Constant Ground Speed Research Articles

Geometry-Induced Acceleration in Case of a Straight-Line Target Trajectory in Bistatic Radar

Bistatic radar signal processing often assumes a constant velocity for target detection. The bistatic geometry alters, however, the effectively measurable velocity in relation to the target position and motion even if the target has a constant ground speed. Hence, the bistatic geometry induces instantaneous bistatic target acceleration. This is especially the case if a straight-line target trajectory cuts across several closely spaced velocity isolines. The letter tries to quantify this geometry-induced instantaneous bistatic acceleration, in general, for straight-line target trajectories. Thereby, position-dependent acceleration values are derived for a practical use case.

The effect of optic flow cues on honeybee flight control in wind.

To minimize the risk of colliding with the ground or other obstacles, flying animals need to control both their ground speed and ground height. This task is particularly challenging in wind, where head winds require an animal to increase its airspeed to maintain a constant ground speed and tail winds may generate negative airspeeds, rendering flight more difficult to control. In this study, we investigate how head and tail winds affect flight control in the honeybee Apis mellifera, which is known to rely on the pattern of visual motion generated across the eye-known as optic flow-to maintain constant ground speeds and heights. We find that, when provided with both longitudinal and transverse optic flow cues (in or perpendicular to the direction of flight, respectively), honeybees maintain a constant ground speed but fly lower in head winds and higher in tail winds, a response that is also observed when longitudinal optic flow cues are minimized. When the transverse component of optic flow is minimized, or when all optic flow cues are minimized, the effect of wind on ground height is abolished. We propose that the regular sidewards oscillations that the bees make as they fly may be used to extract information about the distance to the ground, independently of the longitudinal optic flow that they use for ground speed control. This computationally simple strategy could have potential uses in the development of lightweight and robust systems for guiding autonomous flying vehicles in natural environments.

Bioinspired morphing wings for extended flight envelope and roll control of small drones.

Small-winged drones can face highly varied aerodynamic requirements, such as high manoeuvrability for flight among obstacles and high wind resistance for constant ground speed against strong headwinds that cannot all be optimally addressed by a single aerodynamic profile. Several bird species solve this problem by changing the shape of their wings to adapt to the different aerodynamic requirements. Here, we describe a novel morphing wing design composed of artificial feathers that can rapidly modify its geometry to fulfil different aerodynamic requirements. We show that a fully deployed configuration enhances manoeuvrability while a folded configuration offers low drag at high speeds and is beneficial in strong headwinds. We also show that asymmetric folding of the wings can be used for roll control of the drone. The aerodynamic performance of the morphing wing is characterized in simulations, in wind tunnel measurements and validated in outdoor flights with a small drone.

Optimal swim speeds for traversing velocity barriers: an analysis of volitional high-speed swimming behavior of migratory fishes

Migrating fish traversing velocity barriers are often forced to swim at speeds greater than their maximum sustained speed (U(ms)). Failure to select an appropriate swim speed under these conditions can prevent fish from successfully negotiating otherwise passable barriers. I propose a new model of a distance-maximizing strategy for fishes traversing velocity barriers, derived from the relationships between swim speed and fatigue time in both prolonged and sprint modes. The model predicts that fish will maximize traversed distance by swimming at a constant groundspeed against a range of flow velocities, and this groundspeed is equal to the negative inverse of the slope of the swim speed-fatigue time relationship for each mode. At a predictable flow velocity, they should switch from the optimal groundspeed for prolonged mode to that for sprint mode. Data from six migratory fish species (anadromous clupeids: American shad Alosa sapidissima, alewife A. pseudoharengus and blueback herring A. aestivalis; amphidromous: striped bass Morone saxatilis; and potomodromous species: walleye (previously known as Stizostedion vitrium) and white sucker Catostomus commersonii) were used to explore the ability of fish to approximate the predicted distance-maximizing behaviors, as well as the consequences of deviating from the optima. Fish volitionally sprinted up an open-channel flume against fixed flow velocities of 1.5-4.5 m s(-1), providing data on swim speeds and fatigue times, as well as their groundspeeds. Only anadromous clupeids selected the appropriate distance-maximizing groundspeed at both prolonged and sprint modes. The other three species maintained groundspeeds appropriate to the prolonged mode, even when they should have switched to the sprint optima. Because of this, these species failed to maximize distance of ascent. The observed behavioral variability has important implications both for distributional limits and fishway design.

An Evaluation of the Response of Yield Monitors and Combines to Varying Yields

Comparisons were made between yield measurements with an impact based yield sensor and an electronic scale in adjacent harvest strips and on the same grain stream within a combine. Yield measurements in adjacent strip comparisons were more prone to errors as the segment lengths decreased. Grain yield difference between the yield sensor and electronic scale ranged from 5 to 14%, 4 to 13%, 3 to 12%, and 2 to 11% for 15, 30, 60, and 300 m long segments. The yield differences between neighboring segments might have been caused by yield variability to a degree; however, a consistent decrease in yield differences with increasing segment lengths implied that better accuracies could be obtained in longer management areas. The combine responses to grain yield changes and the effect of varying ground speed on accuracy were also evaluated by creating artificial yield patterns in harvest strips. Grain diffusion within the combine was more obvious when abrupt yield changes were introduced at known locations. Grain mixing and redistribution inside the combine may dictate the selection of segment sizes in the site-specific decision making process. Constant ground speed provided more stable grain flow values than varying ground speed. The average error in yield estimate was 3.4% and 5.2% at constant ground speed and varying speed, respectively. Careful calibration and constant combine speed were important to achieve better accuracy with the grain yield monitor.

Cycling on Earth, in space, on the Moon.

The mechanical power for cycling (P(c)) at constant ground speed (s), in the absence of wind on smooth hard terrain is the sum of the power dissipated against rolling resistance, gravity and air resistance: P(c)=a x s + M x g x s x sin gamma + b x S3, where a and b are constants, M is the mass of the subject plus bike, g is the acceleration of gravity and gamma is the angle of the terrain with the horizontal. The constant b depends upon the drag coefficient (Cd), the overall area projected on the frontal plane (A(f)), and the air density (rhoa): b = 0.5 x C(d) x A(f) x rhoa. In turn, rhoa depends on air pressure (P(B)) and temperature (T): rhoa=rho0 x 0.359 x P(B) x T(-1), where rho0 is the air density at 760 mmHg (101.3 kPa) and 273 K. The metabolic power developed by the cyclist (E(c)) is related to P(c) E(c) = P(c) x eta(-1), where eta is the mechanical efficiency of cycling. The experimental values of a, b and eta are fairly well known so that, if the maximal metabolic power as a function of the performance time is known for a given cyclist, the following set of data can be individually calculated: (1) best performances over any given distance and for any given altitude above sea level, (2) the effects of posture and body size on maximal speeds, and (3) the maximal incline of the terrain that can be overcome at any given speed or coasting speed for any given downslope. The above set of information makes it possible also to calculate the characteristics of a "Twin Bikes System" (TBS) for preventing microgravity deconditioning during long-term space flight. The TBS consists of two bicycles that are mechanically coupled by a differential gearing, which move at the very same speed, but in the opposite sense, along the inner wall of a cylindrically shaped space module. The circular trajectories induce a centrifugal acceleration vector (a(c)) oriented along the head-to-feet direction of each subject: a(c) = v(t)2 x r(-1) where v(t) is the tangential velocity and r the radius of gyration, which is equal to the inner radius of the space module. So, any desired value of a(c) can be achieved by appropriately selecting v(t), wherefrom the mechanical and metabolic powers that the astronauts must generate can be readily calculated. Experiments performed in a ground-based human centrifuge have shown that the discomfort derived from the rotating environment is reasonably low and well tolerated. If the appropriate atmospheric pressure is provided, cycling on circular or elliptical tracks may be useful to reduce cardiovascular deconditioning that occurs due to the reduced gravity in permanently manned lunar bases. Indeed, on the curved parts of the path a cyclist will generate a horizontal outward acceleration: a(c) = s2 x r(-1), where s is the velocity along the track and r is the radius of curvature. To counterbalance a(c), the subject plus bike must lean inwards so that the vectorial sum of a(c) plus the lunar gravity (g(L) = 1.62 m x s(-2)) is applied along a straight line that includes the centre of mass of the system and the point of wheel contact with the ground. For values of s from 10 to 20 m x s(-1) and r from 50 to 200 m, this vectorial sum ranges from 1.05 to 5.03 g(L) (0.17-0.83 g).

Compensation for height in the control of groundspeed byDrosophila in a new, ?barber's pole? wind tunnel

1. All experiments were performed on freely flyingDrosophila in a new, horizontal wind tunnel where the visual input was provided by the apparent axial movement of rotating helical patterns surrounding the cylindrical working section. The flies were flown upwind along a narrow plume of attractant odour in the mid-line of the tunnel so their visual input was specified. 2. When the pattern was at a constant distance from the flies they controlled their groundspeed by flying at an airspeed such that there was a constant angular velocity of image movement rather than a constant frequency of stripe alternation over their eyes. They thus flew at the same groundspeed regardless of wind speed. 3. When the distance of the pattern from the flies changed in a step, the flies adjusted their airspeed at the change so as to keep a relatively constant groundspeed. They did this by changing the angular velocity of image movement that they held constant. 4. When the distance of the pattern from the flies changed gradually, on a cone rather than in a step, the flies did not adjust the angular velocity of image movement that they held constant. Their groundspeed thus did not remain constant. 5. It is suggested that the flies used parallax cues, from the different apparent angular velocities visible juxtaposed on either side of a step in pattern distance, to adjust the angular velocity of image movement that they held constant.

Swimming and orientation inAplysia Brasiliana(Mollusca: Gastropoda)

A description of the swimming behavior of Aplysia brasiliana is provided, and its performance in currents and orientational capabilities evaluated. Aplysia are able to swim for considerable duration, to maintain a fairly straight path, and to make progress against natural currents. Hence this behavior seems to be a functional means of local movement. There is no present evidence that swimming is an escape response. A. brasiliana are able to modify swimming effort depending on their direction of travel relative to the current, so as to achieve a more constant ground speed. Releases over a clear sand bottom in shallow water indicate that swimming is an oriented movement, on an individual and group basis.

The Flight Speed of Honeybees: a Quantitative Approach

SUMMARYForaging bees were trained to feeding stations 210 and 420 m. from their hive, and individually marked. Wind speeds were measured 7 m. from the ground; bees were regarded as flying with or against the wind according to whether the wind struck them at angles of 135–180° or 0–45° from the flight path. Winds at 45–90° were not considered; those at 90–135° were regarded as neutral winds.Wind affected speed of flight, but much less than if wind speed were simply superimposed on a constant air speed. Regression lines gave ground speeds of 7·5 and 6·5 m./sec. respectively for unloaded and loaded bees, the difference being significant (P<0·00l); in neutral winds these speeds were 7·8 and 7·0 m./sec. respectively. At wind speeds above 4 m./sec. bees flew just above the ground instead of at 7·8 m., and thus maintained a fairly constant ground speed. Bees foraging at 420 m. spent less time feeding, and more in the hive (average differences −16 sec., +20 sec.) than bees foraging at 210 m.