Bensen B-8 rotor "grips?"

Patrick'sB8

New member
Hello Everyone,

This is the first of many questions I'm sure! I just bought a 90%+ complete Bensen B-8M from a friend. He needed money for medical expenses and wouldn't take "charity" so sold off some items. He bought it from someone else who had the kit but never put it together. I don't know the history past that. My friend lost his health before finishing the project.

Aside from a horizontal tail surface, the only thing missing as far as I can tell are the "plates" that attach the rotor blades to the hub bar. I have found a drawing showing them on line, but no information. The drawing I'm referring to is below. The rotor head is from Bensen and has the factory sticker on it.

The gyro has come with new, in box, aluminium rotors, but I don't know how to attach them to the hub bar, or even what those plates are called. The drawing looked like there was some amount of pitch up as well. If I knew more about those, I would also strongly consider having them fabricated professionally just to be safe.

This is my first aircraft of any type. I'm in no hurry and of course will get training before ever trying to go up in the air. I'm going to take all the time I need to just get the thing together properly. As a side note, it also has a rebuilt Rotax 532. Also, I'm thinking of installing a seat tank. I would like to raise the CG up closer to the thrust line.

I truly appreciate any input and any support I can get! I've never flown before, despite being fascinated with gyros since I was little. I'm hoping this is an opportunity to not only help out my friend, but to finally realize my dream of getting in the air within a few years!

Thanks,
Patrick
 

Attachments

Gyro28866

David McCutchen
I think you may be referring to the blocks as plates. At each end of the hub bar, there is a pin sticking out about 3/8". This is a centering pin for the blades, as the blades have the corresponding hole for the pin. There are also 4 holes, 2 in the top blocks and 2 in the bottom blocks, that allow for 4 AN6 bolts to attack the blade to the hub bar. (bolts oriented horizontal). The top and bottom blocks are attached to the hub bar and blades also, DO NOT remove these bolts. They are factory installed and adjusted.
The 4 horizontal oriented attachment bolts will allow the blade to rotate on the pin to adjust the pitch of the blade in relation to the hub bar. 1 1/2 degree pitch works best on mine. That will give me approx. 375 Rotor rpm. pitch more and decrease rpm, decrease pitch and increase rpm. I found it easiest to build a set of (blade tracking bars) brackets that clamp to the hub bar and the blade and at approx. 40'ish inches out from the pivot - 1" of separation give me the 1 1/2* I wanted and I can adjust both blades to be the same. you want to get them really close to the same, so the blades tip paths are the same.
It has been 20 plus years ago, and I don't remember exacts. But, the tangent of the angle at a set distance will give you the hypotenuse length of that angle. I made my tracking bars 48" long, and I was able to dial the blades in really close.
 

Gyro28866

David McCutchen
I just took a closer look at the print you posted.
I have never seen a Bensen blade "strapped" to the hub bar. All of mine and all I have ever seen were bolted through 8 blocks total.
I guess this system would work, but you will have to string the blades and also adjust the pitch - each time you assemble the rotor.
But I don't know how you would adjust for pitch with this system.
Unless the pitch was machined into the hub bar attach point. Then you would only have to string the blades to align the rotational center to align over the center of the hub bar. This would be like a set of Dragon Wings for setup.
 

Patrick'sB8

New member
David,
Thanks for the great info. The Hub Bar I have isn't from Bensen. I honestly don't know where it came from. I think I've seen pictures on line of the type you're describing. With blocks on the end. I see from the end of my rotors the hole for the pin you describe. Here is a picture of the rotor end and the Hub Bar I have for clarity.
 

Attachments

Patrick'sB8

New member
I think I have a line on an actual Bensen hub bar. The one with the bolt blocks mentioned. However, on the other end of these blocks are more straight metal (with 3 bolt holes) bars like the end of my hub bar. It looks like no matter what, I'll need to drill holes in my blades and then bolt metal plates to them and then to the hub bar. That looks like what happens on the few pictures I can find on line. So, it seems that I either fab up flat metal plates to bolt to my rotors and the Bensen bar OR fab up plates with the 1.5 degree pitch built in and mount them to my existing bar. Does that make sense? What about the other end of the rotor? Do I need to make a "cap" for it? Both ends are exactly like the picture I posted. Also, that solid, airfoil shaped piece of aluminium just forward of the hole slides back and forth in the blade. Does that need to be secured somehow?

Sorry for all the questions. Pardon the pun, but I'm flying blind! :wink: I got this machine and although I can find some drawings on line, none are full plans and none show the rotors really.

Thanks!
 

Doug Riley

Platinum Member
Jeez, we're starting to lose the lore of the old Bensens that some of us geezers learned on. We have apples, oranges and kumquats here. So...

The print that Dar Dow posted is of the Bensen wood blades.and the Bensen spindle rotor head. The old "woodies" did indeed use steel straps, not pitch blocks. Bensen metal blades OTOH had the little rollpin pivot and pitch blocks. But...

Those blades in the photo aren't Bensen metal blades. They are extruded blades. They could, in theory, be mounted using EITHER straps or pitch blocks.

The adjustability of pitch blocks is a nice feature, but straps are simpler. Bensen also did not build in any coning angle (the upward dihedral you see in the straps in the wood-blades plans) into his metal blades. He allowed the hub bar to flex upward and called it good. This worked with his light, 22-foot blades. I think it's a suspect shortcut for heavier/longer blades. Score one for using straps and bending them into a coning angle of around 3 deg.
 

Patrick'sB8

New member
Thanks, that's good information. I need to figure out how thick of a plate to use and how to bolt them onto the rotor. I think I need to try and come up with some sort of end cap for the rotor as well. Both ends are open.
 

Patrick'sB8

New member
Thanks Doug! I just saw this. Actually, I was thinking about just having some straps fabricated with the 1.5 degree pitch built in.

The guy I bought this from also talked about bending the straight hub bar I have up by the 3 degrees and also adding the 1.5 pitch in at the same time. I would have a machine shop do that for me. Then, just have straight straps to attach to the rotors. I would also have them press in some stainless sleeves into holes in the hub bar for the bolts. I think that would add life to it and also facilitate unbolting the blades for transport. Any thoughts on that? I also need some sort of end cap for the rotors don't I? Both ends are exactly like the picture.

I so appreciate the help! Frankly, I like the idea of using the hub bar I have and building in the pitch and coning angle. Seems straightforward, simple and adjustment free.
 

Doug Riley

Platinum Member
Some makers do bend the hub bar up to "pre-cone" the rotor. If the hub already has holes drilled vertically through ts center (to attach the teeter block), then the bending must be very carefully done so that the bends DON'T occur right at the holes. Stretching the metal at the holes is asking for cracks to radiate from the holes.

To calculate the minimum safe strap thickness, first calculate the centrifugal load on the rotor at its highest possible RPM, then apply this load at some acceptable tensile loading for the strap material. Be sure to account for both the loss of cross-sectional area, and the stress concentration, caused by the holes. Use big margins of safety, for obvious reasons. Don't use stainless steel for straps (more on that below).

End caps improve the performance of hollow blades. Obviously, they must very securely attached. Having one of them come loose in flight would be particularly unpleasant. The end caps, if sealed, will be subject to both centrifugal effect acting the plugs' own mass, and pressure from the column of air inside the blade (the air inside also experiences centrifugal effect). Blade tips are often vented with a small hole for this reason.

Stainless steel in contact with aluminum is problematic. Stainless and aluminum react with each other pretty vigorously in the presence of electrolytes, such as salty water or acidic rain. Ask anyone who owns a sailboat with an aluminum mast and stainless fittings (or who has had a dog pee on his lawn furniture). Most of us don't use bushings in the hub holes. In fact, slightly oversize holes in the blade-to-hub joint are actually beneficial -- they allow you to "string" the blades upon assembly. "Stringing" is a procedure using -- surprise -- a string, stretched from one blade tip to the other. The purpose is the make sure that the blades are exactly 180 degrees opposite each other. A little play in the holes allows you to swing one blade tip or the other back and forth to achieve this goal.

If you're getting the feeling that you have to become an engineer to participate safely in this hobby -- you are nearly correct! There's no factory full of engineers watching out for your safety -- just "us guys and gals," figuring things out for ourselves.
 

Brian Jackson

Platinum Member
Doug Riley;n1144055 said:
… If you're getting the feeling that you have to become an engineer to participate safely in this hobby -- you are nearly correct! There's no factory full of engineers watching out for your safety -- just "us guys and gals," figuring things out for ourselves.
And that is precisely why I pay such close attention to posts and advice by you and a few others here. I hope you are doing well Doug.
 

kolibri282

Active member
Designing the hub bar and the connection for the load generated by the centrifugal blade force is fairly easy. The really hard part is to cover fatigue loads. This is why Dough's advice to include a really large margin of safety is so valuable. I actually don't like the connection of hub and blade via connection blocks because in engineering classes you are taught to never change the direction of forces to be transmitted if you can somehow avoid it. With that hub bar thing you change the direction of your forces four times by an angle of 90°. I know that the system has worked in the past but it still makes me feel very uncomfortable, especially since you have to consider very carefully how you want to support the compression loads of the connection block on the extruded blade.
 

Doug Riley

Platinum Member
K-282: Of course the Bensen grip-block design was intended for Bensen blades. They weren't extruded, they were built up from sheet metal and an extruded nose spar. However, in the area of the blocks, there was a solid metal filler block aft of the spar. No issues with torquing down the vertical bolts. Still, Bensen called out a torque for the vertical bolts of somewhere in the low 20's ft.-lb. The horizontal bolts (the ones that bore he centrifugal loads) IIR were to be torqued to 25 ft.-lb.

I don't quite follow your comment about making forces turn corners. It's true that forces act only in straight lines; making a force "change direction" (as in, say, a Teleflex cable) involves a bunch of reactions, friction and bending.

But in the Bensen hub, nothing quite like that is happening. The forces from the blade's centrifugal reaction, plus its lift, resolve into a mostly-outward, slightly-upward, straight-line load. The bolts and blocks are arrayed nearly symmetrically about the line of this force. The vertical bolts are loaded in shear, and the horizontal bolts are loaded in tension -- both valid loading directions for a bolt. Many helos use a similar setup, though the grip blocks (or flanges) are generally round rather than rectangular.

IMHO, one valid criticism of the Bensen hub is that it accommodates the coning angle by simply flexing upward into a slight dihedral during flight. To make matters worse, there are two 5/16 holes drilled vertically through the middle to secure the teeter block -- right where the bar is subject to tension from the upward bending). Aluminum alloy has such relatively lousy fatigue qualities that ideally we don't want to tempt it to crack by flexing it in this way through an area already weakened with holes. Still, I don't know of any Bensen hub ever cracking.

Bensen was very well-calibrated at predicting where he could simplify and get away with it on a small-light gyro. Many of his "simplicity hacks" don't work on heavier machines.
 
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