Your ability to predict the performance of a helicopter
is extremely important. It allows you to determine
how much weight the helicopter can carry before
takeoff, if your helicopter can safely hover at a spe-
cific altitude and temperature, how far it will take to
climb above obstacles, and what your maximum
climb rate will be.
FACTORS AFFECTING PERFORMANCE
A helicopter’s performance is dependent on the power
output of the engine and the lift production of 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.
DENSITY ALTITUDE
The density of the air directly affects the performance
of the helicopter. As the density of the air increases,
engine power output, rotor efficiency, and aerodynamic
lift all increase. Density altitude is the altitude above
mean sea level at which a given atmospheric density
occurs in the standard atmosphere . It can also be
interpreted as pressure altitude corrected for nonstan-
dard temperature differences.
Pressure altitude is displayed as the height above a
standard datum plane, which, in this case, is a theoret-
ical plane where air pressure is equal to 29.92 in. Hg.
Pressure altitude is the indicated height value on the
altimeter when the altimeter setting is adjusted to
29.92 in. Hg. Pressure altitude, as opposed to true alti-
tude, is an important value for calculating perform-
ance as it more accurately represents the air content at
a particular level. The difference between true altitude
and pressure altitude must be clearly understood. True
altitude means the vertical height above mean sea level
and is displayed on the altimeter when the altimeter is
correctly adjusted to the local setting.
For example, if the local altimeter setting is 30.12 in.
Hg., and the altimeter is adjusted to this value, the
altimeter indicates exact height above sea level.
However, this does not reflect conditions found at this
height under standard conditions. Since the altimeter
setting is more than 29.92 in. Hg., the air in this exam-
ple has a higher pressure, and is more compressed,
indicative of the air found at a lower altitude.
Therefore, the pressure altitude is lower than the actual
height above mean sea level.
To calculate pressure altitude without the use of an
altimeter, remember that the pressure decreases
approximately 1 inch of mercury for every 1,000-foot
increase in altitude. For example, if the current local
altimeter setting at a 4,000-foot elevation is 30.42, the
pressure altitude would be 3,500 feet. (30.42 – 29.92 =
.50 in. Hg. 3 1,000 feet = 500 feet. Subtracting 500 feet
from 4,000 equals 3,500 feet).
The four factors that most affect density altitude are:
atmospheric pressure, altitude, temperature, and the
moisture content of the air.
ATMOSPHERIC PRESSURE
Due to changing weather conditions, atmospheric pres-
sure at a given location changes from day to day. If the
pressure is lower, the air is less dense. This means a
higher density altitude and less helicopter performance.
Density Altitude—Pressure altitude corrected for nonstandard temper-
ature variations. Performance charts for many older aircraft are based
on this value.
Standard Atmosphere—At sea level, the standard atmosphere consists
of a barometric pressure of 29.92 inches of mercury (in. Hg.) or 1013.2
millibars, and a temperature of 15°C (59°F). Pressure and temperature
normally decrease as altitude increases. The standard lapse rate in the
lower atmosphere for each 1,000 feet of altitude is approximately 1 in.
Hg. and 2°C (3.5°F). For example, the standard pressure and tempera-
ture at 3,000 feet mean sea level (MSL) is 26.92 in. Hg. (29.92 – 3) and
9°C (15°C – 6°C).
Pressure Altitude—The height above the standard pressure level of
29.92 in. Hg. It is obtained by setting 29.92 in the barometric pressure
window and reading the altimeter.
True Altitude—The actual height of an object above mean sea level.
ALTITUDE
As altitude increases, the air becomes thinner or less
dense. This is because the atmospheric pressure acting
on a given volume of air is less, allowing the air mole-
cules to move further apart. Dense air contains more air
molecules spaced closely together, while thin air con-
tains less air molecules because they are spaced further
apart. As altitude increases, density altitude increases.
TEMPERATURE
Temperature changes have a large affect on density alti-
tude. As warm air expands, the air molecules move fur-
ther apart, creating less dense air. Since cool air
contracts, the air molecules move closer together, cre-
ating denser air. High temperatures cause even low ele-
vations to have high density altitudes.
MOISTURE (HUMIDITY)
The water content of the air also changes air density
because water vapor weighs less than dry air.
Therefore, as the water content of the air increases, the
air becomes less dense, increasing density altitude and
decreasing performance.
Humidity, also called “relative humidity,” refers to the
amount of water vapor contained in the atmosphere,
and is expressed as a percentage of the maximum
amount of water vapor the air can hold. This amount
varies with temperature; warm air can hold more water
vapor, while colder air can hold less. Perfectly dry air
that contains no water vapor has a relative humidity of
0 percent, while saturated air that cannot hold any more
water vapor, has a relative humidity of 100 percent.
Humidity alone is usually not considered an important
factor in calculating density altitude and helicopter per-
formance; however, it does contribute. There are no
rules-of-thumb or charts used to compute the effects of
humidity on density altitude, so you need to take this
into consideration by expecting a decrease in hovering
and takeoff performance in high humidity conditions.
HIGH AND LOW
DENSITY ALTITUDE CONDITIONS
You need to thoroughly understand the terms “high
density altitude” and “low density altitude.” In general,
high density altitude refers to thin air, while low den-
sity altitude refers to dense air. Those conditions that
result in a high density altitude (thin air) are high ele-
vations, low atmospheric pressure, high temperatures,
high humidity, or some combination thereof. Lower
elevations, high atmospheric pressure, low tempera-
tures, and low humidity are more indicative of low
density altitude (dense air). However, high density
altitudes may be present at lower elevations on hot
days, so it is important to calculate the density altitude
and determine performance before a flight.
One of the ways you can determine density altitude is
through the use of charts designed for that purpose.
[Figure 8-1]. For example, assume you are planning to
depart an airport where the field elevation is 1,165 feet
MSL, the altimeter setting is 30.10, and the tempera-
ture is 70°F. What is the density altitude? First, correct
for nonstandard pressure (30.10) by referring to the
right side of the chart, and subtracting 165 feet from
the field elevation. The result is a pressure altitude of
1,000 feet. Then, enter the chart at the bottom, just
above the temperature of 70°F (21°C). Proceed up the
chart vertically until you intercept the diagonal 1,000-
foot pressure altitude line, then move horizontally to
the left and read the density altitude of approximately
2,000 feet. This means your helicopter will perform as
if it were at 2,000 feet MSL on a standard day.
Most performance charts do not require you to com-
pute density altitude. Instead, the computation is built
into the performance chart itself. All you have to do is
enter the chart with the correct pressure altitude and the
temperature.
WEIGHT
Lift is the force that opposes weight. As weight
increases, the power required to produce the lift needed
to compensate for the added weight must also increase.
Most performance charts include weight as one of the
variables. By reducing the weight of the helicopter, you
may find that you are able to safely take off or land at a
location that otherwise would be impossible. However,
if you are ever in doubt about whether you can safely
perform a takeoff or landing, you should 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 you can make a
landing 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
anytime there is relative airflow over the rotor disc.
This occurs whether the relative airflow is caused by
helicopter movement or by the wind. 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 most 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 manufactur-
ers 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 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.
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) hoverand out of ground effect
(OGE) hover under various conditions of gross weight,
altitude, temperature, and power. The “in ground effect”
hover ceiling is usually higher than the “out of ground
effect” hover ceiling because of the added lift benefit
produced by ground effect.
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In Ground Effect (IGE) Hover—Hovering close to the surface (usually
less than one rotor diameter above the surface) under the influence of
ground effect.
Out of Ground Effect (OGE) Hover—Hovering greater than one rotor
diameter distance above the surface. Because induced drag is greater
while hovering out of ground effect, it takes more power to achieve a
hover. See Chapter 3—Aerodynamics of Flight for more details on IGE
and OGE hover.
Since the gross weight of your helicopter is less than
this, you can safely hover with these conditions.
SAMPLE PROBLEM 2
Once you reach the remote location in the previous
problem, you will need to hover out of ground effect
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 8-3, 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 weight to hover out of ground effect is
approximately 1,130 pounds (point C). Since your
gross weight is higher than this value, you will not be
able to hover with these conditions. To accomplish the
mission, you will have 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
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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 PROBLEM 1
You are to fly a photographer to a remote location to
take pictures of the local wildlife. Using figure 8-2, can
you safely hover in ground effect at your departure
point with the following conditions?
Pressure Altitude..................................8,000 feet
Temperature...............................................+15°C
Takeoff Gross Weight.....................1,250 pounds
R.P.M..........................................................104%
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).
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the entire flight operation. Being able to hover at the take-
off location with a certain 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 ele-
vation, temperature, and/or relative humidity, more power
is required to hover. 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.
TAKEOFF PERFORMANCE
If takeoff charts are included in the rotorcraft flight man-
ual, they usually indicate the distance it takes to clear a 50-
foot obstacle based on various conditions of weight,
pressure altitude, and temperature. In addition, the values
computed in the takeoff charts usually assume that the
flight profile is per the applicable height-velocity diagram.
SAMPLE PROBLEM 3
In this example, determine the distance to clear a 50-
foot obstacle with the following conditions:
Pressure Altitude..................................5,000 feet
Takeoff Gross Weight.....................2,850 pounds
Temperature .................................................95°F
Using figure 8-4, locate 2,850 pounds in the first col-
umn. Since the pressure altitude of 5,000 feet is not one
of the choices in column two, you have to interpolate
between the values from the 4,000- and 6,000-foot
lines. Follow each of these rows out to the column
headed by 95°F. The values are 1,102 feet and 1,538
feet. Since 5,000 is halfway between 4,000 and 6,000,
the interpolated value should be halfway between these
two values or 1,320 feet ([1,102 + 1,538] 4 2 = 1,320).
CLIMB PERFORMANCE
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
will obtain 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 best rate-of-climb speed 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 depends upon the power
available. If there is a surplus of power available, the
helicopter can climb vertically, so the best angle-of-
climb speed is zero.
Wind direction and speed have an effect on climb per-
formance, 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
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affected by atmospheric wind is the angle of climb and
not the rate of climb.
SAMPLE PROBLEM 4
Determine the best rate of climb using figure 8-5. Use
the following conditions:
Pressure Altitude................................12,000 feet
Outside Air Temperature ...........................+10°C
Gross Weight..................................3,000 pounds
Power ...........................................Takeoff Power
Anti-ice ..........................................................ON
Indicated Airspeed .................................52 knots
With this chart, first locate the temperature of +10°C
(point A). Then proceed up the chart to the 12,000-foot
pressure altitude line (point B). From there, move hori-
zontally to the right until you intersect the 3,000-foot
line (point C). With this performance chart, you must
now determine the rate of climb with anti-ice off and
then subtract the rate of climb change with it on. From
point C, go to the bottom of the chart and find that the
maximum rate of climb with anti-ice off is approxi-
mately 890 feet per minute. Then, go back to point C
and up to the anti-ice-on line (point D). Proceed hori-
zontally to the right and read approximately 240 feet
per minute change (point E). Now subtract 240 from
890 to get a maximum rate of climb, with anti-ice on,
of 650 feet per minute.
Other rate-of-climb charts use density altitude as a
starting point. [Figure 8-6] While it cleans up the chart
somewhat, you must first determine density altitude.
Notice also that this chart requires a change in the indi-
cated airspeed with a change in altitude.
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