Mesicopter Progress Report
March 2000

Summary

See the February report for the previous update.

Rotor Development

Aerodynamics (Kunz, Kroo)

Two optimized airfoils have now been developed for operation at Re 6000 and Re 2000. The optimization runs were initialized with a flat plate airfoil, but the converged solutions have been checked by restarting the optimization with a seed geometry near the upper camber limits. Both airfoils exhibit similar features with a prominent droop near the nose, well-defined aft camber, and distinct hump in the camber distribution that begins near 65% chord and reaches its maximum height at 80% chord. Since the spline knots are located between 20% and 80% chord, the regions between the edges of the airfoil and the outermost control points are constrained to be nearly linear.

Optimization at Re 6000 resulted in a maximum camber close to 4%, but the Re 2000 solution increases to 6% camber. The increase is a compensation for the larger reductions in effective camber at lower Reynolds number. The Re 2000 airfoil achieves a maximum L/D of 8.2, 5% higher than best NACA 4-digit section tested at this Re, the NACA 4702. Unfortunately, there is currently no data for 4-digit sections with similar amounts of camber at Re 2000.

The 4% camber of the Re 6000 airfoil is closer to the 4-digit airfoils examined earlier and provides a better point for comparisons. This airfoil achieves an L/D of 12.9, 4% better than the 4702 section and a 16% improvement over the 4402. The L/D versus geometric angle of attack for these three airfoils is shown below. The optimized section begins to show gains past three degrees, increasing until the maximum L/D is reached at 5 degrees. Past this point, the performance of this section falls off sharply.

Small improvements in lift and drag allow the Re 6000 airfoil to outperform the 4702. A small portion of the drag benefit is due to near-zero skin friction over the middle third of the airfoil without any substantial separation. The droop in the nose and the increase in the ideal angle of attack assist this. The majority of the gains in lift and drag are connected to 5% less trailing edge separation on the Re 6000 optimized airfoil compared to the 4702, but this comes at a price. The optimizer is attempting to fully exploit the benefit of limiting trailing edge separation. At 4 degrees, trailing edge separation occurs at 88% chord; this is grows to 86% at 5 degrees. Beyond 5 degrees, the separation point almost immediately moves forward to 30% chord. As the point of separation moves far forward, performance rapidly degrades. The behavior of airfoils such as the optimized Re 6000 airfoil is much less forgiving than more conventional airfoils like the 4402 and 4702. These airfoils exhibit a smooth growth in TE separation and are much more benign in their stall behavior.

Fabrication (Cheng)

As observed in the previous lift tests, the measured thrust degraded over several test runs.  This may indicate that structural deformation is occurring during the operation.  Strategies are being considered to decrease deformation and increase strength and stiffness of rotor.  Possible solutions are summarized as follows:

Stability and Control (Fay)

Testing of the large scale rotorcraft using Westec motors showed that all components are functioning properly using onboard lithium ion batteries. Basic controls for roll, pitch, and yaw have been implemented on a programmable transmitter. The current rotors are not capable of producing sufficient lift for hover however. The latest work has been to build a lighter and also smaller version of the last rotorcraft. This model will use Mabuchi motors which weigh half as much as the Westec motors, but produce less power. The trade off is advantagous because we only need to use two lithium ion cells instead of three. Since battery weight is a large portion of the overall weight, the lift needed per motor is much less. Rotors have been tested that provide the necessary thrust, but only a counter-clockwise rotating version is sold. These rotors are currently being digitized so that pairs of both counter-clockwise and clockwise versions can be manufactured. The new rotorcraft should be assembled and ready to test by the time the rotors are available. This rotorcraft will also incorporate an inward canting of the motor thrust vector to test its effect on passive stability.

Power Supply (Fabian)

My efforts concentrated on the Maxim 1703 DC/DC converter. Schematic of the circuitry was created and a basic PCB of the circuit was routed. The tantalum capacitors used in this converter turned out to have very long lead times (52 weeks) to manufacture. Luckily I was able to obtain enough pieces from a reseller. For testing and weight estimation purposes a minimalist (using SMD components) controller circuit for the Smoovy motors was built.

Continued tests with the DC voltage converter suggest that the required output will be difficult to achieve with a 2.4v input source. A larger inductor may solve this, but further development is needed. The motor controllers with free-form circuitry now work and are very light.

Airframe Design and Manufacturing(Fabian, Cheng)

Experiments were conducted to reduce the weight of air-frame by adding glass bubbles, Micro-Ballons, into polyurethane. Maximum micro-balloon to polyurethane ratio achievable is 1:7.83. This mixing ratio reduces density from 1.12 g/cm^3 (pure polyurethane) to 0.643 g/cm^3, which is 57% of the original. As a result, total weight of air-frame can be decreased from 3.5g to 2.0g.

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