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Variation of inflow velocity (v z ) along the radius using RotCFD and different dynamic inflow models at h = .25R(θ 0 = 8 0 )
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A well-known phenomenon in rotorcraft, the ground effect, occurs when a rotor is at close proximity to the ground creating an obstruction to the trailing wake from the rotor. This results in an increase in thrust and a decreased inflow through the rotor disk, in turn affecting handling qualities. Until recently, analytical modelling of ground effec...
Citations
... Due to the continuous development of computational fluid dynamics (CFD) tools, various simulation methods have been considered in the past to evaluate the influence of the ground on a rotor [43,46], [47][48][49]. Computational Fluid Dynamics is a handy tool for reproducing fluid behaviour around a propeller. Although several simulation techniques exist [50], Reynolds Averaged Navier-Stokes (RANS) models are less expensive and are used to estimate time-averaged parameters. ...
... Although several simulation techniques exist [50], Reynolds Averaged Navier-Stokes (RANS) models are less expensive and are used to estimate time-averaged parameters. In recent years, the number of investigations on propeller performance analysis with RANS models has increased [46][47][48][49][51][52][53][54][55][56][57][58] both high and low Reynolds numbers. Their suitability with experimental sets has been proven in several works [52,55,58]. ...
In recent years, aerial manipulators with fully-actuated capabilities are gaining popularity for being used in aerial manipulation operations such as critical infrastructure inspection or aerial manipulation tasks. Those scenarios usually demand the aerial platform to operate in constrained and narrow scenarios. It is well known that in these situations, the interaction of the wake generated by the propellers with the environment can significantly alter and change the performance of the rotors. Most studies have addressed this problem by considering the ground effect in hover conditions or during the landing maneuver for co-planar multirotor. However, few works analyze the behavior of tilted rotors, which are used in fully actuated multirotor configurations thanks to their omnidirectional motion capabilities. This paper presents a numerical-experimental evaluation of the aerodynamic ground effect for small-scale tilted propellers at low Reynolds numbers. This aerodynamic effect has been experimentally evaluated through an extensive testing campaign in a testbench designed for this purpose which has been complemented by a CFD-based study. CFD results have been validated through a mesh independence study and a CFD-experimental propeller performance comparison. A numerical model has been also proposed to capture the dependence of thrust with distance to the ground and angle of inclination between the propeller and ground planes. We demonstrate that the proximity to the ground of tilted rotors decreases the thrust increment due to the ground effect as the tilt angle (θ) increases. This means that Cheeseman's classical theory is inapplicable, as it only considers the distance from the ground without reference to how the thrust increment changes with the tilt angle. This outcome enables future aerial robotic applications that strongly demand accurate aerodynamic effect models to operate close to obstacles and narrow environments.
... Figure 4 demonstrates the dependence of the main rotor thrust ratio on the ground effect c T /c T∞ (H/R). The obtained result has been compared with the available experimental and calculated data [1], [9], [12], [13], [17], [19], [22], [23], [26] (Fig. 4). The presented calculation results are also in good agreement with the specified data. ...
... Figures 6(b), 6(c) and 6(g). This phenomenon is well known and described in the literature, including the book by Leishman (2006) [1], studied through experimental methods (PIV) in the work by Nathan and Green (2012) [15], through calculation in the work by Brown and Whitehouse (2004) [22], Phillips (2010) [4] and Sugiura et al. (2017) [21]. Figure 5(b) shows that "ground vortex" arises as a result of rolling-up the leading edge of the rotor wake when flying at low speeds in ground effect and makes a significant feature in the velocity field around the rotor. ...
This paper considers the Mi-8 helicopter main rotor aerodynamics when flying near the infinite ground surface. The research is based on the free wake model developed by authors at Moscow Aviation Institute. The distance from the rotor's hub to the ground surface in the range of H = 6-16 m and the values of free stream (flight) velocity in the range of V = 0-15 m/s are considered. The results of the visualization for both rotor wake shapes and streamlines are obtained. The influence of the ground proximity on the rotor wake shape, including the formation of "supervortex" and "ground vortex" structures are analyzed. The induced velocity fields of the main rotor in the area of the tail rotor location for various azimuth positions relative to the main rotor axis are studied. The conclusion is made about the significant influence of the ground effect on the rotor wake structure and induced velocities field, including the area of the tail rotor location. Particularly, at slip flight with speed V = 10 m/s, when the distance to the ground surface H increases from 6 to 12 m, the value of the average induced velocity at the tail rotor plane is growing up to four times. The obtained data have allowed taking into account such effects of aerodynamic interference in the simplified mathematical model of the research flight simulator of JSC Helicopters Mil and Kamov.
... Studies also show that this effect is influenced by the collective pitch (Ref. 7). Although several studies have shown that aerodynamic effect of ground translates to effects on performance parameters, there have been no reported investigations into its role on aeromechanical stability. ...
This paper studies aerodynamic effects of ground on regressive lag mode damping during ground resonance. The experimental investigation is performed on a scaled helicopter model built to simulate the ground resonance scenario. The study involves both stationary as well as dynamic (oscillating) ground conditions. Experiments are conducted by measuring lead-lag damping at different rotor speeds (in the ground resonance regime) for different collective inputs. Results show that ground and its dynamics have a significant effect on regressive lead-lag mode damping during ground resonance.
