Helicopters and gyroplanes both achieve lift through
the use of airfoils, and, therefore, many of the basic
aerodynamic principles governing the production of lift
apply to both aircraft. These concepts are explained in
depth in Chapter 2—General Aerodynamics, and con-
stitute the foundation for discussing the aerodynamics
of a gyroplane.
AUTOROTATION
A fundamental difference between helicopters and
gyroplanes is that in powered flight, a gyroplane rotor
system operates in autorotation. This means the rotor
spins freely as a result of air flowing up through the
blades, rather than using engine power to turn the
blades and draw air from above. [Figure 16-1] Forces
are created during autorotation that keep the rotor
blades turning, as well as creating lift to keep the air-
craft aloft. Aerodynamically, the rotor system of a
gyroplane in normal flight operates like a helicopter
rotor during an engine-out forward autorotative
descent.
VERTICAL AUTOROTATION
During a vertical autorotation, two basic components
contribute to the relative wind striking the rotor blades.
[Figure 16-2] One component, the upward flow of air
through the rotor system, remains relatively constant
for a given flight condition. The other component is the
rotational airflow, which is the wind velocity across the
blades as they spin. This component varies signifi-
cantly based upon how far from the rotor hub it is
measured. For example, consider a rotor disc that is 25
feet in diameter operating at 300 r.p.m. At a point one
foot outboard from the rotor hub, the blades are travel-
ing in a circle with a circumference of 6.3 feet. This
equates to 31.4 feet per second (f.p.s.), or a rotational
blade speed of 21 m.p.h. At the blade tips, the circum-
ference of the circle increases to 78.5 feet. At the same
operating speed of 300 r.p.m., this creates a blade tip
Direction of Flight
Relative Wind Relative Wind
Direction of Flight
Figure 16-1. Airflow through the rotor system on a gyroplane is reversed from that on a powered helicopter. This airflow is the
medium through which power is transferred from the gyroplane engine to the rotor system to keep it rotating.
Resultant Relative Wind
Wind due to Blade Rotation
Upward □
Airflow
Figure 16-2. In a vertical autorotation, the wind from the
rotation of the blade combines with the upward airflow to
produce the resultant relative wind striking the airfoil.
speed of 393 feet per second, or 267 m.p.h. The result
is a higher total relative wind, striking the blades at a
lower angle of attack. [Figure 16-3]
ROTOR DISC REGIONS
As with any airfoil, the lift that is created by rotor
blades is perpendicular to the relative wind. Because
the relative wind on rotor blades in autorotation shifts
from a high angle of attack inboard to a lower angle of
attack outboard, the lift generated has a higher forward
component closer to the hub and a higher vertical com-
ponent toward the blade tips. This creates distinct
regions of the rotor disc that create the forces neces-
sary for flight in autorotation. [Figure 16-4] The
autorotative region, or driving region, creates a total
aerodynamic force with a forward component that
exceeds all rearward drag forces and keeps the blades
spinning. The propeller region, or driven region, gen-
erates a total aerodynamic force with a higher vertical
component that allows the gyroplane to remain aloft.
Near the center of the rotor disc is a stall region where
the rotational component of the relative wind is so low
that the resulting angle of attack is beyond the stall
limit of the airfoil. The stall region creates drag against
the direction of rotation that must be overcome by the
forward acting forces generated by the driving region.
AUTOROTATION IN FORWARD FLIGHT
As discussed thus far, the aerodynamics of autorotation
apply to a gyroplane in a vertical descent. Because
gyroplanes are normally operated in forward flight, the
component of relative wind striking the rotor blades as
a result of forward speed must also be considered. This
component has no effect on the aerodynamic principles
that cause the blades to autorotate, but causes a shift in
the zones of the rotor disc.
As a gyroplane moves forward through the air, the for-
ward speed of the aircraft is effectively added to the
Resultant Relative Wind
Rotational Airflow (267 m.p.h. or 393 f.p.s.)
Upward Airflow□
(17 m.p.h. or 25 f.p.s.)
TIP
Rotor Speed: 300 r.p.m.
Resultant
RelativeWind
Rotational Airflow□
(21 m.p.h. or 31 f.p.s.)
Upward Airflow□
(17 m.p.h. or 25 f.p.s.)
HUB
VERTICAL AUTOROTATION
Figure 16-3. Moving outboard on the rotor blade, the rotational velocity increasingly exceeds the upward component of airflow,
resulting in a higher relative wind at a lower angle of attack.
Driven Region
Driving Region
Stall□□
Region
Driven Region□
(Propeller)
Driving Region□
(Autorotative)
Stall Region
VERTICAL AUTOROTATION
Rotational□
Relative Wind
Lift
Lift
TAF
TAF
Total□
Aerodynamic□
Force Aft□
of Axis of□
Rotation
Drag
Chord LineInflow Up□
Through Rotor Resultant□
Relative Wind
Total□
Aerodynamic□
Force□
Forward□
of Axis of□
Rotation
Drag
Inflow □
Axis of□
Rotation
Axis of□
Rotation
Axis of□
Rotation
(Blade is Stalled)
TAF
Drag
Inflow
Lift
Figure 16-4. The total aerodynamic force is aft of the axis of
rotation in the driven region and forward of the axis of rota-
tion in the driving region. Drag is the major aerodynamic
force in the stall region. For a complete depiction of force
vectors during a vertical autorotation, refer to Chapter 3—
Aerodynamics of Flight (Helicopter), Figure 3-22.
relative wind striking the advancing blade, and sub-
tracted from the relative wind striking the retreating
blade. To prevent uneven lifting forces on the two sides
of the rotor disc, the advancing blade teeters up,
decreasing angle of attack and lift, while the retreating
blade teeters down, increasing angle of attack and lift.
(For a complete discussion on dissymmetry of lift, refer
to Chapter 3—Aerodynamics of Flight.) The lower
angles of attack on the advancing blade cause more of
the blade to fall in the driven region, while higher
angles of attack on the retreating blade cause more of
the blade to be stalled. The result is a shift in the rotor
regions toward the retreating side of the disc to a degree
directly related to the forward speed of the aircraft.
[Figure 16-5]
REVERSE FLOW
On a rotor system in forward flight, reverse flow occurs
near the rotor hub on the retreating side of the rotor
disc. This is the result of the forward speed of the air-
craft exceeding the rotational speed of the rotor blades.
For example, two feet outboard from the rotor hub, the
blades travel in a circle with a circumference of 12.6
feet. At a rotor speed of 300 r.p.m., the blade speed at
the two-foot station is 42 m.p.h. If the aircraft is being
operated at a forward speed of 42 m.p.h., the forward
speed of the aircraft essentially negates the rotational
velocity on the retreating blade at the two-foot station.
Moving inboard from the two-foot station on the
retreating blade, the forward speed of the aircraft
increasingly exceeds the rotational velocity of the
blade. This causes the airflow to actually strike the
trailing edge of the rotor blade, with velocity increas-
ing toward the rotor hub. [Figure 16-6] The size of the
area that experiences reverse flow is dependent prima-
rily on the forward speed of the aircraft, with higher
speed creating a larger region of reverse flow. To some
degree, the operating speed of the rotor system also has
an effect on the size of the region, with systems operat-
ing at lower r.p.m. being more susceptible to reverse
flow and allowing a greater portion of the blade to
experience the effect.
RETREATING BLADE STALL
The retreating blade stall in a gyroplane differs from
that of a helicopter in that it occurs outboard from the
rotor hub at the 20 to 40 percent position rather than at
the blade tip. Because the gyroplane is operating in
autorotation, in forward flight there is an inherent stall
region centered inboard on the retreating blade. [Refer
to figure 16-5] As forward speed increases, the angle of
attack on the retreating blade increases to prevent dis-
symmetry of lift and the stall region moves further
outboard on the retreating blade. Because the stalled
portion of the rotor disc is inboard rather than near the
tip, as with a helicopter, less force is created about the
aircraft center of gravity. The result is that you may feel
a slight increase in vibration, but you would not experi-
ence a large pitch or roll tendency.
ROTOR FORCE
As with any heavier than air aircraft, the four forces
acting on the gyroplane in flight are lift, weight, thrust
and drag. The gyroplane derives lift from the rotor and
Forward
Driven Region
Driving Region
Stall□□
Region
Retreating□
Side
Advancing□
Side
Figure 16-5. Rotor disc regions in forward autorotative flight.
Forward□
Flight at□
42 kt
42kt
42kt
42kt
42kt2'
Area of□
Reverse flow
42kt
Rotor Speed 300 r.p.m.
Figure 16-6. An area of reverse flow forms on the retreating
blade in forward flight as a result of aircraft speed exceeding
blade rotational speed.
rotor blades turn, rapid changes occur on the airfoils
depending on position, rotor speed, and aircraft speed.
A change in the angle of attack of the rotor disc can
effect a rapid and substantial change in total rotor drag.
Rotor drag can be divided into components of induced
drag and profile drag. The induced drag is a product of
lift, while the profile drag is a function of rotor r.p.m.
Because induced drag is a result of the rotor providing
lift, profile drag can be considered the drag of the rotor
when it is not producing lift. To visualize profile drag,
consider the drag that must be overcome to prerotate
the rotor system to flight r.p.m. while the blades are
producing no lift. This can be achieved with a rotor sys-
tem having a symmetrical airfoil and a pitch change
capability by setting the blades to a 0° angle of attack.
A rotor system with an asymmetrical airfoil and a built
in pitch angle, which includes most amateur-built
teeter-head rotor systems, cannot be prerotated without
having to overcome the induced drag created as well.
THRUST
Thrust in a gyroplane is defined as the component of
total propeller force parallel to the relative wind. As
with any force applied to an aircraft, thrust acts around
the center of gravity. Based upon where the thrust is
applied in relation to the aircraft center of gravity, a rel-
atively small component may be perpendicular to the
relative wind and can be considered to be additive to
lift or weight.
In flight, the fuselage of a gyroplane essentially acts as
a plumb suspended from the rotor, and as such, it is
thrust directly from the engine through a propeller.
[Figure 16-7]
The force produced by the gyroplane rotor may be
divided into two components; rotor lift and rotor drag.
The component of rotor force perpendicular to the
flight path is rotor lift, and the component of rotor force
parallel to the flight path is rotor drag. To derive the
total aircraft drag reaction, you must also add the drag
of the fuselage to that of the rotor.
ROTOR LIFT
Rotor lift can most easily be visualized as the lift
required to support the weight of the aircraft. When an
airfoil produces lift, induced drag is produced. The
most efficient angle of attack for a given airfoil pro-
duces the most lift for the least drag. However, the air-
foil of a rotor blade does not operate at this efficient
angle throughout the many changes that occur in each
revolution. Also, the rotor system must remain in the
autorotative (low) pitch range to continue turning in
order to generate lift.
Some gyroplanes use small wings for creating lift when
operating at higher cruise speeds. The lift provided by
the wings can either supplement or entirely replace
rotor lift while creating much less induced drag.
ROTOR DRAG
Total rotor drag is the summation of all the drag forces
acting on the airfoil at each blade position. Each blade
position contributes to the total drag according to the
speed and angle of the airfoil at that position. As the
Lift
Resultant
Thrust
Resultant
Thrust
Lift
Resultant
Drag
Rotor□
Drag
Fuselage□
Drag
Resultant
Weight
Weight
Figure 16-7. Unlike a helicopter, in forward powered flight the resultant rotor force of a gyroplane acts in a rearward direction.
subject to pendular action in the same way as a heli-
copter. Unlike a helicopter, however, thrust is applied
directly to the airframe of a gyroplane rather than being
obtained through the rotor system. As a result, different
forces act on a gyroplane in flight than on a helicopter.
Engine torque, for example, tends to roll the fuselage
in the direction opposite propeller rotation, causing it
to be deflected a few degrees out of the vertical plane.
[Figure 16-8] This slight “out of vertical” condition is
usually negligible and not considered relevant for most
flight operations.
STABILITY
Stability is designed into aircraft to reduce pilot work-
load and increase safety. A stable aircraft, such as a typ-
ical general aviation training airplane, requires less
attention from the pilot to maintain the desired flight
attitude, and will even correct itself if disturbed by a
gust of wind or other outside forces. Conversely, an
unstable aircraft requires constant attention to maintain
control of the aircraft.
Reactive □
Torque on□
Fuselage
Torque □
Applied to□
Propeller
Figure 16-8. Engine torque applied to the propeller has an
equal and opposite reaction on the fuselage, deflecting it a
few degrees out of the vertical plane in flight.
Pendular Action—The lateral or
longitudinal oscillation of the fuse-
lage due to it being suspended
from the rotor system. It is similar
to the action of a pendulum.
Pendular action is further dis-
cussed in Chapter 3—
Aerodynamics of Flight.
There are several factors that contribute to the stability
of a gyroplane. One is the location of the horizontal
stabilizer. Another is the location of the fuselage drag
in relation to the center of gravity. A third is the
inertia moment around the pitch axis, while a fourth is
the relation of the propeller thrust line to the vertical
location of the center of gravity (CG). However, the
one that is probably the most critical is the relation of
the rotor force line to the horizontal location of the
center of gravity.
HORIZONTAL STABILIZER
A horizontal stabilizer helps in longitudinal stability,
with its efficiency greater the further it is from the
center of gravity. It is also more efficient at higher
airspeeds because lift is proportional to the square of
the airspeed. Since the speed of a gyroplane is not very
high, manufacturers can achieve the desired stability
by varying the size of the horizontal stabilizer, chang-
ing the distance it is from the center of gravity, or by
placing it in the propeller slipstream.
FUSELAGE DRAG
(CENTER OF PRESSURE)
If the location, where the fuselage drag or center of
pressure forces are concentrated, is behind the CG,
the gyroplane is considered more stable. This is espe-
cially true of yaw stability around the vertical axis.
However, to achieve this condition, there must be a
sufficient vertical tail surface. In addition, the gyro-
plane needs to have a balanced longitudinal center of
pressure so there is sufficient cyclic movement to
prevent the nose from tucking under or lifting, as
pressure builds on the frontal area of the gyroplane as
airspeed increases.
PITCH INERTIA
Without changing the overall weight and center of
gravity of a gyroplane, the further weights are placed
from the CG, the more stable the gyroplane. For exam-
ple, if the pilot's seat could be moved forward from the
CG, and the engine moved aft an amount, which keeps
the center of gravity in the same location, the gyroplane
becomes more stable. A tightrope walker applies this
same principle when he uses a long pole to balance
himself.
PROPELLER THRUST LINE
Considering just the propeller thrust line by itself, if the
thrust line is above the center of gravity, the gyroplane
has a tendency to pitch nose down when power is
applied, and to pitch nose up when power is removed.
The opposite is true when the propeller thrust line is
below the CG. If the thrust line goes through the CG or
nearly so there is no tendency for the nose to pitch up
or down. [Figure 16-9]
ROTOR FORCE
Because some gyroplanes do not have horizontal stabi-
lizers, and the propeller thrust lines are different, gyro-
plane manufacturers can achieve the desired stability
by placing the center of gravity in front of or behind the
rotor force line. [Figure 16-10]
Suppose the CG is located behind the rotor force line in
forward flight. If a gust of wind increases the angle of
attack, rotor force increases. There is also an increase
in the difference between the lift produced on the
advancing and retreating blades. This increases the
flapping angle and causes the rotor to pitch up. This
pitching action increases the moment around the center
of gravity, which leads to a greater increase in the angle
of attack. The result is an unstable condition.
If the CG is in front of the rotor force line, a gust of
wind, which increases the angle of attack, causes the
rotor disc to react the same way, but now the increase
in rotor force and blade flapping decreases the
moment. This tends to decrease the angle of attack, and
creates a stable condition.
TRIMMED CONDITION
As was stated earlier, manufacturers use a combination
of the various stability factors to achieve a trimmed
gyroplane. For example, if you have a gyroplane where
the CG is below the propeller thrust line, the propeller
thrust gives your aircraft a nose down pitching moment
when power is applied. To compensate for this pitching
moment, the CG, on this type of gyroplane, is usually
located behind the rotor force line. This location pro-
duces a nose up pitching moment.
Conversely, if the CG is above the propeller thrust line,
the CG is usually located ahead of the rotor force line.
Of course, the location of fuselage drag, the pitch iner-
tia, and the addition of a horizontal stabilizer can alter
where the center of gravity is placed.
Propeller Thrust
Propeller Thrust
Center of Gravity Center of Gravity
High ProfileLow Profile
RotorForce
RotorForce
Figure 16-9. A gyroplane which has the propeller thrust line above the center of gravity is often referred to as a low profile gyro-
plane. One that has the propeller thrust line below or at the CG is considered a high profile gyroplane.
Figure 16-10. If the CG is located in front of the rotor force line, the gyroplane is more stable than if the CG is located behind the
rotor force line.
Blade Flapping—The upward or downward movement of the rotor-
blades during rotation.