Help - Rotor Head Design - Double or Single Bearing and undersling?

magilla

Gyro Project Owner
Joined
Dec 17, 2005
Messages
546
Location
Little Rock, AR
Aircraft
UH-1H / UH-60A
Total Flight Time
2000+ RW
Can anyone help me with the math (Chuck Beatty??)

850 lb all-up GWT gyro.
27' DWs (2 ea 12-foot blades and 3' hub bar)
Modified RFD head (Single bearing head)

What is correct undersling for thius setup?
What type of bearing do I need? Single or double?

Short Story - Called Ernie at RFD and sent him some pics of my Rotor Head, as the anti-flap bushings he sent me did not fit (too short).

He looked at the pics and said that the head was a modified single bearing RFD head, and that the all-up GWT coupled with 27' rotors teetering at the top hole in the teeter towers and teeter block would create too much of a load (can't remember what type), and that the head could suffer catastrophic failure withing 250-300 hours.

Yikes!!!!!! :twitch:

Well, needless to say, he got my attention.

This thread is not meant in any way to challenge Ernie's hypothesis - I fully believe him, but I just need more info before I buy a new RFD double bearing rotor head.

So, can any of you math geniuses help me with the numbers?

recommended undersling?
Double or single bearing load limits?

FYI, the top teeter hole is 8.75 inches up from the ring gear in the pics below.
 

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Easy,

1. Where is the balance point of the rotorblade with half the hub bar? (from butt end)

2. Is it a 3 degrees coning angle?

Tell me this, and I'll tell you the correct teeter height.
 
1. Dennis, I don't understand first question... Can you rephrase it so that I know what actions to take to get the measurement you are looking for? (I don't get how to use half a hub bar, and don'tknow which end is butt end))
2. 3 degree coning angle, I believe (it is a stock DW hub bar)
 
1. Dennis, I don't understand first question... Can you rephrase it so that I know what actions to take to get the measurement you are looking for? (I don't get how to use half a hub bar, and don'tknow which end is butt end))
2. 3 degree coning angle, I believe (it is a stock DW hub bar)

When I go back to work tomorrow I'll make a drawing and send you.
 
Your rotor, Spencer, will turn ~347 rpm and ideal undersling is 4.95”. Undersling is the height above the projected apex of the blade centelines. A bit less in practice because the top of the hub bar is somewhat higher than the apex.

If the load on the bearing was pure thrust, there is more than ample bearing capacity. However, in forward flight with the rotor disc tilted aft of the of the rotorhead axis by as much as 5º, the moment load (overturning load) is getting near to the bearing capacity and doesn’t leave a safety factor.

That said, the RAF-2000 has flown many hours on single bearing heads with extended towers without a reported failure. Reputable bearing manufacturers are very conservative with load ratings.

*************

First time around I made a mistake that I've corrected; the spreadsheet was set for 8.5" chord rather than the 7" chord of DWs.
*************
Correction #2. DWs don't weigh 100 lbs.
 
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Alan, I’m embarrassed; you caught me with my head stuck some place where the sun doesn’t shine.

Your spread sheet numbers are correct. I didn’t look at the spread sheet numbers carefully and had some old entries that blew the results. Now, if DWs weighed 100 lbs…….
 
OK Spencer, here is the easy way of doing the job that gets you close enough;

1. I don't know where the Center of Mass is on the Dragonwings blade. I believe it would be worth while to check, because that is the root of everything. I provided a drawing on how to check. The simple way is to take a slab of aluminum half the length of your hub bar and stick it on the butt of your blade as shown. Give me the dimension of where the balance point is from the center of rotation, and I can then tell you what your teeter height should be.

2. What we need to do is determine what your Tower Height needs to be. To do this, I'll need to know where the 3 degrees bend starts on your hub bar from the Center of Rotation, just like the next drawing shows.

3. I will also need to know the width of the Teeter Stop Plate, as shown in the drawing.

After you provide me this information, I can tell you what you need. If the teeter height of what you have is not the same as I calculated, I can tell you if it's close enough, and if it is, I can tell you what the tower height needs to be.

Of course, this is based on if you are turning between 340 and 350 RPM to have the proper coning angle, based on the aircrafts average gross weight you provided.
 

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OK,

So the teeter bolt should go through the teeter block in a hole 4.95" up from the center of the hub bar, which is 1" thick, right? So basically, the top hole in the teeter block should be 4.45" up from where the teeter block mates to the hub bar.

Is my assumption correct?

Next question(s):

How far up should the holes be on the teeter towers?

How much should your blades flap, degree-wise?

And Chuck, as well as everyone else - I really appreciate the help.

Any way to get that Excel program? Would love to see it with my figures in there.

Last question - so a double bearing rotor head is then the correct recommendation, and a prudent course of action for my set up? I'm just wondering how necessary getting a double bearing head is at this moment in time, as I am just about to get it airborne....
 
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Pics of my gyro

Pics of my gyro

By the way, here are some pics of the Jungle Gym at the PRA Chapter 6 Fly-In on June 13th and at home doing some engine runs.

I was really close to getting out there and doing some spin ups and taxiing up and down the runway, but this news from Ernie kind of devastated me...
 

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OK, so I just measured my hub bar, and the top hole is 5.25" from the center of the hub bar, and the next hole down is 4.25."

WHERE DO YOU MEASURE UNDERSLING FROM - TOP OF HUB BAR, BOTTOM, OR CENTER TO CENTER OF TEETER BOLT HOLE?

If the nominal undersling is 4.95 inches, is 5.25 inches too far out? Should I shave off .3 inches from the teeter block, or add a .7 inch spacer and go to the second hole?

Does it matter which hole should I suspend the teeter bolt from in the teeter towers?
 
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Dennis, I cannot get you the info on the blade COG, because I don't have any aluminum blocks laying around, and don't know where to get any. However, the hub bar is 3 foot long, the blades are each 12 foot. Couldn't you figure COG of blades 6 foot from the tip, either side, and add to 1/2 the hub distance?

As to other distances, as depicted in your sheet:

Teeter height: top hole 5.75", another hole at 4.75" (measured from bottom of hub bar to center of teeter bolt hole)
Tower height: 6.1875" (or 6 and 3/16") (measured from top of stop block to center of tower bolt hole)
stop distance: 5.625" (or 5 and 5/8")
3 degree bend starts: 2.5" from center of teeter block.

Do these dimensions help? Thanks for the chart.
 
Dennis, I cannot get you the info on the blade COG, because I don't have any aluminum blocks laying around, and don't know where to get any. However, the hub bar is 3 foot long, the blades are each 12 foot. Couldn't you figure COG of blades 6 foot from the tip, either side, and add to 1/2 the hub distance?

As to other distances, as depicted in your sheet:

Teeter height: top hole 5.75", another hole at 4.75" (measured from bottom of hub bar to center of teeter bolt hole)
Tower height: 6.1875" (or 6 and 3/16") (measured from top of stop block to center of tower bolt hole)
stop distance: 5.625" (or 5 and 5/8")
3 degree bend starts: 2.5" from center of teeter block.

Do these dimensions help? Thanks for the chart.

Without knowing the exact CoM it's all guessing. I don't know the weight of your attachment hardware. But, giving it the old wild-a$$ guess, I would say your CoM could be about 6' out from the Center of Rotation. If so, then you would need a teeter height of 4.250" to the bottom of the hub bar.

If we go with the teeter hight the chart provided, then that would put the CoM out to 7'-10" from Center of Rotation. Now, how can that be anywhere correct? It is only 13'-6" from the Center of Rotation to the tip of the blade! Half way point of that is only 6'-9", so unless you have some mighty bodacious tip weights of depleted uranium on there, there is no way the outer half on the blade is going to be heavier than the inner half of the blade with 1/2 hub bar and blade mounting hardware.

If I'm closer to being correct that the CoM is 6' out, then your 4.75" teeter height may be good, or close. If you use that setting, then your teeter hole must be 5.187" up from the top of the stop block, and that will give you a 8 degree teeter, like you need.

But, if you want to get it really right, I need that CoM.

P.s> The single bearing is OK, don't change it.
 
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Spencer, the undersling is calculated from the center of the teeter bolt to the point where straight lines running along the blade centers would intersect (the vertex in geometry). Ernie locates the hub bends pretty close in so hub center is close enough for government work.

I wouldn’t machine anything off the hub block just yet; you might have to do some fine-tuning. Sometimes you’ll get a better ride with undersling a bit lower than on the coned rotor’s CG. Apparently, lower undersling produces a 2/rev force that is out of phase with the aerodynamic “hits”. I hate diddle tuning but it is sometimes unavoidable.

Whether undersling is touchy or not depends to a large degree on mast stiffness. With curved members in your rotor pylon, it’s unlikely to be totally rigid.

Coning angle works out to be 3.69º at 347 RPM on your rotor. Ernie sometimes uses a bit less pitch on the longer rotors to speed them up and reduce coning angle. Another reason not to finalize your hub block until you learn the true RPM of the rotor.
 
Again, thanks toboth of you for your answers.

I just need to be more detailed in the questions I am asking.

I understand the definition of undersling, and how it is calculated. My real question is this:

"If I am to measure where the teeter bolt hole is on my teeter block, where do I make the measurements so I can give you the correct undersling on my system?"

I don't want to make adjustments to my teeter block,especially if we are talking different coordinates...

I measured the 5.25" from the center of the span-wise thickness of the hub to the center of the teeter bolt hole. It would be 4.75" measured to the top of the hub bar, and 5.75" measured to the bottom of the hub bar.

How far is too far off on undersling? .5" either way OK?

And as I see it, the recommendations for the single bearing vs.double bearing head are this:

One says single bearing head is OK (Dennis), another says it is at the limit but that RAF single bearing heads have flown for years without failures, and that bearing manufacturers are notoriously conservative - which I take to mean as: It's my call; and I have another that says change it (Ernie).

Without knowing the strength of the bearing itself, and the fact that I have only flown helicopters, my limited experience with gyros is forcing me to think conservatively.

That being said, my mind is still open. How much force can an 850lb AUW gyro put on a single bearing head with thrust 5 degrees aft?
 
Magilla, nice to see your project nearing fruition. Congratulations on all your work.

Having arrived at a similar place with ours these questions of yours and the help you are receiving is very interesting.

Am enjoying listening to you sorting your problems out, slip-streaming and picking up some good information from the forums senior members.
 
I can only tell you this --I have been flying a 850 #gyro for over 600hrs with a single bearing rotorhead with no problems .--- I bought the rotorhead from ERNIE -I told him how much my gyro weighed and this is what he sold me--
 
Your rotor, Spencer, will turn ~347 rpm and ideal undersling is 4.95”. .

Your calculation assumes that the blades are straight, right? But can we assume this? The resultant of lift on a blade is further along the radius that the resultant centrifugal. The blade is flexible, it certainly deflects (1° ?). If 3 ° cone in the center, then 4 ° cone at the end ? So, Undersling ideal calculated is not it too little?
 
For a twisted blade, the mean pitch is that at 0.7 radius. Using the pitch at that station will produce the same results as would be the case for an untwisted blade for rpm, cyclic flapping, etc.

The load distribution along the blade span changes between twisted blades and untwisted but it’s mostly second order. DWs have ~ 5º twist.
 
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