Another group of researchers proposed to use the cross-flow fan for vertical lift operation. Gossett [18] proposed to use the cross-flow fan in vertical take-off and landing (VTOL) aircraft applications. He argued that since the vertical thrust requirement is much larger than the forward flight thrust requirement, and since the aircraft aerodynamic design should be optimized for forward flight, the cross-flow fan is a good choice to be used as a vertical thrust augmentation device as it can be easily mounted within the tube fuselage. The cross-flow fan can then be turned off during the forward flight phase, and the fuselage-embedded cross-flow fan introduces no drag penalty. Fig. 55 shows a possible layout of a combined ducted fan and cross-flow fan single-seat VTOL personal air vehicle (PAV). During forward flight mode, the thrust would come from the two ducted fans while lift is entirely generated by canard and horizontal tail surfaces. In the vertical flight mode, the two ducted fans would rotate and become ducted lift-fans deflecting their exhaust in the vertical direction, while the cross-flow fan would be turned on to generate additional vertical lift force. Note that the concept has some similarity to the VTOL commuter airplane developed by Moller (see www.moller.com) in that it also employs ducted fans for both forward and vertical thrust. However, in the concept proposed by Gossett, the forward ducted fans in Moller's design are removed and replaced by the cross-flow fan and a canard wing. Again, the argument being that the combined ducted fan and cross-flow fan configuration would perform more efficiently in the forward flight mode. A similar concept was also proposed by Sharpe [72] and is shown in Fig. 56. In this case, the cross-flow fans are mounted on the wing to provide vertical take-off capability, while a propeller mounted in the rear is employed for thrust.