Why do the intake spikes on the SR71 Blackbird point down and inboard?

[zul8tr]

When I inspected the SR71 at the Dallas Air and Space Museum I noticed that the intake spikes pointed down and inboard toward the fuselage centerline. I am aware that they are adjustable related to Mach speed (out for low Mach in for high Mach at first thought contrare but the pilot explained) for proper shock location and to prevent over pressure of the intake. But why the down and in orientation of the spikes. I am not aware of any adjustment for that spike orientation condition and noticed after. Perhaps total fuselage airflow affects the spike shock that requires that orientation?

 

[anorlunda]

My guess is that it is something they discovered in wind tunnel tests.

But the actual answer might be very hard to find. Let's see, we have many resourceful people here on PF.

 

[CWatters]

This doesn't answer the OP question but has info on the (moveable) intake from 1:50..



Edit: Unless it's for the reason given at 3min.

 

[cjl]

The down and in orientation is because the airplane flies with a nose up orientation, and the airflow in the neighborhood of the spike has been pushed outward by the passage of the aircraft's nose. Down and in orients the spikes to be directly aligned with the local flow, so the shock coming off the spike is perfectly axially symmetric (or as close as reasonably possible).

 

[DQNOK]

I doubt the spikes are intended to provide lift: I suspect they are there exclusively to provide an oblique shock to the inlet. As such, they would (for optimum air-compression performance) be aligned with the local flow streamlines. The local flow streamlines will be directed slightly outward from the fuselage at the engine inlets, when looking down from the top (your second image). Also, the aircraft does not fly "level", it flies at some angle-of-attack, the entire fuselage and engine nacelles providing lift. Hence, the spikes are pointed downward with regard to the fuselage (your first image) -- the spikes ARE flying "level" into the wind, it's the fuselage and engine nacelles that are angled upward with regard to the wind. CWatters' video kind-of shows this at about 2:10 -- the engine is pitched slightly nose-up, but the inlet spike is aligned with the inlet flow. Unfortunately, in the video they colored the inlet air to make it look as though the flow direction aligns with the nacelle, which it does not do in normal flight. They did, however, get the oblique shocks aligned correctly near 2:55.

Now, having said that, I should point out that I never worked on the SR-71, nor at Lockheed. But my experience as an engineer is that you start with nice clean theory (my first paragraph), then in testing you learn a lot, and you tweak a lot. So, the inlet spikes MAY provide some lift, and they may (possibly for several reasons) NOT be exactly aligned with the local flow streamlines. But if I were designing the inlet, what I said in the first paragraph would be where I'd begin the design iterations.

 

[zul8tr]

Thanks, reasonable responses on the orientation of the spikes. I would like to review the wind tunnel data and see some Schlieren photos for different Mach #'s and spike in out position and testing of spike orientation down and in, no doubt still TS. At least in the video the spike is pointing downward.

Just looked at a video of the SR71 in flight but probably at < Mach 1 and there is indeed a noticeable AOA. It would be much less AOA at M3.2? So I suspect the spike orientation was designed for a certain M# or M# range.

 

[cjl]

The flight angle at mach 3.2 isn't necessarily any lower - it's flying much faster, but it's also designed for extremely high altitude where the density is very low. Pretty much everything about that airplane is designed for cruise, so I'd expect spike orientation to be based on the expected attitude at M3-M3.2 at 75-90kft.

 

[zul8tr]

Seems reasonable still would like to see a M3.2 flyby at 70kft viewed from another SR71 at the same speed, not likely .

 

[Locke79]

The inlet cone locks below Mach 1.6 after which the cone is unlocked and movable by up to 26 inches controlled by the inlet computer to account for the orientation of the aircraft (pitch, roll, yaw) and angle of attack. It serves the normal purpose in supersonic flight however the angle and retraction of the cone controls the shape of the shock wave and keeps it optimally positioned for the intake.

 

[Roberto Teso]

Perhaps total fuselage airflow affects the spike shock that requires that orientation?

Not precisely, but the chines in the forward section motivate the geometry of the inlet cones.
Here you can find a document where it's well explained:

DEVELOPMENT OF THE LOCKHEED SR-71 BLACKBIRD

 

[zul8tr]

Thank you for that document reference from the Skunk Works leader Kelly Johnson