers. To better understandthis important area, NASAis sponsoring a research program at Pennsylvania StateUniversity (ref. 13). More detailed information on NASA's low-speed propellerresearch is contained in individual papers given by Keiter, Green, Korkan, andMcCormickat this conference. A summaryof the Purdue University research pro-gram on advanced performance concepts is shownin figure 7. This program, underthe direction of Dr. John Sullivan, includes analytical and experimental re-search on such new concepts as proplets (tip devices) and swept blades (ref.14). A new swept lifting line analysis program is being developed at Purdue,and they are verifying this analysis and determining the performance potentialof advanced concepts through subscale propeller wind-tunnel tests using someimproved test techniques. These new techniques include the laser velocimetersystem shownin figure 7.

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The approach being used by Mississippi State University to develop improvedpropeller-nacelle integration technology is shownin figure 8. They have con-ducted an extensive search of the propeller literature to assist in evaluatingpotential loss mechanismsand to see which of the results apply to modernGApropeller-nacelle geometries. Mississippi State also plans to conduct full-scale propeller wind-tunnel tests (in the Langley full-scale tunnel) in combin-ation with analytical studies to develop the improved propeller-nacelle integra-tion technology. This research should lead to better overall propulsion systemperformance for general-aviation propeller-powered aircraft.

In addition, to the advanced, high-speed propeller wind-tunnel test programdiscussed earlier, NASAis also testing lower speed GApropellers. A 5-foot-diameter model of one of these low-speed propellers is shownin figure 9. Thispropeller, along the with three other designs, was tested in the Lewis i0- by10-foot wind tunnel to comparemeasuredperformance with analytical predictions.Results from this comparison will be used to develop enhancedanalytical pre-diction procedures for this category of propeller.

CONCLUDINGREMARKS

The world energy shortage has led to the need for more fuel efficient GAaircraft. Propeller-powered propulsion, with its inherent high level of effi-ciency, remains an attractive propulsion concept. Current GAaircraft arelimited to maximumcruise speeds near Mach 0.5 because of propeller compressi-bility losses. The NASAresearch program on these lower speed propellers offersthe potential of significant performance improvementsand noise reductionsthrough the development of advanced concepts and new analytical design proce-dures. Extending the current cruise speed limitation to at least Mach0.8, witha large potential fuel saving comparedwith turbofan powered aircraft, maybepossible with the technologies that are being developed under NASA'sAdvancedTurboprop Project.

REFERENCES

I. Corsiglia, V. C.; Katz, J.; and Kroeger, R. A.: Full Scale Wind TunnelStudy of Nacelle Shapeon Cooling Drag. AIAA Paper 79-1820, Aug. 1979.

2. Mikkelson, Daniel C. ; et al. : Design and Performance of Energy EfficientPropellers for Mach 0.8 Cruise. NASATMX-73612, 1977.

3. Dugan, JamesF., Jr.; Gatzen, Bernard S.; and Adamson,William M.: Prop-FanPropulsion - Its Status and Potential. SAEPaper 780995, Nov. 1978.

4. Jeracki, R. J.; Mikkelson, D. C.; and Blaha, B. J. : Wind Tunnel Performanceof Four Energy Efficient Propellers Designed for Mach 0.8 Cruise. SAEPaper 790573, Apr. 1979.

5. Metzger, F. B.; and Rohrback, C. : Aeroacoustic Design of the Prop-Fan. AIAAPaper 79-0610, Mar. 1979.

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6. Cornell, R. W.; and Rothman,E. A.: Structural Design and Analysis ofProp-Fan. AIAA Paper 79-1116, June 1979.

7. Revell, J. D.; Balena, F. J.; and Koval, L. R.: Analytical Study of Inter-ior Noise Control by Fuselage Design Techniques on High SpeedPropeller-Driven Aircraft. (Lockheed-California Company,NASAContract NASI-15427.) NASACR-159222, 1980.

8. Rennison, D. C.; Wilby, J. F.; and Marsh, A. H.; Wilby, E.G.: Interior NoiseControl Prediction Study for High-Speed Propeller-Driven Aircraft. (BoltBeranek and Newman,Inc.; NASAContract NASI-15426.) NASACR-159200, 1980.

9. Bencze, D. P.; et al.: Propeller Slipstream Wing Interactions at MachNo.0.8. SAEPaper 780997, Nov. 1978.

I0. Stolp, Philip C. and Baum, JamesA.: AdvancedTurboprop Propulsion SystemReliability and Maintenance Cost. SAEPaper 771009, Nov. 1977.

ii. Succi, G. P.: Design of Quiet Efficient Propellers. SAEPaper 790584, Apr.1979.

12. Larrabee, E. E.: Practical Design of MinimumInduced Loss Propellers. SAEPaper 790585, Apr. 1979.

13. McCormick, B. W.; et al.: The Anlaysis of Propellers Including InteractionEffects. SAEpaper 790576, Apr. 1979.

14. Sullivan, J.: The Effect of Blade Sweepon Propeller Performance. AIAApaper 77-716, July 1978.

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ADVANCEDTURBOPROP PROJECT

MECHANICALCOMPONENTS

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LOWSPEEDPROPELLERTECHNOLOGYTRENDS

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PROPELLER NACELLE INTEGRATION

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Figure 9

32.5