# Design Process and Environmental Impact of Unconventional Tail Airliners

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## Abstract

**:**

_{x}emissions are even greater. Thus, the methodology has been validated and it can be easily adapted to other unconventional tail configurations.

## 1. Introduction

_{2}, H

_{2}O and SO

_{x}. Thus, just by using the respective emission index, it is possible to calculate them from the fuel flow data. On the other hand, for the other emission species (NO

_{x}, CO, HC and soot), there are several methodologies to estimate them, such as P3T3, DLR and BFFM2, among others [20,21,22]. The use of unconventional tail configurations could reduce aircraft emissions through two main effects: reduction of aerodynamic drag and reduction of weight. Just the fuselage contributes around 28% to the parasite drag of the airplane and the empennage contributes 14% [23]. Regarding the weight, stabilizing surfaces could suppose around 5% of the maximum take off weight of the aircraft [16]. These percentages are indicative, but they show the possible repercussion that a new tail configuration could have.

## 2. Materials and Methods

#### 2.1. Design Strategy and Reference Aircraft

#### 2.2. Aerodynamic Model

#### 2.3. Load Cases for Tails Design

#### 2.3.1. Symmetric Maneuvers

#### 2.3.2. Gusts Loads

#### 2.3.3. Asymmetric Maneuvers

#### 2.4. Weight Estimation Models

- the torsion box bears bending, shear and torsion;
- the torsion box is approximated by a rectangular geometry, as shown in Figure 2;
- the stringers have Z-shape section, and the relation between width and height is a factor of 0.3;
- the spars are stiffened by vertical elements located every certain distance;
- the caps define the extremes of the spars, and their areas are neglected when comparing to total panel area;
- for each section, extrados and intrados panels have the same geometry (this consideration is taken because the tail surface could generate lift upwards or downwards, depending on the flight condition of the aircraft); and
- the panel is sized for a uniform shear load, corresponding to the maximum that appears in the panel.

#### 2.5. Emissions Models

_{2}, H

_{2}O and SO

_{x}. The corresponding emission indices for them are: EI CO

_{2}, 3149; EI H

_{2}O, 1200; and EI SO

_{x}, 0.84 [33].

_{x}, CO and HC emissions do not have constant emission indexes. At this point of conceptual design stage, several techniques can be applied in order to estimate these indexes. For this purpose, the ICAO Engine Emissions Databank [47] is a repository with information on exhaust emissions of those engines that have entered production. The information is based on experimental tests for an idealized landing/take-off cycle (LTO) in International Standard Atmosphere (ISA) conditions. The LTO cycle only assesses the emissions below 915 m (3000 ft) and, therefore, may not be a good guide for analyzing other flight modes, for instance, cruise [21]. The ICAO Databank includes information about the following phases of flight: take off, climb out, approach and taxi/ground idle. These conditions correspond to a throttle setting, in percent of maximum rated output, of 100%, 85%, 30% and 7%, respectively, to the four previous conditions. Nevertheless, there are several correction procedures to take into account altitude effects and estimate emissions in cruise conditions. Among them, Fuel Flow Method 2, developed by Boeing, and DLR (Deutsches Zentrum für Luft- und Raumfahrt e.V., literally German Center for Air- and Space-flight) method are simple methods that provide around 10% accuracy, which are adequate to use in these early stages of aircraft design with reduced available data [48].

_{x}, CO and HC emissions [50]. DLR method is based on the same principle as BFFM2, but the way the parameters are manipulated and the required input data are different. The corresponding inputs in this case are total pressure and temperature which comprehend stagnation and flight speed effects. Furthermore, this methodology just enables the calculation of NO

_{x}[48]. In this study, both methods are considered in order to check both paths.

## 3. Results

#### 3.1. Test Case for Conventional Tail Configuration

- Steady turn at ${V}_{A}$ speed at limit maneuvering load factor at 22,000 ft, as a symmetric steady maneuver.
- Sudden deflection of the elevator in cruising flight at ${V}_{A}$ speed at 22,000 ft, as symmetric unchecked maneuver.
- Lateral balancing of the aircraft after critical engine failure in climbing after taking off with no sideslip angle at sea level altitude.
- Flight at ${V}_{A}$ speed with 10° sideslip angle, without any deflection of the rudder, at 22,000 ft. This maneuver corresponds to Maneuver 2 of those explained in the asymmetric maneuvers section.
- Flight at ${V}_{A}$ speed with 10° of sideslip angle and maximum rudder deflection in the direction opposite to the turn, at 22,000 ft. This maneuver corresponds to Maneuver 3 of those explained in the asymmetric maneuvers section.
- Lateral gust at ${V}_{A}$ speed at 22,000 ft.
- Vertical and positive gust at ${V}_{A}$ speed at 22,000 ft.
- Flight at ${V}_{A}$ speed with zero sideslip angle, but with maximum rudder deflection. This maneuver corresponds to Maneuver 1 of those explained in the asymmetric maneuvers section.

#### 3.2. Unconventional Tail Design

_{x}emissions, just those calculated with BFFM2 are represented because they are the most conservative. As the exhaust gas emissions are somehow proportional to the trip fuel consumption, the behaviors of the lines are qualitatively similar to the behavior of the trip fuel consumption, even the crossing of the lines.

## 4. Discussion

_{2}, is depicted in Figure 9. On the other hand, NO

_{x}emissions are reduced to a greater extent than savings in trip fuel. This is an important conclusion because it could be a stronger reason for considering this unconventional configuration for future aviation. However, HC and CO emissions are not so reduced. In fact, CO emissions are increased with respect to reference aircraft ones. This is justified when analyzing the ICAO databank, since the selected engine emits more CO for lower fuel flows. As this unconventional configuration needs lower thrust levels to develop cruise flight, lower fuel flow is demanded. In consequence, CO emissions are increased.

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**Airplane drag polar of CSR-01 in clean configuration for different Mach numbers (

**left**); and in take-off, climb, approach and final approach with landing gear extended conditions (

**right**).

**Figure 2.**Torsion box drawing, where height is ${h}_{t}$, length is ${b}_{t}$, spar web thickness is ${t}_{L}$ and pitch between stringers is b.

**Figure 3.**Compression load per unit length and shear flux distributions for symmetric loads in horizontal tail surface (

**left**) and for asymmetric loads in vertical tail surface (

**right**).

**Figure 4.**Feasible design space defined by the constraints imposed by static derivatives at cruising conditions, both longitudinal (${c}_{m\alpha}$) and lateral (${c}_{n\beta}$), and by the maximum deflection of the lateral control for crosswind landing conditions, establishing a maximum value for the dihedral angle (CwL MAX).

**Figure 5.**Weight estimation of V-tail for the nine conditions studied, for variation in span and for the parameters $\Gamma $ = 30°, ${c}_{r}$ = 3.7 m and $\lambda $ = 0.32.

**Figure 6.**Design variables combination resulting of intersecting the constraints associated with the static stability conditions, both longitudinal and lateral.

**Figure 7.**Friction drag coefficient as a function of the taper ratio and root chord along the new feasible design space determined intersecting the active constraints.

**Figure 8.**Fuel consumption variation with respect to the reference aircraft for the design route as a function of the taper ratio and root chord along the new feasible design space determined by intersecting the active constraints.

**Figure 9.**Emissions variation with respect to the reference aircraft for the design route as a function of the taper ratio and root chord along the new feasible design space determined by intersecting the active constraints.

Variable | Value |
---|---|

Maximum take of weight | 77,000 kg |

Operating empty weight | 42,100 kg |

Number of passengers | 150 |

Mach number | 0.78 |

Wing area | 122.41 m^{2} |

Horizontal tail area | 32.23 m^{2} |

Vertical tail area | 28.21 m^{2} |

Horizontal tail weight | 682 kg |

Vertical tail weight | 522 kg |

Parameter | kg |
---|---|

CO_{2} | 47,007.8 |

H_{2}O | 17,134.2 |

SO_{x} | 12.54 |

HC | 1.438 |

CO | 39.926 |

NO_{x} | 173.59 (BFFM2)/160.00 (DLR) |

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**MDPI and ACS Style**

Sanchez-Carmona, A.; Cuerno-Rejado, C. Design Process and Environmental Impact of Unconventional Tail Airliners. *Aerospace* **2021**, *8*, 175.
https://doi.org/10.3390/aerospace8070175

**AMA Style**

Sanchez-Carmona A, Cuerno-Rejado C. Design Process and Environmental Impact of Unconventional Tail Airliners. *Aerospace*. 2021; 8(7):175.
https://doi.org/10.3390/aerospace8070175

**Chicago/Turabian Style**

Sanchez-Carmona, Alejandro, and Cristina Cuerno-Rejado. 2021. "Design Process and Environmental Impact of Unconventional Tail Airliners" *Aerospace* 8, no. 7: 175.
https://doi.org/10.3390/aerospace8070175