two blades rotor teeter hinge position

[grayzhu]

Hello everyone.
I have a question.How to calculate the offset of the teeter hinge in the two blades rotor head? I checked some books but did not find the answer.I drew a picture.Thank you.

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[NickLappos]

A true teetering rotor has zero hinge offset. It produces no head moment if there is no positive thrust on the rotor (thus the worries about zero g maneuvers).

[GipsyMagpie]

A true teetering rotor has zero hinge offset. It produces no head moment if there is no positive thrust on the rotor (thus the worries about zero g maneuvers).
RTQ - the offset in the picture is the vertical one. I am not sure there is a means of "calculating" the offset except with a tape measure! Perhaps if you explain the context?

[Arm out the window]

The internet is a great leveller isn't it?? You know who Nick is, Gypsy, right?

[[email protected]]

Perhaps Gypsy is a student of another ppruner who likes to correct Nick;)

[Birdy2]

Hello everyone.
I have a question.How to calculate the offset of the teeter hinge in the two blades rotor head? I checked some books but did not find the answer.I drew a picture.Thank you.

1438

The amount of undersling is easily calculated, but not by me.

[grayzhu]

What is the vertical offset purpose? It must have some good reason.I want to find out.

Thank you.

[Ascend Charlie]

Gray, It is called underslinging, and as the disc tilts, it adjusts the length of the rotor blade by an amount to reduce the "ballerina" effect with conservation of angular momentum. Tilt the (coned) disc forward, and the rear blade apparently is shorter and the front is longer, causing problems with one trying to speed up and the other trying to slow down. Causes stresses in the head. So, the underslinging pokes the rear blade out a bit and pulls the front one in a bit.

A very basic explanation, but maybe of use to you.

[AnFI]

AC that is a nice clear basic explanation of the purpose

grayzhu defines clearly what HE MEANS by the term offset
Nick answers a similar but different question (again) so NL is not wrong just not RTQ as GM politely points out (he misses the F from RTFQ). AOTW points out that some members cannot be wrong because of who they are. Knowing who they are helps you realise that whatever they say is right even if they didn't read the question.

[Arm out the window]

I didn't say he couldn't be wrong, but just that a bit of respect is due. If a term like offset was used which has an accepted meaning in aerodynamics it's not a real surprise that it would be taken as such by people working in the field.

Anyway, it's not worth carrying on over so I will bid you all a Merry Christmas!

[[email protected]]

Oh Dear AnFI - straight in with the insults....

Wonder how long it will be before you are moderated, again.



Merry Christmas

[whoknows idont]

Wow, this one really derailed quickly, didn't it! :)

[NickLappos]

In an attempt to keep this thread purely technical, let me try to explain how I see the vertical undersling's function. I believe the larger that undersling the worse the 1/rev vibration of the system with flapping, since the rotating center of the head moves away from the center of rotation as the undersling gets larger. This means the rotor has a natural 1/rev imbalance as flapping gets large.
The primary reason for undersling I believe is packaging of the several bearings and teetering stops. As the teetering bearings get larger with higher lift forces, the ability to place them along the rotor head axis gets more difficult. The CF forces are passed along a clean loadpath if the teetering bearings are above the path. Otherwise, the bearings cut those beams and potentially call for heavier components as a result. A smooth continuous beam between the blades carries the CF, and the teetering bearings above need only carry the lift forces.

[AnFI]

NL
Well at least you are answering the question this time.
Why don't you rate ACs nice classic answer then?
I had thought that in addition to ACs clean and simple justification of underslinging that there was also an advantage in trying to bring the CoG and the 'Center of Forces' for the blades closer to the same plane with the teeter hinge. ie an attempt to keep the forces inplane with the teeter hinge.
(thereby reducing the moments (with similar considerations for vibration and weight as you allude to) that the teeter bolt has to resolve (in the vertical plane which is parallel to the teeter bolt)), does that sound like it has merit?

Straight loadpaths through the head and 'packaging', hmmm maybe

[NickLappos]

AnFI, what is your problem?

[Lonewolf_50]

All noise and fog all insight, but I'd like to ask the question asker to please clarify what the question is.

@grayzhu:
You are attempting (or wish to know how) to calculate the offset (vertical)... to what end? What is your objective? What do you need to calculate that value for?

Are you trying to design a rotor head, trying to compare one rotor head to another, or are you trying to understand the forces acting on and around the rotor head?

As Gipsy pointed out earlier:
I am not sure there is a means of "calculating" the offset except with a tape measure! Perhaps if you explain the context?
If you explain the context you'll get a better answer to your question.

[megan]

What is the vertical offset purpose? It must have some good reason.I want to find out.What the FAA has to say, assuming I've read your question correctly.A semirigid rotor system is usually composed of two blades that are rigidly mounted to the main rotor hub. The main rotor hub is free to tilt with respect to the main rotor shaft on what is known as a teetering hinge. This allows the blades to flap together as a unit. As one blade flaps up, the other flaps down. Since there is no vertical drag hinge, lead/lag forces are absorbed and mitigated by blade bending. The semirigid rotor is also capable of feathering, which means that the pitch angle of the blade changes. This is made possible by the feathering hinge.

The underslung rotor system mitigates the lead/lag forces by mounting the blades slightly lower than the usual plane of rotation, so the lead and lag forces are minimized. As the blades cone upward, the center of pressures of the blades are almost in the same plane as the hub. Whatever stresses are remaining bend the blades for compliance.

If the semirigid rotor system is an underslung rotor, the center of gravity (CG) is below where it is attached to the mast. This underslung mounting is designed to align the blade’s center of mass with a common flapping hinge so that both blades’ centers of mass vary equally in distance from the center of rotation during flapping. The rotational speed of the system tends to change, but this is restrained by the inertia of the engine and flexibility of the drive system. Only a moderate amount of stiffening at the blade root is necessary to handle this restriction. Simply put, underslinging effectively eliminates geometric imbalance.

Helicopters with semirigid rotors are vulnerable to a condition known as mast bumping which can cause the rotor tends to change, but this is restrained by the inertia of the engine and flexibility of the drive system. Only a moderate amount of stiffening at the blade root is necessary to handle this restriction. Simply put, underslinging effectively eliminates geometric imbalance.

Helicopters with semirigid rotors are vulnerable to a condition known as mast bumping which can cause the rotor flap stops to shear the mast. The mechanical design of the semirigid rotor system dictates downward flapping of the blades must have some physical limit. Mast bumping is the result of excessive rotor flapping. Each rotor system design has a maximum flapping angle. If flapping exceeds the design value, the static stop will contact the mast. It is the violent contact between the static stop and the mast during flight that causes mast damage or separation. This contact must be avoided at all costs.

Mast bumping is directly related to how much the blade system flaps. In straight and level flight, blade flapping is minimal, perhaps 2° under usual flight conditions. Flapping angles increase slightly with high forward speeds, at low rotor rpm, at high-density altitudes, at high gross weights, and when encountering turbulence. Maneuvering the aircraft in a sideslip or during low-speed flight at extreme CG positions can induce larger flapping angles.

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch04.pdf

[grayzhu]

Birdy,
Thank you, I have used your key word 'undersling' to google yesterday.
And I find the answer. Underslung Rotor Design (http://www.helistart.com/underslungrotordesign.aspx)

Ascend Charlie,
Thank you for your nice clear explanation.It was very helpful to me.

Lonewolf,
Yes. I am trying to design a rotor head for small Coaxial helicopter.
I have made one. It can fly but I think it's not very well.

[Birdy2]

I hope you find the formula Gray.

It must be calculated for average AUW, to keep the teeter hinge on the COM with the average cone angle.

And iv always been interested on coaxial, love to hear more about your work.
And i think youll need different amounts of undersling between top and bottom rotors.

[GipsyMagpie]

The internet is a great leveller isn't it?? You know who Nick is, Gypsy, right?

Nick has done a great explanation of undersling now which I was politely trying to prompt. AC's was shorter though :).

And thank goodness for the internet being a leveller. We can all learn no matter how much experience we have.

Thanks Nick for that explanation - added to my knowledge bank should I ever fly a teetering head again.

Such a shame the insults had to be thrown.
GM

[paco]

It's why some people only lurk these days.

phil

[Birdy2]

Teetering and undersling are unrelated.
Teetering allows each blade to compensate equally for the difference in airspeed while traveling horisontaly.
No horisontal travel, no movement on the teeter hinge.
Undersling simply places the teetering hinge in the center of lift/drag, cancelling out the 2per that would be present without it as you add horisontal airflow.

Any increase in teetering angle wont change the cone angle.
Any increase in cone amgle wont change the teetering angle.
The degree of teetering angle is proportinal to airspeed differential.
The degree of cone angle is proportinal to AOA, or load.

[whoknows idont]

Yes. I am trying to design a rotor head for small Coaxial helicopter.
I have made one. It can fly but I think it's not very well.

Fascinating! Care sharing some pictures and details?

[grayzhu]

I made some calculations.The results are not satisfactory.
I choice the undersling let left and right blade CG distances are equal:
If flap angle is 10 degree,blade CG distance moved 13.2 mm.
I do not think it can work well without lag hinges.

ps:some data I use for calculation.
e=794; blade CG distance from blade root (mm)
x=41.555; undersling (mm)
b=80; blade root distance from rotor head center (mm)
a=3; pre cone angle (degree)


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