Advanced Short Takeoff Research Articles

Control allocation for a V/STOL aircraft based on robust fuzzy control

This study addresses the design of an output-feedback H-infinity and fuzzy mixed controller for an advanced vertical and/or short take-off and landing aircraft that exhibits satisfactory global stability criteria. A fuzzy logic based optimization approach is employed to solve autonomously the constrained control allocation problem by suitably adjusting the components of the output vector and finding a proper vector in the attainable moment set. Simulations show that the designed fuzzy controller is better suited to resist external disturbances, and is robust against parameter perturbations.

Remote lift-fan engines

The requirements imposed on advanced short take-off and vertical landing (ASTOVL) aircraft give rise to challenging demands on their propulsion systems. One possible approach is to have a high-performance turbofan of traditional design and an additional, but separate, fan to provide a major part of the lift during the take-off and landing manoeuvres. For such a design, there are several quite different choices of layout for providing the power to drive the remote fan by means of the core engine. These include shaft- and bleed-driven options. The choice will depend on the anatomy and required thermodynamic performance of the whole system. In this paper, several pertinent alternative engine designs are discussed, four of which, based on a high-performance low bypass ratio core engine, are studied in detail and their behaviours compared. Prima facie, the preferred choice is the engine with the shaft-driven fan: a slightly less acceptable choice is the high-pressure turbine exit bleed driven remote fan.

Control of jet engines

Feedback control has always been an essential part of jet engines because they operate at or near their mechanical or aerothermal limitations. In this paper, the basics of controlling an engine while satisfying numerous constraints will be reviewed. The emphasis will be on commercial engines though most of the material is also applicable to military engines. In the first part, a simplified theory of engine operation is discussed in terms of the basic principles and limitations for each component. The second part of the paper discusses overall control requirements and typical sensors and actuators. Finally various control strategies are presented. It is shown that much of the complexity of the control comes from the need to operate the engine as close as possible to its limits. In a commercial engine, this results in a series of single input single output controllers. The case for multivariable control can be more strongly made for military engines especially those with additional flowpath geometry actuators such as a variable cycle or advanced short takeoff and landing engines.

Facilitating Concept Exploration for Configuring Turbine Propulsion Systems

In this Technical Brief we illustrate the efficacy of the Robust Concept Exploration Method as applied to the preliminary design of a turbine lift engine for Advanced Short Take-Off and Vertical Landing (ASTOVL) fighter aircraft. This example is defined in collaboration with the Rolls-Royce Atlanta Engineering Group.

Measurement and empirical prediction of bottom surface acoustic pressures on a hover aircraft model

Acoustic measurements were made on the bottom surface of the 86% scale large-scale propulsion model (LSPM) of an advanced short takeoff and vertical landing (ASTOVL) airplane. The measurements were ob-tained during fixed hover heights where the lower wing surface varied from 5 ft (airplane’s wheels on the ground) through 36 ft above the ground for a wide range of propulsion thrust mixes and airplane control surface configurations. The complete propulsion system provided four exhausts through an integrated unit consisting of an aft located main jet engine, which powered a forward located shaft driven lift fan and was completed with two main engine bleed high-pressure bypass roll jets (in the wings). Flush surface measurements were made at 12 locations on the bottom surfaces and time correlated with the operating propulsion parameters. An empirical acoustic model was made through analyses of the geometry and the data, and was used to predict contours on the bottom of the aircraft. [This work was performed by Lockheed Martin Skunk Works at the NASA-Ames Research Center.]

Robust Control of An Ill-Conditioned Aircraft

A robust controller is designed for a linear model of an Advanced Short Take-Off and Vertical Landing (ASTOVL) aircraft at one operating point. At this operating point the aircraft is unstable and displays significant levels of interaction between its controlled outputs. A controller is designed using μ synthesis which is shown to be robust to prescribed levels of both structured and unstructured uncertainty. The design problem is very poorly conditioned at low frequency which leads to a novel method of selecting the performance specifications using the singular value decomposition.

Actuation System Requirements for Advanced Short Take-Off and Vertical Landing Powerplants

Most of the powerplant concepts currently being studied for advanced short take-off and vertical landing ASTOVL) aircraft require significantly more actuation than conventional powerplants. This paper highlights the challenge being faced by the powerplant and acuation system designers to produce an acceptable solution. Typical concepts and their operation are described, and using this as a basis, it is shown that the actuation system will impact significantly on the viability of the concepts. Most of the actuated features are flight safety critical in the jet-borne flight regime, and considerable care will be needed to achieve a safe system without unacceptable penalties on cost, weight and reliability. The projected timescale of the ASTOVL aircraft (entry into service in 2010) means that there is time for radical solutions to be developed and work to achieve these solutions is already underway.

Field tests of a high‐temperature fiber optic lever microphone

The operating principle of a high‐temperature fiber optic lever microphone (FOLM), rated at 538 °C (1000 °F) continuous service, is based on intensity modulation of light reflected from a vibrating membrane. The frequency response of the microphone, calibrated in the laboratory at ambient temperature and 538 °C by the electrostatic actuator method, is typically 60–70 kHz. Applications to high‐temperature field testing include the thermal acoustic fatigue apparatus at Langley Research Center, an advanced short takeoff and vertical landing model at Lewis Research Center, the Mach 22 helium tunnel at Langley (ambient temperature only), interior mixer‐ejector surfaces at GE Aircraft Engines, Cincinnati, and a high temperature acoustic impedance tube at Langley. In most of these applications the FOLM made possible high‐temperature, high‐frequency acoustic data to be taken for the first time. Typical test results will be discussed.

Aeroacoustic environment of an advanced short takeoff and vertical landing aircraft in hover

The near-field aerooacoustic environment of a 6.02%-scale advanced short takeoff and vertical landing aircraft is investigated in proximity to a stimulated ground plane. The screech and impingement-tone characteristics of twin supersonic round and rectangular plumes are shown to be highly dependent on nozzle geometry and height above the ground plane. Shadowgraph and laser-sheet flow visualization are used.

STOVL Hot Gas Ingestion Control Technology

A comprehensive wind tunnel test program was conducted to evaluate control of Hot Gas Ingestion (HGI) on a 9.2 percent scale model of the McDonnell Aircraft Company model 279-3C advanced Short Takeoff and Vertical Landing (STOVL) configuration. The test was conducted in the NASA-Lewis Research Center 9 ft by 15 ft Low Speed Wind Tunnel during the summer of 1987. Initial tests defined baseline HGI levels as determined by engine face temperature rise and temperature distortion. Subsequent testing was conducted to evaluate HGI control parametrically using Lift Improvement Devices (LIDs), forward nozzle splay angle, a combination of LIDs and forward nozzle splay angle, and main inlet blocking. The results from this test program demonstrate that HGI can be effectively controlled and that HGI is not a barrier to STOVL aircraft development.