Micro Air Vehicles

Definition

Micro air vehicles are either fixed-wing aircraft , rotary-wing aircraft ( helicopter ), or flapping-wing (of which the ornithopter is a subset) designs; with each being used for different purposes. Fixed-wing craft require higher, forward flight speeds to stay airborne, and are therefore able to cover longer distances; however they are unable to effectively manoeuvre inside structures such as buildings. Rotary-wing designs allow the craft to hover and move in any direction, at the cost of requiring closer proximity for launch and recovery. Flapping-wing-powered flight has yet to reach the same level of maturity as fixed-wing and rotary-wing designs. However, flapping-wing designs, if fully realized, would boast a manoeuvrability that is superior to both fixed- and rotary-wing designs due to the extremely high wing loadings achieved via unsteady aerodynamics .

Usages

The Black Widow is the current state-of-the-art MAV and is an important benchmark. It is the product of 4 years of research by Aerovironment and DARPA. The Black Widow has a 6-inch wingspan and weighs roughly 56 grams. The plane has a flight range of 1.8 kilometres, a flight endurance time of 30 minutes, and a max altitude of 769 feet. The plane carries a surveillance camera. In addition it utilizes computer controlled systems to ease control.

The Black Widow is made out of form; individual pieces were cut using a hot wire mechanism with a CNC machine allowing for greater accuracy.

The University of Florida has been very successful over the past five years in the MAV competitions. In 2001 they won in both the heavy lift and surveillance categories. Their plane was constructed of a resilient plastic attached to a carbon fibre web structure. This resulted in a crash resistant airfoil.

Working Principle

Newton's first law states a body at rest will remain at rest or a body in motion will continue in straight-line motion unless subjected to an external applied force . That means, if one sees a bend in the flow of air, or if air originally at rest is accelerated into motion, there is force acting on it. Newton's third law states that for every action there is an equal and opposite reaction . As an example, an object sitting on a table exerts a force on the table (its weight) and the table puts an equal and opposite force on the object to hold it up. In order to generate lift a wing must do something to the air. What the wing does to the air is the action while lift is the reaction.

Let's compare two figures used to show streams of air (streamlines) over a wing. The air comes straight at the wing, bends around it, and then leaves straight behind the wing. We have all seen similar pictures, even in flight manuals. But, the air leaves the wing exactly as it appeared ahead of the wing. There is no net action on the air so there can be no lift. Figure 3.7 shows the streamlines, as they should be drawn. The air passes over the wing and is bent down. The bending of the air is the action. The reaction is the lift on the wing.