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The X-4: The Worst X-Plane Ever?

The X-4 was retired after 81 flights. While the jet performed poorly, there was value gained from the program. It was a solid demonstration of what not to do. A swept-wing, semi-tailless design did not work for Mach-1 speed. 

X-4 Bantam
Image: Creative Commons.

X-4: A Short History – Experimental aircraft are commonly understood to go faster and fly higher than other planes. It is a sound understanding. Chuck Yeager piloted the X-1 right past the sound barrier, and the X-2 flew even faster than the X-1. Today, the X-15 still holds the records for speed and altitude, even though it last flew 60 years ago. 

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Speed and altitude aside, though, sometimes experimental aircraft are developed with humbler objectives in mind – say, to test out a new flight service, or to gain a deeper understanding of some aerodynamic property. The Northrop X-4 Bantam was just such an experimental aircraft. It was never meant to break any records. Instead, the X-4 was developed to test the performance of a plane without horizontal stabilizers. 

A Second Try at an Odd Design

Built in 1948, the X-4 was a prototype twinjet aircraft. The thing was tiny, measuring 23 feet long and 14 feet high. That’s smaller than a Cessna 152, which I can personally attest is small and cramped. To perform maintenance work on the aircraft, workers didn’t even need a ladder or a stool. The plane was so short that a person could look down into the cockpit while standing on the ground next to it. The X-4 was so small, in fact, that it could only carry 45 minutes’ worth of fuel at any time. 

The most distinctive feature of the X-4 was, as you would expect, its lack of horizontal stabilizers. The X-4 relied instead on elevators and aileron control surfaces called elevons that handled pitch and roll adjustments. The X-4’s novel design came about because some aerodynamicists hypothesized that removing a jet’s horizontal stabilizers would decrease the likelihood of a shock stall. (This occurs when supersonic shock waves from the wings and horizontal stabilizers interact in a way that causes a jet to stall.) 

While distinct, the design was not new. The Nazi-built Messerschmitt Me163 Komet also eschewed a horizontal stabilizer. The Me163, a Luftwaffe interceptor used during World War II, remains to this day the only operational rocket-powered fighter jet ever used in combat.

Introduced in 1944, the Komet reached unmatched speeds of 700 miles per hour. While fast, the Me163 did not perform well as an interceptor, and it was dangerous to operate – but its design lived on in the X-4. Since the U.S. gathered up Nazi scientists after the war, during Operation Paperclip, it seems possible that designers of the Me163 might have played a role in the design of the X-4. 

Me 163

Me 163. Image take on October 1, 2022. 19FortyFive.com original image.

Me 163 Komet

A German Messerschmitt Me 163B Komet rocket-propelled fighter (s/n 191095) at the National Museum of the United States Air Force, Dayton, Ohio (USA).

Negative Demonstrator

The X-4 program was short-lived: Only two were ever built, and they were flown between 1950 and 1953 at what today is Edwards Air Force Base. The first X-4 constructed was deemed mechanically unfit for service and was dismantled for parts. The second X-4 was more properly built, allowing NACA (now NASA) to test the horizontal stabilizer-less design. 

X-4 pilots included Scott Crossfield and Yeager. Many of the X-4 pilots noted that the aircraft had a strange pitch in flight. Specifically, as the X-4 accelerated, the jet began to porpoise, or pitch up and down uncontrollably.

The higher the speed, the more intense the porpoising. Also of concern to the pilots was how the X-4’s nose would tuck, or dip downward. But most concerning of all, the X-4 had a tendency at high speed to move around on all three axes. This behavior often leads to inertial coupling – uncontrolled, simultaneous spinning on multiple axes. 

The X-4 was retired after 81 flights. While the jet performed poorly, there was value gained from the program. It was a solid demonstration of what not to do. A swept-wing, semi-tailless design did not work for Mach-1 speed. 

Harrison Kass is the Senior Editor at 19FortyFive. An attorney, pilot, guitarist, and minor pro hockey player, he joined the US Air Force as a Pilot Trainee but was medically discharged. Harrison holds a BA from Lake Forest College, a JD from the University of Oregon, and an MA from New York University. He lives in Oregon and listens to Dokken.

Written By

Harrison Kass is a Senior Defense Editor at 19FortyFive. An attorney, pilot, guitarist, and minor pro hockey player, he joined the US Air Force as a Pilot Trainee but was medically discharged. Harrison has degrees from Lake Forest College, the University of Oregon School of Law, and New York University’s Graduate School of Arts & Sciences. He lives in Oregon and regularly listens to Dokken.

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