Bell 206 38%-30% NR rotor brake application
 

Simple question - in the BH206 series, the rotor brake is to be applied between the above Nr range. I can understand why it shouldn't be applied at the higher range but can someone explain why the lower limit is in place? Will it damage the mechanism if the brake is applied on shutdown under 30%?

Thanks

If you apply the brake too quickly at too low Nr, you can cause damage. If you do apply the brake at lower Nr, be very gentle with the initial application.

And when you do apply it at the higher RPM, apply it fully and leave it on till the rotor stops....
Keeps the disk from chattering.

The reason why damage might occur at low rpm is because the centrifugal force on the blades is a big part of their stiffness. As the blade slows down, the brake force can cause excessive "edgewise" stresses as the blade is forced forward when the hub slows down rapidly. It is actually a double effect - the blade is getting less stiff and the brake is getting proportionally stronger as the rotor energy decays rapidly with rpm.

So what about that electric rotor brake on some S-76s then? I only flew one of those aircraft once or twice - I was horrified to experience how hard it grabbed the rotor down to a standstill; the forces on the transmission and head components must be awesome! :eek:

When we first started with the S76A in 1979 the SOP was to shut down both engines and then apply the rotor brake.
It used to stop in about two revolutions!!!!
Our concern about the loads on the transmission were taken on board and it was changed to apply the brake with one engine still running.

I don't have a rotor brake on my old 206, but I do on my 'new' one due out next week! So I'd better make sure I understand it!

Reading Nick's post above, it has challenged my understanding: Nick, are you saying that you should apply it at 38% and then let go again at 30%? Surely not!

Noone told me to apply the brake at 38% which would reduce the chattering I had to work that out myself.

Lucky boy 206 Jock.

You put the brake on and leave it on until the rotors have stopped. Though I usually release the brake an instant before the blades stop to stop them from rocking back and forth.

My S-76 Training Manual states, that the electric rotor brake builds up 200 psi of hydraulic pressure acting on the disc mounted on the t/r driveshaft right at the main gearbox output - the manual brake procedure calls for two strokes resulting in 200 psi as well?!

Do some company procedures call for a slower manual application or how do you explain the difference fareastdriver describes?

The S76 rotor brake starts off with a partial application, and then goes to full on at a lower rpm (I can't recall the exact numbers offhand, it's been a couple of years since I flew them). The standard application is to engage the brake with one engine out and the other at idle. It won't actually engage until the rotor slows. It does slow quickly, but the problem is engaging at low speeds, not having a constant engagement. As long as the deceleration is constant there should be no damage.

Phil77, the first stroke of the manual RB lever primes the system, the second stroke and therefore the pressure applied is actually controllable by the pilot, depending how far he pulls the lever. The electric brake engagement of A model I flew in had a more violent effect than I could ever bring myself to match. (I was used to doing this in the 3000 hours or so I flew in the 76 models, A++, B, C and C+).

They always say never fly the 'A' model of anything - I did just the one day!

BTW, the B model requires one engine to be shut down before applying the RB, with the other engine at idle. Both engines can be started with the brake on. The C+ rotor can be brought to a halt with both engines at idle due to less residual torque - it's one hell of a rotor brake. :ok:

Thanks guys!

I'll try it in a month from now myself - we're going to get an A++ to play around!

B206biii
 

From the RFM;

ENGINE SHUTDOWN PROCEDURE

CAUTION

AVOID RAPID ENGAGEMENT OF ROTOR BRAKE IF HELICOPTER IS ON ICE OR OTHER SLIPPERY OR LOOSE SURFACE TO PREVENT ROTATION OF HELICOPTER.

Apply rotor brake between 36% and 30% rotor RPM.

As you can see Bell do not expand on the basic instructions, but as noted in the previous posts if you hear or feel chattering of the drive train release the brake to reduce any possible damage. The term "Avoid Rapid Engagement" may also be a good guideline to follow irrespective of the surface conditions.

It seems that a lot of engineers (mechanics) hate rotor brakes but a good guideline has always been;

"I you have one fitted and you need to use it, then use it. If you don't need to use it, then don't use it".:ok:

800

206B rotor brake
 

Can anyone explain to me why the rotor brake shouldn't be applied any slower than 38 to 30%? It is clear above these limits but I cant see the reasons against using it at slower speeds.
I am aware that many Jet rangers dont even have a rotor brake...

MADY

And to ensure they stop at 12' & 6':p

Hi Folks.

I was originally told the reason for the between 38-30% was for the thermal heating of the pads which allowed a clean slowing of the rotor. Where as below the 38-30% say 25ish the pads failed to heat up correctly and glazed therefore causing the transmission to chatter at low speed due to reduction of friction.

Just a thought.:)

Here is what the B407 RFM has to say:

<emphasis added>

" if one leaves the RB fully applied until the rotor stops, it stops very abruptly, possibly putting inappropriate loads on the drive system."

There was a 212 that had a tooth come off somewhere in the main gearbox because of this practice. My own practice is to ease the brake off at about 10%, if i use it at all.

The lower limit may be there for the same reason that the yellow band on the NR gauge is there - to stop inappropriate vibrations on the old one-piece tail rotor drive shaft. Just another thought :)

Phil