Those leg fins on the VF-1

[justvinnie]

I have no idea what the hell they are called since my interest in aviation is nil. But you all know what I'm talking about, right? You can see those black and yellow fins on Skull-1 move when Hikaru has finished entering the atmosphere and initiates propulsion in Ep. 27 Love Drifts Away.

This actually makes sense. Those things would burn up and rip off if undue atmospheric pressure was to be placed on them, so they move to be more aerodynamic in relation to the craft re-entry.

vinnie

[the white drew carey]

I think they're called ventrals. But I could be wrong.

[ewilen]

They're called "ventral fins" according to the Compendium. I see from the Animeigo Liner Notes that Kawamori worked as "Assistant Animation Director" and that Star Pro (who is apparently the real source of many animation errors--whether or not they were part of Animefriends) had nothing to do with the episode.

So you're probably right: the ventral fins are capable of some movement, though whether they only retract for re-entry, or actually act as some kind of active control surface, is unclear.

[Aurel Tristen]

I have no idea what the hell they are called since my interest in aviation is nil. But you all know what I'm talking about, right? You can see those black and yellow fins on Skull-1 move when Hikaru has finished entering the atmosphere and initiates propulsion in Ep. 27 Love Drifts Away.

This actually makes sense. Those things would burn up and rip off if undue atmospheric pressure was to be placed on them, so they move to be more aerodynamic in relation to the craft re-entry.

vinnie

"fins" or "ventral fins" are movable, yes - - mainly for FAST Pack loading

[David Hingtgen]

Real ventral fins don't move. Full aerodynamic explanation available upon request.

(IMHO VF-1's wouldn't need them, but it's possible) VF-1's only have them because F-14's do.

[ewilen]

Real ventral fins don't move. Full aerodynamic explanation available upon request.

Please do--this is the perfect place for it! (Just be sure to save it so we don't have another "lost FSW discourse".)

[David Hingtgen]

Here goes:

Ventral fins are basically "extra" vertical stabilizers, but mounted below instead of above, and they never actually move nor have a rudder attached. There are several reasons you may have ventral fins: (their purpose is for yaw stability--which on most planes is handled soley by the vertical stab--but sometimes the v.stab isn't enough)

1. Can't make the v.stabs big enough without causing a problem. Fairly common on single-finned carrier planes----if a plane needs a 20-foot tall stab, but the hangar deck's only 15 feet tall, you've got a problem. So you make it as tall as you can, and make up the difference by putting ventral fins below. (there's no problem with having a shorter rudder---you can just increase the size or double-hinge it). You generally go with 2 ventral fins, because if you only had one it'd probably have the opposite problem as the fin up top--too big down below, and scrape the ground.

2. High angle of attack/high alpha. Yes, alpha is everything to all aircraft--you cannot fly at zero alpha FYI. Anyways, (and this is THE reason the F-16 has them) at high-alpha with the nose high, the fuselage is almost certainly going to be blocking a lot of the air from reaching the v.stab, or the v.stab will get "stagnant" air, thus you've got a serious loss of yaw stability, in addition to your already probably marginal stability overall at high alpha. So what to do? Add ventral fins. Being underneath, they won't ever have their airflow obstructed at high angles of attack, and in fact usually get their best flow under those conditions.

3. F-14's: due to their widely spaced engines, they need a LOT of fin area to maintain yaw stability incase of an engine failure. The almost-final F-14 design had one BIG fin, and 2 folding/retractable (very much like you see in the aforemention Macross ep) ventral fins, also very large. But as you can imagine, it looked kinda funny, and had "fitting in a carrier hangar deck" problems---would probably have had to fold the v.stab too, if they went with it. But that's a lot of moving parts and mechanisms, and just didn't look right. It was a lot easier to just go to 2 medium-large v.stabs, and 2 smaller non-folding ventral fins, to get the required area. High-alpha may also have an influence here, but my guess is the widely-spaced engines are by far the main reason, especially in light of speed restrictions on Super Tomcats. (Tomcat has as much fin area as it can, would need a third central ventral fin or something to get any more---ventral fins almost scrape the ground the way it is)

4. VF-1: well, it too has widely spaced engines, but with all its verniers etc I don't think it'd need ventral fins. But they can't hurt, and they look cool. (the REAL reason is F-14's have them---VF-1 ventrals look identical to F-14's, which are like no other plane's)

5. Why don't F-15's have them? Really tall v.stabs (no high-alpha probs), and really close together engines (no engine-out yaw probs). The original F-15 design did have ventrals though---when they were removed, they made the v.stabs taller. You'll also note the F-15's v.stabs are mounted on booms extending from the rear fuselage, and are actually aft of the engines--this also helps, being further from the fuselage. Similar for the Hornet (close engines), plus the Hornet's stabs cant outwards and thus aren't blocked at high-alpha, thus they needn't be very tall (they have a lot of area though, their chord is almost as great as their height--Hornet v.stabs are frankly mounted in a weird position, far forward--most planes couldn't have that much chord in a v.stab--exact opposite approach from the F-15).

Edited March 20, 2004 by David Hingtgen

[Aztek]

Jesus David, I got a chub just reading that. I'm going to go tweak my CG ventrals now.

[Hurricane29]

Great explanation as always David.

[Skull Leader]

Damn.... now I need a towel

Pretty darn impressive!

[Demons7th_Skull]

the ventral fins must be retractable, because if they're not , u cant add the GBP armor, that make sense....... i think

[Greyryder]

the ventral fins must be retractable, because if they're not , u cant add the GBP armor, that make sense....... i think

That much would make sense. I know the YF-71 had retractable ventral fins. It couldn't take off or land with them extended, but the nose of the plane caused soem weird aerodynamic effects that necessitated them for regular flight.

That plane never made into production, since the military decided that they really didn't need a fighter that break mach 3. The more streamlined nose on the A-12/SR-71 didn't cause any instability problems, so those planes didn't need the fins. (yes, the SR-71 started out as a fighter prototype)

[David Hingtgen]

YF-12, not YF-71. (Just a nit-pick, they're all Blackbirds)

And it could fold to the side, not retract. (I know, I'm being nitpicky again)

Anyways---the reason for the YF-12 needing the ventral fins was because the tips of the chines on the nose were cut away for the radar. A-12's and SR-71's have full-length chines, to the very tip of the nose. When the YF-12's chine was cut away just that little bit, it needed no less than 3 ventral fins to replace the loss of stability. The Blackbird design really needs that chine on the nose. While the Blackbird's chines do help with stealth, their main function is the overall stability of the aircraft. Smaller versions can be seen on the F-16.

[Greyryder]

I'm terrible with numbers...

I think "retract" was the way I heard it phrased, but it was a long time ago, too. I figured you'd correct any mistakes I made in that post. It's long been obvious to me that you know way more about this, than I.

I do rather wonder if the VF-1 wouldn't really need those fins. It's V-stabs are splayed out to function like a V-tail, but they always seemed a little small to me to give a lot stability. The ventral fins have a similar angle. Seems like the head and shoulders under the plane might cause airflow problems that would need extra surfaces to help counter. (Jeez, was that poorly worded!) But then, with fly by wire they can make just about any shape fly.

[JELEINEN]

YF-12, not YF-71. (Just a nit-pick, they're all Blackbirds)

And it could fold to the side, not retract. (I know, I'm being nitpicky again)

Anyways---the reason for the YF-12 needing the ventral fins was because the tips of the chines on the nose were cut away for the radar. A-12's and SR-71's have full-length chines, to the very tip of the nose. When the YF-12's chine was cut away just that little bit, it needed no less than 3 ventral fins to replace the loss of stability. The Blackbird design really needs that chine on the nose. While the Blackbird's chines do help with stealth, their main function is the overall stability of the aircraft. Smaller versions can be seen on the F-16.

Chine?

[David Hingtgen]

Boy, that's hard to describe. A chine is a chine. Umm, how about:

"Thin, flattened section of structure attached/blended the fuselage, along the sides/nose".

Basically, look at an SR-71 head-on. Now remove the fuselage itself from the center, You're left with the "edges". The chines. F-16 is similar, they go from just aft of the radome to the wing-root. (Not quite LEX's like a Hornet, but close).

BTW---lack of *yaw* stability can't be countered with FBW. F-16 has very large ventral fins, despite being probably the best example of a plane with FBW.

http://www.sr-71.org/photogallery/blackbir...976-2001-08.htm Here--start at the nose-probe, and head right across. That's the chine, heading right and slightly down, until it gets to the wing. It's the line separating the lit and shadowed parts.

Or here http://www.sr-71.org/photogallery/blackbir...976-2002-01.jpg start at nose-probe, and head left. Again, separates the lit and shadowed areas. You can't really draw a line where the fuselage stops and the chine starts. It's just the "edge". Since most planes have nice round fuselage sides, this is the exact opposite.

[Stamen0083]

"Thin, flattened section of structure attached/blended the fuselage, along the sides/nose".

Hmm... Looking at the VF-0S model, looks like that thing has a very small chine too.

[Anubis]

During reentry:

After:

[David Hingtgen]

Yeah, you could say the VF-0 has a chine. And depending on how the animator is feeling that day, maybe the -19 and -21 too. But it's subtle/rounded enough I don't know if it'd really do anything. Sharper is better, basically. I don't really know how "sharp" the VF-0's is.

But the YF-23's is fairly subtle (small, but razor-thin), and it sure works. (You'll find chines are rather stealthy--in addition to the SR-71 and YF-23, the F-16 has a noticeably smaller RCS than the F-18, and most other fighters)

Low quality, but perfect pic of a YF-23's chines:

Edited April 13, 2004 by David Hingtgen