On September 20, 1994, at 1000 central daylight time, a Challenger, unregistered two seat ultralight, was substantially damaged when it impacted the ground in an uncontrolled descent in Princeton, Illinois. The student pilot and passenger reported no injuries. The flight originated in Erie, Illinois, at 0915 with an intended destination of Argos, Indiana. No flight plan was filed and visual meteorological conditions prevailed at the time of the accident. The student pilot failed to submit a NTSB Form 6120.1/2. Numerous attempts were made to reach the pilot both by telephone and letter to no avail. The student pilot did submit a brief written statement several days after the accident. In that statement the student pilot, who was seated in the front seat, reported that at approximately 1,100 feet above the ground, the airplane jolted and began to shudder. He stated the airplane did not require any type of correction initially, but it then began to bank to the left. The bank became progressively steeper and the turn progressively tighter. In an attempt to maintain control, the pilot stated he applied right rudder and right aileron but the airplane did not respond. The airplane then entered into a dive to the left. After retarding the throttle, the passenger, a private pilot who was seated in the back seat, took the controls of the airplane, but was also unable to regain control. The airplane impacted a bean field where it came to rest. During a postaccident examination of the airplane, it was observed that the nose wheel was completely separated at the strut. The actual nose wheel was not at the accident scene. The pilot had taken the wheel from the scene after the accident. Also observed was a break in the connecting rod which links the rudder pedals to the rudder control bar. The pilot had also taken a piece of this connecting rod from the accident scene. According to the builder of this Challenger, he had taken the student pilot for a flight the day before the accident. This flight was the student pilot's first experience flying a Challenger and was to be a practice flight in preparation for his flight to Indiana. The optional doors for the Challenger were not installed at the time of this flight. The builder stated that he did not have the opportunity to take the pilot for another practice flight with the doors installed prior to the pilot's departure. The doors were installed at the time of the accident. The Chief Engineer with Popular Flying Association in the United Kingdom, in a telephone interview stated that the Challenger is directionally unstable when the doors are installed. As a result, several modifications to the Challenger are mandatory before their operation is permitted in the United Kingdom. The modification consist of the installation of a larger tail fin, rudder, and reinforcements. This investigator has spoken to the President of the Challenger manufacturer, Quad City Ultralight Aircraft Corporation, on several occasions concerning the flying characteristics of the Challenger when the doors are installed. In a written statement, the president stated the Challenger II was designed in 1984 as a two place ultralight trainer to be used by Challenger dealers to train their customers to fly the single seat Challenger ultralight. The president stated "We never intended [the Challenger] to have doors." The doors were designed by a Challenger dealer in Canada. A set of these doors were purchased by Quad City Ultralight Aircraft Corporation and installed. "We liked the doors for winter flying and noticed no stability problems other than they made the plane more rudder dominated; i.e., you had to steer more with your feet than previously," the president stated. Soon because of the large requests, Quad City Ultralight Aircraft Corporation started offering the doors in kit form to their customers and dealers. When questioned about the directional stability of the Challenger when the doors are installed, the president stated he believes the problem lies in the inexperience of the pilots who are flying the Challenger. He stated the Challenger is a rudder dominant and sensitive airplane. With the doors installed, a pilot can not take his feet off the rudder pedals. The president stated that inexperienced pilots find it difficult to adapt to this kind of flying.