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Utilization of hydrostatic drives for power transmission, along with hydropneumatic accumulator energy storage can provide significant opportunities for improvements in vehicle design. These advantages are particularly relevant when considering advanced concept vehicles such as flying automobiles. Research and development in this technology has primarily focused on the fuel economy improvements that can be achieved in automobiles using a hydrostatic transmission and hydropneumatic accumulator energy storage. The accumulator permits the engine power to be uncoupled from the road load, thus enabling the engine to be operated at a more efficient point. By using wheel drive units that can operate as either motors (when driving) or pumps (when braking), regenerative braking can also be achieved, with the energy stored in the accumulator. In addition to improved fuel economy, several other significant design opportunities can be exploited.

Many modem aircraft missions require high values of aerodynamic efficiency with aircraft having wings of relatively restricted span lengths. In many of these missions, the aircraft must operate at relatively large values of the lift coefficients, and the large induced drag associated with the small span consequently results in relatively low values for the operational aerodynamic efficiency. In endeavoring to increase the flight efficiency of such aircraft, it becomes necessary to investigate more complex and unconventional wing forms which might offer the possibility of securing appreciable reductions in the induced drag, subject to the restriction of limited span length. Induced drag is associated with the shedding of vorticity along the span of a finite lifting wing and, in particular, in the wing-tip region. For most subsonic aircraft configurations, induced drag contributes about 50 percent of the total drag of the aircraft throughout its flight profile.

The visualization of boundary-layer transition from laminar to turbulent flow plays an important role in flight and wind tunnel aerodynamic testing. Visualization aids in the understanding of specific causes of transition. In the past, the most popular boundary-layer visualization methods for flight and wind tunnel applications included the sublimating chemical technique and the oil flow technique. However, each method has certain advantages and limitations which constrain the applications. This paper discusses a new method for visualizing boundary layer flows including transition, separation, and shock locations, by the use of liquid-crystal coatings. For flight applications, liquid crystals provide transition visualization capability throughout almost the entire altitude and speed ranges for subsonic aircraft flight envelopes. The method is also applicable to supersonic flow visualizaton and for general use in high- and low-speed wind tunnel and water-tunnel testing.

In recent years, natural laminar flow (NLF) has been proven to be achievable on modern smooth airframe surfaces over a range of cruise flight conditions representative of most current business and commuter aircraft. Published waviness and boundary-layer transition measurements on several modern metal and composite airframes have demonstrated the fact that achievable surface waviness is readily compatible with laminar flow requirements. Currently, the principle challenge to the manufacture of NLF-compatible surfaces is two-dimensional roughness in the form of steps and gaps at structural joints. This paper presents results of recent NASA investigations on manufacturing tolerances for NLF surfaces, including results of a flight experiment. Based on recent research, recommendations are given for conservative manufacturing tolerances for waviness and shaped steps.

A research program is in progress to study the effects of the propeller slipstream on natural laminar flow. Flight and wind tunnel measurements of the wing boundary layer have been made using hot-film velocity sensor probes. The results show the boundary layer, at any given point, to alternate between laminar and turbulent states. This cyclic behavior is due to periodic external flow turbulence originating from the viscous wake of the propeller blades. Analytic studies show the cyclic laminar/turbulent boundary layer layer to result in a significantly lower wing section drag than a fully turbulent boundary layer. The application of natural laminar flow design philosophy yields drag reduction benefits in the slipstream affected regions of the airframe, as well as the unaffected regions.

Two major concerns have inhibited the use of natural laminar flow (NLF) for viscous drag reduction on production aircraft. These are the concerns of achieveability of NLF on practical airframe surfaces, and maintainability in operating environments. Previous research in this area left a mixture of positive and negative conclusions regarding these concerns. While early (pre-1950) airframe construction methods could not achieve NLF criteria for waviness, several modern construction methods (composites for example) can achieve the required smoothness. This paper presents flight experiment data on the achieveability and maintainability of NLF on a high-performance, single-propeller, composite airplane, the Bellanca Skyrocket II. The significant contribution of laminar flow to the performance of this airplane was measured. Observations of laminar flow in the propeller slipstream are discussed, as are the effects of insect contamination on the wing.

Design data are presented for a class of high-performance single-engine business airplanes. The design objectives include a cruise speed of 300 knots, a cruise altitude of 10,700 m (35,000 ft), a cruise payload of six passengers (including crew and baggage), and a no-reserves cruise range of 1300 n.mi. Two unconventional aerodynamic technologies were evaluated: the individual and combined effects of cruise-matched wing loading and of a natural laminar flow airfoil were analyzed. The tradeoff data presented illustrate the ranges of wing geometries, propulsion requirements, airplane weights, and aerodynamic characteristics which are necessary to meet the design objectives. very large design and performance improvements resulted from use of the aerodynamic technologies evaluated. Is is shown that the potential exists for achieving more than 200-percent greater fuel efficiency than is achieved by current airplanes capable of similar cruise speeds, payloads, and ranges.

The present status and flight-test results are presented for the ATLIT airplane. The ATLIT is a Piper PA-34 Seneca I modified by the installation of new wings incorporating the GA(W)-1 (Whitcomb) airfoil, reduced wing area, roll-control spoilers, and full-span Fowler flaps. Flight-test results on stall and spoiler roll characteristics show good agreement with wind-tunnel data. Maximum power-off lift coefficients are greater than 3.0 with flaps deflected 37°. With flaps down, spoiler deflections can produce roll helix angles in excess of 0.11 rad. Flight testing is planned to document climb and cruise performance, and supercritical propeller performance and noise characteristics. The airplane is scheduled for testing in the NASA-Langley Research Center Full-Scale Tunnel.