Aero Chapter 01, General Aerodynamics Review

T-45 Aerodynamics Student Workbook

considerably higher than actual field elevation. At a field elevation of 5,000 ft, the standard day

temperature is approximately 5C (41F). With a temperature of 30C (86F), the density altitude is

approximately 7,500 ft, 50% more than actual field elevation. The T-45 NATOPS manuals do not provide

a density altitude conversion chart. However, density ratio must be computed for use with the Takeoff

Charts in the Performance Data section of the T-45 NATOPS.

Aircraft velocity (V) is in true airspeed (TAS). The pitot static system measures dynamic pressure (q) and

gives the pilot indicated airspeed (IAS). The equation for dynamic pressure is q= rV2. At sea level, with

standard day temperature and pressure conditions, IAS equals TAS. At altitude, air density is less than at

sea level and the IAS will be lower than TAS. **BECAUSE THE AIRSPEED INDICATOR IS DIRECTLY**

Consequently, the same indicated airspeed may be used for takeoffs and landings regardless of field

elevation, assuming the same gross weight, configuration, etc.

With most tactical aircraft, wing surface area (S) is not a variable and flap extension will only change the

shape of the wing. However, some types of flaps, such as Fowler flaps, do increase surface area of the

wing. Lowering the flaps changes the wing's shape and possibly surface area, which increases CL.

Drag is the force that acts on the aircraft opposite

the direction of flight through the CG. When

airspeed is constant during straight and level flight,

drag is equal to thrust being produced. For

subsonic flight, total drag can be broken down into

two types, induced drag and parasite drag.

Induced drag is created with and results from the

production of lift. The major factor affecting the

variation of induced drag is the coefficient of lift.

The coefficient of induced drag is illustrated by the

equation:

WHERE: Cdi

= COEFFICIENT OF INDUCED

DRAG

CL

= COEFFICIENT OF LIFT

AR

= ASPECT RATIO (THE RATIO

OF WING SPAN TO AVERAGE CHORD)

As airspeed is reduced in level flight, angle of attack is increased to increase coefficient of lift. As CL is

increased, Cdi increases by the square of the change in CL (Figure 5).

Parasite drag results from skin friction and the form or frontal area of the aircraft. Parasite drag is greatly

influenced by the configuration of the aircraft, external stores, flap or landing gear extension, etc. Parasite

drag is also greatly affected by velocity. As airspeed increases, parasite drag will increase by the square

of the change in airspeed. As aircraft velocity doubles, parasite drag will increase four times. For

example, an aircraft that requires 2,000 pounds of thrust to fly at 200 KTAS will require 8,000 pounds of

thrust to fly at 400 KTAS, ignoring induced drag effects (Figure 5).

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