Low RRPM in cruise

[Aser]

Do you use less than 100% rotor rpm in cruise ?

Yes?
No?
In what type?
and WHY?

Thanks for your comments.

Aser

[Ascend Charlie]

In the Huey, we would beep down to 97% as we were told it was more efficient.

In the S76B we use 107%, because Nick said so!

[eagle 86]

AC
I don't remember ever telling you to beep a Huey down to 97 PERCENT!
Altering RRPM in flight MAY produce a better angle of attack of main rotor blade therefore more rotor efficiency, less power required but depends on number of factors DA, AUW among them - in most cases benefits can be relatively insignificant.
GAGS
E86

[Aesir]

I think the B-407 was supposed to have a "silent mode" where you could flip a switch and the RRPM would decrease some percent! Don´t know if it is standard or what, probably some 407 driver´s will tell us?

I also heard that the AS332 Super Puma has some feature to change RRPM by switch flick, but that is to avoid some phase interference with LLZ on approaches?

Yes in the 212 and 222 you can decrease the RPM to 97-98% inflight for some performance gain, I have been told the gain is neglible though so I have never done that. I keep the RRPM at 99.5 to 100% more or less.

I believe if I remember right that in the Schweizer 300C there is some mention of 3000 ERPM for extended cruise so that´s certainly less than 100% RRPM. One could probably figure out the performance gain from the tables, I think there are tables both for 3000 & 3200 ERPM!

[donut king]

SK76A

Co. proc....beep UP to 107NR for same cruise tq setting resulting in lower blade angle= less drag= lower fuel burn.

Only valid over long distances...ie. offshore flying.

Lower nr to 96% only for single engine flyaway and climbout. Never during dual engine op's!!

That's all I know about that!!

D.K

P.S. Interesting topic though......Nick where you at?????

[Gerhardt]

he'll post when he regains his composure from laughing so hard.

[eagle 86]

GH
and your point is?
GAGS E86

[Ascend Charlie]

Eagle 86, of course you can't remember telling me to beep down. You're an old man now, and your memory is shot - Agent Orange was the first cause, living in Orange is the second, not enough oxygen up there for an old bloke like you. And besides, it was Bach who told me.

It technically wasn't 97%, it was beeping down from 324 RRPM to 310 or something like that.

[Arm out the window]

In my day (younger than you blokes!) it was 6600 to 6400 N2, don't really know if it worked but we would do it on long transits generally.

[Gomer Pylot]

It's standard procedure to beep back to 97% in the 412. IME that results in better airspeed and lower fuel burn. It's a minor improvement, but I think it's there. The RPM reduction is mainly for vibration reduction, though.

[Thud_and_Blunder]

Dunno where all these percentages came from in the Huey - like Arm Out says, it was 6600RPM down to 6400, or 6000 under some-circumstances-or-other which I can't remember after 21 years. The transit from Seeb to Khasab could be done in a one'r (with about 10% reserves) at FL100 with the RPM beeped down. However, most of the Rhodie's had a thing about flying above 2000 feet (something to do with aircraft fires back home?) so usually had to refuel at Sohar.

[eagle 86]

Precisely the point I was trying to make to AC - 66 to 64 - nothing to do with % - I knew you weren't listening all those years ago!!
GAGS
E86
PS Where's big brother these days?

[Ascend Charlie]

Geez, Eagle 86, your brain is totally gone!

Big Brother is on the Gold Coast, and the show finished about 3 weeks ago - the Logan twins won it.

My brain is only partly gone, I was about to edit my post to say 6600 N2 to 6400 N2, but I got beaten to it. Our current Huey IS in percentage RPM, even if it is a Garlick special.


But seriously, big brother IS on the Gold Coast, flying for JetStar.

[magbreak]

We use 107% in our 76B because that's what the fadec sets and we can't change it!

[tottigol]

Howdy Ya'll. BHT recommends a 97% rotor RPM setting for cruise, it reduces vibes and provides a more comfortable ride. Also, beeping RPM down to 97%, mast TQ is reduced, and by adding collective pitch to re-establish cruise setting (more or less 80%), there is additional A/S to gain but no particular savings in fuel economy.
For all those whom are familiar with the 205/212/412 family tree, the XMSN remain largely unchanged. The 6600 RPM refers to the output from either the Lycoming T-53 or the PT-6 Twin Pack's combining gearbox to the main rotor XMSN.
This shaft (also known as long shaft) rotates at 6600 RPM at 100% setting and 6402RPM at......................97%.
So, in the end it's all and the same.

[Aser]

So...
i.e. bell 412
less rrpm(with the same TQ setting) =
more confortable ride.
more knots at the same TQ setting.
and just a few pounds of better fuel burn?

S76
more rrpm(with the same TQ setting) =
less AOA in blades = less drag = NOTICEABLE better fuel burn?

It would be interesting to hear more comments from other helicopter types (ec-120/130 etc.).


Regards
Aser

[delta3]

R44

My simulator roughly predicts 0.75L/hr (0.5-1 depending on precise parameters) saved per percent drop in R44.
So yes, I sometimes use this, but then the L-RPM beeper is really a problem...

Retreating bladestall should be watched though (again depending on speed/load/density)

d3

[donut king]

To add...

76.... reduced AOA=less drag= lower fuel burn...

This practise is noticeable at altitude, above standard conditions and over distance.Helps when flying minimum legal fuel. Every pound/ litre/ gallon counts!!!

Also KTAS increases by a few knots. Play with your GPS fuel planning function, given these parameters, and see if this works.

DK

[delta3]

DK

I am not shure I do understand, but the theory (and my math) show that AOA increases up to the optimal point. Increase is necessary because overall the rotor is not very charged at reasonable cruise speed, and thus below optimal loading. One exception is the retreating tip. Reducing RPM will require you to pull collective to maintain speed, this should convince you that AOA increases.

Added : drag is lower because average speed of blades is reduced by lower RPM. A low loaded profile has near constant drag coefficient, so only dependent on speed and not really on AOA.

d3

[Recuperator]

As said the Bell 407 does have a "silent mode" just for cruising but it is an additional extra fitted separately, with it's own supplement in the Flight Manual.

The EC 120 does not have the facility to beep down it's RRPM.

Neither does the AS350B3 and therefore I don't think the EC130 (AS350B4) has it either.

[donut king]

Hi Delta3!

My bad!!

I should have included....increase rrpm, lower collective to original tq setting. Thus you would have a lower AOA for the same amount of lift (at increased NR).

DK

[delta3]

DK,

That would mean to me that the 76 has a highly loaded disk at cruise, that is on the average over the optimal point (at this point drag indeed increases with AOA, more than lift)
I have no data on this machine (nor math, nor flying experience) I'll leave that to the specialists (Nick..., I remember a track where Nick indeed explained that the 76 design was optimised more for high speed than for hover performance)


d3

[Aser]

Recuperator:
The EC 120 does not have the facility to beep down it's RRPM.

Neither does the AS350B3 and therefore I don't think the EC130 (AS350B4) has it either
I read somewhere that the FADEC on the 130 will do it.. I don't really know.

Regards.

[Ascend Charlie]

Aser comments on the 76 having a "Noticeably better fuel burn" than a Huey.

The Huey burns 600 lb/hr, flogging along at 100kt = 6lb/nm. The 76B burns 700 lb/hr at 140 kt (5 lb/nm) or 800 lb/hr at 150 kt (5.33 lb/nm). I suppose you could call it noticeable, but more noticeable than that is that the passengers from the S76 have already downed two beers before the Huey lands.:8

[John Eacott]

The 407 "quiet cruise" reduces the Nr from 100% to 92% (413rrpm to 374rrpm), which brings the noise footprint down by 6dB.

[Aser]

NOTICEABLE better fuel burn?
Ascend: It was a question ;)

Now that the consecuencies of change from 100% to 97% in i.e. a B412 is clear for me...

What I would like to see is the figures compared between 100%rrpm and 107rrpm in a 76 (asumming it isn't a 76B,so you could change rrpm) , do you have it?

You 76A drivers , how do you feel the vibs when flying from 100% to 107% , an increase?

Excuse me if I'm too boring.:8
Thanks.
Aser

[Collective Bias]

IMHO

On the 76A generally the 4P in cockpit goes down when RRPM is increased from 100 at TO to about 103-104%, I think thats where most of the people have their RRPM during crusise.

How the vibe level is at 107% is very much depending on density altitude and weight, and we are talking mostly 4P. I do not think you can read any noticible 1P diffrence on a track and balancer unit between the two RRPM's.

We fly the A's at 103-104% and increases the RRPM going up in altitude, 104@4000, 105@5000, 106@6000 and 107@7000 and above. It seems to work well in regards of vibes and is also in "sync" with the FF and TAS diagrams.

One benefit is also higher VNE margin at altitude (in reality) but you can not read it somewhere.



CB

[albatross]

Thought I would conduct an experiment:
SK-76A++
FL 045 (4500FT) @1013 0r 29.92
OAT 33 C
IAS 115 kts---------------------------IAS 115 kts

RPM 107% ---------------------------RPM 102%

1 eng Q 59 ---------------------------1 eng Q 64
N1 96.1--------------------------------N1 97.0
ITT 739 -------------------------- -----ITT 753
FF 270 lbs/hr---------------------------FF 280

2 eng Q 60----------------------------2 eng Q 70
N1 96.1---------------------------------N1 97.3
ITT 760----------------------------------ITT 780
FF 282 lbs/hr----------------------------FF 295

Fuel flow total 552 lbs Hr---------Fuel flow total 575 lbs Hr

So it would appear that for a given IAS fuel flow is lower at 107% RPM than at 102%.
In this 76A++ at least.

[Aser]

albatross: very interesting to me, thanks for your time. But it would be nice to compare between same mast-TQ not airspeed

There is any other helicopter that states in the FM the possibility of adjust rrpm to more than 100%? :confused:

Best regards
Aser

[offshoreigor]

Hi All,

S76 C+

107%, NO OPTION. (Normal Ops)

Cheers,

:eek: OffshoreIgor :eek: