Aerodynamics

Question 1

What is inflow roll?

Answer 1

A roll to the side of the advancing blade.

Occurs when moving from a stationary hover to fwd flight once the disc has 10-20kts of airflow.

Due to the front of the disc having a greater horizontal component of airflow producing more lift and the aft of the disc having a greater vertical component of airflow producing less lift.

Gyroscopic precession means that the effect manifests 90 degrees later causing the lateral roll.

Compensated by the cyclic.

Question 2

What causes Flapback?

Answer 2

A dissymmetry in lift due to adv blade experiencing a higher velocity of relative wind and the retreating side a lower velocity of relative wind. Phase lag means that the effect is felt 90 degrees later causing the disc to flapback. Over come by cyclic feathering.

Question 3

Pitch angle

Answer 3

Angle between POR and Chord line

Question 4

Induced Velocity

Answer 4

Speed of air being pushed downwards by the rotor

Question 5

Induced flow

Answer 5

The mass of air being accelerated downwards by rotor action

Question 6

Resultant airflow

Answer 6

Vertical (ind.Vel) and horizontal (blade rotation) components

Question 7

AoA

Answer 7

Angle between RAF and Chordline

Question 8

3 factors affecting AoA

Answer 8

Airspeed, blade pitch angle, Nr

Question 9

Induced drag

Answer 9

Acts parallel to RaF, resultant of producing lift

Question 10

Profile drag

Answer 10

Drag caused by air flowing around airfoil. Skin friction, shape, airspeed are 3 sources.

Question 11

The coning angle is a result of which two forces

Answer 11

centrifugal (larger force) and lift forces

Question 12

How is coning angle measured

Answer 12

Angle between rotor hub and tip path plane

Question 13

What is momentum

Answer 13

Mass x Velocity

Question 14

Flapping to equality

Answer 14

In fwd flight adv blade flaps up as raf increases causing AoA to decrease and lift to decrease slightly.

Retreating side flaps down as raf decreases, AoA increases and lift increases slightly

Use of both flapping and cyclic changes pitch to equalise lift

Question 15

What 2 components make up total reaction?

Answer 15

Rotor thrust ( opposite to axis of rotation) and Rotor drag (TQ)( opposite to blade travel)

Question 16

Difference between rotor drag and aerodynamic drag?

Answer 16

Rotor drag (powered flight) acts in plane of rotation
Aerodynamic drag (autorotation) acts in line with relative airflow

Question 17

What is total rotor thrust?

Answer 17

TRT is sum of all rotor thrust and opposes weight through top of the mast.

Question 18

4 factors effecting rotor thrust

Answer 18

air density
RRPM
blade (pitch) angle
disc area

Question 18

Reduced air density causes pilot to struggle with what?

Answer 18

requirement for larger angles of ask
worsening TRT/rotor drag ratio
requirement for more power

max operating ability and manoeuvrability is directly related to these limiting factors

Question 19

2 forces dictating coning angle

Answer 19

TRT and centrifugal force

Question 20

disc loading

Answer 20

mass of helicopter/disc area
and is proportional to gross weight and total rotor thrust

Question 21

What is conservation of momentum (Coriolis) and how does it affect movement about the lead lag hinge?

Answer 21

It influences the rotational velocity of a blade when it flaps up or down.

Flap up, velocity of blade increases, CoG moves inboard, Blade moves fwd on hinge

Flap down, Velocity decreases, CoG moves out, blade moves aft on hinge

Question 22

What is Hookes joint effect?

Answer 22

Occurs whenever the axis of rotation and shaft axis are not in line. It allows for blade tips to retain their respective positions on the tip path circumference and their velocity. Ex: if cyclic is moved fwd the adv blade moves fwd on hinge and retreating blade moves aft on hinge.