TRex 700 Spindle Guides

[Steve]

I've been noticing a bit of an all over heli wobble at certain head speeds on my 700. It doesn't happen at governed head speeds during the flight envelope at all, but always gives a good shimmy during spool up and also starts to wobble when the head speed decays during an auto, so it's clearly happening at maybe (guesstimate) 1000 rpm head speed.

I've decided to change the dampers this evening for new ones, no big deal there and they are cheap as chips anyway. But, the old ones didn't look to be in a bad state at all so a bit of further inspection seems to show that the two plastic spindle guides that fit inside head yoke and stop the grips from lead/lagging don't seem to be as 'oval' as they used to be, but it's been a long time since I built it. From memory, they seem a lot more round on the inside than when they were new.

So, would these parts being worn cause the symptoms that I'm seeing, and if so, why?

I will try it again with the new dampers and freshly greased thrust races etc at the weekend. Just curious about these guides and their significance in the head.

[Steve]

Anyone?

[coolice]

Hey Steve.

It does sound to me to just be a damper induced wobble which all helicopters can get at certain rotor RPM's. It changes with different damper rubber hardness and also main blades which can effect the rpm the shimmys come in at.

One thing you have not mentioned is if you grease your damper rubbers on assembly? I have always done mine and is a trick which has been about for many years but not widely known in some cases.
Without them being greased they can grab the spindle which is trying to wiggle within the dampers but can't easily, then a big movement comes along and the dampers suddenly allow the spindle to move and you get a wobble. This then happens again until the rotor rpm settles past the point at which it started.

Might be worth adding a light smearing of grease to them, I use the Align kit supplied one way grease when building my models.
.

[Steve]

Thanks Ian.

Yes, I always liberally grease dampers inside and out, but the old ones hadn't been out of the head for quite some time and were a bit grease-free from use.

New ones are well slimey!

[Kozmyk]

Silicon vs Mineral Grease

Reading about greasing O rings reminds me that when airgun shooting (precharged pneumatic) I used to put Molykote33 on the high pressure O rings that sealed the compressed air connections.
IIRC we had to use special grease because of ordinary grease attacking the rubber, low temperatures (as air pressure dropped rapidly) and to eliminate combustion from pressurising the oxygen in air next to a hydrocarbon.
Clearly the last two don't apply to heli flying but the attack on rubber may be significant unless the damper material is attack resistant.
Just a thought.

[ROSSCO]

I always use a silicone based grease, particularly the Tri-Flow Silicone Grease with Teflon. It is designed for use on O-Rings and particularly dampers without attacking the rubber. It seems to last and does'nt get blown out, i can dismantle the head after months of use and still find penty of grease in the thrust bearings and around the dampers.

This is the stuff i use:
Fast Lad Performance

Also used these in the past with good results:
Fast Lad Performance
Fast Lad Performance
Fast Lad Performance

[Steve]

This is the stuff i use:
Fast Lad Performance

Exactly the same stuff as I use already.

[ROSSCO]

Quality Grease your using there then, should'nt have a problem with it.

I've never seen the plastic head spindle guide inserts wear but theres nothing to say they have not done so, usually the dampers wear before anything else.

I'd probably flight test with new dampers and see how it goes from there, the dampers are the usual suspects. With regards to the plastic head inserts if the problem persists then i don't believe they are available as a spare part and are only available with a replacement head block, but if you put out some feelers i'm sure somebody has damaged a headblock and has had a replacement so may have some inserts knocking around.

[Steve]

Well I do use the ultra-heavy FAI Radix blades on my 700, so I wonder if the increased rotating mass could emphasise any lead/lag tendencies and squish the inserts - it's certainly the way it looks.

But I will try the new dampers first and see how it goes.