Daniel,
As you say, one of Fred Weick's design goals was to build a plane that was well behaved, that behaved the same at all times and all power settings. I don't think it was just "noticed" that the twin tail improved the yaw stability, I think he changed to the twin tail specifically to improve yaw stability at all power settings. Similarly, he set the engine canted to the right and down for the same reason. If I recall right, those adjustments were made to the "jeep," the prototype. As far as I know, the prototype was always an experimental. Perhaps the plane to which Tommy refers is Fred's own Ercoupe, the one he kept after he left ERCO and became a professor at Texas A&M University. Fred has described some of his propeller and stability experiments. I can't remember when this one happened - perhaps it was before Fred left ERCO. In one experiment, Fred had a wooden device that he'd prop between the yoke and the instrument panel. It allowed the yoke to turn but prevented fore and aft movement. With this in place, Fred could take off, fly a cross country trip and land without ever moving the yoke fore and aft so the elevator was fixed. Each different stick's length set a particular airspeed. I don't know if Fred put his plane into Experimental category for this simple and instantly removable test, though. It was a joy to listen to Fred talk at our fly-ins. If anyone has any audio or video tapes of him, I'd very much like to get copies and put them up on the web. Ed
