Welcome to the CMAV website!

The objective of the Colorado Micro Air Vehicle (CMAV) project is to conceive, design, fabricate, integrate, test, and verify the smallest achievable flying vehicle that utilizes a membrane wing to fly a flight computer, a radio, and an inertial measurement unit. This objective establishes the best way to advance the current state of MAV technology.
At a university level, MAVs are currently remote controlled (RC), lightweight vehicles engineered with very specific objectives in mind. They are commonly constructed from such materials as high-density foam and carbon fiber. High-density foam is used due to its low weight and its ease in machining shape-specific compartments for housing components. Carbon fiber has the advantage of being lightweight and incredibly strong, but it is more difficult to manufacture. The propulsive systems for MAVs have evolved from very small internal combustion engines to electric motors. Internal combustion engines were originally used due to the inability of the electric motors to produce the necessary power output, but as the energy density of batteries has improved, electric motors are now a nearly ubiquitous solution. The power to the motor and the rest of the components is provided by one of three types of batteries, lithium polymer, nickel metal hydride, or nickel cadmium. Lithium polymer batteries are now the most common choice due to their high energy density. The aircraft are usually controlled by employing RC servos. The advantage of using RC servos is that no additional communication equipment is required onboard the aircraft, thereby minimizing weight. Weight is also minimized by limiting the payload of the aircraft to only that which is required to achieve flight. Typically, MAVs only house a camera that allows the pilot to stably fly the vehicle. These weight minimization design choices, coupled with the miniaturization of electronics, allow some university MAVs to meet the Defense Advanced Research Projects Agency’s size definition of a MAV, which dictates the aircraft can be at most six inches in the greatest dimension and have a mass of at most 100 grams.

Aerodynamically, a plane of this size behaves very differently than typical aircraft due to the low Reynolds numbers encountered during flight. This results in unsteady aerodynamics and requires different design solutions to achieve flight. One such aerodynamic solution for MAVs is the use of thin airfoil, membrane wings. Membrane wings are a newer development in MAVs. Instead of having a fixed, solid wing, incorporation of a membrane for the wing material provides distinct advantages with gust alleviation. With the small physical size and modest flight regime encountered by MAVs, even small gusts can render the aircraft unstable. However, the flexible properties of the membrane allow the wing to deflect, thereby washing out the wing tips and spilling some of the aerodynamic load. This essentially results in a “communication” between the upper and lower surface flows, which helps to dynamically optimize the wing.

MAVs provide unique abilities and thus many future applications are currently under speculation. With the small size of a MAV, one obvious application is reconnaissance or search and rescue. These small aircraft would be able to penetrate areas unavailable to conventional aircraft and also have a very low profile for detection. One other prominent application is incorporation into swarm technology. The idea behind this notion is that many individual units would be able to act in tandem and communicate with each other through wireless ad-hoc networking, or some similar means. This approach allows many units to work in parallel. A practical application example would be deployment of networked MAVs equipped with “electronic noses,” or chemical detection sensors, in an area of chemical spill. The network would then be able to quickly generate a real-time concentration map.

Use of MAVs can also be very cost efficient, because the cost of one is essentially the cost of many since the majority of cost goes into development of the initial design as opposed to manufacturing or parts. The small size of these aircraft would allow many of them to be produced in a short time frame, relatively inexpensively. This also reduces the importance of any single aircraft, so that the loss of one MAV does not terminate the mission.
The design process for the CMAV team began with defining the capabilities or components lacking in current technology that are necessary to achieve some of the future applications mentioned above. First, the team recognized the need for on-board processing and data storage capability. This is an obvious requirement for autonomous flight. The aircraft must have some means of downloading and storing a flight profile and the ability to execute it. Also, the on-board storage would be beneficial to collect flight data, which could be downloaded or analyzed later. The second deficiency the team recognized was the lack of a communications system. This would be necessary to communicate with a ground station or additional aircraft in a swarm environment. The last major shortcoming was the absence of a positional awareness system. In order for the aircraft to effectively report useful data, it must know where it is in either a relative or absolute sense. However, many eligible systems for measuring position are too heavy for a MAV.

As stated in the objective above, the current CMAV design utilizes a membrane wing to fly a flight computer, a radio, and an inertial measurement unit. To accomplish these objectives, the overall aircraft weight will be approximately 150 g with a chord length of 11 in and a span of 13 in. The airframe will have a high-density, machineable foam core used to hold the components in place. A carbon fiber skin will reinforce this foam interior in order to increase durability, a benefit incurring a minimal weight penalty. Similarly, the wing will be made of latex and reinforced by carbon fiber spars. The planform and airfoil shape will be optimized as much as allowed by the limited timeframe of the Senior Project and by the limited information available on low Reynolds number flight and thin airfoils. The comm system is required to have a range of up to 1000 ft from the ground station. The aircraft is also required to fly at least 10 mph, with a desired flight speed of approximately 30 mph. This velocity will be achieved through the use of an electric motor with a 4.4:1 gearbox, a speed controller used to throttle the motor, and a 6x4 propeller. At the desired flight speed, the aircraft will be able to fly from the base station to edge of the comm range in just under 23 seconds and be able to traverse the entire diameter of the comm range in approximately 46 seconds. With this limited amount of straight and level flight, the aircraft will need to be able to respond to control commands on a fairly regular basis. As a result, a human will remotely pilot the aircraft from the ground through inputs from a joystick. The control commands will then be sent to the aircraft through the comm system, where they will be used to actuate two servos, one controlling an elevator and one controlling a rudder. This will give the aircraft pitch and yaw control, with a possibility of achieving some degree of roll control. The servos will receive the information from the comm system by interfacing with the flight computer. The flight computer will read the control commands, or any other subsequent information, from the onboard receiver and direct the information to its proper location after translating it into the proper format. The flight computer selected to perform these tasks is the Naiad, a package created by the Fire-monitoring Uninhabited Aerial Vehicle Avionics (FUAVA) team last year. The Naiad was selected due to its light weight, low power consumption, modular design, and its extensive support of many different interfaces, protocols, and buses. These include I2C, PWM, CAN, and SPI. The Naiad also houses a UART and 8-channel analog-to-digital converter (ADC). The inertial measurement unit (IMU) will be a commercial, off-the-shelf (COTS) product that measures accelerations and angular velocity on all three axes. It will interface with the flight computer and collect the primary data for the aircraft. The power to the aircraft will be provided by four 250mWh lithium polymer batteries.

Experimentally, the CMAV project will measure the forces on the aircraft during flight. This will be done by measuring the aircraft’s acceleration through use of the accelerometers housed in the IMU. However, since the IMU is attached to the airframe, it not only detects the accelerations caused by the external forces on the aircraft, but also the accelerations caused by the control of the aircraft, such as rolling. Since the goal is to measure the external forces on the aircraft, the attitude of the aircraft needs to be determined by using the rate gyros on the IMU so that the sensed acceleration can be translated into true acceleration. With the extraneous accelerations removed, the remaining acceleration can be scaled by the mass to produce the external forces on the aircraft. The aircraft will be designed to have an easily removable, modular wing such that different wing configurations can easily be put on and taken off the aircraft. In doing so, by flying in a controlled environment, the responses to known gust conditions for different wings can be measured and the response of the membrane wing with varying parameters can be characterized. These tests will provide very valuable data to the true nature of membrane wings by quantifying the extent of the communication between the upper and lower surfaces through the different accelerations to a known gust, an area of study with very little empirical data. Secondary goals of the data collection portion of the project include characterizing the motor performance through static tests and tests in the wind tunnel, measuring the battery discharge rates at different current loads, characterizing the accuracy of the determined attitude, and measuring the wing’s aerodynamic performance and gust response in the wind tunnel to validate the experimental data.