Rotorcraft Flying Handbook, FAA-H-8083-21

Approximate Density Altitude – Thousands of Feet
13
12
11
10
9
8
7
6
5
4
3
2
1
SL
°C -18 -12
°F 0 10
12,000
11,000
10,000
9,000
Standard Temperature
8,000
7,000
-7 -1 4 10
20 30 40 50
Pressure Altitude – Feet
6,000
5,000
4,000
3,000
2,000
1,000
Sea Level
Outside Air Temperature
16 21
60 70
-1,000
27
80
32
90
Altimeter
Setting
28.0
28.1
28.2
28.3
28.4
28.5
28.6
28.7
28.8
28.9
29.0
29.1
29.2
29.3
29.4
29.5
29.6
29.7
29.8
29.9
29.92
30.0
30.1
30.2
30.3
30.4
30.5
30.6
30.7
30.8
30.9
31.0
Pressure
Altitude
Conversion
Factor
1,824
1,727
1,630
1,533
1,436
1,340
1,244
1,148
1,053
957
863
768
673
579
485
392
298
205
112
20
0
-73
-165
-257
-348
-440
-531
-622
-712
-803
-893
-983
Figure 8-1. Density Altitude Chart.
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 translational
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 translation
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 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 correctly
and at the appropriate times.
Using these assumptions, the manufacturer develops
performance data for the helicopter based on
actual flight tests. However, they do not test the helicopter
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) hover and 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.
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.
8-3