AER 5: Propulsion

Question 1

Types of compressors

Answer 1

1. Axial

2. Centrifugal

Question 2

Define Specific Fuel consumption (SFC)

Answer 2

SFC is the measure of engine thermal efficiency.
General rule; the lower the SFC, the more efficient.
(The pounds of fuel needed to generate a certain amount of Thrust (Jets) SHP - Shaft horse power (Props))
FF/SHP or FF/Thrust

Question 3

What does SFC Depend on?

Answer 3

As temp decreases, SFC decreases (for both)
As Altitude increases, SFC decreases (for both)
As speed increases, SFC decreases (to a point, then increases due to prop inefficiencies after about 400kts.)

Question 4

How do you achieve max efficiency (low SFC) for Props?

Answer 4

1. At MCrpm (max cruise RPM)
2. At high altitude (better thermal efficiency in colder air)
3. At a TAS between 300 and 400kts (so as to take advantage of the RAM effect of air while prop efficiency is still high)

Question 5

What are the characteristics of a Turbo Prop engine?

Answer 5

Most air passes through the prop
Small amount of air passes through the engine
80% efficiency at 300 - 400mph
Most efficient SFC - FL250 as it is warmer.

Question 6

What is the work cycle of a turbo prop engine?

Answer 6

1. ‘Suck’ - Intake (a: Inlet/accessory gearbox)
2. ‘Squeeze’ - Compression (b: Axial and c: centrifugal compressors)
3. ‘Bang’ - Combustion (d: combustion chamber, e: combustion turbine, f: power turbine, g: reduction gearbox, h: propeller shaft).
4. ‘Blow’ - Exhaust (i: exhaust)

Question 7

What is the thrust produced by turbo Props?

Answer 7

Props move a large amount of air mass but only slightly accelerate it. (f = M x a)

Question 8

What are the types of propeller?

Answer 8

1. Fixed pitch

2. Variable pitch

Question 9

What are the characteristics of a fixed pitch prop?

What is a disadvantage of a fixed prop?

Answer 9

1. Prop blade is an aerofoil in itself
2. the Angle of the blade is set.
3. More thrust is produced by increasing the RPM of the prop.
4. Disadvantage is that a fixed prop is only at its most efficient AoA at one specific speed (usually designed for cruise speed)

Question 10

What does the rotational velocity of a fixed prop depend on?

Answer 10

1. RPM

2. The diameter of the blade (the bigger the blade, the higher the rotational velocity)

Question 11

Define the shape and motion of a fixed Prop

Answer 11

1. Tips of the blades move faster than the rest of the blades.
2. Blade twist (helical), means that the AoA is at its most efficient for each point of the blade. And produces vortices.

Question 12

Discuss optimum AoA of prop blades and how this effects fixed props.

Answer 12

• Props are similar to wings in that they are designed for low drag, because of this they have poor efficiency unless they are at their optimum AoA.
• Control systems are therefore required to adjust RPM during flight to match AoA with required speed to keep efficiency.

Question 13

Define Variable pitch prop

Answer 13

Pitch of blade determines the speed of the aircraft while the RPM is kept constant.

Question 14

Identify the advantages of variable pitch props

Answer 14

1. Prop efficiency
2. Feathering is possible (in the KA350 this is automatic)
3. Reverse pitch is possible.

Question 15

Explain feathering and its uses/advantages.

Answer 15

• Low speed = fine feathering (flat surface towards RAF) for use during lower speeds e.g. cruise
• High speed = coarse feathering (blade 90 degrees/perpendicular) for use during high speeds, e.g. take off.
• Auto feathering is an advantage as this enhances endurance should an engine failure take place.

Question 16

Define blade angle

Answer 16

The angle between the prop chordline relative to the plane on which it rotates.

Question 17

Define blade AoA

Answer 17

The angle between the prop chordline and the relative airflow.

Question 18

Define reverse pitch

Answer 18

Reverse pitch is possible when the pilot manually overrides the system to reverse the pitch of the blades, therefore reversing thrust from the engine. This makes reverse thrust and reverse taxiing possible.

Question 19

What are the characteristics of a jet engine?

Answer 19

1. ALL air passes through combustion
2. 80% efficiency at speeds higher that 800mph
3. most efficient at flight levels above FL250 as it is cooler.