Fuel Foaming

[Steve]

I got fuel tank foaming in one of my helis.

Abviously there's bad vides coming from somewhere, and I need to sort it, so in what order would you recommend starting changing things.

I can start with a different set of blades and paddles at no cost as I have them on another heli, but if that makes no diference, then I'm on to shafts, but which one would you plump for first?

[Ade_Law]

which heli is this?

Ade

[Steve]

Millennium 70 + tuned pipe - 90 size mains and tails.

Could it just be back pressure from the tuned pipe?

[colin n]

not blades.

it will be a high freq vibe from something spinning fast like an engine.
check alignment of motor and then go to balancing the fan or checking the clutch.
ive had it before on start shaft cos there was no runout, that is if u grab the starter coupling and give it a tug there should be a little (tiny ) amount of play.
well when i say should . it cured one of mine.

[colin n]

oh

try backing the pipe off half an inch ( make the gap between pipe and header bigger ) thats a quick experiment. OS dont like pipes , they get a bit harsh.

[Steve]

Who said it was an OS?

Only joking - it is!

Thanks for the input so far.

[robgt]

Do you see any vibration anywhere on the heli, such as on tailboom fins?
Cheers,
Rob

[Steve]

No, none Rob.

In fact, my other Millenium, the 90 version shakes the tail fin quite a bit, but the tank is fine, as is the heli as a whole, apart from the ongoing clutch problems!

[jimbob]

It could be that the tank is just at some resonant frequency and amplifying the inevitable vibrations.
You could try damping the vibration by sticking something to the tank - I know it sounds daft, but try a bit of chewing gum or something similar...
Maybe worth a go if all else fails or as an intermin solution?

[robgt]

Interesting
Is the tank tightly fitted or is it nice and loose in the airframe (not falling out...)?
It is a pumped engine, or muffler pressure system?

Cheers,
Rob

[Steve]

OS 70 engine that's been fully run-in in another heli and is 'sweet as'. It's not pumped, and the pressure nipple on the pipe is right at the start (narrow end!) of the pipe i.e. where you might expect to get maximum back pressure.

The tank is a tight fit between the frames - typical for a Millennium. The tank in my 90 Millennium became loose a while back and it never rippled or foamed. For peace of mind, I stuck some bits of gyro mounting tape between it and the frames to keep it steady, but in reality I don't think it was really necessary.

[robgt]

I was angling for the "too tight" side to be honest.
Pressured system will bloat the tank a little, and if it's too tight in the airframe, it could take frame vibration from the engine and turn it into fuel foaming.
It's a grasp at straws really, but maybe food for thought?

Cheers,
Rob

[Ade_Law]

mainshaft, spindle, tail shaft, tail hub.

when did this start?

got some history on the model?

Ade

[Steve]

Now there's a question Ade.....

I flew this heli for the first time last Saturday, and then again on Sunday.

I've had my Mille 90 for a while, which I bought 2nd hand, and love it. It came with a bundle of spares, and then I made a 2nd purchase of another 2nd hand Mille, also with a bundle of spares on behalf of a clubmate who basically just wanted a single airframe for potential spares as and when he might dink his Millennium.

Anyway, I did a deal with my mate, and ended up with almost enough spares from various sources to build a 2nd helicopter, so with various purchases from BobJ of the bits that were missing, I've ended up with my 2nd Mille.

Now, of course what this means is that many of the parts are of an unknown history, but also I can interchange parts with the 90 which is fine, so can try things without any particular costs.

Hence the original post about what order to approach this.

The main shaft used to be in the my 90, and was fine in there. I only changed it because I had a momentary fit of worrying about it's history and suitability in a 2nd hand model. There was no need for that, but hey, it's history now. Basically I can assume that the main shaft is fine.

The feathering spindle is new - one of the parts I had to buy new from BobJ.

Tail shaft looked good on close inspection, but could be.... I've got a brand new and unused spare so could chenge that.

Tail hub - dunno it's history or straightness - don't have a spare.

Paddles are non - Robbe, not sure what sort they are - they're very square in plan view and look like carbon.

Blades are old open-tipped NHPs. They came to me 2nd hand with white sticky coverrings on them. The sticky coverring had nicks in it so I took it all off to inspect the blades for damage. They looked fine, so I cleaned them down and CGd and balanced them. One blade needed quite a bit of tape to get the balance, and the CG diference between the 2 blades is down to about 5mm, which is acceptable according to the Millennium manual.

[Steve]

An easy place to start might be to just quickly change the whole tail assembly over with the other heli maybe, and similarly with the blades.