Horizontal Stabilizer
• There are basically three types of horizontal
stabilizer design:
– Forward mounted stabilizer
– (Low) Aft mounted stabilizer
– T-tail design
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 101
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).
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 102
Forward mounted stabilizer
• 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
significant performance penalty
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
represents
a
Slide 103
Forward mounted stabilizer
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 104
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.
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 105
Aft mounted stabilizer
• Transition from hover to forward flight can have
a positive pitch (nose up) attitude
Transition to
forward flight
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 106
Aft mounted stabilizer
– Transition from forward flight to hover can have a
negative pitch (nose down) attitude
Transition from
forward flight
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 107
Aft mounted stabilizer
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 108
Aft mounted stabilizer
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 109
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 110
T-tail design
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 111
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 112
Stabilator
• In forward flight the stabilator will have a small
negative AOA producing a downward thrust to
counter the helicopter negative pitch attitude
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 113
Stabilator
• In hover the AOA will be much higher to prevent
the creation of a download
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 114
Stabilator
• In climb the AOA will be small to reduces the
downward thrust adding the climb.
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 115
Stabilator
• In autorotation the AOA will be a negative to
prevent an unwanted up thrust
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 116
Stabilator
• Stabilator position in forward flight
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 117
Stabilator
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 118
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 119
Fin
• Top mounted fin is more efficient in flight since
the bottom mounted fin is inside the tail boom
wake
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 120
Fin
• In descent or autorotation the situation is
reversed
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 121
Fin
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 122
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.
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 123
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 124
Tail Boom
• Sometimes is necessary to increase the slope
angle to accommodate a rear ramp:
• These types of fuselage will have a higher drag
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 125
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:
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 126
Tail Boom
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 127
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 128
Tail Boom
When starting whirling forces from
This
The
resistance
tail
rotor
will
put
resist
the
this
the main rotor will rock the hull from
boomside
under
movement
torsion
to side
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 129
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 130
Tail Boom
• Under the influence of the main rotor downwash
the boom can create a side force.
• This side force could be beneficial:
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 131
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 132
Tail Boom
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 133
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 134
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 135
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 136
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 137
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 138
Tail Rotor
• Physical size:
• Tail rotor =1/6 Main rotor diameter
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 139
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:
  T l
Qr  I zz 
TR TR
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 140
Tail Rotor
• The canted tail rotor does provide some lift
– The allowable centre of gravity position is widen
– Adverse coupling between yaw and pitch
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 141
Tail Rotor
• The tail rotor can be of two kinds:
Tracker- Positioned
Pusheron the right side of
Positioned on the
the fin
left side of the fin
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 142
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 143
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 144
Tail Rotor
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 145
Tail Rotor
Position
• The main purpose of the tail rotor is to provide an
anti-torque stability:
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 146
Tail Rotor
Position
Seen from front end of the helicopter
The tail rotor must be at the same height as the main rotor
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 147
Tail Rotor
Position
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 148
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 149
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
Helicopters / Filipe Szolnoky Cunha
Conceptual Helicopter Design
Slide 150