Hub Bars with coning angles VS. flat

Brian Jackson

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Jul 17, 2004
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Hamburg, New Jersey USA
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GyroBee Variant - Under Construction
For those with a better understanding of loading and forces involved, I'm curious about hub bars with coning angles built in vs. straight (flat) hub bars. I know the blades normally assume approximately this angle in flight, and it would make sense to mount them that way. But regarding bending stresses on the hub bar itself, isn't there almost as much bending load on a coned hub bar as on a flat one? And if that is the case, is the hub's coning angle there to serve another function, like raising the blades to coordinate with teeter bolt height?

It's funny how I'd not considered this until recently having just bought my first set of blades which came with a flat hub bar. and short teeter block. There is discussion in another thread but didn't want to veer off topic with an academic question.
 
Brian, the blade-hub assembly is limber enough that the blades assume an angle dictated by the (vector) sum of the centrifugal reaction and aerodynamic forces. The coning angle isn't (sometimes) built into the hub to raise the blades to the height of the teeter bolt; on the contrary, the blades are going to cone no matter what, and so we locate the teeter bolt to to correspond with the location of the rotor's center of mass once this coning happens.

Igor Bensen was a genius at determining what simplifications and informalities he could get away with in a light gyro. Among these was using an un-pre-coned bub bar on his metal blades (he did build pre-coning into his wood blades). As a result, the metal-blade hub simply bent upward once the blades were loaded. Of course, this stretched the bottom layers of metal in the hub in tension -- and there two 5/16" holes right there in the middle of the hub. Yet I don't know of any ever cracking.

When you scale up a Bensen, these Bensen informalities may no longer be safe. Larger and heavier gyros need pre-coning, IMHO. Skipping it is unwise.

To your question -- there isn't much bending load at all on a properly pre-coned rotor hub. Sketch up a vector diagram, and you'll see that the centrifugal reaction pulls outward, parallel to the tip-path plane. The blades' aerodynamic forces pull upward, roughly perpendicular to the top and bottom surfaces of the blades. The vector sum of these forces is a force parallel to the long axis of the blade, once the blade assumes its coning angle. As long as the hub is built with this same angle as a dihedral, the hub also will largely be in tension in steady flight.

There will be bending loads with sudden changes in disk angle of attack, cyclic pitch changes and similar disturbances.
 
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In my opinion the hub bar acts a spring that needs to respond to changes in coning angle caused by changes in load and rotor rpm.

The Predator appears to me to have a flat hub bar.

As can be seen in the first picture when the rotor rpm does not match the load the coning is evident.

The second picture is more steady state with much less coning.

I would trust the manufacture to understand their system and I would follow their instructions.
 

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Both Vance and Doug got that.
I love my Skywheels because they have a flex hub and react as need.
 
Brian I have noticed that the Gyro-Tech blades tend to run quite a bit flatter than the two blades I am familiar with,RAF/Sportcoptor,the
teeter bolt hole on my hub tower is really low as compared to the other blades.
But the cone angle (dihedral) is still enough that the stability is still very good and the response time is good
considering its a very heavy machine.With your light weight the flat bar should be fine.
 
Doug Riley;n1136751 said:
Brian, the blade-hub assembly is limber enough that the blades assume an angle dictated by the (vector) sum of the centrifugal reaction and aerodynamic forces. The coning angle isn't (sometimes) built into the hub to raise the blades to the height of the teeter bolt; on the contrary, the blades are going to cone no matter what, and so we locate the teeter bolt to to correspond with the location of the rotor's center of mass once this coning happens.

Igor Bensen was a genius at determining what simplifications and informalities he could get away with in a light gyro. Among these was using an un-pre-coned bub bar on his metal blades (he did build pre-coning into his wood blades). As a result, the metal-blade hub simply bent upward once the blades were loaded. Of course, this stretched the bottom layers of metal in the hub in tension -- and there two 5/16" holes right there in the middle of the hub. Yet I don't know of any ever cracking.

When you scale up a Bensen, these Bensen informalities may no longer be safe. Larger and heavier gyros need pre-coning, IMHO. Skipping it is unwise.

To your question -- there isn't much bending load at all on a properly pre-coned rotor hub. Sketch up a vector diagram, and you'll see that the centrifugal reaction pulls outward, parallel to the tip-path plane. The blades' aerodynamic forces pull upward, roughly perpendicular to the top and bottom surfaces of the blades. The vector sum of these forces is a force parallel to the long axis of the blade, once the blade assumes its coning angle. As long as the hub is built with this same angle as a dihedral, the hub also will largely be in tension in steady flight.

There will be bending loads with sudden changes in disk angle of attack, cyclic pitch changes and similar disturbances.

Doug,

That was exactly what I hoped to learn. Thank you for the extended reply and physics lesson. I wasn't giving the centrifugal element its proper due. Got any thoughts on dark matter while we're at it? :)
 
Never build a hub bar out of dark matter, regardless of the coning angle. You won't be able to see it during pre-flight inspection.
 
And having dark matter that close to your head will allow the ailiens to read your mind,I always use a tinfoil

shield on my head. And the other down side is that its really heavy.
 
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I think to fly Gyro's you have to be just a tad off bubble to start with.

I mean think about it, we have actually tried to have a conversation with the Kolibri !
 
eddie;n1136830 said:
I think to fly Gyro's you have to be just a tad off bubble to start with.

I mean think about it, we have actually tried to have a conversation with the Kolibri !

I tried to give you like 564,738,234 likes but it will only allow 1
 
Jake I am not here to be liked,I am here argue about absolutely nothing important,and to really piss off just about
everybody who will listen to me.
Besides that those" likes" are really heavy,and they make your head swell so large you can't get you hat on.
I prefer a "atta boy Bubba"
 
eddie;n1136838 said:
Jake I am not here to be liked,I am here argue about absolutely nothing important,and to really piss off just about
everybody who will listen to me.
Besides that those" likes" are really heavy,and they make your head swell so large you can't get you hat on.
I prefer a "atta boy Bubba"

"atta boy Bubba"
 
Doug Riley;n1136940 said:
Dark matter heats up in the sun.

Not sure how that would be so; It is undetectable by conventional observation including heat signature, only postulated by our inability to explain why galactic rotation doesn't exhibit the slower revolution of outermost stars via classical Newtonian physics. Something's keeping them from flying off into space at those speeds.
 
Brian its because the aliens have planted infalulated convoluted dymanic extrabuluators on those planets

the aliens can be seen if when you are flying you have on polarized sunglass's,oh don't forget to wear your

tinfoil reflectors on your head.

and one final thing, people think the earth is round but as any good well educated gyro-pilot knows its flat.
 
Hey Tim your right sometimes it seems be uphill all the time.
 
I received a reply from Gyro-Tech. Here is the portion pertaining to this thread:

The old type of hub bar is straight (without a cone angle) but it has no influence on installation on a rotor head. There is a different type of fixing the blades because the old blades have a different root and a different location of mounting holes. Also, in the old hub bar there are sliding sleeves whereas in the new block there are 4 bearings installed.
 
Brian I would think that the bushings instead of bearings will work just fine as there is less weight

on them than a two seater and there probably is more bearing surface than ones installed in the

teeter towers themselves.
 
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