Flapping hinges

leech10

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Hi


I do not exactly know how does flapping hinges work in i.e. 3 bladed articulated rotor. Maybe I do understand some principless incorrectly? I know that they are used for compesating dissymentry of lift. That's OK. I understand why one of the blade must go up and second down. But how this is mechanically resolved in 3 bladed articulated rotorhead? For example in the picture I attached there is a connector between blades. If one moves up second one is forced to move down and compensate lift difference. But if there is no connection between the blades both will be pulled up by the lift. So what connects blade system in 3 bladed articulated rotorhead? As I understand if there is no this mechanical connections all blades will be lifted up and system will fail? Or this is done by the blade angle of attack control system? If by the blade angle of attack control system I am wondering why hinges are used as blades will be permanently lifted up. Lift will be equalised by the different angle of attack of each of the rotating blade but they will not go up and down.

And one more. In small RC coaxial helis. Is this task done by flybar?

If there are some mistakes forgive, I am from Poland

Regards
Piotr
 

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The flapping hinges in a fully articulated system are completely independent, without any mechanical connection. (In a teetering two-blade semi-rigid system, it rocks/teeters as a unit).

All blades will cone upward under load, but that doesn't mean they are flapping upward together.

As a blade reaches the advancing side of the disc, it sees more airspeed because the forward motion and the rotating motion add together. The blade will flap up naturally if it is free to do so because of that extra airspeed.

As the same blade moves to the retreating side of the disc, it sees less airspeed because the forward motion of the aircraft is now opposite the direction of the rotating motion, and the two sources of airspeed are subtracted rather than added. The blade will naturally flap downward if it is freely hinged to do so, because it sees less airspeed.

Flapping down increases the angle of attack to compensate for the reduced lift from less airspeed. Flapping up reduces the angle of attack to compensate for the extra airspeed. Both actions together keep the lift well distributed across the rotor disc.

P.S. You're doing fine in English, so just keep asking until we get your questions answered clearly.
 
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On more thought -
the discovery that the blades would naturally flap, without any control mechanism to force that motion, was one of the great insights by Spanish inventor Juan de la Cierva when he developed the autogiro. If you put in hinges that allow the blades to flap, they will flap. It is a lovely fully automatic process that doesn't require pilot intervention or special mechanisms, just the freedom to move. The freedom to move usually comes from hinges, but can be accomplished with appropriate flexing of the blade/hub materials in some designs.
 
Hi

Thanks a lot. To be honest I was watching TV programme about autogiros and there were those hinges shown. And I started to wonder how does it work in complicated rotorhead. So the same, moves freely. I looked at different solutions and saw some flex systems as you wrote. So yes they must be flapping freely :)

In my small RC it is easy to observe how flybar works. All systems are very simple in principle and very effective. Easier than I thought.
 
According to Bruce Charnov’s book,From Autogiro to Gyroplane, although La Cierva discovered blades with flapping hinges alleviated the issue with dissimilarity in lift, it created another problem. As the blade was allow to flap up the center of gravity for that side of the rotor came closer to the point of rotation for the disk. Just like a ballerina spinning faster when she pulls her arms in closer to her body, the flapping advancing blade wanted to accelerate. However the blade was kept from doing so and stresses at the root of the blades resulted in blade failures. So La Cierva added the lead lag hinge to allow the blades to accelerate or slow as needed. The semi ridged teeter system deals with this stress by the simple underslung rotorblade. The blade being under the point of swing will swing out slightly toward the advancing blade thereby canceling out the movement of the blade toward the center of rotation. To me, this is one of the most brilliant yet simple examples of engineering in rotor head design.
 
According to Bruce Charnov’s book,From Autogiro to Gyroplane, although La Cierva discovered blades with flapping hinges alleviated the issue with dissimilarity in lift, it created another problem. As the blade was allow to flap up the center of gravity for that side of the rotor came closer to the point of rotation for the disk. Just like a ballerina spinning faster when she pulls her arms in closer to her body, the flapping advancing blade wanted to accelerate. However the blade was kept from doing so and stresses at the root of the blades resulted in blade failures. So La Cierva added the lead lag hinge to allow the blades to accelerate or slow as needed. The semi ridged teeter system deals with this stress by the simple underslung rotorblade. The blade being under the point of swing will swing out slightly toward the advancing blade thereby canceling out the movement of the blade toward the center of rotation. To me, this is one of the most brilliant yet simple examples of engineering in rotor head design.
This was something I'd never considered before. Thank you for posting it. To the layman a gyro looks like such a simple contraption, unaware of the insane engineering that went into making it simple, as your post illustrates. There are these delicate relationships between seemingly unrelated items I've always found fascinating. A curse too when one design change starts the domino effect.

That was my waxing poetic for the morning. Cheers.
 
This was something I'd never considered before. Thank you for posting it. To the layman a gyro looks like such a simple contraption, unaware of the insane engineering that went into making it simple, as your post illustrates. There are these delicate relationships between seemingly unrelated items I've always found fascinating. A curse too when one design change starts the domino effect.

That was my waxing poetic for the morning. Cheers.
There are some teeter towers on some rotor heads which have multiple points in which you can mount your assembled rotor blades. I would like to know when or which of these different teeter bolt holes is proper and what considerations are needed when choosing where to hang your blades. How do you find the proper mounting point? Seriously, I would like to know this. It has been bugging me for quite sometime.
 
When the teeter bolt is on the line joining the centers of gravity of each blade, then the torque vibration around this line is minimal (not zero!),
But there is in addition the drag vibration which also affects the stick, since the pitch pivot is usually located 8 to 10 inches below the teeter bolt.
So, It can then be interesting not to decrease too much the torque vibration, to better oppose it to the drag vibration, and obtain a smoother total.
I don't think anyone understands very well how it all fits together
 
When the teeter bolt is on the line joining the centers of gravity of each blade, then the torque vibration around this line is minimal (not zero!),
But there is in addition the drag vibration which also affects the stick, since the pitch pivot is usually located 8 to 10 inches below the teeter bolt.
So, It can then be interesting not to decrease too much the torque vibration, to better oppose it to the drag vibration, and obtain a smoother total.
I don't think anyone understands very well how it all fits together
Well, thank you for that piece of the puzzle. So the teeter bolt is in line with the CG of each blade when coned and loaded. Wow, so the rotor is hung high enough to allow for a certain amount of swing which occurs between the towers. The mounting block for the rotor determines which teeter bolt height is used? I’m guessing here.
 
So the teeter bolt is in line with the CG of each blade when coned and loaded.
Yes, to a first approximation. And since, fortunately, the rpm in flight varies with the load, the coning remains constant despite load changes if the atmospheric density does not change
 
So the amount the whole rotor swings to the side of the advancing blade is more or less self regulated dependent of load and atmospheric condition. i understand Bensen had a lot to do with the rotor- pivot -offset but did he pioneer the under slung semi ridged rotor system?
 
Bensen didn't invent the underslung semi-rigid rotor. Arthur Young, designer of the Bell 47 in the WWII years, did. Bensen may have been the first person to apply Young's principles to an autogyro (since autogyros were dying out when Young was at work).

Bensen started with almost zero teeter undersling, but increased it as time went on. Check out the teeter height on some of the 1950's era Bensens in old photos.

The use of a centered flap or teeter hinge on an autogyro is somewhat problematic (even though we all do it). With the hinge in the center, we must rely on rotor thrust for control of the aircraft. If we put the rotor in a situation of zero thrust (roughly speaking, rotor disk edgewise to the airflow), we no longer have control of the airframe.

If the flap hinges are placed a bit outboard of the rotational axis of the rotor, then the rotor will still have some ability to impose moments (torques) on the airframe because of the centrifugal effect. This setup, however, also jacks up the control forces when using a tilt-spindle head (those torques come right through the controls). Cierva's direct-control autogiros had high control forces because of this phenomenon. The designers of the Groen Hawk went back to a helo-style swashplate after trying tilt-spindle.

It's easier to understand the need for, and motions around, the flap and lag hinges if you imagine yourself riding on the tip of one of the rotor blades. From that viewpoint, there's no leading, lagging or flapping; only feathering. Thanks to Chuck Beaty for pointing this out countless times.
 
On a related topic, the farther away from the gimbal you place the center of lift of the rotor disc (across the center of gravity of the blades) the greater the control force required to maneuver the rotorcraft.

Air Command tandem gyrocopters were sold with Skywheels rotors mounted on a single bearing head. The control arm on the torque tube is a fairly narrow 12" and the yoke at the bottom of the push tubes spread is 10". It flies with very light inputs. Once you replace the original single-bearing head with a stacked, two-bearing head, a-la RFD Dragon Wings, the controls become very heavy and slow to respond.

It's my recollection that Chuck Beatty experimented with a rotor head and gimbal design that actually made the distance between these two points negligible, but found that control was impossible without any feedback from the rotor disk.

After exhaustive research, and speaking with several bearing manufacturer's engineers I came to find that the MRC spec double-row angular contact bearings used in rotor heads will be expected to last a minimum of well over 1000 hrs carrying loads averaging 1100 lbs and 1.4 G's over their life. Which is why you've never heard of a single-bearing head bearing failure on any of the multitude of tandem, single-bearing heads since 1990-something. Why anyone decided we needed double bearing heads on a gyro with a TOW of under 1200 lbs is beyond me. It just makes it harder on the Heim joints in the control tube system - and we all know that those have been known to break, resulting in death every time. Seems a whole lot smarter to me to reduce stresses on the control system than it does to needlessly double up on rotor head bearings. Which is why I will be milling down the double-bearing RFD Dragon Wings rotor head for my Air Command, since I am never going to convert the control system to the "double-wide" yoke and arm found on Dominators needed to compensate for the stacked double bearing head sold with that system for two-up gyros.

Which brings up another issue with the earlier RFD Dominator single-bearing tandem airframe heads sold prior to when RFD went with two-up bearing heads. Their single bearing head towers are held in place by two AN4 bolts running transversely through the bearing block. This works "OK" for single-place gyros, but when running 27'+ rotors the towers twist and it is impossible to track them and keep them tracked - unless you just get stupid lucky. The very easy fix is to drill and pin the towers in place. Relying on two AN4 bolts to maintain geometry is never going to work.
 
Hmm. I had a tandem Dominator with a 2-bearing head. It had pleasantly light control forces. The pushrods were 1" dia., and the rod-end bearings were the rather expensive aircraft-grade models. My 1986 Air Command OTOH had 7/8" pushrods and commercial-grade rod ends. Neither setup presented any problems in these areas.

Flexing of the teeter towers in the "chordwise" direction was not an issue, because the towers were joined together by two rectangular gusset plates (one above each blade). The plates represented extra bolts to tighten and loosen when mounting/dismounting blades. I didn't find that task burdensome.

The reason for two-bearing heads has nothing to do with the bearings' pure thrust ratings. Rather, it's the fact that the rotor disk does not fly square to the bearing's rotational axis in any flight mode except a vertical descent. In forward flight, the disk "blows back" 2-3 degrees. The rotor's thrust is then not pulling straight up the bearings' rotational axis -- it's pulling at an angle (it would bend the spindle bolt back if the bolt were soft enough). IOW, the the rotor's thrust is apply a "prying" action the outer race of the bearing, not unlike prying open a paint can. Ask the bearing manufacturers about their bearing's ability to tolerate this type of loading. They'll likely call it an "overturning moment." Turns out that our head bearings aren't especially good at resisting large overturning moments. The heavier the gyro, the higher the overturning moment.

As with so many aspects of the Bensen gyro, Igor Bensen had an exquisite sense of what simplifications he could get away with in a 500 lb. gyro's rotor head. The overturning moment on a single bearing from a gyro of this weight was tolerable.

When you scale up a Bensen, many of these "informal" simplifications just won't do anymore. Everything from using a plywood scrub brake to letting the mast tube do all the flexing is fine on a B-8M -- and not so fine on the half-ton and 3/4-ton models.
 
Hmm. I had a tandem Dominator with a 2-bearing head. It had pleasantly light control forces. The pushrods were 1" dia., and the rod-end bearings were the rather expensive aircraft-grade models. My 1986 Air Command OTOH had 7/8" pushrods and commercial-grade rod ends. Neither setup presented any problems in these areas.

Flexing of the teeter towers in the "chordwise" direction was not an issue, because the towers were joined together by two rectangular gusset plates (one above each blade). The plates represented extra bolts to tighten and loosen when mounting/dismounting blades. I didn't find that task burdensome.

The reason for two-bearing heads has nothing to do with the bearings' pure thrust ratings. Rather, it's the fact that the rotor disk does not fly square to the bearing's rotational axis in any flight mode except a vertical descent. In forward flight, the disk "blows back" 2-3 degrees. The rotor's thrust is then not pulling straight up the bearings' rotational axis -- it's pulling at an angle (it would bend the spindle bolt back if the bolt were soft enough). IOW, the the rotor's thrust is apply a "prying" action the outer race of the bearing, not unlike prying open a paint can. Ask the bearing manufacturers about their bearing's ability to tolerate this type of loading. They'll likely call it an "overturning moment." Turns out that our head bearings aren't especially good at resisting large overturning moments. The heavier the gyro, the higher the overturning moment.

As with so many aspects of the Bensen gyro, Igor Bensen had an exquisite sense of what simplifications he could get away with in a 500 lb. gyro's rotor head. The overturning moment on a single bearing from a gyro of this weight was tolerable.

When you scale up a Bensen, many of these "informal" simplifications just won't do anymore. Everything from using a plywood scrub brake to letting the mast tube do all the flexing is fine on a B-8M -- and not so fine on the half-ton and 3/4-ton models.
Hey, Doug, how's it going?

First: Air Command (no idea what OTOH means) TANDEM, not single place, is the topic of my earlier post. These were sold in the 1990's from what I understand after being corrected elsewhere by Mike Boyette. Once again, these were sold with Skywheels mounted on a SINGLE BEARING head and performed remarkably well with very nominal stick inputs. The yolk and cross tubes are VERY NARROW compared to any Dominator, with the latter having much wider arms thus increasing torque which overcomes the added height of a double bearing head.

When replacing these original Air Command heads with newer double bearing heads, like RFD, e.g., the added 1" height increases required input forces dramatically, resulting in slower response and SIGNIFICANT decrease in maneuverability.

Second: You write, "IOW, the the rotor's thrust is apply a "prying" action the outer race of the bearing, not unlike prying open a paint can. Ask the bearing manufacturers about their bearing's ability to tolerate this type of loading. They'll likely call it an "overturning moment." Turns out that our head bearings aren't especially good at resisting large overturning moments. The heavier the gyro, the higher the overturning moment.

Wrong, Doug. Wrong, wrong, wrong, and...yep, I checked, and STILL wrong. I did ask the bearing manufacturers. MRC, Timken (FAG) and NTN. "Overturning moment"? Can openers? LOL, Doug you are simply repeating something Chuck Beatty wrote or said, you never called a bearing manufacturer, obviously. If you have spoken with any of the major bearing manufacturers, PLEASE send the contact info so I can straighten out MY errors. I welcome to be corrected, and only seek the truth and the FACTS, with no care for personal pride or standing in any such case in which I may be wrong. I don't give a rat's ass what anyone thinks about me, which should be quite obvious to any and all by this late date.

When considering a single deep row bearing this notion of your so-called "can opener" "overturning moments" is true. Absolutely.

When discussing a DOUBLE ROW ANGULAR CONTACT BEARING the argument false. Absolutely.

In fact, this is EXACTLY WHAT THESE BEARINGS ARE DESIGNED FOR. THEY ARE RATED FOR ALL FORCES APPLIED AT VIRTUALLY ANY ANGLE FROM AXIAL (zero degrees) to RADIAL (90°) , AND/OR ANY COMBINATION OF the two, at any time, momentary or constant, SUMMED TO THE MAXIMUM RATED LOADS GIVEN ON THE SPEC SHEET FOR ANY PARTICULAR DOUBLE-ROW ANGULAR-CONTACT BEARING.

Read...my...lips: There is NO DIFFERENCE REGARDLESS OF THE ANGLE OF LOADING. Period. End of argument. You want to argue with SKG, MRC, FAG, NTN whatever, be my guest. Pick the dang phone up and CALL THEM if you will. But PLEASE stop repeating BS 2nd hand rumors just because they originate with Chuck Beatty, whom we all love, trust and respect with great devotion.

Anyone desiring an authentic MRC and/or NTN and/or Timken bearing technical handbook please feel free to email [email protected] and I will GLADLY send you the PDF files I have on hand, no charge.

Anyone else who just wants to go on believing in and repeating second-hand rumors about can openers and other nonsense, be my guest. It does no good to beat a dead horse, after all.

And when you discover the FACTS - after READING THESE HANDBOOKS AND SPEAKING directly with a real, live, major bearing mfr's tech advisor or engineer please come back here with those FACTS, with names and phone numbers, so that I may re-educate myself by speaking with the very same reps you would quote and upon whom you would found your claims.

Please do not belabor the point senselessly with 2nd hand rumor from anyone, including our beloved master of gyrocopter engineering Chuck Beatty, just because we love and do respect him so. Look, I am sorry if this flies in the face of "conventional wisdom" as you think you know it, but this is just wrong, and someone needs to put an end to this silly myth that's been spreading about for the past 15 years or so. Yes, when you get over 1200 lbs TOW you need a two-bearing head. But under that it is unnecessary and in the case of narrow-coupled control systems (Air Command e.g.) it puts unnecessary strain on critical control system parts such as rod end "Heim" joints and is a plain old bad idea.

Finally: Doug writes, "Flexing of the teeter towers in the "chordwise" direction was not an issue". Look, just because one guy did not have a problem does not mean the problem does not exist. In math this is known as a false proof, a failed method to make a point.

Please see photo. THIS IS MOST DEFINITELY AN ISSUE ON SINGLE BEARING RFD DOMINATOR HEADS LIKE THE ONE IN THIS PHOTO, WE HAVE SEEN THIS PROBLEM ON NUMEROUS SIMILAR HEADS both single place as well as tandem rotors. THE TOWERS WILL NOT STAY PERFECTLY VERTICAL BECAUSE THE TWO AN4 BOLTS ARE INSUFFIENT TO KEEP THEM ALIGNED. THESE HEADS NEED TO BE DRILLED AND PINNED IN TWO additional PLACES NEXT TO THE AN4 BOLTS IN ORDER TO PREVENT CORD-WISE MOVEMENT. It's not in all likelihood not dangerous, but it is certainly irritating and frustrating in tuning to eliminate as much stick shake as possible.

I'm going back to my cave. This is no place for me.
 

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