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

[helisphere]

Yes you know, I was at the RHC safety course in june of 2007 and I was planning to ask this very question but wouldn't you know it, Frank, who was almost always present and teaching at the courses, was out of town.

I think that's a good idea Dave, would you know how to contact him? I suppose I could just pop on over to Mutiny Bay and see if he is home sometime, buzz the house a couple times maybe, since I am so close...

[Dave_Jackson]

helisphere,

Lu Zuckerman got FR to post on this forum many years ago. But Lu's method is probably not the best one to use.

Dave

[helisphere]

Yes he did, that guy was absolutely determined. I gave up arguing with him 9 years ago! I don't think my delta hinge question quite has the power to pull that off But maybe a good ole snail mail letter might do the trick, incase he is getting bored in retirement...

Thanks Dave


Any more info Delta3? Engineering is what I would be doing if I could stop flying for money long enough to finish college. I'll find the answer one way or another

Thanks in advance,

Shu

[heole]

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.

One explanation could be the in plane oscillatory loads: A positive delta3 will increase inplane oscillations, when a negative delta 3 will decrease in plane oscillations when the blades are flapping. So , for the drive shaft, couplings, t/r gear boxes, a negative delta 3, means less torsional stress and better component life. The choice is probably only driven by mechanical reasons

[Stuck_in_an_ATR]

Yikes! Before stumbling on this thread, I actually thought I knew how Delta3 hinge worked...

[delta3]

B407 TR drawing

Helisphere

Not yet gotten hold of engineering drawings, only "artistic impressions" from maintenance manual, from which I made the following sketch:



From this it would appear that there is indeed a significant negative delta3...

Due to bad weather no 407 around for the moment, so I can not look for my self. Any one can take some pictures?

thx d3

edited : wrong sketch (must have been the wine yesterday..)

[helisphere]

Here are some pics I took a couple years ago

407 Tail Rotor pictures - Photobucket

[helisphere]

I did find this in Wayne Johnson's book, Helicopter Theory:

Thus pitch-flap coupling reduces the flapping magnitude relative to the rotor shaft. Note that negative coupling is as effective as positive coupling, because the effect of Kp is to remove flap motion from resonant excitation. The sign of the feedback influences the phase of the response, and large negative pitch flap coupling does have an adverse effect on flapping stability. It is common to use 45 degrees of delta three on tail rotors (Kp=1) to reduce the transient and steady state flapping relative to the shaft.

Interesting that it says large negative delta3 does have adverse effects on flapping stability. That would explain why Bell uses a smaller angle on their negative delta3 tailrotors. But I still wonder why they would choose the negative, is there some advantage? Frank Robinson spent two years R&D at Bell in the late 60s where it says he earned a reputation as a tail rotor expert. Yet after leaving Bell he went to Hughes where he helped design a tail rotor with positive delta3 and of course when he built his own helicopters he also chose positive delta3. If there is an advantage to the negative delta3, Frank didn't seem to think much of it. If there isn't some advantage, then why toy the instability factor?

[delta3]

B407 delta3

Helisphere

I finally got to do the math for the tail rotor and beware...

Any delta3 is better than no delta3 !
Negative delta3 can be better than positive !

In approximate numbers (for a B407, without precise Bell data, just my best guesses)

I got a Lock number of 1.72
I computed a dimensional stiffness of 1% because of the bearing.

With a negative delta3 of 40° we get a negative stiffness 18% resulting in a normalized dampening of 12%, and a frequency response (amplifier) at the rotor base frequency of 300%. This is amplifier is relevant for disturbances that have the rotor frequency such as incoming wind.

Assuming a positive delta3 of 40° we get 400% ! This is worse.
Assuming no delta3 we get 500%. This is even more worse.

What happens ?

First: Tail rotors have small Lock numbers and are not very well dampened as compared to Main rotors.
By putting any positive or negative stiffness we gain more by taking the system away from the resonance frequency than by actually changing the dampening (looks a bit like the formula you quote, but this formula look a bit too simple and symmetric, but I did not verify that yet)

Second: both delta3's provoke phase shifts away from the center line of up to 30° (plus and minus)

Third: Tail rotors have also bearing stiffness. In particular this was a major modification Robinson did on the R44 (back in 2003?). Now this creates a positive moment bringing the rotor back to the center.

Putting 1 and 3 together could explain the less efficient positive delta3, because it works together with the bearing stiffness. So I would think that a TR with low bearing stiffness should go for a negative delta3 whereas a TR with some significant bearing stiffness should go for a positive delta3.

So why the change to positive delta3 :
the fact that TR's became mechanically stiffer.

m2c

delta3 (still learning...)