Power Management
1-212th Aviation Regiment
WHAT IS POWER MANAGEMENT?
Operating a helicopter with an awareness of the limitations of
that helicopters’ engine and rotor system.
This allows a pilot to avoid an unintended descent which
results in unwanted contact with the ground or other
obstacles.
Two broad categories:
AVAILABLE ENGINE POWER
ROTOR SYSTEM EFFICIENCY
AVAILABLE ENGINE POWER
Affected by:
Environmental conditions (Density Altitude)
Hot temperatures / High altitudes are conducive to high DA
conditions.
Determined by Performance Planning
Unable to be controlled in flight
BASIC ENGINE OPERATION
Engine air is taken in through the intake
BASIC ENGINE OPERATION
Compressed by the compressor
BASIC ENGINE OPERATION
Passes into the combustion chamber
BASIC ENGINE OPERATION
This energy then passes through the
Gas Producer turbine
BASIC ENGINE OPERATION
Then through the Power turbine and
out of the exhaust.
BASIC ENGINE OPERATION
Some engine layouts may differ, but
the operating principles are the same
Air is ingested by the engine
and compressed by VOLUME
Air is ingested by the engine and
compressed by VOLUME
Approximately 25% of this
VOLUME is used for combustion
with fuel
The approximate
MASS ratio of air /
fuel is 15:1 during
combustion.
The other 75% is used for
cooling the engine
Approximately 25% of this
VOLUME is used for combustion
with fuel
The approximate
MASS ratio of air /
fuel is 15:1 during
combustion.
The other 75% is used for
cooling the engine
DENSITY ALTITUDE EFFECTS UPON
AVAILABLE ENGINE POWER
Power generation depends upon the MASS of fuel + air able to
be combusted.
This is a 15:1 ratio
Engine cooling depends upon the MASS of the cooling charge.
The MASS of a certain volume of air changes depending upon
the Density Altitude.
HIGH DA – Low air density
LOW DA – High air density
LESS MASS
MORE MASS
The MASS of air may change with DA, but the required MASS of
air for engine performance does not change.
All other variables being equal, two different engines burning
the same amount of fuel per hour will create the same amount of
power.
Example: 500 LBS per HOUR = 800 HP
That 500 LBS of fuel requires a certain MASS of air with
which to combust
A certain MASS of air is also required to keep the engine cool
This cannot change if the engine is to provide the performance
for which it was designed
If the engine cannot take in enough air to combust the necessary
amount of fuel, and to cool itself, then a limitation will be
encountered
The question that is answered during Performance Planning is:
While burning the amount of fuel and air necessary to make 800
HP, is there enough air MASS left over from that to sufficiently
cool the engine?
YES – Maximum power is available.
NO – Then you cannot use that much air to burn that much fuel.
There must be a tradeoff.
During HI DA conditions, the amount of air burned with fuel must be
reduced, so that this air may be used for cooling.
The combustion charge within the engine will be smaller.
This will result in decreased torque available.
AVAILABLE
TORQUE
LO DA
MAX
TGT
MAX
TGT
HI DA
AVAILABLE
TORQUE
MAX
TORQUE
MAX
TORQUE
This relationship is calculated during performance
planning.
PPC LIMITATIONS:
BASED UPON STATIC, CONSTANT CONDITIONS WITH NO
WIND.
DOES NOT TAKE INTO ACCOUNT TRANSIENT, IN FLIGHT
CHANGES, SUCH AS WEIGHT INCREASES DUE TO “G”
FORCES.
VARIABLES AFFECTING ROTOR EFFICIENCY
INDUCED FLOW EFFECTS
EFFECTIVE TRANSLATIONAL LIFT
GROUND EFFECT
ROTOR INFLOW
DENSITY ALTITUDE
BLADE CONING
VORTEX RING STATE
TRANSIENT TORQUE
INDUCED FLOW
Has a direct effect on the efficiency of the rotor system.
The greater the amount of induced flow present, the less efficient
the rotor system
WHAT IS INDUCED FLOW?
It is the downward component of air movement across an airfoil.
CL
RESULTANT
RELATIVE WIND
INDUCED
FLOW
ROTATIONAL
RELATIVE WIND
INDUCED
DRAG
LIFT
ANGLE OF
ATTACK
RESULTANT
RELATIVE WIND
TAF
DRAG
INDUCED FLOW
VECTOR DIAGRAMS CAN
APPLY TO INDIVIDUAL
PORTIONS OF THE ROTOR
BLADES
OR TO THE ENTIRE ROTOR
SYSTEM
Both rotor systems are producing
the same amount of lift, because
they have the same overall angle
of attack
However; this one is using more
power to do it
A rotor system requires more engine power to produce a certain
amount of lift if it is operating with an increased amount of induced
flow
Because this higher induced flow creates more induced drag
against which the rotor blades must work
If the rotor system cannot overcome this drag, the result will be:
RPM droop due to excessive drag slowing the rotor
system
Overtorque, overtemp, or both, while the aircraft is forced to provide
the needed lift
If the aircraft has TGT or torque limiting, the aircraft will continue
to descend, because it just WON’T give any more
Or a combination of any of the above
Excess engine power allows the rotor to produce the
needed angle of attack, and in turn, required lift during
conditions of very high induced flow and drag.
While excess power gives the aviator higher margins for
error, it does not make one invincible.
10,000 HP may sound like a lot, but it does no good if
the rotor system is unable to use it.
Control of induced flow:
EFFECTIVE TRANSLATIONAL LIFT
GROUND EFFECT
ROTOR INFLOW
EFFECTIVE TRANSLATIONAL LIFT
When the aircraft is above ETL, the rotor system produces more lift for a
given power setting than during speeds below ETL.
ANGLE OF ATTACK = 5
DEGREES
ANGLE OF INCIDENCE = 25 DEGREES
INDUCED FLOW
ANGLE OF ATTACK = 15
DEGREES
INDUCED FLOW
ANGLE OF INCIDENCE = 25 DEGREES
GROUND EFFECT
When the aircraft is IGE, it’s rotor system operates more efficiently than
when it is out of ground effect.
OGE begins at 1 to 1.25 rotor diameters above the ground
Not a linear relationship with altitude. Most of the efficiency of ground
effect is found within ½ rotor diameters above the ground, with small
decreases in efficiency until the aircraft is OGE.
ROTOR INFLOW
Caused when wind is blown down into the top of the rotor system.
Causes an increase in overall induced flow and drag in the rotor system.
Increases the need for power.
20 KNOT TAILWIND
10 KNOTS FWD SPEED
MINIMIZE DECELERATIVE ATTITUDES WHILE IN A
TAILWIND CONDITION
DENSITY ALTITUDE
In high DA conditions, a higher VOLUME of air must be displaced
downward in order to displace the same MASS of air that it would during
conditions of low air density.
HIGH DA
LOW DA
7000 LBS
LIFT
7000 LBS
LIFT
TWO FOLD: High DA reduces rotor efficiency, and
reduces available engine power.
BLADE CONING
L = CL x p/2 x S x V2
Rotor blades are designed to produce optimum lift with a certain degree
of coning
Excessive blade coning results in loss of rotor efficiency and lift,
because it actually affects the design properties of the blades
themselves
FACTORS THAT CONTRIBUTE TO BLADE CONING
Low rotor RPM - When the rotor system is operating, the blades maintain
their rigidity due to centrifugal force. Loss of this force with collective
pitch applied allows a higher degree of blade coning.
Power droop - Causes low rotor RPM, which causes excessive blade
coning.
** The danger from this can be two fold. If the application of
power causes a droop, the aircraft could descend from
having insufficient power available. Excessive coning in
addition to this will cause an even greater loss of lift.
FACTORS THAT CONTRIBUTE TO BLADE CONING
High gross weight - Increases lift requirements, which causes more blade
coning.
Increased “G” loading - Causes a momentary increase of the gross weight
of the aircraft
THINGS THE PILOT CAN CONTROL WHILE IN FLIGHT
IN or OUT of Ground Effect
ABOVE or BELOW ETL
Transient weight changes
Deceleration rates and
attitudes
Blade coning
Rotor inflow
Example of “G” forces and rotor inflow effects:
AIRCRAFT WEIGHT 10,000
MAX AVAIL POWER 100%
OGE HOVER POWER 100%
MAX TORQUE
AVAILABLE
OGE HOVER
10,000 LBS
Example of “G” forces and rotor inflow effects:
AIRCRAFT WEIGHT 10,000
MAX AVAIL POWER 100%
OGE HOVER POWER 100%
1.2 “G” DECELERATION
BELOW ETL
12,000 LBS
MAX TORQUE
AVAILABLE
OGE HOVER
10,000 LBS
Example of “G” forces and rotor inflow effects:
AIRCRAFT WEIGHT 10,000
MAX AVAIL POWER 100%
OGE HOVER POWER 100%
1.2 “G” DECELERATION
BELOW ETL
12,000 LBS
MAX TORQUE
AVAILABLE
OGE HOVER
10,000 LBS
PERFORMANCE PLANNING DID NOT ACCOUNT FOR:
Transient weight increase
Rotor inflow
Effects of a tailwind on an approach
EFFECTIVE ANGLE
APPROACH ANGLE
Effects of a tailwind on an approach
TAILWIND
EFFECTIVE ANGLE
APPROACH ANGLE
Effects of a tailwind on an approach
COMPOUNDED BY LOWER
ROTOR EFFICIENCY DUE TO
INFLOW
MUCH STEEPER
ANGLE
EXAMPLE OF A POORLY EXECUTED
TAILWIND APPROACH
AIRCRAFT WEIGHT- 12,000 LBS
Speed of the aircraft falls below ETL
while still at an OGE altitude
MAX POWER AVAIL- 100%
IGE HOVER PWR-
75%
OGE HOVER PWR-
87%
This requires at least 87% torque
according to Performance Planning data
OGE
IGE
10 KNOT TAILWIND
BELOW
ETL
OGE
87%
AIRCRAFT WEIGHT- 12,000 LBS
Pilot is late to apply power
MAX POWER AVAIL- 100%
IGE HOVER PWR-
75%
OGE HOVER PWR-
87%
1.25 G’s of deceleration will increase the
aircrafts weight to 15,000 lbs
OGE
IGE
10 KNOT TAILWIND
BELOW
ETL
OGE
87%
WEIGHT
INCREASE
AIRCRAFT WEIGHT- 12,000 LBS
Rotor inflow will reduce the lift
produced by the rotor system even
more
MAX POWER AVAIL- 100%
IGE HOVER PWR-
75%
OGE HOVER PWR-
87%
OGE
IGE
10 KNOT TAILWIND
BELOW
ETL
OGE
87%
WEIGHT
INCREASE
ROTOR
INFLOW
?
AIRCRAFT WEIGHT- 12,000 LBS
Rotor inflow will reduce the lift
produced by the rotor system even
more
MAX POWER AVAIL- 100%
IGE HOVER PWR-
75%
OGE HOVER PWR-
87%
OGE
IGE
EXCESSIVE BLADE CONING AND / OR A
POWER DROOP WILL ONLY WORSEN
THE SITUATION
10 KNOT TAILWIND
BELOW
ETL
OGE
87%
WEIGHT
INCREASE
ROTOR
INFLOW
?
Only two of the four factors involved here were
accounted for during performance planning
BELOW
ETL
OGE
87%
WEIGHT
INCREASE
ROTOR
INFLOW
These need to be planned for while flying
?
Decelerate early, while still above ETL
Give yourself more room in which to
decelerate
OGE
IGE
Each of the factors that would
cause an increased power demand
is countered by a condition that
reflects greater rotor system
efficiency.
10 KNOT TAILWIND
HIGH POWER
CONDITION
OGE
WEIGHT
INCREASE
COUNTERING
FACTOR
ABOVE
ETL
ABOVE
ETL
BELOW
ETL
ROTOR
INFLOW
IGE
MINIMUM
PITCH
ATTITUDE
TERRAIN FLIGHT DECELERATION
ETL
Get most of the deceleration
out of the way early, before
losing ETL
TAILWIND
Remember the times when your rotor is more efficient, and use those
times to make demands from the engine(s).
Don’t stack the variables against yourself
HIGH POWER
CONDITION
OGE
WEIGHT
INCREASE
COUNTERING
FACTOR
ABOVE
ETL
ABOVE
ETL
BELOW
ETL
ROTOR
INFLOW
IGE
MINIMUM
PITCH
ATTITUDE
VORTEX RING STATE
or
SETTLING WITH POWER
A condition in which the helicopter settles into it’s own
downwash.
When the helicopter’s descent matches the descent of the rotor
systems vortices and downwash, it is subject to this
phenomena.
It is a transient state that occurs between normal powered
flight and autorotation
FACTORS THAT ARE CONDUCIVE TO THE
VORTEX RING STATE
20 - 100% of available power applied - With power applied,
vortices and downwash are generated from the rotor system.
Slow forward airspeeds (less than ETL) - At these speeds,
the vortices and downwash descend from the helicopter in a
vertical or near vertical fashion.
300 feet per minute or greater rate of descent - This is the
range where the descent of the helicopter matches the
descent of the vortices and downwash.
Normally, vortices and downwash descend from a hovering helicopter
at 300 to 500 FPM
As the aircraft descends, a region of upflow is created at the center of
the rotor disk
When the rate of descent matches the rate at which the downwash and
vortices descend from the rotor system, the aircraft will experience:
Loss of rotor system lift production
Increased rate of descent
The aircraft is now in the vortex ring state
Rotor is unstable at this point.
Increasing power will only increase the rate of descent.
If the aircraft is at a high enough altitude, and if allowed to continue,
the aircraft will enter an autorotative state, and may continue into a
windmill brake state (overspeeding rotor)
NORMAL
THRUSTING STATE
VORTEX RING
STATE
300 – 500 FPM
WINDMILL BRAKE
STATE
(AUTOROTATIVE)
RECOVERY:
If sufficient power is available, then it should be used EARLY.
If there is sufficient time and altitude, the aircraft may also be flown out
of these conditions with forward or lateral cyclic input.
At low altitudes, the early stages of this phenomena are the most likely
to be encountered.
AIRCRAFT MANEUVERING CONSIDERATIONS
Turning into a tailwind condition
AIRCRAFT BANKING
Sustaining a bank requires more engine power
A 45 bank angle requires 1.4 times the power required for straight and
level flight.
A 60 degree bank requires twice the power.
Bank Angle vs. Power Req.
Bank Angle
Increase in TR
(%)
Load / G
Factor
0
---
1.0
15
3.6
1.054
30
15.4
1.154
45
41.4
1.414
60
100
2.0
70
2.923
80
5.747
85
11.473
90
∞
If adequate excess engine power is available,
increasing collective pitch will enable continued flight
while maintaining airspeed and altitude.
If you do not have the power available, then something must be
traded off, either airspeed or altitude.
BUCKET SPEED
MOST AVAILABLE POWER
LEAST DRAG
BUCKET SPEED
Best maneuvering airspeed
Max endurance airspeed
Minimum rate of descent
airspeed (autorotations)
Transient Torque
This is seen in the cockpit as a momentary increase in torque
when the cyclic is displaced left of center. Conversely, as right
cyclic is applied, a reduction in pitch on the advancing blade
results in a reduction of induced drag that tends to increase Nr
and a corresponding transient torque decrease.
Transient Torque
The amount of total drag within the rotor system is subject to changes
during left and right rolls.
During flight, the types of drag that are affected are:
Advancing Blade – Induced Drag
Retreating Blade – Profile Drag
Advancing Blade – Normally, the advancing blade flaps upward during flight,
creating higher induced flow.
During a left roll, this
induced flow is increased
even more.
DIRECTION OF FLIGHT
This increases the amount
of induced drag on the
advancing blade
DIRECTION OF FLIGHT
Retreating blade – Normally,
the retreating blade flaps
downward during flight, giving
it higher profile drag.
During a left roll, this
profile drag is increased
even more.
During a left roll, the total drag within the rotor
system increases
During a right roll, the total drag within the rotor system
decreases
CONSERVATION OF ANGULAR MOMENTUM
A rotating body will rotate at the same velocity
until some external force is applied to change
the speed of rotation.
UH60 PERFORMANCE CHARACTERISTICS
TRANSIENT ROTOR DROOP -
To minimize transient rotor droop, avoid situations which
result in rapid rotor loading from low Ng SPEED and
% TRQ conditions. Initiate maneuvers with collective
inputs leading or simultaneous to cyclic inputs. During
approach and landing, maintain at least 15% - 20% TRQ
and transient droop will be minimal as hover power is
applied.
4 FACTORS THAT AFFECT TRANSIENT TRQ
1. TRQ transients are proportional with the amount of
power applied.
2. The higher the TRQ setting when lateral cyclic are
made, the higher or lower the transient.
3. The magnitude of cyclic displacement directly
affects the TRQ transient.
4. Drag is increased or decreased by the factor of V2
(Airspeed).
COLLECTIVE APPLICATIONS
1.
During aggressive right turns, RPM R increases, TRQ
decreases – Lead with an increase in collective.
2.
During aggressive left turns, RPM R decreases, TRQ
increases – Lead with a decrease in collective.
3.
During aggressive application of aft cyclic, RPM R increases,
TRQ decreases - Lead with slow increase collective. Be
careful of the transient droop characteristic.
4.
During aggressive application of forward cyclic, RPM R
decreases, TRQ increases - Lead with an decrease in
collective.
Mushing
Mushing results during High G maneuvers when at high
forward airspeeds aft cyclic is abruptly applied. This results
in a change in the airflow pattern on the rotor, exacerbated by
total lift area reduction as a result of rotor disc coning.
Combat Maneuver Do’s and Don’ts
- Every aviator that employs these techniques at the wrong place and time
endangers our ability to continue this critical training.
- Only train maneuvers that have a combat application.
- Taking unnecessary risks when carrying a load of combat equipped
infantry soldiers can be equated to a Commercial Airline pilot showing off
when carrying athletes to the Olympics.
- There is no excuse. Do what the mission requires.
SUMMARY
Pilot controls while in flight:
IN or OUT of Ground Effect
ABOVE or BELOW ETL
Deceleration rates and
attitudes
Do not forget about:
Vortex Ring State
Transient Torque
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3000 Series Tasks for Maneuvering Flight
3005 Demonstrate / Perform Flight Characteristics at Vh-IAS
3006 Perform Maximum Bank Angle
3007 Perform Maximum Pitch Angle
3008 Perform Decelerating Turn