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higher ⇒ slower

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Pt and Ps, respectively measured by the aircraft pitot tube and static probes, are also used to compute the Mach number. Therefore,

The TAS indicated on the navigation display of modern aircraft is then obtained from the Mach number:

TAS(Kt) = 39M 273 + SAT(ºC)

3.6. True Air Speed (TAS) Variations

Figure A11: True Air Speed Variations – Climb profile 300 Kt / M0.78

The above graph (Figure A11) illustrates the TAS variations as a function of the pressure altitude for a climb at constant CAS (300 knots) and constant Mach (M0.78).

The altitude at which a given CAS is equal to a given Mach number is called the cross-over altitude.

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4. FLIGHT MECHANICS

For a flight at constant speed in level flight, the drag force must balance the

engine thrust.

As a general rule, when engine thrust is higher than drag, the aircraft can use

this excess thrust to accelerate and/or climb. On the other hand, when the thrust is

insufficient to compensate for drag, the aircraft is forced to decelerate and/or

descend.

In flight, four forces are applied to an aircraft : Thrust, drag, lift and weight.

If the aircraft is in steady level flight, the following balance is obtained (Figure A12):

• The thrust for steady level flight (T) is equal to drag (D = ½ ρ S V2 CD),

• Weight (mg) is equal to lift (L = ½ ρ S V2 CL).

Figure A12: Balance of Forces for Steady Level Flight

4.1.1.1. Standard Lift Equation

Weight = mg = ½ ρ S (TAS)2 CL (1)

With m = Aircraft mass

g = Gravitational acceleration

ρ = Air density

S = Wing area

CL = lift coefficient

The lift coefficient, CL, is a function of the angle of attack (α), the Mach number (M), and the aircraft configuration.

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4.1.1.2. Standard Drag Equation

Thrust = ½ ρ S (TAS)2 CD (2)

With CD = Drag coefficient

The drag coefficient, CD, is a function of the angle of attack (α), the Mach number (M) and the aircraft configuration.

4.1.1.3. Other Formulas

• As a function of the Mach number:

Lift and drag equations may be expressed with the Mach number M. As a result, the equations are:

Weight = 0.7 PS S M2 CL (3)

Thrust = 0.7 PS S M2 CD (4)

With Ps

= Static Pressure

• As a function of P0:

The pressure ratio δ is introduced into the lift and drag equations:

0 P

P δ = s

With P0 = Pressure at Sea Level

Ps = Pressure at Flight Level

Therefore, the following equations are independent of pressure altitude:

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B. AIRCRAFT LIMITATIONS

1. FLIGHT LIMITATIONS

During aircraft operation, the airframe must endure the forces generated from

such sources as engine(s), aerodynamic loads, and inertial forces. In still air, when

the aircraft is maneuvering, or during in flight turbulence, load factors (n) appear and

thereby increase loads on the aircraft. This leads to the establishment of maximum

weights and maximum speeds.

1.1. Limit Load Factors

JAR 25.301 Subpart C FAR 25.301 Subpart C

JAR 25.303 Subpart C FAR 25.303 Subpart C

JAR 25.305 Subpart C FAR 25.305 Subpart C

JAR 25.307 Subpart C FAR 25.307 Subpart C

JAR 25.321 Subpart C FAR 25.321 Subpart C

JAR 25.1531 Subpart G FAR 25.1531 Subpart G

“JAR/FAR 25.301 Loads

(a) Strength requirements are specified in terms of limit loads (the maximum loads to

be expected in service) and ultimate loads (limit loads multiplied by prescribed factors

of safety). Unless otherwise provided, prescribed loads are limit loads.”

“JAR/FAR 25.321 Flight Loads

(a) Flight Load Factors represent the ratio of the aerodynamic force component (acting normal to the assumed longitudinal axis of the airplane) to the weight of the airplane. A positive load factor is one in which the aerodynamic force acts upward with respect to the airplane.”

Weight

n Lift z =

Except when the lift force is equal to the weight and nz=1 (for instance in straight and level flight), the aircraft’s apparent weight is different from its real weight (mg):

In some cases, the load factor is greater than 1 (turn, resource, turbulence). In other cases, it may be less than 1 (rough air). The aircraft's structure is obviously designed to resist such load factors, up to the limits imposed by regulations.

Apparent weight = nz.m.g = Lift