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Aircraft like the Tu-154 and VC-10 have a "point" on the T-tail, while others like the 727 do not. Is this structure a shock body, or otherwise related to the tail's aerodynamics? And why is it only found on some designs?

Tu-154 Tu-154 Tails
VC-10 VC-10
727 727

zymhan
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    I am somewhat confident this was already discussed, but the search being what it is I can't find it now. – Jan Hudec May 17 '20 at 20:48
  • @JanHudec I had hoped I could find a post (here or elsewhere) discussing the design but I came up empty. I may be using the wrong terms to search with, though. – zymhan May 17 '20 at 21:41
  • Related: https://aviation.stackexchange.com/questions/56900/what-is-the-aerodynamic-purpose-of-an-acorn/ https://aviation.stackexchange.com/questions/26234/what-are-the-spiky-things-on-the-back-of-the-antonov-225-vertical-stabilizers/ https://aviation.stackexchange.com/questions/22237/what-is-the-teardrop-shaped-appendage-on-the-t-6-texan/ – user-781943 May 18 '20 at 04:18
  • @gszavae, the first one is basically duplicate (but now there is an answer here that's better than the accepted one there), the second is somewhat related, the third is not related at all. – Jan Hudec May 18 '20 at 05:38
  • @JanHudec third answer may be interesting to people re: niels nielsen's answer. – user-781943 May 18 '20 at 05:43
  • @gszavae, but its very different. The thing on Texan is really just an antenna housing, and is much smaller. The bullet fairings are never added just for sake of housing antennas, but always to reduce interference and/or compression drag and then possibly utilized for antennas since they need to be there anyway. – Jan Hudec May 18 '20 at 05:47
  • @JanHudec I understand, just thought it would be an interesting example for anyone reading niels nielsen's answer, and it's another example of a bullet fairing in a more general sense. – user-781943 May 18 '20 at 05:49
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    Does this answer your question? What is the aerodynamic purpose of an Acorn? – Peter Schilling May 18 '20 at 06:58
  • It does! I have never encountered the term "acorn" before so I missed that post. I like John's answer on this post a bit more though, he went deeper in to the aerodynamics. – zymhan May 18 '20 at 14:33

2 Answers2

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It's a "bullet fairing". Sometimes you get the pressure fields of the vertical and horizontal surfaces in alignment so the lowest pressure zone of the vertical fin is merged with the lowest pressure zone of the horizontal tail right at the intersection of the surfaces. It results in a more extreme pressure recovery aft and can cause flow separation and vibration, and sometimes shock wave effects at high speed.

The bullet effectively moves the pressure field of the vertical surface forward to bring it out of alignment with the one of the horizontal surface. It's usually an add on resulting from interference issues that are discovered in the wind tunnel, or during flight testing.

John K
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In some aircraft, that protruding spike houses the high frequency comm antenna and its tuning mechanism. In others it contains parts of the elevator angle-of-attack control mechanism.

niels nielsen
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    Neither is reason for adding it though. They are just uses for the space when it is available. – Jan Hudec May 17 '20 at 20:43
  • So, the space isn't designed on purpose, engineers just might put stuff in it "when it is available"? (Because sometimes it is available, and other times, not?) That doesn't make any sense... – Michael Hall May 18 '20 at 03:18
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    @MichaelHall, yes, engineers just put stuff in wherever it fits. They decide on what volume and weight they want for the intended mission, then design the shape to be aerodynamically efficient and then lay out the stuff where it fits. So if they add a bullet fairing for aerodynamic reason, they'll take advantage of it to put some communication equipment there, but if not, they'll stuff it somewhere else. The antenna will probably still go in the tail, but it's not as big and the electronics will just go somewhere else, connected with suitable shielded cable. – Jan Hudec May 18 '20 at 05:43
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    @MichaelHall you misunderstood. That protrusion is designed for a specific purpose, but that purpose is not to actually create some space, but rather to move a pressure field. Once that solution to move a pressure field was implemented and created some space as a side-effect engineers next determined what to put into that space now that it exists. – Dreamer May 18 '20 at 09:41