The years immediately following the end of World War II must have been a heady time to be in aviation. The “sound barrier” had gained its own reputation and the mysteries of flight were being pulled back further and further. Though some of the science could be done in wind tunnels, much of it had to be done in free air, with men at the controls of real airplanes.
The growing Soviet threat made understanding high speed flight and designing aircraft to operate in these regimes of the utmost importance. It was for this reason that many of the X-plane programs overlapped, each trying to extract a performance advantage from some unique aspect of their design. The X-4 Bantam was no exception.
The X-4’s purported advantage came from a semi-tailless design. The “semi” is this case meant no horizontal tail surfaces. The reason for this approach was that it was known that in transonic flight the shockwaves from the wings would disturb the air over the rear horizontal tail surfaces, rendering them either useless, or worse, a source of stability problems.
The (then) Army Air Force contracted Northrop to build the aircraft in 1946. Northrop was selected because they had considerable experience with “flying wing” designs such as the N9M and YB-35. These designs were intended to yield a more efficient aircraft because they dispensed with the drag of the fuselage, incorporating it into a thickened wing. Adapting it for transsonic flight was going to be another challenge.
Pilot’s weren’t enamored with the X-4. Walt Williams, the head of the NACA Muroc Flight Test Unit dubbed the aircraft a lemon, and Scott Crossfield noted that as the X-4’s speed approached Mach 0.88 it began a pitch oscillation which he likened to driving on a washboard. At slightly higher Mach numbers he found the nose would pitch down and the aircraft hunt (oscillate) in all three axes.
The X-4 had a pair of combined flaps/speed brakes on the inner wing edge (see below). These would open symmetrically as speed brakes or asymmetrically for directional control.
The engineers hypothesized they could improve the stability of the design by using these flaps as speed brakes. They installed balsa wood strips between the upper and lower flaps, keeping them open by 5 degrees of more.
Quite why they chose balsa wood is a mystery to me. Why they didn’t choose something more structurally sound I don’t know, but the strips worked. The aircraft did have better handling, but still the oscillation was simply delayed until slightly higher Mach numbers. Supersonic speeds were still out of reach.
The X-4 was retired in the late 50s and semi-tailless designs were ignored for transonic speeds until such aircraft as the Have Blue/F-117 and B-2 (none of which are supersonic). The major difference in these designs was the availability of computer controlled flight systems that artificially stabilized the aircraft, technology that was not available in the X-4’s lifetime. These aircraft also dispensed with horizontal tail surfaces for different reasons than the X-4’s original intent of improving performance in the transonic and supersonic regimes.
It could be said that the 1950’s F7U, which saw service with the US Navy, was the exception to the rule for semi-tailless designs. Indeed, the F7U was the first aircraft to demonstrate weapon release above Mach 1, but the less said about that murderous machine the better.
The X-4’s contribution to flight was largely to demonstrate a negative. I’m sure its designers had high hopes for their hypnosis that semi-tailless aircraft would be more stable above Mach 1, but in the end they gave weight to the need for not just rear surfaces, but correctly designed tail surfaces. Anyone can say “that’s obvious,” but in the 40s it wasn’t, it took an aircraft, the X-4 Bantam, even if it was a lemon.
(Images courtesy of NASA. Further information at NASA’s X-4 page.)