See the November report for the previous update.
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 the current manufacturing process, wax is used as a support material. It is required to melt the wax at high temperature(90-130 degrees C) and clean the residual wax in BioAct solution at the temperature of close to boiling point. According to the material properties provided by the manufacturer, the heat deflection temperature of epoxy is about 48 degrees C. Therefore, it is very likely to generate warpage or some permanent distortion during the wax removal process. With careful handling and precise temperature control, this kind of deformation can be reduced. However, it will be encouraged if the heating-up process can be avoided. In the Rapid Prototyping Lab, there are two different materials that can be removed by water--soldermask and water-soluble wax. Soldermask is a UV-cured material with fair machinability. Water-soluble wax is a lower temperature wax. An initial experiment was conducted to use soldermask as support material to replace wax. The machining quality on soldermask was pretty good, and the boning between soldermask and epoxy is strong enough to hold epoxy during the machining. However, the problem occurred when soldermask is dissolved in the water. The residual stress from the strong boning is too high that the blades of the rotor were distorted into the other direction. The other candidate, water-soluble wax, will be tried next.
For structural analysis of the blade, torsion was considered in addition to the bending effect. The results of pure torsion for both epoxy and polyurethane blades are shown in the chart. The rotation angles are relatively small that we can decouple bending and torsion in the analysis. In the torsion analysis, epoxy and polyurethane are assumed as glassy polymers so that the shear modulus, G, is assume to be 3/8 of modulus of elasticity, E.
In order to verify that the current rotor design is optimized, it is planned to build rotors with different increments in incidence--4, 6, and 8 degrees added to the original design. More lift tests for these rotors will be conducted.
Work on the motor and motor control electronics was minimal this month as we have identified the motors to be used for the prototyping work. We have contacted additional manufacturers, however, for future implementation possibilities.
Our basic layout remains unchanged from last month. Structural analysis has focussed on the rotors as described above.
The optimal design specifications for the 5mm mesicopter have been estimated using previous 2-D analysis work. The two major design specifications are the location of the center of gravity, and the inward tilt of the rotor shafts. The criteria for choosing these specifications were that the helicopter by passively stable in hover with sufficient damping. This was accomplished by analizing the system poles of the mesicopter using the linearized 2-D model of the mesicopter. The plots below show the system pole locations for varying rotor shaft angle (beta which is positive inward), and for varying center of gravity location (h which is positive downward).
In the first plot, poles are shown versus beta using six different values for the center of gravity position. These values range from -1 cm to 4 cm in 1 cm increments with h=0 shown in green. The red triangles indicate the positions where beta=0. The triangle at the origin is a pole for the case where h=0 and beta=0 which is marginally stable. This indicates that for a given center of gravity location, there is a stable range for beta that has good damping properties. The second plot shows the system poles versus h using seven different values of beta. These values range from 0 to .6 rad in .1 rad increments. Another criteria for performance was that we maintain at least 95% of the vertical thrust of the rotors for lift. This corresponds to a beta that is no larger than 18.2 deg or .317 rad. The values in green are for beta=.3 rad which is close to the limit for beta. This plot indicates that there is an unstable range for the center of gravity being above the mesicopter as well as far below it. With the center of gravity even with the rotor hub centers, the damping is approximately .7 for a beta of .3 radians. This plot seems to be more useful in determining the values of beta and h. Current modelling for the rotorcraft have been based on these findings.
The 2-D dynamic behavior of the mesicopter was analyzed using the values beta=.3 rad and h=1 cm. The results of the linear and nonlinear analysis are plotted below. There is good correlation between the two methods, and the values shown have the desired stability and damping qaulities.
Further work is being done to come up with a scalable non-dimensional system model that can be used to build larger or smaller scale mesicopters. A full 3-D model is also under development to make sure the dynamic stability and behavior still holds in three dimensions.
Some experimentation with gyros was undertaken, with details to be reported next month. Prof. Kroo and Peter Kunz visited AeroVironment to discuss their 2g color video system, 2.4GHz comm system, and flight control sensor package (magnetometer, air data, INS). For near-term applications, these systems seem very well suited to the mesicopter. We will continue to work with this group whose work is funded by DARPA's micro air vehicle program.
Prof. Kroo met with researchers at JPL to discuss interesting planetary missions that might be appropriate for the mesicopter. We will be putting together a paper on the potential of such devices and trying to develop a program that will involve a series of tests. We are now hard at work preparing for the first test, scheduled for 1/27/2000 of a rotor in the 7ft Mars environment chamber. Details next month.