lead lag dampners

Ken_Shea

Junior Member
Hi all,
Any reason why you do not see lead/lag dampners (not the rigid lead/lag adjustment) in two bladed systems.
In my admittedly uneducated thinking it seems they would help balance out the total system.
 

WaspAir

Supreme Allied Gyro CFI
It's been done long, long ago (look up the Weir W3, for example), but isn't all that helpful. You would make yourself susceptible to ground resonance without any net benefit, while adding weight and complexity.
 
Last edited:

Ken_Shea

Junior Member
Thanks for the input,
I did not think about the possibility of introducing ground resonance, key there would be to avoid it I guess.
Would not add much additional complexity beyond that necessary for the rigid type lead lag adjustment using a similar design to that of Brantly helicopter. The unit uses no fluids, light weight, not over 4" long and maybe 1 1/2" in diameter threaded connection both ends.
If a fact, not being all that helpful would definitely be a consideration to not waste time.
 

WaspAir

Supreme Allied Gyro CFI
Weir thought it useful because they were building an automatic collective jump take-off machine and thus were already employing a far more complex system than gyros of the current era. I don't know of anybody ever choosing to copy it.
 

C. Beaty

Gold Supporter
Bartram Kelley explains why a see-saw rotor must be stiff in a chordwise direction in part 3: “Birth of the Bell Helicopter” at ~ 5 minutes from the start.


Kelley began as Arthur Young’s assistant and ended his career as VP of Engineering at Bell. Arthur Young invented the underslung see-saw rotor.
Any kind of lead-lag hinges in a see-saw rotor exacerbates the 2/rev vibration problem, but can sometimes be tolerated if the rotor mast is sufficiently limber.
 

bryancobb

Junior Member
Ken,
In plain language, the intuitive spacing of rotor blades... 2=180, 3=120, 4=90, 5=72, etc.

The second idea is a question. Is there ever a situation during flight when the helicopter
would benefit or be smoother by varying those angles? That answer is yes. Anytime a single
blade in the rotor system flies higher or lower than the nominal tip-path plane, it's center of
mass moves closer to the rotational axis and that blade wants to speed up like an ice skater.
It needs to be allowed to do this or the occupants will feel it as a vibration and stress will be
put on whatever component(s) are preventing the acceleration.

The accelerating blade however, needs to have a dampening force on its displacement so it's
always converging on equal spacing instead of diverging. If the spacing between blades were
un-dampened, the difference would quickly destroy the helicopter.

Ground resonance is a serious situation which happens on the ...you guessed it...ground, and
the interaction between the ground and the helicopter causes spacing between blades to diverge
suddenly from equal and it exceeds the damping capability of the lead-lag dampers and the
helicopter ruins itself in a few seconds.

Almost all 2-bladed rotors are rigidly mounted 180 degrees apart and when one blade flies higher
as it swings around to the advancing side because it's developing more lift, the other blade on the
retreating side flies lower the same amount. Their centers of mass move inward the same amount
and both blades want to accelerate THE SAME AMOUNT.

Simplified explanation but it gets the point across.

Real world example. Land a 3-bladed Hughes 269A poorly and when one skid bumps the ground
before the other one, it's likely that will knock the blades away from their 120 Degree spacing. That
makes the helicopter "wobble" or rock on the skids. If timing of that wobble is at the wrong time,
the 125 degrees will become 132 and the 138, and then 140 degrees VERY QUICKLY. Your helicopter
will sustain tens of thousands of dollars in damage unless you react very quickly and pick up to a hover.
It will then go away just as fast. No ground...= no ground resonance.

Moral of the story...KEEP good, serviceable dampeners on your rotor and on your landing gear or
you will regret it.
 

bryancobb

Junior Member
Chuck,
In intelligence and experience, you are galaxies beyond me. The things I post as you know, are concepts that have been widely accepted and taught in the helicopter world for decades. I am an "outside-the-box" thinker myself. When I meet someone like you who challenges the status quo, it impresses me. However, to adopt some of the ideas you and Prouty and others present, much of the rotorheads that have been successful would have to be re-designed. Many FAA test questions must be thrown out. Many ideas taught in US Army flight school and published in FAA instruction manuals would need to be rewritten.

Maybe the word "flapping" needs replaced by "climbing" and "descending" to show that the phenomena is not a sudden occurrence but instead a gradual movement of a blade that puts the tip path plane in an un-natural-looking tilt that is different than the theoretically perfect tip path plane that is perpendicular to the mast. I hope to find a frontal picture I have of a Piasecki twin rotor helicopter approaching head on at 90 kts. You can clearly see that the two rotors' tip path planes have passively assumed that un-natural-looking tilt in opposite directions.

In forward flight, a "flapping" rotor's tip path plane coincides with the perfect tip path plane at precisely two points of rotation. At any other point, the two tip path planes are different. That difference is what I feel demands that blades be allowed to individually speed up and slow down. In turn, dampers are required.



1144570
 

C. Beaty

Gold Supporter
We can imagine rotor blades doing all sorts of things, depending on the angle of view.
Here’s a drawing from Gessow and Meyers (Aerodynamics of the Helicopter), defining the many axes.
1144572
Viewed along the true axis of rotation, the tip plane asis, rotor blades no more flap than a rock being twirled on a string.
When the rotor hub is not permitted to be in alignment with the tip plane axis, then flap and drag hinges are a kinematic necessity, functioning as a universal joint.
 

C. Beaty

Gold Supporter
Years ago, Martin Hollmann, an aeronautical engineer and self proclaimed rotorcraft expert, used to stop by my office and argue about rotor dynamics. Martin was at the time employed by Martin Aircraft’s missile division in Orlando.

As a result of our arguments, I decided to build a hingless, floating hub rotor just to prove my point. It was a triangular aluminum plate with a blade grip/feathering bearing assembly attached to each apex. The hub plate was attached to the rotorhead by a rubber “U” joint made up from 3 Chevrolet engine mounts.

The first flight was during a 1970s Bensen Days flyin. Martin had gathered up an audience and was explaining to them how it was going to fling itself asunder from not havng flap/drag hinges just as I came motoring sedately past. That was related to me by someone present in Martin’s audience.
 

bryancobb

Junior Member
So there was obviously unequal lift between the advancing and retreating halves of your rotor. How was it compensated for? If it was not passively dealt with by flapping, or some other similar means, then you the pilot had to displace the cyclic in a direction that equalized lift.
 

C. Beaty

Gold Supporter
Bryan, a rotor no more flaps than does a rotating bicycle wheel when viewed from its real axis of rotation, the tip plane axis.

From Gessow and Meyers: “An observer riding on the control axis (swashplate axis) and rotating with the blades observes that the blades flap up and down each revolution but that they are fixed in pitch. At the same time an observer who sits in the plane of the tips, rotating with the blades, observes that the blades do not flap at all but do change their pitch-high, then low-once each revolution. The pitch is low on the advancing side of the rotor and high on the retreating side.”

The pitch change, advancing side vs retreating side in forward flight results from rotor “blowback,” the tilt of the tip plane axis relative to the swash plate axis. The pilot need not take action to keep the rotor balanced.

If the rotor hub is aligned with the tip plane axis, there is no need for flap/drag hinges.

Unfortunately, rotor theory as usually presented from the imaginary concept of ”flapping,” does more to confuse than to enlighten.
 

bryancobb

Junior Member
Ok... We have always agreed 100% then. The rotor mast axis and the tip path plane axis are rarely the same.
 

bryancobb

Junior Member
Here's another answer for Ken,

There's several "intermeshing-type" or synchropter helicopters, one of which is the Kaman K-Max. I am by no means a K-Max expert. I have been around ONE and questioned the pilot extensively about the rotor(s).

I'll address lead-lag dampers here. Take one of the rotors and study it alone. The K-Max has a 2-Blade rotor that's not rigid in plane. That means the blades can "hunt" or lead/lag. They are not free to hunt individually though. Each blade has a huge single mounting pin and the blade leads and lags around it. Each pair of blades has large, strong connecting rods connecting one blade's leading edge with the other blade's trailing edge. This connecting rod IS a shock absorber/dampener. When one blade leads, the other blade must do exactly the same thing, except it's dampened a little.

Not real sure why that action is needed but no designer would include it just for fun. 1144575
 
Last edited:
The only rotory-wing time I have is 120 hours in a McCulloch J-2 back in the mid-80s. I am happy to say that I purchased another one last year that is now undergoing an extensive annual before it flies again. I learned about articulated rotor heads from my original J-2. I also learned a lot about lead-lag dampers and ground resonance. The McCulloch flight manual and comprehensive maintenance manual cover the importance of maintaining the landing gear and struts on the J-2. I also had a great instructor. The gear has to absorb any "bump" to the rotor. Ground resonance is a definite possibility with the J-2, but in the 120 hours I flew mine back in the 80s, I never "dropped it in" or had a hard landing. The J-2 was a delight to fly and an even greater delight to land. Nevertheless, maintaining the integrity of the landing struts is paramount to happy landings. After many years of various fixed wings, I am very excited about flying a J-2 again.
 

bryancobb

Junior Member
Notice also that the K-Max rotor blades don't have pitch horns or pitch links. The flight controls are not connected to the blades is any way. The flight controls are connected to little "ailerons" on each blade bu a Teleflex cable and that little aileron's deflection creates an aerodynamic force that feathers the blades like the collective and cyclic does on other helicopters.1144582
 
Top