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enter image description here

What is the name of this fin?

What are these two fins used for?

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  • $\begingroup$ What aircraft is this? $\endgroup$ Oct 16, 2022 at 16:16
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    $\begingroup$ @MichaelHall, JF-17 $\endgroup$
    – user366312
    Oct 16, 2022 at 16:30
  • $\begingroup$ Where did you get the picture from? It's considered polite to indicate your image sources (actually, it's required for copyright reasons). $\endgroup$
    – FreeMan
    Oct 18, 2022 at 17:56
  • $\begingroup$ @FreeMan, I lost the source. Sorry for that! $\endgroup$
    – user366312
    Oct 18, 2022 at 17:59
  • $\begingroup$ That's why it's appropriate to include it when you ask. ;) $\endgroup$
    – FreeMan
    Oct 18, 2022 at 18:04

2 Answers 2

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They are called ventral fins and can be seen as a downward extension of the vertical stabiliser(s). And like any vertical stabiliser(s), their main goal is to provide yaw stability.

They can normally be found on fighters, mainly for the following reasons:

  • if the aircraft is embarked on aircraft carriers, the low height of storage decks and elevators limits the height a vertical stabiliser can have. That's also why many embarked fighters have two short main vertical stabilisers instead of one single big one.

  • at high AoA, the wing can aerodynamically shadow the main vertical stabiliser(s) while the ventral fins remain effective.

  • in respect to the main vertical stabiliser(s), vertical fins have a "squatter" shape (i.e. lower aspect ratio) which if on one hand gives a lower $C_{l_{max}}$, on the other hand gives also a smoother stall and at an higher AoA (and that's why, in general, tailplanes have a small aspect ratio).

Fins don't have to be confused with strakes:

F-18 strake

Strakes resemble fins but they are normally smaller and added after the first flight in order to "correct" small aerodynamic issues which couldn't be foreseen at design phase. In this particular case (red circle on the F-18), they have been added to divert turbolent airflow detaching from the leading edge at high AoA: without strakes this turbolent airflow impinged on the vertical stabiliser generating vibrations.

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  • $\begingroup$ BTW, I promise I'm not being petty, as I appreciate you pointing out my mistaken use of "longitudinal"... But after re-reading your answer, you are similarly mistaking lateral stability for vertical stability. Vertical tail strakes that reduce rudder/vertical stabilizer height would provide vertical stability (which by by definition is yaw stability about the vertical axis), not lateral stability (which is by definition pitch stability about the lateral axis.) And the ventral stability fins on many aircraft are formally called strakes, not just the small "airflow correctors." $\endgroup$
    – Max R
    Oct 16, 2022 at 14:15
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    $\begingroup$ @Max: don't worry, I'm not Einstein nor Kelly Johnson :) With longitudinal I mean pitch and with vertical I mean yaw. But I understand that they might be mistaken and I'm going to correct my answer according to your comment. Formally I don't know, but technically strakes are small surface's added after the first flight in order to correct small aerodynamic issues which couldn't be foreseen at design phase. On the other hand, fins are stabilising surfaces which have been there since the aircraft was still only a model on a computer (or on paper) $\endgroup$
    – sophit
    Oct 16, 2022 at 15:12
  • $\begingroup$ @Max: corrected, thanks for your comment 🖖 Btw roll stability is lateral ;) $\endgroup$
    – sophit
    Oct 16, 2022 at 16:16
  • $\begingroup$ Your Einstein/Johnson answer is the EVER. But, your comment is not correct, LOL! The Lateral Axis runs wingtip to wingtip. The vertical axis is vertical through the plane's CG. The longitudinal axis is tip to tail. Stability is referenced to moments about an axis. By definition lateral stability is pitch stability about the lateral axis. By definition, vertical stability is yaw stability about the vertical axis. By definition, longitudinal stability is roll stability about the longitudinal axis. Illustration: pilotinstitute.com/airplane-stability $\endgroup$
    – Max R
    Oct 16, 2022 at 16:31
  • $\begingroup$ I think I messed things big time :) That's why in my updated answer any reference to lateral/vertical/longitudinal is now gone $\endgroup$
    – sophit
    Oct 16, 2022 at 17:14
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They’re often called ventral strakes, or simply ventral fins. In some high performance fighter aircraft, the Center of Pressure (CP) is located very close to, or even behind the Center of Gravity (CG), to intentionally create a very small, or sometimes even negative pitch stability. This CP placement allows for a lot of very aggressive pitch maneuverability, often coupled with very large elevators, and often stabilators. However, this CP placement also introduces a couple of challenges.

  • It can introduce some undesired nose-down moments in some flight attitudes or maneuvers. This is especially true if the wing is shielding the horizontal tail, which is exacerbated by large stores on wing pylons.

  • It makes the aircraft less stable around it’s vertical (yaw) axis as well. Yaw can be introduced by the asymmetric drag as weapons on either side of the airplane are fired from their wing stores, as well as adverse yaw from large aileron deflections during brisk maneuvering.

Designing in more vertical axis (yaw) stability can involve undesired trade offs… Moving weight forward, a taller vertical tail and larger rudder, or procedural constraints to weapons deployment symmetry.

Another approach is to use strakes.

  • Vertical strakes such as those shown move the vertical axis CP (center of pressure) further back on the aircraft. They also increase spin resistance when the vertical tail is shielded in high-AOA, high-G maneuvers.

  • Angled strakes (think F16 but also many business jets) create both vertical (yaw) stability and lateral (pitch) stability.

Usually when you see strakes it means that wind tunnel testing revealed some undesirable yaw or pitch characteristics in certain configurations or maneuvers that the designers needed to damp out.

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  • $\begingroup$ I think you are confusing lateral stability with longitudinal stability. Ventral fins improve lateral stability, not longitudinal. Plus, strakes are something totally different than the vertical fins in the picture. So except the very first line, the rest of the answer should be updated. $\endgroup$
    – sophit
    Oct 11, 2022 at 10:34
  • $\begingroup$ I changed my answer for clarity, thanks Sophit. Strakes are not definitionally lateral axis devices, a vertical fin as shown would also be called a strake even though positioned differently than on the F16, for instance. I used “longitudinal” incorrectly, but in the case of the photo, lateral is also incorrect, they provide vertical stability (yaw stability about the vertical axis). $\endgroup$
    – Max R
    Oct 11, 2022 at 13:48

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