The Grumman F-14 Tomcat stands out for its variable sweep wings, also known as swing wings, which can alter their sweep angle to suit different flight speeds. This allows the aircraft to fly slowly or quickly as needed. Despite their initial popularity in the 1960s, swing wings are not utilized in commercial aircraft due to the trade-off between aerodynamic benefits and the added weight, which reduces payload capacity.
There have been past efforts to integrate swing wings into commercial designs, like the Boeing 2707 SST project, which ultimately did not meet its design goals due to the weight penalties. In commercial aircraft design, it’s more beneficial to apply aerodynamic principles from the start rather than incorporating a swing wing mechanism.
Swing wings provide military jets with versatility, enabling them to perform both high-speed and low-speed maneuvers, which is essential for various combat scenarios. However, the complexity and heft of the swing wing system are impractical for commercial use, where fuel efficiency and payload are paramount. Maintenance for such intricate systems also adds to the operational costs.
While the Boeing 2707 SST aimed to feature swing wings for supersonic travel, the design couldn’t meet the necessary payload and range due to the weight of the swing wings, leading to the project’s cancellation.
The Need for Speed and Efficiency
Aircraft are designed with various flight conditions in mind. For instance, a straight wing is perfect for low-speed flight due to its efficiency. However, when an aircraft needs to go faster, especially reaching transonic or supersonic speeds, a swept wing is necessary. Fixed-wing designs, like the swept wing or delta wing, are great for high-speed flight but come with trade-offs, such as higher stalling speeds and increased fuel consumption during subsonic cruise. These issues are particularly challenging for carrier-based aircraft that operate from naval carriers and require shorter runways.
Enter the variable sweep wing, which allows pilots to adjust the wing’s sweep angle according to the speed of the aircraft. This adaptability means the aircraft can maintain optimal performance whether it’s flying slow or fast. The benefits of variable sweep wings help balance out the weight and volume penalties that come with mechanical sweep mechanisms.
The F-14 Tomcat
The variable sweep wing on the F-14 was designed to optimize the aircraft’s high-speed performance while also enabling it to slow down sufficiently for safe aircraft carrier landings. The F-14, a large and heavy aircraft, essentially becomes a delta-wing aircraft with its wings fully swept back, minimizing drag at high speeds. However, this configuration also diminishes lift and control at lower speeds. By sweeping the wings forward, lift is increased, and the wing’s slats and control surfaces are exposed, significantly lowering the aircraft’s stall speed.
An interesting piece of history is that the U.S. Navy initially considered the larger General Dynamics F-111B, which also featured a variable sweep wing, to carry the new AIM-54 Phoenix missile and the powerful AN/AWG-9 radar. However, when the F-111B did not meet the Navy’s carrier operation standards, the project was discontinued.
Grumman, which had been collaborating with General Dynamics on the F-111B, went on to develop the F-14, which was slightly smaller but far more agile. The F-14 adopted the same TF30 engines and the same long-range missile and radar systems as the F-111, which expedited its development. However, the replacement for the TF-30 engines, which were intended to address performance issues, was not integrated until ten years after the F-14 had been in service.
Overcoming the Challenges of Variable Geometry
Adjusting the sweep of a wing in flight is not without its challenges. As the wings sweep, the center of lift shifts, which could affect the aircraft’s balance. To counter this, aircraft with swing wings need additional mechanisms, like a sliding wing root or a larger tail stabilizer, to maintain level flight. These mechanisms add weight and complexity, which can eat into performance gains and increase costs and maintenance needs.
Innovative designs have tried to minimize these issues by moving the wing pivots outboard and only sweeping part of the wing. This approach reduces the changes in trim but also limits the variation in span and operational flexibility.
Pioneering the Swing Wing Design
The concept of a wing-controlled aerodyne was developed by British engineer Barnes Wallis, who envisioned a simple fuselage with a variable wing that required no additional control surfaces. The wings themselves could control the direction of flight through subtle movements, while the sweep angle adjusted to maintain balance.
For supersonic flight, a delta-planform lifting body proved more suitable. Wallis tackled the conflict between the necessary wing sweep angle for trim and the optimal angle for supersonic cruise by moving mass, such as engines, to the wingtips. This design allowed for adjustments to the thrust line, maintaining balance even when engines were lost.
The Era of Swing Wing Production
The 1960s saw the first mass production of variable-sweep aircraft. In the US, the General Dynamics F-111 became the first production aircraft with a variable-geometry wing. Despite its innovative design, the F-111 faced challenges, including structural issues with wing attach points that led to redesigns and intensive testing.
The Soviet Union also explored variable geometry wings, leading to the development of aircraft like the Sukhoi Su-17 and the Mikoyan-Gurevich MiG-23. These designs incorporated different pivot spacings to balance aerodynamic effects and operational flexibility.
The Variable Sweep Wing in Strategic Bombers
The variable sweep wing found its way into strategic bombers like the Rockwell B-1 Lancer, which combined high-speed cruising efficiency with fast, low-level supersonic penetration speeds. The B-1’s design allowed for a high lift during takeoff and landing while minimizing drag during high-speed flight.
The Soviet Union’s Tupolev Tu-160, the largest and heaviest combat aircraft with a variable-sweep wing, entered service in the late 1980s. It remains one of the fastest bombers in use today.
The Boeing 2707, intended to be a supersonic transport with a variable-sweep wing, was ultimately abandoned due to design challenges. The introduction of relaxed stability flight control systems in the 1970s made fixed-wing configurations more viable, leading to a decline in new variable-sweep wing aircraft.
However, in 2015, the Russian Ministry of Defense announced plans to restart production of the Tu-160, marking a resurgence in variable sweep wing technology after nearly three decades.
Planes that Use Variable Sweep Wings
| Type | Class | Role |
|---|---|---|
| Dassault Mirage G | Jet | Fighter |
| General Dynamics F-111 / EF-111 | Jet | Attack |
| Grumman XF10F Jaguar | Jet | Fighter |
| Grumman F-14 Tomcat | Jet | Fighter |
| Mikoyan-Gurevich MiG-23 | Jet | Fighter |
| Mikoyan-Gurevich MiG-27 | Jet | Attack |
| Panavia Tornado | Jet | Multirole |
| Rockwell B-1 Lancer | Jet | Bomber |
| Sukhoi Su-17, 20 & 22 | Jet | Fighter-Bomber |
| Sukhoi Su-24 | Jet | Attack |
| Tupolev Tu-22M | Jet | Bomber |
| Tupolev Tu-160 | Jet | Bomber |
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