The average specs
& parameters for a flyable
tractor powered ......
MODEL
AUTOGYRO
This
file will occasionally undergo a few revisions, I will attempt to mark those
revisions as "new". Hopefully I will be able to eventually
present a few important key specs in relative to the design of a "pusher"
type model.
- Except where specifically noted,
this graphic
applies only
to R/C non-winged tractor style (engine in front) model Autogyros
and also does not fully cover all the experiences of every
Autogyro modeler.
An Autogyro.... is very
different in many ways when compared to either an Airplane (fixed
wing) or a Helicopter. An Autogyro is not a
modification of the Helicopter.... The Helicopter is a
modification of the Autogyro.
This information is posted here simply as an aid to those modelers
interested in trying their hand at designing, building, and flying a model of
their own... It can also be of great help in determining why a
model is not performing as well as anticipated. Keep in mind
that if you deviate from the average or norm in one area, it will potentially
/ adversely affect another parameter.
Simply put, the Autogyro depends on an initially high positive angle of
attack of the rotor to initiate acceleration, and while the angle of attack
remains positive (average of about 10 degrees) , throughout the flight, lift
is still produced by down flow from the rotor blades. While
it appears that the airflow is constantly "up" thru the rotor, technically
this cannot produce lift. Since our simple Autogyro models have
an unpowered rotor, and a fixed incidence pitch of the blades, we tilt the
rotor to accomplish a climb, descent, or turn.... whereas the
Helicopter depends on pulling the airflow "down" through the rotor, by varying
the blade incidence, of a powered rotor.. and is able to change attitude by
simply cycling the incidence of the blades. Our autogyros must
have a lifting airfoil on the blades, whereas the helicopter blade
airfoil is generally symmetrical.
To control the direction of our gyros we will tilt the rotor disk just as
you would the wing / tail of an airplane. along with
providing some sort of stabilizing tail fins... We can directly
tilt the rotor via servos, use an elevator and rudder, or a combination
of both. Without the full cyclic blade control of the blades of
the helicopter, we also must provide a propelling engine to pull or push the
Autogyro forward through the air. All these are basically what
separates the gyro from the heli... in as simple terms as I can express.
It is not a simple matter to find the proper balance of angles
and measurements for a model Autogyro. They are far more critical
with this unique type craft than the average fixed wing model
airplane. The angles and measurements mentioned here have been
accumulated thru years of experience and experimentation, and are close
approximates. They do not fully apply to models
that utilize a small to medium sized wing (such as the Gyrace, or even
the Whistler). Nor do they apply to a modified helicopter
such as the Robbe Whopper, Kalt Robin, or
the Micro-Mold Wallis models labeled as Autogyros. Those
latter models are basically helicopter models without a powered rotor, and an
engine installed to compensate for this change..
Keep in mind that a particular model may necessarily deviate from the
above slightly, due to differences in design (tractor versus pusher),
etc. Technically it seems far easier to begin with an all new
model, versus trying to convert a model not originally designed as a
gyro or even a tail controlled gyro not originally designed as a DC
(Direct servo Control) gyro. Please read over
the following explanations in regard to some of these differences.
A: Engine
Downthrust... 5 degrees (3 to 8
degrees)
Engine downthrust is necessary for a model Autogyro to aid in overcoming
the drag of the rotor system, which is located high ( vertically) above the
horizontal centerline and Center of Gravity of the craft.. It
would be nice to have the Thrustline project thru the Center of Gravity
(which, by the way, is virtually necessary with the "pusher" models),
(CG) however this is impractical for most of these models, so we
use just enough to assist in maintaining a level flight attitude.
A Downthrust of 10 to 15 degrees is not unheard of, however this high amount
is indicative of a rotor being placed perhaps too high, creating excessive
drag. Placing the rotor as low as practical should reduce the DT
to the 5 to 8 degree average many of us find works very
well. If you have a model that seems to be flying tail low
all the time, it may have insufficient downthrust, or perhaps it also
needs a "lifting airfoil" on the stabilizer, or the incidence of the
stab needs to be shimmed slightly positive (giving it a slight
lift). Consider also the relative strength of your
motor. If you know your motor is in the small / low power range,
it may require slightly more DT as an aid in countering rotor drag.
B: Mast angle or Aft
Rotor Tilt.(aft of a true vertical line) .. 10
degrees (6 to 14 degrees)
This is the amount (in degrees) the rotor shaft (thus the incidence of the
entire rotor disk / plane of the rotor) is tilted aft for an initial (zero
pitch trim) position. Much seems to depend on the
type/location of the stabilizer system installed, as well as the amount of
downthrust, and the CG location. As an example, With a
lifting stab`, the angle will probably be 10 degrees or more.
With a relatively flat stab` the angle will decrease.
If you find your model requires more aft tilt than this in
the neutral trim position, you may have too much downthrust or the model may
be nose heavy, with a larger than normal "hang angle". A correction
in this situation could mean a slight aft shifting of the CG, and /
or a decrease in downthrust.
C: Rotor Disk
Height... roughly equal to "E" , but no more than "E",
plus perhaps 25%.
This can be a highly variable measurement. One finding is that the
higher the rotor (vertically) the more stable the model, however the more
difficult to accomplish a decent coordinated turn (it may possibly "resist" a
turn). The lower the rotor height, the more maneuverable a model
may be, but less overall stable. A model utilizing a very high
(vertically) rotor position will generally require coordinated
rudder assistance to complete a turn. With totally
direct control (fins without rudder or elevator) models, the rotor height
is more critical to accomplish coordinated flight, therefore are more
difficult to design and fly. However, a properly designed DC
model is more controllable throughout its speed range, and can be
controlled down to zero forward airspeed, versus strictly a rudder/elevator
and no tilting rotor. Some full sized flyers may not find
this true of their craft, however it is something we have found to be true
with these models.
D: Rotor Disk
Diameter... sufficient to equate to a disk load of less
than 5 ounces per sq. foot.
The lower the disk load, obviously the better chance the model has to fly.
Models with loads over 6.0 are relatively heavy, and will
experience problems with hovering in anything less than a strong breeze.
They will also require a longer ground T/O distance and/or be more
difficult to hand launch. Many of the finer flying models have disk
loads in the area of 2 to 3... You should probably review the
following graphic charts on this subject: Model Autogyro Flight
Prediction Chart and Model Autogyro
Calculations. You may think "why not just increase the
rotor size to obtain the lower (better) disk loading?" Well,
that's not the simple solution. By simply increasing the size of
the rotor you also increase the rotor drag, and this is something you must
consider before making such a change.
Rotor Disk
Solidity...3 or 4 blades recommended (number of rotor
blades..)
The actual number of rotor blades is not critical
for model performance. We note that rotor RPM may
decrease some as you add blades, thus simply the adding of
blades does not significantly alter the rotor
performance. A single, counter balanced blade, would
possibly work, however it`s use is not as practical as two or
more. Two rotor blades work fair, but experience with models
tells us they are not as efficient or as easy to pre-spin as three or
four.. Three blades perform exceptionally well, and many
modelers report they seem to add a bit more stability and steadiness to a
model. Four blades work very well, and will improve rotor
efficiency slightly, if you want to go to the effort to built more than
three blades. If your model is an attempt at scaling a full sized
machine using four or more blades, use the correct number,
however do not expect much (if any) increase in efficiency.
Suggestion: It is a good idea when you make blades,
to make an extra identical one for the set...... Sooner or later you
will probably need to replace a blade.
Rotor Blade
Aspect Ratio...10:1 is a good place to begin (length
versus chord-width)
An acceptable range for blades that
always seem to work for these models is sometimes can be anywhere from 8:1 to
12:1. With the ratio of 10:1 being
a good place to begin with. This fact was verified in the spring
2000 wind tunnel
tests. Short / wide ones will work, as
well as long narrow ones, however the most consistently
efficient ones seem to be within the range mentioned above.
Thickness at the maximum lift point of the chord should also be watched.
Blades with a thickness of over 16% of the chord tend to have a
more blunt (higher radius) leading edge, and thus are a bit too thick.
They also tend to be more difficult to pre-spin.
Measuring quite a few of the blades that seem to be more efficient for these
models, the range is 13% to 16%, with the majority closer to the 13% area.
The leading edge should not be highly round or
blunt... A fairly narrow leading edge radius is desirable, along
with a thin trailing edge.. We accomplish this by using the
Clark W, K, or Y airfoil, with a flattened out
bottom, into what we generally refer to as a "flat bottomed
Clark-Y" airfoil. Full sized "rotorcraft" (gyroplanes)
frequently have a slightly "reflexed" trailing edge to their blades (Clark -
YS), which aids in preventing "tuck under".of the leading edge...
This reflex is usually in the form of a slightly up-turned
trailing edge, and has been found to work well on models, however is difficult
to make. If you have the capability of building slightly more
complex airfoiled blades, seriously consider the SG6042 for your
airfoils... This airfoil scored exceptional in the
spring 2000 wind tunnel
tests.
E: Nose (prop) to Rotor
Mast.. roughly equal to "C", keep short, just sufficient
to balance the model just forward of the rotor shaft.
For stability the rotor normally is placed higher than the length of the
nose. If you want to employ direct servo rotor control
(DC) to control the model, extending the nose too far may make it
difficult to impossible for the model to initiate a turn without the
additional use of a rudder. Also remember for each added bit of length or weight to the nose
section, you may have to add at least 1/2 that weight to the tail section to
maintain balance. The point is.... keep the nose short as
practical.
Center of
Gravity....
The model needs to balance just forward of the rotor shaft, and will
necessarily be above the fuselage due to the volume, weight, and drag of the
rotor system. This latter item is one major
difference to the average model airplane, and must be kept in
mind. Due to the configuration of an Autogyro this may place the CG in
a position where you cannot "grasp" the model at this point. To check for
balance/CG location, hold the model by the rotor shaft and note the
nose down attitude of the model in reference to a horizontal line
through the center of the fuselage to the tail. This is what we
call "hang angle" and will vary from model to model, however
the average is around 5 to 10 degrees (nose down). Just as with
disk loading, the higher the hang angle, the heavier the
model may fly. The less, the lighter it will fly, but may
be less stable. Just remember... if the model is balanced
directly at the center of the rotor (shaft) it will be extremely
sensitive in both pitch and roll, therefore it is much easier to
control with a slightly forward CG... A tractor model with
very little or no hang angle may not assume a nose down glide with engine
failure, and thus will be virtually uncontrollable. A model under
these conditions will simply drift with the breeze and hopefully
complete a parachute type descent.
An Autogyro aircraft behaves just like a fixed wing model aircraft
in regard to CG location.. . As the CG is moved aft, the model
will become increasingly control sensitive and perhaps
uncontrollable. However if you elect to move the CG forward too
much (increasing the hang angle ) , the model will begin to fly like it is
overweight. If your model seems to require an excessive amount of
down hang angle, then you are perhaps correcting for an excessive rotor
height (spec "C" above) or some other sort of stability problem in your system
that needs attention? Note that you can fairly well locate the
exact Center of Gravity by noting the vertical line through the model while
holding the model by the propellor shaft, and noting where the hang
line passes through this line...
F: Rotor to Tail
distance.. normally double the distance "E" ..
Extending the tail unnecessarily may create a similar problem as stated
above in regard to extending the nose. Once you establish the
size of the rotor desired, place the tail fin just aft of the max radius
of the rotor (by perhaps 1/2 to 1").. This applies to models where the tail
is aft of the rotor. If your model has an aft fuselage
that can flex upon touchdown on landing, additional clearance may be
needed. Normally under controlled conditions, and with
properly tip weighted rotor blades, the rotor will not flap down on impact,
however with a Fiber tube fuselage it is very possible
for the fuse / tail to flex upward and strike the rotor.
G: Vertical Fin
Area... approximately 2.0 to 3.5 % of the Rotor Disk
Area. (and approximately 45 to 55 % of the horizontal stabilizer
area)
This will vary greatly depending on the particular design,
with this figure applying to the model with the tail aft of the
rotor blade tips. The tail feathers need to be kept close
(but allow at least ½ to 1") blade tip clearance.
Extending the tail well aft of the model may increase its stability, however
decrease its ability to complete a coordinated turn without the need for lots
of rudder assistance. Enlarging the rotor and extending it
aft /over the tail changes several variables, which will not be covered until
a later revision of this page.
Horizontal Stabilizer
Area... approximately 5 to 8 % of Rotor Disk
Area. If the rotor is directly servo controlled, and
elevator + rudder control is not used, the tail can be slightly smaller in
size.
A few DC models have been known to fly without a
horizontal stabilizer, and a smaller vertical fin... however at least
some horizontal stabilzer is desirable, and a good average sized vertical fin
is virtually always necessary.... A few simple tests were completed
with the Horizontal Stabilizer moved forward 1 / 2 the distance
from the rotor mast and highly erratic model behavior was
noted. However moving the Vertical Fin forward had very
little effect, with the model remaining stable.
H: Rotor Pitch control limits
... + / - 8 degrees
Rotor Lateral
Tilt control limits... + / - 7 degrees.
This
applies to rotors that have Direct Servo Control
(DC).. Models seem to be far more sensitive in Roll than
in Pitch.. A new model may be oversensitive in roll if
you install more tilting than mentioned here. However initially
you may want to install slightly more than the 8 degrees of pitch
tilt to provide a cushion of control. NOTE: it is
not unusual for some models to resist a roll/turn to the left, and may
require some left trim. Some new models have been found to be
unable to turn left on the first flight, so it may be advisable to
install an extra 2 to 3 degrees of left roll tilt throw on the initial flight
tests, with a possible subsequent requirement of for a pre-set left tilt trim
a several degrees for all flights..... This may mean a
slight problem involving Dissymmetry of Lift
(unbalanced lift towards one side) and while it is difficult to explain, it is
almost always in the direction of the retreating blade.
Note: most of us employ the counter - rotating (as viewed
from the top) rotor, for several reasons, and in general we are always
speaking of a CCW rotor.
Note: You can possibly detect this
problem prior to the initial flights by conducting a hand-held-nose-high rotor
test with the nose pointed into a good breeze... Allow the rotor
to fully accelerate and note any tendency for the model to tilt to
either side.
If it does, consider pre-trimming a couple degrees
opposite the direction of roll. In otherwords, if it
rolls to the left, trim a degree or two to the right.
As mentioned before, it seems to always be in the direction of the
retreating blade.
NOTE: I have noted an interesting
situation regarding the above in reference to electric powered gyros.
If you replace your electric motor with one of significantly
more power, the model then will often roll to the left on initial
flights. The larger propellor on the electrics produces a
much stronger torque effect on these small models.
Model Weight: This is
critical. Use the lightest building materials available, yet
maintaining reasonable strength. Light Balsa, light Plywood, Carbon
Fiber, etc. Use the smallest servos that will serve the purpose,
such as sub micro and micro for throttles, strong sub-micro
(20 - 25 ounce torque or higher), mini, or lightweight others for
rotor controls under 1 meter ( 39" ). Small, compact
battery packs, receivers, wheels, etc., are best..... anything to keep the
gross weight down. You may be surprised to know that
servo damage usually only occurs on the ground, not in
flight. Tipping the model over on landing, tripping the
blades can damage servos.... Never use more than 1 (one)
hard bolt to mount the blades unless absolutely necessary for
a very large model. A common blade mounting bolt being used is a
4x40... however for the smaller models (1 meter rotor or less) a 2-56 has been
more than adequate (zero known failures to date) and a gram saved is
important! In addition to a single bolt, use balsa shear pins for
the small, and a nylon bolt is good for the larger
models. This is to aid in keeping the blade straight during
initial spin-up. If not, you will need to torque down the
single bolt. I prefer the addition of a balsa pin rather than the
single bolt simply because the torque of the single bolt requires a heavier
bolt mounting and it`s frustrating to keep the blades aligned for pre-spin
with a single bolt. For rotors over 4 feet (1.2 meters) , a
nylon shear bolt works well, rather than the balsa pin. See
this performance Chart...
Engine power: It is not necessary nor
always advantageous to overpower a model Autogyro. The
generally acceptable requirement a few years ago was for the thrust to equal
at least ½ of the weight of the model. If the model weighed
3#, then you needed a bare minimum of 1 ½ pounds of
thrust available. This now appears now to be an
underestimate... Recent practical experience has
demonstrated that the ½ weight in thrust is marginal, especially when flying
well above sea level. I`d suggest about 75% or more of the weight
in thrust . In other words, if the model weighs two pounds, then
you should use at least 1 ½ pounds of
thrust. A 1 to 1 ratio may not be necessary, and may
be unwise unless the additional engine weight does not create
an excessive disk loading. Using a large powerful (heavy)
engine can also create an "overpower" situation at times.
In regard to electric models, it is very difficult
(with the weight of the current batteries being considered) to
obtain sufficient power. Electric models must be
built as light as possible.. For our reference
purposes, a "strong" electric Speed 400 motor is about equal to
.050 in glow motor (IC) power. Refer / to this chart...
To fully understand.. all the information presented
here, you need to read over the many technical articles posted on this web
site. Propellor size and use, as an example is different from a
average / normal model airplane. We must necessarily go
for power (larger diameter) and low speed (low pitch).
The recent (1989-99) research / experimentation with the
Electric versions, the almost necessary larger (than glow size)
propellors has dramatically demonstrated that the torque effect of the larger
prop will tend to turn these (smaller) models with throttle
changes....
Except where
specifically noted, this graphic applies only to
tractor style, (engine in front) R/C models that do not employ an
additional small fixed wing... and also does not fully cover all
the experiences of every Autogyro modeler.
Jim Baxter, revised 22 Dec 2003
To the Technical
Menu
Back to Homepagerev ..12-22-03.. jb