Mickey Nowell provided us with an excellent
detailed answer to this, and in his brief summary he stated: " Tip Weights slow
down the control response, dampen rotor speed variations, increase spin up time.
Other than the increased spin up time the results are all good for models."
If these model rotor blades are kind of
tedious to make, why isn`t there a source of mass produced (balsa, plastic,
carbon fiber, etc.) rotor blades? Have the Gyronuts looked into
this?
Well, we considered this many times... and for
numerous reasons we found it just not feasible, due to the initial costs.
However in 2004, Mike Smith of "AEROBALSA.COM" managed
to come up with what we consider near perfect model autogyro rotor blades.
Mike has them in several sizes, and every set this webmaster has tested were found to
give superior lift to any formerly hand-made blades. Contact Mike at:
Aerobalsa.com
I notice that one blade is not rotating
(tracking) in the same plane (elevation) the others are..? How can I correct
this?
This is generally a problem of one blade not
being shimmed to the same incidence as the others... First you need to
mark each blade with some sort of colored tape, felt tip pen, or paint, etc., at
the tip, and then spin the rotor and note which blade color is off (incorrect)..
Then remove the blades and insure that the incorrect (off track) blade is
shimmed exactly as the others. If the blade is still off, increase
the incidence (+) slightly on the bad blade if it is tracking low, and
vice-versa if high, until all the blades are close in track.
Tracking can be a problem of concern for the helicopter,
however experience has shown that with the considerably slower rotor of
the model autogyro, an out of track blade is not a big problem for us as a
rule.
I just installed a flex hinged hub, and the
model wants to roll to the left on launch.... Any idea as to the problem?
If your rotor rotates CCW (left when viewed from
the top), one strong possibility is insufficient "up" flap (up flex)
allowance in the blades. If you only have perhaps 10 degrees or less
originally, increase this to a good 20 degrees and re-test the rotor and model.
Note: This assumes you are allowing the rotor to reach full
autorotational RPM, and are not attempting to "pull" the model off the ground
prematurely. Because if the rotor is not up to full rpm for safe
launch, it will probably roll into the retreating rotor blade.
Can I use the plastic from a "Tupperware"
container for rotor hub/blade hinging material?
You probably could, however we don`t believe it
is as good as Polypropylene and therefore it may be subject to premature
failure. In otherwords, it may fail/crack/split, etc., after being
subject to the high frequency of blade flexing. Many different
non-Polypro`plastic hinges have been used sucessfully, however
experience tells us to watch carefully for the initial signs of hinge failure.
Is there an advantage to mounting more
blades, such as four, versus just two?
Wind tunnel testing conducted in early 2000
indicate that three blades lift better than two, four blades lift slightly
better than three. And the relative RPM between three and four
is about the same. So, if you feel you have adequate
power (1:1) it would be to your advantage to use 4
blades, however with a potential power problem, it would be best to go
with 3 blades to reduce rotor drag slightly. The rotation rate
of the 4 blader will be slower than the two blade rotor. In heli`s
and prop`s, you add blades to absorb greater amounts of horsepower to lift more
and more weight.
Autorotation..? What is this term
that I hear so much about..?
This is not an easy term to explain without
getting into a lot of technicalities, however to say it as simple as possible,
this is when the rotor "accelerates" due to it`s own aerodynamics....
Before a rotor enters "autorotation", it is simply "windmilling".
Click
here to view a graph we have prepared to assist those who want a more
technical explanation.
How do you know how much engine power is
required for these autogyro models...?
There is a disagreement among the active gyro
modelers we are in contact with... Some years ago, one claimed that:
Weight = 2x Thrust... or in otherwords, the thrust of the engine must
equal (or exceed) 1/2 the total weight of the model. However with
all the recent and more vast experience it is obvious that thrust should equal
weight. The most experienced gyro modeler (Jo Jo
Chaulet, France) agrees that you should have power, up to possibly:
weight = thrust.. Obviously the 2x thrust would be the minimum, and
perhaps may be adequate for indoor modeling, however a nearly 1 to 1 has
been found to be necessary for most all models. However
don`t take the "easy" route and hang a big engine up front just to make sure
though... the larger engine just weighs more and you`ll end up adding
balance weight, and up goes your disk load. Keep engine weight down to
just the amount needed. Check this
chart for some very helpful information on this matter.
Does the above power requirement stated apply
also to the electric models?
Thrust is thrust... so the obvious answer
is yes. Adequately powering an electric model has been found so far
to be a difficult matter, and we are still very much in the experimental area
with the electrics. Model weight / disk loading is
extremely critical.
What in the world is what you fellows call...
"hang angle"?
The "hang angle" is the degree of nose down
attitude a model assumes when you hold it up by the rotor shaft. It
serves the same purpose that suspending a glider or airplane by the wing center
of lift and adjusting the CG to give the craft a nose down attitude when power
fails. A model may fly very well with little or no "hang
angle", However with engine failure it may simply become totally
uncontrollable and float/parachute descend where the wind wants to take
it... with at least some "hang angle" you generally have a little
bit of opportunity to slow/arrest the rate of descent immediately prior to
touchdown.
What's the difference between
"blade pitch" and "blade incidence"?
Nothing... basically for our purposes they
reference the same thing. Both are in regard to the actual angle of attack of
the blade in reference to the incoming air.
I've heard of "Rake" or sometimes "Mast Rake".... what's
this?
This is the same as what we term "aft tilt",
"mast tilt", or even "rotor shaft angle"... This is the angle the rotor shaft is
tilted aft in reference to a horizontal reference line through the fuselage.
What does "direct control" mean?
This when a control device is connected directly
to the rotor, thus allowing for control of the gyro directly thru tilting
the rotor. When we connect a servo directly to the rotor head, we call it
"direct control" or "DC". This could be called the "Bensen" method,
and not being dependent on airspeed, control is maintained down to zero
airspeed. The tiltback of the rotor with respect to the incoming air
can be also accomplished in another way. You could fix the
rotor/mast to the airframe and tilt the body with an elevator and rudder (Cierva
method).. Do not confuse either of these with helicopter control devices.
With a heli, the servo is not connected directly to the rotor, but to an
intermediate device, such as the swashplate.
If the rotor is connected directly to the servo... isn't this
excessive stress on that servo?
Not necessarily... Experience shows that
-if- the rotor system (blades, hub, etc.,) are thoroughly balanced, the stress
on the servo is greatly reduced. Since the servo is "working' only
during the first few revolutions after a rotor position change (until the rotor
assumes a new stable position due to precession), the stress is relatively low.
Servo failure is most likely to be experienced from a blade
striking the ground, etc. , not from in-flight stress. Naturally
one must consider the size of the model, and use higher torque servos as the
size increases, simply to be able to apply enough power to move a rotor.
Micro size servos of limited torque have shown to be adequate for
rotors of up to three feet in diameter. There is more stress
on the pitch servo than the roll servo, so it is generally is a good idea to use
your stronger servo for pitch control.
With the nylon gears on small micro/sub-micro
servos, especially, it is tempting to apply a lube such as "White
Grease", etc. Don`t apply anymore than
the original manufacturers application. It has been noted that with
excessive lube, small servos may 'slip' gears, even without
damage to the gears. I have personally cleaned gears and
sprayed them with an evaporating "cleaner degreaser" to prevent such
slipping.
Do I really have to
balance a blade chordwise and lengthwise both?
All non-symetrical blades must be balanced
chordwise to prevent a nose-down "pitching" or twisting action as the blades
wind up to speed, which decreases the lift of the blade and a loss of
controllability... The balance point is typically at the 1/4 chord
point for Clark-Y airfoils. . If you want to remove vibration, and
prevent possible system failure due to this vibration, lengthwise balancing is a
good idea. Models will and have flown without much rotor &
blade balancing, however system failure, and ulitmate model destruction is
imminent.
We are now finding that at our relatively low RPM
speeds that the need for chordwise balancing is not absolutely
necessary. It certainly won`t hurt though, so consider doing it
anyway if possible. Note also that blades balanced at the
33% (1/3) chord point perform extremely well for our models,
and it is easier to balance a blade at the 33% point than the 25% point.
How fast do these autogyro
rotors spin (rpm) in comparison to a helicopter?
Much slower.... and it will vary with the size
of the rotor. Typically they spin in the 500-800 RPM range for the
average gyro (depending on the solidity and rotor surface finishing). .
Small rotors of 3 feet or less have been tached at over 1000 rpm,
and I have noticed rotors less than 2 feet in diameter clocking over 1500.
Helicopter rotors are turned by an engine, and operate at a
relatively high blade pitch, while gyro rotors operate at a flat pitch
(geometric) and are turned by the air flowing past them. Thus the
term "windmill aircraft".....
"AUTOGYRO"...
versus "AUTOGIRO"... what`s the difference?
The term "Autogiro" is a copyrighted name held
by the Autogiro Company of Spain, and originally obtained by Juan Cierva.
"Autogyro" (or gyro) is the generally accepted English spelling of
the Spanish "Autogiro" and is the term most often used. You
often hear the term "Gyrocopter"..... this is the official FAA (Federal
Aviation Administration) term given to the full sized craft.
You quote rotor disk loads like 2,
3, 4 or 5... ? Is this like wing loading on
an airplane? Are they really that low?
Yes... It is just like a wing loading on a
fixed wing aircraft. Yes, they are necessarily that
low. With these models, an ideal load would be in the area of 2
to 3 ounces- per- square foot. Single rotor models may fly decent up
to 5 or even 6, and multi-rotor models may survive up to about 7 or 8 or
so. Much depends on the design of the model. By keeping
the disk load low, the power-off sink rate is kept low, as well as the required
power and airspeed to maintain level flight.
How do you calculate a disk load?
This is tough to display here... It is the
result of the gross model weight divided by the rotor disk
area. Radius squared, times 3.14, divided by
144, will give you the square feet. Then take the weight
of the model in ounces, and divide this by the square feet, and the result
is the disk loading in "ounces per square feet"...........
Where can I get an autogyro kit? How
about plans?
Kits have been rare & limited up until
recent years.... the market has been small due to the lack of publicity
and the desire of the modeler to tackle a more challenging aspect of
modeling..... which is something we have been slowly changing.
Please check our Plans page ..
for detailed information in regard to plans, and check with Autogyro
Company of Arizona for kit availability.
I'd like to start out with a scale model... where can I get plans
for one? Any kits?
First, forget starting out with a scale
model... unless you are already a highly accomplished modeler, and
have some helicopter experience... If you still feel this is how you
want to start, then check out
"Gyronuts.com " and look for the link to "Kits".
You can also check with Autogyro Company of Arizona.
Why are there so few Autogyro models?
Because there are so few gyro modelers.. to
build and fly them... Autogyros are different, and more challenging......
most modelers are not up to the challenge. and give up at the first sign of
failure... we are trying to change that.
Why can't I just use helicopter blades... and save all that
work?
You could. try them.... but the average
Helicopter blade just won`t work. Heli blades are usually symetric
(neutral lifting) and designed for a powered cyclic controlled rotor.
A heli blade, turning at high RPM tends to experience a "resonance"
or vibration of the blade as it flexes, much as a plucked guitar string dokes if
the natural resonance of the rotor blade falls into a range that could occur
within the normal operating range of the engine. A tip weight
reduces such flexing by increasing the tension on the blade and therefore
raising its natural resonance frequency.....
Why don't you just use fly-bars and such... like a
helicopter?
Fly-bars are a natural for helis in that they
change the lift vector of the rotor, allowing directional flight to occur
without changing the body tilt angle (neglecting the inertial forward motion).
Gyros use direct control of the rotor to accomplish the same effect by
tilting the body (Cierva) or the rotor itself (Bensen), saving a lot of weight
and complexity. The autogyro came before the helicopter, and the
original gyros had -no- flybars. Most of the gyroNUTS are attempting
to basically stick with the original design mechanics of the autogyro, thus no
flybar/cyclic control... it`s more of a challenge.
I'd like to build a "Bensen" type gyro... but I don't see any
information on that type? Why?
One of the big problems with Bensen-type model
autogyros is the pusher prop propulsion. A decent pusher prop itself
(that produces maximum thrust) is hard to find, and starting the engine
can be a problem, due to the position of the engine in relation to the vertical
fin. The "Wallis" (produced by the now defunct MicroMold, Ltd. of
England) solved this problem (?) with a swing-away tail structure.
Engine cooling is also a potential problem. It is a fact that
the airflow over the engine is less when the prop is mounted in a pusher
configuration... So monitor your engine for possible overheating,
especially if you have a prolonged period of ground
operation. If your engine tends to quit on the ground before
launch, overheating just may be the problem.
UPDATE fall 1999: The
mystery of how to design and build a normal, fairly simple, model of the Bensen
type is finally being solved. Check "Gyronuts.com" for the current
information on these models.
Why so much downthrust...? and yet little or no
sidethrust?
Tilting the thrustline of the engine down
reduces the torque generated attempting to raise the nose. A similar
effect can be accomplished by raising the thrustline. That`s when you
physically reise the engine vertically (not tilting the nose up).
Another way to look at this is that with the rotor mass so
high up (vertically) ... it takes more than normal downthrust to overcome the
drag of the rotor. As for side thrust, we have found that side
thrust is just not a big problem, with the tractor powered gyros most of us are
flying.
Why doesn't a gyro just roll over on
it's back when you use a single rotor.. without any cyclic pitch control like a
heli?
For the same reason a spinning "Frisbee" doesn`t
"roll over" ... but tilts back..... gyroscopic action of a spinning
mass! If however, the blades were not hinged or free to tilt, lift
Dysymmetry would indeed cause "roll over".
What is "negative" pitch or incidence... on a
blade?
Negative incidence is where the chordline of the
blade is actually pointing the blade "down" in relation to the
windflow. The chordline is a line from the tip of the leading edge to
the very tip opf the trailing edge. Negative incidence occurs when
the blade zero lift angle of attack is below the plane of rotation, resulting in
a self-starting rotor that is highly inefficient, and has low lift, reaching low
RPM. As the blade incidence is increased to zero (and beyond to a +
angle), the blade requires a faster "pre-rotation" to advance into autorotation,
but turns faster with the same wind speed,becoming more efficient and generating
a higher lift for the same tiltback angle. By Increasing the
incidence to a higher (+) angle, eventually one will not be able to start the
rotor by hand,thus some form of mechanical pre-rotator must be used.
I'm using a good Clark Y blade airfoil, so
why don't the blades want to spin up very well?
The blade is most likely mounted with too much
positive incidence, and will not start due to the explaination given in the
previous question/answer. try setting (shimming) the blade for a
slight negative angle a degree (or less) at a time, . until a satisfactory
spin-up is obtained.
NOTE: We have found that the basic Clark-Y is slightly too thick at its
balance point, and by trimming (flattening) the bottom of the blade, a much more
satisfactory pre-rotation is possible. So, if simply changing the
incidence slightly does not improve your situation, try "flattening" out the
bottom of the airfoil.
Why can`t I get my Kombat Kopter to
fly? . Everytime I try to complete a take-off it either rolls over
before lift or rolls over almost immediately after Take-off?
Unfortunately there are serious design flaws in
this model. Your problem is common with those who have contacted
us. Several of us have KK`s and the general feeling is that the
average modeler will find it very difficult , if not nearly impossible to safely
fly it. Lee Wayman solved most of the mystery thru extensive
modifications which is more than the average modeler may even want to try.
Three recommendations: (1) Check out the Wayman mods posted here
(2) Voice your complaint(s) to the factory (3)
Write it off as a bad deal.
If you want to be unique, and are
up to the challenge, go for an
Autogyro!!!
These questions are being answered by Jim Baxter... with assistance
from other experienced and interested gyromodelers.
Technical
Menu
rev...07-07-03..jb
