2. The design of combat aircraft and flight...

Flight Dynamics & Control, Gerard Leng

2. The design of combat aircraft and flight stabilityor why are the control surfaces located there?

An aircraft is trimmed if there are no nett forces and momentsacting on it

i.e. the nett pitching moment coefficient is zero

M = 1/2 ρ V2 S c Cm = 0

2.1 Trim condition


2.2 Concept of static stability

Static stability refers to the tendency of the aircraft todevelop forces or moments to return to its trimcondition when disturbed

Question : What does that mean ?


2.3 Longitudinal static stability

Longitudinal static stability refers to the tendency of the aircraft toreturn to its trim condition after a nose up or nose down disturbance

This implies that Cm must vary with AOA in a certain manner !!

Question : What manner ?


Figure 2.3.1 : Cm variation with AOA

α

Cm

α

Cm

Statically unstable Statically stable

trim point trim point


Moral

A statically stable aircraft must have ∂Cm /∂α < 0

Question : Is static stability always a good thing ?

Note : Aeronautical engineers usually write ∂Cm /∂α as Cmα


2.4 Tail configuration - statically stable

W

c.g. wing

Lw

Ltlw

lt

tail


2.5 Tail configuration - statically unstable

W

c.g.wing

Lw

Lt

lw lt

tail


Moral

Aircraft stability depends critically on cg location

Note : All aircraft have forward and aft cg limits

Question : What happens if either forward or aft limits are violated ?


2.6 Tail volume ratio

Effectiveness of a tail depends on both its surface area and moment arm

The tail volume ratio is a normalised measure of the tail effectiveness

St lt

VH = -------Sw lw

Question : What are typical values for VH ?


Table 2.6.1 : Horizontal tail volume ratio

0.54 0.47 0.6 0.49 0.52 0.25 0.49 NA 0.41 0.55 0.44

Question : Can you spot the atypical cases ?


Moral

Correct horizontal tail location requires an appreciationof the aircraft’s function and aerodynamics

Question : Is a tail configuration the only possible configuration ?


2.7 Canard configuration

W

c.g. wing

LwLc

lwlc

canard

Idea : Put the “tail” in front

Question : Radical idea ? Any examples ?


Question : Can we do without tails and canards ?


2.9 Lateral static stability

Lateral static stability refers to the ability of the aircraft to generatea yawning moment to cancel disturbances in sideslip

V Question : Which direction should theyawing moment act to align the aircraft withthe velocity vector ?

positive sideslip


2.10 Yawing moment variation with sideslip

β

Cn

1. Cn should be an anti symmetric function of β

2. ∂Cn / ∂β ( denoted as Cn β ) must be > 0 for lateral staticstability


2.11 : Vertical tail volume ratio

Sv lv

Vv = -------Sw lw

Similarly the effectiveness of the vertical tail is related to thevertical tail volume ratio

lv : distance from aerodynamic center of vertical tail to cg


Table 2.11.1 : Vertical tail volume ratio

0.16 0.20 0.17 0.18 0.17 0.13 0.30 0.14 0.26 0.23 0.19

Question : What can you observe ?

2. The design of combat aircraft and flight...

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