Think of the rounded top of an old Beech Staggerwing or Piper Cub. Think of the straight tail of a Cessna 150. Think of the swept-back tail of a dozen different airplanes old and new. Think of the Mooney tail that appears swept forward. Now think of V-tails and T-tails. Throw in an Ercoupe just for fun.
What determines the shape of a tail?
At Zenith Aircraft, best known for producing kits for homebuilt short takeoff and landing (STOL) airplanes, design is guided by the need for the airplane to perform very well while flying very slow and often at high angles of attack. While it would seem obvious that a slow-flying airplane would need a very large tail to deflect enough air, the Zenith designs look small. However, according to Sebastien Heintz, president of Zenith Aircraft Co., “On our STOL designs, as well as our low wing CH 650, the rudder is the full vertical tail section. The vertical tail of the 701/750/650 aircraft is virtually all rudder control and is actually quite a bit larger rudder area than most other aircraft designs. The needed control surface is there despite the overall look [of] a small tail. By designing an all-moving vertical tail we’ve maximized the effectiveness of the rudder for excellent slow flight and crosswind handling capability while also minimizing the weight of the tail and simplifying the overall design with fewer parts.”
What determines the shape of a tail? STOL performance is one goal. Glide ratio is another.STOL performance is one goal. Glide ratio is another. Kevin Bruce, director of airworthiness and quality at Diamond Aircraft, where all but one Diamond design features a T-tail, said, “We designed our aircraft to have excellent gliding and non-powered handling characteristics, which contributes to the ‘passive’ safety system in the aircraft. The Diamond product line began with the HK36, which is a motor glider. The vast majority of the gliders on the market have T-tails because a T-tail adds to a better overall glide ratio for the aircraft. The T-tail is not in the slipstream of the fuselage or wing so there is less drag from this interaction. The T-tail design provides better flow over the elevator, which allows for better pitch control and a more predictable aerodynamic performance.” And then there is brand identity and the fact that a T-tail just looks good. “It is the Diamond image to have a T-tail,” he said.
Jay Hardin, manager in aerodynamics at Textron Aviation, makers of Cessna and Beechcraft airplanes, explains some of the science behind tail design. On the benefits of a T-tail, he says, “The end-plating effect of the horizontal tail on the vertical tail allows the vertical tail to be smaller. With the horizontal tail located above the wing, the effect of the wing’s wake [downwash] on the tail is minimized, allowing the tail to be smaller. Sweeping the vertical tail produces a much longer horizontal tail arm than would be possible with a fuselage-mounted or cruciform tail.” As far as drawbacks, he says a T-tail makes for a heavier vertical tail structure and generally moves the horizontal tail out of the propwash for piston aircraft.
What was the initial motivation for the V-tail design in the Bonanza?
“A reduction in interference drag at the vertical-tail/horizontal intersection and at the vertical-tail/fuselage intersection. A horizontal-tail/vertical-tail configuration has three intersections, whereas a V-tail configuration has two. The drawback is that the V-tail degrades Dutch-roll damping,” he said.
What determines the size of the rudder in a tail? Speed? Weight?
Once the flying is safe, there is art in tail design. The rounded top to the Staggerwing tail was a stylistic decision, as was the move from a straight tail to a swept back version in the Cessna 150/172/182 models in the 1960s.“The rudder and vertical tail are sized together to produce the right combination of stability and controllability, which are counter requirements. On a single-engine airplane, crosswind control is a primary design driver. On a multiengine airplane, one-engine-inoperative control is the primary design driver. In both cases, the rudder must be powerful enough to meet the design requirements, but if it is too powerful, it can generate high sideslip angles that can stall the vertical tail,” Hardin said.
Once the flying is safe, there is art in tail design. The rounded top to the Staggerwing tail was a stylistic decision, as was the move from a straight tail to a swept back version in the Cessna 150/172/182 models in the 1960s. According to Hardin, a more angular design makes the tail more efficient and thus allows it to be smaller. Yet the swept-back design actually lowered directional stability by a small amount. In the 1970s, Cessna tested a T-tail version of the 182, but, he said, “company pilots disliked the loss of elevator authority that resulted from moving the horizontal tail out of the prop wash.”
Perhaps the most recognizable tail in the sky belongs to the Mooney lineup. Instead of swept-back, it appears swept-forward. In truth, though, the leading edge is vertical and the trailing edge narrows toward the top. The airplane is fast and the tail makes it look that way. According to Kevin Hawley, chief engineer at Mooney International, the reason for that design was more practical than exotic. He says, “The main point of the straight leading edges on the wing, horizontal, and vertical stabilizers was that it was easier to wrap the surfaces with wood (plywood). The M18 and the first M20s had wood wings and…horizontal stabilizers. Some people will probably say it had to do with vertical moment arm at high angle of attack, but that is an old wives’ tail.”
Mooney also sports what it calls an all-trimming tail. “The all-moving tail for trim is all about one thing. Al [Mooney’s] favorite topic: performance. It was a cleaner design and ultimately results in less trim drag. Angle of attack is more effective than control surface deflection in terms of aerodynamic force produced,” Hawley said.
Of course there are subtleties and nuances and airflow math involved in tail design. For example, to counteract things such as P-factor and spiraling slipstream, a vertical tail is often slightly offset from the airplane’s centerline. Tabs influence airflow. And just like propeller shapes are now taking things such as noise reduction into their design goals, the future of tail design involves much more than pitch and yaw.
At Zenith, when they hold workshops to introduce people to building their own aircraft, they begin with the tail. Heintz said, “We like to start on the rudder tail section for a number of reasons: First of all, building the rudder is a relatively small project that can readily be completed in less than two days from start to finish, making it a good project for the workshop, while at the same time the rudder assembly introduces the builder to all the elements of the airframe construction. The rudder assembly is similar to a wing assembly: The assembly is made up of a main spar, with rear ribs and a nose rib, and the internal structure is then ‘covered’ by the skins—both a nose skin and a rear skin. Building the entire rudder assembly is easily accomplished in a couple of days, but importantly, introduces the new builder to all the tools and skills needed to complete the rest of the airframe, including reading and understanding the blueprints and assembly instructions, workshop space, et cetera.”
And there is the imagination of future design. Again according to Hardin, “One of the biggest challenges for mechanical control systems is achieving pleasing flying qualities as airplanes continue to get bigger and faster, and regulatory requirements become increasingly more stringent. The challenges lie in achieving the necessary stability while keeping control forces sufficiently low and maintaining good control feel throughout the range of control motion. This requires appropriately sized tails and control surfaces with carefully designed and tailored aerodynamic balances. In the distant future, fly-by-wire and powered flight controls may be within reach of the smaller GA aircraft, and that will change everything.”AOPA
W. Scott Olsen is an aviation writer and private pilot living in Moorhead, Minnesota.