horizontal stabilizer - ulisboa · horizontal stabilizer •there are basically three types of...

Helicopters / Filipe Szolnoky Cunha Slide 101 Conceptual Helicopter Design

Horizontal Stabilizer

• There are basically three types of horizontal

stabilizer design:

– Forward mounted stabilizer

– (Low) Aft mounted stabilizer

– T-tail design


Forward mounted stabilizer

• Avoids the sudden change in download caused

by the wake impingement remaining totally

submerged in the wake (at least until very high

forward velocities).



• Since the are closer to the CG:

– Arm is smaller

– The aerodynamic force must be higher

– Higher area

– Higher structural weight

• The download during hover represents a

significant performance penalty


Aft mounted stabilizer

• Higher arm therefore lower surface area

• On a low mounted stabilizer

– All the loads are carried directly into the tail boom

– Ground clearance can be an issue

– Unsteady separated flow from the upper fuselage and

rotor hub can reduce its efficiency.



• Transition from hover to forward flight can have

a positive pitch (nose up) attitude

Transition to

forward flight



– Transition from forward flight to hover can have a

negative pitch (nose down) attitude

Transition from

forward flight


T-tail design

• As in the aft mounted stabilizer it is positioned as

far way from the CG as possible.

– Lower surface area

– Less weight

• The stabilizer is outside the rotor wake for most

of the helicopter operations

• Vertical fin must carry all the loads meaning

higher overall weight

• Twisting moments may limit the surface area


T-tail design


Stabilator

• A stabilator is a stabilizer that has a variable

incidence capability.

• It can solve the low-speed problems associated

with fixed stabilizer

• The stabilator incidence is automatically based

on airspeed and other measurements.

– Manual override gives the pilot complete control

• Structurally heavier


Stabilator

• In forward flight the stabilator will have a small

negative AOA producing a downward thrust to

counter the helicopter negative pitch attitude


Stabilator

• In hover the AOA will be much higher to prevent

the creation of a download


Stabilator

• In climb the AOA will be small to reduces the

downward thrust adding the climb.


Stabilator

• In autorotation the AOA will be a negative to

prevent an unwanted up thrust


Stabilator

• Stabilator position in forward flight


Stabilator


Fin

• Purpose :

– Provide stability in yaw

• While the stability in yaw is provided by the tail rotor, the vertical stabilizer can (at high forward speeds):

– Alleviate the rotor thrust therefore reducing the power

• Reducing the tail rotor flapping cyclic loads

– Replace the tail rotor in case of failure

• Forms the structural mount of the tail rotor

– Interferes with the tail rotor performance


Fin

• Top mounted fin is more efficient in flight since

the bottom mounted fin is inside the tail boom

wake


Fin

• In descent or autorotation the situation is

reversed


Fin


Tail Boom

• The tail boom must provide structural support for

the:

– Tail rotor

– Fin

– Tail plane

– Other equipment

• We have seen that the helicopter fuselage must

be streamlined to avoid high parasitic drag.


Tail Boom

• Tail boom as a smooth continuation of the

fuselage (also in width)

• This type of boom will suffer a greater download

due to it’s wide dimensions


Tail Boom

• Sometimes is necessary to increase the slope

angle to accommodate a rear ramp:

• These types of fuselage will have a higher drag


Tail Boom

• To decrease the download in hover but at the

same time keep the tail boom strength a

rectangular type of boom is used:


Tail Boom


Tail Boom

• The tail boom provides connection between two

masses:

– Main rotor

– Tail rotor

• Each will experience different forces, some static

some alternating

• This will introduce stresses in the tail boom


Tail Boom

When starting whirling forces from

the main rotor will rock the hull from

side to side

The tail rotor will resist this

movement

This resistance will put the

boom under torsion


Tail Boom

• If this torsional oscillation results from a flight

frequency, the resonant frequency will have to be

change.

– Stiffening the tail boom

– Opening a slot lengthwise and introducing damping

material

• The same kind of attention must be given to

bending frequencies


Tail Boom

• Under the influence of the main rotor downwash

the boom can create a side force.

• This side force could be beneficial:


Tail Boom

• But if the downwash has the wrong direction the effect could be negative:

• This force will be opposite to the tail rotor thrust and therefore might cause lost of tail rotor authority


Tail Boom


Tail Rotor

• Main purpose:

– Provide Anti-torque

– Yaw Stability and directional control about the yaw

axis

• The aerodynamics of the tail rotor provides

weathercock stability:

– Nose-left movement on the helicopter

• Tail rotor in effective climb-> less thrust

– Nose-right movement on the helicopter

• Tail rotor in effective descent-> more thrust


Tail Rotor

• The tail rotor has to operate in a relatively

complex environment.

• It must produce thrust with incoming flow from

every direction.

• The most critical case is when the yaw movement

or side wind forces the tail rotor to operate in an

effective descent.

– If the tail rotor enters the vortex ring state there is a

loss of authority or even a loss of control


Tail Rotor

• Also the tail rotor can operate in the turbulent

separated flow originated:

– Main rotor hub.

– Fuselage

– Main rotor wake itself

• The tail rotor is normally attached to the fin and

there will be a strong aerodynamic interaction

between the two


Tail Rotor

• For these reasons it is very difficult to design a

tail rotor that will meet all the requirements:

– Aerodynamic

– Control

– Stability

– Weight

– Structural


Tail Rotor

• Physical size:

• Tail rotor =1/6 Main rotor diameter


Tail Rotor

• The tail rotor consumes roughly 10% of the total

aircraft power.

• This power is lost since if does result in any lift

production

• The thrust must be equal to:

TRTRzzr lTIQ


Tail Rotor

• The canted tail rotor does provide some lift

– The allowable centre of gravity position is widen

– Adverse coupling between yaw and pitch


Tail Rotor

• The tail rotor can be of two kinds:

Pusher-

Positioned on the

left side of the fin

Tracker- Positioned

on the right side of

the fin


Tail Rotor

• Pusher type:

• The flow is blown way from the fin

• The inflow is distorted by the fin

• Non uniform inflow

• Higher induced power


Tail Rotor

• Tractor type

• The fin is under the wake of the tail rotor

• The fin creates a blockage (ground effect)

• Increase the rotor thrust

• Significant force on the fin

• Force in the opposite direction to the anti-torque requirements

• The net effect is a decrease in thrust compared with an isolated rotor


Tail Rotor


Tail Rotor

Position

• The main purpose of the tail rotor is to provide an

anti-torque stability:


Tail Rotor

Position

Seen from front end of the helicopter

The tail rotor must be at the same height as the main rotor


Tail Rotor

Position


Tail Rotor

• The power requirement depends on the disk loading.

• Large diameter means:

– Less power required

– Heavier design

– Adverse effects of the helicopter CG position

• For certifications it is necessary to sustain a 35kt sideward velocity without entering the vortex state

– A high disk loading is necessary


Tail Rotor

Tail rotor normally have:

• 2 or 4 blades

– The blades are positioned in a “X” rather than a 90º

– Less noise

– There isn’t a dominant blade passing frequency

• Only collective pitch control

• Build in twist to reduce induce power

• Taper to make the inflow even across the blade

– It’s cheaper to produce metal blades without taper

horizontal stabilizer - ulisboa · horizontal stabilizer •there are basically three types of...

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