(However it needs to be explained that there are actually two acceptable ways of describing how and why the rotor disk tilts in one direction even though the - and + pitch of the rotor blades is greatest at the points 90 degrees before and after the lowest point of tilt - these two descriptions are referred to as 1-aerodynamic description, 2-gyroscopic precession description. Both descriptions achieve the same result - Both descriptions are mixed together here - The CC team believe the aero-dynamic description is the most accurate).
Preamble - Helicopters vs Gyros vs Fixed-wing.
As a simple statement, a helicopter moves in a particular direction
by tilting its' rotor disk in the direction it wants to move in. It's
rotor is 'powered' and as such acts as the primary mechanism for vertical
flight as well as directional flight. A 'powered' rotor normaly has air
flowing down through it much like a fan sucks & blows air.
Because the rotor is 'powered', a helicopter requires a mechanism to counter
the 'rotational torque' operating on the engine and frame of the helicopter,
this force is trying to turn the body of the h/c in the opposite direction
to the rotor. This mechanism is (of course) the anti-torque tail rotor. The
tail rotor is also used, at times, to face the body of the helicopter in a
particular direction.
A gyro uses the rotor differently. It is 'unpowered', that is it turns
because air is flowing through it like a 'windmill'. A gyro rotor is
typically tilted rearward to allow the 'wind' to pass up through it and to
keep it spinning. Without this 'wind' the rotor would stop. The gyro uses a
'pusher' or puller ('tractor') prop to propell the craft through the air and
relies on tilting the rotor PLUS use of a tail vertical stabiliser to turn.
The forward thrust in-turn forces air up through the rotor thus spinning it
and providing the lift needed to maintain an altitude. The rotor is
a spinning wing.
The single biggest difference in the appearance of a h/c rotor vs a gyro rotor
in flight, is that as a h/c flies forward, its rotor is tilted forward
whereas a gyro's rotor is tilted rearward. This is the difference between
air passing down through the rotor (h/c) and air passing up through the rotor
(gyro). Also ...
A helicopter with a 'powered' rotor can sit motionless in the air as it's
rotor is sucking air through the top to create lift which holds the craft up.
Without enough forward airspeed, a gyro will slowly sink to the ground - as
it sinks air is still passing up through its rotor which keeps the rotor
spinning. Because the rotor is spinning the 'spinning wing' continues to
create some lift and the spinning also prevents wing 'stall' thus both
helicopters and gyros do not suffer from the greatest plague to fixed- wing
aircraft - wing stall. But that does not mean they are safer - yet.
The following section focuses on the control of a rotary wing and for the
purpose of this write-up accept that both tilt their rotor for some purpose.
Direct and Indirect Cyclic-pitch Control.
Changing the cyclic pitch on rotor blades can be done many ways but
usually divided into 'direct' control of the blade pitch vs 'indirect'
control of blade pitch.
Many (but not all) gyro's including the CarterCopter use a 'tilting spindle'
which causes the tilt of the rotor disk with 'indirect' cyclic-pitch
control.
Helicopters generally use a moveable swash-plate which sits around the main
rotor shaft and when it is tilted it in turn applies 'direct' 'cyclic-pitch'
control to the rotor blades through the use of pitch-control rods to the root
of each rotor blade. As the blades spin in a given plane-of-rotation they
change pitch 'cyclicly' (per cycle) and this induces the blades to fly to
a new 'plane-of-rotation' more or less parallel to the swash-plate.
On craft that use a swash-plate, are 2 sets of control rods,
A swash-plate (of course) allows the pilot to mix collective pitch changes
with cyclic pitch changes (swash-plate vertical movement + swash-plate tilting).
Tilting the swash-plate applies 'direct' cyclic pitch change to the
rotor blades which fly to a new plane-of-rotation which in turn means a
tilt in the rotor disk. Some helicopters can use other methods of applying
cyclic-pitch control such as those based on the 'Hiller control system'
& will not be covered here. Composite Hiller systems can get very
complicated.
This method of rotor disk control I have called 'indirect' control as
the cyclic pitch change that occured to the blades when the spindle
was tilted happened 'indirectly' which differs markedly from the
conventional swash-plate technique where 'direct' control is applied
to the pitch of the rotor blades.
This should be easily visualized by imagining a h/c and a gyro side-by-side.
Both craft are static and their rotor blades are out to left & right.
Accepting a simplified view and assuming couter-clockwise rotation and no
use of delta-3 or shortened blade-pitch arms (explained later), if the h/c
pilot tilts the swash forward 5 degrees, the swash will 'directly' tilt
the blades by + & - 5 degrees, the blade on the left increases its pitch by
5 degrees and the blade on the right decreases its pitch by 5 degrees - if
spinning, this would take full rotational effect 90 degrees later. Which
of course emulates the forward tilt of the swash.
On our gyro with tilting spindle, EXACTLY the same occurs. As the tilting
spindles is tilted forward 5 degrees it in turn forces the rotor blades
assembly to tilt which increases the left blade by 5 degrees and decreases
the right blade by 5 degrees and again if spinning, takes full effect
90 degrees later (as with the swash, emulating the tilt of the spindle).
Repeating, 'Direct' cyclic-pitch control of the blades is usually used by
helicopters and by way of a swash-plate. The swash-plate itself
is a bearing where the inner race turns with the main rotor shaft
and the outer race remains stationary.
The swash-plate assembly is gimbal mounted around the main rotor shaft
and is also allowed to move vertically on the main rotor shaft.
One set of control rods is used to tilt or move the outer race which
of course when tilted, tilts the inner race the same way. A second
set of control rods (the blade pitch change rods, are normally
connected directly to the pitch change arms on the rotor blades from
points on the swash-plate inner race.
Again without going into any more detail here, tilting of the
swash-plate leads to changing the cyclic-pitch of the blades which
(combined with the effects of gyroscopic prescession), leads to
the rotor blades 'flying' to a new plane-of-rotation where the blades
are back in lift equilibrium.
Because the CarterCopter's rotor has such high inertia, it will exhibit
a lag in control when responding to the tilting spindle's shift because
of the gyroscopic inertia of the rotor. The higher the inertia the
greater the lag. This can be countered by using delta-3 (see below for
a more detailed description of delta-3) in the rotor
head teeter. Delta-3 allows a high-inertia rotor to track the spindle
much faster than a conventional 90 degree angled see-saw teeter hinge.
One of the reason lightweight gyros use tilting spindles rather than
swash-plates, is the utter simplicity of the tilting spindle
concept and mechanisms. Comparatively easy to design and manufacture.
But it has its price. The longer the tilting spindle is the harder it is
to tilt it if the gyro is at the same time subject to heavy G forces.
This is because the weight of the gyro especially if magnified by a dive
or other G forces means the pilot is having to fight the leverage created
by the length of the tilting spindle and the instantaneous weight of his
gyro slung beneath the centre of lift which is effectively trying to hold
the spindle straight.
The fact that helicopters are much heavier than gyros has meant that any
type of tilting spindle has usually been out of the question.
Also it is uncommon for a craft to employ a tilting-spindle for
cyclic-pitch control as well as to have collective pitch control (rather
than a fixed pitch head) at the same time. 'Tilting spindle' is usually
synonymous with 'fixed pitch'.
While the CC has taken the 'tilting-spindle' approach and although
the CarterCopter is much heavier than most other like gyros - CarterCopters
have succeeded in getting the tilting-spindle universal gimbal bearings to
with 1/2 an inch of the centre of lift. This is because of its mono
construction of the whole rotor assembly (refer to the Point 1
description).
It is most unusual to try this technique on a craft that weighs so
much but one needs to remember that the rotor gets 'unloaded' the
faster the CarterCopter flys and thus the CarterCopter is not always
dependant on the rotor. The CarterCopter was not designed to spend much
of flying time manouvering at slow speeds (under 100 mph).
The benefit that the CarterCopter has achieved is that the rotor head is
remarkably simple and avoids all the complexity and weight
usually associated with a rotor head (large bearings, swash-plates etc:).
While Gyros with a tilting spindle usually use Delta-3 to dampen flapping
etc:, helicopters have their own technique that achieves the same effect.
Helicopters achieve it in the way the blade-pitch-change-arm is positioned
in relation to the position of the teeter hinge pivot point. If a pitch
change control rod is connected to the blade-pitch-arm directly in line
with the teeter hinge then the blade pitch remains the same when flapping.
If however the blade-pich-arm is shortened so it is NOT in line with the
teeter hinge then as the blade flaps up, the blade-pitch-arm will be pulled
down because it is anchored to the blade-pitch-control-rod, thus reducing
blade pitch. The pitch change increases as the blade flaps up more. This is
exactly what delta-3 achieves. Like delta-3 it can be adjusted by the length
of the blade-pitch-change-arms.
Controlling flapping is an important issue for the CarterCopter when flying
at fast forward speeds. But, bear in mind that when flying over 100 MPH,
the rotor is set to spin slowly with almost no pitch on the blades and
only a slight rearward tilt of the rotor disk, sufficient to keep the rotor
spining at approx 100 rpm. It is actually the flapping that will dictate
the actual rotor rpm.
When a craft uses a tilting-spindle it is applying INDIRECT cyclic pitch
control of the blades When a craft uses a swashplate with blade pitch
control rods it is applying DIRECT cyclic pitch control of the blades
The main points covered then are :-
Doug Marker
A swash-plate will usually control pitch of the blades with an equal
increase or decrease in pitch (for collective pitch control) and
opposing increases in pitch (for cyclic pitch control). Collective
is achieved by moving the swash-plate up or down (without tilting it),
by doing so the blade pitch control arms move up or down the same amount.
Using a Tilting-Spindle vs a Swash-Plate.
Problems Associated with Tilting-Spindles.
But the CarterCopter has both a tilting spindle and collective pich.
Use of Delta-3 and Summary.
The CC because of its unique rotor construction, uses a tilting spindle.
Just how this configuration handles real flying conditions is yet to be fully understood
Go to Main 'Writeups' Menu
THIS SITE = www.internetage.com.au/cartercopters/
D.Marker email: dmarker@zeta.org.au
R.Anderson email:
cartercopter@casagrande.com
- 04 Dec 1998
Created: 01 Dec 1998 - Updated: 2 Nov 1999
Copyright © 1998 Internet Age Pty Ltd