two per rev shake

[Vance]

I am trying to understand the cause and make up of the two per rev shake in an autogyro. It seems to get worse with bigger rotors, more cord and heavy machines. I am trying to understand the fundimental cause and it's direction. What affects it's amplitude and how do you minimise it intrusion into the flight experience. Some of the "fixes" only serve to confuse me further. Is it only vertical, or horizontal as well? I flew with Duane in his RAF and it seemed quite pronounced, two fat people in a heavy gyro. The Sparrow Hawk and other modified RAFs at Buckeye have seemed relitivly smooth. I flew in an RAF at Mentone and the shake seemed to come and go. I would be gratefull for help in getting a better understanding of the two per rev shake, as distinct from out of balance or out of track rotor blades. Thank You, Vance

[gyropilot]

Posted by Greg Gremminger on the old Rotorcraft Forum back in August of 2000...

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 RPM's) 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. Combinations 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

[Jim]

Also see http://www.rotaryforum.com/forum/showthread.php?t=933&highlight=2-per for some good answers (Among others from Chuck Beaty)

Jim

[Vance]

John and Jim, That is wonderfull information! Thank you very much! It helps me to redefine my confusion. Thank You, Vance

[Heron]

Does it have anything to do with the blades been presented to the drag in two different positions (long and transv)?
Heron

[darrellwittke]

How did I miss those posts? I think they already answered my question but I would like to confirm it with an out and out answer.

Does the positive twist (higher angle of attack) of Dragon Wings at the tips cause any increase in H-force? (or two per rev shake)

I suspect not from the previous posts ie. they self adjust to lower tip speeds and rotor rpm. which linearly? equals out to the same shake, I've wondered this for a long time and just have to ask (being relatively obtuse sometimes.)

While I'm at it, some other questions. I understand the positive twist allows the rotors to have 5-7% more efficiency. I think this comes from having more driving area (say 75% of the length of the rotor blade versus 70% for non-twisted blades) to turn the driven (or lifting) section which operates at a higher angle of attack and can thereby turn at a lower speed (producing the same amount of lift.) Since aerodynamic drag increases exponentially with speed this is probaly the main producer of the increase in efficiency of positively twisted rotor blades? Is this correct?

Therefore is the efficiency increase in tapered untwisted (composite) rotor blades equal in their gains of efficiency? I guess their increased performance comes primarily from the larger inboard (drive) section being more adapted to flying forward with the lower airspeed (RPM.) The driven (lifting) section is probaly shifted outwards also (say the same 75% ratio as above) and I would guess the blades run at a higher speed thereby giving up the exponential drag reduction of the slower turning positive twist blades, so where does the increase in efficiency come from? (I do believe they are more efficient ie "tons of lift" but don't understand where they derive their efficiency from.)
Is it because the optimized driving section rams the tapered driven section through the air faster, which imparts higher inertia which can be translated into lift or is their something else in basic physics I am missing?

(Sorry Chuck B., I think you have touched on this before but can you enlighten us once again?)

I am going to take my battery off the mast mounting above the engine and sling it directly below the engine and see what difference this makes. (I have 24 ft Dragon Wings and never understood why they shake more than Bensen blades until now, I missed these links previously.) Should help a little and I have a tall tail and horizontal stabilizer to compensate for the cg move.

Thanks for any replies to shed light on these questions. darrellwittke

[KenSandyEggo]

"In my experience, the major source of 2 per rev shake is improper teeter height" G.G.

Just to show what an inexact science this is, teeter height had nothing to do with the severe shake on my RAF with RAF blades. I mooched 2 additional teeter-blocks and 2 sets of towers to be able to shift my teeter-height all over the place. 2 tower sets were altered in height as were 2 teeter blocks. I also had numerous shims of differing thicknesses available to alter the teeter-height even further. I went from minimal teeter-height to maximum height in small increments by using the 3 different towers, 3 blocks and shims. During the testing, an engineer was riding with me with his RADS vibration analyzer. By the way, he could plot 2-per laterals with his gear. We also tried with and without the RAF "balancing bars," which actually made it worse. We did tracking with a nose-mounted infrared light, weight changes at the tips, pitch changes of each blade, cord adjustments and anything else we could think of. This went on for several months without any meaningful progress as to smoothing out the ride.

Solution? Someone lent me a different set of blades to try out after I took his RAF for a spin and couldn't believe how smooth it was. Got rid of the RAF blades and switched to Sportcopters. From then on to this day, my ride is exceptionally smooth with no cabin-hop or stick-shake. The RAF blades were inconsistent from root to tip in their density, thusly unbalanceable. I saw an RAF blade cut open. There was thick foam next to thin foam next to no foam. If you get smooth RAF blades, it's strictly by accident. Sportcopters are consistently smooth.