Why did so many WW2 Planes have twin tails?

gruad · Mar 21, 2022

Why did so many WW2 Planes have twin tails/stabilizers?

I can see on the Lancaster, Halifax and Me110 that you wouldn't want your mid upper gunner to have an attacker hiding behind a fuselage based tail.

But a single fuselage tail must be stronger and have less vulnerable controls and gives a rear gunner no disadvantages.

Would the twin tail give redundancy? If one was shot off, could you bring the plane home?

As usual I would be very interested to hear your answers.

buffnut453 · Mar 21, 2022

There may be a couple of reasons.
- Turbulence formed by the canopy/fuselage may have reduced the effectiveness of a single fin/rudder installation.
- Blanking of the fin by the fuselage at high angles of attack, again reducing fin/rudder effectiveness.
- To replace a twin rudder configuration with a single rudder, the latter must be large enough to provide the same amount of authority. A larger rudder requires more force to displace it, which is challenging on a multi-engined aircraft...especially if you've lost one or two engines on one wing and are constantly using the rudder to prevent the aircraft from yawing.
- A large single fin does hinder not only gun operation but also visibility for a mid-upper or rear-cockpit gunner.

There's no single answer as to "why" but lots of possible reasons depending on the design.

don4331 · Mar 21, 2022

gruad said:
Why did so many WW2 Planes have twin tails/stabilizers?

As usual I would be very interested to hear your answers.

I assume you mean fin/rudder; not horizontal stabilizer or 2nd tail.

The twin fin/rudder are directly in the line of the thrust from the engines in twin - at least in theory. As the air has been accelerated by the engine, you need 2 much smaller units than a single larger one. In theory, your smaller ones create less drag.

But there is risk - with engine out, the plane might yaw and the fuselage might blank one of the units. And suddenly, you only have a single very small unit to try to control your airplane.

As you have noted, a single tail blanks directly behind, but twin tails blank quartering attacks from both sides.

Howard Gibson · Mar 27, 2022

gruad said:
Why did so many WW2 Planes have twin tails/stabilizers?

I can see on the Lancaster, Halifax and Me110 that you wouldn't want your mid upper gunner to have an attacker hiding behind a fuselage based tail.

But a single fuselage tail must be stronger and have less vulnerable controls and gives a rear gunner no disadvantages.

Would the twin tail give redundancy? If one was shot off, could you bring the plane home?

As usual I would be very interested to hear your answers.

I certainly would not ignore the need for a field of fire for dorsal gunners, as per the Leo-45. Airliners were being converted into bombers, so this may apply there. The Lockheed Electras look to me like a good example of this, but I think Clarence Kelly Johnson implemented them to solve some aerodynamic problems.

I believe that the Lockheed Constellation's triple tail kept the vertical stabilisers lower so that they fit into hangars.

FLYBOYJ · Mar 27, 2022

don4331 said:
I assume you mean fin/rudder; not horizontal stabilizer or 2nd tail.

The twin fin/rudder are directly in the line of the thrust from the engines in twin - at least in theory. As the air has been accelerated by the engine, you need 2 much smaller units than a single larger one. In theory, your smaller ones create less drag.

But there is risk - with engine out, the plane might yaw and the fuselage might blank one of the units. And suddenly, you only have a single very small unit to try to control your airplane.

As you have noted, a single tail blanks directly behind, but twin tails blank quartering attacks from both sides.

Might? It will and this is common on all twins, regardless of the tail configuration. Manufacturers consider engine out performance and procedures to deal with this during flight testing. During WW2 specialized multi-engine training (especially for twins) wasn't taken seriously until you had low time pilots killing themselves because they did not know what to do if they lost an engine, especially on take off.

Howard Gibson said:
I certainly would not ignore the need for a field of fire for dorsal gunners, as per the Leo-45. Airliners were being converted into bombers, so this may apply there. The Lockheed Electras look to me like a good example of this, but I think Clarence Kelly Johnson implemented them to solve some aerodynamic problems.

I believe that the Lockheed Constellation's triple tail kept the vertical stabilisers lower so that they fit into hangars.

This was a requirement from Howard Hughes when purchasing Constellations for TWA

GrauGeist · Mar 27, 2022

gruad said:
Would the twin tail give redundancy? If one was shot off, could you bring the plane home?

Yes, as long as the aircraft didn't have extensive damage to other areas that would compromise the remaining vertical stabilizer/rudder ability to provide control.

To give an example of what it would take for a single tail structure to accomplish comparable control to that of a dual rudder/vert. stab. configuration, compare the B-24 to it's naval version, the PB4Y.

don4331 · Mar 27, 2022

FLYBOYJ said:
Might? It will and this is common on all twins, regardless of the tail configuration.

I said might, because someone on the forum here would point out that the P-38/P-61/Fw.189 don't have fuselage as such.

And yes, light twin still have one of the worst safety records: When the one engine quits just after takeoff and the trees are looming, it's hard to remember to pull throttle off on the good engine, feed in trim, and then re-apply power to the good engine.

FLYBOYJ · Mar 27, 2022

don4331 said:
I said might, because someone on the forum here would point out that the P-38/P-61/Fw.189 don't have fuselage as such.

Well that's incorrect. Each of these aircraft will still yaw adversely during an engine out, especially during take off

don4331 said:
And yes, light twin still have one of the worst safety records: When the one engine quits just after takeoff and the trees are looming, it's hard to remember to pull throttle off on the good engine, feed in trim, and then re-apply power to the good engine.

Training, training, training - with enough training (and maintaining proficiency) this becomes second nature.

EAIAnalog · Mar 28, 2022

A good example of the need for a triple tail is the 747 Shuttle Carrier. With the Shuttle mounted, the vertical stabilizer is really screened, so outboard stabilizers were needed. Another example is the Boeing Model 314 flying boat, with its triple tail. The original design had a standard tail configuration. It was rolled out from the Renton plant and the test pilot flew it up Lake Washington and back down to Renton. He disembarked and reportedly said something like "I have no yaw control. Fix it.".

FLYBOYJ · Mar 28, 2022

EAIAnalog said:
A good example of the need for a triple tail is the 747 Shuttle Carrier. With the Shuttle mounted, the vertical stabilizer is really screened, so outboard stabilizers were needed. Another example is the Boeing Model 314 flying boat, with its triple tail. The original design had a standard tail configuration. It was rolled out from the Renton plant and the test pilot flew it up Lake Washington and back down to Renton. He disembarked and reportedly said something like "I have no yaw control. Fix it.".

On seaplanes and flying boats, you also need additional directional control through the vertical axis. For example, many aircraft will add a fin on the lower empennage. The DHC Twin Otter with the Wipaire STC uses vertical fins mounted on the elevators. I think Viking Aircraft offers this option on their production -400s.

GrauGeist · Mar 28, 2022

Martin's JRM was the opposite.
The prototype had the "twin fin" configuration and it's handling was terrible, so the empennage was replaced by a single vert. stab. set up.

Col Campbell · Mar 28, 2022

I couldn't resist -- because two pieces of tail are better than one!

Jim

gruad · Mar 28, 2022

GrauGeist said:
Yes, as long as the aircraft didn't have extensive damage to other areas that would compromise the remaining vertical stabilizer/rudder ability to provide control.

To give an example of what it would take for a single tail structure to accomplish comparable control to that of a dual rudder/vert. stab. configuration, compare the B-24 to it's naval version, the PB4Y.

Thanks. I believe the final version of the Liberator was to have a single tail fin, but I am not sure if it was ever produced.

swampyankee · Mar 28, 2022

gruad said:
Thanks. I believe the final version of the Liberator was to have a single tail fin, but I am not sure if it was ever produced.

B-24N. The naval version (derivative?) of the B-24, the PB4Y-2, was produced in fairly large numbers. This was done to improve handling at low altitudes.

Corrected. Thank you, GrauGeist

pbehn · Mar 28, 2022

Both the Lancaster and Halifax had twin tails but initially both were supposed to be twin engined designs powered by the Vulture or Sabre. The Halifax was changed at the design stage to a four engined design while the Manchester became the Lancaster, it had three "tails" the centre having no control surfaces. Edit, I have no idea what the original "Halifax" was supposed to be like, I just presume it was twin tailed so the control surfaces were behind the engines.

GrauGeist · Mar 28, 2022

swampyankee said:
B-24N. The naval version (derivative?) of the B-24, the PB2Y-2, was produced in fairly large numbers. This was done to improve handling at low altitudes

Just a quick note:
The PB2Y was Consolidated's flying boat (which had the same tail as the XPB2M, by the way).
The navy's B-24 was the PB4Y.

swampyankee · Mar 28, 2022

GrauGeist said:
Just a quick note:
The PB2Y was Consolidated's flying boat (which had the same tail as the XPB2M, by the way).
The navy's B-24 was the PB4Y.

Thanks. I'll fix my post (note to self: don't post from phone...)

GrauGeist · Mar 28, 2022

swampyankee said:
Thanks. I'll fix my post (note to self: don't post from phone...)

I hate this phone - it seems that for every word I type, I have to type it at least twice to get it right

Maty12 · Mar 28, 2022

As pointed out by others, it depends on a lot of factors. From what I recall the French wanted their version of the Douglas DB-7s to have twin tails for the dorsal gunner to have better visibility, but later ditched this requirement. Both the Martin B-26 and North American XB-28 originally had twin tails that were ditched before either aircraft was built, I don't remember why but suspect stability was a big part. I know for a fact that the reason for the switch is in Wolf's book, but don't remember what it was. I find it interesting that most manufacturers switched to twin tails and then back.

Consolidated PB2Y started as single tail, then single with 2 small additional units, then twin tail.
Consolidated B-24, B-32, B-36 and PB4Y all started with twin tails then moved to singles, the B-36 being the only one where the twin tail didn't make it into even prototype stage.
Martin's M-130 Clipper had a single tail, and the larger M-156 Clipper had a twin tail.
Martin B-26, XB-33, PBM and PB2M/JRM all started with twin tails, the XB-33 was never built and the PBM retained its twin tail. The JRM lost its twin tails because the wing flaps blocked airflow to the bottom half of the stabilizers.

I think manufacturers stick with what's working for them until it stops working.

don4331 · Mar 28, 2022

swampyankee said:
B-24N. The naval version (derivative?) of the B-24, the PB4Y-2, was produced in fairly large numbers. This was done to improve handling at low altitudes.

This sort of surprises me: Twin tails drawback is - they are hanging in the propwash, which while good for low speed control, also means higher drag at higher speeds.
So, a B-26 which is going to be scooting right along to take off just because of the small wings, doesn't need the fin/rudder directly behind the engine as much as a B-25 which can take off from a carrier deck.

M Maty12 : Having 1/2 the fin/rudder on each side of the horizontal stabilizer reduces the torsion at the fin/stab inteface making for reduced structural requirements. Blanking off the bottom 1/2 would put huge loads on the joint, so I can see why JRM would change to single tail.

Why did so many WW2 Planes have twin tails?

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