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Some of the specific areas associated with straight jet and turboprop engine installations are outlined where drag reduction and, thus, improved aircraft system performance is obtained. Specific areas constitute air intake sizing for general aviation aircraft, exhaust duct geometries and cooling system arrangements for propeller powered aircraft.
A set of digital flight control laws were designed for the NASA F-8C digital fly-by-wire aircraft. The control laws emphasize Control Configured Vehicle (CCV) benefits. Specific pitch axis objectives were improved handling qualities, angle-of-attack limiting, gust alleviation, drag reduction in steady and maneuvering flight, and a capability to fly with reduced static stability. The lateral-directional design objectives were improved Dutch roll damping and turn coordination over a wide range in angle-of-attack. An overall program objective was to explore the use of modern control design methodilogy to achieve these specific CCV benefits. Tests for verifying system integrity, an experimental design for handling qualities evaluation, and recommended flight test investigations were specified.
An empirical formula for the steady-state drag coefficient of a 20-degree conical ribbon parachute is developed. The derived expression takes into account the effect of suspension line length and geometric porosity within the limits of practical design. Also included are factors which provide drag reduction due to skirt reefing and the wake behind a primary body. The calculated values are in agreement with the available experimental results.
Assuming Space Shuttle technology to be state-of-the-art, projected technological advances to improve the capabilities of single-stage-to-orbit (SSTO) derivatives are examined. An increase of about 30% in payload performance can be expected from upgrading the present Shuttle system through weight and drag reductions and improvements in the propellants and engines. The ODINEX (Optimal Design Integration Executive Computer Program) program has been used to explore design options. An advanced technology SSTO baseline system derived from ODINEX analysis has a conventional wing-body configuration using LOX/LH engines, three with two-position nozzles with expansion ratios of 40 and 200 and four with fixed nozzles with an expansion ratio of 40. Two assisted-takeoff approaches are under consideration in addition to a concept in which the orbital vehicle takes off empty using airbreathing propulsion and carries out a rendezvous with two large cryogenic tankers carrying propellant at an altitude of 6100 m. Further approaches under examination for propulsion, aerothermodynamic design, and design integration are described.
An empirical formula for the steady state drag coefficient of a 20 degree conical ribbon parachute is developed. The derived expression takes into account the effect of suspension line length and geometric porosity within the limits of practical design. Also included are factors which provide drag reduction due to skirt reefing and the wake behind a primary body. The calculated values are in agreement with the available experimental results.
Surface motion of compliant walls in drag reduction experiments was analyzed. The spectrum of surface motion indicates that membranes over deep cavities respond at low frequencies and large wavelengths. The membrane over a deep cavity is therefore found not to yield the desired reponse predicted by the postulated mechanism. The membrane over a thin air gap is found to act as a wavelength chopper, and analysis of the nonlinear response of the compliant surface indicates its possible suitability for compliant wall experiments. Periodic structures are found to lock in the desired wavelengths of motion. Laminated structures are found to be very ineffective as compliant models, except when there is no bonding between the membrane and the backing. Computer programs developed for these analyses are documented.
Results from recent studies of air transportation energy efficiency alternatives are discussed, along with some of the implications of these alternatives. The fuel-saving alternatives considered include aircraft operation, aircraft modification, derivative aircraft, and new aircraft. In the near-term, energy efficiency improvements should be possible through small improvements in fuel-saving flight procedures, higher density seating, and higher load factors. Additional small near-term improvements could be obtained through aircraft modifications, such as the relatively inexpensive drag reduction modifications. Derivatives of existing aircraft could meet the requirements for new aircraft and provide energy improvements until advanced technology is available to justify the cost of a completely new design. In order to obtain significant improvements in energy efficiency, new aircraft must truly exploit advanced technology in such areas as aerodynamics, composite structures, active controls, and advanced propulsion.
The paper is concerned with the realistic case of two-dimensional random motion of a membrane with bending stiffness supported on a viscoelastic spring substrate and on an elastic base plate under both subsonic and supersonic boundary layer turbulence. The cross-power spectral density of surface displacements is solved in terms of design variables of the compliant wall - such as the dimensions and material properties of the membrane (Mylar), substrate (PVC foam), and panel (aluminum) - so that a sensitivity analysis can be made to examine the influence of each design variable on the surface response statistics. Three numerical examples typical of compliant wall design are worked out and their response statistics in relation to wave drag and roughness drag are assessed. The results can serve as a guideline for experimental investigation of the drag reduction concept through the use of a compliant wall.
Improvements in performance, safety, efficiency, and emissions control in general aviation craft are reviewed. While change is slow, the U.S. industries still account for the bulk (90%) of the world's general aviation fleet. Advances in general aviation aerodynamics, structures and materials, acoustics, avionics, and propulsion are described. Supercritical airfoils, drag reduction design, stall/spin studies, crashworthiness and passenger safety, fiberglass materials, flight noise abatement, interior noise and vibration reduction, navigation systems, quieter and cleaner (reciprocating, turboprop, turbofan) engines, and possible benefits of the Global Position Satellite System to general aviation navigation are covered in the discussion. Some of the developments are illustrated.
Growth projections for aviation technology are put forth for a quarter-century ahead. Three main trends envisaged are towards: great efficiency and economy and longer range and endurance for subsonic aircraft; new generations of short-range fixed-wing craft and rotorcraft with versatile applicability; supersonic and hypersonic speeds. Improvements in lift/drag ratio, specific fuel consumption, structural weight factor, aerodynamic improvements (laminar flow control, increased wing aspect ratio, drag reduction for specified lift, propulsion efficiency, higher bypass ratios, composite structures) are discussed along with V/STOL, controllable twist rotors, circulation control rotors, variable-cycle engines, and higher structural efficiencies.
An optimization procedure for designing wing structures subject to stress, strain, and drag constraints is presented. The optimization method utilizes an extended penalty function formulation for converting the constrained problem into a series of unconstrained ones. Newton's method is used to solve the unconstrained problems. An iterative analysis procedure is used to obtain the displacements of the wing structure including the effects of load redistribution due to the flexibility of the structure. The induced drag is calculated from the lift distribution. Approximate expressions for the constraints used during major portions of the optimization process enhance the efficiency of the procedure. A typical fighter wing is used to demonstrate the procedure. Aluminum and composite material designs are obtained. The tradeoff between weight savings and drag reduction is investigated.
Exploratory wind tunnel tests were conducted on a large chord aircraft wing panel to evaluate the potential for drag reduction resulting from the application of a thin plastic film cover. The tests were conducted at a Mach number of 0.15 over a Reynolds number range from about 7 x 10 to the 6th power to 63 x 10 to the 6th power.
Theoretical analysis of an electrostatically driven wall system for a compliant wall drag reduction program is reported. The electrostatic wall system is capable of producing deflections of many orders greater than the thicknesses and at small wavelengths. An intermediate large response theory was used for structural analysis. The theoretical predictions were compared to bench test results, and good agreement between the two was obtained. The effects of aerodynamic loads and perturbation electric fields on the theoretical solutions were considered. It was shown that for very small wavelengths (approximately 2mm) the aerodynamic effects can be estimated using potential theory without loss of accuracy, and the perturbation electric fields do not affect solutions as long as the deflections are less than one percent of the wavelength. Resonance effects for this type of structure were shown to be fairly small.
Results from recent investigations in the Langley V/STOL tunnel of an externally blown flap and an upper surface blown flap configuration in ground proximity are presented. Comparisons of longitudinal aerodynamic characteristics indicate that in ground proximity, drag is reduced for both configurations, but changes in lift are configuration dependent. Steady state analyses of the landing approach indicate an increase in flight path angle for both configurations in ground proximity because of the drag reduction. Dynamic analyses with a fixed-base simulator indicate that the resultant flight path during landing approach is dependent on the initial flight path angle and the control technique used.
The induced lift characteristics are studied for a vectored-thrust concept in which jet-exhaust nozzles were located in the fuselage at the wing trailing edge. The wing had a supercritical airfoil section. The investigation was conducted at Mach numbers from 0.4 to 0.95, angles of attack up to 14 deg, and thrust coefficients up to 0.35, and nozzle deflection angles of 0 deg and 30 deg. Separate force balances were used to determine both total aerodynamic and thrust forces alone which allowed for a direct measurement of jet turning angle at forward speeds. The Reynolds number per meter varied from 8.20 x 1 million to 12.80 x 1 million. The results show that the configuration with the supercritical wing generally had a better performance with respect to both lift augmentation and drag reduction than the same configuration with a 64 series airfoil.
Theoretical analysis of an electrostatically driven wall system for a compliant wall drag reduction program is reported. The electrostatic wall system is capable of producing deflections of many orders greater than the wall thicknesses and at small wavelengths. An intermediate large response theory is used for structural analysis. The theoretical predictions are compared to bench test results, and good agreement between the two is obtained. The effects of aerodynamic loads and perturbation electric fields on the theoretical solutions are considered. It is shown that for very small wavelengths (lambda almost equals 2 mm) the aerodynamic effects can be estimated using potential theory without loss of accuracy, and the perturbation electric fields do not effect solutions as long as the deflections are less than one percent of the wavelength. Resonance effects for this type of structure are shown to be fairly small.
The NASA Dryden Flight Research Center is flight testing a triply redundant digital fly-by-wire (DFBW) control system installed in an F-8 aircraft. The full-time, full-authority system performs three-axis flight control computations, including stability and command augmentation, autopilot functions, failure detection and isolation, and self-test functions. Advanced control law experiments include an active flap mode for ride smoothing and maneuver drag reduction. This paper discusses research being conducted on computer synchronization, fault detection, fault isolation, and recovery from transient faults. The F-8 DFBW system has demonstrated immunity from nuisance fault declarations while quickly identifying truly faulty components.
The Aerospike drag reduction mechanism was designed and developed for use on the Trident I submarine launched ballistic missile. This mechanism encounters a unique combination of environments necessitating unique design solutions to ensure satisfactory operation over its design life. The development of the Aerospike is reviewed emphasizing the unique and interesting problems encountered and their solutions.