He means the ventral stabilisers.
Ask anything Thread (Air Force)
- Thread starter Jeff Head
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Per ficker22, those are ventral vanes, aka ventral strakes, or entral fins. They don't move or actuate and is there for increasing aerodynamic lift.
The F-16 also has them:
Well Killuminati said the part they were referring to was horizontal, and that made me wonder if they were referring to something else, but your explanation regarding the ventral fins is incorrect.
These fins are there to provide yawing stability under high angles of attack. When pitching up aggressively, some vertical stabilizer designs aren't enough to keep the plane directionally stable, because of pressure loss and flow separation due to the proximity of the wing with respect to the vertical stabilizer. The bottom part of the plane would still have good air flow and very high pressure, so these fins take over from the vertical stabilizer to prevent the plane from yawing uncontrollably.
The designers could make the vertical stabilizer larger to overcome this issue, but this would introduce other issues such as alter the aerodynamic center, become too elastic, negatively impact rolling performance or even prevent the plane from being usable in certain storage facilities such as storage decks inside carriers. The J-8 has a single but larger fin in the center of the fuselage, but that one is large enough to make contact with the runway, so it has to fold during landing and take off, so having two smaller ones is a better choice.
EDIT: There are other ways these fins help increase stability, but that would turn my reply into an essay, so just do more research on its effects if you are interested. It has benefits for supersonic regimes as well as other effects regarding alteration of flight dynamics.
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