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Why did the PB4Y patrol aircraft use a single fin while the B-24 it was derived from used twin fins? I have read that the twin fins were thought to be superior but this was realized to be false. Why did engineers believe the twin fins to be superior and what caused them to reconsider?

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Why did engineers believe the twin fins to be superior?

There doesn't seem to be any single definitive reason for selecting the number of vertical fins on an aircraft design (except, obviously, for the twin-boom designs such as the Lockheed P-38 or Fairchild C-119). I've seen all of the following cited by one authority or another as advantages of multiple vertical fins. (Sorry, I didn't save or remember the sources.)

  • Reduced hangar overhead requirement. This is the most common reason for including total aircraft height in a purchase specification, and it is a sine qua non requirement for carrier deployment.

    Also I remember reading a magazine article that supplied this reason for the triple tail on the Lockheed Constellation.

  • Reduced cantilever stress at the root. This may actually save weight over the single-fin design. It may also help keep a tactical aircraft from disintegrating under the enormous stress of combat manoeuvres.
  • Reduced parasite drag. The lower half of a vertical fin can be twice as thick as the upper half. A twin tail design effectively omits the lower half and uses the upper half twice.
  • For multi-engine designs, operating the rudders in the engine slipstream affords improved rudder authority at low speed. Pilots who taxi loaded bombers into takeoff position between other loaded bombers like lots and lots of rudder authority.

    Also I notice that practically all the big flying boats of the 1940's Flying Clipper era had multiple vertical fins. Those craft had some tricky low-speed steering requirements — they not only had to make their way through a busy harbor, but usually docked and undocked without a tugboat. I'm only guessing at the connection though.

  • A single vertical fin can be masked by the fuselage wake at high angles of attack. You can see how this could be a problem for modern fighter aircraft, which look like flying tennis courts. Traditional bomber designs with their broad wings and heavy aerodynamic loading can also generate quite a troublesome fuselage wake.
  • Improved position and field of fire for a tail gunner. I don't see it myself but I've heard the claim so I'm including it here.

    I concede that the tail gunner slot in the Avro appears to benefit from the absence of bracing structure.

  • Reduced rudder-induced roll due to a shorter rudder offset moment. Fast tactical military aircraft have short wings, and you don't want the rudder to extend further from the roll axis than the ailerons do. On the other hand, the rudder doesn't need to go all the way to the top of the fin, does it.
  • Twin fins can be canted to reduce radar return. They usually are, although I don't understand how the designers know where the hostile radar will be.
  • On the A10, the vertical fins actually mask the hot engine exhaust over a large angular area. Thus in order to aim a ground-to-air heat-seeker up the tailpipes of an A10 you have to be directly behind it. Which means that a few seconds ago you were directly in front of it, so good luck with that.
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  • $\begingroup$ Is hangar height such an issue for carriers? They already have folding wings, rotating the tail sound easier. No hinges in the middle, just rotate the whole assembly to one side. $\endgroup$ – MSalters Jan 1 '16 at 21:51
  • $\begingroup$ +1 for a solid analysis, but the last sentence takes the cake. $\endgroup$ – T.J.L. Mar 12 '18 at 20:42
  • $\begingroup$ One more... The AN-225 had a twin-fin design (compared to the AN-124 on which it was largely based) becaused the Soviet shuttle on top would've masked the original fin (it would not be effective in the turbulent air caused by the load). On the 747 which ferried the US Space Shuttle, the designers added secondary fins on the ends of thehorizontal stabiliser. $\endgroup$ – Anilv Mar 13 '18 at 1:07
  • $\begingroup$ @T.J.L.: Although it does assume that the A-10 had seen you while you were in front of it, which might not be the case if the MANPADder is well-hidden. $\endgroup$ – Sean Jun 1 '18 at 16:04
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The Consolidated B-24 Liberator was designed in something of a hurry. In fact, the Consolidated company had told the USAAF (who had asked them to produce B-17 under license) that they would have the aircraft ready in the same time it takes to adapt B-17 for production in the Consolidated plant.

As a result, the parts of design were 'borrowed' from other aircraft. The twin tails were taken from the Consolidated XP4Y Corregidor.

XP4Y

"XP4Y-1 in flight" by Unknown - This image is available from the United States Commons.

Apparently, the B-24 (which was Consolidated Model 32) used parts straight off from the XPFY (which was Consolidated Model 31), with many parts in vertical fin starting with the 31 prefix of the XPFY. Reportedly, some parts used were from other Consolidated Models, the PB2Y Coronado Flying boat and PBY Catalina.

B-24

Image from acepilots.com

More B-24s were produced than any other heavy bomber but it had its own share of problems. After sometime in service, the USAAF determined that the aircraft would have better stability if it had a single vertical tail and a production aircraft was modified (called XB-24K) for testing this. Tests showed the following improvements:

  • Enhanced lateral stability and improved handling characteristics.

  • The design was better able to cope up with two engines out on one side condition.

  • Increases in field of fire for the ball turret, waist guns and top and tail turrets.

The army was impressed enough that it recommended that single tail be used in all future aircraft, with the model number B-24N. The B-24N also included other improvements like the installation of spherical Emerson model 128 turret in the nose, modified (knife edge) windscreens etc.

B-24N

Image from edwards.af.mil

An order for over 5000 B-24Ns were placed, which were canceled later (in May 1945) as the war drew to an end and only a handful of B-24N with single vertical tails were made.

The PB4Y-2 Privateer was developed from the later models of B-24 and had the single vertical fin in its design as a result. Most of the PB4Y-2s were in fact, delivered after the WWII.

Privateer

"PB4Y-2 Privateer VP-23 in flight" by USN - Dictionary of American Naval Aviation Squadrons Volume 2: The History of VP, VPB, VP(H) and VP(AM) Squadrons 1; U.S. Navy National Museum of Naval Aviation photo No. 2011.003.156.015. Licensed under Public Domain via Commons.

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  • $\begingroup$ My answer may be "nice" but it doesn't really respond to Brinn's main question about the PB4Y-2. Your answer does, and should be accepted. $\endgroup$ – A. I. Breveleri Dec 31 '15 at 2:32
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The additional weight associated with control surface hardware like cables, hinges, a beefy horizontal, etc must be a consideration. The added complexity related to maintenance of the system might be another. On the other hand, battle damage that might completely obliterate a single vertical may only damage one of the two on a plane equipped with a pair of verticals.

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There are multiple reasons for twin vs single fins. In the world war 2 era, twin fins were used to help with engine out performance. The blue line speed on WW2 twins might be 50 mph faster than the take off speed, and even on four engine designs it was a major factor because bombers on take off were heavily loaded. Lose an engine and it would be a very close thing 4 engined or 2. Twin fins helped in this respect by lowering the blue line speed over single finned designs.

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Well, the double vertical stabilizer is still in use in many airplanes like the F-22, F-35, F-15, A-10, and the A300 Super Transporters as well as many other airplanes. So, there is not much "reconsideration" in aircraft design on twin or single stabilizers. The reasons for choosing one over the other is survivability (some military aircraft) or structural requirements compared to simplicity.

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    $\begingroup$ This does not appear to answer the actual question, twin vertical stabilizers are rare nowadays and seem to be pre-dominentaly on planes that don't support a single vertical stabilizer due to the airframe covering it (the mentioned Airbus Beluga for instance) $\endgroup$ – JustSid Dec 30 '15 at 3:00
  • $\begingroup$ @JustSid, also on the Shuttle Carrier due to drag and turbulence introduced by the Orbiter. $\endgroup$ – mins Dec 30 '15 at 13:43
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    $\begingroup$ @JustSid The last sentence does sort of answer the question, though it's not a particularly detailed answer. $\endgroup$ – reirab Dec 31 '15 at 9:12
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Twin fins/rudders reduce yawing from landing or taking off in a crosswind as the second one is blanked and the affected side area is smaller. Twin tails are in the prop wash of the engines on multi-engine aircraft and so are slightly more effective especially at low speeds, and can thus be smaller than fins/rudders that are not. Some aircraft used multiple tails to reduce their height so as to fit into smaller hangars such as the Lockheed Constellation. Twin tails used as endplates on the tailplane also increase the tailplane's efficiency so it can be smaller and produce less drag. As mentioned however - they are more complex, and in an engine out situation yaw control may be degraded slightly. The only aircraft where redundancy was claimed was with the A-10 and that claim is dubious as the rear fuselage remains a weak point. Similar aircraft would be highly dependent on how the controls are designed. The B-24 had snaking issues that could have been resolved by lengthening the fuselage slightly but they chose to go for a redesign.

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