CHAPTER-VII

TAIL GEOMENTRY7.1 INTRODUCTION

Our aircraft has a T-tail configuration, chosen for several reasons.

Despite the T-tail's typical disadvantage of adding to aircraft weight (due

to required extra structural strengthening), its advantages of the T-tail

outweigh the disadvantages. One such advantage is that the T-tail puts the

horizontal tail clear of wing wake and engine exhaust. Another is its

aesthetically-pleasing design. Overall, however, the T-tail results in

higher efficiency and a smaller tail than would be possible if it were of a

different design. Depicted below is our T-tail design

Figure 12.1 T-tail

7.2 HORIZONTAL TAIL GEOMETRY

The sweep of the horizontal tail's leading edge has been set to 25 degrees.

This value was obtained from historical data; a trend in past aircraft has

been to sweep the horizontal tail 5˚ further than the wings. This increase

in sweep angle ensures that the tail stalls later than the wing, important to

maintaining control and maneuverability in adverse conditions. An

increase in tail sweep angle also increases its critical Mach number

relative to the wing; this prevents the loss of elevator effectiveness in case

of shock formation.

7.2.1 TAIL AIRFOIL

NACA 23021

Figure 12.2 TAIL AIRFOIL

Thickness : 21.0%

Camber : 1.9%

Trailing edge angle :28.3o

Lower flatness : 22.2%

Leading edge radius : 6.2%

Max CL : 1.244

Max CL angle : 15.0

Max L/D : 25.219

Max L/D angle : 7.0

Max L/D CL : 0.873

Stall angle : 3.0

Zero-lift angle : -1.0

Figure 12.3 Aero dynamic Characteristics

7.2.2 Lift to drag ratio of NACA 23021

α = 0°

CL =0.1 (from above fig.)

CD =0.007 (from above fig.)CLCD= 0.1

0.007= 14.28

α = 6°

CL =0.7 (from above fig.)

CD =0.0085 (from above fig.)CLCD= 0.7

0.0085= 82.35

α = 12°

CL =0.75 (from above fig.)

CD =0.0087 (from above fig.)CLCD= 0.75

0.0087= 86.206

7.2.3 Horizontal Tail Pitching Moment co-efficient

Cmowf + CL (h – h0) – ηt vt Clt = 0 (7.1)

Cmowf = Cmaf x AR cos ² [ Δ]

AR+2 cos [ Δ] + 0.01 (αt)

(7.2)

Cmaf for NACA 23021 is 0°

From, Equation (7.2) wing pitching moment co-efficient is

Cmowf = 0.12 (7.4)

From, Equation (7.1) Tail lift co-efficient is

Clt = 0.095 (7.5)

7.2.4 Horizontal Tail Area

To calculate the initial size of the horizontal tail, the following

equation was used:

SHTAIL = CW VHT Swing / Lopt (7.6)

where Swing = area of wing, CW =mean chord length of wing, CHT =tail

volume coefficient and LHT = length between wing and horizontal tail.

Our value for the tail volume coefficient was taken from historical

data; this value is 1.1.

St = 10.96 m2 (7.7)

7.2.5 Tail Aspect Ratio

ARt = 2/3 x ARW (7.8)

ARt = 5 (7.9)

7.2.6 Tail Wing Span

Wing Span b2 = AR x St (7.10)

Wing Span b = 7 m2 (7.11)

7.2.6 Horizontal Tail Root Chord and Tip Chord

7.2.6.1 Tail Root Cord

Taper Ratio of tail section λt = 0.26

Crt = (2 x St ) / (bt x (1+λ)) (7.12)

Croot = 2.22 m (7.13)

7.2.6.2 Tail Tip Chord

Ctip = λ x Croot (7.14)

Ctip = 0.58 m (7.15)

7.2.7 Horizontal tail sweep angle

The sweep of the horizontal tail's leading edge has been set to 25 degrees.

This value was obtained from historical data; a trend in past aircraft has

been to sweep the horizontal tail 5˚ further than the wings

Sweep Angle = 25° (7.16)

7.2.8 Horizontal tail Mean aerodynamic chord

Ct = 1.56 (7.17)

7.3 VERTICAL TAIL GEOMETRY

The sweep of the vertical tail has been set to 30 degrees, again obtained

using historical data that indicates that vertical tails are swept 5-10

degrees further than the horizontal tail. The increase in the sweep angle

once again also increases the tail's critical Mach number relative to the

wing, preventing loss of critical yaw control during turbulence. The taper

ratio of the vertical tail has been set to 0.7, based on historical data for T-

tails; T-tail vertical surface taper ratios are in the range of 0.5 to 1.0, to

provide adequate chord for the attachment of the horizontal tail and

associated control linkages.

7.3.1 Vertical Tail Area

To calculate the initial size of the vertical tail, the following equation

was used:

St = b.s.vv / ll (7.18)

St = 6.04 (7.19)

Using a vertical tail volume coefficient taken from historical data (0.09)

7.3.2 Vertical tail wing span

bv2 = AR x St (7.20)

Aspect Ratio for vertical tail is AR = 1.7

b = 3.20 (7.21)

7.3.3 Vertical Tail Chords

7.3.3.1 Vertical tail root chord

Taper Ratio of tail section λt = 0.31

Crt = (2 x Sv ) / (bv x (1+λ)) (7.22)

Croot = 2.88 m (7.23)

7.3.3.2 Vertical tail tip chord

Ctip = λ x Croot (7.24)

Ctip = 0.893 (7.25)

7.3. 4 Horizontal tail Mean aerodynamic chord

The value of mean aerodynamic chord is 2.06