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" While deftly maneuvering between trees and other obstacles, the northern goshawk can reach speeds of up to 55 miles per hour in pursuit of its prey, flying through some of the world&amp;#039;s densest forests, no less. Biologists have long admired the bird&amp;#039;s airborne precision, but engineers are taking notice now too. They&amp;#039;re hoping to emulate the hawk&amp;#039;s unparalleled speed and agility in unmanned aerial vehicles (UAVs).  With the help of Harvard University biologists, researchers at the Massachusetts Institute of Technology calculated theoretical speed limits for a given density of obstacles through which these birds fly unscathed. &amp;quot;Birds are aware of how closely they can get to these things, but we don&amp;#039;t know how they estimate it,&amp;quot; says Emilio Frazzoli, an associate professor of aeronautics and astronautics at MIT.  Presumably, the birds&amp;#039; mechanism is more innate than the engineers&amp;#039;. Frazzoli and his Ph.D. student Sertac Karaman applied an ergodic model\u2014a tool commonly used by ecologists to determine the distribution of trees in a given forest\u2014to calculate the probability of a collision at various speeds and forest densities. (The faster a bird flies, or the denser the forest becomes, the greater the chance of a collision.) Based on this probability, the researchers calculated a theoretical speed above which there is no &amp;quot;infinite collision-free trajectory&amp;quot;\u2014in other words, a speed at which even the most skillful fliers are highly likely to crash.  &amp;quot;Under a very general set of assumptions we can show there exists this critical speed, the exact value of which depends a lot on the specifics of the model,&amp;quot; Frazzoli says. Other such factors include the density of the forest, the size of the trees, and the maneuverability of the bird based on the Harvard biologists&amp;#039; observations.  Determining the critical speed is important in understanding how well different animals, humans, or vehicles can approach this theoretical limit. &amp;quot;It&amp;#039;s an analysis tool for biological systems, but also an engineering tool that will allow us to set a safe speed,&amp;quot; Frazzoli says.  For example, today&amp;#039;s UAVs are used mainly for military purposes, but autonomous aircraft could have applications beyond high-flying surveillance, Frazzoli says. He envisions scenarios in which drones could fly through hazardous buildings such as the disabled Fukushima nuclear power plant, patrol borders, or scope-out potentially dangerous situations prior to human entrance. And if you had a UAV flying through a crowded area, you&amp;#039;d want to know its critical speed for that environment.  It&amp;#039;s important to remember, however, that while a UAV could be programmed with a predetermined speed based on the density or difficulty of the environment, it won&amp;#039;t necessarily be able to reach its critical speed\u2014this is a theoretical limit. &amp;quot;Essentially, the [critical speed limit] is an ideal measure of performance that we can use to assess how well proposed UAV sensing, planning, and control systems would perform,&amp;quot; Frazzoli says. In reality, UAVs rely on limited sensors to detect unforeseen obstacles and are also subject to disturbances in their motion from inclement weather, according to Matt Keennon, project manager of the Nano Aerial Vehicles program, a Pentagon research effort led by the Defense Advanced Research Projects Agency, which created the PM Breakthrough Award-winning hummingbird UAV. Keennon says that though flying robots will be able to move faster than their ground counterparts and through vertical spaces, like an elevator shaft, there are several design challenges to overcome before they can fly too much faster.  Keeping the aircraft from drifting into walls or ceilings, maintaining communication through cement walls, and correcting for air currents inside buildings all present difficulties, Keennon says. &amp;quot;[That&amp;#039;s in addition to] putting all the necessary technological pieces into an aircraft small enough to fly indoors.&amp;quot;  But if everything goes as well for these UAVs as it does for goshawks, Frazzoli would also like to see how close human pilots can come to the theoretical limit. He and Karaman have designed a flight-simulator game to test people&amp;#039;s ability to navigate dense forests at top speeds. However, the difficult part in finding the theoretical limit for human pilots, says Frazzoli, is seeing how closely this limit matches what people actually do in practice.  Like UAVs, people may not be able to reach their critical speed \u2014 or worse, may not be able to stop once they do. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/military/a7429/will-bird-flight-secrets-lead-to-faster-uavs/>