Aircraft Tail Configuration and Its Rationalization
Introduction
Aircraft, both military and commercial/civil, have a variety of tail configurations with different sizes. The tail referred to here is the “fin” or “rudder” that is technically known as the vertical tail plane (VTP).
The determination of the VTP size on an aircraft is based on various parameters. These parameters include the desired VTP volume coefficient, wing area, wingspan, and the distance of the VTP from the overall center of gravity/mass of the aircraft. All these parameters contribute to achieving the ideal VTP area for the aircraft. A larger wing area or longer wingspan requires a larger VTP for good natural yaw stability.
The VTP area is just an initial reference for the design process. From the known ideal area, the aircraft design can freely determine the length and width of the VTP as long as it maintains the same ideal area from the initial calculation.
Number of VTP(s)
The number of VTP(s) on an aircraft is based on the designer’s assessment. An aircraft can have an unlimited number of VTPs, as long as the total area of all VTPs equals the ideal area required by the aircraft.
For example, if the ideal VTP area for an aircraft is 10 square meters, and the designer wants the aircraft to have two VTPs, each VTP would be 5 square meters. The total area of both VTPs remains 10 square meters. In another case, if three VTPs are desired, the designer can create various combinations for each VTPs area, all the while maintaining the total sum of ideal area. For instance, 2.5 square meters for the outer VTPs and 4 square meters for the central VTP, or each VTP could have an area of 10/3 square meters. Both combinations will result in the same total VTP area of 10 square meters. Therefore, different VTP numbers will have the same stability effect as long as they follow the ideal area parameter.
Rationalization of VTP Configuration
The choice of the number of VTPs is based on various reasons:
Structure
In some cases, to adhere to the ideal VTP area, designers may need to create VTPs that are very tall compared to the aircraft’s height. This complicates the VTP and overall aircraft structure design. To simplify and strengthen the structure design, smaller VTP sizes are needed, leading to the designer dividing the VTP area to adjust the size with the number of VTPs required by the aircraft.
Operational Facilities
Aircraft based on aircraft carriers are stored in much narrower hangars than land-based fighter aircraft. To save space and fit into carrier hangars, carried-based aircraft must have smaller dimensions than their land-based counterparts. One way to achieve this is by reducing the aircraft’s heigh through splitting the VTP into two parts, resulting in a 50% reduction in VTP height.
In other cases, existing operational support facilities, such as maintenance hangars, can only accomodate an aircraft of a certain size. To avoid the expense of building new facilities, designers adjust the aicraft size to fit existing operational facility sizes by modifying the number of VTPs.
Mission Requirements
For example, the Antonov An-225 Mriya, a Soviet-built cargo aircraft, was designed to transport the Buran space shuttle on top of its fuselage from the production to the launch site. The An-225 Mriya, an enlarged and modified version fo the An-124 Ruslan, needed to accomodate the large Buran shuttle. The An-124 had a single large VTP in the middile of its fuselage, hindering its ability to carry the Buran. To solve this issue, the An-225’s designer divided he VTP into two and placed them at the end of the aircraft’s tail, freeing up space in the middile for the Buran.
Safety Factors
In flight conditions, structural failure of the VTP due to vibrations or foreign object debris (FOD) impacts could lead to famage or the total loss of the VTP. For an aircraft with a single VTP, such damage could be considered fatal, rendering the aicraft unstable and uncontrollable in yaw axis. However, aircraft with two or more VTPs experience different impacts. If one VTP fails. the remaining VTPs still allow the aircraft to be controlled in yaw axis, albeit with limitations. Additionally, having multiple VTPs ensures smaller VTP sizes, leading to stronger VTP structures.
Conclusion
The number of VTPs on an aircraft is determined by the ideal VTP area parameter needed to make an aircraft stable in its yaw axis. This parameter is then adapted by the designer based on various considerations, from structural demands to safety factors.
