Some of the great wars of the Cold War like the F14, the F11, the tornado, the 222 backfire, the MiG-23 featured a valuable sweep wing. Then almost suddenly the solution disappeared from the drawing boards. Welcome to Millennium 7 Star, the channel that helps you make sense of military history and military technology. Stay with me till the end because the starter we discuss here are not easy to find anywhere else on YouTube. Many aircraft designs of the late 60s and the 70s had chosen the valuable sweep wing. For more than a decade it was a staple feature of many advanced combat jets. The adoption of the terrible sweep wing was caused by the new Air Force requirements that wanted to consolidate in a single airframe the capability of executing different missions. Those missions required different and contrasting aerodynamic designs. Long ferry range and loiter endurance had to be combined with Mach 2 Performance at medium-high altitude and transonic low-level performance all of this operating from short runways or aircraft carriers. The valuable sweep design provided a brilliant solution to this problem adapting the aircraft behavior to the requirement. If you are interested there is an entire playlist covering this subject in detail. So it may appear quite strange that none of the designs of the eight is actually featured a valuable sweep wing. The features had then appeared on the drawing boards just to disappear at the cave afterwards. A new technology in the 70s were entering the arena the relaxed stability. A plane is naturally stable if it just flies a straight with no pilot input. South correct them the bumps and the shakes that it receives from the turbulence and the wind. A plane with a normal configuration is stable if the center of gravity is ahead of the aerodynamic center. The center of gravity is obviously the point where we can think that the force of gravity is applied to the plane. The aerodynamic center is the point where we considered that all the aerodynamic forces are applied to the plane and in this specific case we are interested in lift. In the center of gravity is ahead of the aerodynamic center. Weight and lift create a pitch-down torque that is compensated by the tail negative lift and the plane is in equilibrium and the equilibrium in this case is stable. In case a bump changes the attitude and the angle of attack of the surfaces the lift variations on the wing and the tail but the torque brings back the plane to a straight flight. For the case planes stability has been a major design consideration and planes have been built to be as stable as necessary. Then with the rise of fly-by-wire controls it was to realize that the plane actually did not need to be stable anymore. The computer could keep it flying straight. Design has also realized that having a plane unstable in pitch was very useful because it improved the turn rate. The F-16 was the harbinger of this new era of unstable fighters. Now what has this to do with the abandoning of the variable sweep wing? When the sweep of the wing increases the aerodynamic center of the wing moves back and the excursion is actually quite high. If the plane is already stable stability increases and the torque created by the lift and weight is higher the negative lift on the tail must be higher and the so-called trim drag is also higher. The pilot has the sensation that the plane is nose heavy and the maneuverability is compromised. The F-14 had the famous globe veins to partially compensate this fact. If the plane is unstable the wing sweep could move the aerodynamic center backward. The movement would transition the plane to the stable domain because the center of gravity would go forward from the aerodynamic center and thought purpose of the unstable design would be defeated. Actually a key element of the design of any variable sweep wing is where to place the wing rotation point. Exactly because it influences the aerodynamic center movement. The larger is the fixed segment the shorter is the movement. The smaller can be the tail but the less effective is the change of sweep. For example there is Tukai 17 had a relatively small moving portion of the wing but also relatively small tail plane while the F-14, the F-11 and Ternado all have a large portion of the wing moving but also a very large tail. So while it would be at least in principle possible to make a variable sweep wing that preserves the instability the effectiveness of the change of sweep would be greatly reduced. One of the myths around the variable sweep wing is that it is heavy. Heavy is one of those adjectives that make little sense if you don't specify the comparison term. While it is true that all the mechanism associated with the moving wing weight more than a fixed wing, if we consider the weight and the size required to have the same aircraft performances with a fixed wing the extra weight of the mechanism is almost negligible. The real problems are others. A quite important one is that the mechanism requires extra mechanical maintenance. That is the type of maintenance that you want to reduce the most because it requires time, people and specific equipment. You have to consider that all the pilots located in the mobile part of the wing need to rotate with the wing to stay parallel with the relative wing adding further components that the mechanical maintenance. A large logistic footprint like this reduces the plane overall availability, this strategic mobility, anything increases the hours of maintenance for a brick flight hour. This is definitely something that we don't want. Another related issue is that the moving wing mechanism makes more difficult to store the fuel inside the wing, you need mobile pipes and stuff like that, and the mechanism itself is taking up space inside the wing that could be used for the fuel. It is not a decisive disadvantage but it is a design issue that may have an impact on the range of the plane. A further element was that in the 80s a new generation of engines came along with high-trust and good fuel efficiency. So while designing, for example, the 14th was necessary to have a better refined aerodynamic to obtain the desired performance, the Aú brute force was available in large quantities. Since extreme performances, particularly speed were also being considered less combat-relevant than in the past, then suddenly replacing aerodynamic finesse with brute force became an option. Less efficient aerodynamic configuration that the variable sweep wing became viable. Finally it is basically impossible to have a stout variable sweep wing. The very geometry of the wing makes it inherently not stoutly, and this, at least in the United States, became the final nail in the coffin of the swing wing. If you liked this video, I am sure you will be interested in the videos that are going to appear beside me. In the meanwhile, please like, dislike, subscribe and hit the bell so you won't miss anything in the future, and if you could consider becoming a pattern or a subscribe star, that would be really amazing. In the meanwhile, thank you very, very much for watching and see you in the next video.