PdAVWEPS OD-BOT-80 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYING 
the low pitch limit, the rotor RPM can be in- 
creased only by a sacrifice in altitude or air- 
speed. If insufficient altitude is available to 
exchange for rotor speed, a hard landing is 
inevitable. SufIicient rotor rotational energy 
must be available to permit adding collective 
pitch to reduce the helicopter’s rate of descent 
before final ground contact. 
In the case of most small helicopters, at 
least 300 feet of altitude is necessary for an 
average pilot to set up a steady autorotation 
and land the helicopter safely without damage. 
This minimum becomes 500 to 600 feet for the 
larger helicopters, and will be even greater for 
helicopters with increased disc loading. These 
characteristics are usually presented in the 
flight handbook in the form of a “dead man’s 
curve” which shows the combinations of air- 
speed and altitude above the terrain where a 
successful autorotative landing would be dish- 
cult, if not impossible. 
A typical “dead man’s curve” is shown in 
figure 6.18. The most critical combinations 
are due to low altitude and low airspeed illus- 
trated by area A of figure 6.18. Less critical 
conditions exist at higher airspeeds because of 
the greater energy available to set up a steady 
autorotation. The lower limit of area A is a 
finite altitude because the helicopter can be 
landed successfully if collective pitch is held 
rather than reduced. In this specific case there 
is not sufficient energy to reach a steady state 
autorotation. The maximum altitude at which 
this is possible is approximately ten feet on 
most helicopters. 
Area B on the “dead man’s curve” of figure 
6.18 is critical because of ground contact flight 
speed or rate of descent, which is based on the 
strength of the landing gear. The average 
pilot may have difhculty in successfully flaring 
the helicopter from a high speed flight con- 
dition without allowing the tail rotor to strike 
the ground or contacting the ground at an ex- 
cessive airspeed. A less critical zone is some- 
times shown on this curve to indicate that 
higher ground contact speeds can be permitted 
when the landing surface is smooth. In ad- 
dition, various stability and control character- 
istics of a helicopter may produce critical con- 
ditions in this area. The critical areas of the 
“dead man’s curve” should be avoided unless 
such operation is a specific mission require- 
ment. 
POWER SETTLING. The term “power 
settling” has been used to describe a variety of 
flight conditions of the helicopter. True 
“power settling” occurs only when the heli- 
copter rotor is operating in a rotary flow 
condition called the “vortex ring state.” 
The flow through the rotor in the “vortex 
ring state” is upward near the center of the 
disc and downward in the outer portion, 
resulting in a condition of zero net thrust on 
the rotor. If the rotor thrust is zero, the 
helicopter is effectively free-falling and ex- 
tremely high rates of descent can result. 
The downwash distribution within the 
rotor is shown in figure 6.19 for the conditions 
of normal hovering and power settling. Part 
A of figure 6.19 illustrates the typical down- 
wash distribution for hovering flight. If 
sufficient power were not available to hover 
at this condition, the helicopter would begin 
to settle at some rate of descent depending on 
the deficiency of power. This rate of descent 
would effectively decrease the downwash 
throughout the rotor and result in a redistri- 
bution of downwash similar to Part B of 
figure 6.19. At the outer portion of the 
rotor disc, the local induced downwash veloc- 
ity is greater than the rate of descent and 
downflow exists. At the center of the rotor 
disc, the rate of descent is greater than the 
local induced downwash velocity and the 
resultant flow is upward. This flow condition 
results in the rotary “vortex ring” state. By 
reference to the basic momentum theory it is