Download Retreating Blade Stall - Front Range Helicopters

Retreating Blade Stall
Front Range Helicopters
Definition
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The stalled condition of the retreating
blade when it operates beyond its
critical (stall) angle. The condition is
generally (but not only) associated with
high forward airspeed.
What Occurs as Forward
Speed Increases
As the helicopters forward speed increases the advancing blade gains
the advantage of the aircrafts forward speed plus the speed of rotation.
In this example the forward speed of
the blade in rotation is 400knots.
The forward airspeed is
20 knots. So on the
advancing side, right side
of the aircraft, the blade
tip speed has now gained
20 knots and the tip
speed is now traveling at
420 knots. This allows the
advancing side to
produce more lift.
What Occurs as Forward
Speed Increases
Just the opposite is true on the retreating side, left side, of the
aircraft.
The blade tip which was
moving at 400 knots, now
looses the 20 knots due to the
forward airspeed. Blade tip
speed is only 380 knots on the
retreating side. This would
result in less lift being created
than on the advancing side.
This results in a condition
referred to as Dissymmetry
of Lift.
Angles of Attack in the Plane
of Rotation in Forward Flight
(Angle of Attack)
Without some correction, dissymmetry
of lift would cause the helicopter to roll
to the left in forward flight, a problem
encountered by the early designers of
the helicopter.
To compensate for the increase
in lift on the advancing side and the
decrease in lift on the retreating side,
the angle of attack has to change in
the plane of rotation to make lift equal
on each side of the helicopter. Angle of
attack must be reduced on the right
side and increased on the left side.
This is accomplished by Blade
Flapping, as shown in the diagram to
the left.
Blade Flapping
With the addition of the flapping hinge,
the advancing blade flaps up, changing
the angle of the relative wind and
decreasing AoA and flaps down on the
retreating side increasing the AoA
Compensating for
Dissymmetry of Lift
The blades flap up on the
advancing side and down on
the retreating side changing
the relative wind and angle of
attack. Lift on each side of the
rotor is now in balance. The
approximate angles of attack
throughout the plan of rotation
are shown in figure 23-15.
Angle of attack distribution in forward flight
The degree of flapping will
vary based the forward
airspeed of the helicopter to
maintain this balance………at
least up to a point!
Blade Flapping
Do the Main Rotor Blades “Flap” while in a stationary
hover with calm wind?___________________________
Under what conditions would that be
different?______________________________________
While in a stationary hover, would the blades always flap
up on the left and flap down on the right?____________
Other than wind and forward airspeed, what else would
have an impact the angle of attack throughout the plane
of rotation?____________________________________
What makes the Blades Flap?
2
LIFT = C ½pV S
L
(LIFT FORMULA)
(Produced)
C L= Lift Coefficient as established by the design of the airfoil and it’s given
angle of attack
p= Density of air the blade is moving thru
V = True Air Speed at which the airfoil (blade) fly's thru the air
S = Surface area of the airfoil (blade)
Given the lift formula above, why would the
advancing blade flap up and the retreating
blade flap down??
____________________________________
What component of the Lift formula changes
in the plane of rotation?_________________
Retreating Blade Stall
The ability of the main rotor to flap up and down
compensates for dissymmetry of lift, but as forward
airspeed increases eventually there is a speed
reached where the angle of attack on the retreating
blade reaches its maximum lift coefficient and the
blade is stalled. This stalled region begins at the
blade tip and moves inward and is at it’s maximum
at the 9 o’clock position in the plane of rotation.
Additionally when the retreating blade reaches these
critical airspeeds, the inboard section of the
retreating blade may actually experience a flow
reversal where the airflow is moving across the
trailing edge toward the leading edge of the
blade…….oh my!!!!!!
Retreating Blade Stall
Another Look
What’s Happening
on the Advancing Blade
To complicate matters the advancing blade
tip speed, if forward speed is sufficient, is
now approaching the speed of sound (transsonic). The blade designs on symmetrical
rotor blades were not intended to go transsonic, as a result the blade will experience a
sharp rise in drag associated with shock
waves along the blade section, usually the
tip, resulting in a totally disrupted airflow
and pressure changes that can even cause
structural failure of the blade.
VNE speed limits on helicopters are associated
with both the limitations on the advancing
blade AND the retreating blade, though the
symptoms associated with the retreating
blade will get the pilot’s attention first.
Recognizing
Retreating Blade Stall
After looking at the placard for the Schweizer 300C below, what conditions would
lead to retreating blade stall?
•
________________________
• __________________________
•
________________________
• __________________________
•
________________________
• __________________________
Conditions Leading to
Retreating Blade Stall
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High Forward Airspeed
Temp/Pressure Altitude Combinations (High Density
Altitude)
Maneuvers involving high load factors such as steep
turns and pulling out of a high speed dive
Use of excessive or abrupt control movements,
particularly at high altitudes.
Flight in turbulence where excessive airspeed
changes occur.
Any of the above maneuvers in combination with low
rotor RPM
Recognizing
Retreating Blade Stall
Now that we know airspeed, temperature, pressure altitude
and weight of the aircraft are contributing factors for
retreating blade stall, lets look at some symptoms the pilot
can recognize at onset…..
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First the pilot will experience a roughness and vibration in the main
rotor. This has often been described as a “buzz” on initial onset.
If allowed to progress the retreating side will develop a stalled
condition that will cause the nose to pitch up. This is due to gyroscopic
precession, stall on the right side, reaction is 90 degrees later causing
loss of lift over the tail and the nose to pitch up.
At this point the helicopter may start to roll, usually to the left but not
always.
Recovery
The recovery is accomplished by:
 Reducing collective pitch. This reduces the
angle of attack in the plain of rotation and
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should be done first.
Gently Applying aft cyclic to slow the aircraft.
Application of aft cyclic with out a reduction
in collective pitch will only worsen the stall.
Maintaining the main rotor RPM in the upper
part of the green arc.
Conclusion
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The best way to avoid Retreating Blade Stall is the pilots
recognition of the conditions that may lead to the
occurrence.
Have a clear concept of what the aircrafts VNE is for each
day, in the existing conditions (including potential
turbulence), expected altitudes and at the current aircraft
weight.
Unless corrective action is taken by the pilot, retreating
blade stall can led to the total loss of control of the
aircraft.
Always THINK AHEAD of the AIRCRAFT!!