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I don't know of any bird that has a vertical tailfin, but apparently aircraft need them for lateral stability. Why is this?

I did realize, however, that most fish have a vertical tailfin, or a vertical dorsal fin, or both. What exactly is going on there? Can't see how marine creatures would need lateral stability at low speeds in a thick liquid (compared to air).

If that second paragraph sounds off-topic, let me put it another way. What is it about the air, as opposed to water, that requires this? They are both fluids. One is thicker/more viscous than the other, which seems to imply that the thicker one would require less lateral stability. Yet it seems to be the opposite. I hope this will be taken as a valid on-topic aerodynamics question.

(P.S. I'm leaving out stuff like the B-2 Spirit because it uses active dynamic adjustments to stabilize. Its form is inherently unstable.)

IMPORTANT EDIT: I should have pointed out the large difference in speed. Birds of course are slow. So were the early Wright flyers. Modern airliners fly much faster, around Mach 0.85. I haven't yet looked up Reynold's Numbers, but it certainly could be the case that the faster ones need lateral stability while the slower one's don't.

1 more clarification: When I say "stability", I'm not exactly talking about yaw control. My impression is, just when the plane is flying perfectly straight, there are pressure fluctuations that will cause slips unless there is a large vertical surface area to balance out those fluctuations. I could be wrong, and ultimately I'm trying to find out why most planes seem to need this large vertical area.

1 more: Here's an example model that seems synonymous: Let's say our airplane achieves yaw with butterfly flaps at wingtips, which are aligned with the center of mass (the flaps and CoM have the same height) so there's no side-effect torques along another axis other than the yaw axis (z axis). Let's assume no backup yaw control is necessary. This craft is meant to cruise at Mach 0.85. Is a vertical fin necessary to achieve passive directional stability about the yaw axis? We should probably consider cross-winds too, even at takeoff/landing.

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    You pretty much answered your own question with the PS: animals can continuously adjust their tail very rapidly... airplanes cannot. – abelenky Jan 27 '16 at 03:31
  • @abelenky If that's true then the irony is inescapable: birds and maybe even insects "invented" active dynamic control millions of years before Boeing? – DrZ214 Jan 27 '16 at 03:35
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    Nothing ironic about it: Engineers have long wanted totally dynamic, organic-mimicking control systems, but its only recently that sensors, actuators, and algorithms have gotten good enough. – abelenky Jan 27 '16 at 03:44
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    @DrZ214 What's ironic about that? Insects "invented" flight long before the Wright brothers. Fish "invented" swimming long before we built submarines. Primates "invented" intelligence long before we got anywhere with AI. – David Richerby Jan 27 '16 at 03:47
  • @DavidRicherby All of those sound ironic to me. Pretty much anything that requires advanced science and billions of dollars for us to create, yet is the easiest thing for nature even a million years ago, would be ironic in the way I understand that word. To each his own. – DrZ214 Jan 27 '16 at 04:10
  • Ask nature what you want to know. The science field is named biomimetics. – mins Jan 27 '16 at 06:42
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    Who says that Boeing invented active control? They avoided it like the plague before warming up to the idea. – Peter Kämpf Jan 27 '16 at 08:49
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    Fish typically have a vertical tail fin but marine mammals have horizontal tail fins. This is due to the mammalian spine having a design that bends with more power in the ventral/dorsal direction while fish have a spine that bends with most power in the lateral direction; the tail fin for both is the primary means of generating thrust, unlike an aircraft. The dorsal fin on both fish and mammals helps provide roll stability - an aircraft does this with its wings (fish and marine mamals typically have much smaller front fins so the dorsal fin assists here). – J... Jan 27 '16 at 12:18
  • @PeterKämpf If not Boeing, then idk who first did it. When were they avoiding it? 1960's, 1970'? From what I know about computers, I would also avoid it like the plague unless it was mid-80's bare minimum. – DrZ214 Jan 27 '16 at 23:38
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    @J... thanks for that info. So how would marine mammals do roll stability? I think the blue whale, for example doesn't have any vertical fins anywhere. Of course that's a huge beast so maybe it doesn't need much stability. – DrZ214 Jan 27 '16 at 23:42
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    @DrZ214 Some mammals do have a dorsal fin - like the dolphin or killer whale. The blue whale, porpoises, etc, don't, but they have much larger pectoral fins - rather like an airplane has wings, which it uses for roll control. – J... Jan 28 '16 at 00:29
  • @J... That part makes sense but aren't there also fish/mammals that have a vertical tail and a vertical dorsal fin? I cannot think of any names but there must be so many fish like that. What are their roles (lol) in that case? BTW are you from biology.SE? Welcome to the site. – DrZ214 Jan 28 '16 at 00:41
  • @DrZ214 I linked a relevant paper in a comment below Peter's answer - the dorsal can be used to assist with yaw control for quick turns and, in some fish, actually plays a role in propulsion (setting up constructive vortices that increase propulsive efficiency). And no, haven't set foot in bio SE. I'm a physicist and engineer, but I hail primarily from SO. – J... Jan 28 '16 at 00:48
  • The vertical tail (or horizontal in whales & dolphins) isn't for stability, it's for propulsion. For the rest, you answered your own question in your PS about active stability in the B2 - that's exactly what birds use, though with a much better implementation :-) – jamesqf Jul 28 '17 at 17:45

2 Answers2

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Not all of them need a fin:

Horten IV in flight

This is the Horten IV, a flying wing glider that did not need a fin (picture source). Instead, it used spoilers at the wingtips to create yawing moments, and the swept wing helped in improving its weak directional stability. It could afford to do so because it was a glider. The second prototype of a jet-powered flying wing, the Horten IX V2, suffered a crash when one engine failed and the pilot could not end the resulting spiral dive.

Fins on airplanes

Birds have two advantages over airplanes which make a fin superfluous:

  • A variable geometry wing, where wing span, sweep and incidence can be controlled for each side independently.
  • Propulsion is integrated into the wing, instead of separate power plants which might fail independently.

An extra advantage of birds over older airplane designs lacking a computerized control system is that the bird's brain is wired to adjust the wing on each side subconsciously and continuously. While a fin provides an aircraft with weathervane stability, a bird is inherently indifferent (neither stable nor unstable) and needs continuous tweaking of its wings for directional control.

If a bird wants to control yaw, it simply adjusts its wingtips such that the drag difference between both creates the desired moment. It can do this by reducing wingspan on one side, which also creates a rolling moment for a coordinated turn. Airplanes unfortunately cannot do the same.

Airplanes need positive directional stability because humans cannot adjust the controls continuously - they need to check maps or work the radio, or maybe just want to relax for a moment. Also, multi-engined designs need a stability margin to compensate the asymmetric thrust resulting from a failed engine. With a fin, the mass of the structure behind the center of gravity now needs a compensating mass ahead of it, and the side area of this forward fuselage is de-stabilizing the aircraft in yaw, requiring yet more area at the vertical tail. This is another problem that birds do not face.

B-52H 61-023 on 10.1.1964

The B-52H 61-023 suffered a structural failure of the fin on January 10, 1964, but managed to limp home with almost no lateral stability left. Had it suffered an additional engine failure, it would had crashed (picture source).

Fins on fish

The dorsal fin of fish helps in their propulsion by balancing the areas above and below their vertebral column. By wagging the tail, they create a side force which creates a sideslip condition at the dorsal fin, which in turn produces a correcting side force so the body does not roll while swimming. In this it can be compared to the keel of a sailboat. The hydrodynamic forces on a keel counteract the side force from the sail in crosswind conditions. Its main function is to stabilize the fish from the yawing motion caused by their tail fins, and this stabilization in turn improves the efficiency of the motion of the tail fin.

An additional function is as a roll sensor. Being located at the maximum girth, it gives the best readings of local sideways motion due to roll. This is the main function in marine mammals where the tail fins are horizontally oriented. In fish which have an adipose fin (located aft of the main fin), this fin is most likely their roll sensor.

It is not used for roll control (except as a sensor in a feedback loop), and cannot because it is not adjustable. Roll control is achieved by the adjustable pelvic fins. It also cannot provide roll stability - this would require it to roll the fish back to its initial attitude after a disturbance. All it can do for the roll movement is to dampen it a little and prevent it when the fin is moved sideways.

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  • Thanks for addressing both birds and fish. But I'm surprised at the answer. It seems like you're saying that airplanes don't really need vertical stabilizers for stability (from crosswind or side-slip), but rather that they need/want rudders either as the only yaw control or backup to other yaw control, and tails are just the most convenient rudder location. I was under the impression that the large surface area of the vertical stabilizer was needed to balance out some kind of random pressure fluctuation that comes from airflow over the fuselage. – DrZ214 Jan 27 '16 at 07:52
  • @DrZ214: No, I did not say that. I said that airplanes need positive directional stability, which makes them different from birds which don't, and that is achieved by the vertical. Now you have something hanging out at the back and need to balance it with something hanging out at the front, which needs more area in the back again. Without the need for a tail there would be little fuselage ahead of the cg, which creates instability and requires a fin. – Peter Kämpf Jan 27 '16 at 08:40
  • A variable geometry wing, where wing span, sweep and incidence can be controlled for each side independently. - Propulsion is integrated into the wing, instead of separate power plants which might fail independently. but suddenly, when speaking about wing terminations, birds are the example upon which airplanes should be built. – Federico Jan 27 '16 at 09:11
  • @DrZ214: The vertical stabilizer is not primarily needed for "stability" in the sense of steady-state stability. It is needed to turn. Before the Wright brothers, most gliders and model aircraft and kites were built without vertical stabilizers. And they flew well. Only, they were never intended to turn and was only flown in a straight line. Once people started tackling powered flight they discovered that for most designs turning an aircraft will cause it to crash (it will simply spiral uncontrollably into the ground)... – slebetman Jan 27 '16 at 09:34
  • ... of course, we know from the Horten brothers and Jack Northrop that it is possible to design planes without a vertical fin - just look at the B2. The key it turned out has been known among boat designers for a thousand years: your center of lateral resistance must be behind your center of effort. Or to translate it to airplane-speak: your center of drag must be behind your CG. The vertical fin is one way to achieve this. Split elevons is another (Northrop) and washout/twist is another (Horten) – slebetman Jan 27 '16 at 09:37
  • @DrZ214: Note that the effect crashing when trying to turn I mentioned above has a name: Adverse Yaw. The Wright brothers invented the rudder to solve it and we still teach pilots coordinated turns (another Wright invention) to this day to counter adverse yaw. Planes with extreme washout have negative adverse yaw - proverse yaw. Any aircraft design that have proverse yaw don't need a vertical tail. – slebetman Jan 27 '16 at 09:41
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    The dorsal fin on fish is for roll stability, primarily, not to balance thrust from the tail. In fact, some fish don't have dorsal fins at all (or only very small or undeveloped fins), despite being powerful swimmers. Furthermore, most cetaceans have dorsal fins and, like all marine mammals, have horizontally oriented tail fins and produce thrust in entirely orthogonal directions to fish. They still need the dorsal fin for roll stability, though (also for more powerful yaw in sharp turns). – J... Jan 27 '16 at 12:29
  • @J... and for what are the pelvic fins, if not for roll control? And what about seals, Belugas, or penguins, do they have a dorsal fin? All use their flippers for roll control. No, the dorsal fin counteracts the side force from the tail fin. Of course it helps in stabilizing roll, but it is more important in yaw, together with the anal fins. – Peter Kämpf Jan 27 '16 at 13:51
  • @slebetman The question is about the fin, not the rudder. Flying without a rudder is possible if ailerons (and, ideally, drag rudders) are installed. A rudder merely helps to enable coordinated movement around all three axes. – Peter Kämpf Jan 27 '16 at 13:57
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    @PeterKämpf Naturally each case is unique, but I won't yield the point that your statement in the answer is woefully incomplete and, in most cases, just wrong. Strong tail swimming fish often lack an appreciable dorsal fin - it is not to counteract lateral forces from the tail. Dolphins, on the other hand, have substantial dorsal fins and their tails don't move laterally at all. Where dorsals are absent, usually there are strong pectoral or ventral fins for roll control. Penguins have wings, belugas and seals have strong pectoral fins, etc. – J... Jan 27 '16 at 14:21
  • @Federico which is exactly why early aircraft designs mimicked birds, and why they failed miserably. As mentioned in the original answer, it's only been fairly recently that we've had the computer power to make the multiple, minute corrections that birds do naturally to enable stable flight. – FreeMan Jan 27 '16 at 14:27
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    @PeterKämpf For additional interest, for some fish (at least Teleostei), even use the dorsal fin for propulsion - it assists in creating vortices that react constructively with those from the tail fin to increase propulsive efficiency. This is not strictly in line with what you have suggested (lateral force stabilization), but it is closer to that type of function. This behaviour, naturally, is not universally true of all marine animals with dorsal fins, however. http://www.people.fas.harvard.edu/~glauder/reprints_unzipped/DruckerLauder2001b.pdf – J... Jan 27 '16 at 14:32
  • @PeterKämpf: A rudder is just an adjustable fin. The fin serves the same purpose as the rudder: to improve spiral stability. If all you need is to travel in a straight line then a dihedral is enough to give you stability. I've built lots of free-flight planes that way without a vertical fin and they all fly fine. The problem begins when you try to trim the planes to fly in a circle. An adjustable fin (a rudder) merely allows you to get away with using a smaller fin. – slebetman Jan 27 '16 at 14:52
  • @slebetman (and @PeterKampf) We need to distinguish between low-speed stuff (Wright Bros flyer, model planes) and modern high speed (Mach 0.85 for large commercial airliners). The faster one might require a vertical fin surface for stability (not yaw! not yet), and the slower one might not. I will edit the OP with that. Also Kapmf, Now you have something hanging out at the back and need to balance it with something hanging out at the front. I don't understand. What is hanging out the front, e.g., of a modern jet like a 747? – DrZ214 Jan 28 '16 at 00:01
  • @PeterKämpf The question is about the fin, not the rudder. Yes exactly, but I understand it's easy to get confused especially as we're considering fish, birds, slow aircraft, and fast aircraft. Let me rephrase something. Let's say our airplane achieves yaw with butterfly flaps at wingtips, which are perfectly aligned with the center of mass so there's no side-effect torques along another axis. Let's assume no backup yaw control is necessary. This craft is meant to cruise at Mach 0.85. Is a vertical fin necessary to achieve passive directional stability about the yaw axis? – DrZ214 Jan 28 '16 at 00:17
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    @DrZ214: Yes. Butterfly flaps would need to be actively moved in response to sideslip. At Mach 0.85 you need wing sweep; this alone gives reasonable directional stability if no forward fuselage is present. The fin is really to counteract the negative stability contribution of the forward fuselage (and to have margin for asymmetric thrust). Adding a rudder gives yaw control and the possibility to trim in yaw. – Peter Kämpf Jan 28 '16 at 01:32
  • @J... Where did I say the dorsal fin "balances thrust"? Don't read things into my answer which are not there. I edited it for clarity and will not retract the first sentence. If the fish body would move sideways in response to the tail movement (conservation of momentum; it's not my fault if biologists don't get this), the tail would be less effective. A tall body does the same; this is why most fish are much taller than wide. – Peter Kämpf Jan 28 '16 at 01:48
  • @PeterKämpf: If you have both butterfly flaps open you don't need active control response. Northrop's earlier mechanical planes had a feature to open both for "emergency stability". Granted this is a high drag configuration so if you can afford an active control system you'd prefer using it. Still, the B2 use this passive stable configurations at low speeds. – slebetman Jan 28 '16 at 01:48
  • @DrZ214: Below Mach 1 the rules for tailless aircraft are the same. I'm not sure what changes at mach 1. In your scenario, if the plane is stable at take-off speed with no vertical tail then it will be stable at Mach 0.85 with no vertical tail. Indeed, for some designs like the Hortens the weakness is actually low speed flight. At high speed Hortens are very, very stable. At low speed they can benefit from a vertical tail. – slebetman Jan 28 '16 at 01:52
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    @slebetman: Open butterfly flaps help only when wing sweep puts them behind the CG, a condition that was clearly ruled out ("which are perfectly aligned with the center of mass"). No, they need to be actively moved, a high drag configuration won't help (it does improve damping, though). – Peter Kämpf Jan 28 '16 at 02:01
  • @PeterKämpf Oops, what i meant was, perfectly aligned with the CoM on the lateral axis. In other words, the wing span runs right thru the CoM inside the fuselage, thus the butterfly flaps have the same height as the CoM. Sorry for the inaccurate wording. They could be ahead or behind the CoM as viewed in the xy plane. Behind makes more sense to me just because I know the old rule of thumb that the CoM must be ahead of the aerodynamic center for "weathervane stability". – DrZ214 Jan 28 '16 at 02:05
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    @DrZ214: That was my interpretation as well. Since that was what you intended my answer stands. As I said. The rule is that you simply need more lateral drag behind your CG than in front. Yes, it's like a weathervane. To the best of our current knowledge there are 3 techniques: put drag rudders (the butterfly flaps) behind the CG (Northrop solution). Put washout/twist behind the CG (Horten solution). Put something vertical behind the CG (Wright solution). Of course, nothing stops you from combining them if you still want to increase stability. – slebetman Jan 28 '16 at 05:29
  • @PeterKämpf You did not suggest that the dorsal balances thurst - you're suggesting a cooperative role with the tail and I provided a research paper that is as close to your proposed role for the dorsal fin as I could find. That said, you've several times misinterpreted my statements - I said the dorsal is for roll stability, not roll control. These are very different. As a roll stabilizer the fin reduces the tendency to roll by providing drag about that axis. Roll control implies that it is actively used to adjust the roll angle of the fish - naturally this is not the case. – J... Feb 01 '16 at 11:51
  • @J... So why did you speak of "roll stability" when you mean roll damping? Roll stability means the vehicle moves back to its old attitude after a disturbance. A static fin close to the center of gravity cannot do that, no matter what is written in some papers. All it can do for marine mammals is roll sensing and some roll damping. Please understand the terms first when criticizing things that are correct but seem strange to you. In fish, it helps in reducing the counter-movement of the body on top of the above, thereby increasing propulsive effectiveness. – Peter Kämpf Feb 01 '16 at 20:44
  • @PeterKämpf The term stabilizer does not necessarily imply a direct correlation to dynamic stability (in the sense you mean). In nautical systems it is common to use the term "stabilizer" when referring to a ship feature that provides damping. This even extends to things like gyroscopic stabilizers, which do absolutely nothing for dynamic stability, but provide only damping against rotation about whatever axis. I don't think my meaning was unclear. In any case, to be clear, yes I have always meant its action to be that of roll damping, as you say. – J... Feb 01 '16 at 21:09
  • @J... This being Aviation SE, I prefer to use the aeronautical nomenclature, where stability and damping cannot be exchanged, but have their own meaning. I was really pissed by your repeated assertion that my answer is flawed when in fact your comments were. – Peter Kämpf Feb 01 '16 at 21:12
  • @PeterKämpf I'm sorry you are upset - that wasn't my intent. In any case, the action is very different from a ship's keel - by virtue of its massive weight (and placement at the bottom of the boat), a keel very much is a dynamic stabilizer in the sense you mean (that it tends to combat roll and right the ship in the water). With the keel controlling roll, the ship's rudder provides the primary balancing force against the imbalanced thrust on the sails (yaw control). This is very different from fish in so many ways, and different from what you have suggested in your answer. – J... Feb 01 '16 at 21:20
  • @J You are right, the hydrostatic contribution of a keel is quite different. I guess I need to clarify the answer. However, when sailing close-hauled the keel sees quite some sideslip angle and creates considerable hydrodynamic forces. These are quite similar to the forces on a fin, and that was I was referring to. I was only thinking of dynamic forces. A keel and the rudder on boats are very much like a wing and an empennage on aircraft. – Peter Kämpf Feb 01 '16 at 22:03
  • @PeterKämpf Indeed. Another difference with fish is that the tail side forces, although imbalanced on a single stroke, always alternate with regular frequency - side motion from one stroke is balanced by an opposite motion on the reverse stroke. As you say, the tall and flat profile of the fish's body provides most of the resistance to this motion - particularly in fish, which generally have softer and lighter dorsal fins. Rigid cartilaginous fins, like on dolphins, are particularly ill explained here as they generate no side forces with their tails. They do still need roll damping, however. – J... Feb 02 '16 at 11:50
  • @J... Most mammalian (and all in case of penguins or seals) roll damping is provided by the flippers - they are larger and have a higher aspect ratio. Also, they allow active damping. The little extra from the fin is not really needed - roll damping is not its main purpose. In fish the fin helps in balancing the area above and below the vertebral column, and in this respect it helps against rolling caused by the sideways motion of the body when swimming. If you count this as helping in propulsion or aiding in stability is more a semantic than a scientific difference. – Peter Kämpf Feb 02 '16 at 13:56
  • @PeterKämpf So what would you say is the role of the dorsal fin in mammals that have them? Dolphins, Orca, etc? Again, they do not generate any lateral forces with their tails when swimming. The generally (scientifically) accepted functions in these animals is primarily for roll damping, with an auxilliary function for thermoregulation (they can flood the dorsal fin with blood to release heat, if needed) and, in some cases, to add yaw control for quick turning. I think if you have an alternative idea with merit it is probably worth publishing. – J... Feb 02 '16 at 14:25
  • The vertical fins of fish and sharks may have as much or more to do with propulsion than stability. – quiet flyer May 01 '21 at 18:58
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Not addressing the question as asked, but related: Birds have "active stability control" for lateral stability. Birds don't need a rudder for coordinated turns (like most airplanes do) because they have a more-or-less bell shaped lift distribution curve. See: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110023801.pdf This lift distribution results in proverse (as opposed to adverse) yaw. With other lift distributions the increase in lift on the rising wing in roll produces additional drag, resulting in adverse yaw (there are of course other ways to eliminate this). With the bell shaped lift distribution, the increase in lift on the rising wing reduces drag, while the decrease in lift on the falling wing increases drag. See http://www.nasa.gov/centers/armstrong/news/FactSheets/FS-106-AFRC.html for a modern unmanned aircraft demonstrating this.

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