1. The idea of personal flying machines like flying cars seems to be as old as the aviation itself, but yet continues to attract attentions from general public and media. People hope that eventually flight would be as common a form of transportation as the automobile. Thereare about 70 patents granted for roadable aircraft or flying cars, and some of the designs are illustrated in the online magazine of flying cars and roadable aircraft "Roadable Times" (www.roadabletimes.com). From a more technical viewpoint, researchers at NASA Langley Research Center have surveyed the status and commented the feasibility and problems of personal aircraft (Anders et al. 1994). The various designs can be generally categorized into three groups: propeller-driven winged cars, vectored-thrust vehicles, and personal helicopters. Propeller-driven winged car is a combination of a modified car body with a foldable or convertible wing. In the aircraft mode, propeller-driven winged car takes off and lands like general aviation aircraft and it inevitably needs a runway or an airfield. In a strict meaning, this type of air vehicles is not truly personal aircraft that people envision since it does not the V/STOL capability. Vectored-thrust vehicles use ducted fan system to generate the propulsive lift for VTOL and then transform into forward flight. The canonical examples of this type of vehicles are the British Harrier and the US variant, the AV-8. An attempt of producing the personal vectored-thrust vehicle "Skycar" has been made by Moller International, Inc. (www.moller.com) for years, and a prototype with eight 150 hp rotary engines powering four ducted fans has been built and tested. The Skycar can takeoff vertically and hover about fifteen feet above the ground for minutes, but the transition to forward flight has not yet been demonstrated. In fact, the key design issue in the development of this type of aircraft is the vehicle control particularly in the transition from vertical takeoff to forward flight and from forward flight to vertical landing. Theoretically speaking, with sufficiently powerful engines and ducted fans and sophisticated flight control by powerful computers, it could fly. The aerodynamic configuration of the Skycar indicates that it mainly uses the propulsive lift for flight without significant utilization of the aerodynamic lift from wings. As a result, it is not efficient in terms of fuel consumption. Compared with helicopter, the ducted fans for this type of aircraft require greater power and higher fuel consumption for hover. Although it has been used successfully in military service, vectored-thrust vehicles are difficult to enter civilian service due to high cost and the complexity of the system and operation. Noise is also a serious concern. Therefore, the development of feasible personal vectored-thrust vehicles is still at the very early stage. Technically, personal helicopter is an attractive vehicle with the VTOL capability since the helicopter technologyis considerably mature. However, similar to vectored-thrust vehicles, requirement of special piloting skill and high cost prevent general public from widely using personal helicopters so far. In addition, many people have the perception that a helicopter is not as safe as a fixed-wing aircraft particularly when its engine fails.
2. Certainly, balloons, as the simplest and oldest flight vehicles, can float in the Martian atmosphere. Longduration-flight autonomous balloons with trajectory control capabilities for Mars exploration have been proposed (Pankine 2002). A small wing attached on a balloon was suggested to provide a modest capability of path control in atmospheric winds. Balloon flight is a proven concept in the sense that sufficient buoyancy lift can be generated in the Martian atmosphere. The major shortcoming of balloons as flight vehicles is a lack of the full authority of flight controland maneuver. 1.3.High-altitude flight