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I would like to ask about the function of the lower vertical fin in Mig 23 .enter image description here

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    $\begingroup$ Maybe it's to let Russian pilots know when they're flying too low. $\endgroup$ – Howard Miller Mar 28 '16 at 10:44
  • $\begingroup$ note that other aircraft such as the F-16 has the same feature. $\endgroup$ – Manu H Oct 2 at 11:33
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Directional stability at supersonic speed.

The air above the aircraft is less dense at supersonic speed, and elasticity reduces the effectivity of the fin - it bends and twists from the air loads. Conversely, the lower fin operates in denser air and creates more side force per area.

Also, the lower fin shifts the aerodynamic center of the empennage downwards, which is improving the handling characteristics. Adding area below the centerline helps to reduce the roll contribution of the vertical tail in a sideslip, which improves handling characteristics (more directional stability AND lower yaw-induced roll).

As long as the vertical tail surface sits above the centerline (more precisely the longitudinal axis of inertia - for now we can assume both fall together), any side force will also create a rolling moment. This is undesirable because now a yaw command will not only create the intended yawing moment, but also a rolling moment.

If you want to yaw in order to turn, the vertical tail above the centerline will even roll the aircraft in the wrong direction, so you need more coordinated aileron command than with a symmetrical vertical tail.

The MiG-23 is not alone: Many supersonic combat aircraft use ventral fins. See the picture below for an example where the effects of additional vertical tail area are compared to the effects of a ventral fin of the same area.

comparison of fin and vertical tail area effectiveness

(Source: Ray Whitford: Fundamentals of fighter design)

At supersonic speeds, the stiff, low aspect ratio ventral fin has a nearly constant contribution to directional stability, whereas the wing-like vertical tail loses effectivity in proportion to the Prandtl-Glauert factor $\frac{1}{\sqrt{Ma^2-1}}$ because it bends and twists more due to its higher aspect ratio. The small ventral fin of the F-104 raised directional stability by 30% at Mach 2.

F-104 ventral fin contribution to directional stability

(Source: Ray Whitford: Fundamentals of fighter design)

At high angles of attack, the vertical tail is in the wake of the fuselage which reduces local dynamic pressure and, consequently, effectivity. The ventral fin then is in ideal flow conditions, so it can help to stabilize the aircraft at high angle of attack, just when the fuselage contribution to instability is largest.

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It is called called a ventral fin and is used to provide improved stability. A number of combat aircraft (like F-14, F-8, for example) have them. the image below shows the ventral fin in F-8.

F-8 ventral fin

Image from nasa.gov

Adding a ventral fin helps in increasing directional stability (especially at supersonic speeds) and reduces yaw induced roll (as the roll contribution of vertical fin is reduced in a sideslip). This improves the handling qualities. Another advantage is that they retain their effectiveness at high angles of attack, while the vertical fin may be blanked by the main wing.

The ventral fins in Mig 23 were quite big, which necessiated folding them when landing.

Mig 23

MiG-23 with wing flaps lowered, landing gear deployed, and ventral fin folded aside as it approaches to land; image from aerospaceweb.org

This is not limited to Mig-23; Vought XF8U-3 'Super Crusader' also had quite large ventral fins, which required folding.

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That piece of tail under the belly made it easier for the MiG-23 to be stabilised in high speed conditions. That part would be folded away for ground operations if you are also curious about that.

Source

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Reduce the amount of the side sliping when jet bank. F16 have that.

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