Rotary

Question 1

Solution to Lift Asymmetry

Answer 1

Blade Flapping, where the alpha of the advancing blade is reduced by upwards flapping where as the alpha of the retreating blade is increased by downwards flapping.

Question 2

Consequence of Flapping

Answer 2

Tip path plane needs to be adjusted as there is a phase lag between rotor position and flapping meaning in forward flight the blade would point away from direction of travel.

Question 3

What was used to solve the consequence of flapping?

Answer 3

Cyclic pitch control was used to angle of attack of the blades. This was done via a swash plate at the base of the rotor which would be connected to the blades by a rod of fixed length.

Question 4

How was torque reaction dealt with?

Answer 4

Co-axial rotors, layered
Tandem rotors with opposing spin
Tail rotor
Blade tip jets.

Question 5

Overcoming Speed limits

Answer 5

Design changes,
Blade sharpening to reduce shocks
General tip design changes

Question 6

Figure of Merit

Answer 6

M = Pi/(Pi+Po)
Pi = induced power
Po = power to overcome blade drag
A high Figure of Merit means a large proportion of the power
is being used to induce a downward flow of air (and hence
high thrust)
See notes for more equations.

Question 7

Rotor Config :

Single Main Rotor

Answer 7

Increasing collective pitch: increase in alpha and rotor thrust to move forward.
Cyclic pitch control: control when max and min aerodynamic force is produced via moving swash plate.
Yaw control: via tail rotor done by changing collective pitch of tail rotor.

Question 8

Rotor Config :

Single Main Rotor pros and cons

Answer 8

Pros :
Less moving parts
Smaller Airframe
Better maneuverability
Cons :
Some engine power used to power tail rotor, losses in thrust
Limited by COM range and still being able to trim aircraft

Other option is tip driven a/c:
Pros : no yaw reaction so no tail rotor needed
Cons : complex fuel transfer system, heavy tip weigh, loud.

Question 9

Rotor Config :

Twin Rotor Config

Answer 9

Tandem set up, one after the other that spin in opposing directions so torque cancel out.
Yaws via tilting the rotors in opposite directions.
Differential cyclic pitch, rear rotor faster to move forward.

Question 10

Rotor Config :

Twin Rotor Config Pros Cons

Answer 10

Pros : 
No tail rotor
More freedom with COG location
Larger airframe
Cons : 
Interactions between both rotors, airflow disturbed
More complex
Yaw control can negatively couple roll control

Question 11

Rotor Config :

Side by Side

Answer 11

Control methods same as tandem but roll via differential thrust control.

Question 12

Rotor Config :

Side by Side (Transverse) Pros and Cons

Answer 12

Pros:
Both rotors experience same airflow
Wide COG
Cons :
Additional structure required which induces more drag and weight.

Question 13

Rotor Config :

Coaxial

Answer 13

Counter rotating rotor above each other

Question 14

Rotor Config :

Coaxial Pros and Cons

Answer 14

Pros :
Compact Design
No tail rotor
Cons :
Blades interact so not as efficient
Yaw control revered when in autorotation

Synchrocoper similar to this.

Question 15

What is Blade Elementary theory?

Answer 15

Treat each blade like a wing; work out the lift
generated on each blade; multiply by blade number to get
thrust
Uses assumption that downwash is constant along length of blade.

Question 16

What is the equation for rotor pitch angle, theta?

Answer 16

theta = theta,o - 3/4 * k

where k is the blade twist and theta,o is the blade pitch angle.

Question 17

Relationship between ideal twist and linear twist

Answer 17

For a given thrust requirement, the pitch angle at 2/3 radius for ideal twist is equal to the pitch angle at 3/4 radius for the linear twist case.

Question 18

Autorotation

Answer 18

Where airflow up through the rotor can can use rotor to spin and produce lift to control descent.

In the ideal case (with only induced power to overcome), autorotation occurs when VD = vi.
.
In reality, the upflowing air needs to overcome blade drag, so actual descent value for autorotation occurs for a value of VD > vi.
.

Question 19

What are the two solutions to the solve forward flight using momentum theory?

Answer 19

Assume a small a small disk incidence

or use the Newton-Raphson iteration method. (this will probs not be asked in exam as it is hard to do without computer.)

Question 20

Information about the advance ratio, mu

Answer 20

mu = V/Vt

can be split into x and z components.

Question 21

Define different Blade Reference Planes

Answer 21

Hub plane :
Plane perpendicular to rotor shaft, often used for blade dynamic analysis.
Tip Path Plane :
Surface traced out by the rotor tips as they rotate around the azimuth, no blade flapping is observed from this, Tip Path Axis (TPA) is the axis perpendicular to the TPP.
No Feathering Plane :
Plane in which no cyclic pitch change occurs, looking at a rotor blade in the NFP shows no change in blade orientation as it moves around the azimuth, is about the swashplate.

Question 22

What does H-drag represent?

Answer 22

In forward flight, advancing blade generates more drag than retreating blade => net drag force on the rotor this net drag (termed ‘H force’) is the mean rotor drag.
Has two components : Hi = induced drag and Ho = profile drag.

Question 23

What does the torque coefficient express?

Answer 23

Expresses torque required to overcome blade profile drag and generate lift.

Question 24

Describe the retreating blade limits

Answer 24

Low retreating blade speed needs to operate at high AoA to produce sufficient lift to trim the helicopter.
As flight speed increases, required AoA on retreating blade increases.
Thrust generation capacity lost.
Reverse flow region when Ut<0.

Question 25

Why is a lead-lag hinge needed?

Answer 25

As the blade flaps up the COG moves closer to the shaft and opposite for downwards flapping, this changes the speed of the blade.

Question 26

Rotor Articulation description

Answer 26

If flap and lag freedoms are provided via hinges, rotor is said to be articulated. Naming convention is to order the hinges from hub to rotor.
e.g flap-pitch-lag

Question 27

What is pitch-flap coupling?

Answer 27

Occurs when flap motion induces a (mechanical) pitch change, if pitch horn connects along flap hinge line, effect is minimised.
Tail rotors may require deliberate coupling
Aim is to minimise flapping disturbances (no lag hinge)
No need for cyclic pitch control so pitch motion intentionally coupled with flapping motion

Question 28

Alternate solution to flap and lag hinges? And Pros and Cons

Answer 28

Semi-rigid rotors, use flexible structures so that the blade will flex in the required way for flight.
Pros :
Aerodynamic
Low maintenance
Cons :
Vibrations transmitted from hub to fuselage increases

Question 29

Lag Vs Flap points

Answer 29

Flap frequency > rotor speed, typically 1.05Ω -1.1Ω
Lag frequency &laquo_space;rotor speed, typically 0.2Ω -0.3Ω
Flapping damped with lift
Lag damped with drag (much smaller)
Results in lag dampers to avoid ground resonance

Question 30

Climb rate Power assumption equations

Answer 30

For high climb rates :
Pc = Tc*Vc

For low climb rates :
Pc = Phov + (Tc*Vc)/2

Question 31

Equations for mu x and mu z

Answer 31

mu x = V/Vt * cos(alpha_disk)
mu z = V/Vt * sin(alpha_disk)

where V/Vt is also the advance ratio

Question 32

Lag Vs Flap: Frequency Ratios

Answer 32

𝜆𝜁^2 = 3/2*𝑒𝐿 + 𝜆𝜁0^2
𝜆𝛽^2 = 1 + 3/2*𝑒𝑓 + 𝜆𝛽0^2

Both lambda 0 terms are stiffness terms for semi-rigid blades, so would be 0 for articulated blades.