Single vs. Twin Tails

[Noha307]

So, I think recently learned the answer to a question I had been wondering about for a long time: Why does the Ercoupe have twin tails? Of course, the Ercoupe is famous for it's no rudder pedals design, and it is reasonable to assume that the twin tails is the result of that. However, strictly speaking, there is no reason this has to be the case. A single tail could have equally done the job.

So why does it have twin tails? The short answer is, to make the rudders less effective. Yes, you heard that right, less effective.

One of the old books I had purchased not too long ago provided the answer. In describing a rare airplane I had never heard of before called the General Aircraft Owlet that also has twin tails, the book War Wings stated:

Once in the air, turns can be made steeply in both directions on the stick alone. A violent kick on the rudder bar when flying level merely causes the machine to wag its tail and settle back quickly to the straight and narrow path in rapidly dampened oscillations. There seems to be much said for twin rudders, set outside the slipstream, on single motor types.

Given that the goal of the Ercoupe was to produce a safe airplane for the everyman - specifically to make an airplane that cannot spin - putting the rudders outside of the propwash would have made them less responsive - and therefore less likely to cause a spin.

However, the related question I still don't have an answer to is: Why did the Navy switch from twin tails to single tail when changing the B-24 to the PB4Y-2? I can think of no good reason to do so.

[Kyleb]

However, the related question I still don't have an answer to is: Why did the Navy switch from twin tails to single tail when changing the B-24 to the PB4Y-2? I can think of no good reason to do so.

Aerodynamic theory would tell you that a taller fin/rudder would have a couple of advantages.

- Fewer intersections between surfaces = less intersection drag.

- A longer (taller) fin would gain efficiency through a higher aspect ratio, so you could net out to less vertical surface for the same stability and/or control authority. In theory, that would save some weight, although there are some structural advantages to the twin fin design (lower bending moment, being one of them).

[Matt Gunsch]

The Ercoupe does not have 2 rudders to make them "less" effective, it has 2 rudders that are outside of the influence of the prop slipstream. On a single tail plane, there is more air pushing on one side than the other because of the prop, if the rudders are further out than the prop diameter, there is no influence from the air. On a Ercoupe the rudders are linked to the ailerons, when you turn the wheel to the left, the left aileron rises the same as any other plane, the only difference is the left rudder moves outboard and the right rudder moves slightly inboard. The rudders move 20 degrees out and 3 degrees in.

One a plane like a 24 or 25, they are designed with rudders in the propwash as it make the rudders more effective in the event of an engine out, if the right engine is out, you carry more power on the left, more power means more air over the rudder.

I do know a little about Ercoupes, this one is mine. It is a 1950 model 415G, one of 8 left flying, It was built by Erco, which is the same company that made the nose and waist turrets on the PB4Y-2, and nose turrets on the PB4Y-1.

Attachments

[Chris Brame]

Interesting to learn that. Then there's the Mooney Cadet - which was a single-tailed Ercoupe. How was its performance affected by that change? My late uncle owned this one; I should have asked him:

[wolf]

The twin tails of the B-24 was a fad -- Consolidate figured if one stabilizer / rudder was damaged, the other would be enough to get the aircraft back to base.

What Consolidated and the USN found however, that the twin tails made the B-24 unstable, had to maintain formation and difficult to get on the step. The single tail B-24N (and a retro kit) would have eliminated a vast majority of the handing issues of the B-24.

The PB4Y-2 was a result of the B-24K testing that fixed these problems, giving the USN a much improved patrol aircraft that could be flown low and hands off

[Kyleb]

The twin tails of the B-24 was a fad -- Consolidate figured if one stabilizer / rudder was damaged, the other would be enough to get the aircraft back to base.

What Consolidated and the USN found however, that the twin tails made the B-24 unstable, had to maintain formation and difficult to get on the step. The single tail B-24N (and a retro kit) would have eliminated a vast majority of the handing issues of the B-24.

Can you point to a source for that information?

[wolf]

The twin tails of the B-24 was a fad -- Consolidate figured if one stabilizer / rudder was damaged, the other would be enough to get the aircraft back to base.

What Consolidated and the USN found however, that the twin tails made the B-24 unstable, had to maintain formation and difficult to get on the step. The single tail B-24N (and a retro kit) would have eliminated a vast majority of the handing issues of the B-24.

Can you point to a source for that information?

Read anything on the XB-24K program as well as anything on the PB4Y-2 development - there is several good research papers showing the wind tunnel trials on the Single tail B-24N .

Bombers in Blue: PB4Y-2 Privateers and PB4Y-1 Liberators by Frederick Johnsen I believe has photos of the wind test models for the PB4Y-2 showing the original XPB4Y-2 developed with twin tails vs single tail.

[Matt Gunsch]

Interesting to learn that. Then there's the Mooney Cadet - which was a single-tailed Ercoupe. How was its performance affected by that change? My late uncle owned this one; I should have asked him:

the Mooney M-10 was intended to do everything a Ercoupe could not, it had a defined stall, where the Ercoupe just mushes, and I believe they could spin.

[LysanderUK]

I could be muddying the waters here rather than helping to clear things up, but I have to ask...

In the case of the twin-tail bombers, weren't there some advantages to twin tails in regards to fields of fire for the defensive guns? If you look at the early drawings for something like the Avro Manchester, it has very small twin fins that gave upper and lower turrets a clear sweep without any tail structure blocking the field of fire. Was this a consideration in other designs??

[wolf]

I could be muddying the waters here rather than helping to clear things up, but I have to ask...

In the case of the twin-tail bombers, weren't there some advantages to twin tails in regards to fields of fire for the defensive guns? If you look at the early drawings for something like the Avro Manchester, it has very small twin fins that gave upper and lower turrets a clear sweep without any tail structure blocking the field of fire. Was this a consideration in other designs??

With three tails, the first Manchester's would have had a terrible field of fire.

Just my thoughts, a twin tail design would be more of a hindrance as you loose that arc of fire and end up with a dead zone to both the left and right (especially with a aircraft with a mid-upper turret).

question, did any armed AT-11's (Israel, Turkey, China) see combat.......

[Noha307]

A longer (taller) fin would gain efficiency through a higher aspect ratio, so you could net out to less vertical surface for the same stability and/or control authority. In theory, that would save some weight, although there are some structural advantages to the twin fin design (lower bending moment, being one of them).

I had always worked with the understanding that 2 x twin = 1 x single because surface area would be the same. Thanks for the clarification.

The Ercoupe does not have 2 rudders to make them "less" effective, it has 2 rudders that are outside of the influence of the prop slipstream. On a single tail plane, there is more air pushing on one side than the other because of the prop, if the rudders are further out than the prop diameter, there is no influence from the air.

It took me a couple read-throughs to understand, as the first few times I thought you were saying the same thing I was. You're saying that the Ercoupe does have twin tails to put them outside of the propwash, but this is to eliminate differential effect on the rudder(s), not to reduce their effectiveness.

I do know a little about Ercoupes, this one is mine. It is a 1950 model 415G, one of 8 left flying, It was built by Erco, which is the same company that made the nose and waist turrets on the PB4Y-2, and nose turrets on the PB4Y-1.

I never thought about that connection. Nice plane, by the way!

What Consolidated and the USN found however, that the twin tails made the B-24 unstable, had to maintain formation and difficult to get on the step. The single tail B-24N (and a retro kit) would have eliminated a vast majority of the handing issues of the B-24.

Why would twin tails made the B-24 unstable? As you suggested, a quick search of XB-24K provided mentions of the change being for stability (as well as a suggestion that field of fire was improved), but none went any further as to explaining why it had an effect.

EDIT: Nevermind, I'm an idiot that did not put two and two together with Kyleb's explanation above.

Bombers in Blue: PB4Y-2 Privateers and PB4Y-1 Liberators by Frederick Johnsen I believe has photos of the wind test models for the PB4Y-2 showing the original XPB4Y-2 developed with twin tails vs single tail.

Thanks for providing a reference. I can always appreciate that!

[LysanderUK]

With three tails, the first Manchester's would have had a terrible field of fire.

Yes, the Manchester did end up with three fins, but the early drawings and first prototype had tiny twin fins. Link below is of a picture of the first prototype prior to first flight. (On some drawings, the original fin design was even smaller):

https://imgproc.airliners.net/photos/ai ... .jpg?v=v40

With some of the strange thinking that was going on pre-war, it wouldn't surprise me if some companies/designers were trying to give as much firepower directly to the rear of the aircraft.

I've read that this was also a consideration in the twin-tail design of the Bf110 and Do17, aircraft which didn't have tail gunner positions as such.

[shrike]

There have been a couple of other pluses for twin rudders - at least in theory.

It allows rudder area below the thrust-line, which reduces unwanted roll and adverse yaw effects.

The rudders and fins act as endplates for the horizontal acting to increase it's effectiveness and 'virtual' span (think really big winglets)

Rudders split by the horizontal at midpoint reduce the bending element of the fin. Loads through the horizontal are more or less either tension or compression, which reduces structural weight and complexity.

Two smaller surfaces can use smaller, therefore lighter, hinges and controls. Complexity goes up, but component weight goes down.

Redundancy IS a thing - hence the A-10

[maxum96]

Let me muddy the waters some more. What about the Lockheed Electra? Everyone knows the story about how Lockheed was having major issues during the design phase due to directional instability until a young Kelly Johnson came into the picture and redesigned it with twin tails.

[Kyleb]

Let me muddy the waters some more. What about the Lockheed Electra? Everyone knows the story about how Lockheed was having major issues during the design phase due to directional instability until a young Kelly Johnson came into the picture and redesigned it with twin tails.

I don't think they ever flew one with a single fin. Johnson's twin tail mod has always been explained as a way to get one vertical surface in the propwash so you'd have better single engine control for when (not if) you lost an engine. Of course, Douglas and Boeing were pretty successful with a single vertical on multi engined aircraft.