Hovering MAV Dual Prop with Tilting Payload

Luke Wendt

Summary:

Micro Air Vehicles “MAVs” are both entertaining toys and useful tools. As toys they can be used to explore new methods of flight dynamics; the more elaborate and unconventional they are the more eye catching and interesting they tend to be. As a tool they can be used as scouts for surveillance and recon. MAVs capable of hover and stare operations are of particular interest in an urban setting. They must be capable of controlled outdoor and indoor flight. Initial interest in the subject came from a documentary of several current UAV and MAV designs, several of which used dual propeller technology for controlled hovering. Dual prop technology involves aligning an even number of counter rotating blades in the axes of rotation so there is no net torque.
The dual prop system makes a very stable platform for hovering MAVs and a design goal developed to build a dual prop MAV with a centralized payload directly below the dual prop axis. Rather than using the conventional gyros, thrusters or counter props seen on current military and toy MAVs, the MAV would be controlled by tilting its payload. This would simply tilt the dual prop in the desired direction of flight allowing some of the vertical thrust to provide horizontal thrust. There were some advantages to this design. The tilt actuator has the potential of requiring less energy than the counter props used for tilting if the actuator holds its position when deactivated. If the payload was lowered in resting position, the torque that keeps the MAV upright becomes stronger thereby making the MAV more stable.
Several design concepts were developed for a dual prop with a tilting payload. Some of the main requirements were 3D flight, controlled hover, quite operation, 1 foot maximum dimension, low cost, additional weight support and wireless flight. Due to the complexity of the project, development was broken up into stages. The first stage involved developing or acquiring a dual prop system capable of a controlled hover. The second stage involved control via wire of the dual prop and any actuators selected in the final concept. The third stage involved completely wireless flight.
The concept selected was simply a dual prop with a micro servo mounted underneath. The battery and control circuit would then be attached to an extended shaft off of the servo. Several RC helicopters were discovered with dual props. The one selected was cheep in comparison and fit the size, flight time, and noise requirements. A light micro FUTABA servo was acquired. A piece of Styrofoam was used to house the payload. This project hinged a lot on experimentation as the RC controller had to be treated like a black box. Q-Tips were used for attachment of the payload and landing struts.
Two prototypes were developed. In the first prototype the plan was to control the servo using the signal originally intended to power the counter prop on the helicopter. This would have allowed completely wireless control. A timer circuit was constructed using an NE555 chip, some capacitors, resistors and a BJT. Using a BJT, a threshold DC voltage source was obtained from the battery. The control circuit worked momentarily but the power draw of the servo damaged the controller circuit in the MAV. This resulted in the frustrating end of the first prototype.
The second prototype was simplified by wired control using a hand held servo controller. The rest of the design remained unchanged from prototype 1. The only variation was in the shape of the payload, but the construction material was all the same.