Tail Plane Design

[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.