See the December report for the previous update.
One of the major accomplishments this month was the fabrication and testing of rotors designed for operation on Mars. In cooperation with JPL, we designed, fabricated, and tested 2 rotors intended for use on a Mars-based mesicopter. Because of the very low densities, and JPL's interest in near-term payloads, we are looking at vehicles with masses in the tens of grams. This led to a rather large (20cm) rotor, but the Reynolds numbers are very similar to those of the cm-scale earth-based rotors. Pictures of the rotor and the test set-up at JPL are shown below. More details on the design and test results will be published next month.
Carbon rotor for Mars atmosphere test. (left) View through window of chamber showing lifting rotor on test stand. (right)
The micro-glider test program has begun fabrication of the individual aircraft components. The fabrication process is the same one currently used in the fabrication of the 3mm and 5mm Smoovy rotor blades. Each glider is comprised of three sub-assemblies: fuselage and vertical tail, wing, and horizontal tail. All are constructed of polyurethane resin. Several fuselage pieces and wing pieces have been completed. The horizontal tails are currently being manufactured. The manufactured fuselages are within 2.5% of their predicted mass. The wing assembly is 60% heavier than expected, but a large portion of this discrepancy is due to a wing attachment and alignment structure that was not accounted for in the preliminary weight buildup.
The glider is a conventional aircraft configuration. The wing span is 6.0 cm. All aerodynamic surfaces have a constant chord of 0.5 cm. This provides a constant Reynolds number over the entire aircraft during steady flight. With an expected unballasted mass of 0.145 grams, the predicted trimmed flight speed of 3.8 m/s provides a Reynolds number of 1300. The predicted glide ratio is 3.7, but this value does not yet take into account the latest weight estimate and the 2-D section data for the cambered plate sections. The current model uses airfoil data for NACA 4-digit sections. The wing actually uses a 2% constant thickness airfoil with an NACA 4402 camberline. The tail surfaces use 2% thick flat plates. These corrections will not affect the ability of the aircarft to provide a useful validation of the 2-D airfoil performance data. The maximum L/D will decrease and the glide speed may change slightly, but the stability of the aircraft will be unaffected provided that the c.g. position is maintained.
In order to verify the effect of incidence angle and the optimization of the design, 5-blade rotors with incidence angles increased 4, 6, and 8 degrees from the original design were built and tested. The test results are briefly summarized in table 1. Lift increases when the incidence increased. When the incidence is 8 degrees more than the original design, it required higher voltage and current to reach designed RPM. At this high voltage and current level, the lift was much higher than other lower incidence rotors. This suggests that the maximum lift coefficient may be higher than the assumed conservative value.
(Table 1)
Experiments for other possible material combinations for rotors are being conducted continuously. Besides using the water-soluble UV-cured soldermask as support material, water-soluble wax was tried. Unfortunately, the result was similar to what we obtained previously with soldermask. The residual stress between part and support materials was relatively high causing serious distortion after dissolving the support material, water-soluble wax. The rotor lost camber and desired shape after the water-soluble wax was removed. It was concluded that the combination of yellow wax (support material) and epoxy (part material) with extra care in handling and with precise temperature control during the wax removal process gives best results. Water-soluble support materials introduce more distortion than expected.
More efforts on characterizing the geometry of manufactured rotors were made. Other than mechanical measuring and co-ordinate measuring, laser scanning was considered a better candidate to provide useful information and better resolution. After searching through companies and institutions with capable techniques, we finally found one company can meet our requirements and dimensions. The rotor was sent to GKS Inspection Services, Michigan. They laser scanned the blades with 40,000 points and compared with the design CAD model. The result is shown below. Any red values represent positive side of material (usually meaning excess material, or a bent area of the part), while any blue values represent the negative side of material (usually meaning inside of material). Substantial variations in the desired incidence are observed, perhaps explaining some of the excessive torque values currently measured.
A large 5-blade rotor was fabricated for the Mars environment test at JPL. This large version rotor is 200 mm in diameter. Due to the time constraint and material concern, FDM (Fused Deposition Modeling), a prototyping technology, was first used to fabricate the large rotor. The FDM machine operates by sequentially depositing discrete slices of plastic or wax material. The geometry of the slices corresponds to horizontal cross sections of a part. The parts are rendered using three dimensional computer modeling. ABS plastic, P400 ABS, was used as the part material. The total fabrication time was 11.5 hours. However, the finished part was porous and did not survive when the test began. The main issue was that the uniform slicing steps caused fewer layers at thinner areas. The other option was to use composites as part material. For simplifying the process and increase the rotor strength, the blades were designed with uniform thickness. An aluminum mold was machined with the shape of rotor's bottom surface. Two composite rotors were fabricated out of this mold for the tests at JPL.
The past 2-D analysis was used to develop a preliminary frame design for the mesicopter. A simultaneous effort has begun to build a fully functional large scale version of the mesicopter to fly at higher Reynolds numbers. Fabrication will begin immediately after optimal blade parameters have been calculated since this will size the overall dimensions of the vehicle. The goal is to have the air frame of the larger vehicle be nearly identical to the actual mesicopter design. This is ideal in terms of manufacturing and dynamic and control modelling. Building the frame at a larger scale may give insight into where problems may arise at the smaller scale. Also, if the structures are similar, the free body dynamics should be identical, although the aerodynamic forces acting at different Reynold's numbers may change. The goal is to design a control law for the large scale vehicle that can be used on the actual mesicopter with very few modifications.
During the past month correspondence with scientists at NASA Ames concerning the mesicopter was initiated. The Ames Research Center had already begun an initial investigation into the possibility of vertical lift planetary exploration vehicles prior to this meeting. The NASA proposal is a joint effort between the Army/NASA rotorcraft division and the Center for Mars Exploration. The purpose of the meeting was to collaborate on the development of the mesicopter as well as to determine the interest of Ames in such a project. Considering that the mesicopter project is already under development at Stanford and that one of the primary missions proposed for the mesicopter would be planetary exploration, the scientists at Ames felt reasonably confident in pursuing this venture further. Other issues discussed were the resources that could be made available at Ames for use on the mesicopter project. Some of these resources include tools developed by the Flight Control and Cockpit Integration Branch designed for system identification and control law optimization for rotorcraft. Otherwise, only broad initial design concepts have been discussed which may be incorporated in the preliminary design.
Prof. Kroo met with researchers at NASA Langley who are interested in the use of this platform for research in multiple aerial robots. They have begun the construction of their own system (another 4-rotor electrically powered configuration) with much greater payload capability and all commercial components (incl 13lbs of batteries). They are interested in collaboration, especially in the areas of flight vehicle control.
Prof. Kroo, Peter Kunz, and Gary Fay wrote and presented a paper at the American Helicopter Society meeting in January. This paper will be available on-line soon and described the development of the mesicopter concept to date. Gary presented the paper, which has generated much interest in the rotorcraft community. Researchers at NASA Ames, with whom we have been talking over the past year, also presented a paper on the use of rotorcraft for planetary exploration.