Tail speedup without slotting the frame (confirmed)

[Atomic Skull]

See this post:

https://www.helifreak.com/showpost.p...35&postcount=8

Using the Trex 700 auto gear (H70G007XXW) and front TT drive gear (H70G001AXW) yields a tail ratio of 4.72:1. However the mesh between the auto gear and drive gear is too tight so the screw holes in the frame must be slotted back about 0.5mm to adjust the mesh.

But what if you were to use the 700N front gear (H7NG001AXW) instead? The drive gear has 21 teeth instead of 22 teeth so this should give you the backlash you need without slotting the holes. A 21 tooth mod 1 gear is 23mm in diameter vs 24mm for a 22 tooth gear so it would give you 0.5mm more distance between the two gears, exactly how much the holes need to be slotted with the 22 tooth gear. This would yield a ratio of 4.95:1 so it would work well for low to moderate headspeeds but might not be advisable if you are just looking for more tail authority with high headspeeds.

Please note that I have not tested this, but it seems like it should work unless I am missing something.

[crashcopterman]

Dennis will chime in to see if it's doable.

Steve

[Atomic Skull]

Trex 700N front gear fits perfectly without slotting the frame. There is a tiny, tiny amount of backlash between the spur gear and the auto rotation gear (just the right amount I'd say).

[Baedarlboo]

Nice!

[Atomic Skull]

Ok you need to make one small modification to the front gear. The top of the hub on the spur gear is fatter than the Rush 750 one and will just barely scrap the maingear so you'll need to reduce it a bit. Chuck it up in a power drill and hit it with a file or machine it off with a lathe if you have one. The top of the hub needs to be reduced in diameter to about 10.3mm.

Original gear (forgot to take a picture of mine, this pic is from Amain)



Modded gear, used a power drill and a steady hand with a file then some 2500 grit sandpaper to smooth it out.



Installed, note clearance on main gear. You can see where the main gear scraped the spur gear hub, the gear isn't damaged (I looked at the teeth with a loupe, they're fine) but there is plastic residue from the hub in the teeth which can be scraped off with a fingernail.

[Atomic Skull]

An alternative to modding the TT drive gear might be to use a 110t Trex 700 gear H70G008XXW. This should be 2mm smaller in diameter and give you 1mm extra room between the main gear and tail gears.

EDIT: Or a Ming Da 110T herringbone gear.

http://www.mdhelicoptersusa.com/inde...product_id=228

Details on using this gear with a Trex 700, the same should apply to the Rush 750. This is the older V1 white gear the new black gears are stronger.

I'd recommend reading this entire thread first as there are some non obvious things about using herringbone gears.

https://www.helifreak.com/showthread...ingbone&page=8

[Atomic Skull]

Looking to refine this mod further, If anyone has an Evo or is using the Evo TT front gear please look here.

https://www.helifreak.com/showpost.p...98&postcount=1

I need the diameter of the shaft on the side that holds the spur gear.

[Atomic Skull]

Just an update, if you run this mod you may want to stick to 105mm tail blades. I had some unresolvable tail behavior until I dropped from 115mm to 105mm blades. The tail would make a hissing and spitting noise on CW piros and there was a sort of "vibration" of the tail on hard CW stops. This was sort of a high frequency wag that would decay in a few seconds.

Nothing I did could tune this out (I even tried shortening the servo arm) but as soon as I went to 105mm blades though the problems disappeared.

I'm running 1800rpm and a KST brushless rudder servo btw (BLS 805)

[Atomic Skull]

Looks like the Rush Katana is using a 104t helical auto gear and a 21T drive gear so this mod will probably be obsolete soon.

[robertsm]

Ordered the 700n auto and drive gear. Thanks for this great write up.

[Dawiev]

Originally Posted by Atomic Skull

Looks like the Rush Katana is using a 104t helical auto gear and a 21T drive gear so this mod will probably be obsolete soon.

Soon ............ not a term that has the same general meaning, when used in this forum.

[Dennis P]

I have been flying this 4.9 gear ratio with the Align gears in my Rush and Rail116 tail blades since last year with zero issues and no tail vibes. I did slot the frames slightly as I thought the mesh was to tight . It runs very smooth and very quiet . I ran the Align 4.73 gear set on my previous Rush with Rotor Tech 115s for a few years and the new owner is still running that setup. Very simple to slot the frame slightly and then you can set things up perfectly.

[Atomic Skull]

Originally Posted by Dennis P

I have been flying this 4.9 gear ratio with the Align gears in my Rush and Rail116 tail blades since last year with zero issues and no tail vibes. I did slot the frames slightly as I thought the mesh was to tight . It runs very smooth and very quiet . I ran the Align 4.73 gear set on my previous Rush with Rotor Tech 115s for a few years and the new owner is still running that setup. Very simple to slot the frame slightly and then you can set things up perfectly.

I had about 0.1 backlash so slotting the frame was unnecessary, it might vary from batch to batch though.

When I installed the Align three blade tail I found the mesh was too tight so I tried using the umbrella gear that came with the Align tail shaft and that fixed the problem. I suspect the same would be true if the mesh was too tight for the front drive gear as well, the height the gear sits is a fraction of a mm lower on the Align TT bevel gear than on the Rush one.

[PlaneHazza]

Originally Posted by Dennis P

I have been flying this 4.9 gear ratio with the Align gears in my Rush and Rail116 tail blades since last year with zero issues and no tail vibes. I did slot the frames slightly as I thought the mesh was to tight . It runs very smooth and very quiet . I ran the Align 4.73 gear set on my previous Rush with Rotor Tech 115s for a few years and the new owner is still running that setup. Very simple to slot the frame slightly and then you can set things up perfectly.

Hi Dennis P,

You probably won't see this, but if you do would you mind telling me the part numbers for the autorotation and TT gears I need for my v1 Rush to get 4.9 ratio?

I cannot find H7NG001AXW anywhere, yet I can find H7NG001AXT, which given the specs of tooth count and dimensions, I assume it's just a newer revision? Can anyone confirm please?
Thanks!