Aerobatic model construction is typically quite complex and may require many hundreds of hours. Speed models must be very sturdy to withstand the forces of line tension and to permit a very rigid engine mount for maximum engine performance. Speed models are generally built around an aluminum or magnesium "pan" that forms about half the fuselage. Little or no maneuverability required, as once at speed the model's altitude is maintained by centripetal Acceleration. Racing models need to be both relatively light for good acceleration from the start, or after a pit stop, and to reduce the pitch of the airfoil required to maintain lift. Race Aircraft also be fairly strong to withstand the pit man catching the model after landing.