90 Size Gear Ratios

[Steve]

Curiosity - well, I think I know the answer but would appreciate your input/discussion.

What is the 'ideal' main gear ratio for a modern 90 size heli and engine for 3D?

What is the 'ideal' main gear ratio for a modern 90 size heli and engine for FAI?

What symptoms would an over-ratioed heli demonstrate?

What symptoms would an under-ratioed heli demonstrate?

From a bit of Googling, some ratios are:

JR Vibe - 8:1
TRex700 - 8.1:1
Xcell - Various options between 7.9 and 8.45:1
Aurora - 8.25:1
Srimok - 8.25:1
Raptor 90 - 8.27:1
Millennium - 8:1 (other options available)
Freya - 7.9:1

[mattie-lodge]

This is the way I see it Steve,

You gear the helicopter to match the engine you are using and for the performance you want it to give. If your engine optimum power is at say 16,000 and you wanted to run 1950 on the head you would look for a gearing of 8.20 or as close as.

This is the reason why people were trying to lower the ratio when running YS engines as the torque produced was slightly lower than an OS engine

[Steve]

This is the way I see it Steve,

You gear the helicopter to match the engine you are using and for the performance you want it to give. If your engine optimum power is at say 16,000 and you wanted to run 1950 on the head you would look for a gearing of 8.20 or as close as.

This is the reason why people were trying to lower the ratio when running YS engines as the torque produced was slightly lower than an OS engine

Rumour has it that the YS engines peak at about 15400rpm, so you'd be looking for a lower ratio than say an OS engine, say 7.9:1 for 1950 at the head?

What about the symptoms?

[mattie-lodge]

Correct Steve,

Symptons:- You would find the motor would produce great power than as soon as you bog it, the motor will die instead of loading up and hold its power. If that makes sense

[Steve]

Correct Steve,

Symptons:- You would find the motor would produce great power than as soon as you bog it, the motor will die instead of loading up and hold its power. If that makes sense

If it was over-geared or under-geared?

[mattie-lodge]

I reckon that would be for over gearing. Under gearing would give a more soggy feel, almost like you would want to boot the heli up the ar$e to get it to shift

[Steve]

Under gearing would give a more soggy feel, almost like you would want to boot the heli up the ar$e to get it to shift

Hmmmmmmmm.

[jwatts007]

Been trying 7.6-1 on my Raptor 90, using a novarossi 90, sounds good to me, I like to keep the noise down, will try 8-1 this week.

[barney]

For 3D flying most are trying to extract the max available power.(kinda like driving in 3rd gear) as you tend to run a lot of Collective and Cyclic Pitch

F3C flyers will run say 4th gear as they will run a very high H/S upstairs to get very fast straight line performance but they will not have an over Pitched model like a 3D flyer so a ratio like 7.6-7.8 works but things are erring towards 8:1 I would guess. Drop Steve Roberts on here a PM as he will keep you right on that one for sure.

You need to consider whether you actually need that power for the flying you are doing as its hard on the motor and machine to be screaming all the time. For 3D the general rule is to gear it so RPM is above the max power so when the motor loads and slows a bit its still at or above Max Power.

NOW if you read your motor specs carefully they DONT normally say MAX power but only X power at a certain RPM. If you read on It also gives the recommended usable RPM which extends considerably higher than the power figure given.

Given the above you have plenty of Headroom to run 1900-1950 on 8:2. BUT Remember The shorter your main blades the more RPM you will likely require as its the blade tipspeed thats important for fast, clean stopping power.

Hence a 500 runs 2600 a 50 2100 so a 90 circa 1900

Brian

[trillian]

When talking about relatively small differences in gearing, can't you basically compensate with more or less pitch? Obviously a large difference in headspeed will change the flight characteristics and if you need to make fast changes in direction etc. you need to keep the headspeed up there.

Also some people are using 690s and some are using 710s on 90 machines.

I had my Predator gasser over at the field yesterday and someone thought the headspeed was very high. Not having tached it I don't know exactly what it is but it didn't feel or sound particularly high to me. I know it has the torque to run at a very low headspeed and can deal with the pitch but I think there's always a 'sweet spot' in the combination of pitch and headspeed.