ALLEVIATION OF FUSELAGE FORM DRAG USING VORTEX FLOWS DOE/CE/15277--T1 Final Report on work performed under TI89 004158 U.S. Department of Energy Grant DE-FG01-86CE15277 15 September 1987 DISCLAIMER DR. A. Wortman This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness. or usefulness of any information, apparatus, product, or . process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. ISTAR INC. 406 Aka Avenue Santa Monica, CA 90402 (213) 394-7332 PROGRAM MONITORS: _* T. Levinson D. Mello DISCLAIMER Portions of this document may be illegible electronic image products. Images are produced from the best available original document. FOREWORD AND ACKNOWLEDGEMENTS The concept of employing discrete large vortices, to develop favorable cross-flow and to energize the boundary layer in the aft regions of transport aircraft fuselages, was first proposed by the author almost 10 years ago as an apparently original approach to the reduction of fuselage drag. A series of feasibility demonstration proposals, starting with the 1981 USAF DESAT program was submitted to various U.S. Government agencies, but success did not come until favorable evaluations of the concept by the Office of Energy Related Inventions of the National Bureau of Standards led to a recommendation to the Energy Related Inventions Program of the U.S. Department of Energy and a grant for wind tunnel tests. Special thanks are due to D. Mello and T. Levinson of the Department of Energy who monitored this project and guided it through all its administrative and contractual pitfalls. If this concept is ever deployed commercially, it will be primarily due to M. Rorke of Mohawk Research Corporation and J. Vitullo of the Department of Energy who gave me the opportunity to__ attend~~~ ~ the extremely valuable Commercialization Planning Workshop in August 1987. The California Institute of Technology 10 ft Wind Tunnel personnel (Jerry Landry in particular) were, as always, ex- tremely helpful and efficient. Finally acknowledgement with thanks is made to the superb efforts of Gay1 A. Brinlee without whose assistance and support this concept would not have developed beyond a glimmer of an idea. i P-36 I SUMMARY The concept of using vortex generators to reduce the fuselage form drag of transport aircraft combines the outflow from the plane of symmetry which is induced by the rotational component of the vortex flow with the energization of the boundary layer to reduce the momentum thickness and to delay or eliminate flow separation. This idea was first advanced by the author in 1981. Under a DOE grant, the concept was validated in wind tunnel tests of approximately 1:17 scale models of fuselages of Boeing 747 and Lockheed C-5 aircraft. The search for the minimum drag involved three vortex generator configura- tions with three sizes of each in six locations clustered in the aft regions of the fuselages at the beginning of the tail upsweep. The local Reynolds number, which is referred to the le gth of boundary layer run from the nose, was approximately 107 so that a fully developed turbulent boundary layer was present. Vortex generator planforms ranged from swept tapered, through swept straight, to swept reverse tapered wings whose semi-spans ranged from 50% to 125% of the local boundary layer thickness. Pitch angles of the vortex generators were varied by inboard actuators under the control of an external propor- tional digital radio controller. It was found that certain combinations of vortex generator parameters increased drag. However, with certain configurations, locations, and pitch angles of vortex generators, the highest drag reductions were 3% for the 747 and about 6% for the C-5, thus confirming the arguments that effectiveness increases with the rate of upsweep of the tail. Greatest gains in performance are therefore expected on aft loading military transports. Incremental reductions in fuselage drag translate into approximately 1/3 incremental reductions of overall aircraft drag. Therefore, the overall drag reductions are expected to be 1% for the 747 and 2% for the C-5. For the Boeing 747 this translates into annual operating cost reductions of about $130,000. Preliminary estimates indicate that the installed cost of a set of vortex generators will be under $5,000 and maintenance costs of the 2 ft wing-like vanes will be negligi- ble. In terms of return on investment on a low development risk, low installation cost, no maintenance costs project the concept should.be extremely easy to deploy commercially. ii P-3 6 TABLE OF CONTENTS . INTRODUCTION ................................................ 1 EXPERIMENTS ................................................. 8 Models ................................................. 8 Vortex Generators ...................................... 10 Controls and Actuators .................................13 Procedure.............................................. 13 Wind Tunnel ............................................ 13 Test Aerodynamics ......................................13 RESULTS ..................................................... 16 FULL SCALE IMPLEMENTATION ...................................18 ECONOMIC CONSIDERATIONS ..................................... 21 Initial Cost ........................................... 21 Certification .......................................... 22 Maintenance and Life ................................... 22 Operational Problems ................................... 22 Operating Cost Reductions .............................. 22 Investment............................................. 23 MARKET ...................................................... 24 DISCUSSION .................................................. 25 CONCLUSIONS ................................................. 27 APPENDIX A .Models. Controls and Actuators ................A-1 APPENDIX B .Marketing of the Concept ...................... B-1 APPENDIX C .Wind Tunnel Data .............................. c-1 iii P-36 INTRODUCTION Transport aircraft generally follow the form developed by Sir George Cayley almost 200 years ago in having a wing, a fuselage and empennage. The volume of the fuselage is deter- mined by cargo or passenger load requirements with the overall length being fixed by a compromise between operational con- siderations and aerodynamic efficiency. In general, the aft ends of transport aircraft fuselages are tapered asymmetrically to some minimum base area with a pronounced upsweep of the bottom contour to facilitate rotation in the pitch plane on landing and take-off. The upsweep of the fuselage contour is much more pronounced in aft-loading aircraft such as C-130, C-141, C-5 or the CASA 212. Some representative examples of transport aircraft are shown in Figure 1. Because of the fuselage upsweep, and the rapid decrease of the fuselage cross- sectional area, a strong adverse longitudinal pressure gradient is established. Consequently, the boundary layer on the. fuselage grows very rapidly, and may even separate so that a large volume of low energy flow is established around the fuselage. This in turn results in very high momentum defect in the wake and an increase of the fuselage form drag. The contributions of the various components of an aircraft to the total aerodynamic drag depend on the type of aircraft, mission, and loading. A representative example or a Lockheed C-141 transport at M=0.75 is taken from Nicolai‘ and is shown here in Figure 2. It is seen that the zero-lift drag, CDo is approximately 60% of the total aircraft drag. Nicolai also shows (on p. 2-15) that the zero lift drag CDo may be estimated using with CDF being the skin friction drag coefficient. With interference drag estimated at 5%, the fuselage form drag in cruise condition is therefore about 10% of the total drag. The effect of reducing the fuselage drag on the reduction of the total drag coefficient is readily estimated from the relation ‘D = ‘D,L,t + ‘Do = ‘D,L,t + c~,~+ CD,F (2) with CD - total drag coefficient ‘D,L,t - lift and trim drag coefficient ‘D,W - wing drag coefficient CD,F - fuselage drag coefficient P-36 1 h d h c3z h U 'rn W h I 0 m m 0CY czk W z w CY x W I-e >0 2 100 90 80 a 70 DRAG DUE .TO LlFT e' 50 I- E 30 e 20 IO 0 1.1 Figure 2. Cruise Drag for C-141 Logarithmic-differentiation yields dcdcD = F'ldCD, F/cD, F (3) with F - Influence coefficient = CD,~,t/CDIF+ CD,W/CD,F +1 The quotients in F may be estimated at being approximately equal to unity. Therefore dc~/c, =- 1/3 (~CD,F/~D,F) (4) This shows that an incremental change in the fuselage drag coefficient results in approximately 1/3 the incremental changes in the total drag coefficient. Aircraft designs are being continuously refined to enhance their performance and operating economy. When the basic performance and operating requirements are defined, the con- figurations of the basic components