On-line Forum conversation, back when all the current GyroNut effort began...

For several years, a few gyromodelers have been hashing out the oddities of model Autogyros on computer on-line forums.    Many of the most informative comments have been assembled here and are being posted here for the benefit of all gyromodelers.    The majority of these particular comments on pages one and two are from "PRODIGY", vintage 1993-1995.    With the comments on page three & four we are basically on "COMPUSERVE" , 1996 and into 1997.




  • Page four: Why no Fly-bars?    Centrifugal force, Roll Phenomenon, Windmilling versus Autorotation.


DISK LOADING-Rotor Solidity

All:  Either I'm slipping or *P is! Could have sworn there was a note regarding Disc Loading for 1 rotor, 2 rotors side by side, and 2 rotors Coax/Syncro , but whoosh... not there anymore.

Anyhow, 1 rotor=Weight/area of rotor.
Side by side= Total Weight/Total Area.
Coax= ?

It would SEEM that the coax DL = should be the same as 1 rotor with twice the solidity of one rotor. Does a coax "settle" the same as a single rotor?   Do two parachutes in tandem lower a load slower than 1 'chute? Huh ?    (Ralph Kalb)




To calculate disk loading:
Weight
-------------- = Disk Load
Rotor Area

Step one:
radius\2   x  3.14
-----------------    =   area in sq/ft
   144

Step two:
weight (in ounces)
-----------------   =   Disk Load (oz. sq/ft)
area (in square feet)



In reply to some questions , perhaps this is a means of spreading some information to the whole gang.....

I was asked about disc loading (Weight/Disc Area) and what I thought was the "right" value.  One thing I was hoping to do on this BB was to determine various machines weights and rotor diameters to get a ball park value to answer such a question!   Although it may seem a cop-out, go for the lowest disc loading you can get away with, ie lowest weight and largest rotor you can safely swing!   Decreasing the disc loading is probably one of the most important goals in designing a gyro, since as the disc loading decreases, the minimum AS decreases the maximum AS increases, and the power available for climb (PAFC) really increases, all of which improve performance.   If the disc loading is too high, the gyro won't even get off the ground, although it will be a high speed taxi vehicle!    (Ralph Kalb)

Edited note (1999): See this new chart for newly calculated and charted information..(1998-99).

3 blades vs 2?   Well, all other things being equal, the 3+ blader carries less of a load (ie total weight/# blades), so the coneing angle is smaller, which may affect the stability (sort of like dihederal in a fixed winger). In addition, the three (or more) blade rotor has a higher solidity (total blade area to disc area, and an increase in solidity decreases minimum AS, increases max AS and increases PAFC, so thats a benefit.   However, the extra blade(s) cause a reduction in RPM.   If we are talking about vertical descent (ala Maple Seed), who cares?   However, with forward airspeed, the time for the blade to go from the rudder to the nose is important as to the total amount of flapping we can handle mechanically, and the Tip Speed Ratio (TSR= AS/Tip Speed), since that is a figure of merit that indicates how badly the blade is stalled!   Again, the lower the TSR, the better!    (Ralph Kalb?)


Chord/Diameter ratio: Again, the problems of solidity are addressed as we increase the chord with a constant diameter, since we are increasing the blade area.   Additionally, a large aspect ratio (radius/chord) has a lower drag than a small aspect ratio, so again long, narrow blades seem the best bet!   (Ralph Kalb)




BENSEN-TYPE Gyros. For those out there in gyro land that are contemplating building and attempting to fly a "Bensen"-type gyro, I received a great letter today from Georges Chaulet in France, and the word is not encouraging.
Georges has numerous years of model gyro experience dating way back to free flight days progressing into R/C. He has published many articles, plans, etc. on his designs, research, trials and tribulations, etc.   
Anyway, he states, "The Bensen-type does not work on models.   The reason is that full-size Bensens' have proportionally much less power than models 4 times less).   This means that the prop torque on a model is 4 times higher, and it produces an immediate bank.   On models, rotor and fuselage need some form of liaison, just like the marriage between the wing and fuselage on the airplane..." .
Don't anybody "shoot the messenger".   Just relaying his experience and opinion.   I personally have had much trouble with my scale, 1/4-sized Bensen, but for different reasons.    This may account, however, for several reports lately from at least 3 builders of Bensen-types.   They've all reported to me, that on takeoff, they are experiencing an immediate bank ("hard-over"), all to the ADVANCING side!   This has perplexed me.    I've asked for each to report back on direction of rotor rotation.   (I set my single-rotor gyros to rotate in the opposite direction as the prop.)
Georges also described his unique method of lateral control used on his single-rotor, wingless gyros.   (Details too "wordy" to mention now).   He's been flying "direct control" gyros for years with much success.     He also describes the simplest, "no-moving-parts", head used also on his single-rotor, wingless gyros. By using 7/32" dural aluminum blade tangs, he gets enough flex so each individual blade flaps slightly, compensating for lift dissymmetry.   A simple, triangular piece of this dural becomes the foundation for the 3-bladed head. Each blade straps onto a corner of the triangle with one mounting bolt--so simple!
To further widen my eyes, he showed how he angles each blade about 30 degrees forward into the direction of rotation prior to takeoff.   This becomes a kind of simple "Alpha-1" hinging, promoting quick self-starting and rapid rotor acceleration.   As the rotor accelerates, centrifugal (sp?) force increases, allowing each blade to "lag" back to it's straight-out flight position.   Cool!

(Ralph, you've also suggested this and, evidentally, it is quite effective.)    (Steve Tillson)
Editorial Note (1999): Georges has changed his mind.... and he has now sucessfully built and flown a Bensen Pusher style model, he calls his "SPIRAL"




MODEL PLANS:
Bob Holman Plans, Box 741, San Berdo, CA. 92402 Bob has plans for a few gyros, such as "Al`s gyro" (similar to the Gyro Falcon, and the "Gyrace" (similar to the FA-61)

MODEL BUILDING & BALANCING:
When you get far enuf along... before covering.. (engine w/muffler bolted on, gear w/wheels installed, the .049 rotor cases bolted in place [blades not necessary at this time], and slip the tank in, etc.) .......
(1) Tape (masking) the batt, servos, & rx along the outside of the fuse & tape a tail wheel on the fanny.
(2) Check to see where the CG is.
(3) Remember... covering this thing WILL shift the CG aft perhaps as much as 1/2 inch.
(4) shifting ONLY the batt... see if it brings the CG in place.
(5) If that doesn`t work, shift the servos and/or rx if you can.... Check the CG.
(6) If it is still too far forward, increase the size or material used to brace the stab from the fuse... "you did brace that stab... didn`t you!"
(7) Best tailwheel.... (and you do need a steerable tailwheel!)... is one of those Goldberg/klett small sized ones... Unless you are the greatest flyer in the world, a tail skid is just not going to work if you want to launch from the ground.
(8) If all this won`t work, throw up your hands and grab for the nearest brew.... and change hobbies!
NOTE: I don`t usually install the blades for balancing since I consider their weight distributed equally in all directions.    (Jim Baxter)


BLADE TESTING & THE SIG BALSA SHEETS:
For those following the use of SIG's pre-shaped, airfoil balsa sheets (in several sizes) used for our gyro blades, I have a little testing info to report:
The newest configuration I tested yesterday was a 36"x3" sheet covered in heat shrink and used stock as one rotor blade.   With the 6-inch teeter bar/blade grip assembly, the tested rotor disk was approx. 6.5'.
Trying various angles of incidence of the blades and varying angles of attack on the tripod, one important incident recurred as rotor rpm approached 500:  These long and very flexible blades entered blade flap--or "flutter"--and immediately lost several hundred rpm.
It appeared from observing from the side of the rotor disc, that the leading edge "tucked".  In attempting to configure for autorotation (using less negative incidence settings), the rotors bowed conspicuously, but tracked well. Was never able to coax rotors into autorotation.
They windmilled fine at less than 500 rpm and between 300-400 rpm some of the "bowing" of the blades straightened.
Having ordered a half-dozen sheets, I will try some carbon fiber on the next set of long blades attempting to restrain the flexing and subsequent "flutter".    However, on these present blades, my short-term experimenting led me to return to the workshop and merely slice 5.5" off each tip, using a diagonal cut as I've seen on the low-aspect ratio Whopper blades.    This shortens the leading edge of each blade by 6" and the trailing edge by 5".   More testing today on these "clipped" versions.
Incidentally, the 18"x2.5" blades made of the same SIG stock, currently flying on the WEE WILLIE continue to be my most effective blades yet. Yesterday, while demonstrating gyro flight for two visiting gyro enthusiasts, near-hovering flight was achieved at 1/4 throttle into a 5 knot wind at -3 degrees blade pitch.    (Steve Tillson)



BLADE SHAPE (AIRFOIL):
Yes, that reflex bevel on the underside of the blade will help out in eliminating a nose down pitching moment generated in the Clark Y, causing blades with flexability in torsion to twist.   Trim tab will do the same thing, as will a small "up" stabilizer dragging along on a boom back of the blade.   However, except for experience with light blades by Sig  (balanced waaaaay back from the 1/4 chord point)  "tucking under", I haven't seen any other indications of a problem.  Of course, with that 3.25' radius at 400 RPM does get a rather high tip speed!    (Ralph Kalb)



I was reading your note on the "Big Foche" and how it didn't perform up to expectations, so thought I'd tag a reply on your last message for all.   When you were doing your helicopter "thing", I'm sure you practiced what was called "autorotation"  (but was really windmilling)-   Full down collective to keep the fan going and nose down to keep the airspeed up to give you some translational lift. I say windmilling, because not only did you have to keep the fan going, but also furnish power through the transmission for the tail rotor, all this takes power.
The blades on a chopper typically are symmertical, NACA 0012 or the like, a very low drag, high speed airfoil. Most important is that it did not have a "diving moment" where it tended to pitch down depending on the airspeed and angle of attack. Very docile.   A Clark Y airfoil would never do for a helicopter (except maybe a model), because if this nose-diving characteristic.   Early Bensen blades had a "reflex bevel", about 1/8" x 1" all the length of the underside of the blade, which neutralized this diving tendency.
The NACA 8-H-12 blade (ideal for gyros) is very similar but has the reflex bevel smooooooothed out into a slight upsweepfor the same purpose, as does the Gottingen 606.    Now: If, by scaling up the Gyrace, the blades have become more torsionally flexible, and if you don't have some sort of reflex bevel, the blades will twist nose down very slightly ......... ... at the best L/D ratio for lift!).   If so, the blades can be humming along and you lose the major portion of your lift, which is what seems to be happening.   The Bensen blades also used a trim tab at about the 70% point, advertised to help tracking, but I never saw any trying to point the blade down, only UP!!!   Of course, the faster the blades went, the more UP, and the more drag, which slowed them down, and so they were essentially constant speed blades.
I don't know how you can check the torsional rigidity of the Gyrace vs the Foche, but it would certainly help-- as would a reflex bevel! Luck!    (Ralph Kalb)


More on the source of gears for a 'differential'.   You could use the differential from a model race car.    I picked up a set from my local hobby dealer from a wrecked model.   However, it seems tough to mount the design they use.   I have opted to buy bevel gears from Stock Drive Products in New Hyde Park, NY.,  phone: 516-328-3300,   Fax: 516-326-8827.

They have miter and bevel gears in steel, brass or nylon. Sizes range from 3/8 dia to 2".   Cost is reasonable (can't remember what I paid, but recall being pleased with prices.)  They have a great catalog with all sorts of goodies.    (Bill Friedlander)


ROTOR BLADE TIP WEIGHTS:

You put it so doggone delicately!  Of course you are right in learning to fly a stable machine first, then "tilt at the windmills later"-  Sorry, Don Quixote!   The problem he seems to be having seems to be tied into precession, which if the rotor bearing is REALLY free shouldn't be a problem, as all forces are transmitted through a single point, the hub.   The roll phenomena can also occur if the center of lift of each blade is not at the same level as the teeter pivot bearing.   I got an E-Mail about the lighter blades, and I suggested he use tip weights to keep the inertia up- if dem blades slow down, dey seldom speed up enough- they just FOLD!    (Ralph Kalb)


ROLL PHENOMENON

This roll phenomonon is akin to flying a fixed wing craft with about 2 feet sawed off one wingtip!  Won`t fly hands off, just do endless rolls.  With correct aileron displacement, it can be leveled off, however each attitude or airspeed change means a new correction is needed.  Tilting the mast is an easy fix.   The Wallis has a retreating blade horizontal stab that is 30% bigger than the other side.   Trim tabs have been used.  All KINDA fixes, but they are only good at the speed they are set for.    (Ralph Kalb)




Good Morning...
In regard to my mentioning the blade rotation speed during T/Os... It is nothing really strange, it is mainly due to the changes in the angle of attack of the model during the initial roll.    I think you can understand this when you picture the positive angle of the shaft, the negative angle of the blades, and then throw in your normal control of pitch during the roll.. think about it...
In many of these models, the designers put tri-gear on them because (I think) they normally find a model (airplane) easier to T/O with tri-gear.   However this doesn`t work well with gyros... and one of the problems is getting enuf positive pitch on the angle to really get the blades turning... with a tail dragger you can hold the tail down for a while & get them really turning & then allow the angle to flatten out a bit to prevent to violent of a pitch up when she reaches lift point.    You will hear the other fellows, speak of "windmilling versus autorotation"... Windmillers use angles of -2 to -5 roughly... where the autorotaters seem to be using maybe -2 to possibly a +2.    (Jim Baxter)


WINDMILL VERSUS THE AUTOROTATING ROTOR:
The windmill type rotor (s) is perfectly acceptable if:
1. You don't mind having the rotor tilted way back (Bensen) or the nose way up (Pitcairn).
2. You don't mind flying around slowly at full throttle, due to high drag .
3. Windmills produce torque, Autorotation produces speed! .    (Ralph Kalb)


Recalling back to about 1942, I remember an article in MAN or "AirTrails" called "Props are gyroscopes". This article explained why pylon-mounted-wing, free-flight planes such as the Comet Interceptor, Zombie, Zipper, Playboy etc. wanted to turn to the right under power.
The article went on to say that 'a force acting at right angles to the plane of rotation of a gyroscope is manifested 90 degrees in the direction of rotation.'   Thus the drag of the pylon mounted wing would be experienced as a force which wanted to turn the plane to the right.   (If the props rotated the other way, the force would tend to turn the plane to the left.)   Applying this notion to my Whistler, whose rotor turns clockwise when viewed from above, when I apply up elevator, the plane will want to roll left.  Down elevator would cause a right roll.    Attempting to correct the right roll would cause the plane to pitch down which would cause the plane to roll right etc, etc.
John Kallend talks about pilot induced oscillation and he damps this out with stubby wings and plastic film shroud over the rotor pylon to act as kind of an extra vertical fin.   He also says that rotors made with basswood, being heavier, offer more stability than lighter blades. 
Based on the preceeding analysis, I wonder.... It would seem that lighter blades would have less gyroscopic inertia and would produce less pitching-rolling-pitching forces.
This analysis also would explain why autogyros such as the FA-61, Gyrace, etc. don't experience this kind of "twitchyness"--the two countra-rotating rotors cancel the forces!
In any event, I am going to try lighter weight construction rotors--    ( Bill Friedlander)


The other day I was thinking about the 'ideal angle of incidence" for rotor blades.   In the meantime, your drawings for the Variable Pitch Tester arrived.  (Great minds...etc.!)
However I was thinking about rigging up a fly-ball governor to the blades so that as the rotor spins up, the angle of incidence would become more positive.  The thing could be set so that the angle would be -4 or -6 deg to start then as the rotor's rpm increased, the incidence would go less negative.   Eventually, the incidence could actually become positive and the rpm would no longer increase.
I am not enough of a designer to figure out just how to do this, but perhaps your fertile mind might come up with an idea or two.
Also this past week, I was visiting with Prof. Michael Selig at U of Illinois.    He is doing the wind tunnel testing of airfoils at low speeds, which is a continuation of the work published in "SOARTECH 8".
He has been doing some work on wind turbines and when asked about the ideal airfoil or shape of an autogyro's blade, he thought that the blade should be twisted somewhat like a standard aircraft propeller.
After thinking about this, I'm not sure that he is right.    After all, none of the autogyros have have used twisted blades.    He feels that the region near the hub should have a distinct negative incidence, with the blade twisting less and less so that at the tip it would have nearly zero incidence.
In any event, I have located a model maker with an NC machine who has offered to make a female mold of a twisted rotor blade, if we would just give him the coordinates!
We could try the idea by cutting a blade in sections and then glue it back together giving it progressively more negative incidence from the tip to the hub.   (would have to insert some kind of connectors to keep the thing from flying apart.)   But your Variable Pitch Tester would tell if the idea were any good.   
  (Bill Friedlander)


TRACKING.... (Horizontal 'spinning alignment' of individual blades)   I still prefer the use of small trim tabs on the rotor  (about 70% radius)  rather than warping the rotor strap because it seems to give a finer degree of control.   A strip of different colored tape on the each tip helps tell which is higher/lower.   Look from the "side", because the flapping will give you a wrong answer of you view from front or back!  
You mentioned the blades are in balance, but are they also in dynamic balance?   Cheap check-- hang the individual blades from a nail or wire, and start them swinging at the same time.   They should stay swinging "in sync" for at least 50 swings, if not, the slower one has it's CG too far out.    Lemme know if this is the problem.
When you think the blades are OK, mount them on a motor/electric drill in a no wind situation (garage) and start 'em. If you got vibration then, you still have a static/dynamic balance problem.   If they run nice with no wind, but shake with wind, the flapping up blade is causing that blades CG to shift toward the hub, and throwing the rotor CG off.   More tip weight helps, but a "N"/rev is a common problem (where "N" is the number of blades on the rotor).
You are getting some good speeds there.  Hope you don't lose too much when the rotor is over the big wing!!   Lemme know!    (Ralph Kalb)


I think the "professional" blades are really the way to go, since they are usually machine carved, and more closley matched.   Re your other questions of 5/25 @ 11:10 AM, the static balance (both blades weigh the same) and the dynamic balance (the CG is the same distance from the hub) can be achieved by balancing the entire rotor on a needle point with the cone "point" up, but it isn't as sensitive as swinging the blades like a pendulum.    What you really need is a weight that can be screwed in and out on both blades (so they weigh the same) and the in/out motion is used to get the CG at the same place for all blades WITHOUT changing the static balance.    (Ralph Kalb)


MEASURING BLADE ANGLE:
A friend loaned me a Schluter Universal Einstellwinkel Lehre No. 1366. This gadget is for helicopter blades and can easily measure 0.5 deg.    (Bill Friedlander)


DELTA THREE ROTOR BLADE MOUNTING: ala, J. Kallends "Whistler":
Seriously, a "Delta 3" hinge is where, instead of hinging a blade 90 degrees to its length like a teeter or a Pitcairn, the hinge line is actually pivoted at an angle, so when one blade tilts up, its pitch is reduced as it tries to tilt up and forward. With seperate blades, this throws the CG of the rotor off, so it is used mostly on 2 bladed teetering heli tail rotors to compensate for the unsymmetrical lift of the advancing and retreating blades due to forward airspeed (don't want that rotor tilting around back there out of sight!). With a tail rotor, the rotor is soooo small and the heli soooo large, that a tad of off center lift as the heli maneuvers around doesn't hurt--- on a model, Delta 3 hinging gets very serious real fast, unless you plan on VERY gentle pitch and roll movements.   NOTE: Delta 3 is not recommended for models!    (Ralph Kalb)

Edit note (1999): Delta hubs with angles of 20 degrees have been built and tested with excellent results on small gyros of less than 36" disks.. The slightly negative incidence produced with up-flap aids in quicker pre-acceleration. (JB)



CONING [blade dihedral] has one purpose in life, to minimize the bending forces on the rotor hub. When operating, the blade could be a wet noodle and would still stick out there. I agree that there is an element of stability to be gained from SOME coning. As far as the aspect ratio goes, the higher the better for efficiency. However, increased solidity (total BLADE area to DISC area results in lower RPM, easier starting, lower TO speed, so it's good within reason (Not ping pong shaped blades, however!). A flat bottom Clark Y airfoil has a "zero lift" angle of attack of about -1.5 degrees, so if mounted on the flat surface, it is operating at an angle of attack of 1.5 degrees. The L/D ratio of the rotor governs the angle at which the rotor will fly, and the rotational axis has to pass through the fuse CG. When flying watch the rotor, not the fuse. One of the interesting maneuvers of a gyro is "sliding" left or right, sort of like "slipping" a fixed winger, except the sideways motion tends to roll the fuse (due to the sideways push of air on it), and without the wing surface of a fixed winger, rolls rather rapidly, hence Kallands "roll damping" wing/stab needs. Dunno if this helps, but keep asking and I'll try to answer!    (Ralph Kalb)


Most of the above notes were saved saved from the Prodigy service of 1993 to 1995, with just a few exceptions. It was on Prodigy where the "Gyroheads" first became acquainted and began the quest for the better model autogyro. These notes have been assembled and posted by Jim Baxter.





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