Can someone pleaes explain

[Flyingagain]

Can someone pleae explain what engines can run with different gear rations?

For example lets talk a 91 class heli

there is 8:18, 8:28 and 8:45 I know of, i think those are the numbers are at least, can either hte os or ys engine run in these helis with this gear ratio? What is the draw back/gains of each respectively?

Someone please, let me understand this concept please:)

I know electrics pretty well now, not completly the gearing of nitros though

[Alfred]

Alright
I have a bit of a crack at it.
There are better people here that might add additional info later.

Each type of engine has got different rpm's at which they deliver maximum torque and or horsepower/KW.
The OS develop it's power in general at a higher rpm, while the YS does the same at lower rpm.
8:45 is generally used by the 3D looneys :lol: To stop the engine bogging at the rapid collective/cyclic changes. A bit like a racing car where on a tight circuit with lots of tight turns, lower gearing is used that provides more "punch" out of the corners.

8:28, 8:18 are generally used by FAI pilots or that type of flying.
The higher gearing provides the ability to complete the hovering maneuvers at lower rpms which also reduces the noise. You can imagine that a judge sitting just 15m-25 away from the heli is hardly going to give you a good score if he/she is sitting that close to an engine that's screaming it's brains out.
On top of that FAI aerobatics used to be lots of larger graceful maneuvers that required a high entry speed to be completed. It's all about impression.

Muffler/Nitro/blades combinations also have an influence on it and will determine the final ratios being used. It's not that critical for us mere mortals as we usually let the big boys sort out the best combis on new helies and we then generally follow their suggestions.

So, in simple terms:

You like hard tight 3D..use 8:45
You prefer large aerobatics and precision hovering maneuvers, go for 8.18-7.91:1 ratios.

OS engine develop their power at higher rpm and you find 9.0-9.3:1 gear ratios more suitable.