The Tailless Aircraft Engineering Essay

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This report on tailless aircraft presents the pros and cons of using such an aircraft design for commercial purposes. The report comprises 4 sections discussing the aerodynamics, structural innovations, engines and overall advantages and disadvantages of tailless aircraft. The aerodynamic study of a tailless aircraft highlights the importance of the wave drag and span loading distribution and different designs that can improve the aerodynamic performance effectively. In structural innovations, several existing tailless aircraft are examined to identify how the structures have been designed to create a successful aircraft. In particular, structures used in the control and stability of the aircraft are examined. As regards to engines, the positioning of the engine and the idea of using a Vertical Takeoff has been discussed. The advantages and disadvantages of a tailless aircraft have been detailed.

Introduction

This report has been produced to meet the requirements for the HND graded unit at the request of Mr Iain Ritchie on 17th September 2009. The report was due for submission on 19th May 2010.

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Of the aircraft in use today, the vast majority use a tailplane to house rudder and elevators. Aircraft without such a system remain quite rare. However, the concept of tailless aircraft has long been considered by engineers and aviators as an aerodynamically ideal. In the history of the aircraft design several attempts were made to build an aircraft with reduced tail size which has sometimes resulted in smaller drag and weight but has added to controllability problems. Because of this, tailless designs have mostly been used in military applications. In this report we assess whether it is now possible to seriously use this concept in commercial aircraft.

Methodology

The information contained in this report was primarily gathered from textbooks and internet research. Four different aspects of the subject were identified and each aspect was researched and written up by one member of the group. Additionally, the group were able to examine a harrier jump jet which visited Perth on 7th May 2010.

Results of findings

The following table summarises what the research has revealed:

Positive points

Negative points

Lower profile and interference drag

Lift to drag ratio increases by 20-25%

Engines can be positioned in the centre rear instead of a tail, providing the additional advantage of directional stability

Roll control is more efficient due to large wingspan

The tip of the wing aerofoil is not near the stall angle due to backward sweep along with twisted wing tip

Vertical takeoff is not practical since a large commercial aircraft weighs too much for the thrust available from current engine technology to overcome

Directional control is more difficult to achieve without adding a rudder assembly

The triangular spanwise aerodynamic loading distribution does not give the best aerodynamic performance even though the wave drag is the reduced.

Section 1: Aerodynamics

This section of the report discusses the aerodynamics of a tailless aircraft and various factors affecting the same. A tailless a is a revolutionary conceptual change from the classical design that has been prevailing for the past 50 years i.e. a wing attached to a cylindrical fuselage with a tail to ensure the stability and manoeuvrability of the aircraft.

Lower wetted area (area which is in contact with the external airflow) to volume ratio and lower interference drag is the main aerodynamic advantage of a tailless aircraft in comparison with the conventional aircraft.

On the aerodynamic performance side, the maximum lift-to-drag ratio depends on the ratio of the aircraft span to the square root of the product of the induced drag factor and the zero-lift drag area, which is proportional to the wetted area of the aircraft.

() max =

Where Cf is the average friction co-efficient (mainly dependent on the Reynolds number) over the wetted area Swet and is the friction co-efficient.

Since the tailless aircraft have a lower aspect ratio but also a lower friction co-efficient due to its larger chord, we always get smaller relative wetted area. This provides a substantial improvement in aerodynamic performance by increasing the lift-to-drag ratio of tailless aircraft in cruise to about 20-25% as compared to the conventional aircrafts.

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The BWB-450 and BWB-800 were designed to compare with the existing fleet of conventional aircrafts as Boeing 747 and Airbus 380. BWB-450 was presented with the span and the aspect ratio being reduced to 80 m and 7.55 respectively, thereby concluding a decrease in 30%fuel burn per seat for the BWB models as compared to other conventional aircrafts and thus requiring 3 instead of 4 engines.

Moreover another such design project was successfully completed, which is based on a similar payload and performance as Airbus 380 with over 650 passengers. The configuration of the project is well suited for the application of laminar flow technology (which results in skin friction drag) to the engine Nacelle and potentially to the lifting surfaces. Also an increase in cruise Mach number increases the drag making the design of aircraft unfeasible.

Assessment of aerodynamics of a tailless aircraft:

Geometry and flow conditions:

The geometry of a tailless aircraft consists of the central body, an inner wing and an outer wing to which a winglet is attached. They are blended to form the tailless aircraft geometry. The total span including the winglets is just under 80m.

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Fig 1.1: Isometric view of the aerodynamic surface of a tailless aircraft provided by Delft University.

The above model consists of two lifting bodies:

A thick streamlined centre body where the payload is accommodated to 0-13m span

A pair of inner wings which hosts fuel tanks from 13-23.5m in span and an outer wing from 23.5-38.75m to which winglets are attached.

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Fig 1.2: Planform of a tailless aircraft indicating the dimensions of the aircraft in the spanwise direction

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The leading edge sweep angles are swept back 63.80 for the centre body and 380 for the outer wing respectively. The aspect ratio of the aircraft is 4.26. the wing area is taken as reference area for the aerodynamic coefficient and the mean chord (Cref = 12.3m)is taken as the reference chord for the pitching moment co-efficient and the lift per unit of span definition. The length of the centre chord is C = 50.8m. the wetted area is Swet = 3079m2. The aspect ratio of the reference trapezoidal wing including the winglet is 7.6.

Ideal and low-fidelity drag calculation:

For the given tailless aircraft geometry, the ideal minimum drag can be calculated by using Prandtl lifting line theory for wing generating elliptic lift.

C D ideal = C D friction +

The geometry including the winglets was also modelled using the panel method which is based on the linear potential flow theory with the surface panels representing the tailless aircraft geometry. In the ideal drag calculation, the effect of the winglets is evaluated by increasing the span of an amount equal to the winglet's height.

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Fig 1.3: Comparison between ideal and panel method in prediction of aerodynamic efficiency

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But none of the above methods can be used for the wave drag due to the non-linear compressibility and variation of the skin friction from the flat plate values.

Grid:

At defined transonic cruise condition, the status of the boundary layer is assumed to be turbulent because of high Reynolds number and the high leading edge sweep of the configuration. Different techniques are used in trimming the tailless aircraft through the deflection of the trailing edge surfaces and in shape change in the three dimensional surface optimisation.

Grid sensitivity:

To know how much grid stretching is needed near the aircraft surface to obtain a good prediction of the turbulent boundary layer and therefore the aerodynamic coefficients, a grid sensitivity analysis is done in the grid direction normal to the surface.

The total drag comes from the integration of the pressure and shear stress around the whole geometry surface. The pressure acts normal to the surface while the shear stress is tangential to the surface. Therefore the pressure drag should include the induced drag (vortex drag) due to lift generation, the wave drag due to shock generation and the drag due to boundary layer displacement.

Assessment of aerodynamic performance:

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The outer wing is very highly loaded where the chord is much shorter than the inner wing and the centre body. On the other hand, the local lift for the central body is comparatively much lower than that on the outer wing. The high demand on lift from the outer wing results in shock formation on the upper surface of the outer wing. This shock gets stronger as the incidence increases.

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Fig 1.4: Spanwise local lift

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Fig 1.5: Pressure distribution over the wing surface.

A strong shock wave can be seen in the above figure extending from the junction of the central body and the inner wing to the outer wing tip. Although the central body has the greatest thickness, no significant shock can be seen on this part of the tailless aircraft due to the spanwise lift distribution and the three dimensional effects of high leading edge sweep. The outer wing experiences the strongest shock due to the high local lift demand. Thus the strong wave on the outer wing and the associated wave drag are crucial problems prohibiting the high aerodynamic performance. Moreover, the high outer wing loading also results in a high blending moment, which requires stronger and heavier structures.

Redistribution of the spanwise lift is done through a twist design for the given planform and thickness distribution. The shift of the aerodynamic loading should be inboard and the wave drag is also reduced using twist designs.

A discussion of spanwise lift distribution:

An elliptic lift distribution produces minimum drag for a given lift and an aspect ratio. For a conventional aircraft, an elliptic lift distribution is normally targeted to minimise the induced drag produced by the wing. But if the whole aircraft is treated as an integrated system then the elliptic load distribution on the wing is not optimum for the minimum induced drag.

For a tailless aircraft, it is essential to treat the whole aircraft as an integrated system. The spanwise distribution of a tailless aircraft includes the centre body and the wing as a whole. For a conventional aircraft, the body does not contribute significantly to the lift generation. But for a tailless aircraft should be an intrinsic lift generation surface.

Spanwise loading distribution:

It is desirable to shift the span load inboard in order to off-load the outer wing to reduce the shock strength and the wave drag. It can also be achieved from a reduced bending moment. For the tailless aircraft planform geometry, a movement of the aerodynamic centre forward is required. According to the centre of gravity moving forward, it will result in a reduced trim.

There are three aerodynamic loadings that are imposed on the tailless aircraft apart from winglets as target lift distributions at cruise condition.

1. An elliptic distribution

2. An average of elliptical and triangular distributions

3. Triangular distribution

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Fig1.6: Target Aerodynamic loading

Generally, all the three new twist distributions substantially reduce the pressure drag partially due to the wave drag reduction and partially due to the induced drag reduction. As expected, the variation of skin friction with the span loading change is relatively small. The averaged elliptic/triangular distribution has the minimum total drag and therefore the highest aerodynamic efficiency.

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Fig 1.7: Target lift distribution

Ideally, the elliptic loading should give the minimum induced drag associated with lift generation if there is no transonic shock on the wing, as shown in the panel calculations. The wave drag counteracts this potential benefit. On the other hand, the triangular distribution has the least wave drag but the pressure drag is higher than those from the elliptic and the averaged distributions. This is believed to come from the induced drag penalty. Therefore, from an aerodynamic performance point of view, the best spanwise loading distribution should be a fine balance of the induced drag and the wave drag at transonic conditions.

Section 2: Structural Innovations

This section of the report focuses on current tailless aircraft designs and the innovations and structures found in them; analysing the features, advantages and disadvantages of these designs.

As discussed in the previous section, the idea of removing the tail is to reduce the profile and interference drag of the aircraft. Removing the tail may also allow a special configuration to be used which a tail would obstruct (for example a central engine exhaust or rear-facing propeller). The empennage (see fig 2.1) on current commercial jets is an excellent platform for control surfaces such as the rudder and elevators. It also allows a counter-force to be applied to pitching and yawing movements of aircraft to maintain stability. However, it also presents more of a surface area for airflow to hit, producing drag, and the position downwind of the main wings and fuselage means this airflow can often be turbulent. In some aircraft a deep stall attitude can create airflow that completely disrupt the operation of the elevators and prevent the recovery of the aircraft; a deadly situation.

Fig 2.1: The empennage of a Boeing 747, a common aircraft tail arrangement

Structurally, the empennage is also considered more fragile compared to the rest of the aircraft; it is usually the structure that decides the maximum speed and manoeuvrability the aircraft can achieve. The joins between fuselage, tailplane and fin as well as the connection to control surfaces are all weak points when faced with high velocity airflow. For these reasons aviators and engineers have considered a tailless aircraft an aerodynamically ideal configuration for a long time. With the tail gone, only the wing and fuselage profile would be producing drag. The force of air resistance would also be limited to the leading edge of the aircraft which can be supported and reinforced more easily with internal construction features. A look at the designs in use today will help illustrate this:

The Flying wing:

The most well-known example of a tailless aircraft design is the B-2 stealth bomber (see fig 2.2, below). It has a 'flying wing' or 'blended wing-body' shape where the fuselage and wings are seamlessly combined, resembling a single continuous wing.

Fig 2.2: The B-2 Spirit Stealth Bomber

Basic Design:

The design is simple and provides several advantages to the aircraft. Since the entire aircraft acts as a wing, the design produces excellent lift which contributes to the aircraft's efficiency in aerodynamic terms and in fuel economy. This design also illustrates the aforementioned low drag due to the lack of sharp edges and joins in the face of oncoming airflow. Another advantage to aircraft is the distribution of interior space. For more traditionally shaped aircraft the cargo and passengers are restricted to a long, relatively narrow fuselage section. There are restrictions on the size and shape the fuselage can take such as the strength of joins between the body and wings of the aircraft and the aerodynamic requirements. The blended wing design is does not have a clear boundary between the wing and fuselage. By using a thick wing design the internal structure can be built to house pressurised cabin and cargo space all along the wide body. While the B-2 is an excellent example of the flying wing the basic profile is not the only shape that a flying wing can be. Many of the features are not about aerodynamics but about giving the aircraft its stealth ability and fulfilling military role requirements. The angular top-down profile of the aircraft is designed to reduce the chance of radio waves returning to a radar receiver and the W-shaped trailing edge provides the rigidity and strength needed for low-level manoeuvres during bombing runs. The smooth and constantly changing curves seen on the upper and lower surfaces of the aircraft were developed for scattering RADAR signals first and aerodynamic capabilities second.

Control Surfaces and Systems:

Removing the tail also has problems that need to be addressed. The positioning of control surfaces must be considered since there is no tail to house the rudder and elevators. The elevators can be quite easily repositioned to the main wing. Alternatively, the function of the elevators can be incorporated into the ailerons to make an 'elevon'. Housing a rudder is not such a simple prospect however, without a fin for a vertically aligned moving surface. Adding one or more fins specifically for yaw control rudders would work at the cost of adding back some of the drag that removing the tail saved. The B-2 has an innovation that provides the yaw control of a rudder: The split aileron. The system is placed on the trailing edge of the wing and opens out like a shell (see fig 2.3, below). This disrupts the airflow and increases drag on one wing to produce a yawing motion.

b-2-spirit2splitaileronsindicated

Fig 2.3: B-2 with opened split ailerons (indicated)

This is similar in principle to flaps and the term 'split aileron' is sometimes applied to a combined flap and aileron system on other aircraft.

All the control surfaces of the B-2 operate with heavy computer assistance. The aircraft is naturally unstable and the surfaces need to move with a high degree of synchronisation to fly properly. Earlier designs of flying wings proved difficult for a pilot to control due to the very different way in which it responded to commands in comparison with a conventional aircraft. Details of the flight computer and software of the B-2 are, of course, classified but it has been remarked by pilots of the aircraft that they wouldn't know the difference between controlling the B-2 and a more conventional aircraft if they didn't know what they were flying. As the B-2 has flown for a few decades it is evident modern computers are precise enough to control an aircraft of this design. The only blemish on its record (that is known to the public) was in Feb 2008 when the B-2 'Spirit of Kansas' crashed on takeoff due to a maintenance error.

Delta wing and 'Canard' configuration:

This is a configuration where the tailplane structure is not located at the rear of the aircraft, but at the front. This example (fig 2.4, below) shows how other components, namely a propeller engine in this case, can be placed on the centre rear of the aircraft.

bild1

Fig 2.4: Varieze very light aircraft with canard and rear-facing central engine

As far as aerodynamics goes, canards provide the same pitch dampening and control that a standard tailplane can provide. Canards are in use on several aircraft that use the delta-wing profile. Modern fighter jets in particular make use of all-moving canards which twist to act as elevators. Using the surface in this way also allows the air flowing aft onto the inboard section of the main wing to be directed into optimal flow conditions, improving performance.

200809-eurofighter-typhoon-50000-hours

Fig 2.5: Two Eurofighter Typhoons showing the delta wing and canard horizontal stabiliser

The Eurofighter (fig 2.5, above) is a modern supersonic fighter aircraft used as an interceptor. The swept back profile of the wing has been proved optimal for supersonic aircraft over more than 60 years of development; primarily because the shockwave generated by supersonic airflow over the nose of the aircraft is kept forward of the leading edge of the wing (see previous section). Delta wings also demonstrate excellent structural strength and rigidity; the wings do not suffer greatly from bending and twisting that often occurs on straight-winged aircraft. This is an important requirement for aircraft flying at very high speeds.

Concorde images combined for fig 6.JPG

Concorde (fig 2.6, above) was the first commercial jet to use a delta wing. It suited the design for the supersonic performance required but it differs from fighter craft in a few ways. The body looks longer and thinner in comparison, with an elongated section at the rear on which the fin is positioned. The main reason for this profile is to aid in longitudinal stability since Concorde does not have a horizontal stabiliser.

Notably, Concorde, the Eurofighter and most delta winged aircraft in use still have a fin to house a rudder rather than using an alternative like the B-2's split aileron. Despite the drag from a fin structure it is evident that it is still the best solution for directional control and stability. A split aileron on the main wing only produces drag when it operates but it also has an adverse effect on lift which can be a particularly undesirable when flying at low altitude and speed.

Section 3: Engines

This section deals with the modifications which can be done on the engine of the existing tailless aircraft for its effective application in civil aviation.

The landing and take-off procedures of tailless aircraft reveals its high engine performance when compared over the present day commercial aircrafts. For its usage in commercial purpose, some modifications have to be done on the engines. The idea of Vertical Take-off and Landing can be implemented for the above purpose.

Engines of tailless aircraft (B2 Spirit):

The B2 spirit aircraft is the existing example of a successful tailless aircraft. This aircraft was specifically designed for military purpose.B2 gets its significant advantages from the mixing of low-observable technologies with high aerodynamic efficiency and large payload. Four General Electric F118-GE-100 turbofan engines powers the aircraft. These engines are mounted internally in the body of the wings. This conceals the induction fans which thereby reduces their exhaust signature. A high subsonic speed is provided by the engines which are rated at 77kN.The engines contribute to the effective functioning of the aircraft.

Performance:

Altitude: 50000 ft

Thrust: 17300 pounds, each engine

The characteristics of the B2 are mentioned in the diagram below:

File:NORTHROP B-2.png

Figure 3.1: Characteristics of B2

B2 uses four of F118 non-after burning turbofan engine, which is a derivative of the F110 after burning turbo fan engine. The F118 has a 9 stage HP compressor driven by a single stage HP compressor and a 3 stage fan driven by a 2 stage LP turbine. The combustor chamber is of annular type.

The only factor which raises a thought for modification of the engines on the B2 is that it requires a longer runway to take off. This means more fuel consumption. Implementing the idea of Vertical takeoff and landing (VTOL) would be an added advantage when using the concept of tailless aircraft for commercial purpose. This would reduce the long run required. Study on VTOL type of aircrafts and the performance of the engines would help in understanding the advantages of using VTOL on tailless aircraft.

Engines of VTOL Aircraft (F-35 Join Striker Fighter):

Vertical Takeoff and Landing aircraft achieve vertical lift without the necessity of using a runway. The Harrier jump jets are the best examples for VTOL type of aircraft. Further modifications to the Harrier jump jets lead to F-35.

The Harrier jump jet uses a single jet engine (Pegasus) designed by Rolls-Royce. Though it has only one engine, its provided with four nozzles which direct the thrust downwards for vertical lift (Diagram 1).When the aircraft is airborne, the four nozzles are revolved slowly for the forward movement of the aircraft (Diagram 2).The Pegasus engine is a vectored thrust turbofan engine.2 LP and 2 HP turbine stages drive 3 LP and 8 HP compressor stages respectively. Annular type of combustion chamber is provided with vaporisers. The thrust vectoring system uses 4 nozzles which give the Harrier jump jet thrust for its vertical lift and forward propulsion.

http://www.technologystudent.com/images2/harr2.gif

http://www.technologystudent.com/images2/harr3.gif

Figure 3.2: Harrier jump jet V/STOL

The engine characteristics of the harrier jumpjet are given below:

Thrust: 23800 pounds

Weight: 4260 pounds

The modified version of the Harrier jump jet was the F-35.It proved to be more efficient in Vertical take-off and landing. It's much more advanced, faster and safer.F-35 is powered by the F135 engine which is the main engine and F136 is developed as an alternate engine.F135 engine has a 3 stage fan,6 stage compressor, a single stage high pressure turbine and a 2 stage low pressure turbine. It's provided with an annular combustor. The alternate engine(F136) has a 3 stage fan,5 stage compressor, single stage high pressure turbine and 3 stage low pressure turbine. Rolls-Royce had developed a lift system for the F-35.

http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/F-35B_Joint_Strike_Fighter_%28thrust_vectoring_nozzle_and_lift_fan%29.PNG/300px-F-35B_Joint_Strike_Fighter_%28thrust_vectoring_nozzle_and_lift_fan%29.PNG

Figure 3.3: F-35 Engine and Propulsion Nozzles

Lift fan, two roll posts, drive shaft, clutch and "Three Bearing Swivel Module"(3BSM) form the lift system. The 3BSM is the thrust vectoring nozzle. This allows the exhaust from the main engine to be deflected downward at the rear of the aircraft. A counter-balancing thrust is provided by the lift fan which is placed near the front of the aircraft. Engine's low pressure turbine drives the lift fan via the drive shaft and gear box. The pressurised air from the LP turbine can be diverted through the thrust nozzles called Roll posts. There is a lot of amount of hot, high velocity air which is projected downward during the vertical take-off. This can be reduced by the cool exhaust of the fan as the lift fan extracts its power from the engine. Exhaust temperatures are reduced by approximately 200 degrees. The lift fan of the F-35 makes it more effective than the Harrier.

The characteristics and performance of the F135 engine are given below:

Performance

Max Thrust: 43000 pounds

Short take off thrust: 38100 pounds

Main engine: 15700 pounds

Positioning of the engines:

Engine positioning is very important as it affects the stability of the aircraft. The stability of the aircraft is already discussed in section 1. In the B2 aircraft, the engines are blended within the wing of the aircraft. The mounting of engines at the aft of the fuselage can be considered. For a tailless aircraft, if the engines are placed at the aft, they would replace the tail unit and would provide directional stability as well. When considering about the VTOL type of engine, its positioning on commercial aircraft is looked over.

  

Application of VTOL :

After a study on the engine of tailless aircraft, the reason for its modification is that it requires a longer run and when used for commercial purpose it would be difficult for airports to give space for such long runs. The idea of VTOL suggested some positive benefits.

The study on VTOL reveals that its application on tailless aircraft for commercial purpose is theoretically possible but practically impossible.

When the Harrier fighter first started, one of the problems faced was to train the pilots for horizontal and vertical transition. This transition would be even more difficult to achieve for a commercial aircraft. Next issue to be considered is the engine's thrust versus weight. For vertical takeoff- the thrust of the engine should be more than the weight. Harrier jump jets do not completely do a vertical take -off but they take a short run and then lift-off with the vertical thrust achieved. This is done because Harriers when they are fully loaded their weight exceeds the thrust of their engine and thereby cannot take-off vertically. When implementing the idea of using VTOL for commercial aircraft, very powerful engines are required for VTOL. This concept is best described by an example. Considering the 747 aircraft, it weighs about several 100 tonnes. Each of the engines provides a thrust of about 25 tonnes. The thrust produced is sufficient for horizontal take off but not for vertical lift off. In short the weight of the engine would have to be greater than the weight of the aircraft.

Secondly, considering the positioning of the engine and propulsion nozzles for VTOL on commercial aircraft. Most of the V/STOL (Vertical take-off or short take off and landing) aircraft have their engines within the body of the aircraft. The arrangement of the propulsion nozzles on commercial aircraft would be more difficult. To gain directional control during vertical thrust operation addition of manoeuvring jets may be required for the nose, aft of the fuselage and wingtips

Thirdly, the amount of fuel consumption. For VTOL a lot of fuel is burnt and for larger jets it would be more of a problem.

The economic costs involved for the application of VTOL for a commercial tailless aircraft outweighs the benefits.

.

Section 4: Advantages and Disadvantages

This section of the report discusses several features of a tailless aircraft along with its advantages and disadvantages for each. The report also details the advantages and disadvantages of certain wing configurations as well as the pros and cons that tailless aircraft have during take-off, cruise and landing along with how stability during different aircraft attitudes is maintained.

As the tail plane mainly provides the longitudinal and directional stability in a conventional aircraft, this would have to be compensated due to the lack of the tail plane in a tailless aircraft. This can be achieved by making use of elevons, TVC (Thrust Vector Control) and several others.

As a tailless aircraft is without the tail plane the weight of the aircraft is reduced to some extent (some weight is compensated in large wing span requirements). Due to this the lift coefficient of the aircraft increases. Also loss of the tail plane results in the reduction of the surface area and joints on the aircraft which reduces the profile drag of the aircraft. As a result the thrust increases.

A tailless aircraft can be divided into four different types of wing profiles which are as follows:

Swept and twisted wing profiles

Stable wing profile (force of lift and weight acting from the same point)

Horizontal tail surfaces on the tip of the wing

low centre of gravity wing profiles

Swept and Twisted wing profiles:

By using a swept back wing profile, the tips of the airfoil can be used to achieve some pitch control for the aircraft. The tips can be angled to produce an upwards force that counters the lifting force of the wing root. The reason for doing this is to dampen pitching movements from excess lift. Also, while using backward sweep and twisted wing profile, the root stalls before the tips. If the elevons are placed on tips then this will allow to still be controlled even when the aircraft begins to stall. These are both things a tail would do on a conventional aircraft. However wings with this profile that are flexed out would produce less lift then a normal airfoil. More wing area is required to compensate this loss.

Fig. 4.1 Swept back wing Fig. 4.2 Twist in the wing profile

Stable wing profile:

Fig.4.3 Stable profile of an aircraft (force of lift and weight from the same point)

A stable wing profile requires less work to be done in designing and manufacturing; and is very easy to store in hangars since the connections to the main spar are very easy to undo and the centre of gravity is in the middle of the plane. However, stable wing profiles that are flexed will produce less lift than a normal profile to due reduced wing area; to compensate for the loss of the lift due to the reduced wing area the length of the profile will have to be increased.

Horizontal tail surfaces on the tip of the wings:

Fig.4.4 Horizontal tail surfaces at the wing tips

Wings with tail surfaces at the tip of the profile give similar directional stability as a tail. The tip of the profile acts as the rudder and helps the aircraft to steer. However it also adds to a little bit of profile drag.

Low Centre of Gravity:

In case of low CG type structures the momentum created by the wing is compensated, this method is used for light aircrafts. The advantages of this type of airfoil design, is the fact that the airfoil structure can be designed in an easy manner without concerning twists and sweep. However, for this type of profile, the wing area can not be utilized completely, wasting more space. Another disadvantage to this profile structure is the cockpit position; which increases drag on the aircraft while in flight.

Roll Control:

Control authority in a roll attitude of a tailless aircraft is established by the elevons and little help if any given by the spoilers in assisting the aircraft in rolling. Due to a larger span in a tailless aircraft roll control is more efficient than a conventional aircraft. Due to a minor deflection of the elevons which act as ailerons, control authority is achieved to assist in rolling the aircraft.

Yaw Control:

Directional control in yawing a tailless aircraft can be achieved in three ways:

The Use of TVC (Thrust Vector control) in the direction.

Equipping the wing tips with control Surfaces

Split Flaps

Thrust Vector Control is achieved by deflecting the thrust produced by an aircraft upwards, or downwards and in some cases sideways as well. Thrust Vector Control reduces the control efficiency in Pitch and Yaw, which therefore gives it a lower sweep angle which decreases induced drag and other faults connected with a sweep back wing profile that arise as-well. TVC can be used to direct an aircraft by directing the thrust of the engine in the direction intended to steer the aircraft.

The wing tips can be equipped with control surfaces that deflect air like all moving surfaces. The Surfaces would act like rudders used on aircraft which would help steer the aircraft and attain directional stability. Split Flaps can also be used on tail less aircrafts to achieve directional stability. Split flaps are like conventional rudders, with a minor change. Instead of both flaps moving in the direction the pilot intends to steer the plane, one flap would move; the flap would turn, and as air flows through the airfoil, the drag on one end would increase. The pressure applied on the split flap would slow down one wing surface, while the other wing steers the plane due to having more forward speed.

Drag:

The wing area of a tail-less aircraft compared to a normal aircraft would be similar because the wing of a tail-less aircraft would have to be extended to accommodate space for elevons to be placed which are used to steer an aircraft. However, the friction drag, on the aircraft would be lower as opposed to higher due to having a higher average Reynolds Number over the wing. Form drag on a tail-less aircraft would be less, than an average aircraft (including the horizontal and vertical stabilizer). Induced drag as-well would be lower than a standard aircraft with a fin and horizontal stabilizer because of the wing span being longer.

Weight:

A tail-less aircraft would weigh the same as a normal aircraft even without having a tail. The reason for this is due to the structural integrity needed for making the airfoil. The wing span and sweep needed for a tail-less aircraft plus the integrity needed for mounting the engines and incorporating TVC (Thrust Vector Control, used for directional stability of an aircraft) would make the aircraft weight just as much as a conventional aircraft.

Conclusion:

Compared to a conventional aircraft, a tailless aircraft can be more beneficial. A tailless aircraft has reduced profile and interference drag because it has fewer surfaces facing the airflow and the airflow only passes over one aerofoil. For a given planform and fixed aerofoil sections, the twist inverse designs can improve the aerodynamic performance effectively by manipulating the spanwise loading distribution from our understanding of transonic aerodynamics. Although the triangular distribution gives the most wave drag reduction but the averaged elliptic/triangular distribution gives the best aerodynamic performance. This is due to the less induced drag for averaged elliptic/triangular as compared to the triangular distribution.

On the structural side, moving away from the elliptic towards the triangular distribution will reduce the wing bending moment and therefore the structural weight.

The tailless aircraft in use today demonstrate how the problems of tailless designs can be overcome and advantages exploited. For directional control, the split aileron is an effective solution. It is not, however, considered superior to the fin and rudder system as far as aerodynamics goes; this is proven by their use in delta wing aircraft. For pitch and roll control, elevators can be located on the main wing or combined into ailerons to form elevons. This system has been successful on the B-2 and on delta wing aircraft. From these examples, a blended wing body would be a good basic shape for a commercial aircraft and fins and rudders would be best for directional control.

Conceptually, the idea of vertical take-off and landing for a tailless aircraft design would be advantageous but in reality a commercial tailless aircraft of significant size would be impractical to gain vertical lift. After a detailed study on Harrier jump jet, its characteristics did not meet the requirements for the commercial aircrafts. The tail unit of the tailless aircraft can be replaced by positioning the engines which would not alter the stability of the aircraft but causes maintenance problems.

The sweep back wing profile which positions the elevons behind the centre of gravity helps attain lateral stability during the flight. The sweepback profile coupled with TVC from the engines helps in the roll and yaw aspect of the aircraft.