tail rotor - ulisboa · conceptual helicopter design slide 157 tail rotor ... –heavy lifting ......

Helicopters / Filipe Szolnoky Cunha Slide 151 Conceptual Helicopter Design

Tail Rotor

• Operates at the same tip speed as the main rotor.

Lower speeds are necessary to:

– Minimize noise but

– Requires a higher solidity (stall)

– Increases the torque requirement

– Increases the system weight


Tail Rotor

• Tail rotor normally does not have:

– Cyclic pitch (disk orientation is not required)

– Lead/lag hinges (saves weight)

• But since the flapping movement is allowed they

will have a large amount of pitch-flap coupling


Tail Rotor

• The main rotor wake is known to roll up at its

edges into a pair of vortex bundles.


Tail Rotor


Tail Rotor

• The tail rotor turns in a direction so that the top

of the rotor is retreating aft.


Tail Rotor

• This movement is found to:

– Balance the aerodynamic forces

– Maintain good tail rotor authority over a wider range

of flight conditions

– BVI between tail blades and the main rotor vortices is

decreased and vortex ring conditions are delayed


Tail Rotor

• Due to the high rotational velocity it might be

difficult to see the spinning tail rotor.

• Danger for the ground personnel

• To avoid this:

– Light above the fin when the tail rotor is rotating

– Paint black and white strips on the blades

– Do not approach a hovering helicopter or one with a

rotating main rotor.


Other Anti-torque devices

• Fan-in-fin or fenestron



• Advantages:

– In forward flight the fenestron is protected from the

external flow and main rotor wake

• Performance more predictable

– The aerodynamic interference with the fin are less

important

– Lower power requirements

– Smaller and lighter design

– Reduce the rotor strike possibility when flaying at

low level



• Disadvantages:

– Larger number of blade implies a higher noise

frequency

– Possibility of flow separation at the inlet lip

– The length of the duct must be kept small enough to

avoid a heavy structure



Momentum analysis

• Conservation of mass:

• Conservation of momentum

• Rewriting

iAvm wAadd

i

a

vw

T fanduct TT wm wAvid

i

a

Av 2

eff

di

A

T

A

Tav

2

d

effa

AA

2



• Applying the Bernoulli equation between stations

0 and 1

• And between 2 and 3

• The Thrust on the fan is

2

102

1ivpp

2

0

2

22

1

2

1wpvp i

AwAppT fan

2

122

1



• We can reach :

• The induce power consumed by the fan is:

• or

T

T fan

wAv

Aw

i

2

21

iv

w

2 da2

1

faniP ifanvT

A

Ta

a

T d

d 2 Aa

T

d4

23

TRi

fani

P

P

da2

1



• Ideally ad=0.5 where Aeff=A

• If the shape of the duct is controlled :

– Wake will not contract as much as would occur

naturally

– Less power will be required to produce a given total

thrust.

• If ad=1 (no wake contraction) a ducted fan will

consume 1/√2 of the power of a conventional

rotor.



NOTAR

• The NO TAil Rotor uses a different approach to

the anti-torque and yaw control.

• The anti-torque capability comes from a

circulation control concept which results in a

distributed side-force along the entire tail boom

assembly.



NOTAR

• A jet of air from a pressurized tail boom is blown

tangential to the surface out of narrow slots that

run length wise on one side of the boom.



• The nozzle in the jet

thrusters is rotated using

the conventional action

of the pilot’s pedals

• In forward flight the

circulation control is less

effective as the main

rotor wake moves further

along the boom



• Advantages

– Low Noise

– Safety for ground personnel

– Freedom of blade strikes

– Absence of a vulnerable tail rotor assembly

– High level of redundancy in the event of any boom

damage



McDonnell

Douglas

MD-500

MH-90 Enforcer


Multi-rotor helicopters

• Are significantly less common than the conventional main rotor-tail rotor type.

• This situation is caused by:

– Added complexity

– Additional skills needed to for the development

• Have advantages in same market niches

• When the niche is filled no other manufacturer then seeks to compete

• As result of this situation each type is of machine will be associated with a particular manufacturer



Tandem=Vertol/Boeing



Co-axial=Kamov



Synchropter=Kaman



Side-by-Side=Mil



• The possibilities are numerous:

Non overlapped counter-

rotating tandem

Overlapped counter-

rotating tandem

Non overlapped same-

way tandem



• The possibilities are numerous:

Side by side non

overlapped counter-

rotating

Side by side overlapped

counter-rotating



• Cancelling the torque can be achieved by:

– Counter rotating rotors.

– Rotors rotating in the same direction:



Side by side:

• Advantages in forward flight:

– Improves the aspect ration of the “disc”

– Reduces the induced power

• Disadvantages:

– Pylon structure causes drag

– Difficult to obtain structural rigidity



Coaxial system

• Advantages:

– For uses in naval vessels, with small hangar capacity,

the absences of the tail rotor allows a more compact

hull design.

– As for power consumption it has been studied in

previous chapters



Coaxial system

• For operation two swash-plates are needed

• The cyclic control tilts both rotor in the same sense

• The collective control applies the same pitch to both rotors



Coaxial system

• Yaw control

• Pitch in one rotor is increased, lowered in the other

• There is a net torque rotating the aircraft in the direction

of the rotor where the pitch was lowered



Coaxial system

• Autorotation and the yaw control:

– In Autorotation the pitch is set to the for (L/D)max.

– Increasing or decreasing the pitch there will be a

increase of drag and the rotor will slow down

– If the point is to the left of (L/D)max increasing the

pitch will decrease D and so the yaw control is

reversed

• To overcame this problem coaxial helicopters

need a significant amount of vertical tail area.



Synchropter

• Two counter-rotating synchronized rotors

mounted side by side.

• The rotor shaft are set on pylons and are tilted

outwards so that the blades of one rotor can pass

over the head of the other.

• Although there is no tail rotor there is no safety

advantage since the path of the blade is close to

the ground



Synchropter

• Aerodynamically the synchropter is similar to the

coaxial helicopter

• Since there is a lateral separation of the two

rotors:

– The “disc” area is higher

– The disc loading is smaller

• Limited to a two blade rotor:

– Low solidity



Synchropter

• This is ideal for:

– Low speed

– High altitude

– Heavy lifting

• Yaw control is achieved in the same way as the

coaxial helicopter

– In autorotation Yaw reversal is a possibility



Synchropter

• The need of the large fin area is limited by:

– Trajectory of the retreating blades

– Ground distance in a flared landing



Synchropter

• Power is lost in the cancelling horizontal thrust

components

• The twin rotor heads cause a drag penalty.

• The outward tilt of the rotors means that the

torque cancels in the vertical axis but there is a

component in the horizontal axis that adds up.



Synchropter

• If the advancing blade is on the inside the torque

reaction will tend to pitch the hull nose up:

– If the CM is far enough ahead of the mast it will

counter the torque

– In hover this will require a application of forward tilt

– In forward flight the drag of the hull will act below of

the rotor heads and produce a couple tending to tilt

the nose down



Tandem Helicopters

• A wider range of CM positions is possible



Tandem Helicopters

• In a two motor helicopter is common that either

engine can drive both rotors



Tandem Helicopters

• In forward flight:

– Both rotor are tilted forward

– Remain in the same plane of translation



Tandem Helicopters

• In forward flight:

– The rear rotor will be in the wake of the front rotor

except if it is positioned above the front rotor



Tandem Helicopters

• The rear rotor is mounted on a pylon shaped to act as a fin

• The front rotor is mounted on a pylon shaped not to act as a fin


Other type of rotorcraft

• Compound Helicopters

– Lift compounding

– Thrust compounding

• The idea is to enhance the basic performance

metrics of the helicopter such as:

– Lift-to-drag ratio

– Propulsive efficiency

– Maneuverability


Lift and Thrust compounding

McDonnell XV-1


Lift and Thrust compounding

Sikorsky NH-3 (based on the S-61)


Lift compounding

Boeing-Vertol Model 347


Expanding the envelop


Helicopter

Compound Helicopter

Tilt Rotor

Flapped Tilt Wing

Tilt Duct

Tilt Propeller

0

10

20

30

40

200 400 600

Buried Fan

Jet

Hovering

Time

(Min)

Tradeoff between Hovering Time and Design

Cruising Speed


Rotorcraft- No single configuration optimum

Tiltrotor (but what about $DOC ??

And it scales down poorly)

Autogyro

Light Helicopter

Compound Helo


Sikorsky X-2

tail rotor - ulisboa · conceptual helicopter design slide 157 tail rotor ... –heavy lifting ......

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