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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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    $\begingroup$ You pretty much answered your own question with the PS: animals can continuously adjust their tail very rapidly... airplanes cannot. $\endgroup$ – abelenky Jan 27 '16 at 3:31
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    $\begingroup$ 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. $\endgroup$ – abelenky Jan 27 '16 at 3:44
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    $\begingroup$ @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. $\endgroup$ – David Richerby Jan 27 '16 at 3:47
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    $\begingroup$ Who says that Boeing invented active control? They avoided it like the plague before warming up to the idea. $\endgroup$ – Peter Kämpf Jan 27 '16 at 8:49
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    $\begingroup$ 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). $\endgroup$ – J... Jan 27 '16 at 12:18
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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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    $\begingroup$ 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). $\endgroup$ – J... Jan 27 '16 at 12:29
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    $\begingroup$ @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. $\endgroup$ – J... Jan 27 '16 at 14:21
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    $\begingroup$ @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. people.fas.harvard.edu/~glauder/reprints_unzipped/… $\endgroup$ – J... Jan 27 '16 at 14:32
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    $\begingroup$ @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. $\endgroup$ – Peter Kämpf Jan 28 '16 at 1:32
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    $\begingroup$ @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). $\endgroup$ – Peter Kämpf Jan 28 '16 at 2:01
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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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