max altitude and Mach 1

In article ,
Scott Ferrin writes:


Sorry for the long answer. That sometimes happens with short
questions.



Maybe you could answer a question I've had for a long time. If you
look at the SR-71's inlets from the side they seem to be pointing
somewhat down. I took this to mean that since it seems you'd have to
have the inlet lip on a circular inlet perpendicular to the airflow to
maximize it's efficiency, that at cruise speed and altitude the
Blackbird would be flying at an angle of attack such that the inlet
lip would be at 0 degrees AOA. At that angle the exhaust would exit
in a somewhat downward direction. So my question is is that setup to
maximize the altitude potential (because thrust would be directly
aiding lift)? Do ALL aircraft fly at a certain angle of attack at
their maximum altitude? Is the only reason you see these things on
the Blackbird because it's designed to spend most of it's time in
those conditions? Would a Blackbird's max altitude also be at Mach
0.9?

Well, I'll try - Yes, an A-12/YF-12/SR-71's inlets do face down a bit,
and the reason is to present an inlet face perdenicular to the
airflow, as much as possible. The Blackbirds were intended to cruise
right at the edge of what was possible for an airplane that could also
take off & land, back in the late 1950s. They needed to squeeze every
mit of efficiency out of the airframe & powerplant (Which can be hard
to tell apart, on an SR), and the airplane was intended to get itself
to one point in its flight envelope and stay there. (Mach 3.2/80,000'
or so, around 375 KEAS) At that EAS, an for teh weights that would be
expected, the Angle of Attack range would be predictable, and so it
was dialled in to the inlet design. This maximizes the inlet
efficiancy, and helps alleviate the possibility of the inlet getting
dicombobulated with the complex series of shock waves that it uses to
allow for the maximum pressure recovery. Consider how much of a
problem inlet "unstarts", where the shocks got all tangled up & the
inlet system stopped properly supplying air to the engine, were in the
early stages of the program. Then think about how much worse it would
have been with the inlets getting an uneven flow. Very Ugly Indeed.

While thrust vectoring with AoA does occur, (A good example would be
the F-104. I was told by a CanForce CF-104 pilot that the best way to
ensure a hard landing was to pull back on the throttle during the
flare - the AoA was high enough that a fair chunk of hte airplane's
weight was riding on teh vertical component of the thrust), I don't
think that that was a factor. The angle's too small for there to be
much of a vertical component to the thrust. It might have an effect
on cruise trim, though.

--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster