An Investigation on the Deicing of Helicopter Blades using Shear Horizontal Guided Waves
Open Access
- Author:
- Ramanathan, Srinivasan
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 23, 2004
- Committee Members:
- Vasundara V Varadan, Committee Chair/Co-Chair
Vasundara V Varadan, Committee Chair/Co-Chair
Joseph Lawrence Rose, Committee Member
Thomas B Gabrielson, Committee Member
K A Jose, Committee Member - Keywords:
- piezoelectric actuators
shear horizontal guided waves
helicopter blades
deicing system
acoustics
equivalent circuit - Abstract:
- Despite all the advances that have made air travel safer than ever, the accumulation of ice on airplane and rotorcraft wings continues to be one of aviation’s most challenging problems. Ice accumulates on the wings as an aircraft flies through icing clouds. Icing clouds contain water droplets suspended in a super cooled, unstable liquid state. Water droplets in an icing cloud remain suspended in the liquid state as long as there is no surface for them to crystallize on. When this state is disturbed, such as when an aircraft passes through, the droplets freeze. In-flight ice buildup is a serious issue for propeller aircraft and rotorcraft, which fly at altitudes where icing clouds are likely to be found. The formation of ice on engines, wings and propellers of airplanes, and helicopter blades can seriously degrade the safety of an aircraft. In addition to losing aerodynamic lift, the rotor-blade can become unbalanced if the ice sheds unevenly causing dangerous vibratory loads on the system. Hence the presence of a reliable and efficient deicing or anti-icing system is imperative for their safe operation. The current method used to deice helicopter blades is similar to that available in automobile rear windows. These electro-thermal systems consist of heating coils that run along the span or chord of the rotor-blade. A current source connected via a slip ring configuration heats the coils, which in turn melt the ice on the surface. Due to their enormous power consumption, electro-thermal systems are generally configured to deice one foot of one blade at a time. This makes it hazardous to fly the helicopters under severe icing conditions. Even with the energy saving deicing procedure the electrical power required substantially exceeds the normal helicopter electrical system capacity, necessitating a large secondary electrical system with redundant, dual alternator features. The electro-thermal system for the Bell 412 helicopter weighed 162 lbs and required 26 kW of power for 2 blades! Various types of deicing systems were compared in chapter 1 and electro-mechanical systems were found to be the most energy efficient and practical for in-flight conditions. A novel approach of breaking the ice-substrate bonds by exceeding their adhesive strength using guided shear horizontal waves was chosen as the deicing mechanism. A comparison of the different electro-mechanical actuation systems pointed towards monolithic shear mode piezoelectric actuators as the choice that would satisfy the energy and dimensional requirements. A survey of literature on the mechanics of ice adhesion, in chapter 2, led to the selection of 1.42MPa as the target adhesive bond strength for the refrigerated ice-aluminum interface. The static adhesive strength of naturally occurring forms of ice such as rime ice and glaze ice to aluminum (0.12MPa and 0.4MPa respectively) is much lower than that of the refrigerated ice-aluminum adhesive strength (1.42MPa). Therefore, selecting the static adhesive strength of the refrigerated ice-aluminum interface as the bond strength to overcome would enable the system to deice rotor-blades under natural icing conditions. Equivalent circuit analysis was applied to the actuator, aluminum plate and ice layer system to determine an expression for the shear stress at the ice aluminum interface per unit excitation voltage supplied to the actuator and the corresponding electrical power consumed. All the parameters that affected the stress at the ice-aluminum interface were identified from the equivalent circuit model of the system. The parameters were split into control (can be actively changed by user) parameters and material (no user control over the variation of these parameter due to temperature and electric field) parameters. A statistical approach (Design of Experiments) was used to determine the control parameter settings that resulted in the maximum shear stress at the ice aluminum interface per unit actuator excitation voltage. A material parameter design of experiments was carried out to determine the effect of the deviation in the variable parameters on the stress at the ice-aluminum interface and actuator power consumption. A simplified approach to calculate the shear piezoelectric actuator losses under high excitation fields was presented. The experimental results indicated that the adhesive shear strength of the ice-aluminum bond under high frequency dynamic loads is much lower that its static adhesive strength. This was proved by the fact that the ice-aluminum interface bonds were broken at stress values of 0.73MPa as opposed to the target 1.42Mpa. This can be attributed to inherently stochastic nature of ice and the fact that the ice-aluminum bond fails at a much lower stress under dynamic loading as opposed to static loading. The shear mode actuator has a projected power consumption of 0.6kW for the twin bladed Bell 412 (assuming 6 actuators per foot per blade each consuming 50W) if deiced by station as opposed to 26kW for a corresponding electro-thermal system. The shear mode actuator has a projected power consumption of 3.6kW if both blades are deiced simultaneously over the desired length (1/3rd span from the root) as required in severe icing conditions. The piezoelectric shear mode actuation system (estimated weight of 50 lbs with the actuators themselves accounting for less than 1 lb.) has the potential of delivering this performance while being 70% lighter than a comparable electro-thermal system (weight of 162 lbs).