Bell 407 Nr question
 

I had read, and don’t quote me but I believe it was from a Prouty or Coyle book that said the 407 was most effective and flew the best at a slower Nr but the engineers at Bell wanted to increase the Nr for a reason not known to me. After digging through my books I cannot seem to my notes on this and was hoping someone could enlighten me. Thank you in advance!

That's what I was told as well - apparently it was fashionable to have higher RRPM. But that was noisy so there is an NR reduction facility to be used above 1500 feet to keep the neighbours happy.

could be a confusion of words, effective or efficient

Effective
if you are limited in Torque than 5% increase in Nr will give you a proportional increase in rotor thrust and load carrying capability, I cite for example the H125 having the FADEC increase the Nr to 410RPM when the sling load goes above 150Kg
An increase Nr however goes against your VNE performance (compressibility problem will come faster)

Efficient
With a slower Nr the induced angle will be higher and the rotor system will be more efficient, I cite for example reducing the Nr to stretch the autorotation range and reducing the rate of decent

The 76 increased rotor RPM from 293 (100%) in the A to 312 RPM (107%) in the C, don't know but presumed it was because the gross went from 10,500 to 11,700.

I don't remember the particulars but Bell used the lower Nr capability to market their Quiet Cruise Option which allows ops at around 92%. Look for the RFM supplement which may give more info. However, the whole intent of the mod was not for efficiency but rather noise reduction for certain flight profiles.

"The Quiet Mode kit permits flight operations at 92% NR when above 50 KIAS and 200 feet AGL. Flyover noise level is reduced by 3.8 dBA SEL when in Quiet Mode. The kit consists of an electrical selector switch on the collective (pilot position only), an annunciator."

Allegedly if you ran out of power on the 76 and were below 10400lbs. Beeping from 107 to 100 NR gave a nice torque/N1 boost if you were a bit short.
It was quite substantial, allegedly….

100% Nr must be more efficient. My guess too is the NR related to Gross Take Off. I’d hazard a guess it’s got something to do with the conning angles or RBS. There are plenty here that would know more about that than me and I would be interested in the actual reason if someone wants to share it.

Logically that seems the wrong way round?

Since we are talking Bell here, they could have increased the Nr to improve the TR power.

The 407 has no shortage of tail rotor authority, so unlikely.

I've not flown it but just wondered since it came from the same stable where LTE was invented that they might have used the same calculations for the TR size as before.

Bumping up the Nr is most likely about managing Tq.

IIRC, Nick once said that 76 RRPM was boosted to 107 to improve low speed handling.

I plead the 5th on the logic. The practice was a widespread get out of jail card where I was working. It was halted rightfully so to, still you couldn’t argue that it worked.

Invariably when power was a problem it was hot and you were GTOM limited anyway.

See Posts #304, 305, 1289 https://www.pprune.org/rotorheads/19...ml#post2106445

From Nick Lappos post in Megan's linkand Increasing Nr on a twin to improve OEI performance is normal practice.

​​​​​​​That doesn't answer the 407 question though.