Bell 412 - How Does the 'delta hinge' work?
 

Please can someone explain how the delta hinge works on the tail rotor of the 412?

People say it's to equalise the autorotative forces acting on the blades. When viewed, the hinge mechanism is housed within the tail rotor itself, but this doesn't help me to understand it any more. I also see that the housing onto the spindle is offest by about 30 degrees in plan view compared to the blade line, as I look sideways at the blades. Finally, I was suprised at how little the pitch angle alters when the blades move.

Thanks.

A "delta hinge" is a description of the way the blade is made to reduce angle of attack when it flaps. It is easy to do, just have the pitch change rod apply less pitch as the blade flaps up (increased thrust). The red blade below will flap about the diagonal line (where the hinge bolts are). That means as the blade lifts it has its feathering washed out, and the thrust is reduced.

http://www.scotiabladerunners.ca/Pictures/delta.jpg

http://www.unicopter.com/Delta3%20co...20geometry.gif
The above is courtesy of Dave Jackson, and his excellent site (I hope he doesn't mind the linking, Dave, I'll pull it down in a heartbeat if you wish)

Delta hinges reduce the flapping forces on the blade and hub, so that the TR can be lighter and still withstand flight forces. The delta hinge works mostly in forward flight, were the advancing TR blade sees a bunch of lift increase, and the retreating TR blade sees the opposite. The hinge allows the disk to flap and even out the thrust across the TR disk, so it reduces stresses a bunch.
That flap to pitch coupling is called "Delta 3" by designers.

Ramen,

Thanks for taking the time to reply, with graphics (thanks also to Dave Jackson). All is clear!

Can you tell me the name of Dave Jacksons website

it is called dave jacksons website :E

It has been my understanding that the Delta hinge is used also to reduce flapping amplitude so that the TR can be placed closer to the tailboom without striking the tailboom. Do I have this right?

EN48 asked, " It has been my understanding that the Delta hinge is used also to reduce flapping amplitude so that the TR can be placed closer to the tailboom without striking the tailboom. Do I have this right?"

Not really, since the TR must be placed beyond the maximum flapping range of the blade in any case. The delta hinge cuts down the flapping forces that are caused by disymmetry of lift in forward flight, since without it, the TR blade would flap to the stops and create large forces.

EN48 - I believe you are correct and that by using the delta 3 hinge (or creating the same effect by mounting the pitch change arm ahead of the blades feathering axis) you limit the flapping range of the TR.

I think he named it www.UniCopter.com or www.SynchroLite.com or www.Electrotor.com.
http://www.unicopter.com/Think.gif Or something like that.

Actually, the flapping is limited by the mechanical stops. The delta hinge allows the blade to flap and relieve itself of the excess thrust while the tail rotor is in high speed edgewise flight. In high speed, strong disymmetry of lift forces would make the TR flap wildly, where there no thrust relief for the advancing blade.

The purpose of the delta 3 hinge is to modify the flapping frequency of the rotor away from its natural frequency. A two-bladed T/R has a natural frequency of 1:1, and the aerodynamic loads are 1:1 causing a high flapping amplitude. A positive delta angle (reducing blade angle on up-flap)increases flapping frequency above 1:1. negative delta angle (increase blade angle on up flap)reduces flapping frequency below 1:1, Both accomplish the same goal, both are used on tailrotors, Bell 212 has negative delta, Bell 206 uses positive. Unfortunatley I'm not smart enough to tell you why you would use one over the other.

Very good

Equal loading from the advancing and retrading blade provides equal pull or push on the tailboom, simple and effective.:D:D

Pitch Change Verses Flap Change:

Delta3 angle of +0º. ~ Pitch angle is not changed.
Delta3 angle of +1º to +44º. ~ Pitch angle change is less than flap angle.
Delta3 angle of +45º. ~ Pitch angle change equals flap angle.
Delta3 angle between +46º and +89º. ~ Pitch angle change is greater than flap angle.


Positive and Negative delta3:

"Note that positive coupling δ3 >0 represents negative feedback, decreasing the blade pitch for a flap increase." ~ [Source ~ HT p.239]. This is the common usage.

Re tandem configuration "..speed stability ...... differential pitch-flap coupling (positive on the front rotor and negative on the rear rotor)." ~ [Source ~ HT p.850] This implies that positive delta3 causes an increase of the flap to decrease the pitch and negative delta3 causes an increase of the flap to increase the pitch. In other words, if the reference flapping or teetering hinge is normal to the span of the blade, then rotating this hinge in the opposite direction to that of the rotor's rotation will result in positive delta3. Conversely, rotating this hinge in the same direction as that of the rotor's rotation will result in negative delta3.
To me, having upward flap increases the pitch sounds scary, since this increase in pitch will want to increase the flap even more.



Dave

So Being that the 407 has a negative delta hinge on its tail rotor. Does anyone have any evidence that would support or refute that this might have contributed to the cruise flight tail rotor failures that happened 8 or 9 years ago in the 407s?

I understand that the delta hinge acts like a flapping spring by creating an aerodynamic force on the blade proportional to the flapping angle thereby altering the flapping frequency away from the rotational frequency which reduces the overall amount of flapping and prevents the possibility of the flapping going resonant and destroying the tail rotor. And that it probably doesn't matter whether it's positive or negative. But it still looks wrong to me to see the blade pitch increase with flapping on negative delta3 rather than decrease like with the more conventional positive delta3.

But I am curious if anyone on here can give any good reasons why either positive or negative delta 3 on a tail rotor would have any advantages or disadvantages over the other? The main difference that I could see is that it would change the direction of flapping, this could have an advantage of adding tail structure clearance from the blades. However it doesn't seem to me that any clearance is gained by using the negative delta 3 on 212/412 designs. Although it looks like it might provide for a slight gain on the 407.

Helisphere:
I worked at Transport Canada at the time, and if my rapidly failing memory serves me right, there never was a complete answer as to what caused the problems. At the time, we were all unaware that there was a possibility of the tail rotor flapping and hitting the tail boom.
The modifications, which involved among other things moving the flapping stops solved the problem.

delta3
 

Helisphere

I would like to add to the previous posts

- the "positive delta 3" reduces indeed flapping. This can be understood by the geometry and feedback logic. I also can refer to the graphs I posted on the delta3 of the main rotor of the R44-I (quite a while ago) which confirm this when effectively running the math in a simulator

- the way delta3's are rigged however create secondary effects : they introduce cyclic offsets. Of course the tail rotor has no cyclic steering so why bother about cyclic offsets? A tail rotor will also have not a lot of coning in order to avoid blow backs.

One line of thinking could be as suggested, to create such an offset that the max flap happens aways from the boom. I have to agree that using a destabilizing feedback to achieve this kind of puzzles me. I have asked the details of the 407 TR, and will look at it later this week.

B540

I am not fully following your logic. From a resonance stand point rotors are considered resonant, this is why they listen so fast to control inputs. Where aero forces primarily act at the base frequency, this is not the only one. A coned rotor has a strong second aerodynamic harmonic in forward flight. (note : asymmetries for instance in the blades provoke still other highers harmonics) Looking at a simple second order resonant system dampening will change the resonant frequency, but in the case of a rotor this will always be the base frequency I think, so I think we only have to look at dampening here.

d3

Well thats ok, I'm not sure my logic is completely correct but what I meant by resonant was that the natural flapping and rotational frequencies were equal, 1:1, and that the delta 3 altered the flapping frequency IE making the max flap something other than 90 degrees from where the max lifting force is applied to the blade thereby changing that 1:1 to something else, like 1:0.9 or 1:1.1. Like the cyclic offset you describe, even though the tail rotor does not have a mechanical cyclic flight control input, it does have a cyclic, differential lift input from the horizontal flow and disymmetry of lift causing flapping, and the delta 3 provides a change in phase lag, or cyclic offset(as you put it), so the max flap is other than 90 degrees from the max lifting force. I'm certainly not an expert in harmonics or waves or any kind fluid dynamics, so only describing it the best way I have understood it to be.

Anyway, I have been asking this question for a long time about the negative delta3 tail rotor and never found anyone who could give me a good answer.

I did however just find this, it's page 13 of this document:

http://www.bellhelicopter.textron.co...b/rb_mar04.pdf

But it still does not make sense to me how when a blade has positive flapping that increasing the pitch will reduce that flapping the same way as decreasing the pitch as with positive delta3. And of course while they explain very basically why it works they don't explain why they chose it over a positive delta3 tail rotor or why they chose 30 deg for the neg but 45 deg for the positive ones. And the fact that the 407 had some tail rotor accidents made me wonder even more.
:ugh:

Wasn't Frank Robinson the specialist in tail rotors at Bell?
He may have the time and interest in answering this question, now that he has retired. http://www.unicopter.com/NoIdea.gif

Dave

B407
 

Got some preliminary drawing but awaiting more precise engineering input...

Helisphere

fair enough, I assumed you were talking about the phase shifts and not so much about "different frequencies", because then the rotor would have harmonic vibrations so to speak.

It is specifically the phase shifts you mention which I hope to calculate.

d3