Underslung system

August 15, 2007

Can anyone help me understand the under slung system, I know it is there to correct the effects of Coriolis effect however I don't really understand what the difference between the teetering hinge and the under slung system is. Any help is appreciated.

August 15, 2007

See the teetering hinge bolt in the top middle? The blade bolts are Under the teetering bolt. So when the hub tilts, it still keeps the blades out far enough that coriolis doesn't come into effect. Even when the hub is tilted all the way to the blade stop pad on the mast, the blade bolt will never rise above the teetering hinge bolts plane. So center of mass doesn't move in with power on.

Now when in auto, the blades bend up and the coning hinge together allows the rpm to spin up due to center of mass moving in closer to the rotor mast axis, because wind is driving the blades not the engine.

My 1st cfi made a paper model hub to prove it to me in the class room.

Disclamer, I am not a CFI, I tried to use the same words when it was described to me, but I might have used different words. lol... I am sure one of the more senior pilot cfi's will correct me..

August 17, 2007

Disclamer, I am not a CFI, I tried to use the same words when it was described to me, but I might have used different words. lol... I am sure one of the more senior pilot cfi's will correct me..

Partially correct.

The center bolt is indeed the teetering hinge. the two other bolts are the blade bolts, but they also allow for coning. Robinson choose to allow for a "coning hinge" to reduce stress on the blades and make them lighter.

Now, coning happens any time the blades are producing lift, not just in auto-rotation. The increase in RPM during auto-rotation is mostly caused by in increase in the size of the driving region of the blades during auto.

Now, back to "how" the underslung system works. If the blades were to flap from the hub and up, or the hub and down (not under slung) they would experience larger lead and lag forces as their center of mass moved closer/farther to the center of rotation. However, since they are underslung, the center of mass tends to stay closer to the plane of rotation because the hub is underslung and the blades cone up. In addition, when the blades flap as a unit, the underslung hub tends to push the up flapping blade out, and draw the down flapping blade in. This results in a smaller change in the center of mass.

Do a few quick searches online on "semi-rigid rotor systems" and you should be able to find a bit more info.

August 17, 2007

Looking back on my post I shoulda said, in auto they cone more than when driven by the engine or in powered flight the blades cone less due to centrifugal forces.. The blades of a rigid in plane system bend less, like an articulated system, than a semi rigid system. Part of what makes the rigid in plane system is the blades flap independent vs. a traditional semi rigid system where they flap as a unit with out the coning hinges. In all that I have read about rotor heads, no one book really gives mention to the difference between a rigid inplane vs. semi rigid. I don't know about anyone else, but the difference wasn't so obvious at first, even after the CFI went over all the different types of heads with me. The FAA/NTSB put a footnote in the R22 Accident report, that 500E posted, describing the difference in the rotorheads. See page 7, main rotor system, foot note 9. Seems RHC was trying to get some of both worlds with this head design.

NTSB report

My personnal feeling about the coning hinge, is that this part of the rotor head is what makes it more dangerous in a gusty wind conditions than a traditional semi rigid system and allows the blades to be so light and handle the stress, thus saving weight. In my mind I can see the blade flap wildly in those type of conditions, am I wrong in that statement??

The increase in RPM during auto-rotation is mostly caused by in increase in the size of the driving region of the blades during auto.

I know that is a text book answer, but in regards to KISS (keep it simple silly)hehehee, the reality is that the attitude of the aircraft changes in auto to such and airflow changes direction that the WIND is contacting more of the blade face which creates a larger driven region?? Wind direction is the biggest factor. The rotor head went from being a propeller to being a windmill.

Thanks

P.S. The model my CFI made is the Ill. on page 3-3 in the FAA Rotorcraft Handbook Fig. 3-6.

August 20, 2007

The answers above all dance around the topic, but don't quite hit the nail, because underslinging isn't there for normal coning.

Look at the hub. It "hangs" from the top bolt, and the head can swing underneath it.

Now we will look at forward movement, where the back blade is up and the front blade is down.

In a non-underslung head, if the back blade were to flap up, the conservation of angular momentum means that its apparent shorter radius makes it want to spin faster (skater pulling arms in while spinning). The front blade is apparently longer, making it want to slow down. The end result is a lot of stress on the head and blade grips, with both blades trying to be on the same side of the rotor head at once.

But with the underslinging, when this back blade moves up, you can see that the whole head swings backwards under the top bolt. This "extends" the back blade and "retracts" the front blade, making the apparent radius of the system almost the same, minimising any effects of Conservation of Angular Momentum, and reducing grip stresses.

August 21, 2007

To all intents and purposes the Teetering hinge, Underslung System, and Semi Rigid Rotor Head are all the same thing!!! Just different names!

How to confuse the poor guy. Sorry mechanic, you lost me there!

Eric is spot on!

Purple words? For really simple explanation, try reading only the words in purple!!! A little expriment of mine!

Underslung / Teetering / Semi-Rigid Heads

Simple: As one blade advances into wind it cones upwards, the center of mass moves inwards and it would normally speed up. See the diagram below. This is Coriolis at work (the ice skater's arms).

HOWEVER, the teetering head is shown below. Notice how the whole system is hung on the bolt marked as the 'flapping axis'.

(Just like if you made a V between finger and thumb on one hand, and teetered that V on an upward pointing finger of the other hand)

So as the blade (attached to your fingernail) flaps upwards, the head moves outwards too (black dot in diagram below).

The upflapping does one thing (moves center of mass in), and the outward moving teetering head does the opposite (moves center of mass out)!

Net Coriolis is nil. No lead or lag there.

Of course, if both blades flap upwards the same (maybe due to heavy disc loading) then normal coriolis will happen and the RPM will speed up.

What happens with cyclic input? (Eric Hunt hinted here).

With cyclic input (say forward flight), one blade is doing one thing and the exact opposite is happening to the other blade. X + Y in the diagram below always remains the same. In the diagram below, the aircraft is going from right to left. See how X stays the same. So does Y, so there is no lead-lag during cyclic input!

As there is minimal lead-lag, the hub and blade attachments can be much lighter and you don't need a lead-lag hinge.

Quite clever really.

Well, that's my attempt to add to the confusion!!! Did the purple thing help?

Joker

Edited August 21, 2007 by joker

August 21, 2007

I don't really understand what the difference between the teetering hinge and the under slung system is?

This is all that slick was asking. I think we might be taking it farther than he was asking?? I was trying to give very basic hub and blade movement examples. The text book/engineering/aerodynamic answers are great! But, I had a bit of trouble with rotorhead terminology myself, so I can relate. Once I had a CFI use very basic terms about whats happening, the light bulb came on and then we could start talking in more detail like Joker said. I tend to see things differently, sorry.

Thanks for the input everyone. I appreciate it, hope slick does..

P.S. Slick, did all this confuse you more??? Hope, not.. If I made it worse, accept my apologies. Also, heard that Fedex was buying out UPS. The new name I heard was FedUp..lol..I know, old joke, later.

August 21, 2007

Nope guys, I do believe I understand it. Sometimes I just have to stare at picture and say, what is the purpose of this mechanism, and how can it achieve this purpose. Basically my understanding is that, when a blade cones up it wants to increase in speed due to the Coriolis effect, it is slung outward away from the hub, and thus farther from its axis of rotation. The opposite occurs with the down flapping blade, it wants to decrease in speed as its center of mass is moved away from the axis of rotation, however it is slung in, toward its axis of rotation. Thus the underslung system tends to decrease differences in distances between the axis of rotation, and by doing so, decrease the effects of the Coriolis effect.

August 21, 2007

Slick,

I think you pretty much have it nailed!

If I may, I just want to take your explanation above, and modify it slightly. My edits are in red.

"....Although, when a blade cones up it wants to increase in speed due to the Coriolis effect, it is slung outward away from the hub, and thus farther from its axis of rotation. This will slow it down.

The opposite occurs with the down flapping blade. It wants to decrease in speed as its center of mass is moved away from the axis of rotation, however it the whole blade is slung in, toward its axis of rotation. The two actions cancel each other out.

Thus, the underslung system tends to decrease maintain differences in distances between the axis of rotation and overall centre of mass, and by doing so, decrease cancel the effects of the Coriolis effect."

I think between us, we now have a pretty good (and simple) explanation of the Underslung system!

Joker

August 21, 2007

The Ill. on page 3-3 in the FAA Rotorcraft Handbook Fig. 3-6

This diagram is a basic drawing of whats being said here. And, said in a different way, but maybe stated too simply.

Because of the underslung rotor, the center of mass remains approximately the same distance from the mast after the rotor is tilted.

Later

P.S. Sorry I got off topic on post 4. Best I can tell there are 3 major semi rigid hub designs. Non-underslung, Underslung, and the RHC rigid in plane. Are there more?