View source for Northrop X-4 Bantam

The '''Northrop X-4 Bantam''' is a prototype small [[twinjet]] [[aircraft]] manufactured by [[Northrop Corporation]] in 1948. It had no horizontal tail surfaces, depending instead on combined [[elevator (aircraft)|elevator]] and [[aileron]] control surfaces (called [[elevon]]s) for control in pitch and roll attitudes, almost exactly in the manner of the similar-format, rocket-powered [[Messerschmitt Me 163]] of Nazi Germany's Luftwaffe. Some aerodynamicists had proposed that eliminating the horizontal tail would also do away with stability problems at fast speeds (called [[shock stall]]) resulting from the interaction of [[supersonic]] [[shock wave]]s from the wings and the horizontal stabilizers. The idea had merit, but the flight control systems of that time prevented the X-4 from achieving any success.

==Development==
Two X-4s were built by the Northrop Corporation, but the first was found to be mechanically unsound and after ten flights it was grounded and used to provide parts for the second.<ref name="Air & Space 2014">Wilkinson, Stephan. "Northrop X-4". ''Air & Space Smithsonian.'' Vol.29 No.2. June/July 2014.</ref> 
While being tested from [[1950 in aviation|1950]] to [[1953 in aviation|1953]] at the [[NACA]] High-Speed Flight Research Station (now [[Edwards Air Force Base]]), the X-4's semi-tailless configuration exhibited inherent longitudinal stability problems ([[porpoising]]) as it approached the speed of sound. It was concluded that (with the control technology available at the time) tailless craft were not suited for transonic flight.

It was believed in the 1940s that a design without horizontal stabilizers would avoid the interaction of shock waves between the wing and stabilizers. These were believed to be the source of the stability problems at transonic speeds up to [[Mach (speed)|Mach 0.9]]. Two aircraft had already been built using a semi-tailless design—the [[rocket]]-powered [[Messerschmitt Me 163|Me 163B ''Komet'']] flown in combat by [[Nazi Germany]]'s [[Luftwaffe]] in [[World War II]], and the turbojet-powered [[Great Britain|British]] [[De Havilland Swallow|de Havilland DH.108 Swallow]] built after the war. The [[United States Army Air Forces]] signed a contract with the Northrop Aircraft Company on 11 June 1946, to build two X-4s. Northrop was selected because of its experience with flying wing designs, such as the [[Northrop N-9M|N-9M]], [[Northrop YB-35|XB-35]] and [[YB-49]] aircraft.

The resulting aircraft was very compact, only large enough to hold two [[Westinghouse J30]] jet engines, a pilot, instrumentation, and a 45-minute fuel supply. Nearly all maintenance work on the aircraft could be done without using a ladder or footstool. A person standing on the ground could easily look into the cockpit. The aircraft also had split flaps, which doubled as speed brakes.

==Operational history==
[[File:Northrop X-4 onground colour.jpg|thumb|Preparing for flight]]

The first X-4 (serial number 46-676) was delivered to [[Muroc Air Force Base]], [[California]], in November [[1948 in aviation|1948]]. It underwent taxi tests and made its first flight on December 15, 1948, with Northrop [[test pilot]] [[Charles Tucker (Northrop Test Pilot)|Charles Tucker]] at the controls. Winter rains flooded [[Rogers Dry Lake]] soon after, preventing additional X-4 flights until April [[1949 in aviation|1949]]. The first X-4 proved mechanically unreliable, and made only ten flights. [[Walter C. Williams]], the head of the NACA Muroc Flight Test Unit (now [[Dryden Flight Research Center]]) called the aircraft a "lemon".<ref name="Air & Space 2014"/> The second X-4 (serial number 46-677) was delivered during the halt of flights, and soon proved far more reliable. It made a total of 20 contractor flights. Despite this, the contractor flight program dragged on until February 1950, before both aircraft were turned over to the [[United States Air Force|Air Force]] and the NACA. The first X-4 never flew again, used as spare parts for the second aircraft.

The NACA instrumented the second X-4 to conduct a [[List of X-4 flights|short series of flights]] with Air Force pilots. These included [[Chuck Yeager]], [[Frank Kendall Everest, Jr.]], [[Al Boyd]], [[Richard Johnson (pilot)|Richard Johnson]], [[Fred Ascani]], [[Arthur Murray (pilot)|Arthur Murray]] and [[Jack Ridley (pilot)|Jack Ridley]]. The flights were made in August and September 1950. The first flight by an NACA pilot was made by [[John H. Griffith]] on September 28, 1950.

The initial NACA X-4 flights, which continued from late 1950 through May of [[1951 in aviation|1951]], focused on the aircraft's sensitivity to pitch. NACA pilots Griffith and [[Albert Scott Crossfield|Scott Crossfield]] noted that as the X-4's speed approached Mach&nbsp;0.88, it began a pitch oscillation of increasing severity, which was likened to driving on a [[washboarding|washboard road]]. Increasing speeds also caused a tucking phenomenon, in which the nose pitched down, a phenomenon also experienced by the Me 163A ''Anton'' prototypes in 1941. More seriously, the aircraft also showed a tendency to "hunt" about all three axes. This combined yaw, pitch and roll, which grew more severe as the speed increased, was a precursor to the [[Inertia coupling|inertial coupling]] which would become a major challenge in the years to come.

To correct the poor stability, project engineers decided to increase the flap/speed brake trailing edge thickness. [[Balsa wood]] strips were added between the upper and lower hinged "clamshell"-style flap/speed brake halves, causing them to remain open at a 5° angle. The first test of the blunt trailing edge was flown on 20 August 1951 by NACA pilot [[Walter Jones (pilot)|Walter Jones]]. A second test was made by Crossfield in October. The results were positive, with Jones commenting that the X-4's flight qualities had been greatly improved, and the aircraft did not have pitch control problems up to a speed of Mach&nbsp;0.92.

The balsa strips were removed, and the X-4 then undertook a long series of flights to test landing characteristics. By opening the speed brakes, the [[lift-to-drag ratio]] of the aircraft could be reduced to less than 3:1. This was for data on future rocket-powered aircraft. The tests continued through October 1951, until wing tank fuel leaks forced the aircraft to be grounded until March [[1952 in aviation|1952]], when the landing tests resumed. NACA pilots [[Joseph A. Walker|Joe Walker]], [[Stanley Butchard]], and [[George Cooper (pilot)|George Cooper]] were also checked out in the aircraft.

The thickened flap/speed brake tests had been encouraging, so balsa wood strips were reinstalled on both the flap/speed brake and the elevons. The first flight was made by Jones on 19 May 1952, but one of the engines was damaged during the flight, and it was August before a replacement J30 could be found. When the flights resumed, they showed that the modifications had improved stability in both pitch and yaw, and delayed the nosedown trim changes from Mach 0.74 to Mach&nbsp;0.91. Above Mach&nbsp;0.91, however, the X-4 still oscillated.

In May 1953, the balsa wood strips were again removed, and the X-4's dynamic stability was studied in the original flap/speed brake and elevon configuration. These flights were made by Crossfield and [[John B. McKay]]. This was the final project for the X-4, which made its 81st and final NACA flight on September 29, 1953. Both aircraft survived the test program. The first X-4, AF serial number 46-676, was transferred to the [[United States Air Force Academy]], [[Colorado Springs, Colorado]], before being returned to Edwards Air Force Base.<ref name="Air & Space 2014"/> 46-676 has been restored as of August 2012, and is currently being held in storage pending placement in the Edwards Museum. The second X-4 went to the [[National Museum of the United States Air Force]] at [[Wright-Patterson Air Force Base]] near [[Dayton, Ohio]], where it remains on display.<ref name="Air & Space 2014"/>

The X-4's primary importance involved proving a negative, in that a swept-wing semi-tailless design was not suitable for speeds near Mach 1, although [[Vought]]'s [[F7U Cutlass]] proved to be something of a counterexample—the developed version was the first aircraft to demonstrate stores separation above Mach&nbsp;1. Aircraft designers were thus able to avoid this dead end. It was not until the development of computer fly-by-wire systems that such designs could be practical. Semi-tailless designs appeared on the [[McDonnell Douglas X-36|X-36]], [[Have Blue]], [[F-117 Nighthawk|F-117]], and [[Boeing Bird of Prey|Bird of Prey]], although these aircraft all differed significantly in shape from the X-4. The trend during its test program was already towards delta and modified delta aircraft such as the [[Douglas F4D]], the [[F-102A|Convair F-102A]] derived from the [[XF-92A]], and the [[Avro Vulcan]].

==Aircraft on display==
*X-4 serial number 46-676, is on display at the [[Air Force Flight Test Museum|Flight Test Museum]] on [[Edwards Air Force Base]], [[Edwards, California]].
*X-4 serial number 46-677, is on display at the [[National Museum of the United States Air Force]] in [[Dayton, Ohio]].

==Specifications (X-4)==
[[File:Northrop X-4 3-view.png|right|300px]]
{{Aircraft specs
|ref=''X-4 – The Bantam Explorer'' <ref name="ae3 p23">Hallion 1977, p. 23.</ref>
|prime units?=imp
<!--
        General characteristics
-->
|crew=1
|length ft=23
|length in=3
|length note=
|span ft=26
|span in=10
|span note=
|height ft=14
|height in=10
|height note=
|wing area sqft=200
|wing area note=
|aspect ratio=<!-- sailplanes -->
|airfoil=[[NACA airfoil|NACA 0010-64]]
|empty weight lb=5507
|empty weight note=
|gross weight lb=
|gross weight note=
|max takeoff weight lb=7820
|max takeoff weight note=
|fuel capacity=
|more general=
<!--
        Powerplant
-->
|eng1 number=2
|eng1 name=[[Westinghouse J30-WE-7]] / WE-9
|eng1 type=[[turbojet]] engines
|eng1 lbf=1600
|eng1 note=
<!--
        Performance
-->
|max speed mph=625
|max speed note=
|max speed mach=<!-- supersonic aircraft -->
|cruise speed mph=
|cruise speed note=
|stall speed mph=
|stall speed note=
|never exceed speed mph=
|never exceed speed note=
|minimum control speed mph=
|minimum control speed note=
|range miles=420
|range note=
|combat range miles=
|combat range note=
|ferry range miles=
|ferry range note=
|endurance=<!-- if range unknown -->
|ceiling ft=42300
|ceiling note=
|g limits=<!-- aerobatic -->
|roll rate=<!-- aerobatic -->
|glide ratio=<!-- sailplanes -->
|climb rate ftmin=7700
|climb rate note=
|time to altitude=
|wing loading lb/sqft=
|wing loading note=
|fuel consumption lb/mi=
|power/mass=
|thrust/weight=

|more performance=

|avionics=
}}

==See also==
{{Aircontent|
|related=<!-- related developments -->
|similar aircraft=
*[[de Havilland DH.108 Swallow|DH.108 Swallow]]
*[[Messerschmitt Me 163]]
*[[Lippisch P.15]]
|lists=
*[[List of X-4 flights]]
*[[List of X-planes]]
|see also=}}

[[Category:Northrop aircraft|X-4 Bantam]]
[[Category:Edwards Air Force Base]]
[[Category:1940s United States experimental aircraft]]
[[Category:Tailless aircraft]]
[[Category:Mid-wing aircraft]]
[[Category:Twinjets]]
[[Category:Aircraft first flown in 1948]]