Looking at images and videos displaying B-2 spirit, it seems like split elevons are most of the time deployed, even at high speed. Why is it so?

Having some aerodynamic surface deployed generates drag and one might think it should instead most of the time fly in a clean retracted configuration.

Is this a tradeoff related to stealth capability which tends to reduce or totally suppress vertical stabilizers?

If not, why isn't sweep angle or negative wingtip twist increased, so that it can actually have yaw stability in a more optimized minimal drag configuration?

Are those split elevons actually fully retracted when cruising at high speed in a straight and level flight regime?

(I'm sure it's absolutely state of the art optimised, yet it does not look like it is for the reasons mentioned above)

  • 1
    $\begingroup$ The issue is addressed in this and this answer. Increasing sweep in a flying wing is creating a host of problems of its own, so drag rudders are less damaging. $\endgroup$ Aug 2 at 16:55
  • $\begingroup$ What in the world makes you think that a lazy transit over an air show is high speed? $\endgroup$
    – RonJohn
    Aug 3 at 17:17
  • $\begingroup$ @RonJohn the title of the video says "high speed pass" and I consider all the titles of youtube videos as an absolute truth, my bad.. $\endgroup$
    – qq jkztd
    Aug 3 at 17:41
  • $\begingroup$ @RonJohn joke aside, searching for "B-2 in cruise" leads to this footage, which displays deployed split elevons in straight level flight... so I don't think it's either an accurate title. $\endgroup$
    – qq jkztd
    Aug 3 at 17:50
  • $\begingroup$ actually from 1:20 to 1:26 it seems to fly fast enough for not having to increase yaw stability by deploying elevons, which is satisfying. $\endgroup$
    – qq jkztd
    Aug 3 at 17:57

First, the linked video is not high speed -- that was a slow pass, intended to wow the crowd at an air show. Second, in that pass the aircraft was held in a bank, first toward the crowd, then away from them after the closest pass; the split elevons were deployed only on the down wing (it's a little hard to see the far wing well until the plane has passed, but you can clearly see the near wing splits are closed once the bank is reversed).

As you surmise, these flaps are closed in steady state, high speed, straight and level flight; not only would flying with them open result in increased drag (loss of top speed, increased fuel consumption), it would increase the observability of the airplane due to providing a radar reflection point, increasing exhaust heat, and increasing airflow noise.

The only time you'd expect to see both split elevons open together is when deployed as a drag brake, either to control speed during a rapid descent, or to steepen approach for landing. As noted in comments, the one exception to this is that they might be partially deployed in certain flight regimes (usually for a short time) in order to produce a "virtual vertical fin" by moving the center of drag to the rear when the aircraft develops a yaw instability. This ability may contribute to making the B2 a better bombing platform than the XB-35 and YB-49 of the 1940s to 1950s.

  • 3
    $\begingroup$ Not just drag brake. They provide directional stability during slow speed flight. They are not opened primarily to increase drag but to increase stability by shifting the center of drag backwards (thus creating a virtual vertical stabilizer). During high speed flight the center of drag often shifts naturally rearwards similar to the shift of the center of lift so they can be closed. Even so if the aircraft starts experiencing yaw instability they can be both opened to temporarily improve stability - don't know if the B2 does this but the N9M certainly did. $\endgroup$
    – slebetman
    Aug 3 at 5:10

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