2-per-rev shake

[Jim]

Folks,

I am new to the giro-world, but have learned a lot by reading the forums. Thanks! :)

Hope that I do not show too much of my lack of knowledge by this posting! :confused:


The two-per-rev shake is a problem inherent to all two bladed rotors. In a three-bladed system shakes go up to three per rev. (NACA technical note 764.)

As we, after the achievements of Ron Herron’s Littlewings, approach the era of cross-country-capable giros, this shaking can become a greater problem. In NACA’s technical note 764 they found: (giros with severe vibration) “ . . . are generally regarded as unsuitable for extended cross country flights”

Shaking has a detrimental effect on more than just cross-country gyros. On March 3, 2003, Rusty Nance wrote in Norm’s forum that the blades with the best climb performance in a 1300-lb machine went to 28' fat DW, second place went to 29' skinny DW, third to McCutchen and then RAF. When it comes to shake, the fat DW were the worst, followed by the skinny DW, McCutchen and then RAF. At the end Rusty ended up using the RAF blades with the worst overall performance “ . . . because of vibration levels predictability.” Paul Bruty wrote on March 14, 2003: “In the lead sleds the rotor shake is a major problem.” Don Parnham wrote on Dec 24, 2001: “Paul’s statement that blades over 26 ft cause more problems is very true. It just seems to be almost impossible to get all the 2-per-rev stick shake out of larger diameter blades.”

Blade shake is dangerous:
1. It may wear out components.
2. Aluminum that is continuously shaken may lose its elasticity and break.
4. Chuck Beaty wrote: “Attaching mass to the rotorhead may cause stress levels in the hub and blade root attachments to reach unsafe levels.” (Dec 24, 2001)
3. It causes pilot fatigue.

Where does shake come from?
Two-per-rev shaking is complex and may be attributed to many reasons. Some of these are:
1. Too much or too little undersling to compensate for coning can contribute to shake.
2. The main reason is probably the position of the blades in regards to the airstream. Chuck Beaty wrote on Dec 24, 2001: “There are other sources of 2/rev shake. The aerodynamic drag of the rotor is greatest when broadside to the airstream and least when endwise.” Graphically the drag vectors may be shown as follows:

In Attachment 1 it can be seen that aerodynamic drag is the biggest when the blades are broadside to the air stream.

Attachment 2 is interesting: It shows that as the blades move away from the broadside position the drag becomes smaller. What is further of interest is that as the air hits the blades on one side, the drag vector is not straight to the rear, but also to the opposite side. This causes the shake to be elliptical.

Many different ways have been tried to address shake:
1. Bensen and Bell used soft masts.
2. RAF used rubber bushings and flexible cheek plates.
3. Ron Herron uses rubber bushings where struts attached about half-way with the vertical mast.
4. Chuck Beaty developed the slider head.
5. Ernie Boyette also allowed the hub bar to move sideways at his teeter bolt.
6. Ernie showed off an array of dampening devices such as a brass solid that flexes on a fibre rod.

All these ways are, of course, (good) mechanical devices to neutralize an aerodynamic problem. Has anyone tried to solve this aerodynamic problem with an aerodynamic solution? I am thinking of a solution that was mentioned by Robert Rominger and Ken Rehler on Dec 16, 2002: The use of winglets on rotor tips. At first I was very sceptical, but the more I think of it the more I wonder if it won’t work.

If one makes the winglets to have about the same amount of drag when the airstream hits it on its broad side as the difference between the drag when the airstream hits the rotor on its broadside or endwise, one may be able to eliminate much of the shake.

I can only attach 2 graphics at a time. Hope this description helps!

1. When the rotor is broad-sided to the airstream (most drag), while the winglets are into the airstream (lease drag) – just like the winglets on a jetliner. Winglets reduce the vortexes at the wingtips and the drag on the rotor.

2. As wind hit the rotor on its side, the drag vector is backwards and sideways. Drag on the rotor reduces, but that on the winglets becomes bigger. Side-drag on the winglets is to the opposite side of that of the rotor. Well-designed winglets may reduce shake to the side to zero.

3. The Rotor is at its least drag, while the airstream hits the winglets sideways, causing more drag.


In a well-designed system the difference in drag in 1 and 3 may be very little and therefore reduce shake to small amounts.

Will it work or am I chasing the wind?


Jim

[Jim]

:cool: Here are graphics of the winglets I am talking about

Jim

[eruttan]

I was wondering about winglets from a entirly different place.

I was wondering is the wing "enhancing" effects of a winglet might improve the rotor efficiency.

[KenSandyEggo]

Jim, I beat this to death in other postings, but here's what I found. I had RAF blades that vibrated my gyro like crazy. I had a sophisticated RADS setup installed by a design engineer for RADS (and A&P, and Rotorway builder, and large helicopter pilot...not him, the helos), who balances million dollar+ rotorcraft all over the world for a living.....privately owned and the military. He found that the main culprit was 2-per revolution lateral vibes. He installed vertical and lateral transducers in all planes at the top of the mast and at the bottom of my instrument panel. He also mounted an electronic light on the nose to monitor the tracking precisely. He found that the so-called seemingly vertical-hop in the cabin, manifested by one's feet feeling as though they were bouncing vertically, and up to your hip-bones depending on the amplitude, and the seemingly apparent vertical hopping of the door frames, was actually not caused by vertical vibes but the good old lateral 2-pers. My vertical vibe readings were minimal.

We flew for a period of several months, sometimes 4 to 5 days per week, trying to bring them down from the 6-plus IPS readings he was initially reading, which can cause a lot of damage to many components of a rotorcraft. After adjusting blade tip weights (measured in fractions of grams), tracking, cord, balancing bar weights, pitch settings and infinite coning angles through a series of 3 tower sets and blocks, along with machined shims, I think we got them down to around 5 IPS or maybe a little less.....still in the damaging range.

We believe that the lateral vibes are caused by the inconsistent manufacturing process of the RAF blades. This was discovered when an RAF driver smacked his blades and then did an "autopsy" on them. After peeling them open, he found that the innards were very inconsistent. He found solid portions, portions filled with air spaces and sections where there was nothing but air. RAF also see-saw balances their blades by injecting foam into holes on top of the blades near the roots until they see-saw balance.

Now I profer this. If the addition of a dime-sized washer on the tip of a rotor-blade can dramatically affect the balance, when measured with sophisticated electronic gear, what does one think that inconsistent weights along the entire length of a blade, caused by a variation of gaps inside the blade, and squirting in unmeasured quantities of foam will do.....especially when one blade's inconsistencies do not match the other blade's?

This same pilot that whacked his blades then ordered a set of Sportcopter aluminum blades. I forgot to add that he had whacked 2 sets of RAF blades in his short flying career. He was fed up with the vibrations in both sets and that is why he tried the Sportcopters.....and very fortunate for me. He invited me to fly his RAF with one of my stabs and new Sportcopter blades. As I went around the pattern, I felt something was wrong or different, but I didn't figure it out until I was on downwind. There was no stick-shake or "cabin-hop." It was as smooth as though it were on rails. It took me a minute to come to the realization.

He offered to let me install his blades on mine and take them home for a week, which I did. The ride home was as something I had never felt before... a shake and hop-free ride. I flew his blades heavily for a week....just about every day. All I did to his blades was center them between the towers. I reluctantly brought them back in a week and sent my hard-earned money to Sportcopter.

It appears apparent that the aluminum blades are stick and hop-free because they are extremely consistent in their manufacture. The inside is already filled with air, so no inconsistencies can occur as with foam and Fiberglas. The Sportcopter blades would give just as rough a ride as the RAFs if we inconsistently added ounces of weight to them haphazardly along the entire length of each blade, which is what RAF effectively does with their manufacturing process. Remember, dime-sized washers at the tips of 30 foot blades measurably affects their balance.

The way that the Sportcopter blades attach to the hub-bar is probably also a factor in their smoothness. Through rubber (or elastomeric (sp?)) bushings, the blades are able to seek their natural alignment and coning angle. The lack of this system appears to be why so many RAF blades develop cracks, usually near the root end, where the bending stresses would tend to be more severe. Some will come and argue that their RAF blades fly smoothly. True. This is due mostly to luck when the technician was fabricating them. If each blade was made consistently as to the other one, either filled properly, or inconsistent similarly to the other blade, a smooth ride is possible. Their general inconsistency is apparent by my and others inability to get a smooth ride, even with sophisticated balancing equipment.

I previously mentioned Gary Brewer, a highly experienced builder and CFI. He had his own balancing gear for his RAF blades, adjusted them seemingly forever, and when I flew his ship awhile back, I thought someone was hanging onto the tip of one blade as we took off. That's how bad his stick-shake and hop were. His blades, as mine, were simply unbalanceable due to the inconsistent characteristics of each blade.

I'll add that the first time I saw Sportcopters pre-rotate up, I immediately noticed the absence of upper control-rod shake. Mine did a "happy-dance" each time I pre-rotated and/or flew. With the aluminum blades, that is gone. It was unnerving to me to look up in flight and see the way those rods were vibrating. Something has to give or crack eventually.

As a final note, if anyone has the uncontrollable urge to hop in here and accuse me of sour grapes or RAF bashing, please reread this lengthy post until you finally realize that it is a factual narration of my and other's experiences and impressions with 2 brands of rotor blades and nothing more.

[C. Beaty]

Interesting comments.

Someone at Bensen Days asked what I thought of a vane mounted at the hub center to balance out aerodynamic drag variations; same idea as the winglets.

The amount of shake depends upon the ratio of aerodynamic to mass forces. Heavy blades won't shake as much, other things being equal.

And this may come as a surprise to some; the AAI Sparrowhawk is now using a slider on their rotorheads. I guess they're precluded from using the RAF magic rubber bushing and I suppose rotor shake was intolerable without softening things up.

With something in the neighborhood of 10,000 lb.. of centrifugal force acting on each blade, the "elastromeric" bushings mounted 12"-18" outboard are essentially locked up.

PS: Footnote on dangers of rotor shake.

As I suppose most people know by now, the blade separation suffered by Bill Ortmyer was due to apparent fatigue failure of the 3/4" bolt that retained his RAF blades. The blade that separated sailed several hundred feet and the remaining blade flailed around and mauled the occupants.

The head of the bolt broke off and I understand it showed signs of progressive crack growth.

Double masted Parsons trainers always shake like a jackhammer; possibly compounded by the bolt not being properly torqued down.

In pure tensile failure, a bolt will always fail at the junction of nut and threads or the threads will shear out. There is no point to making the length of threads engaged by the nut stronger than the minor diameter of the bolt.

I have never seen bolt heads fail but am informed by people who pull wrenches for a living that it's not unusual to remove the valve cover from an automobile engine and find several cylinder head bolt heads lying loose on top of the cylinder head. So I suppose fatigue causes this kind of failure.

[KenSandyEggo]

Yes, but don't they lock up when/after the blades are in the best alignment and coning position? Paul claims the ride is O.K. with his metal "bush" if I recall. How the hell can anyone patent a rubber grommet and holes? AAI is merely a parts supplier. RAF patented a holes in pieces of aluminum and a chunk of thick, rubber tubing? The builder is the manufacturer. Oooooh. I wish that I had finished law school. :mad: And.........WELCOME CHUCK!!

[C. Beaty]

Certainly, Ken, the "elastomeric" bushings will permit the blades to fling themselves into alignment but will not permit 2/rev motion that's presently accomplished by your RAF magic rubber bushing.

If my understanding is correct, RAF has a presumably valid patent on the magic bushing.

[KenSandyEggo]

I didn't mean that it would, Chuck. I didn't mean it to sound that way. I don't know what I meant! :confused: Leave me alone! I'm tired and it's past my bedtime. Ever hear from Craig? Is he O.K. and still flying?

P.S. Did you mean "prevent" instead of "permit," or did you mean it permits the absorbing action?

[Al_Hammer]

it can be seen that aerodynamic drag is the biggest when the blades are broadside to the air stream.

It doesn't seem all that obvious to me why that is so. Maybe someone can explain it to me
Granted, the airspeed is greater on the advancing side and less on the retreating side. Lift and drag are both proportional to the square of the airspeed.
This would result in a lift and drag differential at the broadside position, but as we know, flapping equalizes the lift.

Lift is equalized by effectively changing AoA. Reduced AoA on advancing side reduces lift and vice versa on retreating side. The magic of flapping.

When lift is reduced, drag is reduced, but not necessarily by the same amount. It depends on the slope of the lift curve compared to the slope of the drag curve.
If they are equal, then it seems to me, there should be no increase in drag when the blades are broadside to the line of flight. Lift is equalized and drag is also equalized.
For an airfoil like the NACA 0012, I believe the drag actually changes less for a given AoA change at low angles.
There is a region of reversed flow on the retreating side, and this will have an effect, as will the shifting ratios of driving and driven regions.
It appears to be more complicated than it appears at first glance.

I imagine that compressibility effects will be more pronounced on the advancing side, and that might explain some of the drag differential, but the situation sure isn't as clear cut as the diagrams above make it seem.



Regarding winglets:
Many modern blades have
drooped tips, "winglets" which are very beneficial in reducing hover power.
The S-92 and advanced Black Hawk blade has a drooped tip, and it has very
significant hover benefits. The droop affects the tip vortex, and behaves
like an endplate to contain the out wash and therefore the lift loss at the
tip.

Our work shows that a tip plate (above and below the blade) is a very big drag
penalty at high speed, and this offsets any hover advantage. We solve the
structural problems of the droop tip with strong composite structural
attention at the tip. I don't think we could package a metal blade to hold a
drooped tip, without some very heavy structure out there.

Nick lappos
(Sikorsky test pilot)

[davreich]

Hi,
Sorry the picture is so small. It's the only one I could find. I noticed them on a TV special about Marvelous Machines.

(Also, RAF is into a new generation of Main Rotor Blades)

[C. Beaty]

Al, Raoul Hafner, in the paper he delivered to the Royal Aeronautical Sc. in 1937 introducing the first application of feathering cyclic pitch to a rotorcraft, developed some fancy math to explain the periodic 3/rev aerodynamic excitation of 3-blade rotors. So fancy in fact that he had to do the integrations graphically, not having a PC. The excitation is orthogonal rather than simply fore/aft.

Forgetting for a moment the fancy stuff, consider tossing a stick. Tossed spearlike, it will travel farther than when tossed broadside. Tossed with a spin, the deceleration is greater broadside than when endwise. Traveling in formation at the same mean deceleration rate and viewing the center of the stick, you would see a 2/rev displacement of the stick.

If you send me your mailing address, I'll send you a copy of the Hafner paper. I expect you get off on partial differential equations.

[Steven_Kozned]

Chuck
The "elastomeric" bearings on the SC blades do not carry any centrifugal loads.
A 1.5 inch diameter bolt in a bearing does that vector. There is no "lock up".
Have another look!

[KenSandyEggo]

Dave R,, I meant to mention in my copiously-worded post that my experiences with the RAF blades are from those observed about 5 years ago. I apologize for the oversight and have no knowledge of how the newer blades perform. The thought did cross my mind and then I forgot to mention it.

I'm attaching a blowup of the photo you posted of the winglet, but don't know if it's any better. The bigger I made it, the fuzzier it got. I had it in Photoshop and tried to sharpen it up a little.

Didn't you and I used to go head-to-head in the old Forum? It's great having taken a break from posting for awhile. All that bull-crap is gone....I don't remember who I was pissed at or for what and everyone is now just another goofy rotor-head sharing info. I hope we can all keep it that way. I'm trying and doing a pretty good job....I think.

[KenSandyEggo]

Chuck
The "elastomeric" bearings on the SC blades do not carry any centrifugal loads.
A 1.5 inch diameter bolt in a bearing does that vector. There is no "lock up".
Have another look!

THAT'S what I meant to say. :rolleyes:

[C. Beaty]

I know very well how the Sportcopter hub is arranged, Steve, Ken.

The motion required about the pivot bolt (snubbed by the elastomeric bushing) is only about ± ½°.

Now Steve, with 10,000 lb. of centrifugal force acting through the CG of each blade, how hard do you have to push on the hub to deflect each blade by ½° ? That is the amount of cyclically varying force transmitted to an airframe with a rigid mast. Kindergarten level stuff for a guy with an engineering degree.

I base the ½º estimate on observation of the scrub pattern obtained with a slider.

[KenSandyEggo]

Closest I've been to an engineer is riding in the first car on the Illinois-Central R.R.

[C. Beaty]

The fact is Steve, drag hinges do nothing to remove the 2/rev shake of 2-blade rotors because of centrifugal stiffening. It is not a case of one blade leading another; both blades swing fore and aft together in the presence of aerodynamic excitation.

Cierva learned that in the 1930s, I relearned the same lesson years ago and the necessity for AAI to go to a slider in their machines using Sportcopter rotors again reaffirms the superiority of engineering over tinkering.

[KenSandyEggo]

Chuck, in my ignorance, I probably shouldn't have assumed and said that the bushings on the SCs help the vibes. I see what you're saying as having no effect on 2-pers. I think I'm basically correct though that the weight variations along the length of a blade will affect the lateral 2-pers.........yes? That one blade carrying different weights in different places from the other blade, will produce that lateral loping, manifested as stick-shake, cabin "hopping," upper control-rod vibrations and head movement. My rotor-head appeared to be moving in a circular motion as viewed upward from the cabin while in flight.

[KevinKing]

I believe those things on Ken Wallis's blades are weights, used to get the correct chordwise balance on the blades. My Little Nellie model has them. At first I thought they were mold marks, but I saw a better picture of the real thing, and it has them too.

[C. Beaty]

Ken J., there are many factors affecting the smoothness of rotor blades and RAF blades are a piss poor benchmark. Weight distribution, particularly in a chordwise direction has a major effect on shake.

But I suspect, more than anything else that uniformity of surface contour from blade to blade is the biggest culprit with RAF rotors. The ones I've looked at have lumps and waves that seem to to be entirely random. If you're lucky and draw a set of blades with matched lumps, then they might be relatively smooth.

As a general rule, the heavier the blade, the smoother. But remember, weight is the enemy of flying machines and also, heavy (high inertia) blades will result in a heavier stick feel. High inertia rotors follow the stick more slowly so time for the blade to catch up with stick displacement is longer.

[Al_Hammer]

Ken,
you might be right about the non-uniformity of the blades causing some vibration, but don't you think Bell Helicopter and other manufacturers can manufacture rotors that are uniform?
Yet,I've read that 2 bladed ships like the Cobra are real teeth rattlers.
All the 2-bladed ships like Rotorway, Safari, Robinson, etc, have 2-per-rev to one degree or another in forward flight. Large helicopters sometimes use exotic absorbers to deal with it.

According to some on-line searching, it seems that some of the aerodynamaic causes of 2-per rev vibration are:
transonic effects
dynamic stall effects
blade-vortex interactions
spanwise flow
reverse flow
non uniform inflow near the hub
transverse flow (maximum at about 20 kts)

here are some links
http://www.isd.uni-stuttgart.de/sfb409/projectA5.html
http://aerodyn.org/Dstall/dstall.html dynamic stall

Studies have been done to try to eliminate the problem with active blade elements, such as piezo electric servo flaps, and surpising improvements are possible.

Her's a study done an an ultralight helicopter
http://rotorcraft.arc.nasa.gov/publications/files/Shen_AHS03.pdf

[KenSandyEggo]

No. Mark, the application engineer for RADs told me that a certain model of Bell Helicopters (I forget the number) shakes at a rate of 6+ IPS, which is damaging to avionics and components. He said that for whatever reason, that is the best that the Bell engineers could reduce it to, even though they have the finest vibrational analysis equipment available to them.

One of the old(er) airport rats relates how he broke his leg up in the mountains a few years ago. Mercy-Air (a Bell, twin-turbine heli with 2 blades) came to pick him up and the thing vibrated and shook so much that it had him scared to death, and he's an experienced pilot. He said that in hindsight, he would have preferred to walk back down to the hospital. I looked in the thing once and saw springs that I didn't believe could be made that big. It's probably the same one Mark mentioned. I think it was
3_ _ . All I know is that I couldn't ask for a more shake-freee ride than I get with my Sportcopters. Whatever Jim did, he did it right.

[StanFoster]

Ken: I had the option to take RAF on their blade exchange program. I had until Dec.31 2003 to do it though. Everyone that I talked to with the new 24th generation blades said they were smoother.

RAF explained to me that the molds for the older blades were made from 360 individual aluminum blocks that created a lot of surface inconsistencies that needed to be hand sanded. The results were varying in success. Some are rough..some are smooth. Anyway..the new molds are in two pieces..one for the top and one for the bottom. These produce a more consistent and smoother airfoil. There is much less labor and these blades are supposed to be less prone to cosmetic cracking.

My blades are fairly smooth...very little stick vibration..and cabin hop. I have been messing with the chord block and have moved it both ways from a dial indicator measured center...and this resulted in more cabin hop. So for right now...I am back right at the measured center...and actually am content enough with the smoothness...that I am not presently messing with it.

I still am going to try slinging the blades...and then adust the tracking someday just to see how smooth I can get them. But for now...I am getting a fairly smooth ride.

Stan

[KenSandyEggo]

Stan, recall that the vibes we feel are almost always laterals. I doubt that pitch changing will do anything for that. The hop and stick shake is from the laterals. When we slapped the RADS system on mine, the verticals were minimal, it was the laterals that were obscene. I always found that the best ride was always with the block dead-center between the towers. I don't know if I'd call any cracks on a rotor blade "cosmetic." Cracks appear because of stress. Stress no good.

If I recall correctly, Chuck B. was not too impressed with the need for slinging or alignment. He just mentioned that there are about 10,000 pounds of centrifugal force pulling on each blade and no bolts are going to stop that force from aligning the blade. Did I get that about right, Chuck? Close?

[StanFoster]

Ken: That makes sense to me on the 10000 pounds of force. I am listening to you on the tracking as well. You probably have as much experience here as anyone in eliminating rotor vibration.

Like I said, my stick shake is so little,,,you can let go and it looks motionless...the dash has a little shake...and the compass is just fine.

Stan

[Doug Riley]

Dave Reich, the things on Cmdr. Wallis's rotor blades are probably noseweights. Wooden blades built along Bensen's lines typically have them. On Bensen rotors, they consist of a piece of sheet steel bent into the shape of the front half of an airfoil and filled with lead. They bring the chordwise balance point forward. Same idea as the metal clip on the nose of a hand-tossed balsa glider.

[C. Beaty]

Two/rev rotor shake is probably the single most frustrating and intractable problem afflicting gyros with seesaw rotors.

Small rotors –22’ to 23’ diameter generally aren’t too bad when mounted on a gyro with a single, untriangulated mast. The best of the lot, shakewise, are Bensen/Brock limber blades with segmented upper skins.

My first gyro was built from 2.5” diameter round 2024 aluminum tube using Bensen B-8 dimensions. Such a mast is fairly soft in all directions. Never a serious shake problem.

Among the first rotors used was a set of cut down Hughes-269 blades.
The previous owner had sawn a taper in the root ends to mimic Bensen blades and it appeared to me the chordwise strength was too marginal to use a standard seesaw rotor.

I made up an articulated hub without undersling. The hub was in 2 parts, joined at the teeter bolt by an interleaved door hinge arrangement. There were drag hinges about 12” outboard, dampened by friction washers (milk jug plastic). The blades were of course inverted and run backwards from the normal helicopter direction in order for the twist to be in the correct direction for autorotation. Hughes blades have a built in 8º twist; for gyro application, the root ends must be twisted nose down.

Proper direction of twist permits higher pitch settings and lower tip speeds than is the case for untwisted blades. I ran this rotor at about 300 rpm.

Hughes blades use an NACA-0015 symmetrical airfoil, not a particularly good one for gyros (or helicopters for that matter) but as a result of being able to use low tip speed, performance was significantly better than any contemporary gyro blade.

There was a bit of a ground resonance problem. When the critical rpm was reached, the gyro would dance from one wheel to the other but it wasn’t normally severe and usually went through it. Occasionally, ground resonance became so bad that that I’d have to stop and start all over again.

I eventually trashed these blades while skimming the waves 100 yards offshore near Naples, FL and the wire broke on my electric fuel pump. The blades looked as if they had been walked on by a herd of elephants from hitting the water.

That left me with one blade (from a set of 3) but I won’t relate my experience with a one-blade rotor.

I next bought a truckload of military surplus Hughes OH-6 blades out of Ft. Rucker Alabama. Same airfoil and chord as 269s but stronger. Don’t recall if I paid $15 per blade or per set; in any case, highly expendable.

I fooled around with undersling (root ends of OH-6 blades, after machining are highly amendable to seesaw rotors.) but it never seemed critical. As long as the blades were in balance, in pattern and in track, shake was never a problem.

But my partner with structurally bonded double 1x2 mast could never get a set of these blades to run smoothly. We would install my set on his gyro and it would automatically go into jackhammer mode. Swapped rotorheads to no avail. At the time (1970s), it never dawned on me that mast stiffness played such a pivotal role.

I built my present gyro in the early 1990s. With the tail boom running from the rotorhead, there is no alternative to a rigid, triangulated rotor pylon.

The shake with a seesaw rotor was so severe that I wouldn’t fly any higher than I was prepared to fall. Undersling variations made no difference that I could detect.

Tried the door hinge hub with drag hinges without success. I don’t recall whether or not shake was reduced but it was still so severe as to be dangerous.

That’s when I came up with the slider head. Narrowed the “U” block, replaced the lateral needle bearing pivots with plastic bushings and initially tried rubber pads as springs. Big improvement but not perfect.

At someone else’s suggestion, replaced the rubber pads with coil springs and like magic, the shake vanished.

RAF’s rubber bushed and hinged mast is an excellent solution to the rotor shake problem but makes no contribution to stability. It is not, as has been claimed, a “horizontal stabilizer.”

In retrospect, a properly designed flexible mast may be the best and simplest solution. If I ever build another gyro, I’ll most likely use a 1.5” diameter 4130 mast. Equal flexibility in all directions looks to be better than softness in only the fore/aft direction.

The large gyros, 1,000 lb. and up, present a far more difficult problem than the lightweights. The big ones suffer from poor rotor efficiency and would benefit immensely from lower blade loading. But increasing blade chord much beyond the present 9” or so would compound rotor shake problems. Rotors in forward flight are subject not only to periodic aerodynamic forces but the blades must undergo cyclic pitch variations. Increasing the moment of inertia of the rotor about the feathering axis severely compounds the problem. Two/rev stick shake from feathering axis cyclic motion becomes a steady force with 3 or more blades.

Three blade rotors anyone?

[quadrirotor]

one of the best homebuilt three bladed rotor is reported in one of the best magazine:
VOL MOTEUR No 142 février 1998...and you have the pitcairn...

[CLS447]

Chuck B. , What was the situation with the wide cord Dragon Wings? Why did Ernie stop making them? Did they provide better lift?

[C. Beaty]

Ernie only made a few sets of the wide chord blades, Chris. Last I heard, Maxie Wildes had a set and there’s a set on the East Coast –Fort Lauderdale or some such place, owned by CFI Bob Martian.

The main reason he decided not to go into production was very limited demand at the time and of the demand that existed, most such machines were poor designs that would have reflected badly on his business.