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The rotor analysis and design codes have been further developed by coupling the rotor performance program with a non-linear optimizer. In the design mode, the goal is to maximize thrust for a given radius and for a particular motor. The chord distribution, lift distribution, and rotational speed are treated as the free design variables with the primary constraint that the power required matches the motor's power available. The motor data for the DC5-2.4 motors has been determined experimentally. Data for the 5mm Smoovy motors is based on manufacturer's data, but has been validated at several operating points experimentally.
The analysis mode is a different problem and requires the ability to input only the geometry of the rotor, chord distribution and incidence, and the rotor speed. This problem is also solved using the non-linear optimizer, but in this case the specified incidence angle at each station is treated as an equality constraint. There is no constraint on the power required. The design code produces solutions at a single operating point. The analysis code allows these solutions to be evaluated over a range of operating conditions. It also provides a means of validating the method by comparing predicted performance with experimental results for existing rotors and propellers.
Initial results indicate that the both codes do a good job of predicting lifting performance, but the power required is significantly underestimated. Also apparent from initial results is the importance incorporating viscous swirl effects for very small, low Reynolds number rotors. The accompanying figure compares the predicted lift versus RPM, with and without viscous swirl corrections, to experimental data for a 4-bladed, 2.5cm diameter rotor. The prediction with the viscous swirl correction agrees well with experiment, but the prediction without the swirl correction significantly overestimates the lift for a given rotor speed. Although the lift versus RPM prediction is reasonable, the estimation of the power required appears to be underestimated by as much as a factor of 2. It is unlikely that errors in the 2-D section performance predictions could account for this discrepancy. The presence of significant 3-D effects in the flow-field is most likely responsible.
A new testbed for investigation of mesicopter stability and control has been completed. This prototype is lighter and smaller than the last. The longest dimesion is approximately six inches, and the total mass is 60.5 grams. The current motor/rotor combination will produce a total of 80 grams of thrust leaving extra thrust for climbing, maneuvering, and carrying payload. The counter-rotating rotors have also been manufactured and are now mounted on the mesicopter.
Dynamic analysis using the measurements and weights for the prototype was also completed. The operating range of the vehicle is indicated on the root locus plot below. These indicate the natural frequency is approximately 1 Hz with a damping ratio between .8 and .9. The plots indicate that varying the CG location greatly affects the damping. The rotor tilt angle has some effect on the natural frequency. Initial testing with the vehicle on a tether show that some method is needed to trim the thrust so it is equal between the four motors. The rotors have been sanded so that their lift at a given RPM matches. Some method is still needed to match motors and electronic components. Currently trim tabs on the R/C transmitter are used. Also, the quick dynamics of the vehicle might make it extremely difficult for a pilot to control without any feedback. The next step is to determine whether this is true.
Prof. Kroo and Peter Kunz presented papers at the AIAA workshop on Fixed, Flapping, and Rotary Wing Vehicles at Very Low Reynolds Numbers held at Notre Dame in early June. These papers will be published as part of the AIAA progress in aeronautics series, but drafts are available here.
Last update: 15-Jun-00 3:22:32 PM
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