Tail Plane Design

[Jim Oxley]

Why aircraft designers choose a particular tack for design has always fascinated me.

Take for example the twin-tail design, mostly used on bombers (for arguments sake I'm excluding twin-boom aircraft eg P-38, Fw189, Fokker G.1). The aircraft that come readily to mind are the Avro Lancaster, Consolidated B-24 and the Handley Page Halifax. Initially I thought that the reason the twin-tail was adopted was that it facilitated easy installation of a tail gunner's position. But while this is probably correct it can't be the only,or main, reason for it's adoption. After all Boeing managed to install such in it's traditional tailed B-17 and B-29, as did Petlyakov with it's Pe-8, Mitsubishi with it's G4M, Martin with it's B-26 and Vickers with it's Wellington.

No, there has to be specific benefits associated with the twin-tail for it's (fairly) wide use. The French loved it, and used it on the Potez 63, the Bloch 174, Breguet 690, and the Liore LeO 451. British use seemed to rest with specific designers, eg Avro (Manchester, Lancaster), Handley Page (Hampden and Halifax) and Bristol (Buckingham). The same can be said for the Americans, only Lockheed (Hudson, Ventura and Harpoon), North American (B-25) and Consolidated (Coronado and Liberator). The Germans dabbled in it with the Dornier D0 17/217, Heinkel He 219 and the Messerschmitt Me110. As did the Russians with the Pe-2/3 and the Tu-2: the Japanese with the Mitsubishi G3M and the Kawanishi H6K and the Italians with the Fiat B.R.20.

So why was the twin-tail design used? Only a few utilised the space for a rear turret, so while this may have been a benefit it didn't see widespread use. What were the other benefits? Was it ease of manufacture? Improved handling as in rudder response? What were the drawbacks?


Very interested to hear others thoughts on the matter.

[Pilot]

Benefit is double control possibility and penalty is large weight at the end of horizontal tail which cause need of extra stronger structure of horizontal tail and result is extra weight. Whenever is demand for free space in this area, twin tail is used.

[MW Giles]

I always thought it was for three reasons - double the amount of rudder surface area without the need for a single large tail plane (a la B-17), improved sighting to rear for mid upper gunner and the tailplane is usually directly behind the engine in a twin or four engine design thereby increasing effectiveness at start of take off run (prepared to be shot down on this last one)

Regards

Martin

[Bill Walker]

Any design selection is usually the result of several factors, often these are conflicting factors. There is no easy answer to this question.

In general, twin engine aircraft design philosophy underwent a major shift through the late 1930s. At first, rudders and fins were sized for normal cruise, all engines running. The accident record of early twins led people to the concept of single engine controllability at low airspeeds, today we talk about Vmc (minimum controllable airspeed). This can be much higher than the stall speed, leading to excessive takeoff and landing distances because airspeeds must be kept high to maintain controllability after an engine failure at low airspeed.

Designers took several approaches to this, including twin vertical surfaces. As Martin said, keeping at least one vertical surface in an engine slipstream helped keep Vmc low. The designer then had to consider other factors: weight, cost, drag, size of hanger available, rudder pedal control forces, etc. in deciding how to get a bigger vertical surface area without introducing other problems.

Eventually the design process became "give them a low Vmc, however we can". See for example the Liberator to Privateer evolution, or the change from two vertical tales on early B-36 proposals to the final configuration. Things like power boosted controls, bigger factories and hangers, and better understanding of production and ownership costs led to the single fin winning out.

[Jim Oxley]

Fascinating guys. Many thanks.

[Graham Boak]

It is not just a matter of Vmc but more basic stability. The twin fins sit in the propwash, which is considerably faster than the free air thus benefitting rudder power and control, especially on take-off. It can be a different matter for directional stability. When an aircraft yaws - the nose pointing away from the direction of flight - the fin in free air sees this alteration and produced a force to correct it. With twin fins, they will still see the air from the engines, which is necessarily parallel to the fuselage and so the yaw is not noticed. That's why you'll see very tall fins on low-set tailplanes, and high-set tailplanes to get the top of the fin into the free air. Perhaps the He219, with its dihedral tailplane to get the fins high, is the most extreme example of this: you can also see the growth in height of the fins on the Manchester/Lancaster and the Ar240 for this reason.

[Crimea River]

Interesting topic. Learning something here.