Glide Slope Control Research Articles

Control Authority of a Single-Surface Parafoil with Bleed-Air Spoilers

Bleed-air control of ram-air parafoils is a lightweight and cost-effective mechanism that enables significant landing accuracy improvements compared to traditional trailing edge brake control due to the addition of direct glide slope control. Single-surface parafoil canopies have been shown to be lightweight and possess similar flight behavior as their ram-air counterparts. The design of bleed-air spoilers is more challenging for single-surface canopies, which lack the internal ram-air that provides a high pressure differential at any chordwise location. This paper explores bleed-air spoilers on single-surface parafoils with a focus on turn rate and glide slope control capabilities. Through a flight test campaign, it was shown that bleed-air spoilers on a single-surface parafoil provide both turn rate control from asymmetric vent opening and glide slope change from symmetric vent opening. Vents located far forward at the 10% chord location yield substantial control response with a maximum turn rate response of 38 deg/s and a maximum glide slope change of 58%. Vents located farther aft at 30, 50, and 70% chords demonstrated diminished control authority. Varying the spanwise locations of the spoilers revealed that the outermost vents had negligible lateral and longitudinal control authority.

Combined Lateral and Longitudinal Control of Parafoils Using Upper-Surface Canopy Spoilers

Precision placement of guided airdrop systems necessarily requires some mechanism enabling effective directional control of the vehicle. Often this mechanism is realized through asymmetric deflection of the parafoil canopy trailing-edge brakes. In contrast to conventional trailing-edge deflection used primarily for lateral steering, upper-surface bleed air spoilers have been shown to be extremely effective for both lateral and longitudinal (i.e., glide slope) control of parafoil and payload systems. Bleed air spoilers operate by opening and closing several spanwise slits in the upper surface of the parafoil canopy, thus creating a virtual spoiler from the stream of expelled ram air. The work reported here considers the autonomous landing performance of a small-scale parafoil and payload system using upper-surface bleed air spoilers exclusively for both lateral steering and glide slope control. Landing accuracy statistics computed from a series of Monte Carlo simulations in a variety of atmospheric conditions and experimental flight tests were found to be in good agreement. Median miss distances for the combined lateral and longitudinal control logic are on the order of 13 m, indicating an improvement in landing accuracy of nearly 50% over similar systems employing only lateral steering control.

Parafoil Control Using Payload Weight Shift

Autonomous guided airdrop systems based on steerable ram-air parafoils rely on differential deflection of the canopy trailing edge for lateral control with very limited longitudinal control. This work explores an alternative method that achieves both lateral and longitudinal control by shifting the center of gravity of the payload relative to the canopy rigging attachments. A multibody simulation model is required to capture the complex motion associated with the flexible nature of the parafoil, payload, and rigging system and simulate the effects of both longitudinal and lateral shifting of the center of gravity. Simulation results show that longitudinal weight shift can be an effective means of providing airspeed and glide slope control, and lateral weight shift can be an effective means of providing turn rate control. Results demonstrate that, for both lateral and longitudinal control, spreading the attachment points of the rigging to the payload will cause the aircraft to be more sensitive to shifts in the payload center of gravity. It was also found that, for both lateral and longitudinal control, decreasing the vertical distance from the rigging attachments to the center of gravity on the payload causes the aircraft to be more sensitive to shifts in the payload center of gravity. Finally, dynamic results demonstrate that the forces and moments required to induce a weight shift in the payload will often cause an initial response in the opposite direction of the final steady-state response.

Glide Slope Control Authority for Parafoil Canopies with Variable Incidence Angle

The addition of glide slope control to guided airdrop systems has the potential to provide dramatic improvements in landing accuracy. Dynamic rigging incidence control has been demonstrated in flight tests and shown to provide effective glide slope control. The current work presents flight-test results exploring the range of glide slope control that can be achieved with high-performance parafoil canopies of two different aspect ratios. The low- and medium-aspect-ratio canopies have peak glide ratios of 4.4 and 4.9, respectively. By varying the incidence angle in flight, the glide slope can be reduced from these peak values to a glide slope near two for both canopies. Flight tests investigating the relationship between symmetric brake and incidence control demonstrate that symmetric braking produces effective airspeed control with little effect on glide slope, the incidence angle produces significant effects on both glide slope and airspeed, the rapid variation of the incidence angle in flight can lead to significant long period oscillations in both glide slope and airspeed, and the coupled effect of the control mechanisms of incidence angle and symmetric brake can produce dramatic changes in glide slope in any wind condition.

Adaptive Glide Slope Control for Parafoil and Payload Aircraft

The invention of the steerable, gliding, ram-air parafoil enabled the possibility of precision, autonomous aerial payload delivery. Research and development work on guided airdrop systems has focused primarily on evolutionary improvements to the guidance algorithm, although the navigation and control algorithms have changed little since the initial autonomous systems were developed. Recent work has demonstrated the potential for dramatic improvements in landing accuracy through the incorporation of canopy incidence variation to achieve glide slope control for parafoils. The current work presents the development of a control law to implement glide slope control on an autonomous airdrop system. Autonomous landings with the control law in both simulation and flight test demonstrate an improvement in landing accuracy by a factor of two, though the improvement can be even greater in especially windy conditions. Finally, the ability to perform in-flight system identification to adapt internal control mappings to match flight data and provide dramatic improvements in landing accuracy when there is a significant discrepancy between the assumed and actual flight characteristics is demonstrated in both simulation and flight test.

Pilot gaze and glideslope control

We examined the eye movements of pilots as they carried out simulated aircraft landings under day and night lighting conditions. Our five students and five certified pilots were instructed to quickly achieve and then maintain a constant 3-degree glideslope relative to the runway. However, both groups of pilots were found to make significant glideslope control errors, especially during simulated night approaches. We found that pilot gaze was directed most often toward the runway and to the ground region located immediately in front of the runway, compared to other visual scene features. In general, their gaze was skewed toward the near half of the runway and tended to follow the runway threshold as it moved on the screen. Contrary to expectations, pilot gaze was not consistently directed at the aircraft's simulated aimpoint (i.e., its predicted future touchdown point based on scene motion). However, pilots did tend to fly the aircraft so that this point was aligned with the runway threshold. We conclude that the supplementary out-of-cockpit visual cues available during day landing conditions facilitated glideslope control performance. The available evidence suggests that these supplementary visual cues are acquired through peripheral vision, without the need for active fixation.

Use of Variable Incidence Angle for Glide Slope Control of Autonomous Parafoils

Strictly speaking, most autonomous parafoil and payload systems possess only lateral control, achieved by right and left parafoil brake deflection. An innovative technique to achieve direct longitudinal control through incidence angle changes is reported. Addition of this extra control channel requires simple rigging changes and an additional servoactuator. The ability of incidence angle to alter the glide slope of a parafoil and payload aircraft is demonstrated through a flight-test program with a microparafoil system. Results from the flight-test program are synthesized and integrated into a six degree-of-freedom simulation. The simulation model is subsequently used to assess the utility of glide slope control to improve autonomous flight control system performance. Through Monte Carlo simulation, impact point statistics with and without glide slope control indicate that dramatic improvements in impact point statistics are possible using direct glide slope control.

Explicit and Implicit Horizons for Simulated Landing Approaches

In a flight simulator experienced pilots flew landing approaches to a representation of an airport scene in which various sources of information had been distorted or removed. Reasonably accurate approaches could be made to a scene that contained only an aimpoint and a horizon. The addition of a runway outline did not enhance accuracy or stability, which lent credence to the hypothesis that the invariant angle between horizon and aimpoint can support glide slope control. Explicit distortion of this angle by simulation of up-sloping or down-sloping terrain beyond the runway had predictable effects on glide slope control. Implicit specification of a veridical horizon with texture lines parallel to the runway centerline weakened the effect of distortions in the explicit horizon. Thus both explicit and implicit specifications of the horizon contribute to perception of the glide slope angle. Implications of these results for the design of visual scenes for flight simulation are discussed.

Scene Content and Runway Breadth Effects on Simulated Landing Approaches

Eight experienced pilots flew landing approaches in a light-aircraft simulator with a computer-generated landing display. Scene content and runway breadth were manipulated. Approaches were lower with reduced scene content and to the narrow runways, and trial-to-trial variability was higher with reduced scene content. There was no significant interaction between scene content and runway breadth. The data were used to evaluate two projective geometry models for prediction of glideslope control during an aircraft approach to landing. These models did not permit an entirely satisfactory account of the data. We conclude that the landing approach is guided by visual properties other than those entered into these models.

Display Magnification for Simulated Landing Approaches

Previous discussions of simulated landings suggest that biases not found during aircraft flight are induced by one or more features of a simulator. Inadequate representation of texture or of ground detail and distortions in judgments of size from near accommodation to a display are two of the factors that have been thought to contribute to these biases. Addition of surface detail and magnification of the display have been suggested as corrective solutions. The research reported here was a preliminary investigation of the effects of factors that might induce biased landing approaches in a simulator and of a procedure for calibrating a simulator to eliminate biases. Scene content, display magnification, runway size, and start point all influenced glideslope control. With an appropriate start point and appropriate runway dimensions, there was a close correspondence between performance in an aircraft and performance in a simulator in which the display magnification had been set at unity and in which a moderat...