3 Blade Rotor system?

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May 11, 2005
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Burlington Wi
Hi Guys

At Oshkosh I was talking to A helicopter designer / Builder about 3 Blade rotor systems.He was going to try a 3 blade system with no lead lag dampers.
I was an underslung rotor system just like a 2 blade system. but with 3 Blades and double gimbaled (teetering and rocking ) Has there ever been a rotor system like this produced.would it work? The reason I bring this up is I was searching A French helicopter news group and ran into a video Of A 3 blade rotor system on a helicopter and gyro that our very own Chuck Beaty designed and built. Chuck how did the system work.Got any Pictures. this is the kind if stuff that we are dying to see.:flame:

Doug S
 

C. Beaty

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What you’re referring to, Doug, is sometimes called a “floating hub” rotor. If the hub is allowed to self align with the rotor tip plane axis, there is no need for flap or drag hinges. There is a bit of in-plane motion due to drag variations in forward flight so the stress on blade attachments needs to be carefully surveyed with strain gauges.

I think the first helicopter to employ a floating hub was the Austrian Doblhof tip jet helicopter of the 1940s. Doblhof connected the rotor to the airframe via a spherical roller bearing since there was no shaft torque to be accommodated.

The Doman helicopter of the late 1940s was a shaft driven helicopter that was apparently conceived without prior knowledge of the Doblhof machine.

I’ve left the attached picture in large size to better show detail.
 

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Joined
May 11, 2005
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Burlington Wi
3 Blade rotor system

3 Blade rotor system

Hi Chuck

I did see the same drawing in my search on rotor heads with no dampers. The one pictured has no undersling to it.is there any benifit to an undersling?
The bottom line is I would like to build a 3 blade rotor system for an experimental helicopter that has has no lead lag dampers or any other un obtainable materials like elastomerics or flexable hubs. what I am trying to get away from is using a suspension type landing gear to prevent ground resonance.
At Oshkosh Joe Rinke had his Jag helicopter there with a multiblade rotor system on it he was using an elastomeric lead lag damper. not an oil filled damper.He said the damper had basicly rubber bushings in them nothing else.
would that type of damper work or would ground resnonce still be a problem.
3 blade rotor systems have always a mystery to me on how they work but very interesting.with all you know about helicopters what would you use.

Doug S
 

C. Beaty

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Ground resonance is a complex phenomenon upon mathematical analysis. With blades that can get out of pattern, a stirring motion is produced at the rotorhead and when this stirring motion excites the resonance of the machine on its undercarriage, ground resonance can occur if the in-plane frequency of the blades also falls at that same frequency.

Lead-lag dampers can be eliminated by making sure the in-plane resonance of the blades about their lead-lag hinges falls at about 60% of the rotational speed. This is done on the Brantly by locating the drag hinges several feet outboard of hub center. The inner blades look like 50 cal machine gun barrels with plastic fairings. There are rubber bumpers that restrict freedom about the lead-lag hinges but they are in no sense dampers.

Some helicopters with composite blades eliminate drag hinges by making the inner blade sections flexible enough in the lead-lag direction to tune them to about 60% of rotational rpm.

Dick DeGraw is very fond of Honda motorcycle bushings and uses them to tune the frequency of his blades. Here again, they are not dampers but springs.

All such machines, commonly called soft in-plane rotors, require hydraulically damped undercarriages.

And there actually are rubber or elastomeric dampers. They use a lossy, gummy rubber with many of the characteristics of chewing gum and get hot enough to require cooling fins. The are used on the Apache as well as other helicopters.

My 3-blade rotors were all “stiff in-plane” systems that ought not to not be flown higher than the pilot is prepared to fall until surveyed with strain gauges.

The rotorhead below was from a gyro. The rubber shock bushings would bottom out at helicopter torque levels.
 

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RHerron

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Nov 17, 2003
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Mayflower, Ark.
Elastomeric dampers

Elastomeric dampers

Chuck,

As you probably know, the Hughes 300's use elastomeric dampers. There was a time when I could have/should have gathered a few of them from salvage.
I do have a couple of sets of the hydraulic type that are easy enough to maintain. It is very simple to adjust their stiffness.

Ron
 

Elusive

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Feb 28, 2007
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Sofia
Aerokopter (http://www.copter.com.ua/EN/products_ak13.php) has a rotor hub system based on spring straps. According to my humble opinion the advantages of such rotor systems are: simplicity, maintenance-free and easy to build. Disadvantages are: complexity of the rotor blade (different NACA profiles at the root and at the end, the rotor blade is twisted root/end - so it is hard to build), 500h lifetime of the spring straps.
There were plenty of pictures posted on Yahoo helicopter forum(straps, blades manufacturing etc.), but unfortunately I couldn't find them now.

more info can be found here.
http://www.mscsoftware.com/support/library/conf/auc97/p02297.pdf#search="lamin\ated blade stack"


all the best
Miro Arsov
 
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