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The horizontally forward-swept intakes of the Republic F-105 Thunderchief is not a design emulated in other production aircraft of the time, or I think at any time.

If viewed as a more typical intake shape turned 90° it is a little less startling. What was the thinking behind this unique feature, and why was it never copied?

F-105 Thuderchief source

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    $\begingroup$ It was copied once: See the intake of the Crusader III $\endgroup$ Dec 3 '21 at 4:07
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enter image description here

The inlets look like optimal Oswatitsch inlets. Pics in this post are from prof. Wittenberg's course handout on Propulsion of Aeroplanes, Dictaat D-32, TU Delft (in Dutch). Translated (by me) text explains:

Intake efficiency ${\eta}_R$ can be increased by shaping the intake body such that multiple oblique shock waves occur, followed by a normal shock wave. This has led to the idea of achieving isentropic compression via a concave shape, formed such that the compression wavelets (very weak oblique shocks) intersect each other in one point.

This type of inlet has been called the optimal (ideal) Oswatisch inlet after the researcher who during WW2 in Göttingen performed theoretical and experimental research on supersonic intakes.

So this intake design leads to optimum inlet efficiency, but poses two difficulties:

  1. Angle ${\delta}_l$ increases with increasing Mach number, causing the inlet lip to be positioned at increasingly steeper angles and causing ever increasing external drag. Plus above $M_0$ ≃ 1.5, ${\delta}_l$ becomes larger than the angle where the oblique shock detaches at the outside of the intake, so that the design airflow cannot be realised.
  2. The intake geometry is largely fixed, leading to possible sub-optimal conditions outside of the design conditions.

enter image description here

For these reasons, supersonic intakes with variable geometry and partial isentropic compression were designed, and used in subsequent supersonic aeroplanes. As shown in the pic above. The normal shock wave is the only source of loss in this set-up.

------Updates-------

@PeterKämpf linked two pics in a comment, showing some more details of the F-105 intakes:

enter image description here

enter image description here

An adjustable strip can be seen in the last photo, providing some adaptability in intake characteristics.

A further link provided by @mins to this NACA document, where several inlet configurations are measured. The drawing above is from the report, which discusses several intake configurations researched pre-production and it does not provide any statements on variability of the geometry. From the abstract:

A 1:13-scale Republic F-105 wing-root inlet configuration in which inlet components were varied was tested...Inlet performance and engine-face flow distortions as affected by pitch, sideslip, inlet lip sweep, contraction ratio, boundary-layer control, and engine bypass are presented and discussed.

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  • $\begingroup$ The F-105 did have a variable-geometry intake: There was an adjustable ramp inside the intake duct. $\endgroup$ Dec 3 '21 at 4:17
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    $\begingroup$ See also NACA-RM-SL7A28: Wind-Tunnel Investigation of the Republic F-105 Inlet with Various Modifications at Mach 1.41, 1.81, and 2.01, from which previous diagram is extracted. $\endgroup$
    – mins
    Dec 3 '21 at 14:34

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