ACI has successfully ruggedized Thermoelectric Modules (TEMs) for a demanding application in rotary wing aircraft. In conjunction with a shape memory activator (NiTinol), the TEMs enable the twist of the rotor blade to be altered in-flight. For this application, the TEMs must be impervious to the marine (salt spray) environment as well as high vibrational, shock, vertical and centrifugal forces as high as 600G.

Rotary & Hybrid Wing Aircraft:
Rotor Blade Twist Actuator
Rotary and hybrid-wing aircraft rotor design utilizes a fixed geometry with an optimized angle of attack between those most appropriate for various types of operation. This fixed rotor design is aerodynamically sound and proven. It is also less expensive than a rotor having the capability to alter blade twist inflight. However, the fixed geometry is a compromise and is incapable of providing maximum performance among several flight modes.

Payload capacity and flight range can be increased by use of a rotor capable of reconfiguring the span wise distribution of the blade angle (twist) to optimize the flight configurations as needed. The in-flight reconfiguration of the rotor can be accomplished by incorporating a Blade-Embedded Rotational Actuator (BERA) to twist distribution of the rotor blades from the optimum angles for hover to the optimum angle for cruise, and back again. This process employs a Shape Memory Actuator, which is currently in development through a Shape Memory Actuator Demonstrator Project (SMAD) with a multi-participant effort including Boeing as the lead contractor. The design is based on technology developed by the ONR Shape Memory Actuator Consortium (SMAC), also led by Boeing, with support by the U.S. Defense Advanced Research Project Agency (DARPA).

The TEMs, also known as Peltier devices, are small solid-state devices which function as heat pumps. A typical TEM is a few millimeters thick, and is a few millimeters to a few centimeters square, although other device outlines (circles, ovals, triangles, etc) can readily be produced. The TEM is a sandwich formed by two parallel ceramic plates with an array of small Bismuth Telluride cubes (couples) in between. When a DC current is applied, heat is moved from one side of the TEM to the other where it is removed by a heat sink. Thus, the solid-state TEMs, can be an ideal means of controlling and actuating systems that utilize shape memory alloy.

An earlier experimental analysis indicated that for demanding aeronautical applications, a high temperature construction of TEMs was required to meet the reliability requirements of this program. Four commercial-off-the-shelf (COTS) TEM's were evaluated in that study. It was also shown that an epoxy composite fill within the TEM significantly improved shear strength before and after environmental stress testing. This finding was significant as TEM elements, characterized by poor shear strength, required an improved packaging system to withstand 600G forces in a high vibration environment.

TEMs are made from the two elements of a semiconductor (primarily bismuth-telluride), heavily doped to create either an excess (n-type) or deficiency (p-type) of electrons. Heat absorbed at the cold joint is pumped to the hot joint at a rate proportional to the current passing through the circuit and the number of couples.

To prepare a useful device, these couples are connected electrically (in series) and thermally (in parallel). Fortunately, commercial TEMs are available in a variety of sizes, shapes, operating currents, operating voltages and thermal capacities.

TEMs can produce a no load temperature differential of about 67oC. The actual cooling effect, however, is determined by the proper choice of TEM for each specific job. Three specific system factors must be determined before the correct device selection can begin:

In most cases, the cold surface temperature (Tc) is an independent variable - i.e., the target is intended to be cooled to some arbitrary temperature. If the target is in direct and intimate contact with the cold surface of the TEM, the object temperature can be considered to be the same as the temperature of the cold surface of the TEM (Tc). If this is not the case - for instance, where a heat exchanger is required on the cold surface of the TEM - then Tc may need to be several degrees colder than the desired target temperature.

The Hot Surface Temperature (Th) is defined by two major factors: 1. The temperature of the environment to which heat is being discharged, and 2. The efficiency of the heat exchanger between the hot surface of the TEM and the environment.

The factors Tc, Th, and the difference between them must be accurately known to successfully determine the operating specifications of the TEM needed for any given application.

The amount of heat to be removed by the cold surface (Qc) of the TEM is typically the most difficult factor to quantify. This is because all the thermal loads to the TEM must be considered. These include the active (I2R) load from any device, thermal conduction through any object in contact with the cold surface, and all warmer objects in contact with the cold surface such as insulation, electrical leads, mechanical fasteners, air, etc. Sometimes radiant heat from surrounding objects must also be considered.

Once these three basic factors have been quantified, the cooling requirements for each application can be determined with the use of common heat transfer equations. These equations are found in most engineering handbooks, and are usually found in the literature and catalogs from commercial suppliers.

TEMs have unique advantages for thermal control in many harsh environments. But their effective application requires careful engineering, development and confirmation through exhaustive testing to assure thermomechanical performance in addition to the desired thermoelectrical effect.

ACI responded to this problem with an advanced packaging approach for the TEMs in which the free space within the TEMs was filled with an epoxy-composite material to support and add shear strength to the modules. The success of this approach is demonstrated in figure above, which confirms a six-fold increase in the shear strength of filled TEMs.

Shape Memory Alloy
NITINOL (an acronym for Nickel TItanium Naval Ordnance Laboratory) is a family of intermetallic materials, which contain a nearly equal mixture of nickel (55 wt. %) and titanium. Other elements can be added to adjust the material properties. Nitinol exhibits a unique behavior called Shape Memory.

The shape memory effect describes the process of restoring the original shape of a plastically deformed sample by heating it. This is a result of a crystalline phase change known as a thermoelastic martensitic transformation.

Below the transformation temperature, Nitinol is soft and highly deformable, reflecting its martensitic structure. Heating the material converts Nitinol to its high strength austenitic form. While the transformation from austenite to martensite (cooling) and the reverse cycle from martensite to austenite (heating) does not occur at the same temperature, there is a hysteresis curve for every Nitinol alloy that defines the complete transformation cycle and it is highly repeatable. With cold-working and continuous annealing, the metal, in the soft martensite form, cannot become deformed or stabilized in the deformed configuration. Now the thermally reversible conversion from austenite to martensite and back again engenders a controlled and reproducible change in the configuration of the Nitinol. This change in configuration (shape) can now be used to perform physical work - actuate a switch, operate a valve, rotate a mirror, work a lever, advance a gear, etc. - based on a control of the Nitinol temperature.

The currently proposed actuator employs Nitinol, and thermoelectric modules (TEMs) as its primary components. The Nitinol alloys mechanically activate the span wise change in rotor twist when heated above critical temperatures. The TEMs provide an efficient means of thermal management, both heating and cooling the Nitinol alloy electrically on demand.