RunRyder RC
 3  Topic Subscribe
WATCH
 1 page 1213 views
Scorpion Power Scorpion Power
HelicopterMain Discussion › Lets Talk Gear ratios.
03-11-2003 09:51 PM  14 years agoPost 1
vetrider

rrElite Veteran

Daleville, AL​(Ft.Rucker)

My Posts: All  Forum  Topic

OK guys and few gals, I'm wondering about the correct gear ratio for a givin engine performance.
I'm going to relate to the Freya as I see lots of questions on gear ratios on these and I have 2 Freya's myself.

It's said on here, RR and other places, that if you run a 91 you should run the 12T pinion and 95T main gear for a Reduction 7.92 : 1
My new OS91C makes it's peak power at 15000 RPM as per manufaturer.
At that RPM and 7.92 : 1 the rotor is turning 1894 RPM.

With a 8.45 : 1 the rotor would turn 1775 RPM.
With a 8.64 : 1 the rotor would turn 1736 RPM.

Most people on here don't run their rotor RPM over 1850 on a large heli with 680 to 710 blades and probley really average only about 1750.

1750 RPM @ 7.92 : 1 would run the engine at 13860 RPM. How much power are you giving up there???

1750 RPM @ 8.45 : 1 would run the engine at 14787 RPM which is much more closer to peak power.

I'd like to here from the folks here, who KNOW what their talking about and not hear-saying it, as to what they think about this.

Which is the better gearing for the engine to do it's job?

THANKS

Nolan

PM  EMAIL  GALLERY  Attn:RR  Quote
03-11-2003 10:27 PM  14 years agoPost 2
z11355

rrMaster

New England

My Posts: All  Forum  Topic

well, this is just hear-say but.........

(yes, I have a Freya w/ OS91Cspec/MP2)

Yes, I've seen the OS spec say
that the engine produces max power
at 15k. On the MP2, Curtis is emphatic
that the engine produces best power
at 13200 and recommends running
it at 14000. If you crank the numbers,
you end up w/ either 7.75 or 7.92.
Personally, I'm inclinded to believe CY.

You do not want to run the OS at the
Freya's 8.45 setting.

I haevn't tried the 7.75 yet but I will
tell you that 7.92 is KICKASS.

Soooo, a lot depends on flying style
and exhaust selection too. JeffG
recommends and raves about how
well the Freya performs at 1900
for aerobatics. YMMV.

You will not believe how quiet and
powerful the 91Cspec is when running
at 14000 and the fuel economy is
reallly quite good.

My vote: 7.92 but at some point, I will
try 7.75 too.

PM  EMAIL  GALLERY  Attn:RR  Quote
03-12-2003 12:45 AM  14 years agoPost 3
vetrider

rrElite Veteran

Daleville, AL​(Ft.Rucker)

My Posts: All  Forum  Topic

Well said Z

Thanks for a great reply with lots of good info.

YEAH..... I think Jeff really likes his stuff buzzing. Time for a new hair color soon too!

Although I'd be more inclined to run an X-spec at the hi rotor speed rather than my stock headed Freya.

Nolan

PM  EMAIL  GALLERY  Attn:RR  Quote
03-12-2003 01:27 AM  14 years agoPost 4
corey11

rrElite Veteran

Bay Area, California

My Posts: All  Forum  Topic

how about a os 91, or ys 90(is there one?) gear ratio for a vigor cs?? what pinions/gears would i need?

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
03-12-2003 01:43 AM  14 years agoPost 5
Vance

rrVeteran

York ,PA

My Posts: All  Forum  Topic

Well I have always read that BHP(brake horse power) is not as important as torque as far as heli engines are concerned.
Look at the figures below.

YS61ST(June 1997 MHW)
Max BHP- 2.01 @ 19,107 rpm( hatori/30% nitro)
max torque-132 oz/in @ 13,100 rpm( hatori/30%)

Thunder tiger 61(April 1997 MHW)
Max BHP- 2.0 @ 17,100rpm(hatori/5% nitro)
Max torque-132 oz/in @ 13,200rpm(hatori/5%nitro)

Webra Speed 61 P5(March 1998 MHW)
Max BHP- 2.01@ 15,400rpm (hatori/5% nitro)
Max torque-136 oz/in @ 12,900rpm( hatori/5% nitro)

DC-60(Sept. 1998 MHW)
Max BHP- 2.21 @ 16,200(hatori/30% nitro)
max torque- 145 oz/in @ 14,180rpm(hatori/30% nitro)

As you can see the max torque rpm's are always considerably lower than the max HP rpm's, so using the max HP rpms to select gearing isn't necessarily the best way to do it. The torque or "grunt" is what is pulling you through.
The above numbers were taken from Mike Billinton's engine tests as published in various MHW articles.
Vince D

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
03-12-2003 02:13 AM  14 years agoPost 6
Peter Wales

rrElite Veteran

Orlando Fl

My Posts: All  Forum  Topic

I was wondering if the torque V HP myth would​pop up

and sure enough it has.

Vetrider and z have their act together, its the peak HP that counts.

Take a look at these old power curves I found

Lets go with the Hatori Silencer and 30% nitro which gives peak HP at 17500 rpm. The peak torque is at 11500 rpm.

What is needed is torque at the head, and at a headspeed of 1750 for example, given a gear ratio of 10:1 to run at peak HP the torque at the head will be 10x the engine torque. ie 1100 oz in

Now if we gear up to run the engine at peak torque, we get a ratio of 11500/1750 = 6.57:1 so the torque at the head will be 6.57 x 140 = 920 oz in . A loss of 180 oz in. available torque.

Peter Wales
http://scalehelicopters.org

PM  EMAIL  HOMEPAGE  Attn:RR  Quote
03-12-2003 02:24 AM  14 years agoPost 7
EChapkis

rrVeteran

Tampa, FL

My Posts: All  Forum  Topic

gearing

I run my Freya, OS 91SX, Perry Pump, 680 Vblades V1 @ 1750 and I agree with Z, kick butt power.

I was running at 1900 and the I found the Freya to be an all out animal (Jeff Greens words, he he) and it can be just wild. @ 1750, 3D is good and still smooth I also run 15% PM.

When I was assembling the Freya, I was wondering about using a higher gearing and achieving the same rpm and hp, but I figured actual torque would be less. The OS 91 from my observations is a high torque motor.

I wonder if some of the failures of the OS 91 are related to the gearing of the setup.

Last thought......@ 7.92, Hatori SB-15, very, very quiet.

My X Spec, OS 91SX, MP II will fly in a week or so. I am curious to see how the MP II works out.

Evan Chapkis Tampa, Florida

PM  EMAIL  GALLERY  Attn:RR  Quote
03-12-2003 04:04 AM  14 years agoPost 8
Phil Cole

rrVeteran

Menlo Park CA

My Posts: All  Forum  Topic

One other thing to bear in mind:

The manufacturer's ratings generally don't specify an exhaust. Most likely they are with an open exhaust. If you look at the graph Peter put up, you'll see that the power peak is at a lower RPM with just about any exhaust system attached.

You need to know where the power peak is with the exhaust system you will be using.

PM  EMAIL  Attn:RR  Quote
03-12-2003 04:04 AM  14 years agoPost 9
vetrider

rrElite Veteran

Daleville, AL​(Ft.Rucker)

My Posts: All  Forum  Topic

Excellent stuff fella's

I'm an old Motorcycle racer from the 70's and 80's...remember AIR cooled dirt and street bikes???...... I was always told that torque will get you going and HorsePOWER is what will take it farther and keeps it there.
That is,.... torque to accelerate and HP to keep it there.

The rear exhaust motor chart also shows that the straighter the tuned exhaust the better the power characteristics. Nice FAT curve.

Mr Mike Billinton's findings seem good but does not take into consideration the mass of the rotor system. Once in motion it should only take reasonable HP to keep it at that even with the bite of the blades in the air during aggressive manuvers.

Nolan

PM  EMAIL  GALLERY  Attn:RR  Quote
03-12-2003 04:25 AM  14 years agoPost 10
Phil Cole

rrVeteran

Menlo Park CA

My Posts: All  Forum  Topic

Bike and cars are different from helicopters since they require the engine to operate over a range of RPM. A broad torque curve means that you can get good power over a wider speed range, and you don't need a gearbox with zillions of closely spaced ratios.

With a helicopter we can vary the collective pitch continuously, which is like having a infinitely variable transmission. Provided the throttle curve or governor does its job it doesn't matter too much what the torque curve is.

There are limits, of course. A friend of mine has a Bergen twin with tuned pipes. There's quite an art to getting it spooled up since too much stick puts in too much collective and the engine can't get up on the pipe.

PM  EMAIL  Attn:RR  Quote
03-12-2003 04:29 AM  14 years agoPost 11
vetrider

rrElite Veteran

Daleville, AL​(Ft.Rucker)

My Posts: All  Forum  Topic

ctseaplane

I've run my YS80/SB-15 Freya (8.45 ratio) at 1750 and the OS91/MP2 Freya (7.92) at 1850 and they both do very well.
680mm TG's on the YS80 and SAB 690mm on the OS91

I started this thread to get opinions and learn form all the answers. So far, SO good!

Nolan

PM  EMAIL  GALLERY  Attn:RR  Quote
03-12-2003 03:03 PM  14 years agoPost 12
aeromorris

rrApprentice

Murfreesboro, TN

My Posts: All  Forum  Topic

I've been running a stock SST Freya/ OS91sx-h/ Hatori-1piece with the 7.75 gearing for a little under a year. IMPO for FAI style flying it is the only way to go. Loops, rolls, FFF, etc are exceptionally smooth with this gearing and only 1700 on the mains. Sure, this is below the mfg's recommended max RPM/HP rating but it doesn't seem to matter. I'm no 3D master but during flips, tumbles, and other maneuvers that require large collective inputs, the rotor shows now sign of bogging when run between 1750 and 1850rpm.

I'd heard somewhere on this board that the Freya comes "alive" at 1900rpm and that absolutely is the case. At 1900 and 7.75 the engine is up around 14725 and the responsiveness of the heli jumps dramatically. It's probably due to the fact that I'm used to running a significantly lower headspeed 1650ish.

My first few flights of every session usually involve some low hovering work. The tall gearing makes for an extremely quiet heli in a hover. So quiet that hovering becomes an almost relaxing experience. At this point I couldn't imagine using the 8.45 gearing for hover work. The engine would be running almost 14k just to hover at 1650 and that's just overkill IMO. Even if you decide that the taller gearing isn't for you, I think that this is a $25 investment that will actually affect the flight performance of your bird. Unlike a lot of the pretty blue parts. (With that said, I should also note that I have several pretty blue parts on order )
-J

PM  EMAIL  Attn:RR  Quote
03-12-2003 06:16 PM  14 years agoPost 13
RappyTappy

rrProfessor

North Denver,​Colorado

My Posts: All  Forum  Topic

The YS .91 3D gear ratio is 8.45 and the FAI gear ratio is 8.1.

PM  EMAIL  GALLERY  Attn:RR  Quote
03-12-2003 07:13 PM  14 years agoPost 14
corey11

rrElite Veteran

Bay Area, California

My Posts: All  Forum  Topic

Since ive never changed a gear ratio in any heli..would i be able to go from a 60 to a 90 and just change the ratio in the govenor?? i probably have to buy a new main and pinion gear, but wouldnt it be nice if we could!

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
03-12-2003 07:28 PM  14 years agoPost 15
RappyTappy

rrProfessor

North Denver,​Colorado

My Posts: All  Forum  Topic

no you can't just change the gear ratio in the governor. The gear ratio that you put in the governor just does some math of the gearing and determines at 1700rpms, or whatever headspeed you choose, what the engine rpm will be. The govenor works on engine rpm and not headspeed, although that is what we mainly look at and control via the black box.

PM  EMAIL  GALLERY  Attn:RR  Quote
WATCH
 1 page 1213 views
Scorpion Power Scorpion Power
HelicopterMain Discussion › Lets Talk Gear ratios.
 Print TOPIC  Make Suggestion 

 3  Topic Subscribe

Saturday, November 18 - 4:23 am - Copyright © 2000-2017 RunRyder   EMAILEnable Cookies

Login Here
 New Subscriptions 
 Buddies Online