Delta 3 hinges?
 

Are there any helicopters that use delta 3 hinges on their main rotors.
I understand they are often used on tail rotors but are there many on main rotors and whats the reason?

MADY

S-76 has 17 degrees of delta three. It reduces feathering as the blade flaps, so it softens the ride in turbulence.

The Robbie also has some delta three.

Alot of helicopters use flapping hinges and blade horn/pitch link positioning to create a "delta-3 hinge effect", so when the blade flaps the pitch will change appropriately.

The R44 is a good simple example of this. If you look at the head, you will see the pitch link is not in line with the flapping hinge, this generates the "delta-3 effect".

Not the best way of explaining it, no doubt it could be explained easier, but it's a Saturday morning...

TiP:)

The Kaman helicopters also use delta-3 on their main rotors.

More information on Pitch-Flap Coupling; (delta3, δ3)

Dave

Nick, couldn't find the S-76 specifically on this site. How is D3 arranged on this rotorhead? I imagine offsetting the pitch horn along the blade gets used a lot for relatively high offset hinge designs.

http://www.b-domke.de/AviationImages....html#Sikorsky

Understand why Robbie corrects for coning and inflow roll with 18 degree pitch reduction, but never bought the "wee-wa" explanation. I guess 18 deg D3 just makes for convenient pitch link packaging.

Mart

Grav,
Almost always, delta three is induced by making the blade pitch change horn lay inboard of the blade flap hinge (for a leading horn.) This makes the blade flap about an axis that is not normal to the rotation (it flaps about a skewed axis that is 17 degrees from the tangent). This skewed flap axis makes the blade dump lift as it flaps. This make a much smoother and more stable ride for passengers.

One effect of delta three is that the control axis is rotated by that same angle, so that the phase lag (gamma) or rigging angle must be adjusted accordingly. This is what always caused Lu Z concern (rest his soul) because it is a bit hard to fathom.

Thanks, Nick. Wasn't sure if S-76 had flapping hinges - so it does not have such a high offset as i thought. I imagine that hingeless and bearingless designs would need to use flap/pitch flexure in the blade itself (like a fixed wing). The S-300 rotor hub, just above the Sikorsky link, shows trailing pitch links - i didn't realise that trailing designs existed.

Must admit, to understanding Lu's point about the D3 in the R22 (rest his soul). I don't claim to have anything like the experience i would like, and to me it flies fine - as long as cyclic inputs are restricted in reduced g auto entry. However, I can see that if a pilot got himself disoriented when cloud crept up then having the rotor optimised for 1g coning could be problematic. This could probably be said of many designs.

Mart

At the risk of linking pictures directly from someone else's web site :ouch: ~ the S-76 rotorhub;

http://oh1ninja.la.coocan.jp/details..._hub_trims.jpg

http://oh1ninja.la.coocan.jp/details...hub_trims2.jpg

Above Site: http://oh1ninja.la.coocan.jp/details/S76/S76C.htm

Thanks, Dave. Spot on! :ok:

From a dynamic standpoint: The 17 degrees D3 will reduce (possibly remove) gust induced attitude change ~90 degrees later, and increase cyclic following response by several rotations.

Mart

The Kaman helicopter? Cant be much room for flapping with interlocking blades? Just enough I suppose?

MADY

A coax has the real need to avoid self-midair collision, so the mast height and disk separation is a big term in the design equation. Also, the designer will often make the two rotor have different hinge offset (lowe has more) because of flapping and also mast moment considerations, where the long mast runs up to the lower offset rotor head.

Grav, the s-76 has elasotomeric bearings and is a conventional articulated system in that respect. Its hinge offset is about 4%.

I corrected my previous post to make it match the S76 (it was hypothetical, previously).

Mady, You're correct. The 2nd and 3rd sketches on this Web page show the Kaman rotor heads. These teetering rotors also have lead/lag hinges.

For this reason, the continuing increase in the rigidity of rotors is more advantageous for the intermeshing configuration than any other. It allow the 'V' angle between the two masts to move closer to vertical.


Dave

Mart,
If you look closely at the S300 hub, the pitch horn is exactly in line with the flapping hinge. And, the lead-lag hinge is outboard of the flapping hinge. Therefore both lead-lag and flapping have zero effect on pitch angle of the blade, not counting any aeroelastics. I suspect this is why it is feasible to have the pitch horns trail rather than lead.
-- IFMU

Hmmm, flying helicopters is never straight forwards... ;)
 

Thanks for pointing that out, IFMU. I had wondered about control forces for that type of installation. If the blade flexure occurs outboard of the pitch bearing, pitch control moment might become high with lead-lag - i suspect it sorts itself out in flight though. This is also true for a hingeless design, if combined lift&drag force does not always act normal to plane defined by pitch link joint and bearing axis. I imagine this is why hydraulic failure can result in such high cyclic forces for larger machines with say 4% offset hinges.

Nick, how does D3 affect flapback? Being mulling this over while meshing FE. :ugh: I can understand about the need to reduce collective pitch when encountering lift though.

I had also wondered about the S-76 twin bi-filars, but finally put it down to the need for increased pendulum mass. Is one tuned to N+1, while the other tuned to N-1?

Mart

Grav,

some answers:

I imagine this is why hydraulic failure can result in such high cyclic forces for larger machines with say 4% offset hinges. Actually, most of the problem of trying to manually fly a helo with hydraulics off is due to the natural pitching moment of the airfoil section used. As we get sexier with those airfoils, the pitching moment gets harder to control with human power, so we need hydraulics. The old 0012 was sweet in the moment department, but a poor lifter. That is why the new Carson bladews lift so much more than the old 0012 S61 airfoils.
how does D3 affect flapback? It makes it less, because as the blade flaps, it dumps lift due to delta 3.

I can understand about the need to reduce collective pitch when encountering lift though. Actually, it has the opposite effect, as you increase rotor thrust with back stick, the delta 3 washes it out, so you have a weaker pitch-rate to load factor sensitivity with delta 3 (that is also why it makes the ride smoother!)

I had also wondered about the S-76 twin bi-filars, but finally put it down to the need for increased pendulum mass. Is one tuned to N+1, while the other tuned to N-1? The two bifilars are n+1 and N-1 in the rotating system, and pure 4/rev in the stationary system. The N+1 is sometimes deleted, it is the less needed of the two.

Thanks for explaining that, Nick.

Hadn't though about aerofoil pitching moments - of course!

Along with the horizontal stabiliser, i imagine the D3 will also improve longitudinal stability by allowing stabiliser to dominate any airflow attitude change. I guess the penalty is a reduction in lateral stability, since sideslip produces less correcting roll. I imagine a lot of fun was had getting the right balance - was it right from 1st flight?

Useful lesson in defining rotor frequency terms in rotating and stationary system - thanks!

Mart

Well whaddya know, learn something new...the Hughes 269 has trailing pitch links! And look at those MR blade retention pins...what are they, 1/4" bolts?

FH1100 Pilot,
I think that looks about right. But I bet they are glued too.
-- IFMU