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General Aviation
Published in Suzanne K. Kearns, Fundamentals of International Aviation, 2021
However, this has a limit. As a helicopter accelerates, it will reach a speed where it can no longer manage the dissymmetry of lift, as the retreating blade’s angle of attack will increase to a critical angle where a stall occurs and lift is lost. This is called a retreating blade stall, and results in the helicopter pitching up and slowing down. For this reason, all helicopters are forward speed-limited, with an average speed of around 140 knots (260kph, 161 mph).
Spatial Orientation and Disorientation
Published in Anthony N. Nicholson, The Neurosciences and the Practice of Aviation Medicine, 2017
A further force on a fixed-wing aircraft that acts in the long axis of the aircraft approximately at right angles to the lift force comes from the thrust of the engines and the retarding effect of the airbrakes. Though this force contributes to problems with spatial orientation during changes in airspeed, its intensity in all aircraft is substantially less than the lift force that can be generated by the wings. In a helicopter there is no similar longitudinal force. The force that both lifts the aircraft off the ground and drives it forwards (or backwards or sideways) is generated by the main rotor and this force is always vertically upwards with respect to the fuselage of the helicopter, again, whatever the aircraft attitude with respect to the surface of the Earth.
UAS Airframe Design
Published in R. Kurt Barnhart, Douglas M. Marshall, Eric J. Shappee, Introduction to Unmanned Aircraft Systems, 2021
Michael T. Most, Michael Stroup
The aerodynamics and flight physics associated with a helicopter are very complex. A helicopter simultaneously produces both lift, to oppose the weight of the aircraft, and thrust, represented by a vector acting in the direction of flight, by accelerating a mass of air through the rotor disk. The energy imparted to the air mass is provided by the fuel or battery, converted to mechanical power by the aircraft powerplant, and transmitted through gear reduction (e.g., in a transmission) to the main shaft that supports the main rotor blades. The helicopter gear reduction (transmission) is necessary to convert the high rpm, low torque output of the motor to low rpm, for aerodynamic efficiency (generally, the transonic regime should not be entered), and high torque to move the blades through the viscous fluid of the atmosphere. In a hover, 60%–70% of this power is consumed in producing lift (referred to as induced power), while the remainder (referred to as profile drag power) is expended in overcoming parasite drag (Gessow and Myers 1985). Because low Reynolds numbers are characteristic of narrow chord/short span blades (Schafroth 1980) and the energy required to accelerate an air mass increases as the square of the acceleration, increasingly larger diameter rotors become increasingly efficient due to the ability to process greater amounts of air through the rotor disk (Gessow and Myers 1985). Another contributing factor is that increasing the length, and therefore the aspect ratio, of the rotor blades reduces the drag induced by producing lift. The trade-offs are increased parasitic drag and the reduction in rotor rpm, which will reduce dynamic pressure and tend to diminish lift, necessary to keep tip speeds subsonic.
Hardware-in-the-Loop simulation algorithm for helicopter rotor time-varying echo signals
Published in Systems Science & Control Engineering, 2021
Helicopters have garnered considerable interest for military and civil applications, because of their advantages and suitability in searching, detecting, monitoring, identifying, and locating targets (Mu et al., 2014). The echo of the rotor blades of a helicopter features micro-Doppler (m-D) characteristics (Clemente & Soraghan, 2014; Garry & Smith, 2019; Tahmoush, 2015), which are generally inherent for a radar target; the m-D characteristics exhibit less manual controllability (Guo & Sheng, 2010). Therefore, this feature has been widely studied for the classification and recognition of helicopters (Wu et al., 2018; Zuo et al., 2013). However, to study the m-D characteristics of helicopter rotor echoes, it is necessary to conduct a comprehensive study of the echoes of helicopter rotor blades. Owing to the difficulty in obtaining field experimental data, hardware-in-the-loop (HIL) simulations have become necessary for the echo simulations of helicopter rotor blades.
Enhancing cyber-physical security in manufacturing through game-theoretic analysis
Published in Cyber-Physical Systems, 2018
Zach DeSmit, Aditya U. Kulkarni, Christian Wernz
In modern manufacturing environments, cyber-physical systems are interconnected through an Internet of Things (IoT) network, which enables a group of cyber-physical systems to effectively communicate with each other and manufacture products with minimal human involvement [6]. Cyberattacks on cyber-physical systems often exploit vulnerabilities in the IoT infrastructure and compromise the operation of one or more systems within the network [7]. For example, a cyberattack could exploit a communication protocol in the IoT network and gain control of a valve, which when opened spills caustic chemicals onto the surrounding equipment. Another example is the editing or replacing of manufacturing instructions (G-code) for a helicopter rotor, altering the angle of the helicopter rotor resulting in the production of defective rotor blades.
Optimization of helicopter rotor blade performance by spline-based taper distribution using neural networks based on CFD solutions
Published in Engineering Applications of Computational Fluid Mechanics, 2019
One of the main challenging missions for the designers is to increase the helicopter rotor blade performance in hover condition. The helicopter rotor flow is complicated due to vortical motions and instabilities. This means that more accurate analyses with high fidelity tools are required. Computational Fluid Dynamics (CFD) is a strong tool that is able to capture the complicated flow features of the helicopter flows. However, the CFD analysis of rotor blades is harder compared to the analysis of fixed-wing.