Stick Shake post from old forum

[GyroChuck]

Rick had asked in another thread about a Seminar on stick shake. So i decided to post a series of posts on stick shake that I saved fro the old forum.

Hopefully someone will find this useful

 

[GyroChuck]

(Stick shake Greg G.doc)

Rotor head normally 9 degrees tilted back from keel

Keel angle normally 9-12 degrees nose down, 11 degrees preferred.

170lb. Pilot use 9 degrees down, for a heavy 220lb. Pilot use 12 degrees down


Author: Greg Gremminger ([email protected])

Subject: Rotor Shake components

You are perceptive to ask this question. Stick shake pattern is a very interesting subject, although a bit complicated when you get into it. It can be a good tool to help minimize rotor shake! I'll try to explain some of the things stick shake can indicate. But first, you need to appreciate that if there is ANY play in your stick controls, the stick shake may not accurately represent what the ROTOR SHAKE is doing.

For a better representation of ROTOR SHAKE, I have mounted a small laser pen rigidly to the head (on the cross bar or something rigid to the head - using shaped nylon block clamps), and pointed the dot to land on the dash or my knee or somewhere where I can observe it in flight. With the laser pen, there is no slop that might otherwise be in the stick itself! If the stick/controls are very tight (little friction as well), the stick shake can be used like the laser dot can. If the head roll and pitch pivots have a lot of friction, the rotor shake will be transmitted into the airframe, and neither stick shake or laser dot patterns will accurately indicate the sources of the shake.

discussion:

1/per rev circle or oval pattern indicates a balance problem - some combination of span and/or chord balance, and/or a tracking problem:

- Span balance: blade-to-blade dynamic balance - not necessarily the same as static balance if the blades are not mass and geometrically identical.

- Chord balance: Adjustable by blade "string" or by chord balance adjustment screw (on some hub designs). This is not a simple matter of "stringing" the blades! Stringing is a GEOMETRIC balancing. Even if geometrically "strung" most blades will "search" for their MASS center by moving in the vertical blade attachment bolts - essentially "re-stringing" themselves for their mass center! Whether balance chord-wise geometrically or by mass, the aerodynamic center may still not be over the rotor axis. There is no substitute for quality consistent blades! A quality blade has consistent geometry, mass distribution and aerodynamic accuracy. Some blades that "just will not balance" are probably blades where the geometric, mass and aerodynamic centers are not in the same place!

- Tracking: Tracking is not solely the tips visually tracking the same point. If the aerodynamic properties of the two blades are not identical, minimal tracking "bounce" may not occur when the blades track visibly. On the laser dot, tracking appears as a 1 per rev oval or circle which is probably not in phase with the other balance 1-per rev circle or oval. The combination of balance and tracking 1 per rev shake will likely cause oval or even linear shake patterns with the long axis in any direction. This may be a source of side-to-side shake, or shake in any direction.

- Precession effects in the spinning rotor, can complicate analysis considering the direction of shake. If the rotor is essentially flexing the roll and pitch pivots, precession complicates the analysis, because actual movement of the head can lag the actual force applied! So, it can be very difficult to think through all this! 2/per rev shake can come from several components:

- Improper teeter height (for the rotor load). This produces a 2/per rev essentially fore-aft shake. Teeter height criticality can be reduced by designing the blades for lower coning angles (higher weight blades, stiffer blades, and higher RPMs) A shallower coning angle, makes changes in coning angle allow a smaller vertical offset of the rotor CG from the teeter bolt on changing "g" loads.

- Cyclic drag changes on each blade (in the side-to-side position of the rotor) as the gyro moves forward through the air. This produces a 2 per rev essentially fore-aft shake. Not readily reducible except by good efficient blade design. You cannot adjust this away, but, there are schemes such as the Dominator "slider" or the RAF flexible mast to reduce the amount of fore-aft shake that transmits to the airframe. Worse problem on long blades and heavy ships.

- Conservation of momentum of the rigid 2-blade rotor: This produces a 2 per rev essentially fore-aft shake. This comes from one blade teetering up while the other teeters down, essentially like a twirling skater drawing their CG toward the axis of rotation. If a skater could cyclically extend one arm and draw in the other around the spinning circle, they would shake at a 2 per rev rate! This is not reducible, but the slider or flex mast helps minimize transmission to the airframe. Worse problem on long blades and heavy ships.

- Teeter friction. This also produces a 2 per rev shake essentially fore-aft. This can be reduced by keeping the teeter pivot lubricated or otherwise minimal friction.

- Slop (side-to-side, along teeter bolt) in the teeter pivot. Minimize this slop without adding friction to the teeter. Original Bensen guidelines say .010 slop is OK, but, this movement can cause some very intense cyclic shocks and "hard knocks" when the rotor hits the slop stop. It does this twice per rev! On the laser pattern, very sharp and hard hits are usually apparent as a "knot" or sharp turn in an otherwise smooth pattern - the stick can feel like it's hitting hard, maybe even without a lot of movement.

- Teeter tower sway. This is almost the same as "slop" above, but it is a bit less jarring. This will be more of a factor on tall teeter towers. Some people brace the towers with cross bars. Magni type rot hubs and teeter "blocks" eliminate this sway.

So, the laser dot shake pattern may be very complicated, not always intuitive - a combination of 1 and 2 per rev shakes in various directions. If you can identify only 1 per rev or 2 per rev shakes, you are lucky, the problem is easier to identify. If the shake pattern is cleanly circular, oval or linear (in any axis direction), the problem is probably 1 per rev combinations. Start adjusting track, chord and span balance and watch the results for improvements. Experienced rotor people can sometimes differentiate between 1 or 2 per rev shakes - but this is not very easy!

If the shake pattern has extra little loops or tails, the pattern is contaminated with 2 per rev shakes. First, make sure the teeter friction is minimal - clean and lubricate the friction bearings if possible. Also, minimize the teeter slop. Coimbinations of 1 and 2 per rev shake can cause very sharp or irregular shake patterns that feel like hard "knocks". A "hard knock" in the stick can also come from teeter pivot slop (above).

In my experience, the major source of 2 per rev shake is improper teeter height - especially on very flexible aluminum blades and heavy machines. Some combinations require as much as 6 inches of teeter height. CAUTION: extreme teeter heights require double bearing rotor heads to handle the overrunning loads imposed on such a long moment arm. Also, tall teeter heights increase the force or "feel" of the cyclic stick - this is sometimes a good thing though, to give the pilot some heavier stick feedback.

For both 1 per rev and 2 per rev shake, there is no substitute for quality blades. Besides the geometric, mass and aerodynamic center consistency, many of the 2 per rev shakes can be minimized with quality blades. Efficient blades minimize shake due to cyclic drag changes. Low coning blades, minimize teeter height sensitivity. Blades that can "hold" tracking and "string" adjustments may maintain minimized shake better because they don't dynamically misalign themselves.

One more point: only 1 per rev shakes may be analyzed effectively by accelerometers and polar plots. 2 per rev shake is not analyzable by polar plots, and any amount of 2 per rev shake severely confuses the results of trying to do a polar plot! Even if there is no 2 per rev shake, the combination of balance and tracking induced shakes, severely complicates "balance shots" based on polar plots.

I have found that flying with a laser pen dot pattern helps a lot in understanding what is going on. Patterns and amplitudes are readily recognizable, and any improvements are easily noted. The laser dot also helps in isolating the 1 per rev problems from the 2 per rev problems. A severely distorted pattern is a good hint to go after 2 per rev things first!

I do not represent the above mechanisms, patterns or shake directions as totally accurate. This is a very difficult issue to think through and analyze. I surmised the above (sometimes impressions) from long-term laser dot observations on the High Command and on a Dominator that had very severe rotor shake. I invite corrections and additions to the above, but, suggest that you simply install the laser and observe the results of blade adjustments, before you try to "argue" with the specifics above. In the end, it is usually a matter of making an adjustment to see what it does to the dot pattern. It is most helpful to simply be able to see when you make an improvement - which the laser dot helps make apparent!

- Greg Gremminger

 

[GyroChuck]

Balancing the blades

First off you need to remove the blades from the gyro before your able to balance them. Fit a 3/8" rod thru the teeter bolt hole and turn the blades upside down.

Support the 3/8" rod on both sides, suspending the blades high enough off the ground so the tips of the blades don't touch the ground. (Whatever you use to support both sides of the rod can't interfere with the teeter motion of the blades.) The surface of the support should also be level. You should now have the blades set up like a teeter-tauter.

There's a couple of ways to check the balance. If they're off some, you'll notice one blade tip is closer to floor than the other one. You could use a tape measure to note the difference. But if the floors not level this won't help much. You could use a bubble level on the backside of the hub bar. But you have to make sure it's centered or it adds a little of it's own weight to one side or the other giving a false reading. For better accuracy you can use a "smart level" it costs about $100 from Amazon.com tools section.

Or you can go to the local hardware store and purchase about 35' of 1/4" id clear plastic tubing. Fill the tubing with water except about the last foot. You can add some food coloring so it will be easier to see the level of the water. If you want to get fancy, get a couple of 1/4"x 2" pipe nipples and caps. Stick the nipples in the tube ends and cap the nipples. Then when you move the tube the water won't spill out. Of course you have to loosen the caps before using the water level.

Use some sort of a support on the floor to hold the tubing vertically just next to the blade tips. The idea here is to adjust the water level in the tube so the water is at the same level as one of the blade tips. You then compare the level of the water at the other end to see if that blade is higher (lighter) or lower (heavier). You want to add weight to the light blade or remove weight from the heavy blade till both blade tips are equal with the water in both ends of the tube.

As a note, you need to do this in an enclosed space with NO wind, such as a hangar. With your length blades they will teeter for quite a long time before settling down. Every time you make an adjustment to them, be prepared to wait at least 10 minutes for them to settle down again. Even the wind from walking close to the blades will upset them. You'll need a lot of patience.

 

[GyroChuck]

Date: June 06, 2002 07:23 PM
Author: Ken Rehler ([email protected])
Subject: A Chuck Beaty Type Question

But ... anyone can answer.

At the PRA Convention I was talking to Ernie Boyette about rotor blades and he said something that was very interesting. I general, he said that if there is play in the rotor head where the rotor could slide between the teeter towers (allowed to move from side to side on the teeter bolt) that there would be no problem, as the rotor would find the center point naturally when spinning.

If it is correct, then could this help eliminate the 2 per revolution vibration that the "slider" was designed to address? Could we just make the space between the towers a 1/4" wider than needed and put rubber washers on each end to let the rotor move as it wants to while in flight without transferring a vibration to the gyro frame?

I doubt this idea will work, but it's worth asking just to learn something new.

Ken Rehler - New Braunfels, Texas - Ken's Gyro


Date: June 07, 2002 02:13 PM
Author: Ken Rehler ([email protected])
Subject: 1 or 2 per rev

Yes, it seems that it would "automatically" balance the blade "chord wise" - 1 per rev. But it could also allow the blade to move back when at 90 degrees to the flight path and back to center when the blade is parallel to the flight path which might also cure the 2 per rev.

Or it may cause more vibration! If so, it was Ken J's idea not mine.

Ken Rehler - New Braunfels, Texas - Ken's Gyro

Date: June 10, 2002 06:25 PM
Author: Neil Hintz ([email protected])
Subject: Shake

We have done it here in NZ. We are running a Dominator tandam (SUB4) with 28' dragon wings. From the begining there was a problem with rotor shake, we are using Ernies slider system. Ernie was very helpful in solving our problem, first we came to the conclusion that the blades were spinnig too slow, 309 rpm so on Ernies instruction reduced pitch ( one degree ) speeding the blades up to 360 rpm. This resulted in a very smooth control system but you guessed it a slight loss in performance due to incressed rotor drag ( higher blade speed through the air ). On Ernies new instruction we have now constructed a "Teeter Slider" using car valve springs to controll the movement, the result being we have now put the one degree back on the rotors to where they were, although it can't quite be the same as we are looking at 320 rpm now, no two per rev shake or "stirring" smooth system and performance returned!To me this is a very clever yet simple idea with real world results, truly innovative You will need to talk to Ernie about this system as it is important it is set up properly with the right tension, movements and spring rates ,IT WORKS!

Neil Hintz

Date: June 10, 2002 06:57 PM
Author: Ken Rehler ([email protected])
Subject: Car Valve Springs

You stated: "... we have now constructed a "Teeter Slider" using car valve springs".

1. To make sure I understand, are the springs positioned on each end of the teeter bolt so the hub bar can move along the bolt from side to side?

2. Do you think rubber (perhaps like that used for engine mounts) could be used in place of the car valve springs?

3. I assume the total movement is only about 1/16" inch in either direction. Does that sound about right to you?

Thanks.

Ken Rehler - New Braunfels, Texas - Ken's Gyro

Date: June 10, 2002 06:35 PM
Author: Doug Riley ([email protected])
Subject: head slop

Imagine the reaction of a stranger to gyros, seeing the title of this thread. What picture would come to mind?

My experience with axial play in the teeter hinge is limited to the uncontrolled kind (without centering springs). In that case, the 2-per vibrations kind of drift in and out, seemingly at random. Vibrations that come and go, IMHO, are more unnerving than the steady kind.

The more "official" type of slider (down at the lateral pivot) so far hasn't eliminated all the 2-per vibrations in my 28-foot D-W's, although the vib level is reasonably low. I'd be interested in more about the Aussie system.

Date: June 10, 2002 10:35 PM
Author: CA BEATY ([email protected])
Subject: Rotor shake

Paul, here's a simple experiment anyone can do in a few minutes to learn about 2/rev shake.

Take a 2 ft. length of 1/2" PVC pipe, drill a hole through the center, use a bolt for a mandrel and chuck it in your 3/8" variable speed drill. Held edgewise in front of a strong box fan or better still, in the prop blast of a gyro, you'll be able to feel the 2/rev shake.

All 2-blade rotors shake, twisted or not and the only cure is soft mounting.

There are other causes of shake that can't be totally eliminated even with soft mounts. A coned rotor with correct undersling has mass above and below the teeter bolt that resists following the path forced upon it in forward flight, when as a result of cyclic flapping, the tip plane axis isn't concentric with the rotor head axis. This is a tough one to visualize without the aid of a model.

Date: June 11, 2002 11:50 AM
Author: CA BEATY ([email protected])
Subject: Teeter bolt slider

About 10 years ago, when I was developing the slider that gave fore and aft cushioning at the roll axis pivot, Ernie came up with the idea of a teeter bolt slider and did some diddling using rubber pads. Never worked very well, probably for the same reason mine didn't at first.

I tried metal springs just to humor someone but it solved all of my shake problems. I still don't understand why rubber pads don't work; my first thought was that the rubber pads were squeezing down against the roll axle and locking it up but hollow rubber cylinders spaced with shoulder bushings to prevent rubber-axle contact didn't work much better.

Ernie took up the teeter bolt slider again a few months ago using metal springs and had much better results. I think it might be a patentable idea with possible application to helicopters with see-saw rotors but such helicopters are disappearing and the gyro market is too small to do much more that recover patent costs.

There is still work to be done; a modular design is needed that has provisions for adjusting sideways centering and allows the teeter bolt to be removed without having springs go flying in all directions. Teeter bearings need to be replaced with bushings that accommodate axial as well as oscillating motion with a minimum of static (slip-stick) friction. Needle bearings or metal-to-metal (heaven forbid) bushings simply won't do.

I think the teeter bolt slider will solve most of the shake problems of large rotors once the detail design work is completed.

Date: June 11, 2002 12:23 AM
Author: Paul Bruty ([email protected])
Subject: Yes I agree......

.....Chuck, that it is difficult to get a really smooth system with two blades especially if the diameter is over 26'. All blade manufactures have this problem.

The difference being, how they go about minimising this shake. Obviously some manufactures do it much better than others.

Do you agree that the least amount of lift dissymmetry will give less shake? Aussie Paul.

Date: June 11, 2002 12:22 PM
Author: CA BEATY ([email protected])
Subject: Sure, Paul...

...the smaller the lift dissymmetry, the less the shake. In a perfectly vertical descent, lift dissymmetry is zero as is 2/rev shake.

Lift dissymmetry depends upon the ratio of forward speed to rotor tip speed and no one has yet solved that problem.

Twisting a gyro blade root end down reduces the stalled regions near the root end and insures that the tip of the advancing blade doesn't operate at negative lift.

There is a possibility that twisted blades aggravate the shake problem. The blades flex a good bit during forward flight and a twisted blade doesn't flex exactly straight up and down as would be the case of an untwisted blade.Flexible Bensen blades with segmented upper skins have a reputation as being the smoothed of all.

But it's only a possibility; I sure don't know how to analyze it.


Date: June 13, 2002 02:11 PM
Author: Ken Rehler ([email protected])
Subject: I Suggest

Ron,

The steps that need to be taken are: (1) balance the blades end-to-end and add washers (to the bolts farthest out) as necessary so it balances exactly level. Use a 25' long clear tube full of water to see if the blades are exactly level. (2) track the blades exactly while in flight with the reflective tape at the tips. Do it over and over until they track exactly together. (3) balance the blades chord-wise (side-to-side). Stringing is a good "guide", but adjusting the McCutchen like threaded tube to move the head from side to side is needed. This is done by trial and error until the one-per-rev shake stops. (4) calculate the teeter tower height needed then raise or lower it as needed until you get the best results. This can be expensive as you need to change both the towers and the teeter block each time.

You need to do all four in the order shown, and each needs to be done as accurately as possible. Also, before you start I suggest you remove your rotor head and have a machine shop check to see that the center of the teeter bolt is exactly centered when it is spinning - the teeter towers can be off making the spinning teeter bolt center wobble. Mine was off .004" and Ken Brock reset it to zero, saying that .004" was too far off!

 

[GyroChuck]

December 16, 1999

CA BEATY ([email protected])
Subject: 2/rev

A couple of weeks ago, there were some comments about flexible masts which I had intended to take up but was into something else at the time.

As Arthur Young, the designer of the first successful see-saw rotor (Bell-47) learned nearly 60 years ago, fore and aft compliance is an essential requirement of all 2 blade rotor systems.

Cierva had tried 2-blade rotors mounted on a rigid pylon but even with drag hinged blades, 2/rev shake was intolerable.

I repeated Cierva's experiments with my own gyro several years ago. With the tail-boom connected to the upper end of the mast, there is no alternative to a triangulated and rigid structure. With a standard see-saw rotor, 2/rev shake was unbearable at any speed above 40 mph no matter what the undersling- it was almost as bad as some of the 2-place machines. I then tried a non-underslung rotor with an interleaved, door hinge type of joint at the teeter bolt and with a pair of drag hinges about 10 inches outboard; there was no reduction of 2/rev shake whatever! I learned several days ago that Dick DeGraw had repeated the same steps with his wife Karol's (DeBird) gyro, also to no avail.

I solved my 2/rev shake problem by designing a 'slider' mechanism for the rotorhead roll pivot. The normal needle roller bearings were replaced by a pair of plastic lined bushings can which accommodate sliding as well as rotational motion and the gimbel 'U' block is centered with a pair of springs. Almost like magic, the 2/rev shake vanished. The scrub pattern on the roll pivot indicates that periodic motion is only about 1/16 in.

Dick DeGraw eliminated 2/rev shake on 'DeBird' by removing a rotorhead to tail brace and providing compliance in the cabin to rotorhead braces.

When the rotor is broadside to the wind, the aerodynamic drag is higher than when the blades are endwise. This is the excitation that causes the problem (other than incorrect undersling, but that's a different story).

If the rotor is mounted on a rigid structure, a 2/rev 'hit' is produced in forward flight which causes the blades to flex rearward and like any mass-spring system there are resonances which if falling in the operational range of the rotor, can cause violent shake.

The slider mechanism isn't strictly an isolation system, more than anything else, it is a 'detuner' which changes the resonant frequency of the system. It also prevents in-plane rotor flexing- imagine flattening a nail with a hammer: fairly easy if the nail is placed on an iron anvil but doesn't work at all if the anvil in made of rubber. Same with the 'slider,' the equivalent of the rubber anvil.

I think it was Don Parham who had said limber masts can make a gyro unstable: that is quite true if flexing can cause a cyclic pitch input. The geometry of the control pushrods must be very carefully designed to avoid coupling between mast motion and cyclic pitch. This usually requires some sort of parallelogram arrangement.

Take a shoe box and cut the bottom out - two long sides and two short sides which always remain parallel no matter which way it is racked; a parallelogram. Tape one narrow side to a desktop and imagine that one of the long sides is the mast and the other is the pushrods; the top narrow side on which the rotorhead is mounted always stays parallel to the desktop, it makes no difference which long side we designate as mast or pushrods. As the short side that is taped to the desktop is narrowed, then the rotorhead begins to tilt with the mast as the shoe box is racked, representing mast flex to cyclic coupling in an unstable direction.

One of the things contributing to instability in the RAF-2000 is the geometry of the idler arm mounting on the stationary part of the mast. The spacing between the idler arm mount and the mast pivot is narrower than the spacing between the rotorhead pitch pivot and the pushrod ball joints that connect to the rotorhead cross arm, not a parallelogram. The rotor angle of attack increases as the mast flexes rearward. I don't know if there would be a clearance problem but the stability of the machine could be significantly improved if the idler arm pivots could be located farther in front of the mast.

I don't think the RAF story line that the "front pushrods and rubber mast make the machine so stable that a horizontal stabilizer makes it too stable" is deliberate humbug; I think it more likely that they just flunked junior high school geometry. But, humbug non-the-less.

Date: December 17, 1999 11:44 AM
Author: CA BEATY ([email protected])
Subject: 2/rev

2/rev shake is a vexatious problem with teetering rotors and the bigger the rotor, the bigger the problem.

Grasp a 36" stick of welding rod, preferably 1/8" or larger between thumb and forefinger at 8 1/4" from one end and give it a thump in the center- it vibrates at its fundamental resonant frequency; the lowest unrestrained frequency possible. Methods of calculating the resonant frequency can be found in any mechanical engineering handbook so I won't go into it here.

The vibrating welding rod has two nodal points- nodes are points on the welding rod with no to and fro motion, only a pivoting motion which for the 36" rod are about 8 1/4" from each end.

Attaching weights at either the center or ends of the welding rod lowers its resonant frequency. Placing spring restraint at the center or ends raises the resonant frequency. Spinning the rod as in a rotor raises the resonant frequency because of centrifugal stiffening.

This is the resonance which provides most of the headaches with gyro rotors. If the fundamental resonance happens to fall at the operating rotational speed of the rotor, beware! The aerodynamic excitation of a resonant rotor in forward flight can cause structural failure. If the resonant frequency of the rotor is slightly above or below the frequency of the aerodynamic input, vibrations may not reach a catastrophic level but are extremely annoying and with some peculiar phase shifts, for example diagonal to the fore and aft axis.

With teetering rotors, usual practice is to place the fundamental in-plane resonant frequency well above the rotational speed. This requires that the rotor be as stiff in-plane as possible.

Rotor 2/rev shake can sometimes be reduced but never eliminated by stiffening the mast with extra bracing from engine mount to the top of the mast. This slightly raises the resonant frequency of the system but is an incorrect solution.

There was a series on the 'Discovery Channel' a couple of years ago titled "Choppers" which traced the history of the helicopter. The technical content was far better than usual for that sort of program. I think videotapes are still available.

Arthur Young, during development of the Bell Model 30, the precursor of the Bell-47, encountered severe 2/rev shake that was caused by the rotor having insufficient in-plane stiffness. A fix suggested by the test pilot, Floyd Carlson and called the "Swedish Yoke" in his honor was a bridge plate which spanned the inboard section of the rotor and stiffened it enough that development could continue.

The ideal hub, from the standpoint of in-plane stiffness is the SkyWheels type of center section.

The worst possible hub from the in-plane stiffness view is the RAF hub which is necked down in the center.

The in-plane stiffness of the hub varies as the cube power of width: ie, twice as wide is 8 times as stiff.

I have experimented a bit with "Swedish Yoke" type stiffeners and it does work A 1/4" aluminum plate sawn in a shape which mimics the SkyWheels center section and bolted to the hub can very significantly reduce 2/rev shake when nothing else works.


Date: December 21, 1999 09:49 AM
Author: Doug Riley ([email protected])
Subject: 2-per rev

This brings up a few interesting tidbits. Bensen always emphasized the importance of a flexible mast, and certainly the unbraced 2x2 mast qualifies. When double 1x2 masts ("redundant masts") first came out, the oldtimers in our club claimed they could feel more vibration in the frame with them, because of the increased fore-and-aft rigidity add by the two extra walls.

I've always wondered about the flexibility (or lack of it) of triangulated truss masts such as those built up from 4130 tubing (a la Barnett and others). They look as if they violate Bensen's rule by failing to accomodate fore-and-aft movement of the rotor. OTOH, a truss mast can provide far better roll bar protection and a safe attachment point for shoulder belts, as compared to the unbraced tube mast. The latter type is fond of snapping off somewhere around the engine mount in a hard blade strike, leaving the pilot's head to act as the roll bar. The dire consequences of having a shoulder belt attached to such a breakaway mast are obvious... yet the belt should be mounted from above.

It would appear that a triangulated mast ought to be paired with a rotor head using Chuck's slider, but would then be superior to the Bensen type from the crash-safety viewpoint.

 

[GyroChuck]

Date: December 22, 1999 02:01 PM
Author: CA BEATY ([email protected])
Subject: Undersling

Trying to eliminate 2/rev shake is often like wrestling an octopus. Get one tentacle (I once had a young lady in my employ who always confused tentacle with testicle; she now has a Ph.D. in education) pinned down and another pops loose.

As a first cut, the teeter bolt ought to be located on the CG of the coned rotor. Take a length of welding rod and put an abrupt bend in the center so as to create a shallow 'V' to simulate a coned rotor. Lay a straight welding rod on a level surface, place the bent welding rod on top and slide things around until the bent rod balances. Assuming each leg of the bent welding rod touches the straight rod at the same span wise point, the CG of the coned rod is at the center of the straight rod. This is where the teeter bolt ought to go. But life with teetering rotors is seldom that simple.

If the undersling is too little, the rotor CG rotates in a 2/rev circle with cyclic flapping in forward flight. The spatial visualization of combined rotation and flapping is very difficult for left handed people and nearly impossible for right handed people.

Too little or too much undersling produces a 2/rev fore and aft shake in forward flight The shake from too little undersling is out of phase with the aerodynamic input to the rotor and produces some cancellation. Most rotors, depending to a very large extent on mast stiffness, are smoother with undersling a bit less that a consideration of CG alone would indicate.

This is a subject not covered in any text book I've been able to locate and involves a bit of speculation on my part. I'll now wait to be shot down.

I've put together some very simple programs on Excel to calculate undersling, tail volume as a percentage of rotor volume, disc loading, blade loading and HP Vs torque. It's very easy to use and might appeal to those people who don't agree that pie are square. Email me for a copy


Author: CA BEATY ([email protected])

Subject: Shakes

Scott, what you're describing sounds like a 1/rev shake. Although it may not seem so, the only way the stick can move is at rotor speed or a multiple like 2/rev. 2/rev generally gives a pulsing stick, pulsing fore and aft or diagonally. 1/rev is the easiest of all shakes to cure. It is always either out of balance or out of track. A rotor can be out of balance either spanwise or crosswise. Spanwise balance is best checked with a water level. Stick a rod through the teeter bolt hole and place across a pair of sawhorses. A water level is a length of transparent plastic tube filled with colored water (food coloring, Easter egg dye, Cool Aid, etc.), with ends taped to a stick so as to indicate when both blade tips are at the same height. Crosswise or chordwise balance requires that the chordwise CG of the rotor be on the center of rotation. With metal blades, weight is sufficiently uniform so that all required is that the blades be in pattern, i.e.. a string run from tip to tip pass over the center of the undersling block. This is done by loosening the strap bolts and aligning the blades to be in pattern. Plastic blades may not have a uniform distribution of epoxy; that is corrected by the "chord shift" adjustment. The threaded bushings for chord shift usually lack sufficient precision to guarantee repeatability but I suppose the only way is adjust and try. Out of track stirs the stick in a circle and causes a vertical loping motion reminiscent of a galloping horse. Out of track can be seen in flight; view the blade tips at the 1:30 position and if distinctly separate tip streaks can be seen, the blades are out of track. The type of pitch change arrangement used by RAF makes tracking adjustment difficult. Clamp a 2 or 3 ft. length of angle to each side of the pitch adjustment bolt ("wings" & hub) to serve as pointers. A 1 degree pitch differential between the two blades will throw the tips about 3" out of track. One degree is about 5/8" at the ends of 36" pointer bars. If the tips were 3" out of track, you'd begin having trouble keeping your feet on the rudder pedals.


Date: October 28, 2001 12:49 PM
Author: CA BEATY ([email protected])
Subject: Blade balancing

Out of balance, out of pattern and out of track produce 1/rev shake. Out of balance and out of pattern are indistinguishable to the pilot; both stir everything around in a circular path at 1/rev. Out of track produces a vertical lope reminiscent of a galloping horse as well as a circular stick stir. Out of track isn't difficult to see; observe the rotor tips at the 2 o'clock position while in flight and if two separate tip streaks can be distinguished, the blades are out of track.

Two/rev shake is always is a more vexatious problem. Causes are: mast too stiff, undersling incorrect or the two blades aren't matched for pitching moment or chordwise balance.


Date: June 10, 2002 10:35 PM
Author: CA BEATY ([email protected])
Subject: Rotor shake

Paul, here's a simple experiment anyone can do in a few minutes to learn about 2/rev shake.

Take a 2 ft. length of 1/2" PVC pipe, drill a hole through the center, use a bolt for a mandrel and chuck it in your 3/8" variable speed drill. Held edgewise in front of a strong box fan or better still, in the prop blast of a gyro, you'll be able to feel the 2/rev shake.

All 2-blade rotors shake, twisted or not and the only cure is soft mounting.

There are other causes of shake that can't be totally eliminated even with soft mounts. A coned rotor with correct undersling has mass above and below the teeter bolt that resists following the path forced upon it in forward flight, when as a result of cyclic flapping, the tip plane axis isn't concentric with the rotor head axis. This is a tough one to visualize without the aid of a model.

 

[Sv.grainne]

I recently purchased a PB-4, waiting for a 3d printed mount from Nicolas. Once I get that and have a chance to go thru my system I plan to do a writeup.

 

[GyroChuck]

I recently purchased a PB-4, waiting for a 3d printed mount from Nicolas. Once I get that and have a chance to go thru my system I plan to do a writeup.

Too bad you're all the way in Texas. Sure could use use your expertise up here. NE Illinois area. Chapter 18 Lansing, Illinois

 

[Roundwing]

Thanks Chuck.
It will take me some time to read through and digest.

Rick

 

[Eric S]

...or copy the "magic bushing" concept of the RAF gyros. You can get close with balance and tracking and with a balancer even closer, but then as soon as weight or even density altitude changes it's off again.

The RAF guys figured out to let the mast move how many years ago? I had it on my RAF Sparrowhawk and the stick barely moved. It's so simple. I don't understand why every gyro doesn't use it.

 

[Sv.grainne]

While I wait for my rotor mount bracket I'm going to check prop balance.

 

[Sv.grainne]

Mounted tach sensor this morning and captured first data points. Sent email to Mike with .zip data file and some questions.

 

[Kevin_Richey]

Bobby: Is that black line on your prop blades a seam in their construction or a pin stripe?

 

[Sv.grainne]

Kevin: Pretty sure it's a seam

 

[Sv.grainne]

I received the PB-4 bracket from Nicolas and mounted it to the torque tube yesterday.



I mounted the sensors and cables today after talking with Jim while he was working on his Aviomania Duo. I still need to attach a small piece of reflective tape to the back of one rotor blade and finish setting up the tacho sensor.

That's for tomorrow! I'm only spending a few hours at the hangar each day as I'm doggie sitting while my wife visits family in Fort Collins, CO.

I came home (doggie sitting) and fabricated a mount for the PB-4 Brain. I'll get some photos of the finished setup tomorrow and do an initial test before starting the rotor balance process.

Lewis the Basenji kept the lizards at bay while I was working.

 

[Sv.grainne]

I replaced the 3d printed sensor bracket with one I made out of aluminum. The 3d bracket had developed a few cracks (I probably over tightened the sensor bolts.

I completed several test runs getting everything setup correctly.

At first I had the update button in an awkward location so that when I pushed the button to record data, I had either no data recorded or multiple data points.

Had a problem with my phone staying in the cradle, worked that out.

Then this morning I was ready to take off and remembered that I had not disabled my phone screen timeout!

Took off, yanked and banked some to lomber up the rotors the made my data run. Got some pretty good data points but going to run again in the morning to confirm.

Initial results points to a weight imbalance. Slave blade sppears lighter than the master blade. Picked up some 1/4oz adhesive backed tire weights that Mike recommended.

There's a lot going on. Added to my checklist today.