Aerodynamics

Question 1

Aerodynamic center

Answer 1

Is the point about which all changed in lift take place. The aerodynamic center of airfoils in subsonic flight is located at about 25% chord point. As the airfoil becomes subject to supersonic flow, the aerodynamic center changes to the 50% chord point.

Question 2

Subsonic airflow

Answer 2

Can be simplified by assuming the air to be incompressible

Question 3

Speed of sound

Answer 3

Is the rate at which small pressure disturbances will be propagated through the air and this propagation speed is solely a function of air temperature. Since air temperature changes when ascending, the speed of sound also changes when ascending.

Question 4

Mach number

Answer 4

Is the ratio of the true airspeed to the speed of sound.

Question 5

Subsonic
Transonic
Supersonic
Hypersonic

Answer 5

Subsonic Mach below .75
Transonic Mach from .75-1.20
Supersonic Mach from 1.20-5.00
Hypersonic Mach above 5.00

Question 6

Critical Mach number

Answer 6

Is the free stream Mach number at which local Mach number 1.0 is attained at any point on the body under consideration.

Question 7

Drag rise

Answer 7

At 5-10% above the critical Mach number. This is known as drag divergence Mach number (Mcrd). Associated with drag rise are buffet, trim, and stability changes, and decrease in control surface effectiveness.

Question 8

Drag divergence Mach number

Answer 8

At MCRD, drag sharply increases, which subsequently decreases range and economy. Lift decreases and a change in pitching moment occurs.

Question 9

Mach Tuck

Answer 9

The result of an aft shift in the center of pressure causing a nose-down pitching moment.

Question 10

Factors causing Mach tuck

Answer 10

1. Flow separation:
   Shockwave induced flow separation, which normally begins at wing root, causes decrease in the down wash velocity over the elevator, and produces a tendency for the aircraft nose to pitch down.
2. Center of pressure movement:
   This is a result of the shockwave moving aft. The aircrafts cg is now ahead of the aircrafts aerodynamic center of pressure. Pitch down

Question 11

Principle advantages of Sweep

Answer 11

1. Increase the critical Mach number
2. Delay the onset of all compressibility effects
3. Reduce the magnitude of changes in forces due to compressibility.

Question 12

Principle disadvantages of sweep

Answer 12

Wing stalls tip first. Results in serious reduction in lateral control effectiveness and the forward shift of the center of pressure would contribute to a nose up moment

Question 13

Dutch Roll

Answer 13

• Is a coupled lateral-directional oscillation

- Is a combined rolling-yawing oscillation in which the rolling motion preceded the yawing motion.

Question 14

What percent RPM controls more than half of the out put?

Answer 14

The top 20%

Question 15

An increase in altitude will improve power plant performance in two respects:

Answer 15

1. An increase in altitude when below the tropopause will provide lower inlet temperatures.
2. Increased rpm required to furnish cruise thrust at altitude ( an increase in engine speed reduced the specific fuel consumption)

The cruise altitude of the turbofan should be as high as possible.

Question 16

What causes compressor stalls?

Answer 16

• A malfunctioning fuel control. Most apparent during engine acceleration.
• Poor inlet conditions, typical of high angles of attack and sideslip.
• very high altitude flight with associated reduced margins increases the likelihood of compressor stall

Question 17

Compressor stall recovery techniques

Answer 17

• Reduce thrust setting
• Lowering the airplane angle of attack
• Increasing airspeed