A-12/SR-71 Forward Bypass-Door Question


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13 February 2008
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Can the forward bypass-doors operate in such a way that that the doors/louvers on the outboard side or the inboard side could be opened asymmetrically?

I remember a NASA test in which they were able to show that the forward bypass-doors in addition to providing a 50% increase in drag fully open vs closed, actually had as much influence over yaw, and roll (if not yaw, pitch and roll) as the control-surfaces did. The experiment sort of implied the ability to open the inboard or outboard forward bypass-doors asymmetrically of each other. Granted this could have been a special modification for an experimental purpose and may not have been on operational planes, but I'm not entirely sure.

Can anyone clarify?

Do you mean in addition to the sideways asymmetrical drag / thrust affecting the yaw, just like in any twin engine airplane when you use asymmetrical power?
KJ, the SR-71 flight manual is available online at:
click Section 1: - Description and Operation and then click
Forward Bypass.

As you read the section you will find out that the
forward bypass is used to help position the normal
shock near the physical throat (the narrowest point)
of the inlet.

It does this when pressure starts to build behind the
normal shock near the throat of the inlet, by opening
and dumping inlet air behind the throat overboard
through dedicated nacelle spill vents provided for this

This reduces the pressure behind the normal shock so
that a build-up in pressure behind the normal shock
(perhaps due to a engine transient) won't spit the shock
out of the throat and out in front of the inlet cowl
(ie: an unstart).

Active Modes of Operation:
Spike Mode Fwd Bypass Mode Fwd Bypass Function
Auto Auto automatically opens and close as it needs to,
to keep the normal shock near the throat.
each nacelle will operate independently. It would
be interesting to ever ask if this is ever percieved
by the flight crew.
Auto Manual This mode is possible (I suspect this mode is the
mode NASA may have used - they could then manually
modulate the fwd bypass). If it is open 100% that should
be OK. They could then measure drag of different
Manual Auto Fwd Bypass opens to 100%
Manual Manual Default setting when spike is manual unless Fwd Bypass
control setg to AUTO. This means yu need to control
BOTH manually.
I'm not sure if I'm saying it clearly, but to clarify...

On each nacelle can the outboard or inboard set (the vents go all around the nacelle, some are inboard and outboard of the centerline) of bypass vents (forward bypass) be opened independently of each other (i.e. can the inboard side be opened a lot while the outboard side is opened a little) or can the top and bottom set of bypass vents be opened independently of each other?

Or was that a special modification for a NASA test? Or am I making some kind of mistake?

Because I remember reading that they did a test and were able to show not only did the bypass doors when fully opened at speed produced 50% more drag than when closed, they could affect pitch and roll as well as the elevons. To produce pitch (particularly pitch) and roll with the forward bypass doors it sounds like you would have to operate the top and bottom vents at the very least independent of each other...

KJ Lesnick
Personally I didn't find any mention in the dash-1 of such a
capability or the cockpit controls to effect them, unless they
just flew with some of the outlets set as blocked before flight
and took measurements on multiple flights.

But such a thing would increase the flow velocity out the
unblocked outlets because mass flow must be conserved.

The aft bypass can be used to get rid of some of this air as well.

Would you have a pointer to more info on this experiment?

This is actually an interesting idea.

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