Flight safety oriented ice shape modulation using distributed plasma actuator units

Yun WU, Biao WEI, Hua LIANG, Like XIE, Zhe LI, Yinghong LI

Science and Technology on Plasma Dynamics Laboratory, Air Force Engineering University, Xi’an 710038, China

KEYWORDS Anti-icing;Flight safety;Flow control;Icing wind tunnel;Plasma actuator

Abstract Ice accretion on the wings seriously threatens the flight safety of an aircraft. From the perspectives of ensuring flight safety and saving power consumption, the ice shape modulation method using distributed plasma is proposed. Distributed plasma actuator units are designed to modulate the spanwise continuous ice at the leading edge into periodically segmented ice pieces,forming a wavy leading edge. Both airfoil and scaled aircraft model, with continuous and modulated ice, are experimentally investigated and simulated. Compared with the continuous ice, ice shape modulation can significantly improve the aerodynamic performance,flight control characteristics and flight safety. This method can save about half electric power, which is very beneficial for application.

1. Introduction

Ice accretion could happen on the critical components when the aircraft is travelling through the frozen clouds with super-cooled water droplets in cold weather.1–3Especially,ice accretion on the wing or horizontal tail would change the designed aerodynamic shape of aircraft, which may lead to a loss of lift,an increase in drag and be out of control.4,5At present, various aircraft anti-icing technologies (i.e., freezingpoint depressants,6thermal melting,7and surface deformation8)have been developed.Also,several novel anti-icing technologies are under development. Recently, plasma anti-icing technology has received widespread attention for the simple layout, lack of mechanical elements and relatively low energy consumption.9–12

In recent years, research of plasma anti-icing technology is mainly focused on the mechanism, variation law and novel plasma actuation methods.13–16In order to promote the application of plasma anti-icing technology, both anti-icing effect and electric power consumption must be considered. Traditionally,the main evaluating indicator of anti-icing technology is whether there is any ice at the wing leading edge. Since the power for anti-icing of certain area can be hardly reduced,it’s very difficult to apply anti-icing technology in small aircrafts such as unmanned aerial vehicles with limited electric power supply.Driven by the strong demand of reducing power consumption, the purpose of anti-icing method has been reconsidered. The best goal of anti-icing method is no ice at the wing leading edge, while the basic goal is to ensure flight safety, which means that the aircraft can be controlled to fly way from dangerous area even with some ice at the leading edge. From this perspective, the ice at the leading edge must be tailored carefully to ensure that the residual ice is safe for flight. Airfoil with biological wavy leading edge shows better aerodynamic performance than the baseline airfoil, which is a hot topic in biological fluid mechanics17.If the ice at the wing leading edge can be tailored into wavy shape, it is very attractive and interesting.

Therefore,inspired by the airfoil with biological wavy leading edge, this paper proposes a flight safety oriented ice shape modulation method using distributed plasma, which is expected to overcome power constraints and provide innovative ideas for the application of plasma anti-icing technology in aircrafts.

2. Flight safety oriented ice shape modulation method and feasibility analysis

The flight safety oriented ice shape modulation method uses the distributed plasma actuator units to modulate the spanwise continuous ice into periodically segmented ice pieces, forming a wavy leading edge, as shown in Fig. 1. Its connotation is to concentrate the limited power to modulate the dangerous spanwise continuous ice into a safer segmented ice, thus to improve the aircraft’s ability to fly with ice and fly away from the icing area. On the premise of improving the aerodynamic performance and flight safety, the method can significantly reduce the power consumption and widen the flight safety boundary of the aircraft.

In order to verify the feasibility of this method, the investigation is carried out from two aspects,respectively.The one is to analyze whether the‘‘plasma ice shape modulation method”can modulate the continuous ice into the segmented ice, and the other one is to analyze whether the modulated ice can improve the aerodynamic performance and flight dynamics of the aircraft.

The ice wind tunnel experiment in the conditions of Liquid Water Content (LWC)=0.5 g/m3, Medium Volume Droplet(MVD)=25 μm, T=-5 ℃, the free-stream velocity v=65 m/s was conducted on a NACA0012 airfoil to evaluate the plasma ice shape modulation performance. The plasma actuator was driven by nanosecond pulses of the peak voltage Up=7.7 kV,f=6 kHz,which is measured by the probe.The schematic diagram of the experimental arrangement and plasma actuator is shown in Fig. 2.

Fig. 3 shows the typical snapshots per 30 s from the beginning of the plasma ice shape modulation process in 3 min. It can be seen that there is always no ice accretion on the plasma actuator units, while more and more ice accumulates on the unprotected region as time goes on, which represents that the plasma actuator can modulate the continuous ice into segmented ice, forming a wavy leading edge. Compared with the reference,11the input energy is consistent while the protected area is twice.Therefore,about half electric power can be saved.

According to the above ice shape,the modulated ice model is designed and mounted on a NACA0012 airfoil to measure the aerodynamic force at the free-stream velocity of 30 m/s.The accuracy of the force balance in lift coefficient component is 0.05%.The width of the segmented ice and the plasma heating unit is 20 mm and 10 mm respectively.Fig.4 shows the lift coefficient CLof the airfoil with the modulated ice and continuous ice.Compared with the base line with no ice,the lift coefficient of the continuous ice decreases seriously and the stall angle of attack a advances 2°.However,although the lift coefficient decreases with the modulated ice,the reduction is smaller than that with the continuous ice, especially in the linear section. Compared with the continuous ice, the maximum lift coefficient is improved the most by 15.8% and stall angle is delayed by 2° with the modulated ice, which indicates that the modulated ice can significantly improve the aerodynamic performance of the aircraft.

Fig. 2 Schematic diagram of experimental arrangement and plasma actuator for ice shape modulation.

Fig. 3 Snapshots per 30 s from the beginning of the dynamic plasma ice shape modulation process in 3 min.

Fig. 4 Comparison of lift coefficients with modulated ice and continuous ice at free-stream velocity of 30 m/s.

In order to further evaluate the influence of the modulated ice on the aerodynamic performance and flight dynamics of the aircraft, wind tunnel experiments and simulations on a scaled transport aircraft model are carried out at the free-stream velocity of 70 m/s. The ice of 22.5 min after the anti-icing systems fails is chosen and mounted on the horizontal tail, as shown in Fig. 5. The width of the segmented ice and the plasma heating unit is 60 mm and 40 mm respectively.The flap deflection angle on the wings is 35°,which seriously affects the flow flied of the horizontal tail.

Fig. 5 Schematic diagram of modulated ice mounted on horizontal tail.

Fig.6 Rudder effect at different angles of attack with modulated ice and continuous ice at free-stream velocity of 70 m/s.

Fig. 6 shows the rudder effect calculated by the pitching moment in the conditions of the modulated ice and continuous ice. The accuracy of the force balance in pitching moment component is 0.05%. The rudder effect with the modulated ice is much better than that with the continuous ice at every angle of attack. Especially at the angle of -2°, the rudder effect is improved the most by 24.7%. The results show that compared with the continuous ice, the modulated ice can significantly improve the rudder effect, reduce the failure range of the horizontal tail, and enhance the maneuverability and safety of the aircraft.

The aerodynamic data of the above wind tunnel experiments is used as the simulation input to analyze the trim characteristics, static and dynamic stability of the aircraft. Table 1 is the trim information with different ice in conditions of 70 m/s and 400 m height. The positive deflection of the elevator with the modulated ice is larger,namely,the pilot’s usable rudder deflection increases, representing wider manipulation margin.

In the non-manipulated situation, a small disturbance is added at the trim point to conduct open-loop simulation of the aircraft, evaluating its static stability. Fig. 7 shows the time-domain response in gusty condition. The aircraft with the modulated ice re-converges to the trim point slightly faster than that with the continuous ice, which means better static stability.As shown in Table 2,both damping ratio and oscillation frequency at the trim point indicate that the convergence with modulated ice is better.

The dynamic stability of the aircraft is analyzed by increasing thrust ratio of 0.1. The time-domain response with the modulated ice and continuous ice is shown in Fig. 8. The aircraft with the modulated ice responds faster to manipulationthan that with the continuous ice,which means better response characteristics and transient performance. Since the aerodynamic data obtained by wind tunnel experiments is under the most severe icing condition and maximum flap deflection conditions,the improvement of the flight dynamics characteristics with modulated ice is small.The comparison of modulated ice and continuous ice under normal conditions needs further research.

Table 1 Trim information with modulated ice and continuous ice.

Fig.7 Time-domain response in gusty condition with modulated ice and continuous ice at free-stream velocity of 70 m/s.

Table 2 Damping ratio and oscillation frequency at trim point with modulated ice and continuous ice.

Fig. 8 Time-domain response when increasing thrust ratio of 0.1.

3. Conclusions

The flight safety oriented ice shape modulation method using distributed plasma actuator units is proposed. The feasibility of the method is verified from the two perspectives. The first one is the capability of the plasma actuator units to modulate the continuous ice into the segmented ice, and the second one is the capability of modulated ice to improve the aerodynamic performance and flight dynamics of the aircraft. Experiments and simulation of airfoil and scaled aircraft model with continuous and modulated ice show good performance of this novel method. This method breaks the traditional concept of eliminating all the ice at the wing leading edge,which is unpractical with limited power. It is expected to reduce about half power consumption and provide innovative ideas for the application of plasma anti-icing technology in aircraft, especially in unmanned aerial vehicles.

In the future, the periodicity and the size (dimensionless number related to chord length) of the segmented ice will be studied. More detailed work will be conducted to evaluate the flight safety boundary of aircraft with periodically segmented ice pieces.In addition,the idea of this method can also be extended to ice shape modulation using distributed electric heating.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The research work is partially sponsored by the Major Program of the National Natural Science Foundation of China(No. 91941105, 51790511) and the National Basic Research Program of China (No. 2015CB/755802).