Swing-wing airplanes: how and why

You may wonder why some aircraft can adjust the angle (sweep) of their wings. I'll try to explain that in this post.

For each flight phase, one shape is optimal. Straight wings are preferred for slow flight, as they are far more efficient. However, when flying close to or beyond the speed of sound, straight wings will suddenly produce a lot of drag. This is where swept wings come in handy, they allow you to fly faster with less drag.

Pictured above is an F-14 doing tests with asymmetric wing sweep.

As with many technologies, a prototype with swept wings was first made in Nazi-Germany in 1944. This is a Messerschmitt Me-P1101. Its wing sweep could be adjusted only on the ground, but this was only meant for testing. Its actual purpose was to be a cheap fighter which could turn the tide against the allied invasion. It was never completed before the war ended.

When the Me-P1101 was found, it was shipped to the United States, where it formed the inspiration for the Bell X-5, pictured above. This prototype could change sweep angle in-flight, and tests revealed a number of problems. One was that when the wings were swept backward, the airplane would dive nose-down because the lift shifted back as well. The pilot had to compensate quite a lot for this. Also, the mechanism to sweep the wings was quite heavy.

However, in general the concept was found very useful, enabling an aircraft to fly very slow and fast in the same mission.

Another early prototype was the Grumman XF10. This prototype was made for the U.S. Navy. Having good low-speed performance was essential for carrier-based aircraft. To fix the problem of the shifting lift, the wjng itself would translate forward when the sweep angle was increased.

A solution to get rid of the heavy and complicated mechanisms, was to make the wings fixed at a certain angle and just accept the problems this caused at low speeds. The best example is the F-86 Sabre. It was a formidable fighter, and its swept wings allowed it to reach Mach 1 in a dive.

However, the swept wing required a high landing speed, and when the wing stalled, the aircraft would pitch up and make things worse. This phenomenom is called Sabre Dance, there are some videos of it on Youtube.

Because of the succes of the Sabre, the United States wanted to feature swept wings in a high speed bomber. However, to make the landing speed acceptably low, straight wings were preferred. The solution was variable geometry, and resulted in the F-111 Aardvark, the first operational variable sweep aircraft. It was meant to fly under the radar at high speeds to destroy high-profile targets behing the lines.

The Soviet Union meanwhile also saw the advantages and created the Sukhoi 17. Based on the Sukhoi 7, which had 'ordinary' wings, the outer portion of the wing could be adjusted. Because only a part of the wing was movable, the changes in pitch were not so dramatic. It also allowed to carry weapons on the non-moving part of the wing.

Somewhat later, the MiG-23 followed. It was more focused on long-range missiles and a short take-of distance, so it was given a large radar and the extra weight of the sweep mechanism didn't matter that much. A dedicated ground-attack version was also made.

In response to the F-111, the Su-24 was made. It was also meant to have a long range, for which the wings would be unfolded, and be able to do a high-speed dash towards the target at low level, for which the wings would be fully swept aft.

In Europe, the Panavia Tornado was made by England, West-Germany, and Italy. The focus was again on low-level, high-speed attacks to bypass ever more advanced surface to air missiles. In order to let it use short runways, not only wing sweep could be adjusted, it also had thrust reversers, quite unique for fighter aircraft. The wing pylons could swivel to keep the weapons pointed forward and thus reduce drag.

Because the F-111 was also meant to be developed into a carrier-based interceptor, the F-111B was developed. This failed quite badly however, and the iconic F-14 Tomcat was born. It had an excellent range, and a great top speed of over Mach 2. Although it was meant as an interceptor, it was also a maneuverable dogfighter and it has 135 kills on its name.

This is the B-1B Lancer, a heavy supersonic bomber. Originally it had a top speed of Mach 2, but in later versions the emphasis was laid on stealth. This reduced the top speed to Mach 1.2, which is still impressive given the size; it weighs 87,000 kg (190,000 lbs), which is comparable to a Boeing 767 airliner.

This is the Tupolev Tu-160. Although it looks similar to the B-1, it's larger and heavier. Its mission is also a bit different, it mainly carries cruise missiles. Because it is less focused to be stealthy, it is a lot faster. Only 36 were made.

Sadly however, variable sweep wing seem to be a thing of the past. More advanced flaps allow airplanes to fly slower, and stealth technologies has taken away the need to fly low and fast for a good part. I personally think that is a shame, since variable sweep wings look sexy as hell.

Prototype tax: NASA Oblique Wing concept.

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