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The “Frigate Ecojet”: A New Wide-Body Middle-Range Aircraft With Oval Fuselage Cross-Section
24 March 2015

50th 3AF International Conference on Applied Aerodynamics March 30,31 – April 1, 2015, Toulouse - France

V.I. Birjuk(1), A.L. Bolsunovsky(1), N.P. Buzoverya(1), V.I. Chernousov(1), Yu.S. Mikhailov(1), E.A. Pigusov(1)
(1) TsAGI, 1 Zhukovsky St., Zhukovsky, Moscow Region, 140180, Russia, e-mail: skomorohov@tsagi.ru

Abstract

This paper summarizes the preliminary design activity on a new ~300-seat passenger middlerange aircraft, which is being investigated thoroughly by “Russian Avia Consortium” company in a joint cooperation with TsAGI. The oval fuselage concept is a cornerstone of this aircraft, named “Frigate Ecojet” (see Fig. 1).
Due to a wide oval fuselage cross-section a high degree of flexibility in cabin layout is achieved, with, for example, very pleasant triple-aisle 10-abreast seating in passenger layout. Besides passenger comfort, triple-aisle layout adds significantly to the reduction of turn-around time.
Hybrid layout of the aircraft provides rational payload planning at different routes, meeting any market demands and thus making more sustainable profit. Additional advantage of the “Ecojet” is its small size in comparison with current wide-bodies, which reduces the required ground and hangar area.

INTRODUCTION

At present many airlines face the necessity to apply wide-bodies on intense routes with flight range not extending 4500 km. As a result of absence of specialized aircraft in this category airlines have to exploit long-range aircraft such as B767, B777, А300, А310, А330, etc. The cost effectiveness of such “misuse” is far from optimal. Due to their own extra weight the aircraft under consideration have comparably low fuel effectiveness on short distances. Besides, such expenses as take-off and landing charges, aero-navigational and other service fees essentially grow because they are rated in terms of maximum take-off weight. At the same time the employment a couple of narrowfuselage aircraft as well results in a marked growth of the crew and maintenance services expenses.
We should note that market advances high demands of cost/performance ratio and passengerand-airline appeal to any novel air vehicle project.
In such situation an attention may be focused on unconventional layouts.
The “Frigate Ecojet” family represents a new generation of wide-body passenger aircraft whose parameters are optimized for short and middle ranges (1900-4500 km) that results in significant reduction of structure weight (OEW), maximum take-off weight (MTOW) and the attendant operating costs. Unique peculiarity of these aircraft is their specific geometry and fuselage structure with horizontal oval cross-section. This original concept has no analogues throughout the world. The wide oval fuselage (Fig. 1) provides an optimization of overall dimensions in comparison with traditional aircraft of the same capacity.
Fundamental new technical solution of a cabin makes it possible for passengers to get high-level comfort and for airlines to have a wide range of cabin layouts, a wide list of cargo assortment, homogeneous park of aircraft adapted for different transport tasks with minimal expenses. At the moment the “Ecojet” development program has reached its predesign stage.

Frigate Ecojet (Фрегат Экоджет)

Figure 1. “Frigate Ecojet” aircraft

1. DESIGN REQUIREMENTS AND OBJECTIVES

Design requirements were produced on the base of close discussion with the airline representatives for the more thorough satisfaction to the modern and anticipating market demands. In accordance with these discussions the “Frigate Ecojet” family would involve two modifications (of the same dimensions) differentiated by seating capacity and flight range ability.
A baseline modification (“Frigate Ecojet - 300”) has a single-class cabin designed for 300 passengers and flight range about 3500 km. “Frigate Ecojet - 250” would be designed for 250 passengers and 4500-5000 km range (Fig. 2). Thus, seating capacity would depend on modification and cabin layout type and could differ from 260-280 passengers in the single-class cabin to 300-352 passengers in the two-class cabin.

Frigate Ecojet (Фрегат Экоджет)

Figure 2. Payload/range diagram of the “Frigate Ecojet” family with domestic engines PD-18

The cruising speed should correspond the Mach number of M = 0.8. The maximal cruising speed should correspond to M = 0.82.
The “Frigate Ecojet” aircraft with maximum take-off and landing weights should be exploited on airfields of 4D and higher code by ICAO classification. The required (balanced) distance for take-off and landing under conditions of MTOW and ISA+15°C should be no more than 2500 m.
The plane should be able to be exploited on airfields with 3000 m altitude above sea level and with artificial runways.

The aircraft should provide:
- high-level safety, reliability and passenger comfort;
- operating expenses by 20-25% less as compared to the “Airbus” and “Boeing” widebodies;
- fast turnaround time, operative cargo servicing and low ground-servicing costs;
- wide variety of passenger cabin interior, simple conversion into freighter/combi variant;
- minimal maintenance.

Perspective high-economical engines PD-18 with takeoff thrust of 18000 kg or modified engines PS-90A with takeoff thrust increased to 20000 kg (PS-90A20 version) were considered as base.
Existing foreign engines of the required thrust are as well under consideration.

2. “FRIGATE ECOJET” CONCEPT ADVANTAGES

The original fuselage cross-section is a brand new element and a cornerstone of the proposed widebody.This cross-section has a shape of a horizontal oval and allows a possibility of the more efficient volume utilization. The wide oval fuselage provides a pleasant seating and the higher comfort level (as compared to rivals) for passengers during flight (Fig. 3).

Frigate Ecojet (Фрегат Экоджет)

Figure 3.Cabin layout and typical cross-section of the “Frigate Ecojet”

There is a possibility to organize a triple-aisle cabin with 10-abreast seats. Any variant of the layout (see Fig. 4) suggests a very attractive cabin interior.

Frigate Ecojet (Фрегат Экоджет)

Figure 4.Variants of cabin interiors

All proposed cabin layouts meet the requirements of safety. The availability of three aisles facilitates the tasks of food on-board servicing, passenger loading and emergency evacuation. The “Frigate Ecojet” freighter version could carry all kinds of aircraft containers and pallets of 96"×125" and provide more flexible plane loading due to a wide cargo deck (Fig. 5).

Frigate Ecojet (Фрегат Экоджет)

Figure 5.Cargo variant

The ability to change aircraft purpose, seating arrangement and cabin interior freely, allows for carriers to build a flexible aerial system meeting to any market demands even within the framework of the unified structural layout, although the principal possibility exists to stretch the basic aircraft by inserting fuselage plugs.
Due to the application of innovative technical and layout solutions, it is possible to obtain significant improvements in a number of “Frigate Ecojet” operating characteristics as compared with rival jet-types of the similar capacity. These improvements include:
- reduction in harbor dimensions and increase in ground maneuverability;
- reduction in turnaround time;
- increase in fuel efficiency.

Reduction in harbor dimensions is provided due to fuselage shrinkage as compared with the airplanes of the similar capacity (Fig. 6).

Frigate Ecojet (Фрегат Экоджет)

Figure 6. Fuselage length comparison between “Frigate Ecojet” and its rivals

It should be noted that the “Frigate Ecojet” fuselage surface wetted area per passenger is by 4-14% (percentage depends on layout type) less in comparison with the modern wide-bodies such as B-777-200, IL-86, IL-96, A-340-300. This mentioned feature makes it possible to decrease fuselage drag per passenger by this magnitude and improve (in conjunction with innovative structural layout) fuel efficiency.
Summarizing the above mentioned it could be concluded that oval fuselage scheme permits to realize a new conceptual design of a modern civil aircraft which is passenger-oriented and at the same time possesses high cost-effectiveness.

3. STRUCTURAL DESIGN

The aim of the structural design is to find the airframe layout with a minimum structural weight.
The following design criteria were utilized:
- ensuring static strength of the structure under extreme loads;
- ensuring fatigue strength for prescribed service life;
- meeting the damage tolerance criteria for standardized damages;
- ensuring safety with regard to aeroelastic phenomena including flutter, divergence, etc.

Several variants of wing and fuselage structures and their integration have been investigated. Usual two-spar wing structure with wing box passing the lower part of the fuselage was chosen. As for the fuselage, there exists a difficult task of approving the strength under pressurization.
The “Frigate Ecojet” fuselage cross-section has a shape of a symmetrical 7,75×4,94 m horizontal oval formed by the conjugation of arcs of 2 m and 6 m-diameter circles. Conventional resolution of the internal pressure problem involves mounting vertical longitudinal struts which divide in-fuselage space and lead to impossibility of rational arrangement of passenger cabin and cargo compartments. Within the current project framework this challenge was resolved in another way.
The original structural layout realized for “Frigate Ecojet” fuselage perceives internal pressure by the skin connected to the frames of variable rigidity along the perimeter, passenger and cargo floors, and additional sealing beams (see Fig. 7). In fact they are the same struts but horizontally arranged.

Frigate Ecojet (Фрегат Экоджет)

Figure 7. The “Frigate Ecojet” fuselage structural layout in principle

Detailed finite-element calculations (Fig. 8) showed that the weight of the oval fuselage may be close to the weight of its cylindrical counterpart.

Frigate Ecojet (Фрегат Экоджет)

Figure 8. Stress and strain distributions in fuselage structural elements

4. AERODYNAMIC DESIGN

At designing “Frigate Ecojet” aerodynamic configuration the authors used a well-established four-stage aerodynamic design procedure [1]. At the beginning, the initial wing geometry is selected with chosen on a conceptual design stage planform, sweep and mean relative thickness. Then, by means of an inverse method new geometry is generated with improved pressure distribution and small wave drag at basic cruise regime. At the third stage the parametric variation of the configuration obtained is made, and the optimization procedure defines an optimum set of parameters which maximizes chosen objective function with the account of numerous restrictions of different origin. This stage is labor-consuming, not only because search of an optimum demands large computing expenses, but also owing to numerous repeated changes of a kind of object function and restrictions for achieving maximum project efficiency by many criteria. Fine "tuning" of configuration by the inverse and optimization methods is carried out in a number of successive cycles. After the third stage usually the manufacturing of aerodynamic model is started to receive experimental confirmation of estimated characteristics. At last, local aerodynamics adjustment is made at fourth stage (fairings, fillets, wingtips etc.) aimed at unveiling the last reserves of configuration and preventing deterioration of aerodynamic characteristics owing to technology factors. Here it is possible to use not so fast computational methods (for example, RANSmethods) with detailed modeling of all airplane elements, as well as wind tunnel studies.
Parameters of the wing planform are as follows: wing area (trapezoid) Str=205м2 , aspect ratio λ=10, ¼=26º, taper ratio η=3.5, kink position y=0.482. The wing is shaped with the seven baseline sections, chosen in such a way that to account for the wide body and engine influence and to minimize adverse aerodynamic interference. The relative thickness of the wing equals t/c=16.2÷12.2÷10% at the root, kink and tip accordingly.
During the multi-regime optimization the following flight conditions were considered: М=0.8 Сl=0.6 (basic regime); М=0.8 Сl=0.525; М=0.78 Сl=0.6; М=0.82 Сl=0.55; М=0.8 Сl=0.72 (Re=30mln) and М=0.8 Сl=0.6 (Re=3mln, fixed transition condition).
According to the calculations, the designed wing really guarantees small level of wave drag at prescribed regimes.
On the basis of the fulfilled studies the CAD model has been developed and several aerodynamic models have been manufactured for tests in different wind tunnels. The aerodynamic cruise model of “Frigate Ecojet” in the TsAGI’s Т-106 wind tunnel is shown in Fig.9.
Wind tunnel studies confirm fine tuning of the designed wing for the prescribed cruise regimes (Fig.10). Favorable pitching moment behavior is observed until buffet onset with sufficient margin counting from cruise regimes (Fig.11).

Frigate Ecojet (Фрегат Экоджет)

Figure 9.The aerodynamic model in T-106 wind tunnel

Frigate Ecojet (Фрегат Экоджет)

Figure 10. Lift-to-drag ratio at different Mach numbers

Frigate Ecojet (Фрегат Экоджет)

Figure 11.Pitching moment behavior at M=0.8

Effective high-lift devices have been designed consisting of a full-span slat and a single-slotted Fowler flap. Double-slotted inner flap was considered also. Wing tunnel studies conducted at Reynolds numbers of Re=0.95-4.75*106 and Mach number М=0.2 demonstrated high lift capability of the configuration (Clmax=2.5/2.9 at take-off and landing accordingly, Re=4.7*106, М=0.2) and good behavior of the lift and pitching moment characteristics including those near the ground. Airfield length utilization of no more than 2500m is possible.
After preliminary wind tunnel studies the aerodynamic configuration was modified slightly. Vertical winglets have been added to the wing in order to increase lift-to-drag ratio. The shape of the fuselage tail was refined also. New aerodynamic model for testing at cryogenic temperatures and flight Reynolds numbers in the European ETW wind tunnel has been manufactured (see Fig.12).
The results of the ETW campaign are under study at present.

Frigate Ecojet (Фрегат Экоджет)

Figure 12. “Frigate Ecojet” aerodynamic model in the ETW wind tunnel

CONCLUSION

The oval fuselage concept layout enables the designer to realize naturally a new concept of advanced medium-range aircraft, focused at passengers and being of high economic efficiency in operation at the same time.
As follows from the fulfilled aerodynamic and weight estimates, the “Frigate Ecojet” performance data correspond to the state-of-the-art of newly developed aircraft of its class and will allow efficient operation on short- and medium-haul routes.

REFERENCES

[1] Bolsunovsky, A.L., Buzoverya, N.P., Karas O.V., Skomorokhov, S.I. An experience in erodynamic design of transport aircraft. ICAS 2012.

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