Cruciform wing

Lore & Background

Cruciform wing missiles, often called Cruciform wing weapons (CWW), use four identical thin, low aspect ratio wings equally spaced around a long, slender body. For wings of equal size and shape, this gives constant aerodynamic characteristics whatever the aircraft's angle of roll or direction of turn. However, because only half the total lift of the four surfaces is available in any given attitude, the configuration is less efficient than a conventional planar wing. The missile may also have small cruciform canard foreplane surfaces set at 45° to the main wing to minimise interference. The aerodynamic properties of such a slender wing-plus-body configuration are different from those of the individual elements and need to be evaluated as a unified form; off-axis side forces are relatively independent of the angle of pitch or yaw.

The horizontal cruciform rotor wing, also known as the X-wing, is a form of the Stopped rotor. Teledyne Ryan studied the concept in the 1970s and took out patents. The X-Wing circulation control rotor was developed in the mid-1970s under DARPA funding, first developed by the David W. Taylor Naval Ship Research and Development Center, with an experimental rotor built by Lockheed for testing on the Sikorsky S-72 RSRA. Intended to take off vertically like a helicopter, the rigid rotor could be stopped in mid-flight to act as an X-shape cruciform wing providing lift during forward flight. Instead of controlling lift by altering blade angle of attack, the craft used compressed air fed from the engines and expelled from its blades to generate a virtual wing surface. Computerized valves ensured the compressed air came from the correct edge of the rotor as it rotated. In late 1983 Sikorsky received a contract to modify the S-72 RSRA as a demonstration testbed, rolled out in 1986, but the program was cancelled two years later after the X-wing had been installed but before it had flown.

Other proposed applications include a solar-powered aircraft studied by NASA around 1980, using a very-high-aspect-ratio cruciform fixed wing with solar panels along one plane, allowing the craft to roll at any angle to follow the sun. Variable-geometry studies by Matra shortly after World War Two, patented by Robert Roger in 1946, provided two sets of wings—one for low-speed takeoff and landing, the other for high-speed flight—with the fuselage section rotatable 90° to swap them. The bi-directional wing is a similar approach, comprising a long-span low speed wing and a short-span high speed wing joined in an unequal cross, studied in the form of a bi-directional flying wing.

Reader's Guide

The cruciform wing configuration has significance in both missile and rotorcraft applications, as described in the source article. For missiles, the cruciform arrangement provides constant aerodynamic characteristics regardless of roll angle, a key advantage for maneuvering weapons, though it is less efficient than a planar wing because only half the total lift is available in any given attitude. The article notes that the aerodynamic properties of the slender wing-plus-body combination must be evaluated as a unified form, with off-axis side forces relatively independent of pitch or yaw. The cruciform rotor wing, or X-wing, represents an attempt to combine vertical takeoff and landing capability with high-speed forward flight by stopping the rotor in mid-flight to act as a fixed wing. This concept was developed under DARPA funding in the mid-1970s, with an experimental rotor built by Lockheed and tested on the Sikorsky S-72 RSRA. The program was cancelled in the late 1980s before the X-wing flew. The article also describes other proposed applications: a solar-powered UAV studied by NASA around 1980 that could roll to follow the sun, and variable-geometry designs from Matra and the bi-directional wing concept, which aimed to address the conflicting requirements of low-speed and high-speed flight by rotating the wing set. These examples illustrate the versatility of the cruciform layout, though the article does not indicate that any of these concepts beyond missiles achieved operational status.

Did You Know?

• Cruciform wing missiles are sometimes called Cruciform wing weapons (CWW) in contrast to planar wing weapons (PWW).
• The X-wing rotor used compressed air expelled from its blades to generate lift, rather than changing blade angle of attack.
• The X-wing program was cancelled two years after the Sikorsky S-72 RSRA testbed was rolled out in 1986, before it had flown.
• A variable-geometry cruciform wing design was patented by Matra's chief designer Robert Roger in 1946.