Perhaps the most startling piece in Roman Signer’s recent show was a seven-foot-high wooden crate that sat in the middle of the room and contained the debris of a helicopter crash. The helicopter was a toy model, however, and the artist staged the crash to occur at the opening of his show. Operated by remote control, the helicopter whizzed around inside the crate until it hit one of the sides, flew out of control, and, with a last high wail of its little motor, destroyed itself within the narrow confines of the box. Viewers could observe the action through a doorway-size opening in the crate,