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04-24-2014 03:15 PM  6 years ago
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epryor

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Grandview mo,

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91 engine
It's says that the gearing for my heli is 8.2:1:4.54 I know nothing about gear ratios and target head speed, this is a new ball game for me. I want to get low head speeds with this 91 engine no more than 1400 tops with 800mm blades and that might be to much maybe someone can help me on this page?
Batman - Scale it up
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04-24-2014 03:22 PM  6 years ago
jharkin

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Holliston, MA - USA

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Gear ratios are expressed as engine rpm:head rpm: tail rpm. So at 1400 head speed with those ratios you would be running 11,480rpm on the motor and 6,350 on the tail.

Im no expert on heli nitro motors but I'm fairly sure thats out of the power band (too low) on most .91 nitros.

The nitro experts will be along to help shortly I am sure. It will help them if you can post the exact make and model of your engine and your heli. Are alternate pinions available to change the gear ratio? What fuel are you going to run, etc. Any other info that might help us provide answers.
-Jeremy
Whiplash-G
Helix 700G
T-Rex 450 fbl conversion
alot of planks
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04-24-2014 03:47 PM  6 years ago
epryor

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Grandview mo,

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It is a Trex 700LE, Wildcat 30% fuel, OS91HZ engineBatman - Scale it up
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04-24-2014 04:19 PM  6 years ago
xcellgasman101

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WOODWARD, OKLA....

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to get the HS and RPM you will need something in the range of 10.71:1

15,000RPM
-:-
1400 Head Speed
=

10.71:1 Ratio,

Its a simple math, If you know what your head speed is, (1400)
and RPM on the engine (Output: 3.4 PS, 3.35 HP @ 15,000 RPM Practical RPM Range: 2000-16,000)

So 15,000 RPM divided by 1400 HS = 10.71:1 gear ratio,, I'm not for sure if Algin has that or not, but if so,,,,

14,000 RPM
1400 HS
10.0:1 Ratio

13,050 RPM
1400 HS
9.3:1 Ratio

13,000 RPM
1400 HS
9.28:1 Ratio

And so on,,,,,, Hope this helps... XGM/VGM
John Crotts
www.soonerhelicamproductions.com
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04-25-2014 12:46 AM  6 years ago
Einzelganger

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Campbell, Texas

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You want the lowest gear ratio that you can find that will fit your bird if you want power.

Wayne
I love the smell of nitro in the morning.
RIP Roman
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04-25-2014 01:07 AM  6 years ago
Einzelganger

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Campbell, Texas

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If ever Align had a gearing set up for O.S. 70's or YS 80's, that would be the kind of setup you would want for running large blades at low head speeds with a .91. If not, you are gonna be stuck below the engine's comfort zone. It will be quiet when it runs at that speed.

Thunder Tiger makes a 9.3:1 gear set for Raptors that would give you 13,020 rpm at 1400 head speed.

Wayne
I love the smell of nitro in the morning.
RIP Roman
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04-25-2014 04:18 AM  6 years ago
epryor

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Grandview mo,

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I have a Raptor 90 with a 91 engine in it also, maybe I should use it instead of the Trex 700.Batman - Scale it up
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04-25-2014 04:25 AM  6 years ago
epryor

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Grandview mo,

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This is what I found - Raptor 60 Specifications:
Main Rotor Dia: 1540 ~1580 mm (using 660 or 680 mm
blades)
Gear Ratio : 9.3 : 1: 4.6
Full equipped weight : 9.8 lb
(Optional 11 teeth and 12 teeth pinion gears are
available to make the main gear ratio into 7.75 and
8.45 to 1 for operation with the 80 and 90 size heli
engines.)
Remember I will be using 800mm blades.
Batman - Scale it up
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04-25-2014 03:04 PM  6 years ago
xcellgasman101

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WOODWARD, OKLA....

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You will have to extend the boom for sure, but I'm sure Raptor makes a longer boom and torque tube... Good Luck,, XGM/VGMJohn Crotts
www.soonerhelicamproductions.com
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04-25-2014 08:57 PM  6 years ago
ticedoff8

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Morgan Hill, CA. USA

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The OS 91HZ (OSMG1974) is rated at 3.35HP @ 15,000 RPM.

For a 1400 RPM head speed, you need 10.7:1 main gear ratio to run the motor close to 15,000 RPM.

The Raptor 60 & 90 models had several different main gears and pinion gears that allowed you to mix & match to get the best gear ratio for the motor. The gears were interchangeable between the R60 & all R90.

There was a 91, 93, 94 & 95 main gear (the 91 was white plastic).
And, there was a 10, 11 & 12 pinion.

No combination of them could get a 10.7:1 ratio.

But, the 9.5:1 will give you 1578 at 15,000 RPM.

__________91_____93_____94_____95 <-Main Gears
10.00____9.10____9.30____9.40____9.50
11.00____8.27____8.45____8.55____8.64
12.00____7.58____7.75____7.83____7.92
| -----------Gear Ratios
|- Pinions

Then, of course, there is the consideration of the tail authority at the low head speed.
The tail rotor gear ratio on the Raptor 60 & 90 is 4.65:1.
At 1400 RPM head speed, the tail is turning at 6,510 RPM.
My guess is that is too slow for the 800mm main blades even with a longer tailboom.
So, you will need to use the 115mm tail blades - not the 95mm blades that are standard for the R90SE.

If you try to run the motor at a lower RPM to get to 1400, with the 9.5:1 gear ratio, you are at 13,000 RPM. At that RPM and the load of the 800mm blades, there is a pretty good chance of pre-ignition or detention. At the very least, the motor will run hot.

You may want to consider a 0% to 5% Nitro fuel instead of 30%. The 30% nitro is for higher RPM.
You could also look into adding shims in the head to lower the compression for the lower RPM.
+ > ÷
Believe 1/2 of what you see and none of what you hear.
Fake News will be the downfall of our Republic!
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04-25-2014 11:08 PM  6 years ago
epryor

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Grandview mo,

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I wonder should I just go with a Helibug conversion kit and a gas 231 engine?Batman - Scale it up
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