A tailless transonic experiment: Flight-testing the Northrop X-4 Bantam

Flight Journal, Feb 2002 by Tucker, Charles

In the fall of 1947, I received a phone call that was one of those calls every test pilot dreams about: Northrop was looking for a pilot for its new X-4 Bantam, and was I interested? It isn't often that a test pilot gets to make the first flight of a new design, especially one as radical as the X-4, so I couldn't believe my good fortune when Northrop gave me the nod.

At the time, I had only been flying for five years, but it had been a busy five years. I graduated from the Army Air Corps flying school in January 1942 as a "pursuit" pilot. With its typical logic, the Army had me flying a C-47 from West Palm Beach, Florida, to China via South America, Africa, India and over the "Hump" to China. I made seven cargo flights over the Hump in the C-47 before transferring to Gen. Chennault's 14th Air Force in China in August 1942. I flew 75 combat missions in P-40s and scored four victories before ground fire hit my cooling system during a strafing run in July 1943, and I was forced to bail out. After a week of trekking, I made it home and then became ill from the bad food and water I had while evading the Japanese, and I was evacuated to the U.S. for treatment in October 1943.

After recovering, I was assigned to the 412th Fighter Group. This unit was equipped with the Bell P-59 Airacomet, P-38s, P-63s, P-51s, the Lockheed XP-80 and various other aircraft. Later, the Air Corps ran accelerated service tests on the P-80 at Muroc Army Airfield. Wright Field pilots were flying these airplanes, and I was invited to join this elite group in those tests.

After the War, I went to work for Lockheed as a civilian production test pilot on P-80s, and that lasted until September 1947. I wound up with quite a number of hours on that aircraft. It was then that Northrop hired me to fly the X-4, as I had quite a bit of jet experience and was rather small of stature; my size was important because the X-4 cockpit was small. The X-4 hadn't yet been completed, so they started me flying the XB-35 and YB-49 Flying Wing. By the time the X-4 was ready to fly, I had already completed the stall program on the YB-49 and had spun the airplane.

The X-4 was designed to explore the flight characteristics of tailless aircraft in the transonic speed range. After WW II, all airframe manufacturers were aware of Germany's efforts with jet propulsion and flying-wing aircraft. As Northrop was a flying-wing specialist, the government selected it to build an airplane with no horizontal tail. As far as I know, no German designs influenced Northrop's design. After the initial problems had been sorted out, the aircraft performed especially well, and was fun to fly; it gave Northrop, NACA and the Air Force much-needed data about tailless-aircraft performance in high-speed flight.

The aircraft (two were being built) were not finished when I arrived at Hawthorne. The first time I saw them, the beauty of the little birds stunned me; I could hardly wait to get my hands on one. I watched their assembly and closely studied the control systems as they were installed. Northrop used the hydraulic actuator system from the YB-49 bomber for the elevons of the X-4. Using the control system from a heavy bomber in an airplane as small as the X-4 resulted in high stick friction and an excessive breakout force of approximately 12 pounds. These problems were primarily caused by the 3/16-inch-size bomber control cables and the 400 pounds of high cable tension in conjunction with the small, control-cable pulleys and the resultant acute-cable radii required in the small airframe of the X-4. These problems would later cause me much grief, as the friction in the system made it impossible to make small control inputs, thus making the airplane almost uncontrollable.

The rudder was electrically controlled. The rudder's engineering team had been thinking about high-speed flight, and they designed the actuating motor with a very slow response rate of approximately 25 degrees per second. While this is fine for high-speed flight, it is not at all desirable for low-speed flight such as during takeoff and landing. Once again, engineers were thinking like engineers, and it worried me; I wished more engineers were pilots.

The first X-4, No. 6676, was transported to Muroc in November 1948. The slow rudder response made itself felt immediately by making the airplane very difficult to control in the yaw axis. On the ground, this translated into an inability to track straight on the runway. I argued repeatedly that this rudder system was inadequate, but the engineering team thought I was just being "sour grapes" and should be able to handle anything they could build; after all, I was a test pilot, wasn't I? As the engineers refused to alter the rudder control system, I had a line painted on the lakebed for use as a yaw reference indicator. I felt this "centerline" would be beneficial in tracking the yaw movements because there was little else for visual reference on the lakebed.

I continued to make taxi runs on the lakebed, and over time I increased the speed in small increments. The slow rudder response was almost intolerable. The aircraft would yaw right, so I would apply left rudder and wait. As soon as the aircraft began to go left, I would apply right rudder and wait. This induced an oscillation that was difficult to damp. Still, the engineers thought it wasn't a serious problem. Quite a number of those guys were young and relatively inexperienced; some of them had their own agendas, and none were pilots. I continued making taxi tests until I had learned enough about controlling the slow response time that the engineers thought I should try a takeoff. I was certain that the aircraft wasn't ready for that phase of the program, but the X-4 project was way behind schedule, and the company was losing money on it daily. I was furious about the rudder, but I also wanted to get this first flight off and running. As a conscientious employee, with my employer's well-being foremost in my mind, I consented to try a takeoff.