(NOTE: World speed record for helicopter is 249 mph over 15 km - for Gyro it is 120mph over 3km)
The maximum RPM reached with the propeller also running was 540 RPM, at approximately 88 horsepower. Without the propeller, the rotor went to 580 RPM (Mach .91) at approximately 145 horsepower. Compressibility effects at the rotor tip were causing a steep increase in power required for a small increase in RPM. Therefore, jump takeoffs will be performed at 525 RPM (Mach .825).
Restating this problem :- Rotor blades CAN travel faster than
the speed of sound, it is the energy (horsepower) needed to drive them
through the air at above about mach .8 that is a major problem.
A secondary problem comes from the stresses acting at each end
of the rotor blade. It doesn't really matter if the force fighting
compressibility is from a powered rotor or the indirect force from
a pusher or puller engine.
(There is another problem that does emerge when the rotor tips
exceed mach 1 - the problems caused when one part of a flying craft
is creating mach 1 shockwaves that hit other parts of a craft that
are not travelling faster than mach 1 - this problem is avoided by
the CarterCopter so will not be explored in this write-up).
Several rotorcraft have been built over the years, that 'unload'
their rotors by using stub wings, in order to cope with retreating
blade stall (and the compressibility problems).
The prototype Lockheed Cheyenne AH-56A helicopter used rotor
'unloading' and a pusher prop for this very purpose.
Put simply it is the effect that occurs as a rotor craft flys
faster and the retreating blade loses lift or 'stalls' in parts of
its swept area, due to not enough airflow (or a reverse airflow)
occuring.
Pictures & Write-up of the Cheyenne AH-56A
Problem 3
Another challenge is the issue of rotor stability and flapping at
forward high speeds.
One additional issue for a rotor is that with a conventional 2-bladed
rotor, in particular, there is the need for the rotor edgewise natural
frequency to always be higher than the frequency of the highest RPM
the rotor may spin to.
Also, a rotor while spinning and moving forward at very high speeds,
presents a maximum resistance profile (drag) while the blades
are out sideways and its minimum resistance profile while the blades
are fore and aft. So 2 times per one full revolution, the resistance
goes from max to min - this resistance can be plotted as a sine-wave
where the peaks up & down are the maximum profile resistance and the
sine-wave crossover points are its minimum resistance. If the
natural harmonic frequency of the whole rotor matched the frequency
of spin or a multiple of it, the rotor could quickly vibrate
itself to destruction because of the harmonic buffeting as the rotor
profile resistance frequency matches its inherent harmonic frequency.
These issues poses a special challenge for a fast flying craft
like the CarterCopter - an entirely new rotor design approach was
required and this was done by building a 2-bladed rotor as a single
mono-constructed unit with high in-plane edgewise stiffness.
Regarding flapping, in normal forward speeds, blade 'flapping' occurs
to help achieve lift equilibrium - an advancing blade moves upwards
while a retreating blade moves downwards as a way of compensating for
the different lift forces acting on each blade in forward flight.
Flapping is allowed to occur by fitting either a teeter (see-saw) hinge
for 2-bladed rotors or by fitting individual flapping hinges to each
blade root (usually when there are 3 or more blades).
Blade flapping does not occur in a normal hover when there is zero wind
as all blades are experiencing the same wind speed. As soon as the craft
starts to move through the air or if a wind starts to blow through the
rotor system, the blade advancing into the wind experiences more lift
than the retreating blade hence the need to allow the 'flapping' movement
to counter the resulting lift imbalance between the blades.
Increased flapping activates other forces that can cause vibration and
control problems. Some helicopters have quite complicated blade hinges
and damping mechanisms to minimise the direct and indirect impact of
flapping. The CarterCopter is no exception and has employed its own
various damping mechanisims which are described later.
Many helicopters (more than 2 blades) use a 'fully-articulated' head -
one that has hinges for flapping plus hinges that allow the blades to
creep forward and backward during flight. The forward & backward creep
occurs more during heavy flapping. The CarterCopter's composite I-beam
design deals with many of these forces (described again later).
A rotor flying forward through the air not only has to overcome the
drag experienced by rotor blades as they spin in the air, but also
has to cope with the effects of the spining rotor itself flying at
speed through the air.
Spinning unpowered rotors are driven because of an effect that causes
a section of the inner part of the rotor blades to want to move forward
(thus spinning around the rotor hub and mast) rather than upward. The
lift characteristics of the outer portion of the blades want to move
upward and slightly rearward rather than forward. So the rotor drive
comes from the inner area (called the 'Driving Region') and the main
lift comes from the outer area (called the 'Driven Region'). So the
faster a rotor is pushed through the air and allowing for the rearward
tilt of the rotor disk and the pitch of the blades, the rotor will try
to spin faster which also increases the profile drag of the entire rotor.
The speed of a autorotating rotor is the speed at which the Driven
Region opposes the force of the Driving Region. That is the Driving
Region torque is matched by the retarding torque of the Driven Region.
Gyro (or auto-rotating) rotors need to be 'loaded' all the time in
order for the Driving Region of the blades to keep driving the Driven
portion of the blades. Lightweight rotor blades that become 'unloaded'
will decay in speed much faster than heavy blades due to the different
inertia characteristics. They will also accelerate faster when loading
increases.
The blade moving forward will be experiencing approx 500MPH forward
airflow at its tip while the retreating blade will experience about
300MPH reverse airflow at its tip.
This problem is one of the more controversial that the CarterCopter
deals with.
To do this the rotor is 'unloaded' at high forward speeds, that is,
the pitch or angle-of-attack is reduced to near zero degrees and
the rotor disk is tilted almost parallel to the airflow. This means
that at this time the rotor is now providing a very small amount of
lift at this high speed, so the craft requires wings that will have
by this point taken over that role. While it would be good to stop
the rotor altogether, it is a big challenge to do so.
The CarterCopter has very high aspect ratio wings that are close to
the same overall length as the rotor diameter (high aspect means they
are very long compared to their width - a bit like a rotor blade which
is in effect a very very high aspect ratio wing).
This wing shape also provides reduced drag when compared to a low
aspect ratio wing. Low aspect ratio wings are needed for slow flight
in craft that must have a low stall speed (in fixed-wing craft, there
are legal requirements for minimum stall speed). On the CarterCopter
the rotor provides primary lift in slow flight (below 100 MPH) and as
with all Gyro rotors (at the correct reward disk tilt and collective
settings) has no real stall speed.
The CarterCopter's wings will normally take over the burden of lift
at a speed close to just above 100 mph depending on what the flying
gross weight actually is.
By slowing the rotor down also greatly reduces the rotor drag.
The horsepower reduction gained by reducing the CarterCopter rotor
speed to 1/3 what it was, is 1/27th of the horsepower! (the Fan Law).
Direct quote from Jay Carter : "The profile horsepower
required in flight due to the rotor is essentially a function of rpm3
(rpm cubed) and the forward velocity of the rotor.
Dropping the rotor rpm from 300 to 100, for example, reduces the
rotational aspect of the rotor profile horsepower to approximately
(1/3)3 or 1/27 of the rotational profile hp @ 300 rpm.
Because the surface area of the rotor can be small in comparison to
a fixed wing aircraft for the same lift, the forward velocity drag
component on the rotor is small relative to a fixed wing.
Since the rotor provides lift at slow and intermediate speeds, the
wing can be sized very small and still provide all the lift for
cruise conditions.
This combination of a very small high aspect ratio wing with a slow
turning rotor results in significantly less net lifting surface
drag than a comparable fixed wing propeller driven aircraft. "
The CarterCopter rotor spining at 100 RPM at near zero blade pitch
only takes 3hp, this is a very small penalty to pay compared with
the complexity of trying to stop the rotor altogether. So applying
the fan law ...
Quote 1 from CC Engineer Adrian Nye : - The main technique
is the tip weights - they increase the centrifugal force so much
that a pretty low RPM is enough to keep the rotor stable.
Our rotor can be thought of as a 55 pound weight on the end of a 16
foot string. We are not relying on the stiffness of the string to
keep it stable.
In flight we can measure flapping which will give us a good idea of
when the rotor RPM is getting too low for a particular forward speed.
Quote 2 from CC Engineer Adrian Nye : - Flapping is safe
up the flapping limits of the rotor head.
The stability of the blades at high speeds is based on the centrifugal
force of the tip weights holding the rotor blades straight out,
a force that has to remain greater than the aerodynamic forces trying
to destabilize it.
The pilot will have a flapping gauge and warning, which is how we
will safely find out what the rotor RPM has to be at each forward
airspeed to maintain rotor stability.
To prevent the blades from stopping altogether the CarterCopter rotor
takes some small advantage of the anemometer effect by which the
airflow will cause the rotor to spin due to different drag coefficients
on the advancing and retreating blade.
The anemometer effect is used in devices that measure wind speed.
But to keep the rotor spinning at the required minimum of approx
100 RPM mostly requires the use of the windmill effect.
To explain this effect we should also look at what happens at slow
forward speed (again from Jay Carter Jnr) ...
The core of the rotor is an I-beam that is torsionally soft if twisted
but edgewise (in-plane) is very stiff. The I-Beam is flexible
(out-of-plane) to reduce loads due to rotor coning.
The purpose of twisting the core I-beam is that the actual airfoil part
of each rotor blade (on the inner half of the I-beam spar) is hollow but
the outer section lengthways - from the middle to blade tip - is rigidly
attached to the I-beam spar it is fitted on.
The I-Beam spar in this hollow inner section of each blade, gets twisted
when collective pitch is applied to the outer airfoil part.
Another way to explain this - picture a 33.5 foot long rod (our I-beam
spar), in the middle of the spar there is a doughnut shaped moulding that
becomes part of the hub. Over each end of the spar we slip a hollow 16
foot airfoil blade as sleeves.
This is a very simple pitch change technique and a simple mechanism
compared to most other rotor craft.
The rotor is further stiffened by the inclusion of 55Lbs of depleted
uranium in each tip - this is 1.7 times heavier than lead and is
moulded as slugs, into a 7ft segment of each rotor blade near the tip.
At 100 RPM the rotor maintains the required rigidity to withstand
buckling from the airflow. One needs to remember that the air is
less dense the higher one goes - at 40,000 Ft it is approx 1/5 th
as dense as at sea level.
The entire rotor is close to 240 Lbs including the 110 Lbs of depleted
uranium. This is lighter than most similar size helicopter rotors are
even including the weight of the depleted uranium. Most helicopters
concentrate their weight in the center while the CarterCopter
advantageously concentrates its weight at the rotor tips.
The collective pitch control is very simple because of the novel use
of the twisting I-Beam in the center of the blades (no big hub and
massive bearings).
A properly designed rotor (symetrical front-to-rear) can handle the
reverse airflow that is going to take place as the craft speeds up
to 400 MPH.
Some people want to see this work before they will believe it and
that is understandable.
Many people have difficulty grasping that at these high speeds the
rotor pitch is near zero and while there will be a small amount
of flapping taking place, the rotor is essentially 'unloaded'. One
also needs to visualize that the air at high altitude, is a fraction of
the density of air at sea-level.
CarterCopter are satisfied it will work exactly as predicted
and tested on their 1/6th scale rig.
Because they are able to shape the composite material in the middle of
the monoconstructed rotor I-beam they have shaped a hollow that allows
them to place the tilting plane within 1/2 inch of the centre-of-lift.
The tilting spindle is very short and provides satisfactory control to
the pilot operating the cyclic (rotor tilting) stick. This tilting
spindle and its mounting are designed to allow the normal oscillating
forces that can appear in a rotor, to move the tilting spindle without
affecting the current tilt angle or collective setting.
The control rod that is used to apply collective is actually inside
the tilt spindle and also passes through the hollow rotor shaft
which is connected via a universal coupling to the tilt spindle.
One other important design factor is that the teeter hinge (the see-saw
used to allow blade flapping) is pivoted at 30 (or 60 depending on how
you look at it) degrees rather than 90 in much the same way a normal
two-bladed helicopter tail-rotor is hinged (called Delta-3). As the
blades flap the offset pivot causes blade pitch to alter in opposition
to the flapping effect. This rotor flapping damping technique is feasible
on the CarterCopter because it also has its high aspect ratio wings which
have flaperons plus there is the tail stabilator, both of which provide
pitch and roll control in addition to the rotor. However the tail
stabilator only comes into operation during high-speed flight.
Firstly the tilting rotor spindle is only inches long, is hollow and
rotates inside a universal gimbal that is mounted to the airframe
pylon within 1/2 inch of the centre of lift. The gimbal bearing
mounting is the top side of a shock absorbing mechanism (a parallelogram
device) that absorbs fore/aft movement of the whole rotor assembly.
(See photo in the Pictures section).
This parallelogram shock absorbing mechanism is inside the top of the
CarterCopter pylon that is between the main fuselage and the rotor hub.
The assembly can move fore/aft without changing either the collective
setting or pitch/roll tilt settings.
The hollow rotor spindle allows the collective to be controlled by a
pushrod that moves up and down inside it and through the appropriate
use of bearings it can spin with the rotor spindle.
A transmission universal joint connects the tilting rotor spindle to
the hollow main rotor shaft - the main shaft is used for spinning
up the rotor prior to take-off. A drag reducing cover over the rotor
control mechanism greatly reduces drag as compared to helicopters or
other gyroplanes.
Hopefully this has explained why the CarterCopter will be able to
travel at high-speed with a slow turning unloaded rotor.
One comment I have added to this topic but which crops up from some
people, is what is it about Jay Carter that he understands rotors so
well vs all the many highly paid experts of rotary wings both current
and over the years - It is this .... Jay Carter ran one of only two
major US companies that constructed composite variable-pitch rotors, up
to 80 ft in diameter for use in his own company's wind turbines. These
rotors had to work in extreme conditions in extreme locations. A 32 ft
composite monoconstructed rotor using a warping I-beam and with depleted
uranium in the tips is no great mystery to him. It quite reasonably may
be a mystery to many other rotor experts.
Cheers
Doug Marker
In order for the rotor to maintain adequate stability, the rotor
blades need to provide equal lift at all times and to resist the
destabilising forces acting on the rotor at high speeds.
Problem 4
Problem 5
When at this speed and with a rotor speed as low as 100 RPM there would
be approx 200MPH difference between airflow over the advancing and
retreating blades.
The way CarterCopter sets out to solve these problems is :-
Solving Problems 1-2-3
At 300RPM if the hp energy needed to spin the rotor is 81 hp then,
At 100RPM the hp energy needed is one 27th of 81 = 3 hp.
Rotor Stability - Solving Problem 3
A big challenge for the CarterCopter is how to prevent the slow
turning rotor blades from buckling (that is, blades bending if the
airflow gets under or over them and they are not stiff enough to
resist folding over).
CarterCopter solves this by both the way the rotor is designed
and the use of heavy tip weights
The windmill effect occurs as air passes through the free-wheeling
rotor blades.
If we then firmly attach the outer lengthwise half of each blade
to its section of spar but leave the inner half lengthwise free, then
by applying a twist at the root of either blade (pitch-change) to the
outer airfoil sleeve, the inside unattached part of the I-beam spar
will twist with the outer sleeve thus effectively providing a pitch
change. Also, because this pitch change technique is used collectively
(not cyclicly) the spring tension in the twisted sections of the spar
are equal so there are no tension imbalances to be dealt with.
When at this speed and with a relative rotor tip speed of say
100 MPH there would be approx 200 MPH difference between airflow
over the advancing and retreating blade tips. The blade moving
forward will be experiencing approx 500 MPH forward airflow at its
tip while the retreating blade will experience about 300 MPH reverse
airflow at its tip.
Other Aspects of the CarterCopter Design
The rotor shaft is needed to pre-rotate the rotor before take-off
(sounds complcated but looks remarkably simple when looked at).
(There is a photo of the rotor head in the Pictures section).
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 © 1999 Internet Age Pty Ltd