This is Forum Notes.... Page Two
From Ralph Kalb: Re: the hang angle.....
From what I have been able to "glean" from the "experts (no two of which
agree!), the hang angle should be at or near the angle that represents the best L/D of the rotor.
Using Alan Nagel's "up the shaft" experiments of a while ago, a rotor "on
a stick" will fly at an angle that is the best L/D ratio if the contraption has a counterbalance to offset the weight of the rotor, and the non-rotor drag is low.
Since both the lift and drag vary as the wind velocity squared, the L/D ANGLE measured shouldn't vary much with
windspeed.
What we are dealing with in gyros are portions of the performance curve. At takeoff,
we would like to get airborne at a low airspeed and climbout quickly,
so the rotor is operated at a high angle to the incoming wind, the angle being the one that gives the maximum vertical lift. However, depending on the maximum thrust available we may have to compromise; it
might be that with marginal thrust, sufficient lift will not be generated to get "up",
and we spend the day taxiing around with the rotor cocked way back!
By tilting the rotor at a lesser angle, we would not be at a point of maximum vertical lift,
BUT with the lesser rotor drag, thrust could accelerate the craft faster,
and since lift varies as the SQUARE of the velocity (ie doubling the airspeed quadruples the lift!),
and eventually we will go "up" (but we will be flying at the minimum forward airspeed!).
As we continue to reduce the tiltback angle, we go higher and higher on
the L/D curve, and you will climb like a homesick angel with full power,
or be able to throttle back to maintain a constant altitude.
Contining to reduce the tiltback angle, one soon reaches the peak of the
curve (best L/D ratio) and starts descending. At full power, this means the
rate of climb decreases, or if level flight is desired more power must be
added, until level flight is maintained with full-bore engine performance!
At this point, any further decrease in rotor angle will cause descent,
since the lift is less than the weight.
From: Steve Tillson:
Fellow Gyro-Heads, Did some testing last week on a premise expounded by our "French Connection"
(Georges and Alain) and our very own "Ralph" from Florida. Briefly, theconcept is simple:
The less negative pitch, the more lift generated--with limits, o f course.
We all pretty much know this, but there is little actual flight data.
Over the years, I've studied rotor behavior using a common tripod out in the
desert in the blowing wind. Using lots of different airfoils, blade weights,
balances, chords and lengths, I've got a fuzzy idea of what works and what doesn't.
I have tached rotor rpm's and wind speeds, but it's all been fairly haphazard and my record keeping is a myth.
One thing that I've learned for sure from the 3 guys mentioned above is that
the least amount of negative pitch DOES produce the fastest rotor and the most
lift--if you can ever get the darned rotors to start! Normally what most of
us do is find a pitch setting that "feels" right (-2 to -5) and allow the
rotors to self-start with a modest hand flip. nbsp; I've even experimented with
prerotation devices to allow a flatter pitch setting and hopefully get the
rotors going--with only minor success. (The prerotation clutch on the Robbe
WHOPPER is a marvelous device and works the best of all--but is so complicated
for our smaller, more modest gyros.) With 2 to 4 degrees negative in most of
our little gyros, the result is a self-starting rotor(s) that spins up to
create enough lift to fly the gyro, but requires a more than optimal forward
airspeed to gain and maintain altitude. Our gyros perform too much like a
"starch wing" model for my tastes.
So, last week, I started with a new set of my usual SIG airfoil-shaped balsa
blades on the "Al's" co-ax gyro, but with no negative pitch in the hubs. At
"0" blade pitch the rotors would not self start; in fact would begin slowly
turning backwards in a moderate breeze. Would not start even with a hand flip.
Using a suggestion learned from our French buddies, each blade was "advanced"
by hand approx. 45 degrees (preset "lead" as in "lead/lag") prior to the
takeoff roll. One must have the single blade mounting bolt tightened faily
snugly in order to accomplish this. Anyway, it works! I gave each rotor a
little hand flip and began a slow takeoff roll. In almost no time at all, the
rotors (with flat pitch) began accelerating quickly into autorotation. The
centrifical (sp) forces allow the "cocked" blade angle to soon straighten out
and off she went. Enormous difference in the effective lift--can't say for
sure if there was more rotor rpm--my guess is that there was.
In conclusion, this "aerodynamic" finesse to start rotors that will not start
by hand works well and will allow us to use flatter pitch settings and get
more lift. CAUTION: As seems to always be the case in experimentation with
these little devils, you change one stupid thing and something else changes
that becomes a new puzzle, and on and on. My gyro had a significant pitch
attitude/trim change as a result of the new blade pitch settings. It required
an enormous increase in the amount of "up" elevator to break ground (just the
opposite of what was expected), and then an application of full down to stop
the nose up as she finally lifted. It was a mess for awhile. Normally on
this "tried and true" gyro (75+ flights), slight configuration changes only
produce small results. Not so here. Anybody got any ideas as to why? (I've
been toying with the notion that if the center of lift is aft of the center of
rotation, this may have kept the nose down on takeoff--pure speculation on my
part.)
Hope someone else will try this technique for starting rotors with flat pitch
settings and report their impressions. Steve
From Ralph Kalb:
Re "Behind the power curve" 2/9/97
Yes, although I think one of the later curves may be of more help in the
understanding of what is going on.
In level flight, as the airspeed increases from a low value, less and less
power from the engine is required from the engine to maintain a constant altitude. At some moderate airspeed, the power required curve flattens out
due to the ever increasing parasitic drag on the gyro (or plane!), and the
power required curve starts rising.
As you can imagine, the minimum of this "U" shaped curve represents the
airspeed at which minimum power (read: fuel) is required, and therefore is
the maximum endurance airspeed you would chose to fly at if you had to
stay aloft as long as possible with the fuel aboard! Lower airspeed
operation (usually referred to as being on the "backside of the power
curve") results in more and more fuel being burned. This kind of operation
is referred to as "slow flight" and is a perfectly valid maneuver, however
there is a problem if carried too far!
If the wing/rotor is tilted too far back, eventually a point is reached
where the power required by the machine will be greater than the power the
engine can provide at that airspeed, resulting in an inability to maintain
altitude with the throttle wide open!
Recovery from "normal" slow flight is accomplished by dropping the nose,
picking up airspeed (remember, we are sliding down the curve now) and
reducing the power to cruise. Doing this when the airspeed is to the left
of the crossover point (HP available/HP required) worsens the problem, and
the descent rate increases! If we have the time (and altitude) to let the
airspeed build back up, no sweat (well, a little!). If time (and altitude)
runs out, in the words of Steve, "Splat", a belly-flop onto the tarmac!
This is the reason I have concerns when I see a gyro flying nose waaaaay
high, engine wide open, and low altitude. No more options left!
The only possible recovery is to slowwwwly try to lower the nose (yawing
the flight path into the prevailing wind- rudder only!) and hoping for a
gust of wind to help boost the airspeed!. An option on R/C is to keep the
throttle trim at a minimum... that way, you have some chance of getting a
bit of increased power if/when you need it to "escape". Ralph
From Jim Baxter: 2/9/97
Hi Alan,
This is always a problem... that`s why I started & endorsed the use of
"Ernst thrust plates" (thin shims) to set blade incidence several years
ago. Bending the blade holding tangs -accurately- is very difficult, and
also leaves you not really not knowing if they are set correctly.
Any thin, accurate, shim would work, but the Ernst thrust plates are
simple & easy to work with, and I secure them in in place with just a bit
of clear silicone, to keep them from shifting. They are sandwiched
directly between the blade and the holding tang. It is easy to add &
subtract blade incidence by simply removing or adding additional shims.
My opinion: If one single blade is off by a degree or more, it could
really hinder the efficiency of the entire rotor. Just how much I don`t
know, but one time I accidentally left the incidence out of one blade, and
could -not- get the rotor to function. Jim
Hey Ralph-- Can you tell us what you really think about a one-bladed rotor?
How would it perform? What changes would need to be made in the chord and blade length
(if any?) Bill Friedlander/WI
Bill:.... re the "one blader"....
Did you get my "Gyro Seed" at Christmas? Besides being "cute" and "really
neat" they demonstrate that two or more blades aren't really needed, and basically a single blade is more
efficient. Cone angle goes up but thats about all.
One of the biggest problems with a 1 blader is the use of a counterweight
to balance out the working blade since it is "dead weight". If the weight
is only 1/2 as far out as the blades CG, it would have to weigh twice as much to balance.
What a loser!!!
However, if instead of a "dead weight" one were to make that weight "go
to work" by being the battery pack for an electic ducted fan for instance, the game changes,
since we would have to take the batteries up anyhow and we can leave the "dead weight" on the ground!
Same thing goes for R/C equipment or an IC engine + fuel (although burning fuel would
cause the center of rotation of the "thing" to shift only slightly!
One design difficulty is keeping the pitch of that one blade where it is supposed to be,
and a simple reflex bevel on the airfoil takes care of that.
Control? A set of co-axial tubes linked to servos on the top and offset
weights on the bottom, along with a basic fuseledge with rudder & landing
gear would when moved up and down shift the CG for pitch and roll
control, like a hang glider! The rudder would only give yaw control to
keep it aimed in the same direction while the rotor spun! Possible!
Chord and Span?? Hobson's choice! I have a 3" x 36" SIG balsa Clark Y
airfoil just waiting for me to run it up using a small electric duct fan
with a thrust controller. One of these days...... Ralph
Basically, increasing the solidity of a rotor decreases that rotors RPM
(and therefore increases its cone angle and roll/pitch stability). Ralph
From Mickey Nowell: 5/96
Re: Aerodynamic Incidence!
Not because I'm picking on you Bill ( got your package by the way, thanks) but
just to try to add some standard nomenclature to this forum from the standard
conventions of the full scale engineering world I want to try to explain/define
some standard terms.
1) Chord : the line drawn from the aft most trailing edge point to the forward
most leading edge point. This is chosen because it easy to define geometrically
so that engineering data can be readily transferred and results duplicated.
2) Angle of attack(s). Measurement of the angle between a reference line and
the relative wind.
3) Relative wind : the velocity and direction of the air flowing toward the
airfoil/wing/etc that is not influenced by the airfoil etc. We call that
V(infinity) in aero terms. I would like to be able to write a capital V with a
sideways "8" for infinity because that is standard but I'll abbreviate here
with Vi. For a whole aircraft it is the opposite from the direction the
aircraft is flying in ( not where the propellor points but the direction that
the center of gravity is moving).
4) Geometric angle of attack : angle between the chord line and the relative
wind (Vi)
5) Zero lift line : line drawn through the airfoil, through the trailing edge
and parallel to the relative wind, when the airfoil is making zero lift. Note
that the ZLL is only the chord line for fully symmetrical airfoils. In general
the ZLL and the chord line are not the same. For example the zero lift line is
about 2 degrees above the chord line on the clark y, which is in turn about 2
degrees above the "flat" bottom portion.
6) Absolute or true angle of attack : angle between the ZLL and the Relative
wind.
7) Incidence angle : angle between the chord line and some arbitrary reference
line. Note that in aerodynamics the incidence angle is basically meaningless in
determining what the airfoil is going to do. For example if you want to call
the reference line on the gyro the line perpindicular to the mast, then the
incidence is the angle between the chord line and this line. Suppose that you
have an aircraft with 10 degree backwards mast tilt, clark y blades rigged at
+2 degrees (geometric) with respect to the shaft. The rotor speed is 500 and
the gyro speed is 30 feet per second. Gyro is flying 3 degrees nose up. Blades
are 2 foot radius. The real angle of attack of any section of that blade is the
result of all these things and whether or not you are on the advancing or
retreating side. An approximation of the real angle of attack on the tip
section on the advancing side is obtained by first obtaining the blade speed (
500 / 60 = rev/sec * 2 * pi = rad /sec * 2 feet = tip speed )
So tip speed is about 100 feet / sec. At 3 degrees nose up ( meaning that the
line used to determine the 10 degree tiltback angle is flying along inclined up
at 3 degrees) and + 10 degree mast angle you get +13 degrees with respect to
the 0 line perpindicular to the shaft. THe blade is at geometric +2, true +4 (
for the clark y ) So the airfoil is seeing 30 feet per second at +17 degrees
and 100 feet per second (tip speed) at +4 degrees due the rotation. If you draw
a little force triangle you see that the airflow is - 6 degrees with respect to
the line drawn perpidicular to the main shaft. This -6 is what keeps the blade
moving forward. The -6 combined with the fact that the blade is rigged to +4
incidence with respect to the zero lift line, yields a true blade angle of
attack of +10 degrees. If you recompute for 1 foot radius or halfway out on
the blade, tip speed is 50 feet per second everything else is the same thus
airfoil sees 30 fps at +17 and 50 fps at +4 go through the force triangle again
and you get +14 degrees ( the -6 turns to -10 and the +4 true is still the
same) true angle of attack. From this you might see that the blade at half span
is probably stalled at 500 rpm, you probably need to lower the angle of attack
, increase the rotor speed or increase the aircraft speed. Any way this has
been a digression off into a tangent. The original point is that incidence
angle is an arbitrary rigging angle between the chord line and some reference.
For rotor craft the reference is usually chosen to be the line perpindicular to
the main shaft. The angle of ATTACK is something else altogether and while the
incidence angle contributes to the angle of attack so do many other factors,
and the real angle of attack takes some work just to get a good estimate. Note
also that the angle of INCIDENCE of a non twisted blade is the same down the
full span of the blade but the angle of ATTACK of any particular section sees
is continously variable based on span, where the blade is on the circle and
what speed the craft is flying at. You generally cannot talk about THE angle of
attack unless you are talking about one little section at one azimuth position
at one aircraft velocity, etc.
Hope this monologue provides some information........ Mickey
From Emilio Cabezas:
Ref: Rotor tilting at neutral........ 3 May 96
Good information, Steve!! Your test flights with CCw blades are a confirmation that on flapping rotors its is necessary to tilt slightly the rotor towards the retreating side for
level flight. As you know, the rotor on my direct control gyro is tilted 2o or 3o
left (retreating side).
Full size gyros of the thirties also had the rotor tilted 3o towards retreating....
At the Cierva and the Autogiro exhibition last March in Madrid there
were several pages of the "Engineering Theory of the Autogiro" exposed
on the wall... one of them said that the center of lift of a rotor was slightly
shiftes forward and towards the retreating side (there was no explanation
as to why on that page) and a drawing clearly showed the lateral tilt needed...
The text also said that the more the hinge offset the more lateral shift of the
center of lift... This may be the reason why you need the 5o lateral tilt; I
remember your C.4 has a high value of hinge offset, which on the other hand
may be good for lateral stability... Emilio
From Ralph Kalb:
Ref: Hinge Location...... 19 May 96
Emilio: It has been mentioned that you advocate "keeping the Cierva hinges
close in" (and I am assuming you mean close to the rotor axis by this). Correct
me if I am wrong, but with Cierva hinged blades acting as a gyroscope, the
fuseledge if displaced (and being fixed firmly to the rotor hub) the hub should
tilt also if the body is displaced in pitch or roll (or both!).
Being a gyroscope, the blades would stay in the same PLANE of rotation, but would be
displaced vertically from one another. The centrifugal force and the moment arm
would form a couple to help the normal pendulum response, resulting in a more
stable gyro than by the use of pendulum action only (ie the teetering rotor).
Regarding the reports of excess lift by the retreating blade (Cierva blades),
is it possible that with a large "pre-bent-in" cone angle, the difference in
disc area on the advancing side actually results in less VERTICAL lift on that
half of the disc as compared to the lift of the almost flat retreating disc
side?
Even with low values of cone angle, the Cierva hinged blades would
produce a slight unbalanced force toward the retreating blade. That side force,
coupled with a rotor "blowback" caused by Coriolis forces of the free rotors
being transformed into a precession force could be the explanation for Cierva's
comments regarding the "shifting" of the center of lift on a forward moving
rotor system. Comments? Ralph
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