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enter image description hereenter image description hereJenkins and Landis write in their 2004 book about the XB-70, regarding its inlet duct: "Three movable panels, positioned by two hydraulic actuators, opened or closed the throat area to meet engine air requirements; the maximum opening was 48 inches wide (doors set at 25.5 degrees) while the minimum was 19 inches (3.6 degrees)." The doors here are the bypass doors just before the engines, jettisoning the excess air from the duct. (There is a drawing showing four, not three panels, but this is not important fot the question.) So as far as I can understand this, when the panels opened fully (48 in), the doors too (25.5 deg). But this means the inlet sucking in lots of air but then the doors jettisoning the max amount they can. What is the reason behind it? Wouldn't be better a smaller duct from the beginning? Or I misunderstood the method here somehow?

Update: I tried to upload this pic before, but it was not working somehow. The bypass doors are somewhere near the right, about the "32.0" mark.

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  • $\begingroup$ Can you include an image of the Illustration question for more clarification? $\endgroup$ Commented May 13, 2023 at 10:50
  • $\begingroup$ Interesting question. Possibly aerodynamicists found that it formed turbulent air in that section of the intake, duct at high speed, or some other places in the flight envelope, and therefore needed a door to spill this layer overboard.. $\endgroup$ Commented May 13, 2023 at 17:00

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Maybe this will help.

From SD 72-SH-0003 B-70 Aircraft Study Final Report Volume IV, available here: https://ntrs.nasa.gov/search?q=19950002361

The following is from pages IV-6 and IV-7. The image is from page IV-23.

My note: AIS = Air Induction Subsystem.

"The AIS of the B-70 was a multi-shock, variable area, convergent-divergent inlet with essentially four regimes of operation...

...During take-off and low subsonic flight (up to local Mach number 0.6), the bypass doors were closed and the throat panels fully retracted to provide maximum air capture area. At high subsonic-low supersonic speeds (up to local Mach number 1.1), the bypass doors were still closed while the throat panels moved from the maximum width of 48 inches to approximately 46 inches. (Note: Throat width is sometimes identified as throat height.) At the intermediate supersonic speeds (up to local Mach number 1.8), the bypass doors were opened a fixed increment while the throat panels had scheduled to an approximate width of 41 inches. Above local Mach number 1.8 and at high supersonic speeds, the bypass doors were modulated for terminal shock control while the throat panels were scheduled as a function of local Mach number to a minimum width of approximately 21 inches.

To maintain efficiency throughout the wide range of operating conditions from take-off to Mach 3 cruise, the variable geometry features of the B-70 AIS provided continuous match of inlet and engines for maximum inlet recovery and engine thrust. As previously stated, for take-off, the inlet was at maximum air capture area with the bypass doors closed to provide the volume of air demanded by the engines."

The image might clarify the position of the bypass doors:

enter image description here

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Old discussion, but: As the text says, they exist to fine-tune the position of the shock. Jettisoning air achieves that by reducing the pressure behind the inlet throat. The SR-71's inlet design has a similar mechanism, for similar reasons.

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The doors are auxiliary bypass doors, most likely to provide extra air to the engines during the takeoff roll, or low speed flight, where ram air pressure at the inlet is insufficient to provide air for the engines to operate at high thrust settings. This isn’t anything new to an XB-70. Many aircraft use auxiliary bypass doors near the main intake to provide extra air in the conditions described above.

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  • $\begingroup$ Those are not auxiliary intakes, as I mentioned (and the cited book also clear about this) the doors are for jettison (i.e. to let go out) the air, and not to let air inside. $\endgroup$
    – magalenin
    Commented May 13, 2023 at 12:02
  • $\begingroup$ Judging by the illustration, they sure do appear to be auxiliary air intakes. $\endgroup$ Commented May 13, 2023 at 17:01
  • $\begingroup$ I finally able to find a better picture, I uploaded it too. That clearly shows the doors are opening to the back, so no intakes. They are between the vertical stabilizers and the "fuselage". As the book wrote, they are jettisoning air. Anyway, thank you for thinking about it :) $\endgroup$
    – magalenin
    Commented May 13, 2023 at 18:08

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