Brantly's Lead/Lag Dampers

[bryancobb]

I have an idea that some "tribal-knowledge" about the Lord elastomeric dampers on the 2-piece blades of Brantly helicopters may be lost forever if that topic is not discussed in a public forum. I owned and flew a B-2b from 2000-2007 and my dampers were in bad shape. Lord could not justify making any new ones since there was only a few hundred flying Brantlys IN THE WORLD so each set would very, very expensive (several thousand dollars at least). Brantly owners would be left to fly their aircraft with decades-old, marginally serviceable rubbers.

Although I sold my ship before my research was complete, I did collect a fair amount of data and learned about how they were made. After that I worked as a manufacturing engineer for seven years for a major aerospace manufacturer that produced ballistic self-sealing polymeric fuel cells that had multiple aluminum and titanium flanges and mountings BONDED and cured into the bladders and MUST NOT LEAK. This gave me a strong understanding, through intimate, daily familiarity and experience with attaining strong, permanent bonds between rubbers and metals on flight critical parts. This is the precise description of the dampers on Brantly rotor blades.

I will try to display here, all things that I learned about these dampers, just for documentation purposes.

 

[bryancobb]

I held my old dampers in my hands and studied them for several days. I measured them with calipers and completed AutoCAD drawings. I downloaded the factory Service Bulletin/Service Instruction. I can't find it right now because I have no idea which hard drive to look for it on.
I took notes and poked and prodded at these things and discussed them with the factory folks in Vernon, TX.
It's been 15 years so memory has dimmed but I hope things come back to me.
Here's some general comments about them.
* They were light and the metal parts were Aluminum, probably 2024 T3. I never instrument-measured the tan rubber's durometer but from what I remember about my 1970's Kryptonics and G&S Wonkers skateboard wheels, the damper rubber was soft, probably in the 60-range. Again, my rubber was 50 years old so who knows how hard it was when new. In one of the pictures below, a supposed "new" set of black dampers was obtained from the guy who had became the "go-to parts man," and the owner of the helicopter said they flew awful because they were much too soft, and scared him to death.
* N.O. Brantly chose to surround the damper with a thick aluminum "canister" and slightly compress it and keep it that way with a snap ring. On a few occasions, I removed mine from the canisters for cleaning and inspection and in putting them back in, I had to use a vise to compress them approximately 0.125" to get the snap rings back in the groove. Because of this, they had a self-centering/neutral finding characteristic to them. They had no oil or hydraulic damping. There was no "notch" or detent feel when it reached neutral but neutral was there.
* The canister kept the diameter of the rubber confined and constant during compression cycles. I don't think its design ever made it be in tension but different "zones" get compressed depending on whether the blade was leading or lagging.

* Now I'll post several pictures that I'll discuss later. I'm running out of time right now.

 

[DennisFetters]

Bryan, this is an extremely valuable thread, and thank you for recording this early technology for history. I have done much research to learn about the ingenious-yet-simple/durable Brantly dampers and can find NOTHING! A 75-year-old technology that lasted this long and still works, what an achievement.

The pictures you posted here I have seen from you in some very old threads. I can see how they attached to the inner hub bar, however, I can't wrap my mind around how they fully worked, with the respect to how they were attached to the blades to allow feathering.

Also, the orange color of the rubber looks like silicon rubber?

In picture "damp1.jpg", are they broken or come as two parts?

I sure wish there was a 3D rendering of the hub, damper, and blade somewhere so I could get the full understanding of it as a system.

Thanks again for this most valuable information.

 

[bryancobb]

Keep in mind, Dennis. The Brantly blade is a limp spaghetti-noodle. It has a "flap-and-feather-only" hinge at the root/hub but has no damper there. The inner (40%) blades are very thick airfoil (close to 4") and hollow and built like the wing of a Cessna 150 (thin aluminum riveted to ribs and a 2.5" diameter aluminum tube). That very light "wing" feathers around a VERY HEAVY, solid steel axle that is over 1.5" in diameter. That "pylon shaft" is internally threaded at its tip. A super-precision "Outer Pylon Bearing Shaft" (see picture below) screws in those close-tolerance internal threads and acts as the mount for the 4-pack of angular contact feathering bearings . This OPBS has a short life limit (1200 if I remember right). The Outer Pylon Bearing Shaft Nut retains the four bearings. This is all at 40% span and the dampers are there at about the 40% span station too. That joint is called the "universal joint." From here outward, the outer 60% of the blades can flap and hunt but it cannot feather by itself. The inner 40% and outer 60% are held together at the same angle of attack by the universal joint and always feather together "as a unit."

 

[bryancobb]

Bryan, this is an extremely valuable thread, and thank you for recording this early technology for history. I have done much research to learn about the ingenious-yet-simple/durable Brantly dampers and can find NOTHING! A 75-year-old technology that lasted this long and still works, what an achievement.

The pictures you posted here I have seen from you in some very old threads. I can see how they attached to the inner hub bar, however, I can't wrap my mind around how they fully worked, with the respect to how they were attached to the blades to allow feathering.

Also, the orange color of the rubber looks like silicon rubber?

In picture "damp1.jpg", are they broken or come as two parts?

I sure wish there was a 3D rendering of the hub, damper, and blade somewhere so I could get the full understanding of it as a system.

Thanks again for this most valuable information.

* I never saw the factory documents so the material is unidentified, but I suspect it was raw, uncured neoprene that was autoclave cured ONTO THE EMBEDDED ALUMINUM parts under 90 PSI at 250-350 DegF for several hours (REMEMBER, the black one shown in my first post was modern SILICONE and it didn't work very well). This is when the crosslinking of the rubber and metal happens, forming a very strong mechanical bond between them. The metal gets media-blasted and prepped chemically with alkaline cleaner and adhesion primer to enhance the strength.
* damp1.jpg does show a FAILED damper. Presumably it failed because of is 50 year old age. They are 1-piece items as new.
The midpoint of the damper has failed. Since the damper can't be stretched inside its aluminum barrel where it's held captive in a compressed state, the only explanation for failure would be age or inadequate bond prep or expired material or poor bagging/autoclave process when manufactured. A greasy fingerprint left by the builder during assembly would compromise the bond. Lord has been THE DAMPER GENIUSES since before WWII, so I'd suspect their process/quality in the 1960's was impeccable.
* I did draw everything in 2-d while I had the parts in my hands but never did a 3-d model. Stay tuned.

 

[bryancobb]

Here is the universal joint at 40% and the inner retention end of the outer blade showing its lead/lag bearing and piece of rubber tubing that limits the amount of max lead and max lag allowed.

 

[Brent Drake]

I've had several of these. There is a guy that was currently making them. I know Gary was selling them for $800 each, These were made out of a higher grade plastic than originals. I'm thinking of a type of silicone possibly. They worked much better than the originals.