# What inspired the unique design of the F-105 Thunderchief intakes?

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?

• It was copied once: See the intake of the Crusader III Dec 3 '21 at 4:07

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.

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.