Introduction
A pilot’s ability to predict the performance of a helicopter is
extremely important. It helps to determine how much weight
the helicopter can carry before takeoff, if the helicopter
can safely hover at a spe cific altitude and temperature, the
distance required to climb above obstacles, and what the
maximum climb rate will be.
Helicopter Performance
Chapter 7
Factors Affecting Performance
A helicopter’s performance is dependent on the power output
of the engine and the lift produced by the rotors, whether
it is the main rotor(s) or tail rotor. Any factor that affects
engine and rotor efficiency affects performance. The three
major factors that affect per formance are density altitude,
weight, and wind. The Pilot’s Handbook of Aeronautical
Knowledge, FAA-H-8083-25 (as revised), discusses these
factors in great detail.
Moisture (Humidity)
Humidity alone is usually not considered an important factor
in calculating density altitude and helicopter performance;
however, it does contribute. There are no rules of thumb used
to compute the effects of humidity on density altitude, but
some manufacturers include charts with 80 percent relative
humidity columns as additional information. There appears
to be an approximately 3–4 percent reduction in performance
compared to dry air at the same altitude and temperature,
so expect a decrease in hovering and takeoff performance
in high humidity conditions. Although 3–4 percent seems
insignificant, it can be the cause of a mishap when already
operating at the limits of the helicopter.
Weight
Weight is one of the most important factors because the pilot
can control it. Most performance charts include weight as one
of the variables. By reducing the weight of the helicopter, a
pilot may be able to take off or land safely at a location that
otherwise would be impossible. However, if ever in doubt
about whether a takeoff or landing can be performed safely,
delay your takeoff until more favorable density altitude
conditions exist. If airborne, try to land at a location that has
more favorable conditions, or one where a landing can be
made that does not require a hover.
In addition, at higher gross weights, the increased power
required to hover produces more torque, which means more
antitorque thrust is required. In some heli copters during high
altitude operations, the maximum antitorque produced by the
tail rotor during a hover may not be sufficient to overcome
torque even if the gross weight is within limits.
Winds
Wind direction and velocity also affect hovering, take off, and
climb performance. Translational lift occurs any time there
is relative airflow over the rotor disk. This occurs whether
the relative airflow is caused by helicopter movement or by
the wind. Assuming a headwind, as wind speed increases,
translational lift increases, resulting in less power required
to hover.
The wind direction is also an important consideration.
Headwinds are the most desirable as they contribute to the
greatest increase in performance. Strong crosswinds and
tailwinds may require the use of more tail rotor thrust to
maintain directional control. This increased tail rotor thrust
absorbs power from the engine, which means there is less
power available to the main rotor for the production of
lift. Some helicopters even have a critical wind azimuth or
maximum safe relative wind chart. Operating the helicopter
beyond these limits could cause loss of tail rotor effectiveness.
Takeoff and climb performance is greatly affected by wind.
When taking off into a headwind, effective trans lational lift
is achieved earlier, resulting in more lift and a steeper climb
angle. When taking off with a tailwind, more distance is
required to accelerate through transla tion lift.
Performance Charts
In developing performance charts, aircraft manufacturers
make certain assumptions about the condition of the
helicopter and the ability of the pilot. It is assumed that
the helicopter is in good operating condition, calm wind,
and the engine is developing its rated power. The pilot is
assumed to be following normal operating procedures and
to have average flying abilities. Average means a pilot
capable of doing each of the required tasks cor rectly and at
the appropriate times.
Using these assumptions, the manufacturer devel ops
performance data for the helicopter based on actual flight
tests. However, they do not test the hel icopter under each
and every condition shown on a performance chart. Instead,
they evaluate specific data and mathematically derive the
remaining data.
Height/Velocity Diagram
The height/velocity (H/V) diagram shows the combinations
of airspeed and height above the ground, which will allow
an average pilot to successfully complete a landing after
an engine failure. By carefully studying the height/velocity
diagram, a pilot is able to avoid the combinations of altitude
and airspeed that may not allow sufficient time or altitude to
enter a stabilized autorotative descent. Refer to Figure 7-1
during the remainder of the discussion on the height/velocity
diagram.
In the simplest explanation, the H/V diagram is a diagram in
which the shaded areas should be avoided, as the pilot may be
unable to complete an autorotation landing without damage.
The H/V diagram usually contains a takeoff profile, where the
diagram can be traversed from zero height and zero speed to
Feet AGL
KIAS
Height/Velocity Diagram
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Avoid operation in shaded areas
8500 FT Density Altitude at 2500 LB
Sea Level at 2500 LB
Recommended
Takeoff Profile
Figure 7-1. Sample height/velocity diagram for a Robinson Model
R44 II.
cruise, without entering the shaded areas or with minimum
exposure to shaded areas.
The grey portion on the left side of the diagram marks a flight
profile that probably does not allow the pilot to complete an
autorotation successfully, primarily due to having insufficient
airspeed to enter an autorotative profile in time to avoid
a crash. The shaded area on the lower right is dangerous
due to the airspeed and proximity to the ground resulting
in dramatically reduced reaction time for the pilot in the
case of mechanical failure, or other in-flight emergencies.
This shaded area at the lower right is not portrayed in H/V
diagrams for multiengine helicopters capable of safely
hovering and flying with a single engine failure.
The following examples further illustrate the relevance of
the H/V diagram to a single-engine helicopter.
At low heights with low airspeed, such as a hover taxi, the
pilot can simply use the kinetic energy from the rotor disk
to cushion the landing with collective, converting rotational
inertia to lift. The aircraft is in a safe part of the H/V diagram.
At the extreme end of the scale (e.g., a three-foot hover
taxi at walking pace) even a complete failure to recognize
the power loss resulting in an uncushioned landing would
probably be survivable.
As the airspeed increases without an increase in height, there
comes a point at which the pilot’s reaction time would be
insufficient to react with a flare in time to prevent a high
speed, and thus probably fatal, ground impact. Another thing
to consider is the length of the tailboom and the response
time of the helicopter flight controls at slow airspeeds and
low altitudes. Even small increases in height give the pilot
much greater time to react; therefore, the bottom right part of
the H/V diagram is usually a shallow gradient. If airspeed is
above ideal autorotation speed, the pilot’s instinct is usually
to flare to convert speed to height and increase rotor rpm
through coning, which also immediately gets them out of the
dead man’s curve.
Conversely, an increase in height without a corresponding
increase in airspeed puts the aircraft above a survivable
uncushioned impact height, and eventually above a height
where rotor inertia can be converted to sufficient lift to enable
a survivable landing. This occurs abruptly with airspeeds
much below the ideal autorotative speed (typically 40–80
knots). The pilot must have enough time to accelerate to
autorotation speed in order to autorotate successfully; this
directly relates to a requirement for height. Above a certain
height the pilot can achieve autorotation speed even from a
zero knot start, thus putting high OGE hovers outside the
curve.
The typical safe takeoff profile involves initiation of forward
flight from a 2–3 feet landing gear height, only gaining
altitude as the helicopter accelerates through translational
lift, as airspeed approaches a safe autorotative speed. At this
point, some of the increased thrust available may be used to
attain safe climb airspeed, which will keep the helicopter out
of the shaded or hatched areas of the H/V diagram. Although
helicopters are not restricted from conducting maneuvers
that will place them in the shaded area of the H/V diagram,
it is important for pilots to understand that operation in
those shaded areas exposes pilot, aircraft, and passengers to
a certain hazard should the engine or driveline malfunction.
The pilot should always evaluate the risk of the maneuver
versus the operational value.
The Effect of Weight Versus Density Altitude
The height/velocity diagram [Figure 7-1] depicts altitude and
airspeed situations from which a successful autorotation can
be made. The time required, and therefore, altitude necessary
to attain a steady state autorotative descent, is dependent on
the weight of the helicopter and the density altitude. For this
reason, the H/V diagram is valid only when the helicopter is
operated in accordance with the gross weight versus density
altitude chart. If published, this chart is found in the RFM
for the particular helicopter. [Figure 7-2] The gross weight
Density altitude (thousands of feet)
Gross weight (pounds)
2,300 2,400 2,500 2,600 2,700 2,800
Figure 7-2. Gross weight versus density altitude.
versus density altitude chart is not intended to provide a
restriction to gross weight, but to be an advisory of the
autorotative capability of the helicopter during takeoff and
climb. A pilot must realize, however, that at gross weights
above those recommended by the gross weight versus density
altitude chart, the values are unknown.
Assuming a density altitude of 8,500 feet, the height/velocity
diagram in Figure 7-1 would be valid up to a gross weight of
approximately 2,500 pounds. This is found by entering the
graph in Figure 7-2 at a density altitude of 8,500 feet (point A),
then moving horizontally to the solid line (point B). Moving
vertically to the bottom of the graph (point C), with the existing
density altitude, the maximum gross weight under which the
height/velocity diagram is applicable is 2,500 pounds.
The production of performance charts and diagrams for
helicopters are regulatory as set out in Title 14 of the Code
of Federal Regulations (14 CFR) Part 27, Airworthiness
Standards. These charts establish safer parameters for
operation. Although not regulatory, the pilot should carry
out a full risk assessment to carefully consider the higher
risk before operating within the shaded areas of the height/
velocity diagram.
Autorotational Performance
Most autorotational performance charts state that autorotational
descent performance is a function of indicated airspeed (IAS)
and is essentially unaffected by density altitude and gross
weight. Keep in mind that, at some point, the potential energy
expended during the autorotation is converted into kinetic
energy for the flare and touchdown phase of the maneuver. It is
at that point that increased density altitudes and heavier gross
weights have a great impact on the successful completion of
the autorotation. The rotor disk must be able to overcome the
downward momentum of the helicopter and provide enough
lift to cushion the landing. With increased density altitudes
and gross weights, the lift potential is reduced and a higher
collective pitch angle (angle of incidence) is required.
During autorotation gravity provides the source of energy
powering the rotor by causing upflow up through the rotor
during descent. This is the same as saying that potential
energy is being traded for kinetic energy to turn the rotor as
the aircraft descends.
In Figure 7-3, the S-300 curve shows the various combinations
of horizontal and vertical speeds that supply the required
energy to keep the rotor turning at a constant 471 rpm.
For example, an airspeed of 54 mph with a corresponding
vertical speed of 1,600 feet per minute (fpm) will provide
enough kinetic energy to maintain the rotor at a 471 rpm. The
rotor does not care if the air is coming from the front or the
bottom so long as the total is sufficient to maintain the rpm.
Any point on the curve will maintain rotor speed. However,
the pilot does care because if he or she, for example, glides
at 30 knots, the corresponding rate of descent will be over
2,200 fpm. Since there is little airspeed for a deceleration
(or “flare”) to reduce the rate of decent before touchdown,
the collective pitch application (increasing blade pitch and
giving a final temporary increase in lift before the blades
slow down) may be insufficient to arrest the rate of descent.
Students who fully comprehend this relationship understand
why training autorotations are usually limited to airspeeds
between the minimum rate of descent airspeed and the
maximum range airspeed (usually about 25 percent faster
than the minimum rate of descent airspeed).
Referencing a curve similar to the one shown in Figure 7-3 is
useful to understand the consequences of not maintaining the
target airspeed when executing an autorotation. Simply put,
the pilot should know why airspeed is the most significant
factor affecting the rate of descent.
Hovering Performance
Helicopter performance revolves around whether or not the
helicopter can be hovered. More power is required during the
hover than in any other flight regime. Obstructions aside, if
a hover can be maintained, a takeoff can be made, especially
with the additional benefit of translational lift. Hover charts
are provided for in ground effect (IGE) hover and out of
ground effect (OGE) hover under various conditions of
True airspeed - mph
Autorotation, 471 Rotor rpm
2,400
2,200
2,000
1,800
1,600
1,400
20 30 40 50 60 70 80 90
Rate of descent - ft/min
Figure 7-3. An autorotation curve for the S-300 shows the various combinations of horizontal and vertical speeds that supply the required
energy to keep the rotor turning at a constant 471 rpm.
gross weight, altitude, temperature, and power. The IGE
hover ceiling is usually higher than the OGE hover ceiling
because of the added lift benefit produced by ground effect.
See Chapter 2, Aerodynamics of Flight, for more details on
IGE and OGE hover. A pilot should always plan an OGE
hover when landing in an area that is uncertain or unverified.
As density altitude increases, more power is required to
hover. At some point, the power required is equal to the
power available. This establishes the hovering ceiling under
the existing conditions. Any adjustment to the gross weight
by varying fuel, payload, or both, affects the hovering ceiling.
The heavier the gross weight, the lower the hovering ceiling.
As gross weight is decreased, the hover ceiling increases.
Sample Hover Problem 1
You are to fly a photographer to a remote location to take
pictures of the local wildlife. Using Figure 7-4 , can you
safely hover in ground effect at your departure point with
the following conditions?
A. Pressure Altitude................................8,000 feet
B. Temperature..........................................+15 °C
C. Takeoff Gross Weight.........................1,250 lb
rpm..............................................104 percent
First enter the chart at 8,000 feet pressure altitude (point A),
then move right until reaching a point mid way between the
+10 °C and +20 °C lines (point B). From that point, proceed
down to find the maximum gross weight where a 2 foot
hover can be achieved. In this case, it is approximately 1,280
pounds (point C).
Since the gross weight of your helicopter is less than this,
you can safely hover with these conditions.
Sample Hover Problem 2
Once you reach the remote location in the previous problem,
you will need to hover OGE for some of the pictures. The
pressure altitude at the remote site is 9,000 feet, and you will
use 50 pounds of fuel getting there. (The new gross weight is
now 1,200 pounds.) The temperature will remain at +15 °C.
Using Figure 7-5, can you accomplish the mission?
Enter the chart at 9,000 feet (point A) and proceed to point
B (+15 °C). From there, determine that the maxi mum gross
Max continuous or full throttle
OGE hover ceiling vs. gross weight
Pressure altitude (PA x feet in thousands)
900 1,000 1,100 1,200 1,300 1,400
425 450 475 500 525 550 575 600 625
Density altitude 12,600 feet
°C °F
−20
−10
+10
+20
+30
+40
−4
+14
+32
+50
+68
+86
+104
OAT
GROSS WEIGHT (LB)
OUT OF GROUND EFFECT
Full throttle (or limit manifold pressure) and 104% rpm
GROSS WEIGHT (KG)
−20°C
−10°C
0°C
+10°C
+20°C
+30°C
+40°C
Standard day
Figure 7-5. Out of ground effect hover ceiling versus gross weight
chart.
IGE hover ceiling vs. gross weight
Pressure altitude (PA x feet in thousands)
Density altitude 12,600 feet
900 1,000 1,100 1,200 1,300 1,400
425 450 475 500 525 550 575 600 625
°C °F
−20
−10
+10
+20
+30
+40
−4
+14
+32
+50
+68
+86
+104
OAT
GROSS WEIGHT (LB)
IN GROUND EFFECT AT 2-FOOT SKID CLEARANCE
Full throttle and 104% rpm
GROSS WEIGHT (KG)
−20°C
1,370
−10°C
0°C
+10°C
+20°C
+30°C
+40°C
Standard day
Figure 7-4. In ground effect hovering ceiling versus gross weight
chart.
weight to hover OGE is approximately 1,130 pounds (point
C). Since your gross weight is higher than this value, you will
not be able to hover in these conditions. To accomplish the
mission, you will need to remove approximately 70 pounds
before you begin the flight.
These two sample problems emphasize the importance of
determining the gross weight and hover ceiling throughout
the entire flight operation. Being able to hover at the take off
location with a specific gross weight does not ensure the
same performance at the landing point. If the destination
point is at a higher density altitude because of higher
elevation, temperature, and/or relative humidity, more power
is required to hover there. You should be able to predict
whether hovering power will be available at the destination
by knowing the temperature and wind conditions, using
the performance charts in the helicopter flight manual, and
making certain power checks during hover and in flight prior
to commencing the approach and landing.
For helicopters with dual engines, performance charts provide
torque amounts for both engines.
Sample Hover Problem 3
Using Figure 7-6, determine what torque is required to hover.
Use the following conditions:
A. Pressure Altitude . . . . . . . . . . . . . . . . . . . ..9,500 feet
B. Outside Air Temperature .............. . . . . 0 °C
C. Gross Weight. . . . . . . . . . . . . . . . . . . . . . . . . 4,250 lb
D. Desired Skid Height . . . . . . . . . . . . . . . . . . . . . . 5 feet
First, enter the chart at 9,500 feet pressure altitude, then
move right to outside air temperature, 0 °C. From that point,
move down to 4,250 pounds gross weight and then move left
to 5-foot skid height. Drop down to read 66 percent torque
required to hover.
Climb Performance
Mast torque - percent
Det mast torque - percent
Density altitude - feet
Pressure altitude - feet
Engine torque - percent
100 90 80 70 60 50 40
100 90 80 70 60 50 40 -5000 0 5000 10000 15000
50 (OGE)
Skid height - feet
- 60FAT °C
- 40
- 20 0
+ 20
+ 40
+ 60
Gross weight - pounds
A B
66%
Figure 7-6. Torque required for cruise or level flight.
Most of the factors affecting hover and takeoff per formance
also affect climb performance. In addition, turbulent air, pilot
techniques, and overall condition of the helicopter can cause
climb performance to vary.
A helicopter flown at the best rate-of-climb speed (VY) obtains
the greatest gain in altitude over a given period of time. This
speed is normally used during the climb after all obstacles
have been cleared and is usu ally maintained until reaching
cruise altitude. Rate of climb must not be confused with angle
of climb. Angle of climb is a function of altitude gained over
a given distance. The VY results in the highest climb rate, but
not the steepest climb angle, and may not be sufficient to clear
obstructions. The best angle of climb speed (VX) depends upon
the power available. If there is a surplus of power available,
Legend
Maximum range
Maximum rate of climb or maximum end
Continuous torque
Torque (%)
Fuel flow (pounds per hour)
30 40 50 60 70 80 90 100
PRESSURE ALTITUDE = 8,000 FEET
Indicated airspeed (knots)
True airspeed (knots)
OAT 15°C
160 200 240 280 340 380 420
0 10 20
∆Torque - %
Maximum torque
available
Transmission limit
Maximum rate of climb
or maximum end
Maximum range
Continuous torque available
10 SO F1 AF
55003500 5000450040003000
GW (LB)
Torque (%)
1,500
1,400
1,300
1,200
1,100
1,000
0 5 10 15 20 25 30 35 40
Rate of climb or descent (feet per minute)
GROSS WEIGHT - (LB)
Figure 7-7. Maximum rate-of-climb chart.
Figure 7-8. Climb/descent torque percentage chart.
the helicopter can climb vertically, so VX is zero.
Wind direction and speed have an effect on climb
performance, but it is often misunderstood. Airspeed is
the speed at which the helicopter is moving through the
atmosphere and is unaffected by wind. Atmospheric wind
affects only the groundspeed, or speed at which the helicopter
is moving over the Earth’s surface. Thus, the only climb
performance affected by atmospheric wind is the angle of
climb and not the rate of climb.
When planning for climb performance, it is first important to
plan for torque settings at level flight. Climb performance charts
show the change in torque, above or below torque, required
for level flight under the same gross weight and atmospheric
conditions to obtain a given rate of climb or descent.
Sample Cruise or Level Flight Problem
Determine torque setting for cruise or level flight using
Figure 7-7. Use the following conditions:
Pressure Altitude............................................... 8,000 feet
Outside Air Temperature...................................... +15 °C
A. Indicated Airspeed........................................80 knots
B. Maximum Gross Weight................................5,000 lb
With this chart, first confirm that it is for a pressure altitude
of 8,000 feet with an OAT of 15°. Begin on the left side
at 80 knots indicated airspeed (point A) and move right to
maximum gross weight of 5,000 lb (point B). From that point,
proceed down to the torque reading for level flight, which
is 74 percent torque (point C). This torque setting is used
in the next problem to add or subtract cruise/descent torque
percentage from cruise flight.
Sample Climb Problem
Determine climb/descent torque percentage using Figure 7-8.
Use the following conditions:
A. Rate of Climb or Descent ............. . 500 fpm
B. Maximum Gross Weight ............... 5,000 lb
With this chart, first locate a 500-fpm rate of climb or descent
(point A), and then move to the right to a maximum gross
weight of 5,000 lb (point B). From that point, proceed down
to the torque percentage, which is 15 percent torque (point C).
For climb or descent, 15 percent torque should be added/
subtracted from the 74 percent torque needed for level flight.
For example, if the numbers were to be used for a climb
torque, the pilot would adjust torque settings to 89 percent
for optimal climb performance.
Chapter Summary
This chapter discussed the factors affecting performance:
density altitude, weight, and wind. Five sample problems
were also given with performance charts to calculate
different flight conditions and determine the performance
of the helicopter.
Introduction
Once a pilot takes off, it is up to him or her to make sound,
safe decisions throughout the flight. It is equally important
for the pilot to use the same diligence when conducting
a preflight inspection, making maintenance decisions,
refueling, and conducting ground operations. This chapter
discusses the responsibility of the pilot regarding ground
safety in and around the helicopter and when preparing to fly.
Ground Procedures and Flight
Preparations
Chapter 8