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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WaspAir

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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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WaspAir

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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.
 

leech10

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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.
 
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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.
 

Brian Jackson

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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.
 
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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.
 

Jean Claude

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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
 
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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.
 

Jean Claude

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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
 
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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?
 

Doug Riley

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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.
 
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