The Functioning Of A Tailless Aircraft Engineering Essay

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The history of the aircraft design says that 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 the controllability problems. This section of the report discusses the stability of a tailless aircraft and different ways to achieve it.

The advantage of the tailless aircraft over a conventional one is the reduced drag and weight. For a tailless aircraft to be successful, it should have the same longitudinal and directional stability as the conventional configuration and should have the same control authority in all directions. It should also be capable to handle the same centre of gravity envelope

An aircraft does not need a tail to attain stability. If we locate the centre of gravity far enough forward, we can possibly get any level of stability.

Go in detail wid the aerodynamics of the aircraft…tailless to be more precise.

Add a few more equations and explain dem in detail.

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The aircraft is subjected to three different types of movements i.e. pitch, roll and yaw. These three terms describe the motion of an aircraft in relation to the three dimensions.

For a tailless aircraft (with or without fuselage) the condition for stability is:

dCm/dCl = Xc.g./Xa.c ; which must be less than or equal to zero.

Where, dCm/dCl is the parameter representing the static longitudinal stability of an aircraft.

The neutral point (or the position of the c.g. (centre of gravity) with respect to the a.c. (aerodynamic centre) is satisfied when dCm/dCl = 0

The above equation then becomes:

Xc.g./Xa.c = 0;

This equation thus tells us that for a stable flight, the centre of gravity should be ahead of the aerodynamic centre.

Now to achieve stability, dCm/dCl < 0 or we can say dCm/dCa < 0, where 'a' (angle of attack) is used in place of Cl because of the direct relationship between these two at their lower values.

This means that the pitching axis moments respective of the angle of attack changes must be less than zero. Thus for Positive stability we need negative (nose heavy) producing moments.

Scientists have already proved that stability depends upon the sign of dx/da where x is the fore and aft distance from the centre of gravity. Now dx/da is the travel of the centre of pressure for the wings. When dx/da moves forward the wing system becomes unstable. Therefore for positive static stability, dx/da>0.

For tailless aircraft to have cambered airfoils (produces more lift), the tail plane's stabilizing action can be replaced by incorporating the tail directly into the wing system.

To achieve dx/da >0, we can use an aircraft having heavily reflexed camber sections all along the span or a moderate swept back swap plan shape with wash out at the tips.

A reflexed airfoil is the one where the trailing edge has been turned upward as shown in figure 1. This kind of airfoil gives a stable centre of pressure travel over the whole useful range of incidences. The only limit for such stable airfoil sections is the invert, simply cambered airfoil.

Figure 1: Reflexed Airfoil Stability Chart

The drawback of reflexed airfoils is their performance qualities. They suffer from the low values for the maximum lift co-efficient and technically reflexed portion is supposed to balance the lift produced by the front part of the airfoil. This means that static stability cannot be attained without the loss of maximum lift which is about 9-15% of the total lift produced. It is clear from the above discussion that reflexed airfoil is beneficial in their lower minimum profile drag.

In the conventional aircraft designs, the unstable wings have always been made stable with the use of sweepback and washout. But for a tailless aircraft to be more efficient and stable, large degrees of sweepback and washouts are needed.

The function of the sweep and washout is to move the aerodynamic centre rearwards in order to keep the centre of gravity ahead of it and to provide a positive pitch moment respectively. The angle of sweep helps the centre of gravity of the aircraft to move fore and aft as shown in figure 2.

Figure 2: Aft movement of CP with an increase in the Sweep angle

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The drawback with the swept wings is their tendency to stall prematurely at the tips. But the washout on the other side lowers the angle of attack of the tips thereby decreasing the tendency of tip stalling at high angles of attack.

The flying wings that rely solely on the wing planform shape for stability are inferior to conventional aircrafts as the drag produced will be high and the maximum lift is low. To overcome this we can use modern sweepback with washout combined with the use of stable airfoil sections which could be a combination of reflexed and symmetrical airfoil cross sections.

Moreover, adding a fuselage to the tailless flying wing gives the designer more liberty in positioning the aircraft centre of gravity as shown in figure 3.

Figure 3: Adding a fuselage moves the CG forward in a Tailless Aircraft

To make the aircraft stable with swept forward or oblique wing planforms, the fuselage is the only way to move the centre of gravity far enough forward as shown in figure 4.

Figure 4: Adding a fuselage is the only way to achieve appropriate CP-CG relationships for Tailless Aircraft

Lateral Stability: The lateral stability on a tailless aircraft is controlled in the same way as on a conventional aircraft by adding dihedral to the wing as shown in figure 5.

Figure 5: Adding dihedral to an aircraft increases the roll stability

Sweeping the wings also add roll stability and if a swept wing is used the dihedral is reduced or even eliminated.

Another way to achieve the same is to lower the centre of gravity by mounting the wing high on the fuselage.

Directional Stability: It is hard to attain yaw stability on a tailless aircraft unless a vertical tail is added which should be placed as aft as possible so that there is large moment arm to the centre of gravity for the forces to act.

Figure 6: Tip Fins should be placed as far aft as possible to increase their effectiveness.

To cut down on the induced drag and so that the tips of the tails are out of the relatively thick boundary layer, planforms of higher aspect ratio on the verticals should be used.

Close attention should be paid to both the fins of the vertical tails placed on the wing tips so that each of them produces the same drag and that they have equal weight. The drag produced on the tips acts with a far larger moment arm and any imbalance due to these can throw the aircraft out of trim.

Giving the vertical tails some initial Toe-In increases the ability of the aircraft to return to the intended direction of flight as shown in figure 7

Figure 7: Vertical Fin 'Toe-In' can provide yaw stability

When the aircraft experiences some yaw, the forward wing presents more of its profile to the wind thereby creating a lot of drag. The opposite fin on the trailing wing is now at lower angle of attack aknd thus its drag is lower. The resulting force imbalance creates a moment about the centre of gravity that turns the aircraft back into the direction of flight.

Figure 8: Drag on 'Toed-In' Vertical Fins results in a way restoring force

The direction of toe can be set by either physically mounting the vertical fin at an angle relative to the direction of flight or by sanding some camber into the fin so that convex side is towards the centre of the aircraft as shown in figure 9.

Figure 9: Aerodynamic 'Toe-In' from airfoiled vertical fins

Yaw stability can also be achieved on swept wing configurations with negative dihedrals which is required just at the tips. Thus the directional stability is manifested through the outward lift developed on the wing tips.

Figure 10: Directional Stability increased with Downward Drooping Wing Tips

Also the gains made in the directional stability must be balanced against the losses in the roll stability as the negative dihedral does not help the roll stability of the aircraft.

Another element for the same purpose is a diffuser tip. A diffuser tip wing is the one where the tip is bent downward and at the same time is tilted forward. This downward tilt is for the yaw stability and the forward tilt (a form of washout) is for the longitudinal stability.

Figure 11: Diffuser Tip Flying Wing

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The only disadvantage is that a diffuser tip is an element which produces drag and down lift itself. But since it can take the place of fins and rudders and the wing twist, the deficiency in performance is pretty tolerable.

LOG BOOK:

DATES

ACTIVITIES

10-09-2009

Project Introduction by Iain Richie

17-09-2009

Decided to do the project on Tailless Aircraft

Group members selected

18-09-2009

Topics decided by the group members.

23-09-2009

Meeting held at the college library.

Discussed topics with each other

24-09-2009

Used internet to study the tailless aircraft concept

25-09-2009

Referred to http://www.owlnet.rice.edu/~mech594/handouts/aircraft_stability_control.pdf

to have a clear idea of the aircraft stability

28-09-2009

Started doing the project

29-09-2009

Group meeting held to discuss the progression of the report

6-10-2009

Researched on the advantages and disadvantages of the tailless aircraft

7-10-2009

Group meeting held at library

18-10-2009

Gathered more information on the tailless aircraft stability

28-10-2009

Submitted the report