Why does the MiG-15 have a cruciform tail?

Question

The horizontal stabilizers of the MiG-15 are halfway up the tail. This is called a cruciform tail.

Why did they put them there? Why not a T-tail or the normal tail where the stabilizers are on the fuselage?

Answer 1

The tail position is a compromise which is influenced by these factors:

• Interference with the wing's wake: The tail should be outside of the boundary layer flow coming off the wing root at all normal angles of attack. A high tail can cause a deep stall, something that was not known in 1947. Today all similar designs try to get the tail surface below the wing's wake.
• Lever arm: By mounting the tail higher up on the swept vertical tail, its lever arm (in horizontal direction) can be increased without lengthening the tailpipe. A longer tailpipe means more mass and more internal drag and is best avoided.
• Flutter: The higher the horizontal tail is along the vertical, the higher the danger of flutter. Here the longer lever arm (now in vertical direction) will lower the eigenfrequency of the bending mode of the vertical and the torsion mode of the rear fuselage. I suspect that for that reason the full T-tail was not chosen.

Mounting the horizontal smack in the middle of the tailpipe would require a heavy spar there to carry the bending moment across the fuselage. Mounting it on the vertical tail allows for a much lighter construction.

Answer 2

For a couple of reasons.

First the shaping of the aft fuselage does not offer the space to accommodate the structure for the tailplane and flight controls without adding on an additional blisters to provide space. The routing of the jet pipe further uses up space as compared to an F-86. See images below.

A low mounted tailplane would not have a clean airflow over it due to the MiG-15's middle mounted wing as opposed to the F-86's low wing design.

Answer 3

It’s a good question. I remember that during lectures on aircraft structures, it was stated that the horizontal tail should be mounted either at the fuselage or at the top of the vertical tail. Not in between, since that involves having to implement the downsides of both methods.

The case of the Bae 125 was mentioned: this was designed as a full T-tail, but during flight tests it turned out that additional vertical tail surface was required which was put on top of the existing structure.

Wikipedia mentions that the design of the MIG 15 was based on that of the Focke-Wulf TA 183 Huckebein, which was only built as a wind tunnel model implementing a serious T-tail, not a cruciform. It seems probable that development of the MIG 15 vertical tail had a similar history to that of the Bae 125.