The N-1M "Jeep"
The N-1M was designed to be able to change configuration on the ground by varying dihedral, sweep, and wing tip "droop". This flexibility allowed an enormous amount of data to be collected during the test program with the construction of a single airplane.
Control of the N-1M used elevons for pitch and roll control. Rudder control was accomplished by the use of drag rudders at each wingtip. By opening both drag rudders at the same time, they acted as air brakes. The B-2 uses the same features today.
During the flight test program, the plane was reported to have excellent handling characteristics, but the Lycoming engines were inadequate. They were unable to generate enough power to raise the plane out of ground-effect. Moye Stephens, the test pilot, said that "In the initial flights with the Lycoming engines the ship would climb to about five feet and the increased induced drag associated with attempts to force it higher would bring it down to a landing. Continuous flight called for maintenance of a precise angle of attack. Any increase in the angle of attack and the ship would land. Any decrease in the angle of attack and the ship would land. The situation was complicated by a "dead area" in elevator effectiveness. In order to nose down it was necessary to move the wheel forward a disturbing amount with no response, and then the elevons would suddenly take over. In order to keep from banging into the ground it was then necessary to traverse the dead elevator area in the opposite direction to find the start of effectiveness. This was moderately unsettling while flying five feet off the ground. I temporarily overcame the difficulty by use of the longitudinal trim flap: a control surface spanning the trailing edge of the center section. With this adjusted to create a nose heavy condition, flight was maintained with a constant back pressure of the wheel. To nose down it was simply necessary to ease off the back pressure".
Dr. Theodore von Kárman was one of the experts consulting with Northrop, and found the solution. The thick wing created flow separation which did not join until aft of the wing. He suggested extending the trailing edge of the elevons into the closure of the airflow.
Replacing the Lycomings with the 117 HP Franklins solved the power problem (although still underpowered), but engine overheating in the totally enclosed wings remained a problem.
After two years, about 200 test flights had been flown in the N-1M, the longest of which was over an hour. The original drooped wing tips were straightened, after it was discovered that they did not contribute to directional stability and instead decreased lift considerably.
In 1945 the N-1M was given to the Army Air Forces for display. It was placed in temporary storage at Freeman Field. In 1946 it was delivered to the Museum Storage Depot at Park Ridge, Illinois. It has been restored by the staff of the Smithsonian National Air & Space Museum.