Mesicopter Progress Report
February 2000
Summary
See the January report for the previous update.
Rotor Development
Aerodynamics (Kunz, Kroo)
The two-dimensional section optimization has yielded some interesting initial
results. The results are based on a seed airfoil which is a 4-digit, 2% thick
NACA thickness distribution. The free parameters represent the shape of the
camberline. The code has been running at a fixed Reynolds number of 6000.
Maximizing the lift to draft ratio is the objective function. The constraints
on maximum local camber are +6.0 percent and -1.0 percent. The maximum lift to
drag ratio is taken as the best solution within the range of steady-state
convergence.
Due to the maximum lift to drag ratio occurring very close to where the steady-state analysis fails, if angle of attack was included as a design variable, a candidate geometry would likely fail to converge at some point. For this reason, angle of attack is not considered by the optimizer; it is addressed within each function evaluation. This is the simplest method of avoiding invalid simplex vertices that would require a restart. A polar is generated for each candidate, beginning at a low angle of attack where the geometry is much more likely to converge. This provides a valid design point for the optimizer even if the performance of the candidate section is very poor and stalls at a low angle of attack. This is a computationally intense approach, but it is straightforward and computationally manageable for the two-dimensional case.
The early results of the optimization indicate that unconventional camberlines may offer benefits over more conventional designs, such as the NACA 4-digit series. The first optimized geometry for Re 6000 is shown in accompanying figure, the original seed section is on the right. The most interesting features are the 'humps' at the leading and trailing edges. The leading edge geometry is likely an attempt to reduce the local angle of attack at the nose, while the aft camber appears to enhance lifting performance. The purpose of the concave region at mid-chord was unclear.
The optimizer indicated that the lift to drag ratio for this section should be slightly over 13. The best of the NACA 4-digit sections analyzed only reached 10.5 to 11. The predicted 18% improvement in L/D was substantial, but further investigation indicated that the surface grid density was insufficient. The predicted results were not repeatable at higher grid densities and the section was only able to match, but not improve upon, the performance of the best-tested NACA 4-digit section.
Further runs have been completed using higher grid densities for the CFD calculations. The pronounced humps in the section have dissipated, but the localized forward and aft camber remains a feature in the new solutions. The section shown in the accompanying figure reaches a computationally repeatable L/D of 12.9 at Re 6000. The forward camber reaches 3%, then increases nearly linearly to 4% at the 80% chord station. Two separate optimizations, starting from very different initial airfoils, reached the same final geometry. This indicates that this is section is not a highly local optimum, but covers a significant portion of the design space.
Micro-Glider
The first micro-glider has been assembled and flown. Preliminary flights of have exhibited stable, steady-state gliding flights suitable for experimental determination of vehicle flight performance. Methods for experimental flight path determination and analysis are currently under investigation. The previously mentioned 60% increase in wing weight from the design value is due to a manufacturing error in the wing cross section. The manufactured wing is 3.5% thick. The design indicated 2% thick sections. Airfoil drag polars and overall vehicle performance predictions are being recalculated for the as-built geometry.
Fabrication (Cheng)
Detailed analysis was done on the laser-scanning result shown in the January progress report. It was found that there was excess material in the leading edges and less material in the trailing edges. Blades were twisted clockwise and this may result in more incidence than we need. Therefore, we analyzed the twist angle (in degrees) at 50% and 75% of radius of the rotor in each blade from the picture. By measuring the differences between leading and trailing edges and knowing the chord, the twist angles (degrees) of each blade are shown in the following table.
The average twist angle is 4.29 degrees at 50% R and 5.07 degrees at 75% R.
However, it varies from blade to blade. In order to reduce this twist effect introduced
during manufacturing, we decided to increase the thickness in the first 25% of
the rotor radius. The modified rotor was built and sent out for laser scanning.
The result is shown in Figure 1. The anomalous twist has been substantially
reduced. 
Right: modified rotor, and left: previous result for comparison
Another rotor with additional incidence was fabricated and tested. The lift test result showed similar behavior to the previous increased incidence rotor: at the designed rpm (~45,000), it can generate 2.6 gram lift, but required high voltage (~10.2V) and high current (~0.19A) inputs. Since we expect up to 4g of thrust with lower torque we are continuing to look at the drag assumptions and details of the as-built geometry.
Wings of the micro-glider were fabricated using the same SDM methodology as rotor. Polyurethane was used to build micro-glider to reduce weight. Fuselage and wings are manufactured separately and later assembled. Due to the thin structure and waker bonding between polyurethane and wax, additional machining tricks, such as the addition of removal anchors, are used during the fabrication, especially for the tail surfaces that are flat thin plates.
Stability and Control (Fay)
A large scale model of the mesicopter has been built as a proof of concept testbed for flight tests and control law development. The overall dimensions of the model are 6 inches along each side not including rotors. The motors have been tested while the vehicle was rigidly constrained, but an open loop free flight test has not yet been made. The final vehicle weight is 114 grams. The rotors which are currently mounted on the vehicle are not capable of lifting the weight of the vehicle, but new rotors capable of lifting more than 120 grams total weight are being manufactured. The model is built as a modular planform with replaceable motor mounts so that the cant angle of the rotor blades may be changed. The current cant angle is 10 degrees. Power comes from three 3V lithium ion cells rated at 430 mAh each. The weight due to batteries alone is 37 grams
.
Power Supply (Fabian)
According to latest motor/rotor testing the minimum power required for the 5mm smoovy-based mesicopter prototype is about 5W or 8-9V and 600mA. 6g limits the total weight for the power supply. The following table compares the performance of various rechargeable battery technologies.
|
Battery System |
Energy Density |
Battery Voltage |
Self Discharge |
Cycle Life |
Electrolyte |
Discharge Profile |
Memory Effect |
|
|
|
Wh/kg |
Wh/liter |
V |
%/month |
cycles |
|
|
|
|
Lead Acid |
30-35 |
80 |
2 |
8 |
100-500 |
Liquid (Aqueous) |
Flat |
No |
|
NiCd |
30-40 |
100 |
1.2 |
10-20 |
500-1000 |
Liquid (Aqueous) |
Flat |
Yes |
|
NiMH |
70-100 |
180 |
1.2 |
10-20 |
500-1000 |
Liquid (Aqueous) |
Flat |
No |
|
Lithium Ion |
100-125 |
260 |
3.6 |
9-12 |
300-800 |
Liquid (Organic) |
Sloping |
No |
|
Lithium Ion Polymer Electrolyte |
100-110 |
240 |
3.6 |
<10 |
300-800 |
Solid-Liquid (Organic) |
Sloping |
No |
|
Lithium Metal |
150-200 |
300-400 |
3.6 |
2-3 |
100-150 |
Liquid (Organic) |
Sloping |
No |
|
Lithium Metal Polymer Electrolyte |
150-200 |
300-400 |
3.6 |
1-2 |
200-300 |
Solid-Liquid (Organic) |
Sloping |
No |
|
LITHIUMPOWER Lithium Metal Dry Polymer Electrolyte |
250-350 |
450-550 |
3.6 |
<<1 |
>>1000 |
Solid (Polymer) |
Sloping |
No |
(source: Lihiumpower Inc.)
New high energy density lithium based battery chemistries are coming to the market. The most interesting ones are still in development phase and first engineering samples are expected at the end of this year. Several battery manufacturers were approached with the above mentioned battery specifications. None of the approached battery companies was able to satisfy our requirements. Therefore our current investigation concentrates on solutions with existing batteries combined with a DC/DC step-up converter. MAXIM 1703 based DC/DC step-up converter is being investigated as a prospective candidate.
Other Items
Air-frame Design and Manufacturing (Fabian, Cheng)
Substantial efforts were put into the design of the mesicopter frame. In this design we added shrouds around the propellers in order to increase the performance and provide some protection. A 3D model defined in a parametric way (to the extent possible) was built. The parametric approach enables easy implementation of design changes. Two different configurations are shown in the figures below.
The first configuration consists of three parts (shrouds, connecting pins, and motor holders) that are later assembled. The second configuration consists of a one-part airframe. The structure is stiffer and the alignment between shrouds and motor holders is precise. However, this will be more challenging in manufacturing. After feedback from other members of the team we decided to adopt the second one. This design was further modified and the weights computed. The overall size of this design is 65.6 mm. The total weight of the airframe is 3.24 gram out of polyurethane. This calculated weight is much higher than the initial prediction (~1 gram) and we are looking for solutions reduce it.
Edit
Last update: 15-Mar-00 12:59:57 PM
WebEdit servlet by I. Kroo