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On a lot of fighter jets, I see a heavily slanted intake, such as the F-15.
I always thought this is to help ingest air at high angles of attack. However, what about something like the MiG-21?
There's no slanted intake there. Yet on Wikipedia it does say, "Its G-limits were increased from +7Gs in initial variants to +8.5Gs in the latest variants." And it has a citation for it.
So my question is, do nose cones work well at high angles of attack? And if so, how?
No, they don't.
The slanted intake is slanted because the top part produces a compression shock wave that hits the lower intake lip so all of the compressed air is ingested and nothing is wasted. In order to do this reliably up to Mach 2.5 (where the Mach cone angle is 73.8° from the vertical), the whole intake needs to be slanted by those 73.8° (actually a bit less due to the intake wedge). In order to adjust the intake for different Mach numbers, one or both parts are adjustable, as shown in the F-14 example below.
Cross sections of the F-14 intake at subsonic, transsonic and supersonic speed. Note that the shocks of the supersonic example do not fit well with the intake geometry – the geometry is optimized for a much higher Mach number than shown. See here for a better example.
Here is (admitted a quite messy) cross section of the F-15 intake. The adjustable upper wedge is clearly visible (picture source):
In order to minimize losses, the upper intake uses adjustable ramps in order to force additional shocks so the compression happens over a sequence of weak, oblique shocks.
Regarding spike intakes: They were the lightest and simplest way to produce efficient supersonic intakes and were ill suited to large sideslip angles or angles of attack. Note that they were a characteristic of early supersonic flight when speed was the main objective and have since been replaced by ramp or even pitot intakes.
Interestingly, a high G maneuver may have lower AOA than a stall maneuver. The MiG 21 spike inlet is 1950s vintage, also seen on the SR-71, used to solve the problem of slowing supersonic inlet air to subsonic flow that was then fed into the turbojet.
The MiG 21 was designed more for hit and run and is not as versatile as the F-15 Eagle, which suceeded it and it's F-104 Starfighter and F-4 Phantom contemporaries in the 1970s.
It is doubtful a spike inlet would be anywhere near as effective as the F-15s inlets during low speed, high AOA maneuvers, but neither would the rest of the plane. It would be far better off to stay with its strengths.
The MiG 29 and Sukhoi Su 27, developed and put into sevice just after the F-15, feature similar air intakes, as does the fastest of them, the MiG 25.
In seems that the designers reasoned that the nose of the plane itself could be the "spike", placing the turbojet intakes on the side (further back) inside the shock wave "cone".
For subsonic high AOA maneuvering, the advantages of placing angled intakes on the side (or underneath) are obvious, and could lead too ... a better pitot tube design for GA aircraft.
