R22 Corner

[Lu Zuckerman]

Lead-Lag on the Robinson head

To: slowrotor

The Robinson head like those on the Bell machines is underslung to minimize if not eliminate any tendency for the blades to lead and lag. With forward cyclic and with the blades equally coned there is no leading and lagging. However in a maneuvering situation or during gust alleviation the blades will flap about the cone hinge and then there is leading and lagging. Since there is no vertical hinge to permit the leading and lagging the forces are absorbed in spanwise blade bending. These loads are transmitted to the head and from there to the mast, which results in a cyclic twisting of the mast. All of the lead lag loads are transmitted via the cone hinges with resultant elongation of the cone hinge, which is in the form of an ellipse. All of these loads are generated at twice the rotor speed during maneuvering.

[delta3]

Hub resonance

Dave, bugdevheli

Your reasoning would point to the existance of a mast bending resonance (whole system including hud and blades), with a resonance frequency equal to the main rotor rpm (or first multiples ,say 2 and/or 3). Only in this case small excitated disturbances, such as lift inbalances) can provoque large oscillations.

It is my understanding that the mast is on rubber bearings, making the likely resonance frequency pretty low, so very unlikely in mho. Take for instance also small inbalances in the geometry of the mast it self, these would also be amplified. So this system can not realistically be resonant at these frequencies.

I'll put it on my simulation todo list...

Delta3

[slowrotor]

Lu,
Glad your back, its been a little slow here without you.
You said the blades absorb the lead and lag loads in a manuever or gust. Are the blades designed for this (in your opinion)?

The R-22 blades are attached at the root with a small hub. And several photos posted on this forum show cracked and separated blades at the root. Seems like a highly stressed blade attachment without the lead and lag hinge. Other teetering rotors (Bell, Hiller) are much larger with drag link struts.

I suspect Mr. Robinson used coning hinges to allow for a lighter rotor system. A good idea as long as both blades cone together. But if one flaps alone for a fraction of a second then it would be stressed in lead/lag ,I think. The R-22 rotor just looks weak in the lead and lag plane to me.

[Dave_Jackson]

If someone wanted to build a rotor similar to theR22/44 but eliminate the potential problem under discussion, why not simply link the coning hinges so that they cone in unison.

Alternative, just take the weight of the above modification plus the weight saving from the removal of the coning hinges and put this weight into stronger composite blades and simple hub.
Then call it a Bell 47.

Dave

__________________

Edited to add the following thought.

1/ Teetering rotors are more difficult to control than rotors with offset flapping hinges.
2/ Lighter helicopters are more difficult to control than heavier helicopters.
3/ A new US helicopter does not have to comply with the new FAA regulations if it maintains commonality with a previously certified helicopter. (This statement is based on very limited knowledge.)

Perhaps number 2 + 3 is one of the reasons why Frank Robinson is rumored to be stopping the production of the R22. He can use his approved rotorhead in the R44 and have the advantage of a more controllable craft.

[Chiplight]

Delta 3, thanks for trying to explain.

I'm still "collating", as Ash said in the movie Aliens.

[flyingbull84]

rhm, thanks for your reply.

maybe frank robinson knows the right answer...

regards,
Franz

[madman1145]

Funny enough you should ask - was tolled the other day why the tailrotor is pre-coned on the R22 ..

The tailrotor has to produce a certain amount of thrust to one side all the time so the helicopter doesn't start spinning around itself ..
To limit the stress of the bending moment on the tailrotors blades hinges/center the natural coning would make when it produces thrust, it is pre-coned to the side where it would cone anyway to minimize the stress ..


- madman

[delta3]

Dave

As many times suggested in similar topics perhaps only Frank knows the answer.....

The others perhaps speculate...

My speculation

- coning seams good in order to have tailboom clearance, it allows greater pitch angles of the MR. In order to have the same angles the MR hub would have to be higher. Making it higher creates other problems/compromises

- the fact that the R44 (not R22 as far as I know and as we discussed some time ago, cfr your pictures) has a positive coning delta3, induces me to speculate that the 'designer wanted that', that is at high loads the MR cones even more, what surprised me. I am speculating that this is for the above reason, he really want it coned upwards (difference could be that the heavier R44 blades would cone less that the lighter R22 blades)

- making the thing rigid would mechaninally speaking
- create too big stresses at the foot (you could make it semi-ridig, but I let the creativity on that route over to you...)

- make it very unpractible on the ground (could use your spring)

As I read this back one possible design alternative comes to mind
- have a UH-1 style of configuration, with an angle in the boom, but this would be heavier


My simulator is now in 'Full 3D validation mode'. As soon as I'am comfortable I will release it (target date august-september). Before that I want to check the yaw and low-G induced tail boom strikes (my original goal)


Delta3

[delta3]

Madman

As the rotor is more rigid, indeed a preconing in the direction where it would be on average if coning were free will reduce stresses at the foot

But, since tail rotor has no cyclic and still takes a lot of different aerodynamic disturbances, the aerodynamic design is more subtile than it would appear (see previous threats and answer of Nick several months ago).


Delta3

[bugdevheli]

Robinson rotor head question.

Is it not the case that lead and lag on any rotor system would occur regardless of whether blades were coned or running flat, in any condition other than a no wind hover.

[Lu Zuckerman]

Three cheers for the axis.

To: bugdevheli

There are two axes of rotation on the rotor system. One is the driving axis and the other is the driven axis. The driving axis is coincident with the centerline of the mast. The driven axis is an imaginary line that is coincident with and perpendicular to the center of the rotor disc. As long as the two axes are coincident with each other such as when there is no cyclic input and the cyclic is in the neutral position there is no leading and lagging On the Robinson head with forward cyclic and equal coning of the blades the two axes will separate but since the head is underslung leading and lagging will be at an absolute minimum. However in maneuvering situation the blades will flap about the cone hinge resulting in leading and lagging because the blades have deviated from the coned position.

[Dave_Jackson]

delta3,

Robinson's coning hinges serve the same general function as the precone angle on other teetering rotors. They reduce the stresses in the blades.

If you are referring to the comment in my previous post, the idea is not to 'freeze' the coning hinges. The idea is to allow the blades to still cone about their hinges BUT insure that both hinges have the same amount of cone at all times. This should theoretically eliminate the concern that was raised by bugdevheli in this thread.


Lu,

You said "Three cheers for the axis."

Is this the old axis of evil, Iraq, Iran and North Korea?
Or, is this the newer axis of evil, Cuba, Libya and Syria?

[bugdevheli]

Robinson rotor head question.

Mr Zuckerman. Sorry to spit hairs but. Two blades travelling foreward subject to different loads either side how do they remain directly opposite each other. Cyclic input or no cyclic input.

[Flingwing207]

Hokay, I've been following this debate for quite awhile. There seems to be two (or three, but I'll focus on two) somewhat seperate issues. Let's see if I have a handle on them.

1. Symmetrical blade coning: If the blades cone excessively, the CG of the blades will "rise above" the optimally designed position when referred to the underslung rotor hub. This will lead to increased lead/lag forces as the CG/driven axis wil be "above" the driving axis. The contention is that the coning hinges on the Robinson allow this to happen to a greater degree than will happen on a "rigid-in-plane" system common to other 2-bladed systems.

But does this happen? I would think that Frank Robinson's team would have designed the underslung hub to be aligned with the normally coned axis of the blades. Perhaps, though, this is why the Robbie is so twitchy about vibration during startup and shut down - when the blades are not coned, the driven axis is "below" the drive axis. (Could this also be an issue in low-"G"?)

However, I would think that the Robbie would be no more prone to "over-coning" than any 2-bladed system. Perhaps with the hinge allowing the whole blade to cone up rather than just cone by flexing (as happens in other 2-bladed systems), the over-coned CG for the disk would be slightly higher than in a system like the 206.

2. Asymmetrical blade coning: Aerodynamic forces cause a coning hinge to activate instead of teetering the rotor. While theoretically possible, I see the enormous coupling force across the coning hinges versus the very small force that inhibits teetering. Even in a very steep turn, there might be 2.5 tons of force felt across the almost infinitesimal couple at the teetering hinge versus 10 tons across the much larger couple of the coning hinges. So would the Robbie actually be more susceptable to asymmetric coning than any 2-bladed system?

Now about that phase angle...

[Lu Zuckerman]

Blade relationship. Opposites do not attract.

To: bugdevheli

Mr Zuckerman. Sorry to spit hairs but. Two blades travelling foreward subject to different loads either side how do they remain directly opposite each other. Cyclic input or no cyclic input.

I don’t know if I fully understand your question. On most helicopters with an even number of blades with the blades at rest the centers of pitch rotation are not directly opposite each other. The blade root attachment is offset forward in direction of rotation so that the center of pitch rotation does not intersect the center of the mast. This offset is usually about a ¼” possibly more on larger helicopters. Uneven blade rotor heads are similarly constructed. This is done to minimize spanwise bending of the blades thus reducing structural stresses in the blades.

In forward flight with no maneuvering the blades will be coned and in the pure radial position due to (if I may use the term) centrifugal force. With maneuvering the blades will flap about the cone hinges and due to the different forces (lift) acting on them they will flap to equality (to use a British phrase) and the degree of flap may not be the same for each blade. Under ideal (design) conditions the disc is equally loaded on both the advancing side and retreating side however the helicopter is not operating under ideal conditions resulting in the pilot making corrections with the cyclic stick.

If this does not fully answer your question please restate it.

[Dave_Jackson]

bugdevheli

Two blades travelling foreward subject to different loads either side how do they remain directly opposite each other.

There is symmetry of lift (actually it is symmetry of moment) between the advancing and the retreating blades. The lift/drag ratio is quite constant across the angles of attack seen by the blade elements. Therefor there is symmetry of drag.

Corollas is, by far, the largest contributor to lead/lag. When a 2-blade teetering disk is tilted, during 'mean' flight conditions, the two blades will accelerate and decelerate together, twice per revolution. The remaining causes of lead/lag on a teetering rotor are relatively insignificant and the long flexible mast absorbs them.

On a 'theoretical' 4-blade teetering rotor, lead/lag would be a big problem because one pair of blades would be accelerating as the other pair was decelerating.

If interested, more information is available on the web page;
OTHER: Flight Dynamics - General - Lead/Lag


Flingwing207


Dave

[bugdevheli]

Robinson rotor head question.

Lu Zuckerman. Upon reading your earlier post again, you did in fact verify my suggestion that there must be some degree of lead and lag.


On the Robinson head with forward cyclic and equal coning of the blades the two axes will separate but since the head is underslung leading and lagging will be at an absolute minimum.


I was making the point that some degree of lead and lag is present in this hub. Thanks Bug.

[Musket097]

R22 Servicing Costs

Can anyone quote the appoximate servicing costs for the R22.

50Hr ? 100Hr ? 200 ? Annual ? etc.

[Chiplight]

No, there is not necessarally any lead lag force at all in normal flight conditions.
Undersling puts the teeter hinge above the blade root, at the imaginary point where a straight line could be drawn through the cg of each blade and through the teeter joint. Now, when the rotor tilts about the teeter hinge, which is another way of saying flapping, the cg will not move off the mast, it will stay at the hinge point.
Without undersling, when the coned rotor tilts or flaps, the cg of the rotor, being above the hinge, will move away from the mast or towards it- and vibration will result as the cg of the rotor resists being moved away from it's center by the rotor head which is now spinning in a different plane.
With more than two blades, there is no hinge that can float in a way that keeps the cg of the rotor on the mast. Each blade rises and falls seperately and the resulting cg shift creates a coriolis force and the need for a lead lag hinge.

[UwantME2landWHERE!]

Contact Hugh @ PDG Helicopters, Kintore, Aberdeen.

I know that they do fixed service costs for pretty much most types.

Not suggesting you go there, but at least it will give you a ball park figure!