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Swept wings increase the critical mach number for aircraft with otherwise identical wings (Source 1). The U-2 Dragon Lady is limited in altitude by its critical mach number, as it needs more speed in order to go higher (Source 2), but going faster would put it into its "coffin corner" (Source 3). However, looking at the following photo of a modern U-2 from Lockheed's website, the wings seem to have little to no significant sweep. The top speed (presumably what is used during the highest-altitude cruising) is only Mach 0.67 (Source 4), while modern aircraft like the Boeing 737 (which do have swept wings) often cruise at speeds between Mach 0.75 and 0.8 (Source 5).

Why does the U-2 not have swept wings?

Lockheed U2 in flight

I know that wing sweep was first developed in Germany during WWII; had the technology not taken hold in the USA by the time the U2 was developed in the 1950s? Or is there a deeper technical reason I'm missing?

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    $\begingroup$ Flying fast isn't exactly what they want to do. They want to fly high to avoid being intercepted, but they don't really want to fly fast because they need to loiter around a target (or targets) to gather intelligence. $\endgroup$
    – Ron Beyer
    Commented Jun 28, 2017 at 17:23
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    $\begingroup$ Perhaps for the same reason sailplanes seldom have swept wings? The U2 is pretty close to a jet-powered sailplane... $\endgroup$
    – jamesqf
    Commented Jun 28, 2017 at 18:05
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    $\begingroup$ I believe, there was simply no desire at all for this plane to go fast. $\endgroup$
    – Fattie
    Commented Jun 29, 2017 at 0:31
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    $\begingroup$ @RonBeyer, It is true that the re is a benefit to flying high to avoid anti-aircraft measures, however the benefit of sensor coverage is significant. The speed was determined to be limited to subsonic for several reasons, which included complexity and acoustic stealth. But the notion that a U-2 loiters around a target is perhaps a mis-understanding. I have personally reviewed hundreds, if not thousands of U-2 missions, and they are planned for linear flight line. Loitering increases risk, and does not substantially augment the intelligence value of the flight. $\endgroup$
    – mongo
    Commented Nov 17, 2017 at 21:16

4 Answers 4

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Sweeping a wing costs weight, for several reasons. Therefore, if the goal is to fly high and achieve long range, an unswept wing is the better choice. Only when transport performance needs to be maximized will a moderately swept wing look better.

Why is a swept wing heavier?

  1. Sweep reduces aerodynamic efficiency, so a larger wing is needed to create the same amount of lift.
  2. Sweep introduces torsion when a straight wing will mostly see bending only. Adding torsion strength needs more structure.
  3. The stall characteristics of a swept, high aspect ratio wing are extremely unpleasant and require a healthy margin from the stall angle of attack, when flying high requires to fly just at the edge of stall.

In the end, Kelly Johnson decided to cut weight and drag as much as possible but not to pursue maximum speed to achieve the greatest flight altitude. Another reason was that the U-2 begun as a modification of the XF-104 with extremely stretched wings. Adding sweep would had resulted in too many modifications to the U-2 fuselage, which was initially built in the same tooling than that of the XF-104.

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Basically, because of its mission requirements. It wasn't intended to fly fast, and because it didn't need to go particularly fast, there was no need to sweep the wing. Instead, it was supposed to provide a massive amount of data (by 20th century standards) along its flight path, which necessitated long flight times: thus the long, straight wings providing a lot of lift at low airspeeds were ideal for its mission.

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A swept wing would most likely adversely affect the handling of a U2 during a typical mission profile, and would adversely affect the structure required (weight) of the aircraft.

A swept wing adversely impacts the stall characteristics and stability when flying near a stall.

It should be noted that while a swept wing helps manage effects due to the compressibility of air near mach speed. The primary effect is a swept wing delays the near mach shock wave development. This results in reduced drag near transonic speeds. The U-2 is designed to operate just below transonic speeds, so in theory, there could be a benefit of a swept wing on the U-2.

The benefits of a swept wing comes at a price. In addition to requiring a beefier structure to handle the torsional load stresses of a swept wing, the stall characteristics of the wing are different.

The resultant stall characteristics can become more pronounced and more difficult to recover from. In the case of the U-2, which at higher altitudes can fly with as little as a 2.5 to 7.5 knot gap between stall and MMO, the pilot must be sensitive to the pre-stall buffet.

A swept wing would make the sensing of that buffet potentially a little more difficult, but more importantly, it would make stall recovery much more difficult. In many aircraft that would be manageable, however, in the case of the U-2 stalls are not docile, and have been known to cause tail structural failures and separation.

The risks of a swept wing on the U-2 outweigh any benefit of reduced aerodynamic drag from a swept wing, given that there was a decision to have the U-2 be a subsonic aircraft. The additional weight of the structure needed for the swept wing could also be construed as to further close the stall to MMO gap.

Therefore, not implementing a swept wing on the U-2 helps control the stall characteristics, and should the onset of a stall occur, it is easier and with less risk, to recover from a stall.

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The limiting factor for U2's speed is not the wing, but the engine.

According to Unlimited Horizons (also U2 Developments ), engine performance is only 6% of sea level at 70000 ft due to oxygen starvation. Before engine is improved to the point that the same airframe can reach M0.8, there's no need to sweep the wing.

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