Most split/V tail designs I see are either perfectly vertical, or radiate OUTWARDS. Why did the SR-71’s split vertical stabilizer radiate INWARDS?
In: Engineering
The airflow that the vertical stabilizers need to interact with is split and heavily deflected both outboard and inboard by those huge engine nacelles. The stabilizers pretty much have to lean one way or the other to get into that flow. So why not cant outward like a typical fighter?
Canting away from vertical also reduces the aircraft’s radar cross section. An outward cant tends to deflect radar downward, and helps prevent detection from aircraft at similar or higher altitudes. For typical fighter or tactical aircraft, detection by another (hostile) aircraft is a major concern. And such a hostile interceptor is likely going to be at as high or higher altitude, so it makes sense to prefer deflecting incoming radar waves downward, away from that hypothetical interceptor. (Ground based radar is still a concern, but can be dealt with other ways, like strikes ahead of time, or flying very low).
But in the case of the SR-71, there’s virtually no chance of a radar being transmitted at it from above due to its extremely high operating altitude. So instead it has an inward cant, which helps deflect radar upward, enhancing the reduction in radar cross section for sources below the aircraft, which is basically all of them.
At least that’s how it was explained to me once.
The SR-71 was a very unusual airplane. It had two giant engines, far from the centerline. Engines go out from time to time, particularly when you try to fly so high there isn’t much oxygen. This puts a huge torque on the plane, which of course you’d like to correct with the tail. Tail fins pointed out induce a roll, making a spin even harder to get out of, so that’s not going to be acceptable. Tail fins pointed in work better while you get the working engine turned off and then point at the ground to restart both engines. (Yes, that’s the procedure.)
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